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1. Vertical GaN Fin JFET: A Power Device with Short Circuit Robustness at Avalanche Breakdown Voltage

   https://doi.org/10.1109/IRPS48227.2022.9764569  

   Zhang, R. ; Liu, J. ; Li, Q. ; Pidaparthi, S. ; Edwards, A. ; Drowley, C. ; Zhang, Y. ( March 2022 , Vertical GaN Fin JFET: A Power Device with Short Circuit Robustness at Avalanche Breakdown Voltage)

   GaN high-electron-mobility transistors (HEMTs) are known to have no avalanche capability and insufficient short-circuit robustness. Recently, breakthrough avalanche and short-circuit capabilities have been experimentally demonstrated in a vertical GaN fin-channel junction-gate field-effect transistor (Fin-JFET), which shows a good promise for using GaN devices in automotive powertrains and electric grids. In particular, GaN Fin-JFETs demonstrated good short-circuit capability at avalanche breakdown voltage (BV AVA ), with a failure-to-open-circuit (FTO) signature. This work presents a comprehensive device physics-based study of the GaN Fin-JFET under short-circuit conditions, particularly at a bus voltage close to BV AVA . Mixed-mode electrothermal TCAD simulations were performed to understand the carrier dynamics, electric field distributions, and temperature profiles in the Fin-JFET under short-circuit and avalanche conditions. The results provide important physical references to understand the unique robustness of the vertical GaN Fin-JFET under the concurrence of short-circuit and avalanche as well as its desirable FTO signature. 

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2. Exceptional Repetitive-Short-Circuit Robustness of Vertical GaN Fin-JFET at High Voltage

   https://doi.org/10.1109/ISPSD49238.2022.9813618  

   Zhang, R. ; Liu, J. ; Li, Q. ; Pidaparthi, S. ; Edwards, A. ; Drowley, C. ; Zhang, Y. ( May 2022 , Exceptional Repetitive-Short-Circuit Robustness of Vertical GaN Fin-JFET at High Voltage)

   The limited short circuit (SC) capability of GaN high-electron-mobility transistors (HEMTs) has become a critical concern for their adoption in many power applications. Recently, breakthrough SC robustness was demonstrated in a 650-V rated vertical GaN Fin-JFET with a short circuit withstanding time of over 30 µs at 400 V bus voltage (V BUS ), showing great potential for automotive powertrain and grid applications. This work presents the first study on the repetitive SC robustness of this GaN Fin-JFET at a V BUS of 400 V and 600 V. The GaN Fin-JFET survived 30,000 cycles of 400 V, 10 µs SC stresses without any degradation in device characteristics. At a 600 V V BUS , it survived over 8,000 cycles of 10 µs SC stresses before an open-circuit failure. This open-circuit failure signature allows the GaN Fin-JFET to retain its avalanche breakdown voltage and is highly desirable for system safety. Besides, an increase in gate leakage was observed during the 600 V repetitive test, which can be used as a precursor to predict device failure. As far as we know, this is the first report of an exceptional repetitive SC robustness in a power transistor at a V BUS close to its rated voltage. 

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3. 10 kV GaN Power Diodes and Transistors with Performance beyond SiC Limit

   https://doi.org/10.1109/EDTM53872.2022.9797920  

   Zhang, Yuhao ; Xiao, Ming ; Ma, Yunwei ; Cheng, Kai ( March 2022 , 10 kV GaN Power Diodes and Transistors with Performance beyond SiC Limit)

   Medium-voltage (MV) power electronic devices are widely used in renewable energy processing, electric grids, pulse power systems, etc. Current MV devices are mainly made of Si and SiC. This paper presents our recent efforts in developing a new generation of MV devices based on the multi-channel AlGaN/GaN platform and many new device designs involving charge balance, fin, and Cascode. The specific on-resistance of our 10 kV-class GaN Schottky barrier diodes and normally-OFF transistors is ~40 mΩ•cm 2 , rendering a Baliga’s figure of merit exceeding the 1-D unipolar SiC limits. We show the great promise of GaN in medium and high-voltage power applications. 

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4. GaN-on-GaN p-i-n diodes with avalanche capability enabled by eliminating surface leakage with hydrogen plasma treatment

   https://doi.org/10.1063/5.0107677  

   Fu, Kai ; He, Ziyi ; Yang, Chen ; Zhou, Jingan ; Fu, Houqiang ; Zhao, Yuji ( August 2022 , Applied Physics Letters)

   Traditional mesa terminations require precise angle design to reduce the electric field at the edge and surface treatment to reduce etch damage. Otherwise, the device usually suffers a premature breakdown. This work proposes the use of easy-to-implement hydrogen plasma treatment to solve the premature breakdown caused by mesa and demonstrates the avalanche capability in GaN-on-GaN p-i-n diodes. The breakdown electric field when the avalanche occurred was ∼2.3 MV/cm at room temperature for a GaN drift layer with a doping concentration of ∼7 × 1015 cm−3, which is consistent with the theoretical value. The temperature coefficient of the avalanche breakdown voltage of the devices was 4.64–4.85 × 10−4 K−1. This work shows a simple and effective approach to achieve avalanche capability in vertical GaN power devices, which can serve as an important reference for the future development of efficient and robust GaN power electronics.

    

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5. Impact of Residual Carbon on Avalanche Voltage and Stability of Polarization-Induced Vertical GaN p-n Junction

   Elena Fabris, Carlo De ( January 2020 , IEEE transactions on electron devices)

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   We demonstrate that the residual carbon concentration in the drift region can have a significant impact on the reverse leakage, breakdown voltage, and breakdown stability of GaN-on-GaN vertical diodes. Two generations (Gen1, Gen2) of polarization-doped p-n junctions with different C concentrations were compared, in terms of avalanche voltage, avalanche instability, and deep-level concentration. The original results collected within this paper show that: 1) both generations of devices can safely reach the avalanche regime; diodes with a lower residual CN have a higher reverse leakage and a lower avalanche voltage, due to an uneven distribution of the electric field; 2) the presence of residual carbon can lead to breakdown walkout, i.e. a recoverable increase in breakdown voltage under reverse-bias stress. Specifically, devices with higher C concentration show a fully-recoverable breakdown walk-out, whereas the breakdown voltage is stable in devices with lowerC concentration;and 3) steady-state photocapacitance measurements confirm the presence of CN in both generations, and are used to assess the relative difference in concentration between Gen1 and Gen2, even for levels below secondary ion mass spectroscopy (SIMS) sensitivity. The results described in this paper indicate the existence of a trade-off between breakdown voltage (increasing by improving compensation) and breakdown stability (improving by reducing CN concentration) and are of fundamental importance for the optimization of GaN power devices. 

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