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The frequency dependence of nonquasistatic (NQS) operation in MOS transistors is studied. With the help of a two-dimensional device simulator, a time varying sine function is applied to the gate at different frequencies. The inversion channel charge variation in response to the input signal at high frequency is compared with that at dc. By observing the frequency that generates an observable delay in channel charge response to the applied signal, the onset frequency of the NQS effect can be determined, which gives a limit to the valid quasistatic (QS) assumption. It was found that the onset frequency of the NQS effect is very close to f/sub T/, the unity gain frequency of a transistor, and the QS approximation is sufficient for most of the practical applications.