Light Emitting Diode (LED)
A light emitting diode (LED) is a forward biased PN junction diode, which emits visible light when energized.
When a junction diode is forward biased, electrons from N-side and holes from P-side move towards the depletion region and recombination takes place. When an electron in the conduction band recombines with a hole in the valence band, energy is released. In the case of semiconducting materials like
gallium arsenide (GaAs), gallium phosphide (GaP) and gallium - arsenide phosphide (GaAsP), a greater percentage of energy is given out in the form of light. If the semiconductor material is transluscent, light is emitted and the junction becomes a light source (turned ON). The LED is turned ON, when it is forward biased and it is turned OFF, when it is reverse biased. The colour of the emitted light will depend upon the type of the material used. By using gallium arsenide phosphide and gallium phosphide, a manufacturer can produce LEDs that radiate red, green, yellow and orange. Fig shows the symbol of LED. LEDs are used for instrument displays, calculators and digital watches.
PN Junction diode
If one side of a single crystal of pure semiconductor (Ge or Si) is doped with acceptor impurity atoms and the other side is doped with donor impurity atoms, a PN junction is formed as shown in Fig. P region has a high concentration of holes and N region contains a large number of electrons.
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