Ga-In-Zn-O (GIZO) thin film transistors (TFTs) with disparate passivation structures were fabricated and their stabilities were compared. The devices were subjected to a negative bias stress with simultaneous exposure to visible light. TFT that incorporates a dual passivation composed of a SiOx layer grown at a relatively high temperature with an additional SiNx film deposited shows only -0.8 V V-th shift, whereas a -5.7 V shift was observed for a TFT covered by a single SiO2 film. The device degradation is susceptible to the ability of protecting external moisture, which may adsorb on the surface of the GIZO semiconductor to create donor states therein. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3481710] All rights reserved.