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.