The mechanism controlling the memory function of the floating-gate amorphous silicon thin film transistor, i.e., the hysteresis behavior of the transfer characteristics, has been identified. The magnitudes of the forward and backward gate voltages determine the charge trapping and detrapping processes including the curve of the transfer characteristics, the hysteresis direction, and the memory window. The thickness of the channel-contact layer also controls the charge transport distance and the memory function. The memory window can be well defined and optimized by avoiding excessive trapping of holes and enhancing the trap of electrons. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3487929] All rights reserved.