The high-temperature operation of a GaN MOSFET is reported. The MOSFETs were operated up to 250 degrees C, best reported to date. The MOSFETs showed good dc characteristics with field-effect mobilities of 138 cm(2)/Vs and 133 cm(2)/Vs at room temperature and 250 degrees C, respectively. The field-effect mobility, threshold voltage, and sub-threshold slope did not changed significantly up to 250 degrees C. Also, we compared the activation annealing condition for n(+) layer fabrication. A high-temperature annealing condition of 1300 degrees C led to a low contact resistance, but caused slight degradation of the field-effect mobility compared with the 1100 degrees C annealing condition. (c) 2007 Elsevier Ltd. All rights reserved.