Ion implantation technigues have been used to produce ZnS diodes which give different colours of electroluminescence emission if the excitation parameters are changed.One type of ZnS diode has Nd

3+

implanted in one surface and Er

3+

implanted in the opposite surface. Emission occurs only at the negatively-biased surface and stripe geometry electrodes allow green (Er

3+

) or orange (Nd

3+

) emission to be observed from one face

according to the polarity of applied voltage. A second type of ZnS diode has both Er

3+

and Nd

3+

implanted into one surface at different depths and gives different colours in the green (Er

3+

) to orange (Nd

3+

) range according to the magnitude of the applied voltage.