In this article, we report on the temperature-dependent GaN HEMT behavior. To evaluate the potentialities of this kind of active solid-state electronic device at its best, the experiments are performed at extreme operating conditions. As a case study, we consider a large 0.25-mu m GaN HEMT with a gate periphery of 1.5 mm, providing a high dissipated power of 5.1 W. The tested semiconductor device is characterized by measuring scattering parameters at high frequencies up to 65 GHz and at high ambient temperature up to 200 degrees C. To assess the impact of the thermal effects on high-frequency GaN HEMT performance, an equivalent circuit is analytically extracted and then used to determine the main RF figures of merit. The achieved experimental results show evidence that the increase of the temperature causes a significant degradation in device performance.