A numerical simulation of the electrostatic properties of metal / semi-insulating GaAs contacts has been carried out for different surface potentials at different temperatures. Specific results have been obtained for the three most common compensation mechanisms within the semi - insulating GaAs material: (a) a deep donor and a shallow acceptor, (b) a shallow donor and a deep acceptor, and (c) a deep donor and a deep acceptor. These results are compared to those from the one-sided Schottky abrupt junction model, and some discussions on the correlation of the electrostatic properties with Schottky barrier formation are presented.