We describe physically based analytical models for n-channel amorphous silicon thin film transistors and for n- and p-channel polysilicon thin film transistors. The models cover all regimes of transistor operation : leakage, subthreshold, above-threshold conduction, and the kink regime in polysilicon thin film transistors. The models contain a minimum number of parameters which are easily extracted and can be readily related to the structural and material properties of the thin film transistors. The models have been verified for a large number of devices to scale properly with device geometry.(d)