OFF-State Drain-Voltage-Stress-Induced VTH Instability in Schottky-Type p-GaN Gate HEMTs

In this article, we systematically investigate the OFF-state drain-voltage-stress-induced threshold voltage (
$V_{\mathrm {TH}}$
) instability in Schottky-type p-GaN gate high electron mobility transistors (HEMTs). OFF-state drain-voltage stress and recovery tests were conducted under various temperatures and different drain biases. A sharp increase in
$V_{\mathrm {TH}}$
was observed at the beginning of the stress, and
$V_{\mathrm {TH}}$
kept shifting positively during the stress until it reached saturation. Further experiments showed that two different mechanisms dominated the
$V_{\mathrm {TH}}$
shift, which were distinguished by the temperature dependence, degradation/recovery process and affected locations in the gate region. The hole deficiency caused by hole emission from the p-GaN layer is suggested to be the dominant reason for the
$V_{\mathrm {TH}}$
instability at the beginning of the stress, while with increasing stress time, electron trapping in the barrier and buffer layers gradually dominates the
$V_{\mathrm {TH}}$
shift. Based on the identified mechanisms, physics-based analytical calculations and empirical fitting are conducted to describe the
$V_{\mathrm {TH}}$
behavior during the OFF-state drain-voltage stress. The fundamental mechanisms can provide a guide to develop corresponding methods to address the drain-induced
$V_{\mathrm {TH}}$
instability issue.