A computer program SUPREM 3.5, has been developed to simulate processes used to manufacture ion-implanted GaAs integrated circuits. The processes modelled in the present version of the simulator include ion implantation, diffusion, and activation. The simulator includes a routine to calculate the threshold voltage of a MESFET device based on the simulated processing results and on substrate properties. The models used for each dopant and process are based on physical mechanisms when possible, but empirical parameter fitting is sometimes used. Pearson IV distributions are used for the implantation modeling, concentration-independent diffusivities are used for n-type dopants, concentration-dependent diffusivities are used for p-type dopants, and concentration-dependent activation efficiencies are used for all dopants. The simulator models each process individually, as well as integrating the various processes and models together.>