Abnormal hysteresis was found to exist in amorphous Hf-In-Zn-O thin film transistors under illumination. Unlike common hysteresis, a disparity in the sub-threshold swing of the transfer curves was observed. This distinction showed a linear increase according to light intensity, and is thought to come from the light enabled motion of ionized oxygen vacancies (V-O(2+)). The positively charged nature of these defects causes them to show a different distribution in the channel according to sweep direction. The hysteresis decreased when the sweep range was decreased, and vanished when the sweep range fell in the positive region only. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3589244] All rights reserved.