We describe a new, unified model for MEtal Semiconductor Field Effect Transistors (MESFETs) which covers all ranges of operation, including the subthreshold regime. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics are described by continuous, analytical expressions with relatively few, physically based parameters. The model includes effects such as velocity saturation, parasitic series resistances, the dependence of the threshold voltage on drain bias, finite output conductance in saturation, and temperature dependence of the device parameters. We also describe a parameter extraction routine which allows the model parameters to be derived in a straightforward fashion from experimental data. The model has been incorporated into our new circuit simulator AIM-Spice. The new device characterization is applied with good results to a typical ion-implanted GaAs MESFET and a delta-doped MESFET.