We report on contact-free measurement of channel temperature of AlGaN/GaN HEMTs under different operation modes. Micro-Raman spectroscopy was successfully used to measure the temperature of operating HEMT devices with < 1 mu m spatial resolution. Channel temperatures at various load lines were compared. The effective operating temperatures of AlGaN/GaN HEMTs were accessed from the calibration curve of passively heated AlGaN/GaN structures. A linear increase of junction temperature was observed when DC dissipated power was increased. The temperature of the center of the channel in the HEMT was higher than that near the edge by similar to 30 degrees C at 800 mW DC power. The temperature range of class A operation mode is higher than that of class B operation mode. The operating device reached temperatures as high as 115 degrees C at 800 mW DC input power. (c) 2005 Elsevier Ltd. All rights reserved.