LIGHT EMITTING DIODE

                             

  • LED is a p-n junction semiconductor device which emits light when it is forward biased.

principle:

LED is a device which converts electrical energy into the light(visible) energy. Light is emitted across the p-n junction when the recombination of electrons and holes takes place. The below figure shows Recombination of electrons in forward biased condition.
What is Light Emitting Diode (LED)? - Definition, Working ...
     
 The energy gap present between the valance band and conduction band determines the energy of photons which are emitted by the LED. When the recombination takes place, the excess energy will be  ejected out as light. This process is called as electroluminescence.

what type of diodes can emit light?:

          The colour of emitted light depends on the type of material used. The semiconducting material used for manufacturing LED's are compound semiconductors like gallium arsenide(GaAs),gallium arsenide phosphate(GaAsP) and gallium phosphide(GaP). The colour or wavelength of light emitted by LED depends on the percentage of doping of  phosphide in GaAsP or GaP.


SEMICONDUCTING MATERIAL COLOUR/WAVELENGTH
Gallium phosphide(GaP) Red-7000A*
Green-5650A*
Gallium Arsenide phosphide(GaAsP) Yellow-5850A*
Red-7000A*
Gallium Nitride(GaN) Blue-4800A*
Gallium Arsenide(GaAs) IR-9000A*

How to select semiconducting material?  

                    Semiconducting materials are selected according to the colour we need, this is done on the basis of energy gap of P-N junction diode. Energy gap of semiconductor is equal to 'h' (planks const.) times of the frequency of radiation  emitted from semiconductor.
                                Eg=hv

In visible light:

violet- 4000A*

required energy gap=hv=3.1v

Red-7000A*

required energy gap =hv=1.77v

                          on calculating 'hv' for above wave lengths (converting them to frequencies) we can conclude that the semiconductors having energy gaps between  >1.77v to >3.1v can be used to make LED .


construction of LEDs:

            LEDs emit light in a direction perpendicular to the p-n junction plane in the case of surface emitting LED ,the construction of surface emitting LED is shown in figure .


Sketch of the internal structure of a LED with n-type substrate ...

                     An substrate layer is grown on  'n' type layer  and 'p' type layer is grown on substrate by the process of diffusion. The photons that are generated at the P-n junction must pass through the semiconducting materials to reach the surface and become visible .In this process some of light rays  reach the surface may not be able to leave the semiconductor owing to the total internal reflection(TIR) . The refractive index of LED materials is about 3.3 to 3.8 and critical angle corresponding to these refractive index(s) are 15 to 18degrees respectively(According to snell's law ).All the rays of light striking the surface with angle greater than the critical angle gets internally reflected .There fore the device is encapsulated in  a clear epoxy resin of suitable refractive index.
  • P-type layer is made very thin to prevent loss of photons due to the absorption of the layer .
  • Metal connections are made at the edges of the P-layer in order to allow more central surface to escape.
  • A metal film is deposited at the bottom of the substrate for reflecting as much light as possible towards the surface of the device and also provide electrode connection.
 

V-I characteristics   :

           

What is Light Emitting Diode (LED)? Definition, Construction ...

  • The forward voltage across an LED ranges from 1.8v to 3.2v 
  • The reverse breakdown voltage of led is of order of 5v.
  • Every LED is fixed with its energy gap based on required colour , on increase in voltage the intensity of light emitted is increased.
If it is possible to change the energy gap of led(assumption), then frequency of emitted light is changed .  

why LED should not connected in reverse biased :

            LED should be connected in forward bias because in reverse biased condition ,the depletion layer is more which offers more resistance and on further increase in voltage current suddenly raises ,at break down voltage (5v) which leads to the damage of LED . That is at breakdown voltage LED fuses off and at voltage less than breakdown voltage LED does not glow as there is no generation of photons ,so connecting LED in unbiased way is not preferred .

Figure of merit:

  • Output is bright and the intensity can be controlled easily by varying current .
  • They can be operated over a wide range of temperatures (0 to 70 degrees)
  • very fast response time in the order of nano sec. and hence very useful for optical communication 
  • Has long life about 10^5 hours and high degree of reliability 
  • The viewing angle is not limited .




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