LED (light emitting diode)

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          LED(light emitting diode) 

It is a heavily-doped forward-biased p-n junction which spontaneously converts the biasing electrical energy into optical energy, like infrared and visible light. 

Symbol








Working

A p-n junction made from a translucent semi conductor like gallium arsenide or indium phosphide is provided with metallised contacts as shown in diagram. 
When it is forward biased through ba series resistance R, light photons are emitted from the non - metallised surface of the n-region.
The series resistance R limits the current through the LED and hence controls the intensity of light emitted by it.



When the p-n junction is forward biased, electrons are sent from n-region to p-region (where they are minority carriers) and holes are sent from p-region to n-region (where they are minority carriers). Near the junction, the concentration of minority carriers increases as compared to the equilibrium concentration (i.e., when there is no bias). On either side near junction,  the excess minority carriers combine with the majority carriers. On recombination, the energy is released in the form of photons. Photons with energy equal to or slightly less than the band gap are emitted. When the forward bias of the diode is small, the intensity of emitted light is small. As the forward current increases, intensity of light increases and reaches a maximum. Further  increase in forward current decreases the light intensity. 



The general shape  of the I-V characteristics of an LED is similar to that of a normal p-n junction diode, as shown in diagram. However, the barrier potential changes slightly with the colour. 

Two important features of LEDs are :
1. The colour of light emitted by an LED  depent on its band-gap energy. 
2. The intensity of light emitted is determined by the forward current conducted by the p-n junction. 

Choice of the semiconductor material used in LED


The wavelength of visible light ranges from 0.4micrometer to 0.7 micrometer (energy from 3eV to 1.8eV). For a semiconductor to emit visible light, the minimum band gap must be 1.8eV. The compound semiconductor gallium - arsenide - phosphide (GaAs(1-x) Px) is used for making LEDs of different colours. GaAs(0.6) P(0.4) (Eg = 1.9eV) is used for red LED. GaAs (Eg = 1.4eV) is used for infrared LED. 


Advantages of LEDs over conventional incandescent lamps : 

1. Low operational voltage and less power consumption. 
2. Fast action and no warm up time required. 
3. The bandwidth of emitted might is 100 angstrom to 500 angstrom i.e., the light is nearly monochromatic. 
4. Long life and ruggedness. 
5. Fast ON/OFF switching capability. 

Uses of LEDs

1. Infrared LEDs are used in burglar - alarm systems. 
2. In optical communication. 
3. In image scanning circuits for picture phones. 
4. LEDs are used in indicator lampsbin radio receivers and other electronic equipment. 
5. In remote controls. 
6. In digital display lights of calculators, cash registers, digital clocks, etc. 













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