A light-emitting diode (LED) is based on a forward-biased heterojunction and emits narrow-band radiation whose wavelength is determined by the bandgap of the semiconductor material. Recently-developed white LEDs are typically based on blue LEDs (InGaN, indium gallium nitride) coated with doped YAG (yttrium aluminum garnet) crystals which act as a phosphor, down-converting part of the blue radiation to a broad spectrum centreed in the yellow. The resulting spectral mix has appearance of white light to the human eye. Shown here is a white LED shining on a red screen.
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