Abstract 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 °C for 30 s in Ar ambient. The ohmic contact and sheet resistance of n+ contact layer were 1.1 × 10−7 Ω cm2 and 53 Ω/sq, respectively. As a result, we achieved a normally-off operation GaN MOSFETs and moreover high temperature operation of 250 °C for the first time. Threshold voltage (Vth) was +3 V. Drain current (Id) was over 100 mA/mm at gate voltage (Vg) of 10 V and drain-to-source voltage (Vds) of 10 V. Maximum field effect mobility (μFE) was 210 cm2/V s at Vds = 0.1 V.