The DC and RF-characteristics of novel AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) were studied at elevated temperatures up to 300 degreesC, after a 36 h continuous operation at 200 degreesC and after a 1 min thermal stress at temperatures up to 850 degreesC. At 300 degreesC, the gate-leakage current remains about four orders of magnitude lower than that for regular HFETs. At zero gate-bias, the saturation current decreased by only about 20% after 36 h of continuous operation at 200 degreesC. After a 700 degreesC, 1 min thermal stress, the gate leakage remained as low as 5 nA/mm, whereas the peak current and DC transconductance showed a 20% reduction. In spite of the decrease in the peak-current, the RF saturation power remained nearly constant for operation at temperatures up to 200 degreesC. We attribute this to a reduction in the current collapse. (C) 2002 Published by Elsevier Science Ltd.