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Ohmic contacts to n-type GaAs have been developed using epitaxial Ge films on GaAs alloyed with Ni overlayers by solid-state diffusion at temperatures of 450°C-650°C. These contacts have applications to high reliability, high temperature microwave devices. Reflection electron diffraction of the Ge layers prior to deposition of the Ni overlayers reveals the presence of high quality single-crystalline films. Even after sintering, there is very little penetration of Ge into GaAs in the absence of Ni. With the presence of a Ni overlayer, significant interdiffusion between Ge and GaAs is revealed by Auger electron spectroscopic profiles. These results, together with the current-voltage characteristics of similar contacts prepared on p-type GaAs, indicate the presence of a Ge-doped n/SUP +/ layer at the Ni/Ge-GaAs interface. Ohmic contacts using epitaxial Ge films with Ta and Mo overlayers have also been investigated.