Gate Reliability of p-GaN HEMT with Gate Metal Retraction

In this paper, we present an analysis of the gate degradation induced by long-term forward gate stress in GaN-based power HEMTs with p-type gate, controlled by a Schottky metal-retracted/p-GaN junction. In particular, time-dependent gate breakdown and threshold voltage instability are investigated as function of different geometries, gate biases, and temperatures. The introduction of a gate metal retraction (GMR) process step improves the device lifetime because it suppresses the onset of the leakage current flow occurring at the gate edges for relatively high gate voltage. However, biasing GMR p-GaN HEMT at VG > 8 V and T > 80 °C, a new degradation mechanism shows up, possibly altering the lifetime even at low VG operation. Main results in this paper demonstrate that, although at high-VG and high-T a localized degradation effect ascribed to the device isolation region is responsible for time-dependent gate breakdown, thanks to GMR higher operating voltages compatible with 10-years continuous operation is attained. Finally, the longer device lifetime at moderate VG values brought by GMR, allows to evaluate the threshold voltage instability for long stress times (≈ 112 hours) at relatively high-VG and high-T, leading to the observation of a saturation of the long-term positive threshold voltage shift, and providing additional information about the underlying physical degradation mechanisms. Overall, the saturated 0.65 V ΔVTH under worst-case condition (VG = 7 V at 150 °C, i.e. corresponding to 10 years lifetime), reveals a reliable and fairly stable technology with respect to forward gate stress.