The floating-gate a-Si:H thin film transistor provides the memory function that is not available with the conventional a-Si:H thin film transistor. Mechanisms of charge trapping and detrapping in the floating-gate a-Si:H TFT are discussed based on the transfer characteristics curve. Influences of two critical factors, i.e., the starting gate voltage of the forward transfer characteristics and the channel-contact layer thickness, on the memory window were investigated. The amount of electrons trapped to the gate dielectric layer is dependent on the maximum gate voltage at the end of the forward curve but independent of the original amount of holes trapped. The memory window can be well defined when no excessive holes are trapped to the gate dielectric layer in the forward curve.