We have determined that silicon films have nearly ideal properties for use as a diffusion mask and encapsulation coating for InP and GaAs. The Si films, composed of a single element, are easily and reproducibly deposited by electron beam evaporation at low temperatures. Sharp features can be defined by standard photolithography and freon‐plasma etching. The thermal coefficient of expansion of silicon nearly matches that of InP and GaAs so that problems due to film stress are avoided. Additionally, the interaction of Si with InP and GaAs crystals, under severe thermal treatments often used in device fabrication, was found to be negligible. Finally, we found that the Si film acts as a good diffusion mask for Zn which is a commonp‐type impurity for formingp‐njunctions in III‐V compounds.