p-CdS:Cu/n-CdS thin-film diodes were fabricated by an all-vacuum deposition process. Light emissions were observed in a forward current at 77 K. The colors of emitted light changed within the range of blue-green to red depending on the Cu concentration and the forward current densities. The spectra of the lights were broad, and the peaks shifted to shorter wavelengths with increases in current densities. Also, spectra of the lights had small peaks near the absorption edge of CdS. It is thought that the change in the color of emitted lights is caused by mixing of two transition processes in US films: the interband and/or near-interband transitions and the transition through deep and broad Cu levels. (C) 2003 Elsevier Science B.V. All rights reserved.