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