In this work, Y2O3 was evaluated as a gate insulator for thin film transistors fabricated using an amorphous InGaZnO4 (a-IGZO) active layer. The properties of Y2O3 were examined as a function of various processing parameters including plasma power, chamber gas conditions, and working pressure. The leakage current density for the Y2O3 film prepared under the optimum conditions was observed to be similar to 3.5 x 10(-9) A/cm(2) at an electric field of 1 MV/cm. The RMS roughness of the Y203 film was improved from 1.6 nm to 0.8 nm by employing an ALD (Atomic Layer Deposition) HfO2 underlayer. Using the optimized Y2O3 deposition conditions, thin film transistors (TFTs) were fabricated on a glass substrate. The important TFT device parameters of the on/off current ratio, sub-threshold swing, threshold voltage, and electric field mobility were measured to be 7.0 x 10(7), 0.18 V/dec, 1.1 V, and 3.3 cm(2)/Vs, respectively. The stacked insulator consisting of Y2O3/HfO2 was highly effective in enhancing the device properties. (C) 2009 Elsevier B.V. All rights reserved.