Abstract We report detailed results on the temperature dependence and thermal stability of the planar-integrated enhancement/depletion-mode (E/D-mode) AlGaN/GaN high-electron mobility transistors (HEMTs). Compared to the standard mesa etching technique, the plasma treatment can achieve the same device isolation results. The E/D-mode HEMTs and the corresponding digital integrated circuits fabricated by the planar process exhibit stable operation from room temperature up to 350 °C. No degradation in device performance was observed after a 140-h thermal stress at 350 °C, implying excellent thermal stability of the planar process. The direct-coupled FET logic inverter, realized by planar-integration of E/D-mode HEMTs, presents larger noise margins (NM S) at high temperatures than the previously reported work, demonstrating promising potential for GaN-based high-temperature digital ICs. The NM improvement can be attributed to the higher threshold voltage and the improved gate turn-on voltage of the E-mode HEMTs that is achieved with larger plasma treatment dose.