Nuova ricerca

Alessandro CHINI

Professore Ordinario
Dipartimento di Ingegneria "Enzo Ferrari"

Home | Curriculum(pdf) | Didattica |


Pubblicazioni

2024 - Microwave and Millimeter-Wave GaN HEMTs: Impact of Epitaxial Structure on Short-Channel Effects, Electron Trapping, and Reliability [Articolo su rivista]
Zanoni, Enrico; Santi, Carlo De; Gao, Zhan; Buffolo, Matteo; Fornasier, Mirko; Saro, Marco; Pieri, Francesco De; Rampazzo, Fabiana; Meneghesso, Gaudenzio; Meneghini, Matteo; Zagni, Nicolo'; Chini, Alessandro; Verzellesi, Giovanni
abstract

Application of gallium nitride high-electron-mobility transistors (GaN HEMTs) to millimeter-wave power amplifiers requires gate length scaling below 150 nm: in order to control short-channel effects, the gate-to-channel distance must be decreased, and the device epitaxial structure has to be completely redesigned. A high 2-D electron gas (2DEG) carrier density can be preserved even with a very thin top barrier layer by substituting AlGaN with AlN, InAl(Ga)N, or ScAlN. Moreover, to prevent interaction of hot electrons with compensating impurities and defects in the doped GaN buffer, the latter has to be separated from the channel by a back barrier. Other device designs consist in adopting a graded channel (which controls the electric field) or to adopt nitrogen-polar (N-polar) GaN growth (which decreases the distance between gate and channel, thus attenuating short-channel effects). The aim of this article is to review the various options for controlling short-channel effects, improve off-state characteristics, and reduce drain–source leakage current. Advantages and potential drawbacks of each proposed solution are analyzed in terms of current collapse (CC), dispersion effects, and reliability.

2023 - Dynamic Behavior of Threshold Voltage and ID-VDS Kink in AlGaN/GaN HEMTs Due to Poole-Frenkel Effect [Articolo su rivista]
Gao, Z; De Santi, C; Rampazzo, F; Saro, M; Fornasier, M; Meneghesso, G; Meneghini, M; Chini, A; Verzellesi, G; Zanoni, E
abstract

The kink effect in field-effect transistors (FETs) consists in a sudden increase in drain current, during a drain voltage sweep and leading to a higher drain current saturation value. We report new experimental data concerning the dynamic behavior of the "kink" in AlGaN/GaN HEMTs and correlate them with deep levels. The results demonstrate the role of the Poole-Frenkel effect in determining the occurrence of the kink and identify the experimental conditions that make it observable.

2023 - GaN on Si Power and RF Devices and Application [Abstract in Atti di Convegno]
Castagna, M. E.; Marchesi, M.; Miccoli, C.; Moschetti, M.; Giorgino, G.; Cioni, M.; Tringali, C.; Chini, A.; Iucolano, F.
abstract

2023 - Gate-Bias Induced RON Instability in p-GaN Power HEMTs [Articolo su rivista]
Chini, Alessandro; Zagni, Nicolo'; Verzellesi, Giovanni; Cioni, Marcello; Giorgino, Giovanni; Nicotra, Maria Concetta; Castagna, Maria Eloisa; Iucolano, Ferdinando
abstract

In this letter, we investigate the on-resistance ( RON ) instability in p-GaN power HEMTs induced by a positive or negative gate bias ( VGB ), following the application of a quasi-static initialization voltage ( VGP ) of opposite sign. The transient behavior of this instability was characterized at different temperatures in the 90–135 °C range. By monitoring the resulting drain current transients, the activation energy as well as time constants of the processes are characterized. Not trivially, both RON increase/decrease were found to be thermally activated and with same activation energy. We attribute the thermal activation of both RON increase/decrease to the charging/discharging of hole traps present in the AlGaN barrier in the region below the gate.

2023 - Impact of Gate and Drain Leakage on VTHDrift and Dynamic-RONof 100V p-GaN Gate AlGaN/GaN HEMTs [Relazione in Atti di Convegno]
Cioni, M.; Giorgino, G.; Chini, A.; Parisi, A.; Cappellini, G.; Modica, L.; Luongo, G.; Miccoli, C.; Castagna, M. E.; Moschetti, M.; Tringali, C.; Iucolano, F.
abstract

The impact of gate and drain leakage on VTH drift and dynamic-RON of 100 V p-GaN gate AlGaN/GaN HEMTs is investigated in this work. Devices presenting two different AlGaN barrier layer designs are characterized by means of DC gate/drain leakage and Pulsed I-V measurements. Results show that a larger gate leakage yields a reduced positive VTH drift under off-state stress at large VDS, coherently with hole injection in the floating p-GaN gate. Conversely, a larger off-state drain leakage current exacerbates the RON degradation at high VDS,stress due to hot-electrons effects. The application of a negative VGS,stress has been demonstrated to solve this issue, thanks to a more pinched-off channel that avoids hot-electrons related issues.

2023 - Impact of Process Variations on Back-Bias Effect in 100V p-GaN Gate AlGaN/GaN HEMTs [Relazione in Atti di Convegno]
Cioni, M.; Giorgino, G.; Chini, A.; Marletta, G.; Miccoli, C.; Castagna, M. E.; Luongo, G.; Moschetti, M.; Tringali, C.; Iucolano, F.
abstract

In this paper, we investigate the impact of Buffer resistivity and AlGaN barrier design on back-bias stress performed on 100 V p-GaN gate AlGaN/GaN HEMTs. To this end, we compare the results obtained in terms of (i) vertical leakage, (ii) back-bias stress on Transmission Line Measurements (TLM) structures and (iii) back-gating on real transistors. Concerning the latter, a novel test sequence is implemented to monitor the drain current evolution during the stress and evaluate the impact on V TH and R ON parameters after 1000 s stress with V SUB =-50 V. Results indicate that high resistive buffer can significantly reduce the back-bias effect, but also the AlGaN barrier design can affect the parameters drift due to a different two-dimensional electron gas (2DEG) density.

2023 - Improved High Temperature Behaviour of On-Resistance in 100V p-GaN HEMTs [Abstract in Atti di Convegno]
Giorgino, G.; Cioni, M.; Cappellini, G.; Iucolano, F.; Miccoli, C.; Castagna, M. E.; Moschetti, M.; Tringali, C.; Chini, A.
abstract

2023 - Mechanisms of Step-Stress Degradation In Carbon-Doped 0.15 μm AlGaN/GaN HEMTs for Power RF Applications [Articolo su rivista]
Zagni, Nicolo'; Zhan, Veronica Gao; Verzellesi, Giovanni; Chini, Alessandro; Pantellini, Alessio; Natali, Marco; Lucibello, Andrea; Latessa, Luca; Lanzieri, Claudio; Santi, Carlo De; Meneghini, Matteo; Meneghesso, Gaudenzio; Zanoni, Enrico
abstract

We discuss the degradation mechanisms of C-doped 0.15-μm gate AlGaN/GaN HEMTs tested by drain step-stress experiments. Experimental results show that these devices exhibit cumulative degradation effects during the step stress experiments in terms of either (i) transconductance (gm) decrease without any threshold-voltage (VT) change under OFF-state stress, or (ii) both VT and gm decrease under ON-state stress conditions. To aid the interpretation of the experiments, two-dimensional hydrodynamic device simulations were carried out. Based on obtained results, we attribute the gm decrease accumulating under OFF-state stress to hole emission from CN acceptor traps in the gate-drain access region of the buffer, resulting in an increase in the drain access resistance. On the other hand, under ON-state stress, channel hot electrons are suggested to be injected into the buffer under the gate and in the gate-drain region where they can be captured by CN traps, leading to VT and gm degradation, respectively.

2023 - Microwave and millimeter-wave GaN HEMTs: impact of epitaxial structure on short-channel effects, electron trapping and reliability [Abstract in Atti di Convegno]
Zanoni, Enrico; Gao, Zhan; Fornasier, Mirko; Saro, Marco; Rampazzo, Fabiana; De Santi, Carlo; Meneghesso, Gaudenzio; Meneghini, Matteo; Cioni, Marcello; Zagni, Nicolo'; Chini, Alessandro; Verzellesi, Giovanni
abstract

2023 - Modelling and Simulation of ON-Resistance Instability due to Gate Bias in p-GaN Power HEMTs [Abstract in Atti di Convegno]
Zagni, N.; Chini, A.; Verzellesi, G.; Cioni, M.; Giorgino, G.; Nicotra, M. C.; Castagna, M. E.; Iucolano, F.
abstract

2023 - Study of 100V GaN power devices in dynamic condition and GaN RF device performances in sub-6GHz frequencies [Articolo su rivista]
Giorgino, G.; Cioni, M.; Miccoli, C.; Gervasi, L.; Giuffrida, M. F. S.; Ruvolo, M.; Castagna, M. E.; Cappellini, G.; Luongo, G.; Moschetti, M.; Constant, A.; Tringali, C.; Iucolano, F.; Chini, A.
abstract

2023 - Temperature Effect on RON-degradation induced by Off-state Drain Voltage Stress [Abstract in Atti di Convegno]
Cioni, M.; Giorgino, G.; Cappellini, G.; Chini, A.; Miccoli, C.; Castagna, M. E.; Moschetti, M.; Tringali, C.; Iucolano, F.
abstract

2023 - Unveiling the Role of Hole Barrier Traps on ON-Resistance Instability after Gate Bias Stress in p-GaN Power HEMTs [Relazione in Atti di Convegno]
Zagni, Nicolo'; Chini, Alessandro; Verzellesi, Giovanni; Cioni, Marcello; Giorgino, Giovanni; Nicotra, Maria Concetta; Eloisa Castagna, Maria; Iucolano, Ferdinando
abstract

Gate bias effects on p-GaN power HEMTs stability are primarily investigated in terms of threshold voltage (V T ) drifts. However, applying positive and negative gate bias for long time can also induce ON-resistance (R ON ) instability which is often overlooked in the literature. In this work, we characterize I D -V GS after applying a –6/+6 V gate initialization (VGP) bias for 3000 s at different temperatures and reproduce the same characteristics by means of numerical simulations. The comparison between measurements and simulations allowed attributing the observed R ON dispersion to barrier acceptor traps emitting/capturing holes (injected/removed by the Schottky contact on the p-GaN layer) after V GP = –6/+6 V, respectively. Characterization of I D /R ON transients at different temperatures obtained after initialization at +6 V while keeping a gate baseline bias V GB = –6 V allows extracting an activation energy E A ≈ 0.4 eV. This feature is also reproduced by the simulations, further indicating that the underlying mechanism of R ON increase is the charge of acceptor traps in the barrier mediated by hole removal through the Schottky gate contact on the p-GaN region.

2022 - A Novel Temperature Estimation Technique Exploiting Carrier Emission from Buffer Traps [Relazione in Atti di Convegno]
Cioni, Marcello; Zagni, Nicolo; Chini, Alessandro
abstract

We propose a novel technique for temperature estimation in electron devices based on the mutual correlation between emission time constant from traps ( τ ) and temperature ( T ). Arrhenius equation is employed as the physical model relating τ and T The reference system used to present the technique is AlGaN/GaN high electron mobility transistors (HEMTs) with Fe-doping in the buffer. Drain Current Transients (DCTs) are used for extracting the emission time constant ( τ ) from Fe traps and non-linear regression through Trust Region Reflective (TRR) optimization algorithm is used to learn the model parameters from data and infer device temperature. Electro-thermal device simulations are employed for validating the proposed technique, showing that this method is able to provide an improved accuracy with respect to conventional electrical techniques (e.g., McAlister method) promoting it as a valid alternative to state of-the-art optical techniques in GaN HEMTs.

2022 - Experimental and numerical investigation of Poole-Frenkel effect on dynamic RON transients in C-doped p-GaN HEMTs [Articolo su rivista]
Zagni, Nicolò; Cioni, Marcello; Iucolano, Ferdinando; Moschetti, Maurizio; Verzellesi, Giovanni; Chini, Alessandro
abstract

In this paper, we investigate the influence of Poole-Frenkel Effect (PFE) on the dynamic RON transients in C-doped p-GaN HEMTs. To this aim, we perform a characterization of the dynamic RON transients acquired during OFF-state stress (i.e., VGS,STR = 0 V < VT, VDS,STR = 25–125 V) and we interpret the results with the aid of numerical simulations. We find that dynamic RON transients at room temperature accelerate with VDS,STR1/2, which is signature of PFE, as further confirmed by the simultaneous decrease of the activation energy (EA) extracted from the Arrhenius plot of the dynamic RON transients at VDS,STR = 50 V and T = 30–110 °C. Results obtained by means of calibrated numerical simulations reproduce the exponential dependence of transients time constants (τ) on VDS,STR1/2 and consequent EA reduction only when including PFE enhancement of hole emission from dominant acceptor traps in the buffer related to C doping. This result is consistent with the model that considers hole emission from acceptor traps (rather than electron capture) as the mechanism underlying dynamic RON increase during OFF-state stress.

2022 - Failure Physics and Reliability of GaN-Based HEMTs for Microwave and Millimeter-Wave Applications: A Review of Consolidated Data and Recent Results [Articolo su rivista]
Zanoni, E.; Rampazzo, F.; De Santi, C.; Gao, Z.; Sharma, C.; Modolo, N.; Verzellesi, G.; Chini, A.; Meneghesso, G.; Meneghini, M.
abstract

Herein, the results are reviewed concerning reliability of high-electron mobility transistors (HEMTs) based on GaN, which currently represent the technology of choice for high-efficiency microwave and millimeter-wave power amplifiers. Several failure mechanisms of these devices are extensively studied, including converse piezoelectric effects, formation of conductive percolation paths at the edge of gate toward the drain, surface oxidation of GaN, time-dependent breakdown of GaN buffer, and of field-plate dielectric. For GaN HEMTs with scaled gate length, the simultaneous control of short-channel effects, deep-level dispersion, and hot-electron-induced degradation requires a careful optimization of epitaxial material quality and device design.

2022 - Fe-Traps Influence on Time-dependent Breakdown Voltage in 0.1-μm GaN HEMTs for 5G Applications [Relazione in Atti di Convegno]
Cioni, M.; Zagni, N.; Chini, A.
abstract

Scaled (LG = 0.1 μm) GaN HEMT technology is currently pursued for high-frequency applications (such as 5G), requiring high current/speed and blocking capability. However, traps introduced with intentional Fe doping yield time-dependent breakdown voltage (VBR), seriously affecting reliability. Here, we investigate the role of Fe traps by pulsed I-V characterization performed at different pulse durations (TOFF). A TOFF-dependent VBR is observed on tested devices and is ascribed to the time-dependent occupancy of deep acceptors in the buffer layer. More specifically, the decrease in VBR for short pulses is attributed to the increased leakage due to the reduced ionization of Fe-traps. This interpretation is supported by 2D numerical simulations.

2022 - Identification of Interface States responsible for VTHHysteresis in packaged SiC MOSFETs [Relazione in Atti di Convegno]
Cioni, M.; Fiorenza, P.; Roccaforte, F.; Saggio, M.; Cascino, S.; Messina, A.; Vinciguerra, V.; Calabretta, M.; Chini, A.
abstract

We investigate the mechanism governing threshold voltage (VTH) hysteresis in packaged SiC MOSFETs. A double-ramp measurement method was employed for this scope, being able to accurately evaluate the time-dependent recovery of the positive VTH shift induced by the sweep-up of the gate voltage. Particularly, we studied the effect of the (i) gate driving voltage (VGH), (ii) recovery time (TOFF) and (iii) temperature (T) on the VTH hysteresis. No appreciable differences were observed among data collected at different VGH, whereas a recovery speed-up was observed at higher T values. Temperature dependent measurement of VTH recovery yielded a 0.3 eV activation energy, that has been associated to SiC/SiO2 interface traps located ~0.3 eV below the SiC conduction band.

2022 - Role of carbon in dynamic effects and reliability of 0.15-um AlGaN/GaN HEMTs for RF power amplifiers [Relazione in Atti di Convegno]
De Santi, Carlo; Zanoni, Enrico; Meneghini, Matteo; Meneghesso, Gaudenzio; Rampazzo, Fabiana; Gao, Veronica Zhan; Sharma, Chandan; Chiocchetta, Francesca; Verzellesi, Giovanni; Chini, Alessandro; Cioni, Marcello; Zagni, Nicolò; Lanzieri, Claudio; Pantellini, Alessio; Peroni, Marco; Latessa, Luca
abstract

This paper presents results concerning the dynamic performance and reliability of Fe-doped and C-doped 0.15-m gate AlGaN/GaN HEMTs. Step-stress tests at increasing drain-source voltage and different gate-source voltages are specifically reported. Fe-doped HEMTs exhibit, under both off- and on-state conditions, excellent parametric stability up to breakdown. C-doped devices are instead affected by enhanced degradation effects during the step stress experiments compared to Fe-doped ones, consisting of RON increase during off-state stress and both threshold-voltage and RON increase under on-state conditions. 2D hydrodynamic device simulations are used to validate hypotheses on the physical mechanisms underlying the observed, distinctive degradation effects. The role of C doping in causing additional degradation compared to Fe-doped device is explained with the aid of device simulations as follows: 1) under off-state conditions, hole emission from the CN acceptor traps in the gate-drain region of the buffer leads to an RON increase which is not completely recovered during the typical recovery time interval following each stress phase and therefore accumulates during the step stress experiment; 2) under on-state conditions, channel hot electrons are injected (besides towards the surface) into the buffer where they can be captured by CN traps under the gate and in the gate-drain region, inducing semi-permanent threshold-voltage and RON increases.

2022 - Symmetrical VTH/RONDrifts Due to Negative/Positive Gate Stress in p-GaN Power HEMTs [Relazione in Atti di Convegno]
Zagni, N.; Cioni, M.; Castagna, M. E.; Moschetti, M.; Iucolano, F.; Verzellesi, G.; Chini, A.
abstract

2022 - Temperature-Independent Current Dispersion in 0.15 μm AlGaN/GaN HEMTs for 5G Applications [Articolo su rivista]
Zagni, Nicolo'; Verzellesi, Giovanni; Chini, Alessandro
abstract

Thanks to high-current densities and cutoff frequencies, short-channel length AlGaN/GaN HEMTs are a promising technology solution for implementing RF power amplifiers in 5G front-end modules. These devices, however, might suffer from current collapse due to trapping effects, leading to compressed output power. Here, we investigate the trap dynamic response in 0.15 μm GaN HEMTs by means of pulsed I-V characterization and drain current transients (DCTs). Pulsed I-V curves reveal an almost absent gate-lag but significant current collapse when pulsing both gate and drain voltages. The thermally activated Arrhenius process (with EA ≈ 0.55 eV) observed during DCT measurements after a short trap-filling pulse (i.e., 1 μs) indicates that current collapse is induced by deep trap states associated with iron (Fe) doping present in the buffer. Interestingly, analogous DCT characterization carried out after a long trap-filling pulse (i.e., 100 s) revealed yet another process with time constants of about 1–2 s and which was approximately independent of temperature. We reproduced the experimentally observed results with two-dimensional device simulations by modeling the T-independent process as the charging of the interface between the passivation and the AlGaN barrier following electron injection from the gate.

2021 - Effect of Trap-Filling Bias on the Extraction of the Time Constant of Drain Current Transients in AlGaN/GaN HEMTs [Relazione in Atti di Convegno]
Zagni, Nicolo; Cioni, Marcello; Chini, Alessandro
abstract

2021 - Electric Field and Self-Heating Effects on the Emission Time of Iron Traps in GaN HEMTs [Articolo su rivista]
Cioni, Marcello; Zagni, Nicolo; Selmi, Luca; Meneghesso, Gaudenzio; Meneghini, Matteo; Zanoni, Enrico; Chini, Alessandro
abstract

In this paper we separately investigate the role of electric field and device self-heating (SHE) in enhancing the charge emission process from Fe-related buffer traps (0.52 eV from Ec) in AlGaN/GaN High Electron Mobility Transistors (HEMTs). The experimental analysis was performed by means of Drain Current Transient (DCT) measurements for either i) different dissipated power (PD,steady) at constant drain-to-source bias (VDS,steady) or ii) constant PD,steady at different VDS,steady. We found that i) an increase in PD,steady yields an acceleration in the thermally activated emission process, consistently with the temperature rise induced by SHE. On the other hand, ii) the field effect turned out to be negligible within the investigated voltage range, indicating the absence of Poole-Frenkel effect (PFE). A qualitative analysis based on the electric field values obtained by numerical simulations is then presented to support the interpretation and conclusions.

2021 - Evaluation of VTH and RON Drifts during Switch-Mode Operation in Packaged SiC MOSFETs [Articolo su rivista]
Cioni, Marcello; Bertacchini, Alessandro; Mucci, Alessandro; Zagni, Nicolò; Verzellesi, Giovanni; Pavan, Paolo; Chini, Alessandro
abstract

In this paper, we investigate the evolution of threshold voltage (VTH) and on-resistance (RON) drifts in the silicon carbide (SiC) power metal-oxide-semiconductor field-effect transistors (MOSFETs) during the switch-mode operation. A novel measurement setup for performing the required on-the-fly characterization is presented and the experimental results, obtained on commercially available TO-247 packaged SiC devices, are reported. Measurements were performed for 1000 s, during which negative VTH shifts (i.e., VTH decrease) and negative RON drifts (i.e., RON decrease) were observed. To better understand the origin of these parameter drifts and their possible correlation, measurements were performed for different (i) gate-driving voltage (VGH) and (ii) off-state drain voltage (VPH). We found that VTH reduction leads to a current increase, thus yielding RON to decrease. This correlation was explained by the RON dependence on the overdrive voltage (VGS–VTH). We also found that gate-related effects dominate the parameter drifts at low VPH with no observable recovery, due to the repeated switching of the gate signal required for the parameter monitoring. Conversely, the drain-induced instabilities caused by high VPH are completely recoverable within 1000 s from the VPH removal. These results show that the measurement setup is able to discern the gate/drain contributions, clarifying the origin of the observed VTH and RON drifts.

2021 - GaN-based power devices: Physics, reliability, and perspectives [Articolo su rivista]
Meneghini, Matteo; De Santi, Carlo; Abid, Idriss; Buffolo, Matteo; Cioni, Marcello; Khadar, Riyaz Abdul; Nela, Luca; Zagni, Nicolò; Chini, Alessandro; Medjdoub, Farid; Meneghesso, Gaudenzio; Verzellesi, Giovanni; Zanoni, Enrico; Matioli, Elison
abstract

Over the last decade, gallium nitride (GaN) has emerged as an excellent material for the fabrication of power devices. Among the semicon- ductors for which power devices are already available in the market, GaN has the widest energy gap, the largest critical field, and the highest saturation velocity, thus representing an excellent material for the fabrication of high-speed/high-voltage components. The presence of spon- taneous and piezoelectric polarization allows us to create a two-dimensional electron gas, with high mobility and large channel density, in the absence of any doping, thanks to the use of AlGaN/GaN heterostructures. This contributes to minimize resistive losses; at the same time, for GaN transistors, switching losses are very low, thanks to the small parasitic capacitances and switching charges. Device scaling and monolithic integration enable a high-frequency operation, with consequent advantages in terms of miniaturization. For high power/high- voltage operation, vertical device architectures are being proposed and investigated, and three-dimensional structures—fin-shaped, trench- structured, nanowire-based—are demonstrating great potential. Contrary to Si, GaN is a relatively young material: trapping and degradation processes must be understood and described in detail, with the aim of optimizing device stability and reliability. This Tutorial describes the physics, technology, and reliability of GaN-based power devices: in the first part of the article, starting from a discussion of the main proper- ties of the material, the characteristics of lateral and vertical GaN transistors are discussed in detail to provide guidance in this complex and interesting field. The second part of the paper focuses on trapping and reliability aspects: the physical origin of traps in GaN and the main degradation mechanisms are discussed in detail. The wide set of referenced papers and the insight into the most relevant aspects gives the reader a comprehensive overview on the present and next-generation GaN electronics.

2021 - Impact of Soft- and Hard-Switching transitions on VTH and RON Drifts in packaged SiC MOSFETs [Relazione in Atti di Convegno]
Cioni, M.; Chini, A.
abstract

2021 - Investigation on VTH and RON Slow/Fast Drifts in SiC MOSFETs [Relazione in Atti di Convegno]
Cioni, M.; Bertacchini, A.; Mucci, A.; Verzellesi, G.; Pavan, P.; Chini, A.
abstract

RON and VTH drifts in TO-247 SiC packaged MOSFETs are investigated in this paper. The use of a novel on-the-fly measurement setup able to capture their variation over a 100µs to 1000s time range revealed the presence of two separated fast and slow mechanisms affecting the VTH and RON stability. Particularly, fast drain-induced mechanisms were found to negatively shift VTH, whereas no appreciable fast drifts were observed on RON. Conversely, slow drifts were found on both parameters, yielding a decrease in VTH and an RON increase. To investigate their origin, measurements were carried out for either i) different Duty Cycles and ii) several on-state current levels, proving that device self-heating (i.e., temperature increase) is responsible for the observed slow instabilities.

2021 - Mechanisms Underlying the Bidirectional VT Shift After Negative-Bias Temperature Instability Stress in Carbon-Doped Fully Recessed AlGaN/GaN MIS-HEMTs [Articolo su rivista]
Zagni, Nicolo; Cioni, Marcello; Chini, Alessandro; Iucolano, Ferdinando; Puglisi, Francesco Maria; Pavan, Paolo; Verzellesi, Giovanni
abstract

In this brief, we investigate the bidirectional threshold voltage drift (VT) following negative-bias temperature instability (NBTI) stress in carbon-doped fully recessed AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs). Several stress conditions were applied at different: 1) gate biases (VGS,STR); 2) stress times (tSTR); and 3) temperatures (T). Both negative and positive VT (thermally activated with different activation energies, EA) were observed depending on the magnitude of VGS,STR. In accordance with the literature, observed VT < 0 V (EA ≈ 0.5 eV) under moderate stress is attributed to the emission of electrons from oxide and interface traps. Instead, VT > 0 V (EA ≈ 0.9 eV) under high stress is attributed to the increased negatively ionized acceptor trap density in the buffer associated with carbon doping.

2021 - On the Modeling of the Donor/Acceptor Compensation Ratio in Carbon‐Doped GaN to Univocally Reproduce Breakdown Voltage and Current Collapse in Lateral GaN Power HEMTs [Articolo su rivista]
Zagni, Nicolo'; Chini, Alessandro; Puglisi, Francesco Maria; Pavan, Paolo; Verzellesi, Giovanni
abstract

The intentional doping of lateral GaN power high electron mobility transistors (HEMTs) with carbon (C) impurities is a common technique to reduce buffer conductivity and increase breakdown voltage. Due to the introduction of trap levels in the GaN bandgap, it is well known that these impurities give rise to dispersion, leading to the so‐called “current collapse” as a collateral effect. Moreover, first‐principles calculations and experimental evidence point out that C introduces trap levels of both acceptor and donor types. Here, we report on the modeling of the donor/acceptor compensation ratio (CR), that is, the ratio between the density of donors and acceptors associated with C doping, to consistently and univocally reproduce experimental breakdown voltage (VBD) and current‐collapse magnitude (ΔICC). By means of calibrated numerical device simulations, we confirm that ΔICC is controlled by the effective trap concentration (i.e., the difference between the acceptor and donor densities), but we show that it is the total trap concentration (i.e., the sum of acceptor and donor densities) that determines VBD, such that a significant CR of at least 50% (depending on the technology) must be assumed to explain both phenomena quantitatively. The results presented in this work contribute to clarifying several previous reports, and are helpful to device engineers interested in modeling C‐doped lateral GaN power HEMTs.

2021 - Partial Recovery of Dynamic RON Versus OFF-State Stress Voltage in p-GaN Gate AlGaN/GaN Power HEMTs [Articolo su rivista]
Cioni, Marcello; Zagni, Nicolo; Iucolano, Ferdinando; Moschetti, Maurizio; Verzellesi, Giovanni; Chini, Alessandro
abstract

Dynamic Ron dispersion due to buffer traps is a well-known issue of GaN power high electron mobility transistors (HEMTs), critically impacting their performance and stability. Several works show that the dynamic Ron reaches a maximum for some off-state drain-source voltage ( VDS,off ) value typically in the range of several hundred volts and then partially recovers to smaller values. In this work, we propose a quantitative explanation for this behavior, attributing it to the charging/discharging dynamics of carbon (C)-related buffer traps. We characterize the dynamic Ron in packaged p-GaN gate AlGaN/GaN HEMTs with a custom measurement setup. We find that in these devices, the relative Ron increase reaches a maximum of 60% for VDS,off≈100 -200 V, partially recovering to about 30% as VDS,off is raised to 500 V. We ascribe this behavior to the partial neutralization of C-related acceptor traps in the buffer due to trapping of holes produced by a high-field generation mechanism. This explanation is supported by calibrated 2-D numerical simulations, that successfully reproduce the experimentally observed Ron reduction only when including a hole generation mechanism.

2021 - T-CAD simulations study on drain leakage current and its correlation with gate current for AlGaN/GaN HEMTs [Relazione in Atti di Convegno]
Miccoli, C.; Cerantonio, V.; Chini, A.; Iucolano, F.
abstract

2021 - “Hole Redistribution” Model Explaining the Thermally Activated RON Stress/Recovery Transients in Carbon-Doped AlGaN/GaN Power MIS-HEMTs [Articolo su rivista]
Zagni, Nicolo'; Chini, Alessandro; Puglisi, Francesco Maria; Meneghini, Matteo; Meneghesso, Gaudenzio; Zanoni, Enrico; Pavan, Paolo; Verzellesi, Giovanni
abstract

RON degradation due to stress in GaN-based power devices is a critical issue that limits, among other effects, long-term stable operation. Here, by means of 2-D device simulations, we show that the RON increase and decrease during stress and recovery experiments in carbon-doped AlGaN/GaN power metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) can be explained with a model based on the emission, redistribution, and retrapping of holes within the carbon-doped buffer (“hole redistribution” in short). By comparing simulation results with front- and back-gating OFF-state stress experiments, we provide an explanation for the puzzling observation of both stress and recovery transients being thermally activated with the same activation energy of about 0.9 eV. This finds a straightforward justification in a model in which both RON degradation and recovery processes are limited by hole emission by dominant carbon-related acceptors that are energetically located at about 0.9 eV from the GaN valence band.

2020 - Double-channel hemt device and manufacturing method thereof [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

An HEMT device, comprising: a semiconductor body including a heterojunction structure; a dielectric layer on the semiconductor body; a gate electrode; a drain electrode, facing a first side of the gate electrode; and a source electrode, facing a second side opposite to the first side of the gate electrode; an auxiliary channel layer, which extends over the heterojunction structure between the gate electrode and the drain electrode, in electrical contact with the drain electrode and at a distance from the gate electrode, and forming an additional conductive path for charge carriers that flow between the source electrode and the drain electrode.

2020 - From T-CAD simulations to large signal model for GaN RF device [Abstract in Atti di Convegno]
Cerantonio, V.; Giuffrida, M.; Miccoli, C.; Chini, A.; Iucolano, F.
abstract

The large-signal RF power performance of an AlGaN/GaN High Electron Mobility Transistor (HEMT) is studied starting from technology computer-aided design (TCAD) simulations in terms of DC characteristic and S parameters. A clear procedure of physical parameters calibration is described. Trapping effects and GaN mobility model are included. Good agreement between simulations and measurements is reported, giving a meaningful starting point for the large signal model with the aim to optimize the efficiency of GaN HEMTs. The large signal model extracted is crucial to predict the device performance and give an essential contribution for the device maturity at industrial level.

2020 - Hemt transistor including field plate regions and manufacturing process thereof [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

HEMT transistor (50; 100; 150) having a semiconductor body (52) forming a semiconductive heterostructure (54, 56); a gate region (60), of conductive material, arranged above and in contact with the semiconductor body (52); a first insulating layer (58) extending above the semiconductor body, laterally to the conductive gate region (60); a second insulating layer (62) extending above the first insulating layer (58) and the gate region (60); a first field plate region (84), of conductive material, extending between the first and the second insulating layers (58), laterally spaced from the conductive gate region (60); and a second field plate region (85), of conductive material, extending above the second insulating layer (62), vertically aligned with the first field plate region (84).

2020 - Hemt transistor including field plate regions and manufacturing process thereof [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

An HEMT transistor includes a semiconductor body having a semiconductive heterostructure. A gate region, of conductive material, is arranged above and in contact with the semiconductor body. A first insulating layer extends over the semiconductor body, laterally to the conductive gate region. A second insulating layer extends over the first insulating layer and the gate region. A first field plate region, of conductive material, extends between the first and the second insulating layers, laterally spaced from the conductive gate region along a first direction. A second field plate region, of conductive material, extends over the second insulating layer, and the second field plate region overlies and is vertically aligned with the first field plate region.

2020 - The Role of Carbon Doping on Breakdown, Current Collapse and Dynamic On-Resistance Recovery in AlGaN/GaN High Electron Mobility Transistors on Semi‐Insulating SiC Substrates [Articolo su rivista]
Zagni, Nicolò; Chini, Alessandro; Puglisi, Francesco Maria; Pavan, Paolo; Verzellesi, Giovanni
abstract

In this work, the critical role of carbon doping in the electrical behavior of AlGaN/GaN High Electron Mobility Transistors (HEMTs) on semi-insulating SiC substrates is assessed by investigating the off-state three terminal breakdown, current collapse and dynamic on-resistance recovery at high drain-source voltages. Extensive device simulations of typical GaN HEMT structures are carried out and compared to experimental data from published, state-of-the-art technologies to: i) explain the slope of the breakdown voltage as a function of the gate-to-drain spacing lower than GaN critical electric field as a result of the non-uniform electrical field distribution in the gate-drain access region; ii) attribute the drain current collapse to trapping in deep acceptor states in the buffer associated with carbon doping; iii) interpret the partial dynamic on-resistance recovery after off-state stress at high drain-source voltages as a consequence of hole generation and trapping.

2020 - The effects of carbon on the bidirectional threshold voltage instabilities induced by negative gate bias stress in GaN MIS-HEMTs [Articolo su rivista]
Zagni, Nicolò; Chini, Alessandro; Puglisi, Francesco Maria; Pavan, Paolo; Verzellesi, Giovanni
abstract

In this paper, numerical device simulations are used to point out the possible contributions of carbon doping to the threshold voltage instabilities induced by negative gate bias stress in AlGaN/GaN metal–insulator–semiconductor high-electron mobility transistors. It is suggested that carbon can have a role in both negative and positive threshold voltage shifts, as a result of (1) the changes in the total negative charge stored in the carbon-related acceptor traps in the GaN buffer, and (2) the attraction of carbon-related free holes to the device surface and their capture into interface traps or recombination with gate-injected electrons. For a proper device optimization of carbon-doped MIS-HEMTs, it is therefore important to take these mechanisms into account, in addition to those related to defects in the gate dielectric volume and interface which are conventionally held responsible for threshold voltage instabilities.

2020 - Trap Dynamics Model Explaining the RON Stress/Recovery Behavior in Carbon-Doped Power AlGaN/GaN MOS-HEMTs [Relazione in Atti di Convegno]
Zagni, Nicolo; Chini, Alessandro; Puglisi, Francesco Maria; Pavan, Paolo; Meneghini, Matteo; Meneghesso, Gaudenzio; Zanoni, Enrico; Verzellesi, Giovanni
abstract

In this paper, we present simulation results that reproduce stress and recovery experiments in Carbon-doped power GaN MOS-HEMTs and explain the associated R ON increase and decrease as the result of the emission, redistribution and re-trapping of holes within the Carbon-doped buffer. The proposed model can straightforwardly clarify the beneficial impact of the recently proposed p-type drain contact on R ON degradation as being a consequence of enhanced hole trapping and reduced negative trapped charge within the buffer during stress.

2020 - 包括场板区域的hemt晶体管及其制造工艺 [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

HEMT晶体管包括具有半导体异质结构的半导体主体。导电材料的栅极区域被布置在半导体主体上并且与半导体主体接触。第一绝缘层在半导体主体之上侧向延伸到导电栅极区域。第二绝缘层在第一绝缘层和栅极区域之上延伸。导电材料的第一场板区域在第一绝缘层与第二绝缘层之间延伸，沿着第一方向与导电栅极区域侧向分离。导电材料的第二场板区域在第二绝缘层之上延伸，并且第二场板区域覆盖第一场板区域并与第一场板区域垂直对齐。

2019 - Double-channel hemt device and manufacturing method thereof [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

An HEMT device, comprising: a semiconductor body including a heterojunction structure; a dielectric layer on the semiconductor body; a gate electrode; a drain electrode, facing a first side of the gate electrode; and a source electrode, facing a second side opposite to the first side of the gate electrode; an auxiliary channel layer, which extends over the heterojunction structure between the gate electrode and the drain electrode, in electrical contact with the drain electrode and at a distance from the gate electrode, and forming an additional conductive path for charge carriers that flow between the source electrode and the drain electrode.

2019 - Experimental and numerical analysis of VTH and RON drifts in E-mode GaN HEMTs during switch-mode operation [Articolo su rivista]
Chini, Alessandro; Iucolano, Ferdinando
abstract

The evaluation and investigation of electrical parameters drifts during device operation is one of the mandatory task that has to be performed in order to improve GaN-power devices stability. In this work the authors would like to present novel results concerning the analysis of V TH and R ON drifts in E-mode GaN HEMT observed during their switch-mode operation. Two different mechanisms are observed for the induced positive V TH shift. The first one is related to electron injection from the 2DEG into dielectric/semiconductor traps beneath the gate region of the device, while a second one linked to electron injection from the gate terminal induces a V TH shift linked to a localized negative trapping mechanism in the drain-edge of the gate terminal. The observed R ON increase is instead linked to a hole-emission process taking place in the gate-drain access region within the device carbon-doped buffer layers. The negative charge build-up related to the ionized acceptor traps leads to the 2DEG depletion and consequent R ON increase.

2019 - HEMT transistor of the normally off type including a trench containing a gate region and forming at least one step, and corresponding manufacturing method [Brevetto]
Iucolano, Ferdinando; Patti, Alfonso; Chini, Alessandro
abstract

A method forms an HEMT transistor of the normally off type, including: a semiconductor heterostructure, which comprises at least one first layer and one second layer, the second layer being set on top of the first layer; a trench, which extends through the second layer and a portion of the first layer; a gate region of conductive material, which extends in the trench; and a dielectric region, which extends in the trench, coats the gate region, and contacts the semiconductor heterostructure. A part of the trench is delimited laterally by a lateral structure that forms at least one first step. The semiconductor heterostructure forms a first edge and a second edge of the first step, the first edge being formed by the first layer.

2019 - Insights into the off-state breakdown mechanisms in power GaN HEMTs [Articolo su rivista]
Zagni, Nicolo'; Puglisi, F. M.; Pavan, P.; Chini, A.; Verzellesi, G.
abstract

We analyze the off-state, three-terminal, lateral breakdown in AlGaN/GaN HEMTs for power switching applications by comparing two-dimensional numerical device simulations with experimental data from device structures with different gate-to-drain spacing and with either undoped or Carbon-doped GaN buffer layer. Our simulations reproduce the different breakdown-voltage dependence on the gate-drain-spacing exhibited by the two types of device and attribute the breakdown to: i) a combination of gate electron injection and source-drain punch-through in the undoped HEMTs; and ii) avalanche generation triggered by gate electron injection in the C-doped HEMTs.

2018 - A novel GaN HEMT degradation mechanism observed during HTST test [Relazione in Atti di Convegno]
Iucolano, F.; Parisi, A.; Reina, S.; Chini, A.
abstract

The maximum drain current (Imax) reduction after high temperature short term (HTST) tests in RF-GaN HEMT was investigated. A “critical-voltage” like degradation was observed with voltage levels correlated with the pinch-off voltage of the MIS structure formed by the device field-plate terminal (FP). The recoverable nature of the observed phenomena and the positive temperature dependence of both the Imax reduction and leakage currents within the passivation layer allowed us to propose a novel interpretation based on a charge-injection process from the FP into SiN/AlGaN electrons trap within the device gate-drain access region.

2018 - Evolution of on-resistance (RON) and threshold voltage (VTH) in GaN HEMTs during switch-mode operation [Articolo su rivista]
Chini, Alessandro; Iucolano, Ferdinando
abstract

The evaluation and investigation of electrical parameters drifts during device operation is one of the mandatory task that has to be performed in order to improve GaN-power devices stability. In this work the authors would like to present a novel characterization method that allows the simultaneous monitoring of both RONand VTHduring the switch-mode operation. By applying the proposed measurement method with different operating parameters it has been possible to provide a preliminary interpretation of the physical mechanisms leading to the observed RONincrease and VTHpositive shift. In particular, the observed RONincrease, which is thermally activated with a 0.83eV activation energy, has been ascribed to the presence of Carbon-related traps within the device buffer layer. On the other hand, the positive VTHshift has instead been related to interface defects at the dielectric/III-N interface and/or to bulk traps in the gate dielectric.

2018 - GaN RF and GaN Power – Device Parameter Drifts Analysis [Relazione in Atti di Convegno]
Chini, Alessandro
abstract

Although GaN devices are expected to provide great advantages in RF- and Power applications, they often suffer from trap related parameter instabilities. Results obtained on the analysis of the RF-gain collapse phenomena observed in GaN-on-SiC devices will be presented. Furthermore, a dedicated measurement setup for the monitoring of VTH and RON drifts during switch-mode operation of normally-off GaN-on-Si devices will be proposed and the experimental results obtained will be discussed.

2018 - HEMT transistor with high stress resilience during off state and manufacturing method thereof [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

An HEMT includes a buffer layer, a hole-supply layer on the buffer layer, a heterostructure on the hole-supply layer, and a source electrode. The hole-supply layer is made of P-type doped semiconductor material, the buffer layer is doped with carbon, and the source electrode is in direct electrical contact with the hole-supply layer, such that the hole-supply layer can be biased to facilitate the transport of holes from the hole-supply layer to the buffer layer.

2018 - High electron mobility transistor and manufacturing method thereof [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

HEMT (1; 21; 31; 51) including a buffer layer (4), a hole-supply layer (6) on the buffer layer (4), a heterostructure (7) on the hole-supply layer (6), and a source electrode (16). The hole-supply layer (6) is made of P-type doped semiconductor material, the buffer layer (4) is doped with carbon, and the source electrode (16) is in direct electrical contact with the hole-supply layer (6), such that the hole-supply layer (6) can be biased to facilitate the transport of holes from the hole-supply layer (6) to the buffer layer (4).

2018 - On-wafer RF stress and trapping kinetics of Fe-doped AlGaN/GaN HEMTs [Articolo su rivista]
Rzin, M.; Chini, A.; De Santi, C.; Meneghini, M.; Hugger, A.; Hollmer, M.; Stieglauer, H.; Madel, M.; Splettstößer, J.; Sommer, D.; Grünenpütt, J.; Beilenhoff, K.; Blanck, H.; Chen, J. -T.; Kordina, O.; Meneghesso, G.; Zanoni, E.
abstract

In this paper, we investigate the trapping effects, of iron doped AlGaN/GaN HEMTs, before and after on-wafer 24 hour RF stress test. First, we study the trap centers responsible of the current collapse at different on-state bias and temperature conditions. Second, we investigate 24 hour RF stress effect on the trapping kinetics. By filling traps under off-state condition with high drain-source voltage, we have identified two prominent traps labelled E 1 and E 2 with activation energies of 0.7 eV and 0.6 eV under the conduction band, respectively. An increase of the amplitude of the trap centers E 1 and E 2 by 22.9% and 15.8% respectively is noticed during the RF stress. This result suggests that the degradation observed during RF stress might have induced a density increase of the traps involved in the E 1 and E 2 trap signatures responsible on the current collapse.

2018 - Optimization of 0.25µm GaN HEMTs through numerical simulations [Relazione in Atti di Convegno]
Chini, Alessandro; Verzellesi, Giovanni; Lanzieri, Claudio; Pantellini, Alessio; Rzin, Mehdi; Rampazzo, Fabiana; Meneghini, Matteo; Meneghesso, Gaudenzio; Zanoni, Enrico
abstract

0.25µm GaN HEMTs performance dependence from epitaxial and geometrical parameters has been investigated by means of numerical simulations. A single-heterojunction GaN HEMT structure with an iron doped buffer layer also including a mushroom-gate layout forming a gate-connected field-plate over the device SiN passivation layer was considered. Numerical simulations including static-IV characteristics and breakdown voltage estimation, small signal analysis and double pulse-IV characteristics have been carried out on more than 400 different structures. Simulations results showed that contact resistance, gate-source spacing, barrier thickness and AlGaN/SiN interface trap density are critical for improving device RF gain. Field-plate extension and passivation layer thickness were found to be parameters that can be used for trading off between device breakdown voltage and RF gain. Increasing iron-doping in the buffer layer leaded to larger breakdown voltage and RF gain but, due to the enhanced trapping effects, also to poorer large-signal operation.

2018 - 在断态期间具有高应力顺应性的hemt晶体管及其制造方法 [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

本公开涉及在断态期间具有高应力顺应性的HEMT晶体管及其制造方法。HEMT包括缓冲层、缓冲层上的空穴供应层、空穴供应层上的异质结构、以及源极电极。空穴供应层由P型掺杂半导体材料制成，缓冲层掺杂有碳，并且源极电极与空穴供应层直接电接触，使得空穴供应层可以被偏置以促进空穴从空穴供应层到缓冲层的传输。

2018 - 高电子迁移率晶体管 [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

本公开涉及高电子迁移率晶体管。HEMT包括缓冲层、缓冲层上的空穴供应层、空穴供应层上的异质结构、以及源极电极。空穴供应层由P型掺杂半导体材料制成，缓冲层掺杂有碳，并且源极电极与空穴供应层直接电接触，使得空穴供应层可以被偏置以促进空穴从空穴供应层到缓冲层的传输。本公开的实施例使得能够在不降低击穿阈值以及在应力之前不增加通态电阻的值的情况下，消除应力对通态电阻的影响。

2017 - A novel test methodology for RONand VTHmonitoring in GaN HEMTs during switch-mode operation [Relazione in Atti di Convegno]
Iucolano, Ferdinando; Parisi, Antonino; Reina, Santo; Meneghesso, Gaudenzio; Verzellesi, Giovanni; Chini, Alessandro
abstract

One of the critical issues limiting the performances and reliability of GaN power devices is the degradation of their on-resistance (Ron) when they are operated at high drain-source voltages. Being able to monitor RON variation during device operation thus becomes a necessary task in order to investigate the physical mechanisms leading to the observed drifts. Standard measurement equipment does not allow to perform easily this task, specifically when Ron variation should be monitored when the device operates in switch-mode. Moreover, to better understand the physical mechanisms involved it is very important to monitor other device parameters such as its threshold-voltage VTH which is typically not monitored during device switch-mode operation. Therefore, in this work is presented a novel testing methodology which allows the simultaneous monitoring of device Ron and Vth during switchmode operation with the ability to capture the variation of said parameters starting from the very first switching cycles.

2017 - Experimental and Numerical Evaluation of RON Degradation in GaN HEMTs during Pulse-Mode Operation [Articolo su rivista]
Chini, Alessandro; Iucolano, Ferdinando
abstract

The on-resistance (RON) degradation in normally-OFF GaN high electron mobility transistors has been evaluated both experimentally and by means of numerical simulations by analyzing its drift during device pulse-mode operation. Experimental data showed that the device RON measured during the on-time interval of the switching period increased with time resulting in a thermally activated process with an activation energy $EA=0.83$ eV. For the first time, numerical simulations have been carried out in order to evaluate the device RON drift during pulse-mode operation and to understand the physical phenomena involved. A good qualitative agreement between experimental and simulated data has been obtained when considering in the simulated device simply a hole trap located at 0.83 eV from the GaN valence-band, an energy level which has been linked in previous works to carbon-doping within the GaN buffer.

2016 - A comprehensive reliability evaluation of high-performance AlGaN/GaN HEMTs for space applications [Abstract in Atti di Convegno]
De Santi, Carlo; Dalcanale, Stefano; Stocco, Antonio; Rampazzo, Fabiana; Gerardin, Simone; Meneghini, Matteo; Meneghesso, Gaudenzio; Chini, Alessandro; Verzellesi, Giovanni; Grünenpütt, Jan; Lambert, Benoit; Schauwecker, Bernd; Blanck, Hervé; Zanoni, Andrew Barnes and E.
abstract

A systematic analysis of the reliability of GaN HEMT technologies for space applications in L-, S- and C- band up to 6 GHz (GH50, gate length 0,5 um) and C-, X- and Ku-band up to 20 GHz (GH25, gate length 0.25 um) has been carried out by means of on-wafer short-term (<24 h) tests, long-term high temperature storage and life tests. Specific long-term tests included: (1) 4000h thermal storage at three different temperatures (300°C, 325°C and 350°C); (2) 4000h DC life-test at 250°C, 300°C and 350°C junction temperature at a VDS value of 50 V and to 30 V respectively for GH50 and GH25 and at other bias points, up to VDS=100 V (GH50) and 60 V (GH25); (3) 500h RF life-tests at Tj=350°C, VDS=50V, IDS=50mA/mm, 1.7 GHz for GH50, and at Tj=325°C, VDS=30V, IDS=100mA/mm, 9 GHz for GH25, respectively. For both RF tests the input power corresponding to the maximum PAE value was selected. For all tested devices, degradation of main DC and RF parameters has been very limited. Even at 350°C, the highest temperature adopted for both thermal storage and life tests, the decrease of drain saturation current or transconductance did not exceed 20% after 4000 hours.

2016 - Correlation between dynamic Rdson transients and Carbon related buffer traps in AlGaN/GaN HEMTs [Relazione in Atti di Convegno]
Iucolano, F.; Parisi, A.; Reina, S.; Patti, A.; Coffa, S.; Meneghesso, G.; Verzellesi, Giovanni; Fantini, Fausto; Chini, Alessandro
abstract

The on resistance increment observed when the device is operated at high drain-source voltages is one the topics that limits the performance of the AlGaN/GaN HEMT devices. In this paper, the physical mechanisms responsible of the RDSon degradation are investigated. The dynamic RDSon transient method is used in order to get insight to characterize the traps states. By calculating the Arrhenius plot associated with the RDSon transients an activation energy of 0.86eV was extracted, that can be correlated to the traps due to the incorporation of Carbon inside the buffer. This hypothesis was further supported by the analyses performed on a simpler structure (TLM). By applying a negative substrate bias the effect of only the buffer traps was studied. A fairly close value of the activation energy (0.9eV) to the one extracted when analyzing the RDSon transient was obtained.

2016 - Double-channel HEMT device and manufacturing method thereof [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

An HEMT device, comprising: a semiconductor body including a heterojunction structure; a dielectric layer on the semiconductor body; a gate electrode; a drain electrode, facing a first side of the gate electrode; and a source electrode, facing a second side opposite to the first side of the gate electrode; an auxiliary channel layer, which extends over the heterojunction structure between the gate electrode and the drain electrode, in electrical contact with the drain electrode and at a distance from the gate electrode, and forming an additional conductive path for charge carriers that flow between the source electrode and the drain electrode.

2016 - Double-channel hemt device and manufacturing method thereof [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

An HEMT device (1), comprising: a semiconductor body (15) including a heterojunction structure (13); a dielectric layer (7) on the semiconductor body; a gate electrode (8); a drain electrode (12), facing a first side (8') of the gate electrode (8); and a source electrode (10), facing a second side (8") opposite to the first side (8') of the gate electrode; an auxiliary channel layer (20), which extends over the heterojunction structure (13) between the gate electrode (8) and the drain electrode (12), in electrical contact with the drain electrode (12) and at a distance from the gate electrode, and forming an additional conductive path for charge carriers that flow between the source electrode and the drain electrode.

2016 - Experimental and Numerical Analysis of Hole Emission Process from Carbon-Related Traps in GaN Buffer Layers [Articolo su rivista]
Chini, Alessandro; Meneghesso, G.; Meneghini, M.; Fantini, Fausto; Verzellesi, Giovanni; Patti, A.; Iucolano, F.
abstract

The role of carbon-related traps in GaN-based ungated high-electron mobility transistor structures has been investigated both experimentally and by means of numerical simulations. A clear quantitative correlation between the experimental data and numerical simulations has been obtained. The observed current decrease in the tested structure during backgating measurements has been explained simply by means of a thermally activated hole-emission process with E-A = 0.9eV, corresponding to the distance of the acceptor-like hole-Trap level from the GaN valence band. Moreover, it has been demonstrated by means of electrical measurements and numerical simulations that only a low percentage of the nominal carbon doping levels induces the observed current reduction when negative substrate bias is applied to the tested structure.

2016 - Fabrication of single or multiple gate field plates [Brevetto]
Chini, Alessandro; Kumar Mishra, Umesh; Parikh, Primit; Wu, Yifeng
abstract

A process for fabricating single or multiple gate field plates using consecutive steps of dielectric material deposition/growth, dielectric material etch and metal evaporation on the surface of a field effect transistors. This fabrication process permits a tight control on the field plate operation since dielectric material deposition/growth is typically a well controllable process. Moreover, the dielectric material deposited on the device surface does not need to be removed from the device intrinsic regions: this essentially enables the realization of field-plated devices without the need of low-damage dielectric material dry/wet etches. Using multiple gate field plates also reduces gate resistance by multiple connections, thus improving performances of large periphery and/or sub-micron gate devices.

2016 - HEMT transistor of the normally off type including a trench containing a gate region and forming at least one step, and corresponding manufacturing method [Brevetto]
Iucolano, Ferdinando; Patti, Alfonso; Chini, Alessandro
abstract

A HEMT transistor of the normally off type, including: a semiconductor heterostructure, which comprises at least one first layer and one second layer, the second layer being set on top of the first layer; a trench, which extends through the second layer and a portion of the first layer; a gate region of conductive material, which extends in the trench; and a dielectric region, which extends in the trench, coats the gate region, and contacts the semiconductor heterostructure. A part of the trench is delimited laterally by a lateral structure that forms at least one first step. The semiconductor heterostructure forms a first edge and a second edge of the first step, the first edge being formed by the first layer.

2016 - Hemt-transistor des normalerweise ausgeschalteten typs mit einem graben, der einen gatebereich enthält und mindestens eine stufe bildet, sowie entsprechendes herstellungsverfahren [Brevetto]
Iucolano, Ferdinando; Patti, Alfonso; Chini, Alessandro
abstract

A normally-off type HEMT transistor comprising: a semiconductor heterostructure (4, 6, 200) having at least a first layer (4) and a second layer (6), the second layer being on top of the first layer is arranged; a trench (15) extending through the second layer and a portion of the first layer; a gate region (10) of conductive material extending into the trench; and a dielectric region (18) extending into the trench, covering the gate region and in contact with the semiconductor heterostructure. A portion of the trench is bounded laterally by a lateral structure (LS) forming at least a first stage (Pb1, P11, Pb2). The semiconductor heterostructure forms a first edge (E1) and a second edge (E2) of the first stage, wherein the first edge is formed by the first layer.

2016 - Study of threshold voltage instability in E-mode GaN MOS-HEMTs [Articolo su rivista]
Iucolano, F.; Parisi, A.; Reina, S.; Meneghesso, G.; Chini, Alessandro
abstract

In this work, the threshold instability in E-mode GaN MOS-HEMTs was investigated. In particular, the shift of VTH as a function of the applied positive gate voltage during device characterization was monitored, resulting in positive VTH shifts up to 1 V. A complete VTH recovery required more than one day of unbiased storage, but a partial recovery of the observed VTH shift was observed after few seconds. These results could be related to different positions of trap states: fast states, localized at the dielectric/GaN interface and slow states, the traps inside the dielectric layer. Moreover, VTH shift of 0.2 and 0.8 V for fast and slow states, respectively, was obtained. To gain insight into the physical mechanism involved in the observed phenomena, numerical simulation were also carried out. A VTH shift was obtained adding the interface states. Moreover, three different distributions of traps were compared. In particular, the concentration of filled traps was monitored to understand the impact of the distribution on the electrical behaviour. An increment in filled trap concentration at the increasing of the applied VGS, which in turns correlates with experimentally evaluated device behaviour, was obtained if the distribution of traps states is also above the GaN conduction band energy.

2016 - 双沟道hemt器件及其制造方法 [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

本公开的实施例涉及一种双沟道HEMT器件及其制造方法。该HEMT器件包括：半导体主体，包括异质结结构；在半导体主体上的电介质层；栅极电极；漏极电极，面向栅极电极的第一侧；和源极电极，面向栅极电极的与第一侧相对的第二侧；辅助沟道层，其在异质结结构之上在栅极电极与漏极电极之间延伸，与漏极电极电接触并且与栅极电极相距一定距离，并且形成用于在源极电极与漏极电极之间流动的电荷载流子的附加导电路径。

2016 - 常关断型hemt晶体管 [Brevetto]
Iucolano, Ferdinando; Patti, Alfonso; Chini, Alessandro
abstract

本实用新型涉及常关断型HEMT晶体管。一种常关断型HEMT晶体管包括：半导体异质结(4、6、200)，其至少包括一个第一层(4)和一个第二层(6)，第二层布置在第一层的顶部上；沟槽(15)，其延伸穿过第二层和第一层的一部分；导电材料的栅极区(10)，其在沟槽中延伸；以及介电区(18)，其在沟槽中延伸，涂覆栅极区并且接触半导体异质结。沟槽的一部分由形成至少一个第一台阶(Pb1、Pl1、Pb2)的横向结构(LS)横向定界。半导体异质结形成第一台阶的第一边缘(E1)和第二边缘(E2)，第一边缘由第一层形成。

2016 - 常关断型hemt晶体管以及对应的制造方法 [Brevetto]
Iucolano, Ferdinando; Patti, Alfonso; Chini, Alessandro
abstract

本发明涉及常关断型HEMT晶体管以及对应的制造方法。一种常关断型HEMT晶体管包括：半导体异质结(4、6、200)，其至少包括一个第一层(4)和一个第二层(6)，第二层布置在第一层的顶部上；沟槽(15)，其延伸穿过第二层和第一层的一部分；导电材料的栅极区(10)，其在沟槽中延伸；以及介电区(18)，其在沟槽中延伸，涂覆栅极区并且接触半导体异质结。沟槽的一部分由形成至少一个第一台阶(Pb1、Pl1、Pb2)的横向结构(LS)横向定界。半导体异质结形成第一台阶的第一边缘(E1)和第二边缘(E2)，第一边缘由第一层形成。

2016 - 高电子迁移率晶体管hemt器件 [Brevetto]
Iucolano, Ferdinando; Chini, Alessandro
abstract

本公开的实施例涉及一种高电子迁移率晶体管HEMT器件。该HEMT器件包括：半导体主体，包括异质结结构；在半导体主体上的电介质层；栅极电极；漏极电极，面向栅极电极的第一侧；和源极电极，面向栅极电极的与第一侧相对的第二侧；辅助沟道层，其在异质结结构之上在栅极电极与漏极电极之间延伸，与漏极电极电接触并且与栅极电极相距一定距离，并且形成用于在源极电极与漏极电极之间流动的电荷载流子的附加导电路径。

2015 - Effects of buffer compensation strategies on the electrical performance and RF reliability of AlGaN/GaN HEMTs [Articolo su rivista]
Bisi, D.; Stocco, A.; Rossetto, I.; Meneghini, M.; Rampazzo, F.; Chini, Alessandro; Soci, Fabio; Pantellini, A.; Lanzieri, C.; Gamarra, P.; Lacam, C.; Tordjman, M.; Di Forte Poisson, M. A.; De Salvador, D.; Bazzan, M.; Meneghesso, G.; Zanoni, E.
abstract

The effects of buffer compensation strategies on the electrical performance and RF reliability of AlGaN/GaN HEMTs have been studied by means of static and dynamic I-V measurements, drain-current transient spectroscopy, XRD, and RF stress tests. Devices equipped with C-doped and Fe-doped GaN buffer feature improved subthreshold behaviour (lower source-to-drain leakage current, and lower DIBL) and improved RF reliability. As a drawback, devices equipped with Fe- and C-doping experience higher dynamic current dispersion, ascribed to higher concentration of the deep levels E2 (0.56 eV/10- 15 cm2) and E4 (0.84 eV/10- 14 cm2).

2015 - Hot-Electron Degradation of AlGaN/GaN High-Electron Mobility Transistors During RF Operation: Correlation With GaN Buffer Design [Articolo su rivista]
Bisi, D.; Chini, Alessandro; Soci, Fabio; Stocco, A.; Meneghini, M.; Pantellini, A.; Nanni, A.; Lanzieri, C.; Gamarra, P.; Lacam, C.; Tordjman, M.; di Forte Poisson, M. A.; Meneghesso, G.; Zanoni, E.
abstract

Comprehensive RF stress-test campaign has been performed over AlGaN/GaN high-electron mobility transistor employing different GaN buffer designs, including unintentional doping, carbon doping and iron doping. No signature of gate-edge degradation has been found, and good correlation emerges between the buffer composition, subthreshold leakage current, and permanent degradation of the RF performance. The degradation mechanism, more pronounced in devices with parasitic buffer conductivity, involves the generation of additional deep trap states, the worsening of the dynamic current collapse, and the subsequent degradation of RF output power.

2014 - Buffer traps in Fe-doped AlGaN/GaN HEMTs: Investigation of the physical properties based on pulsed and transient measurements [Articolo su rivista]
Meneghini, M.; Rossetto, I.; Bisi, D.; Stocco, A; Chini, Alessandro; Pantellini, A.; Lanzieri, C.; Nanni, A.; Meneghesso, G.; Zanoni, E.
abstract

This paper presents an extensive investigation of the properties of the trap with activation energy equal to 0.6 eV, which has been demonstrated to be responsible for current collapse (CC) in AlGaN/GaN HEMTs. The study was carried out on AlGaN/GaN HEMTs with increasing concentration of iron doping in the buffer. Based on pulsed characterization and drain current transient measurements, we demonstrate that for the samples under investigation: 1) increasing concentrations of Fe-doping in the buffer may induce a strong CC, which is related to the existence of a trap level located 0.63 eV below the conduction band energy and 2) this trap is physically located in the buffer layer, and is not related to the iron atoms but-more likely-to an intrinsic defect whose concentration depends on buffer doping. Moreover, we demonstrate that this level can be filled both under OFF-state conditions (by gate-leakage current) and under ON-state operation (when hot electrons can be injected to the buffer): for these reasons, it can significantly affect the switching properties of AlGaN/GaN HEMTs

2014 - Effect of Gate Field-Plate Geometry on On-Resistance in AlGan/GaN HEMTs for Power Applications [Abstract in Atti di Convegno]
Soci, Fabio; Pozzovivo, G.; Silvestri, M.; Curatola, G.; Detzel, T.; Haeberlen, O.; Chini, Alessandro
abstract

On-resistance (RDSon) degradation is a well-known issue in AlGaN/GaN HEMTs technology for power applications [1,2], and is usually observed when the device is rapidly switched from off- to on-state condition. The introduction of field-plate terminals has proven to be a viable solution in order to mitigate the off-state electric fields and consequently improve the dynamic on-resistance behavior [2]. In this work the effect of different field-plate geometry will be investigated. Moreover, by using different characterization techniques, some insights on the trapping mechanisms causing the RDSon degradation will also be presented.

2014 - PARASITIC EFFECTS OF BUFFER DESIGN ON STATIC AND DYNAMIC PARAMETERS OF ALGAN/GAN HIGH ELECTRON MOBILITY TRANSISTORS [Abstract in Atti di Convegno]
Bisi, D.; Stocco, A.; Rossetto, I.; Meneghini, M.; Rampazzo, F.; Chini, A.; Soci, F.; Pantellini, A.; Lanzieri, C.; Gamarra, P.; Lacam, C.; di Forte-Poisson, M. -A.; De Salvador, D.; Bazzan, M.; Meneghesso, G.; Zanoni, E.
abstract

GaN-based HEMTs are excellent candidates for next-generation high-power microwave applications. Nevertheless, due to the spontaneous n-type conductivity of GaN crystals, devices equipped with unintentionally-doped buffer experience detrimental short-channel effects, undermining both the device performances and their long-term stability. Technological solutions involve the introduction of carbon and/or iron compensating species which compensate the unintentional donor species, render the GaN buffer layer semi-insulating, and improve the confinement of electrons in the 2DEG. Nevertheless, the presence of foreign impurities and the related peculiar growth conditions, may give rise to enhanced crystallographic defect density. Within this work, we comprehensively investigate the static and dynamic parasitic effects related to the GaN-buffer design, and we discuss the implications on the rf performance and reliability.

2014 - Reliability Investigation of GaN HEMTs for MMICs Applications [Articolo su rivista]
Chini, Alessandro; Meneghesso, G.; Pantellini, A.; Lanzieri, C.; Zanoni, E.
abstract

Results obtained during the evaluation of radio frequency (RF) reliability carried out on several devices fabricated with different epi-structure and field-plate geometries will be presented and discussed. Devices without a field-plate structure experienced a more severe degradation when compared to their counterparts while no significant correlation has been observed with respect of the different epi-structure tested. RF stress induced two main changes in the device electrical characteristics, i.e., an increase in drain current dispersion and a reduction in gate-leakage currents. Both of these phenomena can be explained by assuming a density increase of an acceptor trap located beneath the gate contact and in the device barrier layer. Numerical simulations carried out with the aim of supporting the proposed mechanism will also be presented.

2014 - Role of buffer doping and pre-existing trap states in the current collapse and degradation of AlGaN/GaN HEMTs [Relazione in Atti di Convegno]
Meneghini, M.; Rossetto, I.; Bisi, D.; Stocco, A.; Cester, A.; Meneghesso, G.; Zanoni, E.; Chini, Alessandro; Pantellini, A.; Lanzieri, C.
abstract

The aim of this work is to quantitatively investigate the influence of buffer doping on the current collapse of AlGaN/GaN HEMTs, and to analyze the contribution of trap states to the increase in current collapse detected after reverse-bias stress. The study was carried out on GaN-based HEMTs with increasing levels of iron doping in the buffer, which were submitted to drain current transient measurements and reverse-bias stress. Results demonstrate that the use of Fe-doping may significantly impact on current collapse; moreover, we demonstrate that the increase in current collapse detected after reverse-bias stress is not due to the generation of new types of defect, but to the increase in the signal of the defects which were already present before stress.

2014 - The Italian perspectives on the application of GaN technology in future SAR and RADAR systems [Abstract in Atti di Convegno]
Lanzieri, Claudio; Pantellini, Alessio; Romanini, Paolo; Marziale, Feudale; Colantonio, Paolo; Giannini, Franco; Limiti, Ernesto; Cucinella, Giovanni; Bisi, Davide; Zanoni, Enrico; Chini, Alessandro; Formaro, Roberto
abstract

GaN technology providing at the same time high power density, efficiency, low noise performance combined with the robustness to high RF signals, wideband operation, represents the best solution for the development of very compact and efficient Transmit Receive (TR) Module. For this reason the Italian Space Agency (ASI) has supported the creation of a national team of competences that, starting from the availability of technological processes from the Selex ES Foundry and the experience of Thales Alenia Space Italy on SAR Systems development, promotes a European GaN technology for the next Space missions.

2014 - Trapping and High Field Related Issues in GaN Power HEMTs [Relazione in Atti di Convegno]
Meneghesso, Gaudenzio; Meneghini, Matteo; Chini, Alessandro; Verzellesi, Giovanni; Zanoni, Enrico
abstract

Gallium Nitride HEMTs grown on Si substrates are the most promising solution for the future technologies in the power electronics industry. Compensation of unintentional GaN n-type conductivity is specifically mandatory in the buffer for an optimum device blocking function. Carbon (C) or Iron (Fe) doping are the most common solutions that however are responsible also for the introduction of traps in the buffer, that induce large charge trapping and current collapse when devices are biased at high voltages as well as affect breakdown behavior of these devices. This paper reviews the main high field related issues recently reported in GaN-on-Si devices for power applications.

2014 - Traps localization and analysis in GaN HEMTs [Articolo su rivista]
Chini, Alessandro; Soci, Fabio; Meneghesso, G.; Meneghini, M.; Zanoni, E.
abstract

A simple experimental technique aimed at the spatial localization of the dominant trap states involved in drain current dispersion in GaN HEMTs will be presented. By analyzing the dependence of current dispersion from the gate voltage base-line (VGbl) used in double pulse I-V measurements it is possible to observe a different trend of said dispersion when caused by buffer or barrier/surface traps. Devices whose dynamic characteristics are mainly affected by buffer traps are showing the largest dispersion when VGbl approaches the device threshold voltage (VTH) while a reduction in dispersion is observed when VGbl is lowered below VTH. On the other hand, when dynamic characteristics are mainly affected by barrier/surface traps, the opposite trend is observed, i.e. dispersion increases when VGbl is lowered below VTH. Numerical simulations supporting the proposed measurement technique and traps characterization in Fe-doped buffer devices will also be presented.

2013 - AlGaN/GaN-Based HEMTs Failure Physics and Reliability: Mechanisms Affecting Gate Edge and Schottky Junction [Articolo su rivista]
Zanoni, E.; Meneghini, M.; Chini, Alessandro; Marcon, D.; Meneghesso, G.
abstract

This paper presents a comprehensive review of AlGaN/GaN high electron mobility transistor failure physics and reliability, focusing on mechanisms affecting the gate-drain edge, where maximum electric field and peak temperatures are reached. Physical effects at the origin of device degradation (inverse piezoelectric effect, time-dependent trap formation and percolative conductive paths formation, and electrochemical AlGaN and GaN degradation) are discussed on the basis of literature data and unpublished results. Thermally activated mechanisms involving metal-metal and metal-semiconductor interdiffusion at the gate Schottky junction are also discussed.

2013 - Analysis of self-oscillating switched-mode circuit for low-voltage energy harvesting [Articolo su rivista]
Chini, Alessandro
abstract

The operation of a self-oscillating switched-mode circuit topology for low-voltage thermal energy harvesting is analysed and experimental efficiency results are presented. Circuit self-start-up function as well as high-efficiency power transfer are obtained by using only one transformer without additional inductors. It is shown that even without using a maximum power point tracking (MPPT) tracking circuit, it is possible to achieve efficiencies up to 65% over a relatively broad input voltage range which can be optimised by properly varying the transformer turns ratio n.

2013 - Deep levels characterization in GaN HEMTs - Part II: Experimental and numerical evaluation of self-heating effects on the extraction of traps activation energy [Articolo su rivista]
Chini, Alessandro; Soci, Fabio; Meneghini, M.; Meneghesso, G.; Zanoni, E.
abstract

In this paper, the effects of device self-heating on the extraction of traps activation energy are investigated through experimental measurements and numerical simulations. Neglecting the device temperature increase during the experimental measurements can lead to an underestimation of traps activation energies as well as nonoverlapping Arrhenius plots. It will be shown that said artifacts can be removed once device thermal resistance is known and used to correct the temperatures data points at which trap time constants are extracted. The correctness of the proposed method is also supported through numerical simulations carried out both by neglecting and considering thermal effects during the drain current transient measurements. Finally, the experimental results obtained are also suggesting a novel method for the extraction of device thermal resistance, which yielded comparable results with respect to those obtained with other experimental techniques.

2013 - Deep-Level Characterization in GaN HEMTs-Part I: Advantages and Limitations of Drain Current Transient Measurements [Articolo su rivista]
Bisi, D.; Meneghini, M.; De Santi, C.; Chini, Alessandro; Dammann, M.; Bruckner, P.; Mikulla, M.; Meneghesso, G.; Zanoni, E.
abstract

This paper critically investigates the advantages and limitations of the current-transient methods used for the study of the deep levels in GaN-based high-electron mobility transistors (HEMTs), by evaluating how the procedures adopted for measurement and data analysis can influence the results of the investigation. The article is divided in two parts within Part I. 1) We analyze how the choice of the measurement and analysis parameters (such as the voltage levels used to induce the trapping phenomena and monitor the current transients, the duration of the filling pulses, and the method used for the extrapolation of the time constants of the capture/emission processes) can influence the results of the drain current transient investigation and can provide information on the location of the trap levels responsible for current collapse. 2) We present a database of defects described in more than 60 papers on GaN technology, which can be used to extract information on the nature and origin of the trap levels responsible for current collapse in AlGaN/GaN HEMTs. Within Part II, we investigate how self-heating can modify the results of drain current transient measurements on the basis of combined experimental activity and device simulation.

2013 - Degradation of AlGaN/GaN Schottky diodes on silicon: Role of defects at the AlGaN/GaN interface [Articolo su rivista]
Meneghini, M.; Bertin, M.; Stocco, A.; Dal Santo, G.; Marcon, D.; Malinowski, P. E.; Chini, Alessandro; Meneghesso, G.; Zanoni, E.
abstract

We report on a detailed investigation of the degradation of AlGaN/GaN Schottky diodes grown on silicon, submitted to high reverse-bias. The analyzed devices have a vertical structure; thanks to this feature, it was possible (i) to characterize the effects of stress by means of capacitance-voltage (C-V) measurements, therefore, identifying and localizing the trap states generated as a consequence of the stress tests; (ii) to accurately control the intensity and distribution of the electric field over stress time. Results indicate that stress induces an increase in the leakage current, which is well correlated to the increase of a new capacitance peak in the C-V characteristics. Based on experimental data and bidimensional simulations, degradation is ascribed to the generation of donor traps in the GaN buffer, close to the AlGaN/GaN interface.

2013 - Experimental Technique for Traps Spatial Localization in GaN HEMTs [Abstract in Atti di Convegno]
Chini, Alessandro; Soci, Fabio; Meneghesso, G.; Zanoni, E.
abstract

An experimental technique based on pulsed I-V measurement for the trap spatial localization within GaN-HEMTs is presented and discussed. By evaluating the dependence of the drain current-collapse versus the gate voltage base-line used for pulsed I-V measurement it is possible to spatially localize trap level responsible for the device performance degradation.

2013 - Fabrication of single or multiple gate field plates [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

"This invention relates to semiconductor devices, and more particularly, to the fabrication of single or multiple gate field plates."

2013 - Impact of field-plate geometry on the reliability of GaN-on-SiC HEMTs [Articolo su rivista]
Chini, Alessandro; Soci, Fabio; Fantini, Fausto; Nanni, A.; Pantellini, A.; Lanzieri, C.; Meneghesso, G.; Zanoni, E.
abstract

GaN on SiC HEMTs fabricated with different gate-connected field plate structures have been tested by means of RF reliability tests. The increase in field-plate length yielded an improvement of both the dynamic and the reliability performance during RF testing. Results are thus suggesting that reliability in field-plate devices can be improved by a proper design of the field-plate geometry.

2013 - Influence of device self-heating on trap activation energy extraction [Relazione in Atti di Convegno]
Soci, Fabio; Chini, Alessandro; Meneghesso, G.; Meneghini, M.; Zanoni, E.
abstract

In this paper results obtained by drain current transients measurement on GaN-based high electron mobility transistors (HEMTs) are presented. It will be shown that neglecting device self-heating effects during the calculation process can lead to an underestimation of said energies and to non-overlapping Arrhenius plots, when the emission time constants are extracted at different device dissipated power levels. Thanks to the estimation of the mean channel thermal resistance, thermal effects were taken into account by correcting the measured data. Higher activation energy values have then been extracted and a reasonable overlap of the Arrhenius plots was obtained amongst measurements carried out at different dissipated powers. The experimental results are also suggesting a novel method for the extraction of device thermal resistance, which yielded similar results with respect to other experimental techniques.

2013 - Influence of properties of Si3N4 passivation layer on the electrical characteristics of Normally-off AlGaN/GaN HEMT [Relazione in Atti di Convegno]
Iucolano, F.; Miccoli, C.; Nicotra, M.; Stocco, A.; Rampazzo, F.; Zanandrea, A.; Cinnera, M. V.; Patti, A.; Rinaudo, S.; Soci, Fabio; Chini, Alessandro; Zanoni, E.; Meneghesso, G.
abstract

In this paper a comparison between two samples with two different SiN passivation layers was performed. An increment of the drain leakage current increasing the Si concentration in the SiN layer was observed. On the other hand, a similar behavior of dynamic RON was recorded. T-CAD simulations supported our hypothesis on the factors that cause parametric yield loss.

2013 - N-polar GaN epitaxy and high electron mobility transistors [Articolo su rivista]
Wong, M. H.; Keller, S.; Dasgupta, N. S.; Denninghoff, D. J.; Kolluri, S.; Brown, D. F.; Lu, J.; Fichtenbaum, N. A.; Ahmadi, E.; Singisetti, U.; Chini, Alessandro; Rajan, S.; Denbaars, S. P.; Speck, J. S.; Mishra, U. K.
abstract

This paper reviews the progress of N-polar (000 1) GaN high frequency electronics that aims at addressing the device scaling challenges faced by GaN high electron mobility transistors (HEMTs) for radio-frequency and mixed-signal applications. Device quality (Al, In, Ga)N materials for N-polar heterostructures are developed using molecular beam epitaxy and metalorganic chemical vapor deposition. The principles of polarization engineering for designing N-polar HEMT structures will be outlined. The performance, scaling behavior and challenges of microwave power devices as well as highly-scaled depletion- and enhancement-mode devices employing advanced technologies including self-aligned processes, n+ (In,Ga)N ohmic contact regrowth and high aspect ratio T-gates will be discussed. Recent research results on integrating N-polar GaN with Si for prospective novel applications will also be summarized.

2013 - The influence of interface states at the Schottky junction on the large signal behavior of copper-gate GaN HEMTs [Articolo su rivista]
Esposto, Michele; Di Lecce, Valerio; Bonaiuti, Matteo; Chini, Alessandro
abstract

The large signal characteristics of Cu-gate and Ni/Au-gate AlGaN/GaN high-electron-mobility transistors (HEMTs) were compared. The tested devices were fabricated on two different parts of the same wafer following the same fabrication steps, the only difference being in the Schottky contact material. Comparison of the direct-current (DC) and radio frequency (RF) characteristics points out a critical drain current collapse in the Cu-gate devices, with detrimental effects on the RF performance, whereas the Ni/Au-gate HEMTs performed properly during DC, pulsed, and RF measurements. Investigations on the drain current transients and on the I D-V GS characteristics, obtained by pulsed signals, suggest the presence of an acceptor trap density in the AlGaN barrier, beneath the Cu Schottky gate contact, responsible for the poorer performance of the Cu-gate device; an activation energy of 430 meV was extracted. This defectivity seemed to be due to a copper diffusion event, activated by thermal stress induced in the sample during the plasma-enhanced chemical vapor deposition (PECVD) of SiN. Numerical simulations carried out on the tested structure qualitatively support the presence of a trap density explaining the nature of the observed drain current transients.

2013 - Threshold voltage shift investigation and oxide trap profile extraction in AlGaN/GaN MIS-HEMTs [Abstract in Atti di Convegno]
Soci, Fabio; Tedaldi, P.; Iucolano, F.; Patti, A.; Meneghesso, G.; Zanoni, E.; Chini, Alessandro
abstract

High breakdown voltage and low on-state resistance make AlGaN/GaN High Electron Mobility Transistors (HEMTs) very attractive for switching applications. However, because to its strong piezoelectric effect the conventional GaN HEMT structure is unsuitable for enhancement-mode (E-Mode) operation, which is usually required to improve the safety of the switching systems. In this work we present a GaN HEMT, where a gate recess in combination with Metal Insulator Semiconductor (MIS) structure have been used to obtain a positive threshold voltage (VTH). Insulator layer effectively helps to reduce the parasitic gate leakage currents, but unfortunately MIS-HEMT devices suffer of threshold voltage instability due to the charge trapping that occurs in the insulator layer [1,2,3]. Our aim is to investigate the trapping mechanisms in MIS-HEMT structure exploiting different characterization techniques.

2012 - A novel degradation mechanism of AlGaN/GaN/Silicon heterostructures related to the generation of interface traps [Relazione in Atti di Convegno]
Meneghini, M.; Bertin, M.; Dal Santo, G.; Stocco, A.; Chini, Alessandro; Marcon, D.; Malinowski, P. E.; Mura, G.; Musu, E.; Vanzi, M.; Meneghesso, G.; Zanoni, E.
abstract

With this paper we report on an extensive analysis of the degradation of AlGaN/GaN Schottky diodes, submitted to reverse-bias stress tests. The analysis is based on combined electrical measurements and 2D simulation. Results indicate that stress may induce a gradual increase in the leakage current of the devices. Capacitance-Voltage investigation was used as a tool for the analysis of the modifications in device structure generated during the stress tests. We demonstrate a new failure mechanism which consist in the formation of donor-like traps on the GaN-side of the AlGaN/GaN interface, and can explain the observed increase in diode leakage. 2D simulation was used to support the hypothesis on degradation, and to extrapolate the parameters of the trap responsible for the degradation process.

2012 - Analysis of GaN HEMT Failure Mechanisms During DC and Large-Signal RF Operation [Articolo su rivista]
Chini, Alessandro; DI LECCE, Valerio; Fantini, Fausto; Meneghesso, G.; Zanoni, E.
abstract

An analysis of the reliability of GaN high-electron-mobility transistors during dc and RF stress tests is presented. Competing degradation mechanisms have been observed during RF operation, demonstrating the dependence of device reliability on device RF driving conditions. DC tests revealed only one degradation pattern related to defect formation at the high-electric-field region of the gate contact. RF signals, however, allow for other physical phenomena to take place before the said defect formation. Electron injection on the gate–drain region decreases the electric field value, thus counteracting the electric-field-induced defect formation at the gate edge.

2012 - Correlation between Drain Current Transient and Double-Pulse Measurements in AlGaN/GaN HEMT Trap Analysis [Abstract in Atti di Convegno]
Bisi, D.; Stocco, A.; Rampazzo, F.; Meneghini, M.; Soci, Fabio; Chini, Alessandro; Meneghesso, G.; Zanoni, E.
abstract

Trap characterization represents an important step towards the improvement of the performances and reliability of GaN-based HEMT devices. Double-pulsed measurements and long drain current transient analysis are effective methods to gather complementary information on traps identification and localization: activation energy and time constant of the traps may be extrapolated through drain current transients measurements [1-3]; on the other hand, the analysis of the dynamic VTH shift and gm peak reduction, based on double-pulsed measurements, allows to understand whether the trap states are located below the gate (VTH shift) or in drain-gate access region (gm peak reduction) [4]. The aim of this work is to experimentally investigate the correlation between these two characterization techniques by applying them to a series of identical devices and comparing results on individual samples.

2012 - Enhancement-/depletion-PHEMT device and manufacturing method thereof [Brevetto]
Chini, Alessandro; Lanzieri, Claudio
abstract

The present invention relates, in general, to enhancement/ depletion Pseudomorphic High Electron Mobility Transistors (PHEMTs) and, in particular, to an enhancement/depletion PHEMT device and a method for manufacturing enhancement/depletion PHEMT devices that finds advantageous, but not exclusive, application in the production of integrated circuits operating at millimetre-wave and microwave frequencies.

2012 - Enhancement/depletion PHEMT device [Brevetto]
Chini, Alessandro; Lanzieri, Claudio
abstract

An embodiment of the present invention concerns a layered epitaxial structure for enhancement/ depletion PHEMT devices, an enhancement/depletion PHEMT device and a method for manufacturing an enhancement/ depletion PHEMT device that finds advantageous, but not exclusive, application in the manufacturing of integrated circuits operating at millimeter-Wave and microWave frequencies.

2012 - Experimental and numerical correlation between current-collapse and fe-doping profiles in GaN HEMTs [Relazione in Atti di Convegno]
Chini, Alessandro; DI LECCE, Valerio; Soci, Fabio; D., Bisi; A., Stocco; M., Meneghini; G., Meneghesso; E., Zanoni; A., Gasparotto
abstract

In this work, for the first time, the dependence of the GaN HEMTs current collapse from the profile of the Fe-doped semi-insulating GaN buffers has been demonstrated both experimentally and by means of numerical simulations based on the SIMS measured profile of the GaN buffer Fe-doping concentration.

2012 - Field plate related reliability improvements in GaN-on-Si HEMTs [Articolo su rivista]
Chini, Alessandro; Soci, Fabio; Fantini, Fausto; Nanni, A.; Pantellini, A.; Lanzieri, C.; Bisi, D.; Meneghesso, G.; Zanoni, E.
abstract

State of the art GaN on Silicon HEMTs fabricated with and without a field-plate structure have been tested by means of DC and RF reliability tests. The introduction of the field-plate structure greatly improves device reliability both during DC as well as RF testing. Results are thus suggesting that reliability in NOFP and FP devices is mainly limited by the high electric fields within the device structure causing an increase in traps concentration.

2012 - Metal-oxide barrier extraction by Fowler-Nordheim tunnelling onset in Al2O3-on-GaN MOS diodes [Articolo su rivista]
Di Lecce, Valerio; Krishnamoorthy, S.; Esposto, M.; Hung, T. H.; Chini, Alessandro; Rajan, S.
abstract

Investigation of the properties of Al2O3-on-GaN metal-oxide-semiconductor diodes is reported. A new method is shown to calculate the metal-oxide barrier height based on the onset of the Fowler-Nordheim tunnelling current regime in direct bias. The Ni/Al2O3 barrier was extracted, for the first time with this method, and it was found to match other reports in the literature. The dependence of the effectiveness of this method on the oxide thickness is discussed. The breakdown field for Al2O3 was also measured and found to be in agreement with previous reports.

2012 - Time-dependent degradation of AlGaN/GaN high electron mobility transistors under reverse bias [Articolo su rivista]
Meneghini, M.; Stocco, A.; Bertin, M.; Marcon, D.; Chini, Alessandro; Meneghesso, G.; Zanoni, E.
abstract

This paper describes a detailed analysis of the time-dependent degradation kinetics of GaN-based high electron mobility transistors submitted to reverse-bias stress. We show that: (1) exposure to reverse-bias may induce recoverable changes in gate leakage and threshold voltage, due to the accumulation of negative charge within the AlGaN layer, and of positive charge at the AlGaN/GaN interface. (2) Permanent degradation consists in the generation of parasitic leakage paths. Several findings support the hypothesis that permanent degradation is due to a defect percolation process: (2(a)) for sufficiently long stress times, degradation occurs even below the “critical voltage” estimated by step stress experiments; (2(b)) before permanent degradation, gate current becomes noisy, indicating an increase in defect concentration; and (2(c)) time to breakdown strongly depends on the initial defectiveness of the samples.

2012 - 單一或多重閘極場平板之製造 [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

本發明係關於半導體裝置，且更特定言之，本發明係關於單一或多重閘極場平板之製造。

2011 - An Investigation of the Electrical Degradation of GaN High-Electron-Mobility Transistors by Numerical Simulations of DC Characteristics and Scattering Parameters [Articolo su rivista]
DI LECCE, Valerio; Esposto, Michele; Bonaiuti, Matteo; Fantini, Fausto; Meneghesso, G.; Zanoni, E.; Chini, Alessandro
abstract

The effects of direct-current (DC) stress on GaN high-electron-mobility transistors (HEMTs) are investigated by means of numerical simulations, by which the creation of an acceptor trap in the AlGaN barrier layer was correlated to the observed experimental degradation. An increase in the trap concentration induces a worsening of the saturated current IDSS, transconductance g m, and output conductance gO. An increase in the length of the trapping region induces a degradation of IDSS and gm, but can reduce gO. Analysis of scattering parameters in the saturation region shows that the cutoff frequency fT matches the trend of gm.

2011 - Analytical Model for Power Switching GaN-Based HEMT Design [Articolo su rivista]
Esposto, Michele; Chini, Alessandro; Rajan, S.
abstract

The GaN high-electron mobility transistor (HEMT) structure has been widely investigated, particularly for radio-frequency applications. This structure is suitable for high-frequency switching applications in power electronics because of the high breakdown field of GaN and the high mobility of the channel. In this paper, a physical model for predicting the power-switching operation of GaN-based HEMTs as a function of material and device parameters is proposed. Analytical equations for total losses and power dissipation density are derived and discussed both qualitatively and quantitatively.

2011 - DC and Pulsed Characterization of GaN-based Single- and Double- Heterostructure Devices [Abstract in Atti di Convegno]
Zanandrea, A.; Rampazzo, F.; Stocco, A.; Zanoni, E.; Bisi, D.; Soci, Fabio; Chini, Alessandro; Ivo, P.; Wuerfl, J.; Meneghesso, G.
abstract

In this study, we have characterized 5 different GaN-based HEMTs: 3 single-heterostructure (SH) and 2 double- (DH) hetero-structures.

2011 - Dependence of Static and Dynamic GaN HEMT Characteristics from Fe-doped GaN Buffer Parameters [Relazione in Atti di Convegno]
Chini, Alessandro; C., Lanzieri; E., Zanoni
abstract

The influence of Fe-doped buffer parameters on the static and dynamic characteristics of GaN HEMTs have been analyzed by means of two-dimensional numerical simulations. Results obtained on the static characteristics of the simulated devices have shown that thick and highly doped buffers yield better performances in terms of device output conductance and subthreshold conduction, while leaving mainly unaffected other DC parameters. On the other hand, when dynamic characteristics are evaluated, a significant current collapse has been observed pointing out the importance of the correct design of Fe-doped buffer parameters in order to obtain state-of-the-art GaN HEMT performances.

2011 - Effects of Thermal Annealing on Current Degradation in Enhancement Mode Pd gate InAlAs/InGaAs/InP pHEMTs [Relazione in Atti di Convegno]
Lai, Y.; Ian, K. W.; Bisi, Davide; Chini, Alessandro; Missous, M.
abstract

In this work, we have investigated a novel quasi-enhancement mode Pd gate delta-doped InGaAs/InAlAs pHEMT, which has the advantages of high breakdown voltage.

2011 - Electroluminescence analysis of time-dependent reverse-bias degradation of HEMTs: A complete model [Relazione in Atti di Convegno]
M., Meneghini; A., Stocco; M., Bertin; N., Ronchi; Chini, Alessandro; D., Marcon; G., Meneghesso; E., Zanoni
abstract

This paper analyzes the reverse-bias degradation of GaN-based HEMTs. Experimental evidence collected within this work demonstrate that, (i) when submitted to reverse-gate stress, HEMTs can show both recoverable and permanent degradation. (ii) recoverable degradation consists in the decrease in gate current and threshold voltage, which are ascribed to the simultaneous trapping of negative charge in the AlGaN layer, and of positive charge close to the AlGaN/GaN interface. (iii) permanent degradation consists in the generation of parasitic leakage paths. Results indicate that permanent degradation can occur even for stress voltage levels significantly lower than the “critical” voltage identified by step-stress experiments. Time-dependent analysis suggests that permanent degradation can be ascribed to a defect generation and percolation process. Results supports the existence of a “time to breakdown” (tBD) for HEMT degradation, which significantly depends on the stress voltage level.

2011 - High Level of Automated Process for Broadband and X-Band MMIC's Production [Relazione in Atti di Convegno]
P., Romanini; A., Bettidi; Chini, Alessandro; W., Ciccognani; S., Colangeli; D., Dominijanni; E., Limiti; A., Nanni; C., Lanzieri
abstract

In this paper we propose and advanced fully automated high yield process to fabricate MMIC based on 0.25um T-gate technology. The industrialization of such process will make available a mature technology suitable to satisfy the increasing demand for military and space wide-band and X-band T/R modules applications. RF device performances have been evaluated focusing our attention to Power Amplifier (PA) and robust Low Noise Amplifier (LNA)

2011 - Impact of Hot Electrons on the Reliability of AlGaN/GaN High Electron Mobility Transistors [Abstract in Atti di Convegno]
Rampazzo, F.; Stocco, A.; Silvestri, R.; Meneghini, M.; Ronchi, N.; Bisi, D.; Soci, Fabio; Chini, Alessandro; Meneghesso, G.; Zanoni, E.
abstract

The increase of gate leakage current induced by high voltage gate reverse bias is one of the main reliability concerns for Gallium Nitride HEMTs [1]-[4]. In this work we tested AlGaN/GaN HEMTs with off-state critical voltage > 100 V. We show that the on-state degradation of those devices can be unambiguously attributed to hot-electrons.

2011 - Improvement of breakdown and DC-to-pulse dispersion properties in field-plated InGaAs-InAlAs pHEMTs [Relazione in Atti di Convegno]
Saguatti, Davide; M., Mohamad Isa; K. W., Ian; Chini, Alessandro; Verzellesi, Giovanni; Fantini, Fausto; M., Missous
abstract

We report on novel, high voltage InGaAs-InAlAs pHEMTs, which have been processed on an optimized epilayer and incorporate field-plate structures of different dimensions. Fabricated devices demonstrate great improvements in break-down voltage and gate leakage, while keeping the same DC and RF behaviour with respect to baseline devices, i.e. with no field-plate implemented. In addition, the field plate strongly attenuates DC-to-pulse dispersion, making these devices suitable for high-power-density RF power amplifiers.

2010 - Analysis of DC and rf degradation of AlGaN/GaN High Electron Mobility Transistors based on pulsed measurements and spectroscopic techniques [Abstract in Atti di Convegno]
Zanoni, E.; Chini, Alessandro; Stocco, A.; Rossetto, I.; Meneghini, M.; Rampazzo, F.; Ronchi, N.; Tazzoli, A.; Verzellesi, Giovanni; Meneghesso, G.
abstract

The experimentally observed DC and rf degradation of AlGaN/GaN High Electron Mobility Transistors has been analyzed and discussed by means of pulsed measurements and spectroscopic techniques.

2010 - Boost-converter-based solar harvester for low power applications [Articolo su rivista]
Chini, Alessandro; Soci, F.
abstract

A novel implementation of a solar harvester circuit based on a boost DC/DC converter topology is presented. The proposed circuit is able to harvest solar energy down to a power level of 1.6 mW while maintaining comparable performance with previously presented results at higher power levels.

2010 - Design of GaN HEMTs for Power Switching Operation [Abstract in Atti di Convegno]
Esposto, Michele; Park, P. S.; Nath, D. N.; Krishnamoorty, S.; Akyol, F.; DI LECCE, Valerio; Chini, Alessandro; Rajan, S.
abstract

A model based on the key parameters of the device, such as the 2DEG carrier concentration and its mobility, as well as the active area geometry, will be presented and discussed.

2010 - Experimental and simulated dc degradation of GaN HEMTs by means of gate-drain and gate-source reverse bias stress [Articolo su rivista]
DI LECCE, Valerio; Esposto, Michele; Bonaiuti, Matteo; Meneghesso, G.; Zanoni, E.; Fantini, Fausto; Chini, Alessandro
abstract

In this paper, the degradation of AlGaN/GaN high-electron mobility transistors (HEMTs) is investigated by means of dc stresses performed on fresh devices either on the gate-drain junction only (i.e., with the source terminal floating) or on the gate-source junction only (i.e., with the drain terminal floating). In both cases step-stresses were carried out by increasing VDG and VSG respectively up to 35 V: the saturated drain current decreased in both cases, and a significant increase in the output conductance was found for the drain-stressed devices, whereas it was negligible for the source-stressed devices. The reason for these different behaviors was believed to be the creation of acceptor traps in the AlGaN layer underneath the stressed side of the gate junction, their influence being different in the two cases because of the high horizontal electric field at the drain end of the gate during on-state operation. We carried out numerical simulations showing that the presence of a defective region with an acceptor trap concentration underneath the gate-drain or gate-source junction fits our hypothesis.

2010 - Fabrication of novel high frequency and high breakdown InAlAs-InGaAs pHEMTs [Relazione in Atti di Convegno]
M., Mohamad Isa; Saguatti, Davide; Verzellesi, Giovanni; Chini, Alessandro; K. W., Ian; M., Missous
abstract

This paper presents a Novel low noise, high breakdown InAlAs/InGaAspseudomorphic High Electron Mobility Transistors (pHEMTs). Theimprovements in breakdown voltage are brought about by a judiciouscombination of epitaxial layer design and field plate techniques. No significant degradations of DC and RF characteristics are observed for devices with field plate structures. An outstanding improvement in breakdown voltages of >30% is attained by field plate devices which should allow their usage in efficient high-added power efficiency amplifiers design.

2010 - Fabrication of single or multiple gate field plates [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

A process for fabricating single or multiple gate field plates using consecutive steps of dielectric material deposition/growth, dielectric material etch and metal evaporation on the surface of a field effect transistors. This fabrication process permits a tight control on the field plate operation since dielectric material deposition/growth is typically a well controllable process. Moreover, the dielectric material deposited on the device surface does not need to be removed from the device intrinsic regions: this essentially enables the realization of field-plated devices without the need of low-damage dielectric material dry/wet etches. Using multiple gate field plates also reduces gate resistance by multiple connections, thus improving performances of large periphery and/or sub-micron gate devices.

2010 - Field Plate Devices for RF Power Applications [Capitolo/Saggio]
Chini, Alessandro
abstract

The aim of this chapter is to provide to the reader insights into field plate operation and itsgeometrical optimization. After giving some basic definitions concerning the operation of anRF-power device, which will be used in order to quantify the performance of the devicesstudied, the optimization of a gate-connected single field-plate GaAs-based pHEMT will bepresented. Field plate geometrical parameters will be varied in order to show how they canaffect device properties such as breakdown voltage, maximum output power and smallsignal performances. It will be thus possible to quantify the maximum improvement thatcan be achieved by using a gate connected single field plate. Finally, some advanced fieldplate structure will be discussed and compared in order to point out their advantages withrespect to the gate connected single field plate structure.

2010 - High Robustness GaN HEMT Subject to Reverse Bias Stress [Relazione in Atti di Convegno]
Stocco, A.; Ronchi, N.; Chini, Alessandro; Nilsson, P. A.; Meneghesso, G.; Zanoni, E.
abstract

In this paper we report a reliability study of optimized AlGaN/GaN HEMTs submitted to high bias voltages. In these devices, thanks to the improved epilayers structures, gate Field Plate and GaN cap layer, no significant degradation has been observed after drain-gate biased up to 200 V. Three SiC substrates have been adopted, with different quality properties, and unexpected high device robustness has been observed in all three wafers.

2010 - High Voltage Breakdown pHEMTs for C-band HPA [Relazione in Atti di Convegno]
Lavanga, S.; Chini, Alessandro; Coppa, A.; Corsaro, F.; Nanni, A.; Pantellini, A.; Romanini, P.; Lanzieri, C.
abstract

High Voltage Breakdown (HVB) HPA MMIC based on GaAs pHEMTs technology represents a useful way for high power RF application. To increase the breakdown voltage respect to conventional device, a field-plate (FP) gate structure is implemented in our standard 0.5 μm process. With this solution the off-state breakdown voltage of the device has been improved from 18V to 28V, while keeping constant the drain current. As expected, FP devices showed smaller drain current collapse than standard devices under pulsed DC measurement conditions and a significant increase in power density, i.e. 1.4 W/mm. To guaranty a full advantage for real HPA applications we have carried out numerical simulations, to optimise device thermal behaviour as a function of GaAs thickness and gate pitch. On the basis of this technology development, reliable and reproducible HPA have been fabricated with an output power of circa 30 W and power added efficiency (PAE) of circa 35% in the 4.9-6.1GHz frequency range. Therefore this proposed technological solution enables the realization of very high power T/R modules with reliable GaAs technology, waiting for more disruptive and less mature GaN HEMT one.

2010 - Micro-power photovoltaic harvester based on a frequency-to-voltage MPPT tracker [Articolo su rivista]
Chini, Alessandro; Rovati, Luigi
abstract

A micro-power circuit for photovoltaic energy harvesting applications is presented. The system can work in different lighting power ranging from a few tens of microwatts to milliwatts maintaining tracking of the maximum output power operational conditions of the photovoltaic cells. The energy harvesting efficiency ranges from 30 to 70%.

2010 - Reliability issues of Gallium Nitride High Electron Mobility Transistors [Articolo su rivista]
Meneghesso, G.; Meneghini, M.; Tazzoli, A.; Ronchi, N.; Stocco, A.; Chini, Alessandro; Zanoni, E.
abstract

In the present paper we review the most recent degradation modes and mechanisms recently observed in AlGaN/GaN (Aluminum Gallium Nitride/Gallium Nitride). High Electron-Mobility Transistors (HEMTs), as resulting from a detailed accelerated testing campaign, based on reverse bias tests and DC accelerated life tests at various temperatures. Despite the large efforts spent in the last few years, and the progress in mean time to failure values, reliability of GaN HEMTs, and millimeter microwave integrated circuits still represent a relevant issue for the market penetration of these devices. The role of temperature in promoting GaN HEMT failure is controversial, and the accelerating degradation factors are largely unknown. The present paper proposes a methodology for the analysis of failure modes and mechanisms of GaN HEMTs, based on (i) DC and RF stress tests accompanied by an (ii) extensive characterization of traps using deep level transient spectroscopy and pulsed measurements, (iii) detailed analysis of electrical characteristics, and (iv) comparison with two-dimensional device simulations. Results of failure analysis using various microscopy and spectroscopy techniques are presented and failure mechanisms observed at the high electric field values typical of the operation of these devices are reviewed.

2010 - Study of GaN HEMTs degradation by numerical simulations of scattering parameters [Abstract in Atti di Convegno]
DI LECCE, Valerio; Esposto, Michele; Bonaiuti, M.; Meneghesso, G.; Zanoni, E.; Fantini, Fausto; Chini, Alessandro
abstract

In this paper, a study of the dependence of the transconductance gm on the trap concentration and on the length of the trapping region is carried out by means of numerical simulations, and particular attention is dedicated to the correlation between gm and the device AC performance by simulation of scattering parameters.

2010 - Study of GaN HEMTs electrical degradation by means of numerical simulations [Relazione in Atti di Convegno]
DI LECCE, Valerio; Esposto, Michele; Bonaiuti, Matteo; Fantini, Fausto; Chini, Alessandro
abstract

In this paper, we investigate the effects of dc stress on GaN high-electron mobility transistors (HEMTs) by means of numerical simulations. Following stress tests showing a degradation of static characteristics (dc), the formation of an electron trap in the AlGaN barrier layer was related to the observed degradation according to the results obtained from numerical simulations carried out by introducing a trapping region underneath the gate edge. The worsening of the device dc performance is evaluated by changing the extension of the degraded region and the trap concentration while studying the variation of parameters like the saturated drain current IDSS, the output conductance g O, and the device transconductance gM. An increase in the trap concentration induces a worsening of any of the abovementioned parameters; an increase in the extension of the degraded region induces a degradation of IDSS and gM, but can reduce gO.

2010 - TCAD optimization of field-plated InAlAs-InGaAs HEMTs [Relazione in Atti di Convegno]
Saguatti, Davide; Chini, Alessandro; Verzellesi, Giovanni; M., Mohamad Isa; K. W., Ian; M., Missous
abstract

High-voltage InGaAs-InAlAs HEMTs featuring optimized field-plate structures are being developed. A TCAD approach has been adopted for their design. Two-dimensional device simulations have preliminarily been calibrated by comparison with DC and RF measurements from the baseline InP HEMT technology into which the field plate is being incorporated. Simulations have then been used to design field-plate structures with optimal length andpassivation thickness.

2009 - Comparison of Cu-gate and Ni/Au-gate GaN HEMTs large signal characteristics [Relazione in Atti di Convegno]
Esposto, Michele; DI LECCE, Valerio; Chini, Alessandro; De Guido, S.; Passaseo, A.; De Vittorio, M.
abstract

In this paper a complete comparison between Copper (Cu) gate and Nickel-Gold (Ni/Au) gate passivated AlGaN/GaN High Electron Mobility Transistors (HEMTs) is presented. DC and Radio Frequency (RF) performance was compared in order to evaluate the behaviour of the two Schottky contacts in the standard HEMT structure. From the obtained data a critical drain current collapse was observed in the Cu-gate devices, with detrimental effects on the RF performance, while the Ni/Au-gate performed nicely both during pulsed I-V and RF measurements. An investigation on the drain current transients and on ID - VGS characteristics, obtained by pulsed signals showed that an acceptor trap at the Cu/AlGaN interface, with activation energy of about 0.43 eV, could be responsible for the Cu-gate HEMT poorer performance. The results suggest that a detailed investigation on surface treatments, gate metal quality and deposition methods is needed in order to fabricate Cu-gate GaN HEMTs.

2009 - Correlation between DC and rf degradation due to deep levels in AlGaN/GaN HEMTs [Relazione in Atti di Convegno]
Chini, Alessandro; Fantini, Fausto; DI LECCE, Valerio; Esposto, Michele; Stocco, A.; Ronchi, N.; Zanon, F.; Meneghesso, G.; Zanoni, E.
abstract

We investigate the role of the 0.5 eV traps in determining GaN HEMT degradation by means of DC and rf testing, and 2D numerical simulation. We demonstrate that generation of deep levels, having an activation energy of 0.5 eV, is responsible for the degradation observed during rf aging; we show that the occurrence of trap-induced degradation depends on rf driving conditions. We also show that degradation can be explained by the generation of a damaged region within the AlGaN layer at the gate-drain edge, and that the DC and pulsed device degradation effects have a different dependence on the width and depth of the damaged region.

2009 - Design of field-plated InP-based HEMTs [Abstract in Atti di Convegno]
Saguatti, Davide; DI LECCE, Valerio; Esposto, Michele; Chini, Alessandro; Fantini, Fausto; Verzellesi, Giovanni; Boulay, S.; Bouloukou, A.; Boudjelida, B.; Missous, M.
abstract

The design of field-plated InP-based HEMTs is presented by means of numerical simulations tools aimed at the optimization of device breakdown voltages.

2009 - Evaluation and Numerical Simulations of GaN HEMTs Electrical Degradation [Articolo su rivista]
Chini, Alessandro; Di Lecce, Valerio; Esposto, Michele; Meneghesso, G.; Zanoni, E.
abstract

In this letter, the effects of dc stress on GaN high-electron-mobility transistors' performance are investigated by means of experimental measurements and numerical simulation. A degradation of both dynamic (pulsed I-V) and static characteristics (dc) has been observed on stressed devices, and it has been experimentally related to the formation of an electron trap in the AlGaN barrier layer. Numerical simulations carried out on the tested structure by introducing a trapping region at the gate edge of the device barrier confirm the experimentally observed device degradation. The worsening of the dynamic performance is induced by both an increase in trap concentration and/or depth of the trapping region while the degradation of the dc characteristics can be explained by an increase in the trapping-region depth.

2009 - Evaluation of GaN HEMT degradation by means of pulsed I-V, leakage and DLTS measurements [Articolo su rivista]
Chini, Alessandro; Esposto, Michele; Meneghesso, G.; Zanoni, E.
abstract

The effects of short-term step-stress on the performance of GaN HEMTs have been evaluated for the first time by means of current deep-level-transient-spectroscopy (DLTS) measurements. When subjected to high reverse gate bias the devices experienced an increase in the drain current dispersion as well as in the gate current. Current DLTS measurements carried out during the stress experiment show that the device degradation can be associated to the formation of a defect that is thermally activated with an energy of 0.5 eV.

2009 - False surface-trap signatures induced by buffer traps in AlGaN-GaN HEMTs [Relazione in Atti di Convegno]
Verzellesi, Giovanni; Faqir, Mustapha; Chini, Alessandro; Fantini, Fausto; Meneghesso, G.; Zanoni, E.; Danesin, F.; Zanon, F.; Rampazzo, F.; Marino, F. A.; Cavallini, A.; Castaldini, A.
abstract

Buffer traps can induce “false” surface-trap signatures in AlGaN-GaN HEMTs, namely the same type of current-mode DLTS peaks and pulse responses that are generally attributed to surface traps. Device simulations are adopted to clarify the underlying physics. Being aware of the above phenomenon is important for both reliability testing and device optimization, as it can lead to erroneous identification of the degradation mechanism, thus resulting in inappropriate correction actions on the technological process.

2009 - GaN Hemt Degradation induced by Reverse Gate Bias Stress [Abstract in Atti di Convegno]
Meneghesso, G.; Meneghini, M.; Tazzoli, A.; Ronchi, N.; Stocco, A.; Zanoni, E.; DI LECCE, Valerio; Esposto, Michele; Chini, Alessandro
abstract

The degradation mechanisms induced in GaN-based HEMTs when reverse gate-bias stress are applied have been presented and discussed.

2009 - Influence of RF drive and switching frequency on degradation mechanisms in GaN HEMTs [Abstract in Atti di Convegno]
DI LECCE, Valerio; Esposto, Michele; Bonaiuti, Matteo; Fantini, Fausto; Chini, Alessandro; Meneghesso, G.; Zanoni, E.
abstract

GaN-based high-electron mobility transistors (HEMTs) have received increasing attention for their outstanding performance in RF-power applications [1]–[3]. Gallium nitride’s wide energy gap (3.49eV) and breakdown field (3MV/cm) allow HEMTs to tolerate high voltages and high temperatures, while its higher saturation velocity and the absence of doping atoms in the channel make them very fast switching devices compared to silicon-based ones. However, HEMTs still suffer from reliability problems like current collapse, gate-lag, and RF degradation, to be solved before they can be used for commercial purposes [4]–[5]. In this paper a review of RF and pseudo-RF stress campaigns carried out on GaN HEMTs is presented.

2009 - Influence of interface states at Schottky junction on the large signal behaviour of Cu-gate standard AlGaN/GaN HEMTs [Abstract in Atti di Convegno]
Esposto, Michele; DI LECCE, Valerio; Bonaiuti, Matteo; Fantini, Fausto; Verzellesi, Giovanni; De Guido, S.; De Vittorio, M.; Passaseo, A.; Chini, Alessandro
abstract

Copper (Cu)-gate AlGaN/GaN High Electron Mobility Transistors have been already proposed by Ao et al. [5] and more recently by Sun et al. [6]. Gate leakage currents, Schottky barrier heights and other small signal parameters have found to be very interesting for Cu-gate devices. The aim of our work is to evaluate the large signal characteristics, giving a direct comparison between Cu-gate device and Ni/Au-gate one.

2009 - Long-term stability of Gallium Nitride High Electron Mobility Transistors: a reliability physics approach [Relazione in Atti di Convegno]
E., Zanoni; G., Meneghesso; M., Meneghini; A., Tazzoli; N., Ronchi; A., Stocco; F., Zanon; Chini, Alessandro; Verzellesi, Giovanni; A., Cetronio; C., Lanzieri; M., Peroni
abstract

Several groups have demonstrated nitride-based High Electron Mobility Transistors with excellent rf output power, with a constant increase in performances. However, despite the large efforts spent in the last few years, and the progress in MTTF (Mean Time To Failure) values, reliability of GaN HEMTs (High Electron Mobility Transistors) and MMICs (Millimeter Microwave Integrated Circuits) still has to be fully demonstrated, due to the continuous evolution of adopted processes and technologies, and to the lack of information concerning failure modes and mechanisms. The role of temperature in promoting GaN HEMT failure is controversial, and the factors accelerating degradation are largely unknown. This paper proposes a methodology for the analysis of failure modes and mechanisms of GaN HEMTs, based on the extensive characterization of deep levels using Deep Level Transient Spectroscopy (DLTS) and pulsed measurements, on the detailed analysis of electrical characteristics, and on comparison with two-dimensional device simulations. Results of failure analysis using various microscopy and spectroscopy techniques are presented and failure mechanisms observed at the high electric field values typical of the operation of these devices are reviewed.

2009 - RF degradation of GaN HEMTs and its correlation with DC stress and I-DLTS measurements [Relazione in Atti di Convegno]
Chini, Alessandro; DI LECCE, Valerio; Esposto, Michele; Meneghesso, G.; Zanoni, E.
abstract

The reliability of GaN HEMTs under RF stress has been evaluated and correlated with DC stress and current DLTS measurements techniques. During RF operation a degradation of output power, PAE and an increase in reverse gate current has been observed. A similar degradation has been observed by applying reverse biases to the gate terminal under DC operation, resulting in an increase in drain current-collapse as well as in the gate reverse current. DLTS measurements showed that the physical mechanisms involved in the RF- and DC- degradation are the same and that can be related to a defect which is thermally activated with an energy of 0.5eV. The strong correlation between RF- and DC-stress results suggests that the RF degradation is related to the formation of localized defects at the gate contact. These defects degrade device performances by increasing both drain current collapse and reverse gate current.

2009 - Reverse gate bias stress induced degradation of GaN HEMT [Abstract in Atti di Convegno]
Zanoni, E.; Meneghini, M.; Tazzoli, A.; Ronchi, N.; Stocco, A.; DI LECCE, Valerio; Esposto, Michele; Chini, Alessandro; Meneghesso, G.
abstract

GaN HEMT degradation induced by reverse gate bias stress methods has been investigated by means of experimental measurements. Experimental data will be presented and discussed.

2009 - Trap analysis on GaN HEMT after DC accelerated tests [Abstract in Atti di Convegno]
Stocco, A.; Ronchi, N.; Zanon, F.; Zanoni, E.; Meneghesso, G.; Chini, Alessandro; Peroni, M.
abstract

Several groups have demonstrated nitride-based High Electron Mobility Transistors (HEMTs) with excellent rf output power and with constant increase in performances. However, despite the large effort spent in the last few years, and the progress in MTTF (mean time to failure) values, reliability of GaN HEMTs still has to be fully demonstrated, due to the continuous evolution of adopted process and technologies. This paper analyses the effects of accelerated DC tests at different temperatures in static and dynamic characteristics.

2008 - Characterization and Numerical Simulations of High Power Field-Plated pHEMTs [Relazione in Atti di Convegno]
Chini, Alessandro; Esposto, Michele; Verzellesi, Giovanni; S., Lavanga; C., Lanzieri; A., Cetronio
abstract

This paper presents the results obtained both by experimental measurements and numerical simulations carried out on state-of-the-art Field-Plated GaAs-based pHEMTs. The effect of field-plate length on DC and RF operation of pHEMTs will be discussed showing that the adoption of an optimal fieldplate structure can significantly boost the device RF power performance, resulting in power density up to 2W/mm measured under continuous wave RF signals at 2GHz. The physical origin of the DC-to-RF dispersion in the fabricated devices has been associated with a hole-trap located at 0.65eV from the valence band as obtained from current-DLTS measurements. The experimental results will also be supported and validated by numerical simulations. It will be shown that the beneficial effects arising from the adoption of the field-plate structure lie in its control on the trapped charge population responsible for the DC-to-RF dispersion mechanism.

2008 - Effects of Surface and Buffer Traps in Passivated AlGaN-GaN HEMT [Abstract in Atti di Convegno]
Faqir, Mustapha; Verzellesi, Giovanni; Chini, Alessandro; Fantini, Fausto; Danesin, F.; Rampazzo, F.; Meneghesso, G.; Zanoni, E.; Labat, N.; Touboul, A.; Dua, C.
abstract

The effects of surface and buffer traps in passivated AlGaN-GaN HEMT on their pulsed I-V characteristics has been investigated by means of numerical simulations.

2008 - Effects of surface and buffer traps in passivated AlGaN/GaN HEMTs [Abstract in Atti di Convegno]
Faqir, Mustapha; Verzellesi, Giovanni; Chini, Alessandro; Fantini, Fausto; F., Danesin; F., Rampazzo; G., Meneghesso; E., Zanoni; N., Labat; A., Touboul; C., Dua
abstract

The physical mechanisms underlying RF current collapse effects in AlGaN-GaN HEMTs are investigated by means of measurements and numerical device simulations. Our study suggests that 1) both surface and buffer traps can contribute to RF current collapse through a similar physical mechanism involving capture/emission of electrons tunneling from the gate; 2) surface passivation strongly mitigate RF current collapse, by reducing the surface electric field and inhibiting electron injection into traps; 3) for surface donor trap densities lower than 9e12 /cm2, surface potential barriers in the 1-2 eV range can coexist with surface traps having much a shallower energy depth and inducing, therefore, current-collapse effects characterized by relatively short time constants.

2008 - High Power Performances of GaN HEMT on SOPSIC substrate [Abstract in Atti di Convegno]
Zanon, F.; Danesin, F.; Tazzoli, A.; Montanari, G.; Chini, Alessandro; Thorpe, J.; Gaquière, C.; Meneghesso, G.; Zanoni, E.
abstract

In this paper we will present an experimental characterization of AlGaN/GaN HEMTs grown on SopSiC composite substrates. Tested devices yielded breakdown voltages up to 90 V (on-state) and 300 V(off-state), power densities up to 4 W/mm and power added efficiencies up to 54%. These results clearly suggest that the adopted composite substrate represent a very promising solution for the fabrication of low-cost high-power microwave transistors for wireless communication systems.

2008 - Influence of Device Self-Heating on the Activation Energy Extraction During Current-DLTS Measurement [Abstract in Atti di Convegno]
Chini, Alessandro; Esposto, Michele; Bonaiuti, Matteo; Verzellesi, Giovanni; Zanon, F.; Zanoni, E.; Meneghesso, G.
abstract

Results obtained by current-DLTS measurements on GaN-based HEMTs are presented. It is shown that device self-heating can significantly influence the extraction of trap ionization energy leading to a large underestimation of the latter.

2008 - Mechanisms of RF Current Collapse in AlGaN–GaN High Electron Mobility Transistors [Articolo su rivista]
Faqir, Mustapha; Verzellesi, Giovanni; Chini, Alessandro; Fantini, Fausto; Danesin, F.; Meneghesso, G.; Zanoni, E.; Dua, C.
abstract

The physical mechanisms underlying RF current collapse effects in AlGaN-GaN HEMTs are investigated by means of measurements and numerical device simulations. Our study suggests that 1) both surface and buffer traps can contribute to RF current collapse through a similar physical mechanism involving capture and emission of electrons tunneling from the gate; 2) surface passivation strongly mitigates RF current collapse by reducing the surface electric field and inhibiting electron injection into traps; 3) for surface-trap densities lower than 9e12 cm-2, surface potential barriers in the 1-2 eV range can coexist with surface traps having much a shallower energy and therefore inducing RF current collapse effects characterized by relatively-short time constants.

2008 - Reliability aspects of GaN-HEMTs on composite substrates [Relazione in Atti di Convegno]
Zanon, F.; Danesin, F.; Tazzoli, A.; Meneghini, M.; Ronchi, N.; Chini, Alessandro; Bove, P.; Langer, R.; Zanoni, E.; Meneghesso, G.
abstract

This paper shows electrical characterizations and reliability analysis performed on AlGaN/GaN HEMTs processed on epitaxial layers grown on composite substrates. The results are very promising for the fabrication of low cost high power microwave transistors for wireless communication systems. The composite substrates constitute a valuable alternative to the silicon since better thermal properties are expected.

2008 - Trapping phenomena in field-plated high power GaAs pHEMTs [Abstract in Atti di Convegno]
Chini, Alessandro; DI LECCE, Valerio; Esposto, Michele; Verzellesi, Giovanni; Lavanga, S.; Cetronio, A.; Lanzieri, C.
abstract

This paper presents the results obtained both by experimental measurements and numerical simulations carried out on state-of-the-art field plated GaAs-based pHEMTs.The effect of the field-plate length on DC and RF operation will be discussed showing that the adoption of an optimal field-plate structure can significantly boost the device RF power performance, resulting in power density up to 2W/mm measured at 2 GHz.

2007 - Analysis of High-Electric-Field Degradation in AlGaN/GaN HEMTs [Relazione in Atti di Convegno]
Faqir, Mustapha; Chini, Alessandro; Verzellesi, Giovanni; Fantini, Fausto; Rampazzo, F.; Meneghesso, G.; Zanoni, E.; Kordos, P.
abstract

High-electric-field degradation phenomena are investigated in GaN-capped AlGaN-GaN HEMTs by comparing experimental data with numerical device simulations. Simulations indicate that the stress-induced amplification of gate-lag effects and the correlated gate-leakage-current reduction can be ascribed to the generation of acceptor traps at the gate-drain surface and/or in the device barrier. The drop in DC drain current observed after stress should rather be attributed to trap accumulation within the GaN buffer region. Only the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the observed degradation modes. Simulations suggest also that under power-state stress traps should accumulate over a wide region extending laterally from the gate edge towards the drain contact, whereas, under off-state stress, trap generation should rather take place in a narrower portion of the drain access region close to the gate edge and should be accompanied by a significant degradation of the channel transport parameters. Channel hot electrons and electric-field-induced strain-enhancement are finally suggested to play major roles in power-state and off-state degradation, respectively.

2007 - Design, Fabrication and Characterization of Γ Gate GaN HEMT for High-Frequency/Wide-Band applications [Relazione in Atti di Convegno]
Peroni, M.; Romanini, P.; Pantellini, A.; Cetronio, A.; Mariucci, L.; Minotti, A.; Ghione, G.; Camarchia, V.; Limiti, E.; Serino, A.; Chini, Alessandro
abstract

We report a study on AlGaN/GaN HEMT performances optimization by using Γ-Gate technology. The influence of the adopted technological parameters, like the field plate extension, through the analysis of small/large signal RF performances, including the comparison of the obtained results with conventional LG=0.5µm gate-length devices is investigated. The introduction of a field plate overhang of 0.2µm over the underlying SiN layer at the drain side of the gate foot, has brought approximately to a factor two improvement on the device breakdown voltage. On the other hand, a reduction of RF gain has been observed in presence of the FP metallization, especially if longer than 0.2µm, even though such negative effect is less significant if drain bias higher than 30V has been applied.

2007 - N-polar GaN/AlGaN/GaN high electron mobility transistors [Articolo su rivista]
Rajan, S.; Chini, Alessandro; Wong, M. H.; Speck, J. S.; Mishra, U. K.
abstract

We describe the development of N-polar GaN-based high electron mobility transistors grown by N2 plasma-assisted molecular beam epitaxy on C-face SiC substrates. High mobility AlGaN/GaN modulation-doped two-dimensional electron gas channels were grown, and transistors with excellent dc and small-signal performance were fabricated on these wafers. Large-signal dispersion was observed, and the trap states responsible for this were identified, and layer designs to remove the dispersive effects of these traps were demonstrated. Finally, an AlGaN-cap layer was used to reduce gate leakage in these devices, and a low-dispersion high breakdown voltage device was achieved. This detailed study of dispersion and leakage in N-polar GaN-based transistors establishes a technological base for further development of field effect devices based on N-polar III-nitrides.

2007 - Very High Power Field-Plate GaAs PHEMT technology for C and X -band applications [Relazione in Atti di Convegno]
Lavanga, S.; Lanzieri, C.; Peroni, M.; Romanini, P.; Cetronio, A.; Chini, Alessandro; Mariucci, L.
abstract

Very high voltage breakdown pHEMTs have been successfully developed by implementing a field-plate (FP) gate structure. Devices with and without FP, in which the FP is simply connected to the gate contact, have been fabricated on the same wafer in order to compare the improvements induced by adopting the FP. As expected FP devices showed smaller drain current dispersion than standard devices under pulsed DC measurement conditions. Moreover off-state breakdown voltage improved from -25 V, for the devices without FP, up to -40 V for the field-plated devices. Electron Beam Lithography of the gate with Lg=0.25µm and i-line Stepper Lithography with Lg=0.5µm devices results have been compared. FP devices with effective gate length of 0.6 µm yielded output power levels as high as 1.6W/mm CW @ 4 GHz with PAE up to 50%. FP devices with effective gate length of 0.3 µm yielded a maximum available gain @10 GHz as high as 13 dB (Vds=8V) and 10 dB for Vds=15V. The fabricated structures were also evaluated by carrying out 2D numerical simulations. Experimental results on MISpHEMT devices have been explained by means of a donor trap located at the dielectric/semiconductor (namely SiN/GaAs) interface.

2007 - シングルゲートまたはマルチゲートフィールドプレート製造 [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

電界効果型トランジスタの表面に、誘電性材料の堆積／成長させ、誘電性材料をエッチングし、および、メタルを蒸着させる、連続的なステップを用いる、シングルゲートまたはマルチゲートプレートの製造プロセス。本製造プロセスのは、誘電性材料の堆積／成長が、典型的には、非常によく制御できるプロセスなので、フィールドプレート動作を厳しく制御できる。さらに、デバイス表面に堆積された誘電性材料は、デバイスの真性領域から除去される必要はない。このため、乾式または湿式のエッチングプロセスで受けるダメージの少ない材料を用いることなく、フィールドプレートされたデバイスを、実現することができる。マルチゲートフィールドプレートを使うと、マルチ接続を使用するので、ゲート抵抗を減らすこともでき、こうして、大周辺デバイスおよび／またはサブミクロンゲートデバイスの性能を向上することができる。

2006 - Fabrication, Characterization and Numerical Simulation of High Breakdown Voltage pHEMTs [Relazione in Atti di Convegno]
Chini, Alessandro; S., Lavanga; M., Peroni; C., Lanzieri; A., Cetronio; V., Teppati; V., Camarchia; G., Ghione; Verzellesi, Giovanni
abstract

High Breakdown Voltage pHEMTs have been successfully developed by implementing a field-plate (FP) structure. Devices with and without FP have been fabricated on the same wafer in order to compare the improvements induced by adopting the FP. Both kind of devices showed little or no current dispersion under pulse measurement conditions. Moreover the off-state breakdown voltage improved from 23V, for the devices without FP, to 38V for the field-plated devices. At 4GHz an output power as high as 1.6W/mm was measured for a FP device, resulting in a 60% improvement with respect to the device without FP. The fabricated structures were also evaluated by carrying out 2D numerical simulations. Experimental results on MISpHEMTs have been explained by means of a donor trap at the SiN/GaAs interface located at 0.18eV from the GaAs conduction band. Finally, a good agreement between experimental and simulated device characteristics was obtained.

2006 - Off-state breakdown optimization in field plated GaAs-pHEMTs by means of two-dimensional numerical simulation [Abstract in Atti di Convegno]
Chini, Alessandro; Verzellesi, Giovanni
abstract

The optimization of a field plated pHEMT structure has been presented. It has been shown that by optimizing the FP length (LFP) and the SiN layer thickness the off-state breakdown voltage can be improved as much as four times for the selected pHEMT structure. Choosing the right SiN thickness (in this case in the 40-60nm range) and LFP (in this case in the 0.8-1.2um range) is crucial in order to obtain a significant benefit in the device off-state breakdown.

2006 - Physical investigation of high-field degradation mechanisms in GaN/AlGaN/GaN hemts [Relazione in Atti di Convegno]
Faqir, Mustapha; Chini, Alessandro; Verzellesi, Giovanni; Fantini, Fausto; Rampazzo, F.; Meneghesso, G.; Zanoni, E.; Bernat, J.; Kordos, P.
abstract

High-electric-field degradation phenomena are investigated in GaN-capped AlGaN-GaN HEMTs by comparing experimental data with numerical device simulations. Simulations indicate that the stress-induced amplification of gate-lag effects and the correlated gate-leakage-current reduction can be ascribed to the generation of acceptor traps at the gate-drain surface. The drop in DC drain current observed after stress should rather be attributed to trap accumulation within the GaN buffer region. Only the simultaneous generation of surface and buffer traps can account for all of the observed degradation modes. © 2006 JEDEC.

2006 - Study of high-field degradation phenomena in GaN-capped AlGaN/GaN HEMTs [Abstract in Atti di Convegno]
Faqir, Mustapha; Chini, Alessandro; Verzellesi, Giovanni; Fantini, Fausto; Rampazzo, F.; Meneghesso, G.; Zanoni, E.; Bernat, J.; Kordos, P.
abstract

High-electric-field degradation phenomena are studied in GaN-capped AlGaN-GaN HEMTs by comparing experimental results with numerical device simulations and assessing the suitability of different degradation scenarios to account for the observed electrical stress effects.

2006 - Transient Phenomena in GaAs and GaN Devices, including Electroluminescence and Emission Spectroscopy, for Future THz Applications [Abstract in Atti di Convegno]
Chini, Alessandro; Verzellesi, Giovanni; Meneghesso, G.; Zanoni, E.
abstract

Transient Phenomena in GaAs and GaN Devices are reviewed focusing on the physical mechanisms limiting their performance. Device characterization by means of Electroluminescence and Emission Spectroscopy techniques are also presented.

2006 - Use of Double-Channel Heterostructures to Improve the Access Resistance and Linearity in GaN-Based HEMTs [Articolo su rivista]
Palacios, T.; Chini, Alessandro; Buttari, D.; Heikman, S.; Chakraborty, A.; Keller, S.; Denbaars, S. P.; Mishra, U. K.
abstract

Double-channel structures have been used in AlGaN/GaN high electron mobility transistors to reduce the access resistance. Carrier densities as high as 2.9×1013 cm-2 and mobilities in the 1300 cm2/V·s range have been obtained in the access region. Also, the correct design of the potential barrier between the different channels allowed tailoring the differential access resistance to enhance the linearity of the transistors. This increase in linearity has been measured as a flatter profile of the transconductance and cutoff frequency versus current and as an improvement of more than 2 dB in large-signal two-tone linearity measurements.

2006 - Very High Performance GaN HEMT devices by Optimized Buffer and Field Plate Technology [Relazione in Atti di Convegno]
Romanini, P.; Peroni, M.; Lanzieri, C.; Cetronio, A.; Calori, M.; Passaseo, A.; Poti, B.; Chini, Alessandro; Mariucci, L.; Di Gaspare, A.; Teppati, V.; Camarchia, V.
abstract

One of the main expected benefits of AlGaN HEMT technology for microwave applications is related to the higher operating bias voltage achievable with these devices. However, various technological issues, concerning material properties and device technology must be properly tailored to fully exploit the potential of that kind of devices. In this work we report on the realization of HEMT device showing improved performance in terms of breakdown voltage, device isolation and reverse current leakage achieved by improved epilayer buffer properties and optimized field plate gate geometry. In particular the low defect density and the high resistivity obtained by using an HT-AlN crystallization layer for the growth of the GaN layer has lead to an effective 2DEG carrier concentration of 8 times 1012 cm-2 with related mobility of 1700cm2/Vs and corresponding devices with a very high voltage breakdown (VB > 200V), excellent active device isolation and limited reverse current leakage.

2006 - Very high power low-cost field-plate GaAs PHEMTs for X-band applications [Abstract in Atti di Convegno]
Ghione, G.; Lanzieri, C.; Peroni, M.; Lavanga, S.; Chini, Alessandro; Verzellesi, Giovanni; Camarchia, V.; Cappelluti, F.; Angelini, A.; Limiti, E.; Serino, A.
abstract

A low cost fabrication process based on field-plated GaAs pHEMT for X-Band applications is presented by also focusing on the device performances evaluated by means of experimental measurments.

2006 - p-GaN/AlGaN/GaN Enhancement-Mode HEMTs [Abstract in Atti di Convegno]
C. S., Shu; Chini, Alessandro; Y., Fu; C., Poblenz; J. S., Speck; U. K., Mishra
abstract

GaN-based enhancement-mode (E-mode) HEMTs are attracting significant interest for integration of control circuitry and for the added safety of a normally-off device in power switching applications. While previous work reports excellent performance by gate-recessing1 and Fluorine-based plasma treatment2, the Schottky gate turn-on voltage of these devices are at most 2 V. Because high-power switching applications require a threshold voltage of over 1 V for gate signal noise immunity, increasing the gate turn-on voltage is crucial. Utilization of p-GaN barrier below the gate3 depletes the channel and increases the gate turn-on voltage to 3 V, rendering it attractive for high-power applications. In this report we present a p-GaN/AlGaN/GaN E-mode HEMTs with a 3 V gate turn-on and maximum output current exceeding 0.3 A/mm. In addition, pulsed I-V measurement and small-signal performance of these devices are presented and the design space of p-GaN gated E-mode HEMTs are investigated for high-power switching applications.

2006 - 单个或多个栅极场板的制造 [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

一种制造单个或多个栅极场板的方法，其利用了下列连续步骤：在场效应晶体管表面上进行介电材料沉积/生长、介电材料蚀刻以及金属蒸镀。由于介电材料沉积/生长是一种充分可控的处理，因此这种制造方法允许对场板工作进行严格控制。而且，沉积在器件表面上的介电材料无需从器件本征区中去除：这实质上使得无需低损伤介电材料干法/湿法蚀刻便能实现场板器件。使用多个栅极场板还通过多个连接来减少栅极电阻，从而改善了大周边和/或亚微米栅器件的性能。

2005 - DC-to-RF dispersion effects in GaAs- and GaN-based heterostructure FETs: performance and reliability issues [Articolo su rivista]
Verzellesi, Giovanni; Meneghesso, G.; Chini, Alessandro; Zanoni, E.; Canali, Claudio
abstract

The performance and reliability implications of DC-to-RF dispersion effects are addressed. The proposed physical explanations and technological counteractions are reviewed. GaAs- and GaN-based FET technologies are considered, trying to point out both similar and peculiar aspects.

2005 - Effect of CF4/O2 plasma damage on AlGaN/GaN HEMTs [Abstract in Atti di Convegno]
Chini, Alessandro; Peroni, M.; Romanini, P.; Lanzieri, C.; Teppati, V.; Camarchia, V.; Passaseo, A.; Verzellesi, Giovanni
abstract

We report on the pulsed and RF power measurements of passivated GaN HEMTs obtained by two different gate evaporation process. Due to the SiN lateral overetch prior to the gate metalization step, devices with a self-aligned gate evaporation showed larger dispersion with respect to those where an angled evaporation was performed. The poorer performances of the self-aligned devices have to be related to the surface regions that are not covered by the gate metal, and that have been exposed to the SiN CF4/O2 dry etch process.

2005 - Fabrication and Characterization of N-Face AlGaN/GaN/AlGaN HEMTs [Relazione in Atti di Convegno]
Chini, Alessandro; Rajan, S.; Wong, M.; Fu, Y.; Speck, J. S.; Mishra, U. K.
abstract

We report on the characteristics of N-face AlGaN/GaN/AlGaN HEMTs. Ohmic contact optmization experiments based on the Ti/Al/Ni/Au metallization scheme commonly used for Ga-face AlGaN/GaN HEMTs were carried out and a low contact resistance of 1.3Ohm/mm was achieved. The devices were then characterized before and after SiN passivation. Before passivation, large current dispersion was observed in 80us pulsed I-V measurements compare to the DC IV curves. The adoption of a SiN passivation layer improved the I-V pulsed characteristics at 80us but current dispersion was still severe when using shorter (200ns) pulse widths. © 2005 IEEE.

2005 - Fabrication of single or multiple gate field plates [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

A process for fabricating single or multiple gate field plates using consecutive steps of dielectric material deposition/growth, dielectric material etch and metal evaporation on the surface of a field effect transistors. This fabrication process permits a tight control on the field plate operation since dielectric material deposition/growth is typically a well controllable process. Moreover, the dielectric material deposited on the device surface does not need to be removed from the device intrinsic regions: this essentially enables the realization of field-plated devices without the need of low-damage dielectric material dry/wet etches. Using multiple gate field plates also reduces gate resistance by multiple connections, thus improving performances of large periphery and/or sub-micron gate devices.

2005 - Fabrication of single or multiple gate field plates [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

"A process for fabricating single or multiple gate field plates using consecutive steps of dielectric material deposition/growth, dielectric material etch and metal evaporation on the surface of a field effect transistors. This fabrication process permits a tight control on the field plate operation since dielectric material deposition/growth is typically a well controllable process. Moreover, the dielectric material deposited on the device surface does not need to be removed from the device intrinsic regions: this essentially enables the realization of field-plated devices without the need of low-damage dielectric material dry/wet etches. Using multiple gate field plates also reduces gate resistance by multiple connections, thus improving performances of large periphery and/or sub-micron gate devices."

2005 - Hot-electron-stress degradation in unpassivated GaN/AlGaN/GaN HEMTs on SiC [Relazione in Atti di Convegno]
Meneghesso, G.; Pierobon, R.; Rampazzo, F.; Tamiazzo, G.; Zanoni, E.; Bernat, J.; Kordos, P.; Basile, Alberto Francesco; Chini, Alessandro; Verzellesi, Giovanni
abstract

Long term on-state and off-state stress on GaN-AlGaN-GaN HEMTson Sic substrates are presented. Hot carrier effects and theirdependence on bias conditions are evaluated withelectroluminescence measurements. Both hot-electron stressconditions produce drain current gate-lag dispersion and gate current decrease. However on- and off- state stresses induce degradation in different gate-to drain surface device regions, i.e. close to the drain contact for the on-state stress and close to the gate contact in the off-state stress. Furthermore a correlation between gate-leakage current and gate-lag dispersion is also observed.

2005 - Light sensitivity of current DLTS and its implications on the physics of DC-to-RF dispersion in AlGaAs-GaAs HFETs [Articolo su rivista]
Verzellesi, Giovanni; Basile, Alberto Francesco; Cavallini, A.; Castaldini, A.; Chini, Alessandro; Canali, Claudio
abstract

The light sensitivity of current deep-level transient spectroscopy (I-DLTS) is analyzed with the aim of gaining insight about the physics of surface-trap related dc-to-RF dispersion effects in AlGaAs-GaAs heterostructure field-effect transistors. I-DLTS experiments under dark reveals three surface-trap levels with activation energies 0.44 eV (h1), 0.59 eV (h2), and 0.85 eV (W), as well as a bulk trap with activation energy 0.45 eV (e1). While the I-DLTS signal peaks associated with the two shallower surface traps h1 and h2 are suppressed by optical illumination with energy larger than the AlGaAs bandgap, that which is associated with the deepest surface trap h3 is nearly unaffected by light up to the highest intensity adopted. Two-dimensional device simulations assuming that surface traps behave as hole traps provide an interpretation for the observed different light sensitivity of surface traps, explaining it as the result of the temperature dependence of surface hole concentration and negative trap-charge density, making trap-charge modulation at increasing temperature less and less sensitive to excess carriers generated by light.

2005 - 單一或多重閘極場平板之製造 [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

本發明係關於半導體裝置，且更特定言之，本發明係關於單一或多重閘極場平板之製造。

2004 - 12 W/mm power density AlGaN/GaN HEMTs on sapphire substrate [Articolo su rivista]
Chini, Alessandro; Buttari, D.; Coffie, R.; Heikman, S.; Keller, S.; Mishra, U. K.
abstract

Record power performance at 4 GHz has been obtained using field-plated AlGaN/GaN HEMTs on sapphire substrate. High power density (12 W/mm) as well as high efficiency (58%) have been measured. A comparison between devices with and without field plate on the same sample showed a significant reduction in knee-voltage walk-out for the field-plated device, thus enabling high power and efficiency operation.

2004 - A C-Band High-Dynamic Range GaN HEMT Low-Noise Amplifier [Articolo su rivista]
Xu, H.; Sanabria, C.; Chini, Alessandro; Keller, S.; Mishra, U. K.; York, R. A.
abstract

A C-band low-noise amplifier (LNA) is designed and fabricated using GAN HEMT power devices. The one-stage amplifier has a measured noise figure of 1.6 dB at 6 GHz, with an associated gain of 10.9 dB and IIP3 of 13 dBm. it also exhibits broadband operation from 4-8 GHz with noise figure less than 1.9 dB. The circuit can endure up to 31 dBm power from the input port. Compared to circuits based on other material and technology, the circuit shows comparable noise figure with improved dynamic range and survivability.

2004 - A new field-plated GaN HEMT structure with improved power and noise performance [Relazione in Atti di Convegno]
Xu, Hongtao; Sanabria, Christopher; Chini, Alessandro; Wei, Yun; Heikman, Sten; Keller, Stacia; Mishra, Umesh K.; York, Robert A.
abstract

Field-plated structures can dramatically improve power capacity of GaN HEMT devices. In this paper, two different field-plated GaN HEMT structures will be demonstrated and compared to each other. The results show that a new GaN HEMT structure improves both power and noise performance without additional processing or costs.

2004 - Effect of gate recessing on linearity characteristics of AlGaN/GaN HEMTs [Relazione in Atti di Convegno]
Chini, Alessandro; Buttari, D.; Coffie, R.; Shen, L.; Palacios, T.; Heikman, S.; Chakraborty, A.; Keller, S.; Mishra, U. K.
abstract

The planar and recessed gate devices were fabricated and compared for their linearity characteristics. AlGaN/GaN heterostructures were formed using a 290 Å thick Al 0.22Ga0.78N barrier layer on sapphire substrate. Gate recessing has been proven to be a viable solution to improve device linearity characteristics. It is observed that further optimization of gate recessing may results in higher efficiency operation while maintaining low distortion level.

2004 - Fabrication of single or multiple gate field plates [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

A process for fabricating single or multiple gate field plates using consecutive steps of dielectric material deposition/growth, dielectric material etch and metal evaporation on the surface of a field effect transistors. This fabrication process permits a tight control on the field plate operation since dielectric material deposition/growth is typically a well controllable process. Moreover, the dielectric material deposited on the device surface does not need to be removed from the device intrinsic regions: this essentially enables the realization of field-plated devices without the need of low-damage dielectric material dry/wet etches. Using multiple gate field plates also reduces gate resistance by multiple connections, thus improving performances of large periphery and/or sub-micron gate devices.

2004 - High Linearity GaN HEMT Power Amplifier with Pre-Linearization Gate Diode [Abstract in Atti di Convegno]
S., Xie; V., Paidi; S., Heikman; Chini, Alessandro; U., Mishra; S., Long; M. J. W., Rodwell
abstract

A high linearity MMIC RF power amplifier is reported in the AlGaN/GaN HEMT technology. In order to obtain high linearity, a pre-linearization gate diode is added at the input to compensate for the nonlinear input capacitance C/sub gs/ of the GaN HEMT device. Another single-ended class B power amplifier without the gate diode is also designed for comparison. The circuit with the pre-linearization gate diode demonstrates at least 4dB improvement on 3rd order intermodulation distortion (IMD/sub 3/) performance over the one without the diode over the useful power range in two-tone measurement.

2004 - High breakdown voltage AlGaN-GaN HEMTs achieved by multiple field plates [Articolo su rivista]
Xing, H.; Dora, Y.; Chini, Alessandro; Heikman, S.; Keller, S.; Mishra, U. K.
abstract

High-voltage Al0.22Ga0.78N-GaN high-electron mobility transistors have been fabricated using multiple field plates over dielectric passivation layers. The device breakdown voltage was found to increase with the addition of the field plates. With two field plates, the device showed a breakdown voltage as high as 900 V. This technique is easy to apply, based on the standard planar transistor fabrication, and especially attractive for the power switching applications.

2004 - High linearity GaN HEMT power amplifier with pre-linearization gate diode [Relazione in Atti di Convegno]
Xie, S.; Paidi, V.; Heikman, S.; Shen, L.; Chini, Alessandro; Mishra, U. K.; Rodwell, M. J. W.; Long, S. I.
abstract

A high linearity MMIC RF power amplifier is reported in the AlGaN/GaN HEMT technology. In order to obtain high linearity, a pre-linearization gate diode is added at the input to compensate for the nonlinear input capacitance Cgs of the GaN HEMT device. Another single-ended Class B power amplifier without the gate diode is also designed for comparison. The circuit with the pre-linearization gate diode demonstrates at least 4dB improvement on 3rd order intermodulation distortion (IMD3) performance over the one without the diode over the useful power range in two-tone measurement.

2004 - High-power polarization-engineered GaN/AlGaN/GaN HEMTs without surface passivation [Articolo su rivista]
Shen, L.; Coffie, R.; Buttari, D.; Heikman, S.; Chakraborty, A.; Chini, Alessandro; Keller, S.; Denbaars, S. P.; Mishra, U. K.
abstract

In this paper, a high-power GaN/AlGaN/GaN high electron mobility transistor (HEMT) has been demonstrated. A thick cap layer has been used to screen surface states and reduce dispersion. A deep gate recess was used to achieve the desired transconductance. A thin SiO2 layer was deposited on the drain side of the gate recess in order to reduce gate leakage current and improve breakdown voltage. No surface passivation layer was used. A breakdown voltage of 90 V was achieved. A record output power density of 12 W/mm with an associated power-added efficiency (PAE) of 40.5% was measured at 10 GHz. These results demonstrate the potential of the technique as a controllable and repeatable solution to decrease dispersion and produce power from GaN-based HEMTs without surface passivation.

2004 - Improved high power thick-GaN-capped AlGaN/GaN HEMTs without surface passivation [Relazione in Atti di Convegno]
Shen, L; Buttari, D.; Heikman, S.; Chini, Alessandro; Coffie, R.; Mccarthy, L.; Chakraborty, A.; Keller, S.; Denbaars, S. P.; Mishra, U. K.
abstract

The improvement in the gate-drain leakage current densities and breakdown of thick GaN-capped AlGaN/GaN HEMTs was discussed. It was found that by lowering the Si doping sheet density, the electric field in the GaN cap in the was decreased, and thus reduced the gate leakage. 8.5W/mm with a power-added efficiency (PAE) of 57% was achieved from unpassivated HEMTs on sapphire. It was also found that by replacing the GaN cap by a thick graded AlGaN layer had reduced gate leakage and increased breakdown while retaining high carrier density.

2004 - Power and Linearity Characteristics of Field-Plated Recessed-Gate AlGaN–GaN HEMTs [Articolo su rivista]
Chini, Alessandro; Buttari, D.; Coffie, R.; Shen, L.; Heikman, S.; Chakraborty, A.; Keller, S.; Mishra, U. K.
abstract

Record power density and high-efficiency operation with AlGaN-GaN high-electron mobility transistor (HEMT) devices have been achieved by adopting a field-plated gate-recessed structure. Devices grown on SiC substrate yielded very high power density (18.8 W/mm with 43% power-added efficiency (PAE) as well as high efficiency (74% with 6 W/mm) under single-tone continuous-wave testing at 4 GHz. Devices also showed excellent linearity characteristics when measured under two-tone continuous-wave signals at 4 GHz. When biased in deep-class AB (33 mA/mm, 3% Imax) device maintained a carrier to third-order intermodulation ratio of 30 dBc up to a power level of 2.4 W/mm with 53% PAE; increasing bias current to 66 mA/mm (6% Imax) allowed high linear operation (45 dBc) up to a power level of 1.4 W/mm with 38% PAE.

2004 - Power and linearity characteristics of GaN MISFETs on sapphire substrate [Articolo su rivista]
Chini, Alessandro; Wittich, J.; Heikman, S.; Keller, S.; Denbaars, S. P.; Mishra, U. K.
abstract

The improvement of device performance arising from the adoption of a MIS gate structure in GaN field-effect transistor (FET) is presented. GaN MISFET/MESFET devices were fabricated on sapphire substrate with and without the insertion of a thin SiN layer on device surface. The MISFET device showed improved device characteristic due to significant reduction in device gate leakage with respect to the standard MESFET structure. Measured power and linearity performance showed promising results. Under single-tone testing at 4 GHz, device yielded saturated output power 6.2 W/mm with 55% peak power added efficiency. When tested with two-tone signal device maintained a carrier to third order intermodulation ratio of 30 dBc up to power levels of 1.8 W/mm with 40% power added efficiency.

2004 - Selective dry etching of GaN over AlGaN in BCL3/SF6 mixtures [Relazione in Atti di Convegno]
Buttari, D; Chini, Alessandro; Chakraborty, A.; Mccarthy, L.; Xing, H.; Palacios, T.; Shen, L.; Keller, S.; Mishra, U. K.
abstract

Inductively coupled plasma (ICP) etching of GaN with high selectivity over Al.22Ga.78N in BCl3/SF6 mixtures has been studied. Selectivity and surface morphology were investigated over a wide range of pressures (3.75-37.5mTorr), RF powers (30-120 W), ICP powers (100-400 W), and SF6/BCl3 ratios (0.1-0.7). Higher pressures, lower dc biases, and higher SF6/BCl3 ratios increased the GaN to AlGaN selectivity. Selectivities up to 25 were measured by laser interferometry. A root mean square (rms) surface roughness of 0.67 nm was measured by atomic force microscopy (AFM) after removal of 0.5 μm from a GaN template (process selectivity: 15, as-grown rms surface roughness: 0.56 nm). A degradation in surface morphology, with the gradual formation of pits, was observed for selectivities above 10.

2004 - Surface-related drain current dispersion effects in AlGaN-GaN HEMTs [Articolo su rivista]
Meneghesso, G.; Verzellesi, Giovanni; Pierobon, R.; Rampazzo, F.; Chini, Alessandro; Mishra, U. K.; Canali, Claudio; Zanoni, E.
abstract

Drain current dispersion effects are investigated in AlGaN-GaN HEMTs by means of pulsed, transient, and small-signal measurements. Gate- and drain-lag effects characterized by time constants in the order of 10-100 us cause dispersion between dc and pulsed output characteristics when the gate or the drain voltage are pulsed. An activation energy of 0.3 eV is extracted from temperature-dependent gate-lag measurements. We show that two-dimensional numerical device simulations accounting only for polarization charges and donor-like traps at the ungated AlGaN surface can quantitatively reproduce all dispersion effects observed experimentally in the different pulsing modes, provided that the measured activation energy is adopted as the energetic distance of surface traps from the valence-band edge. Within this hypothesis, simulations show that surface traps behave as hole traps during transients, interacting with holes attracted at the AlGaN surface by the negative polarization charge.

2004 - Trap-related effects, passivation and hot-carrier aging in GaN-based MESFETs and HEMTs [Relazione in Atti di Convegno]
E., Zanoni; G., Meneghesso; R., Pierobon; F., Rampazzo; Chini, Alessandro; Verzellesi, Giovanni
abstract

2004 - Unpassivated GaN/AlGaN/GaN Power High Electron Mobility Transistors with Dispersion Controlled by Epitaxial Layer Design [Articolo su rivista]
Shen, L.; Coffie, R.; Buttari, D.; Heikman, S.; Chakraborty, A.; Chini, Alessandro; Keller, S.; Denbaars, S. P.; Mishra, U. K.
abstract

In this paper, a novel GaN/AlGaN/GaN high electron mobility transistor (HEMT) is discussed. The device uses a thick GaN-cap layer (∼250 nm) to reduce the effect of surface potential fluctuations on device performance. Devices without Si3N4 passivation showed no dispersion with 200-ns-pulse-width gate-lag measurements. Saturated output-power density of 3.4 W/mm and peak power-added efficiency (PAE) of 32% at 10 GHz (VDS=+15 V) were achieved from unpassivated devices on sapphire substrates. Large gate-leakage current and low breakdown voltage prevented higher drain-bias operation and are currently under investigation.

2004 - Use of multichannel heterostructures to improve the access resistance and f T linearity in GaN-based HEMTs [Relazione in Atti di Convegno]
Palacios, T; Chini, Alessandro; Buttari, D.; Heikman, S.; Keller, S.; Denbaars, S. P.; Mishra, U. K.
abstract

The improvement in the access resistance and the linearity of AlGaN/GaN HEMTs with the use of high conductivity modulation doped AlGaN/GaN multichannel heterostructures was discussed. The sheet conductivity and contact resistance of each individual channel of the heterostructures were determined from the measurement of TLM structures with different recess patterns. The electrical properties of the structures were modeled using an equivalent electrical circuit. The use of heterostructures had resulted in the reduction of access resistance and increase in the linearity of the HEMTs, and thus also allowed improvement of their DC and rf characteristics.

2004 - 단일 또는 다중 게이트 필드 플레이트의 제조 [Brevetto]
Chini, Alessandro; Mishra, Umesh K.; Parikh, Primit; Wu, Yifeng
abstract

본 발명은 전계 효과 트랜지스터의 표면에 절연 물질 증착 또는 성장, 절연 물질 식각 및 금속 증발의 연속된 단계를 사용한 단일 또는 다중 게이트 필드 플레이트의 제조 방법에 관한 것이다. 이 제조 방법은 절연 물질의 증착/성장이 전형적으로 잘 제어가능한 방법이므로 필드 플레이트 작동시 엄격한 제어를 허용한다. 게다가, 소자 표면에 증착된 절연 물질은 소자 고유 영역으로부터 제거될 필요가 없으며, 이는 전형적으로 저손상 절연 물질의 건식/습식 식각법의 필요 없이 필드-플레이팅된 소자의 구현을 가능하게 한다. 다중 게이트 필드 플레이트의 사용은 다중 접속을 통해 게이트 저항을 감소시키므로 대형 주변부 및/또는 초미세한 게이트 소자들의 성능을 향상시킨다.

2003 - 2.1 A/mm current density AlGaN/GaN HEMT [Articolo su rivista]
Chini, Alessandro; Coffie, R.; Meneghesso, G.; Zanoni, E.; Buttari, D.; Heikman, S.; Keller, S.; Mishra, U. K.
abstract

The electrical performance of high current density AlGaN/GaN HEMTs is reported. 2 × 75 μm × 0.7 μm devices grown on sapphire substrates showed current densities up to 2.1 A/mm under 200 ns pulse condition. RF power measurements at 8 GHz and VDS=15 V exhibited a saturated output power of 3.66 W/mm with a 47.8% peak PAE.

2003 - Current collapse in AlGaN/GaN HEMT’s analysed by means of 2d device simulation [Abstract in Atti di Convegno]
Meneghesso, G.; Verzellesi, Giovanni; Pierobon, R.; Rampazzo, F.; Chini, Alessandro; Canali, Claudio; Zanoni, E.
abstract

In this work, we analyze for the first time, by means of 2D numerical device simulations, the influence of surface states on the DC and pulsed characteristics of AlGaN/GaN HEMT’s, and we show that the concomitant presence, at the ungated surface, of polarization induced negative charge and surface hole traps can explain, without invoking any other hypothesis, all dispersive effects in AlGaN/GaN HEMT’s and, in particular, both gate- and drain-lag experiments. In the presence of polarization charge densities of the order of 1013 cm-2, bands are upward bent at the ungated surface and consequently the dynamics of surface states is governed by hole exchange with the valence band.

2003 - High linearity and high efficiency of class-B power amplifiers in GaN HEMT technology [Articolo su rivista]
Paidi, V.; Xie, S.; Coffie, R.; Moran, B.; Heikman, S.; Keller, S.; Chini, Alessandro; Denbaars, S. P.; Mishra, U. K.; Long, S.; Rodwell, M. J. W.
abstract

A 36-dBm high-linearity single-ended common-source class-B monolithic-microwave integrated-circuit power amplifier is reported in GaN high electron-mobility transistor technology. We also describe the design and simulation of highly linear and highly efficient common-source and common-drain class-B power amplifiers. Single-ended class-B amplifiers with bandpass filtering have equivalent efficiency and linearity to push-pull configurations. The common-source class-B circuit demonstrates high linearity, greater than 35 dBc of third-order intermodulation (IM3) suppression and high power-added efficiency (PAE) of 34%. Simulations of common-drain class-B designs predict a PAE of 54% with a superior IM3 suppression of more than 45 dBc over a wider range of bias due to the strong series-series negative feedback offered by the load resistance.

2003 - High performance AlGaN/GaN HEMTs with a field plated gate structure [Relazione in Atti di Convegno]
Chini, Alessandro; Buttari, D.; Coffie, R.; Shen, L.; Heikman, S.; Chakraborty, A.; Keller, S.; Mishra, U. K.
abstract

Record performance at 4GHz has been obtained by using field plated AIGaN/GaN HEMTs. For devices on sapphire substrate, high power density (12W/mm) as well as high efficiency (58%) have been measured. Devices on Sic substrate yielded power density up to I8.8W/mm and efficiency up to 74% (with 6Whm). Excellent linearity performance was also achieved: while maintaining a carrier to third-order intermodulation ratio of 30dBc device yielded 2.4W/mm with 53% PAE.

2003 - High-linearity class B power amplifiers in GaN HEMT technology [Articolo su rivista]
Xie, S.; Paidi, V.; Coffie, R.; Keller, S.; Heikman, S.; Moran, B.; Chini, Alessandro; Denbaars, S. P.; Mishra, U. K.; Long, S.; Rodwell, M. J. W.
abstract

A 36-dBm, high-linearity, single-ended Class B MMIC power amplifier is reported in GaN HEMT technology. The circuit demonstrates high linearity, greater than 35 dBc of third-order intermodulation (IM3) suppression and high power added efficiency (PAE) of 34%. We demonstrate experimentally that Class B power amplifiers can achieve IM3 suppression comparable to Class A, while providing approximately 10% improved power added efficiency.

2003 - Instabilities and degradation in GaN-based devices [Relazione in Atti di Convegno]
Meneghesso, G.; Verzellesi, Giovanni; Pierobon, R.; Rampazzo, F.; Chini, Alessandro; Zanoni, E.
abstract

Drain current instabilities and degradation in GaN-based devices are reviewed by discussing the experimental techniques adopted for their characterization as well as the physical mechanisms leading to the observed phenomena.

2003 - Origin of etch delay time in Cl2 dry etching of AlGaN/GaN structures [Articolo su rivista]
Buttari, D.; Chini, Alessandro; Palacios, T.; Coffie, R.; Shen, L.; Xing, H.; Heikman, S.; McCarthy, L.; Chakraborty, A.; Keller, S.; Mishra, U. K.
abstract

The etch delay time commonly found during dry etching of AlGaN and GaN has been experimentally proven to be due to the presence of hard–to–etch surface oxides. A BCl3 deoxidizing plasma, followed by a Cl2 etching plasma, was found to give dead-time-free aluminum-mole-fraction-independent etch rates. No selectivity between GaN and AlGaN has been observed up to an aluminum mole fraction of 35%. The aluminum-mole-fraction-dependent etch rates commonly reported in literature have been related to the different dead-times associated with dissimilar surface oxides, disproving the more common explanations in terms of the higher binding energy of AlN compared to GaN and/or the lower volatility of AlClx compared to GaClx.

2003 - Pulsed measurements and circuit modeling of weak and strong avalanche effects in GaAs MESFETs and HEMTs [Articolo su rivista]
Meneghesso, G.; Chini, Alessandro; Maretto, M.; Zanoni, E.
abstract

An extensive characterization of the on-state break-down characteristics of GaAs based MESFETs and HEMTs has been carried out by means of dc and pulsed measurements and of circuit simulations. A computer-controlled, three-terminals Transmission Line Pulse (TLP) system with 50-100 ns pulse width and sub-ns risetime has been developed, which allows automated pulsed measurements of device I-V characteristics. The TLP system has been adopted for nondestructive measurements of the on-state breakdown characteristics of GaAs MESFETs and HEMTs up to unprecedented values of gate current density (IG/W = 30 mA/mm has been reached), in strong avalanche conditions. The device behavior in strong avalanche conditions is dominated by a parasitic bipolar effect (PBE) similarly to SOI and bulk Si MOSFETs. By taking into account this and other parasitic effects, an equivalent circuit model, suitable for SPICE simulations has been developed. The proposed model is capable of predicting the exact behavior of the gate and drain currents in both weak and strong avalanche conditions.

2003 - Reliability aspects of GaN microwave devices [Relazione in Atti di Convegno]
E., Zanoni; G., Meneghesso; Verzellesi, Giovanni; R., Pierobon; F., Rampazzo; Chini, Alessandro
abstract

We address the current collapse effects and hot-carrier degradation phenomena observed in GaN based MESFETs and HEMTs.

2003 - Robust C-band MMIC Low Noise Amplifier using AlGaN/GaN HEMT Power Devices [Abstract in Atti di Convegno]
Xu, H.; Sanabria, C.; Chini, Alessandro; Keller, S.; Mishra, U. K.; York, R. A.
abstract

The design and the characterization results of a robust C-band MMIC Low Noise Amplifier using AlGaN/GaN HEMT Power Devices have been presented.

2003 - Study on the origin of dc-to-RF dispersion effects in GaAs- and GaN-based heterostructure FETs [Relazione in Atti di Convegno]
Verzellesi, Giovanni; Mazzanti, Andrea; Canali, Claudio; Meneghesso, G.; Chini, Alessandro; Zanoni, E.
abstract

In the present work a consistent set of experimental and numerical results are presented, addressing dc-to-RF dispersion effects in FETs of two different technologies, namely AlGaAs/GaAs heterostructure FETs (HFETs) and AlGaN/GaN HEMTs. Numerical device simulations suggest that, differently from what commonly assumed, surface traps can behave, during the switching transients of both device types, as hole traps interacting with holes attracted at the ungated surface by surface band bending.

2003 - Temperature dependence of the current-voltage characteristics of AlGaN/GaN HEMT [Relazione in Atti di Convegno]
Shen, L.; Chini, Alessandro; Coffie, R.; Buttari, D.; Heikman, S.; Keller, S.; Mishra, U. K.
abstract

In this paper, we report on the DC and gate-lag pulsed (200ns) I-V characteristics of GaN-based HEMTs with and without SiN passivation between 77 K and 300 K.

2003 - Unpassivated p-GaN/AlGaN/GaN HEMTs with 7.1 W/mm at 10 GHz [Articolo su rivista]
Coffie, R.; Shen, L.; Parish, G.; Chini, Alessandro; Buttari, D.; Heikman, S.; Keller, S.; Mishra, U. K.
abstract

A novel gate process utilising SiO2 to cover the recess sidewall on the drain side of a p-GaN/AlGaN/GaN high electron mobility transistor is presented. Improvements in breakdown voltage and output power are demonstrated with no degradation in small-signal performance.

2002 - Characterization of GaN-based metal--semiconductor field-effect transistors by comparing electroluminescence, photoionization, and cathodoluminescence spectroscopies [Articolo su rivista]
Armani, N.; Grillo, V.; Salviati, G.; Manfredi, M.; Pavesi, M.; Chini, Alessandro; Meneghesso, G.; Zanoni, E.
abstract

We report on a methodological comparison between photocurrent (PC), electroluminescence (EL), and cathodoluminescence (CL) investigations on GaN metal-semiconductor field-effect transistors. The purpose of this work is to show the effectiveness and the complementarity of these experimental techniques and to investigate the presence and nature of electron traps which limit the performances of the devices. PC measurements reveal four distinct energy levels, located at 1.75, 2.32, 2.67, and 3.15 eV, responsible for current collapse. The 1.75 eV level has also been observed in low temperature EL curves. The 2.32 and 2.67 eV levels, on the basis of the comparison with CL and EL results, can be correlated with the so-called "yellow band," located at 2.2 eV. The origin of 1.75 and 3.15 eV levels is at present unknown, however a nonradiative nature has been attributed to the 3.15 eV level, due to the absence of this signature in both CL and EL spectra. The luminescence measurements also reveal the presence of the donor-acceptor pair emission at 3.27 eV and the near-band-edge transition at 3.45 eV. EL measurements show a series of emission peaks in the energy range between 1 and 1.4 eV, while the CL spectra reveal a broadband at 2.8 eV, which arises mainly from the semi-insulating layer. This result has been obtained by increasing the energy of the CL electron beam, allowing us to investigate both the conduction channel and the layers underneath it.

2002 - Dry-Etch gate recessing in AlGaN/GaN HEMTs [Poster]
D., Buttari; Chini, Alessandro; S., Heikman; L., Mccarthy; A., Chakraborty; S., Keller; S. P., Denbaars; U. K., Mishra
abstract

Cl-based dry etching techniques on AlGaN/GaN HEMT structures are presented and discussed.

2002 - Experimental/numerical investigation on current collapse in AlGaN/GaN HEMT’s [Relazione in Atti di Convegno]
Verzellesi, Giovanni; Pierobon, R.; Rampazzo, F.; Meneghesso, G.; Chini, Alessandro; Mishra, U. K.; Canali, Claudio; Zanoni, E.
abstract

RF current collapse is investigated in AlGaN/GaN HEMT's by means of pulsed, transient, and small-signal measurements. Numerical device simulations are presented, showing that the concomitant presence, at the ungated device surface, of polarization-induced charges and hole traps can explain, without invoking any other hypothesis, all dispersion effects observed experimentally.

2002 - High Linearity Class B Power Amplifiers in GaN HEMT Technology [Abstract in Atti di Convegno]
S., Xie; V., Paidi; R., Coffie; S., Keller; S., Heikman; Chini, Alessandro; U. K., Mishra; S., Long; M., Rodwell
abstract

The design, simulation and measurement results on a highly linear single-ended Class B PA based on GaN RF power devices have been presented.

2002 - P-GaN/AlGaN/GaN high electron mobility transistors [Relazione in Atti di Convegno]
Coffie, R.; Heikman, S.; Buttari, D.; Keller, S.; Chini, Alessandro; Shen, L.; Zhang, N.; Jimenez, A.; Jena, D.; Mishra, U. K.
abstract

Despite the considerable improvement in GaN-technology and material quality, RF-dispersion is still one of the main issues hampering device progress. RF-dispersion affects device output power and device power added efficiency (PAE) due to a reduction in saturation current and an increase in knee voltage at high frequencies and high biases. Surface passivation, using silicon nitride, has been found to mitigate RF-dispersion and microwave power degradation (B.M. Green et al, IEEE Electron Dev. Lett., vol. 21, pp. 268-270, 2000; S.C. Binari et al, IEEE Trans Electron. Dev., vol. 48, pp. 465-471, 2001; R. Vetury et al, ibid., vol. 48, pp. 560-566, 2001). This paper discusses a novel AlGaN/GaN high electron mobility transistor (HEMT) device structure which has been developed to reduce RF-dispersion prior to silicon nitride passivation. The device structure uses a p-doped GaN cap layer to screen surface potential changes (regardless of origin) from affecting the gate-drain access region resistance, reducing the amount of RF-dispersion in the device. The epilayers of AlGaN/GaN devices were grown by metal organic chemical vapor deposition (MOCVD) on a c-plane sapphire substrate. Sheet electron concentration and electron Hall mobility of the as-grown wafer were /spl sim/1.35/spl times/10/sup 13/ cm/sup 2/ and 1,475 cm/sup 2//V-s at room temperature.

2002 - Systematic characterization of Cl2 reactive ion etching for gate recessing in AlGaN/GaN HEMTs [Articolo su rivista]
Buttari, D.; Chini, Alessandro; Meneghesso, G.; Zanoni, E.; Chavarkar, P.; Coffie, R.; Zhang, N. Q.; Heikman, S.; Shen, L.; Xing, H.; Zheng, C.; Mishra, U. K.
abstract

High electron mobility transistors (HEMTs) with different gate recess depth were fabricated on an Al0.33Ga0.67N/GaN heterostructure, utilizing low power Cl-2 reactive ion etching. An increase in extrinsic transconductance and a positive threshold shift were observed with an increase of etching time. The etch depth was measured by atomic force microscopy (AFM) and determined to be nonlinear with etching time. The two terminal gate-drain leakage increased from about 0.005 mA/mm to 0.05 mA/mm. The destructive three-terminal breakdown voltage was about 120 V for all devices, etched and un-etched. Power measurements were performed in class A/B at a frequency of 8 GHz. The output power varied between 2.5 and 4.5 W/mm with the increase of bias voltage from 25 to 50 V. Independently of etch depth, there was no evidence of device failure even for the highest bias. The low increase in leakage, and no change in breakdown voltage support that low power RIE etching is a viable solution for low damage gate recess etch.

2002 - Systematic characterization of Cl2 reactive ion etching for improved ohmics in AlGaN/GaN HEMTs [Articolo su rivista]
Buttari, D.; Chini, Alessandro; Meneghesso, G.; Zanoni, E.; Moran, B.; Heikman, S.; Zhang, N. Q.; Shen, L.; Coffie, R.; Denbaars, S. P.; Mishra, U. K.
abstract

Pre-metal-deposition reactive ion etching (RIE) was performed on an Al0.3Ga0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFNI). The contact resistance decreased from about 0.45 Ohmmm for unetched ohmics to a minimum of 0.27 Ohmmm for 70 A etched ohmics. The initial thickness of the AlGaN layer was 250 Angstrom. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs.

2002 - p-Gan cap layer for dispersion control in AlGaN/GaN HEMTs [Poster]
Robert, Coffie; Stacia, Keller; Lee, Mccarthy; Chini, Alessandro; Dario, Buttari; Sten, Heikman; Likun, Shen; Umesh K., Mishra
abstract

The role of using p-doped cap layers with the aim of reducing GaN-HEMTs current collapse is presented and discussed.

2002 - p-capped GaN-AlGaN-GaN high-electron mobility transistors (HEMTs) [Articolo su rivista]
Coffie, R.; Buttari, D.; Heikman, S.; Keller, S.; Chini, Alessandro; Shen, L.; Mishra, U. K.
abstract

A novel p-capped GaN-AlGaN-GaN high-electron mobility transistor has been developed to minimize radio-frequency-to-dc (RF-DC) dispersion before passivation. The novel device uses a p-GaN cap layer to screen the channel from surface potential fluctuations. A low-power reactive ion etching gate recess combined with angle evaporation of the gate metal has been used to prevent gate extension and maintain breakdown voltage. Devices with gate lengths of 0.7 mum have been produced on sapphire. Current-gain cutoff frequencies (f(T)) of 20 GHz and maximum frequencies of oscillation (f(max)) of 38 GHz have been achieved. Unpassivated device demonstrated a saturated output power of 3.0 W/mm and peak power-added efficiency of 40% at 4.2 GHz (V-DS = +20 V).

2001 - Dependence of impact ionization and kink on surface-deep-level dynamics in AlGaAs/GaAs HFETs [Relazione in Atti di Convegno]
Mazzanti, Andrea; Verzellesi, Giovanni; Vicini, L.; Canali, Claudio; Chini, Alessandro; Meneghesso, G.; Zanoni, E.; Lanzieri, C.
abstract

The physical origin of the kink and its dynamics are investigated in AlGaAs/GaAs HFETs both through measurements and two-dimensional device simulations. The kink arises from the interaction of recess surface defects with impact-ionization-generated holes, screening the negative trapped charge and partially discharging surface deep levels. Under pulsed operation, kink dynamics is governed by hole emission and capture phenomena, prevailing at low and high drain-source voltages, respectively. At high drain-source voltages, in particular, the drain-current time constant depends on impact ionization, the latter controlling surface hole density and, through it, hole capture rate.

2001 - Electrostatic Discharge and electrical overstress on GaN/InGaN Light Emitting Diodes [Relazione in Atti di Convegno]
Meneghesso, G.; Chini, Alessandro; Maschietto, A.; Zanoni, E.; Malberti, P.; Ciappa, M.
abstract

Results of ESD testing (HBM and TLP) carried out on commercially available GaN LEDs grown on sapphire or Silicon Carbide will be presented. A non optimal design of layout leads to current crowding phenomena determining premature failure. Devices grown on SiC, adopting vertical current flow, and optimized layout and technology, achieved maximum ESD robustness in excess of 8 kV HBM, 5 A TLP.

2001 - Influence of surface-trap dynamics on impact-ionization and kink phenomena in AlGaAs/GaAs HFETs [Abstract in Atti di Convegno]
Mazzanti, Andrea; Verzellesi, Giovanni; Canali, Claudio; Chini, Alessandro; G., Meneghesso; E., Zanoni; C., Lanzieri
abstract

The kink effect is a detrimental phenomenon for the performance of III-V compound semiconductor FETs, resulting in output-conductance increase, transconductance compression, and dispersion between DC and RF characteristics. Although several papers have been dedicated to it, the physical origin of this effect is still a debated issue [1-4]. The aim of this work is to provide insight about the physical origin and the dynamic behavior of the kink in AlGaAs/GaAs HFETs.

2001 - Long Term Stability of InGaAs/AlInAs/GaAs Methamorphic HEMTs [Articolo su rivista]
Meneghesso, G.; Chini, Alessandro; Zanoni, E.
abstract

The long term stability of M-HEMTs has been evaluated by means of hot electrons stress test carried out at high ambient temperature. Devices demonstrate a good stability under high temperature thermal storage (T = 200 degreesC) or hot electron stress test at room temperature. Moreover, under very severe stress conditions (hot electron stress test at very high temperature), devices show large degradation in the I-V characteristics i.e. positive threshold voltage shift, a degradation of the gate Schottky diode, and increase of the parasitic drain resistance. The degradation are activated by both high temperature and high electric field.

2001 - Measurements of the InGaAs hole impact ionization coefficient in InAlAs/InGaAs pnp HBTs [Articolo su rivista]
Buttari, D.; Chini, Alessandro; Meneghesso, G.; Zanoni, E.; Sawdai, D.; Pavlidis, D.; Hsu, S. S. H.
abstract

The hole multiplication factor in pnp InAlAs/InGaAs single heterojunction bipolar transistors (HBTs) has been measured as a function of the base-collector bias. The hole impact ionization coefficient beta (p) has been estimated taking into account the Early effect, I-CBO, and thermal effects. Numerical corrections for dead space were made. The importance of considering second order effects is highlighted, showing that rough approximations can lead to an overestimation of the coefficient beta (p). At low electric fields, the extracted coefficient agrees with the most recent photomultiplication measurements available in the literature, At high electric fields, hole impact ionization coefficient is estimated up to values previously not reported in the literature (beta (p) approximate to 10(4) cm(-1)).

2001 - Trap characterization in buried-gate n-channel 6H-SiC JFETs [Articolo su rivista]
Meneghesso, G.; Chini, Alessandro; Verzellesi, Giovanni; Cavallini, A.; Canali, Claudio; Zanoni, E.
abstract

We present a detailed characterization of deep traps present in buried gate, n-channel 6H-SiC JFETs, based on transconductance measurements as a function of frequency. Four different deep levels have been identified, which are characterized by activation energies of 0.16, 0.18, 0.28 and 0.54 eV. Furthermore, based on the transconductance frequency dispersion features (upward or downward dispersion), we have been able to infer that three deep levels (0.16, 0.18 and 0.54 eV) are hole traps localized in thep-gate layer and one (0.28 eV) is an electron trap localizedin the n-channel.

2001 - Trap-related effects in 6H-SiC buried-gate JFETs [Abstract in Atti di Convegno]
G., Meneghesso; Chini, Alessandro; E., Zanoni; Verzellesi, Giovanni; Tediosi, Erika; Canali, Claudio; A., Cavallini; A., Castaldini
abstract

In this paper, deep levels are characterized in 6H-SiC, buried gate, n-channel JFETs by means of capacitance-mode (C-) and current-mode (I-) Deep Level Transient Spectroscopy (DLTS) and transconductance frequency dispersion measurements. Moreover, the drain-current transients following a gate-to-source voltage step are analyzed both experimentally and through two-dimensional device simulations allowing the dierent deep levels to be localized both energetically and spatially.

2000 - Diagnosis of trapping phenomena in GaN MESFETs [Relazione in Atti di Convegno]
Meneghesso, G; Chini, Alessandro; Zanoni, E.; Manfredi, M.; Pavesi, M.; Boudart, B.; Gaquiere, C.
abstract

In this work we present results related to the current/voltage characteristics collapse in GaN MESFETs. Two different failure mechanisms were observed, one of them clearly consists in a positive threshold voltage shift. Traps responsible for the "current collapse" were also characterized by means of different measurement techniques. Phototransient experiments show the presence of a capture barrier of 0.1 eV while by a spectroscopic technique four photoionization energy levels were estimated. Finally, electroluminescence measurements shows the presence of visible and UV light, suggesting the presence of a self-recovery mechanisms.

2000 - Hole impact ionization coefficient in (100)-oriented In0.53Ga0.47As based on PNP InAlAs/InGaAs HBT's [Relazione in Atti di Convegno]
Buttari, D; Chini, Alessandro; Meneghesso, G.; Zanoni, E.; Sawdai, D.; Pavlidis, D.; Hsu, S. S. H.
abstract

The hole multiplication factor in pnp In0.52Al0.48As/In0.53G0.47As single heterojunction bipolar transistors (HBT's) has been measured as a function of the base-collector bias. Hole impact ionization coefficient βp has been estimated by taking into account the Early effect, the collector-base leakage current ICBO, thermal effects and the spread in the nominal device processing parameters. Numerical corrections for dead space and current-induced collector charge density variations were made. The data obtained in this way agree with the most recent photomultiplication measurements available in literature.

2000 - Parasitic effects and long term stability of InP-based HEMTs [Articolo su rivista]
Meneghesso, G.; Luise, R.; Buttari, D.; Chini, Alessandro; Yokoyama, H.; Suemitsu, T.; Zanoni, E.
abstract

A study of InP based HEMTs implemented with different process options will be reported. It will be demonstrated that devices with an InP etch stopper layer or with a narrow lateral gate recess region do not present any kink effect, neither any transconductance frequency dispersion, g(m)(f) and a stable behavior with respect to hot electron aging is observed. The opposite occurs in devices without the InP etch stopper layer and a wide lateral gate recess region. The data presented confirm the effectiveness of an InP passivating layer in improving the reliability of advanced InP-HEMTs, and point out at the free InAlAs surface as responsible for the observed instabilities (kink effects, g(m)(f) dispersion).