We report the epitaxial growth of GaAs films on bulk Si, Ge and Ge coated Si substrates by MOCVD. This work is directed at comparing the structural, electrical, optical and optoelectronic properties of the GaAs films grown on these substrates. The quality of the GaAs films was evaluated by a number of techniques including X-ray, TEM, RBS, I-V and C-V. We have also obtained good photoresponse for Au-GaAs/Si, Au-GaAs/Ge and Au-GaAs/Ge/Si Schottky diodes at 0.87μm. We have observed significant differences in defect density, strain, crystalline quality, structural order, doping density depth-profile, dark current, diode ideality factor, photoresponse, inter-diffusion and interface structure among these three different structures. The GaAs/Ge structure exhibited the best structural and electrical characteristics of all. The GaAs/Ge/Si structure showed material properties and Schottky diode performance that are better than those of the GaAs/Si stucture. Although Ge and Si both out-diffuse into GaAs, that the out-diffusion of Si into GaAs appears to cause more degradation in the electrical and optoelectronic properties of GaAs.