We report on a high temperature forming of iridium oxides (IrO2) gates of circular AlGaN/GaN high electron mobility transistors (C-HEMTs) to be predetermined for high temperature applications. IrO2 gate interfacial layer is formed by high temperature oxidation (T = 500-800 degrees C, for 1 min) of 15 nm thick Ir gate contact layer. A comprehensive microstructural and electrical characterization of the IrO2 gates is carried out to explain the improved transport properties and thermal stability of the gate interfaces. It is found that the transformation of Ir gate layer into its crystalline IrO2 phase at oxidation temperatures of 700 C and 800 C provides the high barrier gate interface with prevailing thermionic emission transport mechanism. Accelerated reliability tests are used to confirm C-HEMT thermally stable performance deduced from both the gate interface and 2DEG channel stability. The introduced AlGaN/GaN C-HEMTs with high temperature grown IrO2 gates seem to be very attractive for both the high temperature operated electronic and sensor devices. (C) 2013 Elsevier B.V. All rights reserved.