We have fabricated III-nitride metal-oxide field effect transistors (MOSFETs) using high-k HfO2 as a gate oxide. Two types of MOSIFETs were studied; GaN MOSIFETs and AlGaN/GaN MOSFETs. In the case of GaN MOSIFETs, the maximum transconductance of 45 mS/mm has been obtained. This is seven times larger than the best-reported value, to our knowledge, for the normally-off GaN MOSIFETs with SiO2 gate oxide. In order to improve the performance of the device, AlGaN/GaN MOSFETs in which high-quality AlGaN/GaN heterointerface is used as a channel have been fabricated. The maximum transconductance and drain current were as high as 160 mS/mm and 840 mA/mm, respectively. (C) 2009 Elsevier Ltd. All rights reserved.