Heterodiodes have been prepared by the growth of epitaxialn‐type CdS films from the component vapors onp‐type CuGaS2substrates. In dc operation at 77°K, green light is emitted with an external quantum efficiency of 0.1%. For pulsed operation at room temperature, the efficiency is[inverted lazy s]0.001%. The near‐band‐gap radiation results from electron injection into a semi‐insulating CuGaS2region near the CdS/CuGaS2interface. This region also controls the forward current of the diodes.