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 degrees C. No degradation in device performance was observed after a 140-h thermal stress at 350 degrees C, implying excellent thermal stability of the planar process. The direct-coupled FET logic inverter, realized by planar-integration of ED-mode HEMTs, presents larger noise margins (NMs) 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. (C) 2008 Elsevier Ltd. All rights reserved.