Based on a closed form of the base-emitter voltage of the parasitic bipolar transistor, a physical model of floating body effects is proposed for polysilicon thin film transistors, which takes into account the polysilicon graded pn junction and the generation rate including the Poole-Frenkel effect. Simulated results by this model are in good agreement with experimental data. It is shown that the action of a parasitic bipolar transistor should be taken into account only when the channel length is short enough due to the much smaller carrier mobility in polysilicon compared with single crystalline silicon. Whereas, the parasitic bipolar transistor gain (beta) increases sharply with decreasing the channel length when the channel length is less than 5 mu m, which is due to the rapid increase of the base transport factor (alpha(T)) (c) 2008 Elsevier Ltd. All rights reserved.