Provided is a silicon-wafer-based germanium semiconductor photodetector configured to be able to provide properties of high gain, high sensitivity, and high speed, at a relatively low voltage. A germanium-based carrier multiplication layer (e.g., a single germanium layer or a germanium and silicon superlattice layer) may be provided on a silicon wafer, and a germanium charge layer may be provided thereon, a germanium absorption layer may be provided on the charge layer, and a polysilicon second contact layer may be provided on the absorption layer. The absorption layer may be configured to include germanium quantum dots or wires.