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An iterative technique based on device transport and continuity equations is used to formulate a unified nonquasi-static model for the long-channel four-terminal MOSFET for both transient and small-signal analyses in all regions of operation (weak, moderate, and strong inversion). The model is physically derived without resorting to the concept of channel charge partitioning or the use of a priori assumptions about the functional form of the channel charge density. It is shown that the Ward charge-based model is only a 0th-order solution of this formulation. A first-order solution is presented that holds for arbitrary time-varying input voltages and can be reduced exactly to a small-signal nonquasi-static admittance model. Relaxation due to the channel resistance is included to account for the device nonquasi-static transient behavior. The first-order model consists of simple ordinary differential equations, which can be easily discretized for solution. Results from the proposed model are examined and compared with numerical simulation results and experimental data. Good agreement has been obtained.