Method to Distinguish Between Buffer and Surface Trapping in Stressed Normally-ON GaN GITs Using the Gate-Voltage Dependence of Recovery Time Constants

Draincurrent recovery transientsare analyzed in normally-ON (NON) gate injection transistors (GITs) subjected to different kinds of stress. The recovery ismeasured as a function of forward gate bias, V-G, which controls the number of holes injected from the gate and speeds up the recovery. Unlike conventional normally-OFFGITs, the newly designed NON GIT test structures allow simple characterization of buffer trapping during back-gating. By comparing the V-G-dependence of recovery time constants for OFF-state and semi- ON state stress with those obtained from backgating experiments we are able to distinguish between buffer and surface trapping. Our approach is simple and does not require time-consuming temperature-dependent measurements. It is found that OFF-state stress causes negative charge accumulation in the buffer, while semi- ON state stress leads to accumulation in both, buffer and surface. The use of NON GITs enables to measure the recovery time constants related to the buffer over awide span of six orders of magnitude (10(-4)-100 s).