The effect of interface treatment of ZnO/gate insulator on the variations in both the electrical properties and the trap density of bottom-gate ZnO thin film transistors (TFTs) were investigated. Two kinds of interface treatments, as a diluted HF wet treatment and a nitrous oxide (N2O) plasma treatment, are individually applied to the gate-insulator surface of the ZnO TFTs. The threshold voltage uniformity of ZnO TFTs with the N2O plasma-treated gate insulator was drastically improved. The trap densities extracted from the ZnO TFTs revealed that the variation in the trap density in deep energy level is reduced significantly by the N2O plasma treatment. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3290741] All rights reserved.