Construction and Working Of Light Emitting Diode LED

Light Emitting Diode

The LESD is basically a device which changes input electrical energy into output optical radiation in the visible or infrared position of spectrum depending on the semiconductor material used. LEDs have replaced incandesced lamp in many applications because of low voltage long life and fast on off switching. He materials used in manufacturing the LEDs are

1.       Gallium Arsenide Phosphide (GaAsP): It provides either red light or yellow light.

2.       Gallium Phosphide (GaP): It provides red or green light.

3.       Gallium Arsenide (GaAs): It provides infrared radiation

Principle: When a P-N junction forward biased, charge carrier recombination take place at a junction as electron cross from the n- side and combine with holes on the p- side. Free electrons are in the conduction band of energy level while holes are in the valence band.
Therefore electronics are at place some of its energy is given up in the form of heat and will be emitted and junction becomes light source which called as “Light Emitting Diode”.

Construction and Working

The cross sectional view of a typical LED is shown in fig (or) the semiconductor material employed is gallium arsenate or Gallium Arsinatic phosphate or Gallium phosphate. A n type bar is grown up on a substrate and p region is created by diffusion. Since the charge carrier recombination occurs in the p-region it must be kept uppermost the metal film anode connection must be patterned to allow most of the light to be emitted. A gold film is applied to the bottom of substrate to reflect as much as possible of the light towards the surface of the device and to provide cathode connection.

                                                                             Cross Section View of LED

 The below fig shows the circuit symbol of LED. The outward arrow indicates emission of light and the below fig shows the VI characteristic of LED. From VI characteristics one can expect a typical the operating voltage range of LED is from 1.7V to 3.0V.

Circuit Diagram

                                                                                      V-I Charecteristics of LED