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The effect of temperature on the small-signal radio-frequency (RF) performance of submicron AlGaN/GaN high-electron-mobility transistors on SiC has been studied from room temperature (RT) up to 600 K. A relation between ambient and channel temperatures has been established by means of finite-element simulations. The thermal behavior of the intrinsic parameters C gs, C gd, gm, int, and g ds has been extracted accurately from RF measurements by means of the small-signal equivalent circuit. Main dc parameters (ID, gm, ext) show reductions close to 50% between RT and 600 K, mainly due to the decrease in the electron mobility and drift velocity. In the same range, fT and f max suffer a 60% decrease due to the reduction in gm, ext and a slight increase of C gs and C gd. An anomalous thermal evolution of C gd at low ID has been identified, which is indicative of the presence of traps.