We fabricated an enhancement-mode thin film transistor (TFT) using ZnO as an active channel layer deposited by radio frequency (rf) magnetron sputtering. The NH3 plasma passivation was performed in order to improve the electrical properties of the ZnO TFTs. We observed that the NH3 plasma treated ZnO TFTs revealed improved device performances, which include the field effect mobility of 34 cm2/Vs, threshold voltage of 14 V, subthreshold swing of 0.44 V/dec, off-current of 10-11 A and on to off ratio higher than 105. These results demonstrate that NH 3 plasma treatment could effectively enhance the performance of the ZnO based TFT device. © 2011 Elsevier Ltd. All rights reserved.