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Light Emitting Diode
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A light-emitting diode is a
two-lead semiconductor light source. It is a p–n junction diode, which emits
light when activated.
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When a suitable voltage is applied to the
leads, electrons are able to recombine with electron holes within the device,
releasing energy in the form of photons.
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This effect is called electroluminescence, and
the color of the light is determined by the energy band gap of the
semiconductor.
Operation
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When forward biased the
electrons and holes in the p-n junction recombine.
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Some of the free electrons have
higher energy than holes and dissipation of that energy will be in the form of
heat and light.
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If the semiconductor is
translucent the light is emitted.
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Semiconductor materials used
are:
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GaAsP
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GaP
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| Circuit Symbol |
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On a substrate an n type
epitaxial layer is formed. P region is diffused into it.
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Recombination occurs in p
region and hence it is kept on top. Anode connection is metal film for light to
escape.
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A gold film is applied to
bottom of substrate to reflect as much light as possible and to provide cathode
connection.
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GaAs – infrared (invisible)
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GaAsP – red or yellow light
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GaP – red or green
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Junction is mounted on cup
shaped reflector, wires for anode and cathode connection is given. Device is
encapsulated in a epoxy lens.
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The
color of emitted light is solely based on p-n junction material.
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