Carrier Transport Mechanisms Underlying the Bidirectional ${V}_{\mathrm{{TH}}}$ 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 (AV
_TH
) with the critical gate voltage (V
_G
) of 6 V was observed. The carrier transport mechanisms underlying the AV
_TH
were 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 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 electrontrapping 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 holeinjection 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.