LIGHT EMITTING DIODE (LED)
A PN junction diode,which emits light when forward biased known as a light emitting diode(LED)
Fig.6.9 (a) and 6.9 (b) shows a symbol and construction respectively.
It consists of N-type layer placed on a P-type substrate by a diffusion.Then a thin P-type layer is placed on the N-type layer.The metal connection to both the layer forms anode and cathode terminals.when light falls on the junction recombination of electrons with holes take place.After passing through the P-region the light is emitted through the window arranged at the top of the surface.
When the LED is forward biased,electronics and holes move towards the junction and the recombination takes place.After recombination,the electrons,lyin in the conduction bands of N-region,fall into the holes lying in the valence band of
P-region.The difference of energy between the conduction band and valence band is radiated in the form of light energy.
The LED's radiate light in different colors such as red,green,yellow,blue,orange etc.some of the LED's emits infrared (i e invisible) light also.The color of the emitted light depends upon the type of the semiconductor used.Thus gallium arsenide emits infrared radiation gallium arsenide phosphide produces either red or yellow light,gallium phosphde emits red or green light and gallium nitride produces blue light.
ELECTROLUMINESCENCE IN LED:
In the PN junction of LED electrons are majority carriers in N-type material and holes are the majority carriers in P-type material.Electrons (in the N-type materials) exist in the conductionband and the hples (P-type materials) are in the valence band.
Therefore electrons in the conduction band have higher energy values than holes in a valence band when electrons move across a PN junction and recombine with a hole they release excess energy,the combining electrons release energy in the form of heat and light.LED's are constructed in a such a way that PN junction is contained within a translucents material and a large exposed surface area on one layer of semiconductor material permits the photons to be emitted as visible light.This process is called electro luminescence.
The light that result from the conduction of these PN function is reason for the name light-emitting diode( LED )
SEMICONDUCTOR MATERIALS USED FOR LED:
LED's are made from doped gallium.Various impurities are added during the doping process to establish the wavelength of the emitted light remember that wavelenght determines the color of the light A Gallium arsenide (GaAs) will have a green glow or infrared. Gallium arsenide sulphide (GaAsS) glows red.gallium phosphide (GaP) glows yellow and gallium nitride (GaN) glows blue.
LED's are used to:
(1) Displays are used to indicate alphanumeric characters and symbols in various system such as digital clocks microeave ovens stereo tuners calculators etc.
(2) Indicating power ON/OFF conditions.
(3) In optical switching applications.
(4) Are also to send light energy to fiber optical cable,which transmits energy by means of total internal reflection.
(5) For image sensing circuits in picture phones.