Monolithic integration of III-V materials on silicon appears as the most promising, cost-effective and versatile method for the next generation of optoelectronic devices [1-3]. Several research efforts have been devoted to understand the epitaxial growth of high quality III-V semiconductor on low cost Si substrates. However, the growth process has proven difficult due to number of fundamental problems. Thus, it is essential to develop a growth process that produces III-V films with low surface roughness and an acceptable defect density at low deposition temperature. In this work we demonstrate GaAs metal-semiconductor-metal photodetector (MSM PD) epitaxially grown on Si. The growth of GaAs is carried out using the low temperature thin Ge interlayers which was previously demonstrated . In the present device architecture, a two-step direct growth of Ge-on-Si acts as a “virtual” substrates to reduce the overall threading dislocation density in the GaAs device layers.