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This paper presents a four-terminal small-signal dc-to-high-frequency model, valid in weak, moderate, and strong inversion regimes, for the intrinsic part of the long-channel MOS transistor. A charge-sheet approximation is used. Basic MOSFET equations are separated into parts corresponding to dc and ac small-signal components. The former are used to evaluate the drain current under dc conditions; the latter, describing the "transmission-line" behavior of the MOSFET, are solved to arrive at a complete set of admittance parameters. Based on different approximations of these parameters, various models are presented, each of different upper frequency limit of validity. For each model parameter, a single continuous expression is used which is valid in all regions of operation (weak inversion, moderate inversion, strong inversion; nonsaturation and saturation). The frequency range of validity of these models and the inadequacies of the quasistatic models at high frequencies are discussed. It is shown that at low frequencies the high-frequency model reduces to a quasistatic model which is widely verified by experimental results; at high frequencies the model agrees with available measurements.