Volterra series analysis is used to determine gain and output power of GaN HEMT amplifiers. Gain compression defined as the difference between linear and nonlinear gain is reported for varying temperatures. Measured 1-dB gain compression of 17.5 dBm for a 1 mum x 500 mum Al0.15Ga0.85N/GaN HEMT at 300 K and at 2 GHz is in excellent agreement with the calculated value of 17 dBm. With the operating frequency increasing from 1 to 6 GHz the 1-dB gain compression point decreases from 20.5 to 13.8 dBm at 300 K. At 2 GHz the 1-dB gain compression point decreases from 17.5 dBm at 300 K to 6.5 dBm at 600 K. (C) 2002 Elsevier Science Ltd. All rights reserved.