We have investigated the effect of the deposition of an HfO2 thin film as a gate insulator with different O-2/(Ar + O-2) gas ratios using RF magnetron sputtering. The HfO2 thin film affected the device performance of amorphous indium-gallium-zinc oxide transistors. The performance of the fabricated transistors improved monotonously with increasing O-2/(Ar + O-2) gas ratio: at a ratio of 0.35, the field effect mobility of the amorphous InGaZnO thin film transistors was improved to 7.54 cm(2)/(V s). Compared to those prepared with an O-2/(Ar + O-2) gas ratio of 0.05, the field effect mobility of the amorphous InGaZnO thin film transistors was increased to 1.64 cm(2)/(V s) at a ratio of 0.35. This enhancement in the field effect mobility was attributed to the reduction of the root mean square roughness of the gate insulator layer, which might result from the trap states and surface scattering of the gate insulator layer at the lower O-2/(Ar + O-2) gas ratio. (C) 2012 Elsevier Ltd. All rights reserved.