This paper investigates the impact of N(2)O plasma treatment on the light-induced instability of InGaZnO thin film transistors with a SiO(2) passivation layer deposited by plasma-enhanced-chemical-vapor-deposition (PECVD). For the untreated device, because the deposition of the SiO(2) passivation layer by PECVD causes extra trap states, the anomalous subthreshold leakage current can be attributed to a lowering of the source side barrier due to trap-assisted photogenerated holes. In contrast, the N(2)O plasma treatment applied to both the gate insulator and the active layer effectively suppresses the device instability under illumination. In order to clarify the influence of the N(2)O plasma treatment, this study investigates a device with treatment of only the gate insulator. This device shows a slight decrease of light-induced subthreshold leakage current. This demonstrates that N(2)O plasma treatment on IGZO active layer after its deposition is critical in preventing damage from the subsequent SiO(2) passivation deposition process. In addition, the instability of threshold voltage (V(T)) under negative bias illumination stress (NBIS) is significantly improved by the N(2)O plasma treatment. Furthermore, a different dark recovery rate follows NBIS for untreated and N(2)O plasma-treated devices, indicating different hole-trapping levels exist in the energy band. (C) 2011 Elsevier B.V. All rights reserved.