English: Diagram showing the mapping of typical red, green and blue LEDs onto the visible light spectrum. Both ZnSe and GaP produce sharp blue and green spectra respectively. AlGaAs is a crystal which contains two metals – aluminium and gallium in varying quantities. The ratio of the two metals determines the energy levels in the crystal and hence the wavelength of the light. The spectrum from AlGaAs LEDs covers red light and extends into the infra-red part of the spectrum.
It should be noted that there are many other substances that are also used for colour LEDs.