In this article, heat generation distribution and maximum device temperature of gallium-nitride (GaN) high-electron-mobility transistors (HEMTs) are investigated by using the 2-D electrothermal and finite-element method (FEM) simulations. Devices with different gate lengths and source-to-drain spacing are investigated. It is observed that the maximum device temperature (T MAX) depends on the drain-to-source spacing and is almost independent of the gate length and that the assumption of a uniform heat generation region, under the gate, is not accurate; this is contrary to conventional calculation methods. Moreover, based on the results, a new approximation is proposed to use in the FEM simulations that can estimate T MAX more accurately. This method does not require physics-based technology computer-aided design (TCAD) simulations and can work with a low mesh density. The performance is compared with prior methods. Index Terms-2-D device simulations, AlGaN, channel temperature, finite-element analysis, gallium nitride (GaN), high-electron-mobility transistors (HEMTs), hot point, selfheating, technology computer-aided design (TCAD), thermal analysis, thermal resistance. I. INTRODUCTION G ALLIUM-NITRIDE (GaN)-based high-electronmobility transistors (HEMTs) are high-performance devices that show superior performance in high-power amplifiers and switching applications [1]-[3]. The high bandgap leads to an elevated breakdown voltage [4]. The 2-D electron gas (2DEG) provides excellent electron mobility Manuscript