Reliability Concern of 650-V Normally-OFF GaN Devices Under Reverse Freewheeling Stress

This brief first reports the behaviors of three different kinds of 650-V commercial normally- OFF GaN devices under repetitive reverse freewheeling stress, and the mechanisms are illustrated by experiments and mix-mode technology computer-aided design simulations. After the stress, unlike the stable behaviors of the device with ohmic-type metal/p-GaN gate stack and the device with cascode structure, there are degradations on the threshold voltage, the ON-state resistance, and the source-to-drain reverse voltage for the device with Schottky-type metal/p-GaN gate stack. During the freewheeling stress, there is a dynamic positive gate to drain voltage on the devices. Especially, the Schottky-type contact leads to a barrier height at metal/p-GaN junction and makes the p-GaN layer to become floating, which can lock the injected carriers. Worse more, there is a high electric field at the gate contact, which leads to the trapping of injected electrons. Once the stress is removed, more positive gate voltage is needed during the turn-on process to suppress the influences of unreleased electrons; as the results, the electrical parameters shift. Finally, the mix-mode simulation results prove the carrier injection, and the measured segmental shifts of gate leakage current prove the trapping process in p-GaN layer successfully.