Performance and reliability of a high power amplifier are correlated with its thermal behavior. Thermal model development and suitable temperature measurement systems are necessary to quantify the channel temperature of devices in real operating conditions. In this paper, temperature measurements applying X-band RF dynamic signal are presented. Infrared microscopy and Raman spectroscopy performed on 8×125μm-wide with 0.25μm-long gates AlGaN/GaN on SiC based HEMTs will be discussed. Measurements will be compared with simulation results to extract the operating thermal resistance.