© The Electrochemical Society We observe strong room-temperature electroluminescence from Ge-on-Si. A Ge p-i-n junction is formed within the epitaxial Ge by the precise in-situ doping method and additionally a high density P delta doping is performed in the vicinity of the surface to create a low resistivity Ohmic contact. An EL device is mesa-defined by the photo-lithography and following reactive ion etching process in order to suppress leak currents. As a result, current-voltage characteristics reveal a good diode rectifying property with an on/off ratio of over 105. The EL spectrum appears at around 2.0 kA/cm2 injection current, and the EL intensity drastically increases with the increase in the injection current. These results indicate that the Ge-on-Si p-i-n structure has a high potential to realize high efficiency lasers monolithically integrated on a Si platform.