Ge/Si heterojunction light emitting diodes with 20-bilayers undoped or phosphorus in situ doped GeSi islands were fabricated on n(+)(-)Si(001) substrates by ultrahigh vacuum chemical vapor deposition (UHV-CVD). Enhanced room temperature photoluminescence (PL) and electroluminescence (EL) around 1.5 μm were observed from the devices with phosphorus-doped GeSi islands. Theoretical calculations indicated that the emission is from the radiative recombination in GeSi islands. The intensity enhancement of PL and EL is attributed to the sufficient supply of electrons in active layer for radiative recombination.