LED is a semiconductor device which work on the property of Electroluminescence in LEDs which is a particular diode which generates photons (light) when a stream of electrons passes through it. To build a diode we use a crystal (electric insulator) which is doped by atoms which have one more electron on their valence band (N doping) or missing one electron on their valence band (P doping). For N doping electron donor an atom which has 5 electrons on its valence band. Four electrons will participate to the crystal structure the fifth will stay free capable of moving in the crystal as a negative charge. For P doping electron acceptor element which has 3 electrons on its valence band. These will participate in the crystal structure but is fails one electron which creates a fixed hole like a positive charge. Examples of P-doping elements: boron (B), aluminum (Al), gallium (Ga), indium (ln).

  LED Construction And Working Principle

  LED diode convert current to light when a forward bias voltage is applied across a p-n junction, electron and hole flow across the space-charge region and become excess minority carriers. The excess minority carriers diffuse into the neutral semiconductors regions, where they recombine with majority carriers. If the semiconductor is a direct bandgap material such as GaAs the electron and hole can combine with no change in momentum, and a photon or light wave can be emitted. Conversely, in an indirect bandgap material, such as silicon, whe an electro and hole recombine, both energy and momentum must be conserved. Therefore LED is fabricated from GaAs or other compound semiconductor material. In an LED, the diode current is directly proportional to the recombination rate, which means that the output light intensity is also proportional to the diode current i-e, forward current. Thus the higher the diode current higher the light output. The schematic symbol and basic structure of LED is shown figure.

  Here an N-type layer is grown on P-type substrate by diffusion process. The metal connection to both layers make anode and cathode terminal as shown in figure. The light energy is released at the junction, when the recombination of electrons with holes takes place.