Dependence of Impact Ionization and Kink on Surface-Deep-Level Dynamics in AlGaAs/GaAs HFETs

The physical origin of the kink and its dynamics are investigated in AlGaAs/GaAs HFETs both through measurements and two-dimensional device simulations. The kink arises from the interaction of recess surface defects with impact-ionization-generated holes, screening the negative trapped charge and partially discharging surface deep levels. Under pulsed operation, kink dynamics is governed by hole emission and capture phenomena, prevailing at low and high drain-source voltages, respectively. At high drain-source voltages, in particular, the drain-current time constant depends on impact ionization, the latter controlling surface hole density and, through it, hole capture rate.