An Analytical Subthreshold Model for Polysilicon Thin-Film Transistors by a Quasi-Two-Dimensional Solution

An analytical expression of the surface potential for polysilicon thin-film transistors (poly-Si TFTs) working in the subthreshold region is obtained, following a quasi-2-D Poisson's equation, and then, an analytical subthreshold current model is subsequently developed based on the processes of diffusion and thermal emission. Furthermore, the kink effect is also taken into account in the subthreshold current model. Consequently, an analytical expression of the subthreshold swing is obtained from the surface potential equation and the subthreshold current expression. It is easily shown in our model that the subthreshold swing of poly-Si TFTs decreases with reducing the trap states or enlarging the grain size, and then, the switching performance of poly-Si TFTs will be improved. Moreover, this model has a simple functional form, and it can reduce to that of the conventional long-channel MOSFET in the case of large grain size and low trap states. The model has been verified by comparing simulated results with experimental data.