A method for providing a dual layer diffusion mask or encapsulation coating for use with III-V compound semiconductors, the dual layer coating comprising an inner layer of silicon which closely matches the coefficient of thermal expansion of the III-V compound semiconductor and an outer layer of silicon nitride which is relatively impermeable to subsequent metallization and for thereafter applying metallized contacts to the III-V compound semiconductor through selectively etched openings in the diffusion mask or encapsulation coating.