The effect of adding hydrogen gas (H(2)) when depositing a zinc oxide (ZnO) thin film in a thin film transistor (TFT) using the ZnO as the channel layer on the electrical characteristics of the ZnO-TFTs, particularly the change in the characteristics according to long-term exposure to air, was investigated. As the amount of added H(2) gas was increased, the resistivity of ZnO films was monotonously decreased and their crystallinity was weakened. Compared with the TFT using a ZnO without H(2) addition, the threshold voltage (Vth) decreased and the on/off current ratio (I(on)/I(off)) greatly increased, if the amount of H(2) entry was small (<= 0.3 sccm). However, when an excessive (>= 0.5 sccm) amount of H(2) was added, the TFT's properties deteriorated. In addition, the ZnO TFTs showed a positive Vth-shift with increased air exposure time. The analysis using the X-ray photoelectron spectroscopy (XPS) confirmed that this was attributed to the reduction of oxygen vacancies due to air exposure. It was noticed that the TFTs that were manufactured using ZnO films with H(2) addition showed significant suppression of the Vth-shift according to air exposure. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.