Carrier Transport Mechanisms Underlying the Bidirectional VTH Shift in p-GaN Gate HEMTs Under Forward Gate Stress

The threshold voltage (V-TH) instability of p-GaN/AlGaN/GaN HEMTs was investigated under forward gate stress. A unique bidirectional V-TH shift (Delta V-TH) with the critical gate voltage (V-G) of 6 V was observed. The carrier transport mechanisms underlying the Delta V-TH were extensively investigated through the voltage-dependent, time-resolved, and temperature-dependent gate current. The gate current is decomposed into electron and hole current in three distinct regions with respect to V-G, which are off-state for V-G < 1.2 V (V-TH), on-state for 1.2 V < V-G < 5 V and gate-injected region for V-G > 5 V. In off-state, the electrons were thermally activated and transport towards the gate, while electron-trapping governed by the space charge limited conduction (SCLC) in AlGaN barrier was observed in on-state and gate-injected region. Such an electron-trapping effect results in the positive V-TH shift for V-G < 6 V. Meanwhile, the marginal hole transport from gate by thermal activation was also captured by gate current, which features negligible impact on V-TH. However, for V-G > 6 V, a drastic hole injection triggered by high V-G takes place that causes subsequent hole-trapping in AlGaN barrier and hole-injection into GaN buffer. The injected holes enhance the positive charge in the gate region and turned the positively shifted V-TH into a negative shift.