A planar avalanche photodetector (APD) is fabricated by forming a, for example, InGaAs absorption layer on a p+-type semiconductor substrate, such as InP, and wafer-bonding to the absorption layer a second p-type semiconductor, such as Si, to form a multiplication layer. The layer thickness of the multiplication layer is substantially identical to that of the absorption layer. A region in a top surface of the p-type Si multiplication layer is doped n+-type to form a carrier separation region and a high electric field in the multiplication region. The APD can further include a guard-ring to reduce leakage currents as well as a resonant mirror structure to provide to wavelength selectivity. The planar geometry furthermore favors the integration of high-speed electronic circuits on the same substrate to fabricate monolithic optoelectronic transceivers.