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

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The threshold voltage (VTH) instability of p-GaN/AlGaN/GaN HEMTs was investigated under forward gate stress. A unique bidirectional VTHshift (AVTH) with the critical gate voltage (VG) of 6 V was observed. The carrier transport mechanisms underlying the AVTHwere extensively investigated through the voltage-dependent, timeresolved, and temperature-dependentgate current. The gate current is decomposed into electron and hole current in three distinct regions with respect to VG, which are off-state for VGTH), on-state for 1.2 VGG> 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 electrontrapping effect results in the positive VTHshift for VGTH. However, for VG> 6 V, a drastic hole injection triggered by high VGtakes place that causes subsequent hole-trapping in AlGaN barrier and holeinjection into GaN buffer. The injected holes enhance the positive charge in the gate region and turned the positively shifted VTHinto a negative shift.