We present an accurate small signal model for thin film transistors (TFTs) taking into account non-idealities such as contact resistance, parasitic capacitance, and threshold voltage shift. The model gives high accuracy in s-parameters, and the predicted cutoff frequency yields 1% discrepancy compared with measurement results. In contrast, the conventional CMOS small signal model adapted for TFTs yields 12.5% error. The TFT’s cutoff frequency is also evaluated under bias stress to examine the effect of device instability on small signal behavior.