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- Vertical GaN Fin JFET: A Power Device with Short Circuit Robustness at Avalanche Breakdown Voltage
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- 1 to 8
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- National Science Foundation
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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 tomore »
Power devices are highly desirable to possess excellent avalanche and short-circuit (or surge-current) robustness for numerous power electronics applications like automotive powertrains, electric grids, motor drives, among many others. Current commercial GaN power device, the lateral GaN high-electron-mobility transistor (HEMT), is known to have no avalanche capability and very limited short-circuit robustness. These limitations have become a roadblock for penetration of GaN devices in many industrial power applications. Recently, through collaborations with NexGen Power Systems (NexGen), Inc., we have demonstrated breakthrough avalanche, surge-current and short-circuit robustness in NexGen’s vertical GaN p-n diodes and fin-shape junction-gate field-effect-transistors (Fin-JFETs). These large-area GaN diodes and Fin-JFETs were manufactured in NexGen’s 100 mm GaN-on-GaN fab. The demonstrated avalanche, surge-current and short-circuit capabilities are comparable or even superior to Si and SiC power devices. Additionally, vertical GaN Fin-JFETs were found to fail to open-circuit under avalanche and short-circuit conditions, which is highly desirable for the system safety. This talk reviews the key robustness results of vertical GaN power devices and unveils the enabling device physics. Fundamentally, these results signify that, in contrast to some popular belief, GaN devices with appropriate designs can achieve excellent robustness and thereby encounter no barriers for applications in electric vehicles,more »
GaN-on-GaN p-i-n diodes with avalanche capability enabled by eliminating surface leakage with hydrogen plasma treatment
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.
Impact of Residual Carbon on Avalanche Voltage and Stability of Polarization-Induced Vertical GaN p-n JunctionWe 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 (improvingmore »
This work demonstrates a novel junction termination extension (JTE) with a graded charge profile for vertical GaN p-n diodes. The fabrication of this JTE obviates GaN etch and requires only a single-step implantation. A bi-layer photoresist is used to produce an ultra-small bevel angle (~0.1°) at the sidewall of a dielectric layer. This tapered dielectric layer is then used as the implantation mask to produce a graded charge profile in p-GaN. The fabricated GaN p-n diodes show a breakdown voltage ( BV ) of 1.7 kV (83% of the parallel-plane limit) with positive temperature coefficient, as well as a high avalanche current density over 1100 A/cm 2 at BV in the unclamped inductive switching test. This robust avalanche is ascribed to the migration of the major impact ionization location from the JTE edge to the main junction. This single-implant, efficient, avalanche-capable JTE can potentially become a building block of many vertical GaN devices, and its fabrication technique has wide device and material applicability.