The threshold voltage of HfO2/AlGaN/GaN MOSHFETs was dependent on the time after the bias stress and the transconductance decreased at large gate voltages. These behaviors were explained based on the two-dimensional device simulation assuming three trap levels of E-C-E-T = 0.4, 0.765, 1.65 eV with short, medium and long time constants at the HfO2/AlGaN interface, which were obtained based on the analytical consideration of the experimental results. (C) 2010 Elsevier Ltd. All rights reserved.