Two-dimensional device simulations of a HfO2/AlGaN/GaN metal-oxide-semiconductor-heterostructure FET (MOSHFET) have been carried out based on the drift-diffusion model focusing on the effects of HfO2/AlGaN interface properties. In the case of MOSHFETs with no trap at the HfO2/AlGaN interface, the transconductance was found to decrease at large V-GS due to channel formation at the HfO2/AlGaN interface, resulting in a plateau structure of g(m). When the interface states were incorporated at the HfO2/AlGaN interface, g(m) decreased due to electron capture by the trap at a smaller gate voltage than the onset of g(m) decrease for the case with no trap at the HfO2/AlGaN interface. This is because the trap level reached E-F earlier than the channel formation at the HfO2/AlGaN interface. This resulted in a peak structure of the g(m) when the interface states were deep, which is consistent with experimental results. It was pointed out that if the trap concentration was less than 4 x 10(11) cm(-2), the threshold voltage shift was less than 0.3 V and the g(m) decrease was less than 10%. (C) 2010 Published by Elsevier Ltd.