(Invited) Vertical Gallium Nitride PN Diodes for Grid Resiliency and Medium-Voltage Power Electronics

Abstract

Due to their fast avalanche breakdown response, gallium nitride (GaN) diodes with vertical device geometry hold promise for protection of the electric grid against fast (<1 us) voltage transients induced by electromagnetic pulses (EMPs). Additionally, comparable critical electric field and superior mobility relative to silicon carbide (SiC) suggest that vertical GaN devices compare favorably to SiC for medium-voltage (defined here as 1.2 to 20 kV) power electronics, with the advantage becoming more pronounced as the voltage rating exceeds 10 kV. However, vertical GaN power devices are significantly less mature than their SiC counterparts, and further work on epitaxial growth, characterization of fundamental material properties, device processing, manufacturability, and reliability / failure analysis is needed. This talk will describe our team’s efforts to advance the state-of-the-art for vertical GaN PN diodes. One key challenge is the growth of thick (tens of um), controllably low-doped (<1016 cm-3) drift layers on native GaN substrates by metal-organic chemical vapor deposition (MOCVD), and our team has achieved >50 um thick layers with net doping in the low-1015 cm-3 range suitable for medium-voltage devices. Additionally, significant challenges exist in the processing of such devices, notably in the edge termination used to prevent premature breakdown. Using the aforementioned drift layers coupled with a multi-step-etched junction termination extension (JTE), our team has demonstrated PN diodes with breakdown voltage >4 kV and specific on-resistance <4 mOhm cm2. Future work aims to extend the growth and fabrication such that devices up to 20 kV breakdown are achieved. Additionally, a parallel foundry effort aiming to establish the manufacturability of vertical GaN technology, focused thus far on 1.2-kV-class devices, has performed extensive metrology on incoming epitaxial GaN wafers prior to fabrication of PN diodes with areas ranging from ~0.1 up to 1 mm2. Using various combinations of implanted guard rings and JTEs as the edge termination, devices with breakdown voltages >1.2 kV have been achieved, and the yield has been evaluated based on a combination of both breakdown voltage and the forward current characteristic. The foundry effort will be extended to 3.3 and later 6.5 kV devices, possibly using alternative edge termination approaches such as bevels that are currently under development. Further, in order to understand the fundamental degradation mechanisms in vertical GaN devices and ultimately to mitigate them, extensive reliability testing of 1.2-kV-class vertical GaN PN diodes has been underway, including tests such as high-temperature reverse bias and high-temperature operating life. Such testing has revealed several distinct degradation behaviors, and failure analysis has been performed on select devices to determine the root cause(s) of degradation and/or failure. Finally, a transmission-line-based setup has been used to quantify the time response of the avalanche breakdown of 1.2-kV-class GaN PN diodes to determine whether it is fast enough for the diodes to be used for protection of the grid against EMPs, and the measured breakdown time of ~1 ns is indeed sufficient for such an application. This work was supported by the ARPA-E OPEN+ Kilovolt Devices Cohort directed by Isik Kizilyalli. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.