A thermally stable AlGaN/GaN heterostructure field-effect transistor (HFET) using an iridium oxide (IrO2) gate contact was demonstrated, compared with conventionally used Pt Schottky contact. The Schottky barrier height of the Pt contact significantly decreased from 0.71 to 0.52 eV and the reverse leakage current at -20 V increased by two orders of magnitude when the device was annealed at 450 C for 24 h. This was due to the indiffusion of the Pt atoms into the AlGaN layer during the annealing. However, no electrical degradation of the contact was found in the IrO2 Schottky contact. This was due to the fact that the IrO2 suppressed the indiffusion of the contact metals into the AlGaN/GaN heterostructure. As a result, the sheet-carrier concentration at which the two-dimensional electron gases are confined was not degraded at high temperature. It was found that the electrical properties of the HFET using the IrO2 gate contact are thermally stable and no distinct change of device performances was observed even after annealing at 450 C for 24 h. It is suggested that IrO2 is a promising candidate as gate electrode for a high-temperature AlGaN/GaN HFET. (c) 2006 American Vacuum Society.