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The thermal stability and electrical characteristics of GaN high-electron-mobility transistors (HEMTs) were investigated. Storage tests were carried out at 400°C for 48 h to study the ohmic-contact stability by means of the transmission line model. It was found that Ti/Al/Ni/Au ohmic contacts were stable and had superior thermal performance, but the Schottky contact may be more sensitive to the temperature. After thermal storage for 48 h at 400°C, the Schottky barrier height was increased, and the ideality factor decreased. Two types of isolation structures were also investigated under the same condition. DC tests were implemented to study the phenomenon and provide feedback for potential process improvements.