A process for forming GaAs on a silicon substrate with very low levels of unintended silicon doping. First, a dielectric layer of silicon dioxide, silicon nitride, or both is grown or deposited on the substrate. Next, a window is opened in the dielectric layer exposing the silicon substrate in the regions in which the GaAs is to be formed. The GaAs layer is then formed on the substrate using conventional techniques with the gas phase transfer of silicon contamination from the edges and back of the silicon substrate to the GaAs region inhibited by the dielectric layer or layers.