A capacitance voltage model that is continuous across the linear and saturation regimes is developed for a polysilicon thin film transistor, incorporating all the effects like the field dependent mobility and the channel length modulation. The expression developed explains the capacitance behavior in the kink region as well. Further, transconductance in saturation region is evaluated. The results obtained are then used to calculate the cut off frequency of the device in all the regions of device operation. The C-V results so obtained are matched with the available experimental results. The results are analyzed to obtain high cut off frequency by varying the grain size. (C) 2003 Elsevier Ltd. All rights reserved.