TCAD Analysis of GaN HEMT Output Conductance Through Trap Rate Equation Green’s Functions

An efficient, in-house developed, TCAD simulator is used to investigate the effects of buffer traps in 150 nm gate length GaN HEMTs. The developed TCAD allows to compute not only the sensitivity of DC and AC Y parameters towards variations of the trap physical parameters, but also the local sensitivity, showing the device areas where traps influence most the HEMT behavior. The technique is applied to analyze the dependency of the output impedance (Y DD ) of a Fe-doped HEMT versus the buffer trap energy and concentration. We demonstrate that the two trap parameters impact differently on the output resistance in terms of frequency dispersion and of absolute values. The local source is also different, showing that buffer trap energy variations are also important when traps are located below the saturated channel, while trap concentration perturbations are important only for traps located under the ohmic portion of the channel

Electric Field and Self-Heating Effects on the Emission Time of Iron Traps in GaN HEMTs

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

TCAD Modeling of GaN HEMT Output Admittance Dispersion through Trap Rate Equation Green’s Functions

We present a novel and numerically efficient approach to analyse the sensitivity of AC parameters to variations of traps in GaN HEMTs. The approach exploits an in-house TCAD simulator implementing the drift-diffusion model self-consistently coupled with trap rate equations, solved in dynamic conditions with the Harmonic Balance algorithm. The capability of the model is demonstrated studying the low-frequency dispersion of a 150 nm gate-length AlGaN/GaN HEMT output admittance YDD as a function of the trap energy of Fe-induced buffer traps. The real part of YDD exhibits strong frequency dispersion and an important degradation of the output resistance at high frequency. The imaginary part is characterized by a peak at a frequency decreasing with trap energy deeper in the gap, in agreement with experimental data on similar structures. Distributed local sources show that YDD is most sensitive to trap energy variations localized in the buffer region under the gate, peaking under the unsaturated portion of channel towards the source. Trap variations affect the output admittance when localized in depth into the buffer up to a 100 nm distance from the channel

OFF-State Reliability of pGaN Power HEMTs

The concern for climate changes and the increase in the electricity demand turned the attention towards the production, sorting and use of electric energy through zero emission (CO2) and highly efficient solutions (e.g. for electric vehicle), respectively. As a consequence, the need for high performance, reliable and low cost power transistors adopted for power applications is increasing as well. Gallium nitride seems to be the most promising candidate for the next generation of devices for power electronics, thanks to its excellent properties and comparable cost with respect to Si counterpart. The main and most adopted GaN-based device is the high electron mobility transistor (HEMT). In particular, in the case of switching power applications, HEMTs repeatedly are switched between high current on-state and high voltage off-state operation. For both operation modes a good reliability must be guaranteed. This thesis is focused on the reliability issues related to the off-state operation. The results have been obtained during a six months research period at imec (Belgium) on 200V p-GaN gate AlGaN/GaN HEMTS. Different devices have been investigated, differing for gate-to-drain distance, field plates lengths, AlGaN and GaN layers properties. Time-dependent dielectric breakdown and hard breakdown tests have been performed in combination with TCAD simulations. It has been demonstrated that the gate-to-drain distance (LGD) impacts the breakdown voltage and the kind of failure mechanism. If LGD ≤3um the breakdown occurs through the GaN channel layer due to short channel effects. In this case, by reducing the thickness of the GaN channel layer such behaviour can be attenuated, eventually leading to longer time-to-failure. If LGD≥ 4um the breakdown occurs between the 2DEG and the source field plates, where the properties of the AlGaN barrier layer (i.e. thickness and Al concentration) and the field plates configuration play the main role on the time-to-failure

Buffer Trap Related Knee Walkout and the Effects of Self-Heating in AlGaN/GaN HEMTs

Mixed-mode simulations of a class A amplifier is used to study the DC/RF dispersion commonly observed in AlGaN/GaN based HEMTs. We show that the observed knee walkout at frequencies greater than the emission rates of buffer traps (time constants tae > 1 week) is related to the steady state trap density and spatial location due to the DC operational bias. An increase in the drain bias point and an initial distortion of the RF signal, that is expected to disappear as the device global temperature reduces, is observed when a self-heating model is included. Finally, we propose that a reduction in the DC/RF dispersion is possible with a suitable location and concentration of an acceptor doping in the buffer

A Fast Extraction Method of Energy Distribution of Border Traps in AlGaN/GaN MIS-HEMT

MIS-HEMT is one of the most promising structures to prohibit the unfavorable gate leakage in conventional AlGaN/GaN HEMTs. However, the extra insulator layer introduces massive border traps at insulator/AlGaN interface and results in the poor reliability. In this brief the energy distribution of border traps in AlGaN/GaN MIS-HEMT gate stack is extracted and investigated through a discharging-based trap energy profile technique. The technique adopts spot-Id sense measurement with 1 millisecond measurement time to capture the “whole (both fast and slow)” border traps. The results are beneficial to improve the reliability of AlGaN/GaN MIS-HEMT

Impact of Gamma-Irradiation on the Characteristics of III-N/GaN Based High Electron Mobility Transistors

In this study, the fundamental properties of AlGaN/GaN based High Electron Mobility Transistors (HEMTs) have been investigated in order to optimize their performance in radiation harsh environment. AlGaN/GaN HEMTs were irradiated with 60Co gamma-rays to doses up to 1000 Gy, and the effects of irradiation on the devices\u27 transport and optical properties were analyzed. Understanding the radiation affects in HEMTs devices, on carrier transport, recombination rates and traps creation play a significant role in development and design of radiation resistant semiconductor components for different applications. Electrical testing combined with temperature dependent Electron Beam Induced Current (EBIC) that we used in our investigations, provided critical information on defects induced in the material because of gamma-irradiation. It was shown that low dose (below ~250 Gy) and high doses (above ~250 Gy) of gamma-irradiation affects the AlGaN/GaN HEMTs due to different mechanisms. For low doses of gamma-irradiation, the improvement in minority carrier diffusion length is likely associated with the irradiation-induced growing lifetime of the non-equilibrium carriers. However, with the increased dose of irradiation (above ~ 250 Gy), the concentration of point defects, such as nitrogen vacancies, as well as the complexes involving native defects increases which results in the non-equilibrium carrier scattering. The impact of defect scattering is more pronounced at higher radiation, which leads to the degradation in the mobility and therefore the diffusion length. In addition for each device under investigation, the temperature dependent minority carrier diffusion length measurements were carried out. These measurements allowed the extraction of the activation energy for the temperature-induced enhancement of the minority carrier transport, which (activation energy) bears a signature of defect levels involved the carrier recombination process. Comparing the activation energy before and after gamma-irradiation identified the radiation-induced defect levels and their dependences. To complement EBIC measurements, spatially resolved Cathodoluminescence (CL) measurements were carried out at variable temperatures. Similar to the EBIC measurements, CL probing before and after the gamma-irradiation allowed the identification of possible defect levels generated as a result of gamma-bombardment. The observed decrease in the CL peak intensity after gamma-irradiation provides the direct evidence of the decrease in the number of recombination events. Based on the findings, the decay in the near-band-edge intensity after low-dose of gamma-irradiation (below ~250 Gy) was explained as a consequence of increased non-equilibrium carrier lifetime. For high doses (above ~250 Gy), decay in the CL intensity was observed to be related to the reduction in the mobility of charge carriers. The results of EBIC are correlated with the CL measurements in order to demonstrate that same underlying process is responsible for the changes induced by the gamma-irradiation. DC current-voltage measurements were also conducted on the transistors to assess the impact of gamma-irradiation on transfer, gate and drain characteristics. Exposure of AlGaN/GaN HEMTs to high dose of 60Co gamma-irradiation (above ~ 250 Gy) resulted in significant device degradation. Gamma-rays doses up to 1000 Gy are shown to result in positive shift in threshold voltage, a reduction in the drain current and transconductance due to increased trapping of carriers and dispersion of charge. In addition, a significant increase in the gate leakage current was observed in both forward and reverse directions after irradiation. Post-irradiation annealing at relatively low temperature was shown to restore the minority carrier transport as well as the electrical characteristics of the devices. The level of recovery of gamma-irradiated devices after annealing treatment depends on the dose of the irradiation. The devices that show most recovery for a particular annealing temperature are those exposed to the low doses of gamma-irradiation, while those exposed to the highest doses results in no recovery of performance. The latter fact indicates that a higher device annealing temperature is needed for larger doses of gamma-irradiation