To analyze short-channel effects of MOS transistor ac characteristics, a two-dimensional device simulator has been used to extract MOS transistor capacitances. The results of simulation and measurements agree quite well. Several causes of short-channel effects are explained by the simulations. Velocity saturation effects are found to play a key role in the gradual increase in Cgd. Also holes in the accumulation region and the two-dimensional effect or the influence of the back-gate field from the drain are important in explaining the short-channel effect of MOS transistor capacitance.