The impact of 15 nm thick Schottky gate contact layer based on conductive Ir oxides grown by thermal oxidation in O-2 ambient at 500 degrees C for 1 and 10 min, respectively, on performance of circular high electron mobility transistor (C-HEMT) has been investigated. Besides the effects of the gate barrier height increase (similar to 0.32 eV) and gate leakage current decrease (2-4 orders) with the time of oxidation, the shift of both threshold voltage (similar to 1 V) and peak of transconductance (similar to 2 V) of the C-HEMT device towards the zero gate voltage has been introduced. X-ray diffraction and depth profiles of secondary ion mass spectroscopy are employed to analyze the microstructure and composition of the gate contacts. They revealed the mixture of Ir and Ir oxides (IrO, IrO2) that have not been formed in the whole gate contact layer thickness. All results confirmed the oxidation proceeding from the top of the Ir gate contact layer up to the interface. A promise of Ir oxides based gate contact layers for design of thermally stable HEMT devices with an enhancement mode of operation could be available. (C) 2012 Elsevier B. V. All rights reserved.