Drain bias stress effects on amorphous Zinc Indium Oxide (a-ZIO) Thin Film Transistors (TFTs) are important in flexible electronic systems. The drain bias impacts the overall threshold voltage (V-th) shift more so in the saturation stress mode than in the linear stress mode. Localization of degradation region in channel results in asymmetry in post stressed drain current in forward and reverse operations. This brief studies the impact of drain bias on V-th degradation, and also the effect on post stressed forward and reverse currents in a-ZIO TFTs, under both positive and negative gate bias stress in different regions of operation. Based on the measured results, an empirical expression incorporating drain bias effect on V-th degradation is derived. Also the measured results point to charge trapping in the insulator-semiconductor interface as the dominant degradation mechanism. (C) 2011 Elsevier Ltd. All rights reserved.