Observation and Analysis of Anomalous VTH Shift of p-GaN Gate HEMTs Under off-State Drain Stress

In this work, time-dependent threshold voltage (
${V}_{\text {TH}}$
) instability of p-GaN gate high electron mobility transistors (HEMTs) under OFF-state drain stress is systematically investigated. An anomalous
${V}_{\text {TH}}$
shift behavior is observed with an initial negative shift followed by a kink and a positive shift upon prolonged stress time. Through bias- and temperature-dependent stress/recovery experiments, it has been found that the process of electron releasing by donor states at the p-GaN/AlGaN and AlGaN/GaN interface is the primary reason for negative
${V}_{\text {TH}}$
shift in the initial stage. The hole deficiency in the p-GaN layer gradually dominates the change in
${V}_{\text {TH}}$
and finally leads to a positive
${V}_{\text {TH}}$
shift. Moreover, a continual leakage path between the gate and drain terminal during the stress stage is further revealed which is mainly ascribed to the trap-assisted electron tunneling across the AlGaN layer. In addition, the effect of OFF-state drain stress on static ON-resistance (
${R}_{\text{ON}}$
) is also studied, and it is found that the trapped electrons at the passivation/AlGaN interface and/or in the GaN buffer layer results in the
${R}_{\text{ON}}$
degradation.