Principle:

• In silicon and germanium the recombination energy is absorbed by the crystak as heat but in certain other semiconductors, for instance gallium arsenids, a photon is emitted when recombination occurs. This is the basis of the light-emitting diode (LED).

Construction of LED:

• One of the popular methods of LED construction is to deposit 3 semiconductors layers on the substrate. The active region exists between the p and n regions.
• The light emerges from the active side in all the directions when electrode hole pairs recombine.
• The disadvantage of the structure is that LED emits light in all the directions. This problem can be solved by placing the basic structure inside a small reflective cup so as to focus the light in the desired direction.
• Such a structure is called as a cup type construction. The circuit symbol of an LED is identical to a p-n junction diode with 2 arrows indicating that it emits light.
• LEDs are made of Gallium Arsenide (GaAs), Gallium Arsenide Phosphate (GaAsP) and Gallium Phosphide (GaP).
• Silicon and Germanium are not used because they are essentially heat producing materials and are very poor in producing light.

Working:

• When the LED is forward biased, the electrons in the n-region will cross the juntion and recombine with the holes in the p-type material.
• These free electrons reside in the conduction band and hence at a higher energy level than the holes in the valence band.
• When the recombination takes place, these electrons return back to the valence band which is at a lower energy level than the conduction band.
• While returning back, the recombining electrons give away the excess energy in the form of light. This process is called as electroluminescence. In this way an LED emits light. This is the princilpe of operation of LED.