LIGHT EMITTING DIODES:
It is a semiconductor p-n junction diode which emits light when it is forward biased.
The electrons injected into the p- region make a direct downward transition from the conduction band into valence band and they recombine with holes and emit photons of energy Eg.
We know that the forbidden gap energy is given by
Figure shows crossectional view of a LED.
A n- type layer is grown on a substrate and a p- type layer is deposited on it by diffusion. Since carrier recombination takes place in the p-layer, it is deposited upper most.
For maximum light emission, a metal film anode is deposited at the outer edges of the p-layer. The bottom of the substrate is coated with metal (gold) film for reflecting most of the light surface of the device and also to provide connection with n- type layer.
When the p –n junction diode is forward biased, the barrier width is reduced, raising the potential energy on the n-side and lowering that on the p-side.
The free electrons and holes have sufficient energy to move into the junction region. If a free electron meets a hole, it recombines with each other resulting in the release of a photon
Thus light radiation of the LED is caused by the recombination of holes and electrons that are injected into the junction by forward bias voltage.
Advantages of LED:
1. Light output is proportional to the current. Hence, the light intensity of LEDs can be controlled easily by varying the current flow.
2. LEDs are rugged and therefore withstand shocks and vibrations.
2. Varieties of LEDs are available which emit in different colours like red, green, yellow etc.
3. It has long life time and high degree of reliability.
4. It has low drive voltage and low noise.
5. It is easily interfaced to digital logic circuits.
6. It can be operated over a wide range of temperatures.
Disadvantages of LED:
1. It requires high power.
2. Its preparation cost is high.
3. LED is not suitable for large area display because of its high cost.
4. It cannot be used for illumination purposes.