Effect of source field plate on the characteristics of off-state, step-stressed AlGaN/GaN high electron mobility transistors

The effects of source field plates on AlGaN/GaN high electron mobility transistor reliability under off-state stress conditions were investigated using step-stress cycling. The source field plate enhanced the drain breakdown voltage from 55 to 155 V and the critical voltage for off-state gate stress from 40 to 65 V, relative to devices without the field plate. Transmission electron microscopy was used to examine the degradation of the gate contacts. The presence of pits that appeared on both source and drain sides of the gate edges was attributed to the inverse piezoelectric effect. In addition, a thin oxide layer was observed between the Ni gate contact and the AlGaN layer, and both Ni and oxygen had diffused into the AlGaN layer. After step-stress cycling, additional threading dislocations were observed.