Modelling Velocity Saturation Effects in Polysilicon Thin-Film Transistors

By using full-two dimensional (2D) numerical simulations, it is found that, similarly to crystalline silicon, velocity saturation effects occur also in polysilicon thin-film transistors (TFTs). Therefore, precise modelling of output characteristics in short channel polysilicon TFTs should take into account velocity saturation effects, which influence the saturation current. Since full-2D numerical simulations are time consuming and unpractical for circuit simulations, we introduced velocity saturation in the gradual channel approximation (GCA), but still we found a non satisfactory agreement with experimental results, especially for short channel devices. We attributed this behaviour to the presence of a strong longitudinal electric field, that is neglected in GCA, when velocity saturation occurs. So we have developed a new quasi-2D model, that takes in account both carrier velocity saturation and the effects of the longitudinal field, which nicely reproduce the experimental data.