This study investigates the effects of ambient atmosphere on electrical characteristics of Al2O3 passivated InGaZnO thin film transistors during positive bias temperature stress. Under H2O vapor environment, the Al2O3 passivated device exhibited stable electrical behaviors (Delta V-th < 0.5 V), while the unpassivated device showed an apparent hump effect in the transfer curves under bias stress. The hump phenomenon was attributed to the absorption of the H2O molecule which can serve as a donor to develop a conductive back channel. The experiment results suggest that Al2O3 is an effective passivation layer to suppress water vapor absorption in the InGaZnO back channel. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3534828] All rights reserved.