We tried normally-off operation GaN metal-oxide-semiconductor field effect transistors (MOSFETs). n+ Contact layers as a source and drain region were fabricated using a Si ion implantation technique with the activation annealing at 1300 degrees C for 30 s in Ar ambient. The ohmic contact and sheet resistance of n+ contact layer were 1. 1 X 10(-7) Omega cm(2) and 53 Q/sq, respectively. As a result, we achieved a normally-off operation GaN MOSFETs and moreover high temperature operation of 250 degrees C for the first time. Threshold voltage (V-th) was +3 V. Drain current (I-d) was over 100 mA/mm at gate voltage (V-g) of 10 V and drain-to-source voltage (V-ds) of 10 V. Maximum field effect mobility (YFE) was 210 cm(2)/V sat V-ds= 0.1 V. (c) 2007 Elsevier Ltd. All rights reserved.