Light-emitting diode emission frequencies

In a light-emitting diode (LED), which is used in displays on electronic equipment, watches, and clocks, a voltage is imposed across an n-p semiconductor junction. The electrons on the n side combine with the holes on the p side and emit light at the frequency of the hand gap. This process can also be described as the emission of light as electrons fall from levels in the conduction hand to empty levels in the valence band. It is the reverse of the production of electric current by illumination of a semiconductor. 

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