We explore the physical mechanisms responsible for hole generation in GaN HEMTs in the semi-on state by experimentally investigating dependence of gate-collected hole current on VDS , VGS and temperature. Three key observations contradict conventional impact ionization: the hole current 1) saturates at high electric fields; 2) exhibits a positive temperature dependence; and 3) has a low trigger voltage of VDS=2.7V at 5 K, which is significantly below the GaN bandgap energy. We therefore propose trap-assisted hole generation as the underlying mechanism and implement a corresponding behavioral model in TCAD. TCAD simulations accurately reproduce the observed dependencies of hole generation on VDS , VGS and temperature.
