An exact time-domain technique to analyze RF performance considering thermal effects of the GaN/AlGaN HEMT amplifier is reported. The measured maximum output power of 28 dBm with a power gain of 8.2 dB, power added efficiency (PAE) of 19% a 1 mum x 500 mum Al0.15Ga0.85N/GaN HEMT at 2 GHz are in excellent agreement with theoretical calculations. The calculated third-order intermodulation (IM3) and carrier-to-IM3 ratio at the peak operating power/PAE at 2 GHz are - 16.5 dB and 13.5 dBc, respectively. With output power decreasing to 16 dBm at 2 GHz the above quantities are -37 dB and 22.5 dBc, respectively. For a 0.45 mum x 250 mum Al0.14Ga0.86N/GaN HEMT, IM3 and carrier-to-IM3 ratio at 16 dBm output power and 2 GHz are -50 dB and 36 dBc, respectively. With increasing output power level and frequency these quantities degrade at a higher rate compared to the longer-gate length device. (C) 2002 Elsevier Science Ltd. All rights reserved.