The investigation of the DC and microwave characteristics versus the ambient temperature is a key issue for active solid-state devices, since the operating temperature can strongly affect not only the transistor performance but also its lifetime to failure. In light of the fact that such a study is of crucial importance especially for high-power devices, the present paper is focused on investigating the temperature dependent behavior of a GaN HEMT. This purpose is accomplished by analyzing measurements performed under different ambient temperatures at DC and microwave frequencies. The latter characterization is carried out under both small- and large-signal conditions. The results of this analysis show that thermal phenomena and traps are responsible for a marked self-heating effect, a slight threshold voltage shift, and an evident kink effect. The main benefit of the developed large-signal measurement system consists of allowing one to obtain not only the magnitude but also the phase spectra of all traveling voltage waves under realistic operating condition for different ambient temperatures. Hence, the thermal dependence of the time-domain waveforms can be investigated. The present analysis clearly shows that the thermal phenomena have a marked impact especially on the magnitude of the wave phasors. (C) 2011 Elsevier Ltd. All rights reserved.