Thermal behavior of a multifinger AlGaN/GaN/s,apphire HEMT with a metal aithridge connecting five source contacts is investigated in the transient state using optical and electrical methods and using a 2D thermal model. The gate layout consists of eight fingers, each having 50 mu m width/0.3 mu m length, the pitch is 35 mu m and the thickness of the electroplated airbridge is 7 mu m. The device drain contact was pulsed by 10 mu s long 10 V pulses, corresponding to similar to 11.9 W/mm dissipating power density. The electrical characterization method shows that at the end of the pulse the temperature increase in the HEMT channel is similar to 185 K while the transient interferometric mapping (ITM) optical method indicates that the airbridge structure serves also as a cooler removing approximately 10% of the heat energy. Surface temperature maps are constructed by using the TIM for a time window of 2-6 mu s. An asymmetry in the temperature profiles was observed, e.g. the source contact was colder by similar to 25% than the drain contact at t = 6 mu s. (c) 2007 Elsevier Ltd. All rights reserved.