A nonlinear lumped Circuit model for GaAs MESFET which includes the effect of Gunn-domain formation under the gate, mobility modulation, and diffusion process in the channel boundary is presented. The important design parameters such as Cin,g_{m}, I_{dsat}, and Ft, etc. can be derived from the model. The model not only predicts realistically the nonlinearI-Vcharacteristics, but it also provides a closed form design criterion for avoiding instability due to Gunn oscillation. Moreover, a small-signal Circuit model having the same basic circuit structure as the existing empirical small-signal model in [2], [10], [14] can be derived from this nonlinear model. In addition, a two-segment piecewise-linear dc model is derived. This piecewise-linear model contains only four model parameters which can be determined very easily by measurement. Because of its simplicity, this model can be implemented into any CAD system to simulate complex circuits with significantly less CPU time. The simulation results are in excellent agreement with experimental data, and with results obtained by a much more time-consuming two-dimensional calculation.