A new dc and temperature nonlinear GaAs MESFET device model is presented. This offers improved accuracy over existing models by simulating the bias dependency of the device transconductance (beta) and pinch-off point (V(TO)) parameters. The effect of these bias dependencies becomes more important as a departure from room temperature is made. New expressions for simulating the temperature dependency of V(TO) and beta are also presented and these provide improved accuracy over existing techniques. The new model effectively couples together the bias and temperature dependency of the device.