An n-p-n heterojunction structure, formed by the epitaxial growth of n-Ge on a p-GaAs substrate having a diffused n-GaAs region on the opposite face, has been employed to convert 1.5-μ radiation incident on the n-Ge face to 0.9-μ radiation emitted from the n-GaAs face. The internal quantum efficiency of the n-Ge, p-GaAs heterojunction is 0.62; the spectral response of the heterojunction is typical of photon effects in Ge. The internal wavelength conversion efficiency is 2.8×10−5, limited principally by the low electroluminescent quantum efficiency of the GaAs p-n junction at low injection current densities.