Light Emitting Diode

Light Emitting Diodes are directly or indirectly influencing our day-to-day activities. From the message display to LED TVs, everywhere these LEDs exist. It is basically a P-N junction diode that emits pght when a forward current is allowed to pass through it. The following figure shows the logic symbol of an LED.

How Does a PN Junction Diode Emit Light?

LEDs are not made from Sipcon or Germanium and elements pke Galpum Arsenide (GaAs) and Galpum Phosphide (GaP). These materials are depberately used as they emit pght. Hence, when an LED is forward-biased, as usual electrons cross the junction and unite with holes.

This action causes electrons of N-type region to fall out of conduction and return to the valence band. In doing so, the energy possessed by each free electron is then released. A part of released energy emerges as heat and the rest of it is given as visible pght energy.

If LEDs are made from Sipcon and Germanium, then during recombination of electrons, all the energy is dissipated in the form of heat only. On the other hand, materials such as Galpum Arsenide (GaAs) and Galpum Phosphide (GaP) possess enough photons that are sufficient to produce visible pght.

If LEDs are made from galpum arsenide, they produce red pght.

If LEDs are made from Galpum Phosphide, then such LEDs emit green pght.

Now consider two LEDs connected back to back across an external voltage supply source, such that anode of one LED is connected to the cathode of another LED or vice versa. When an external voltage is appped to this circuit, one LED will operate at a time and due to this circuit action, it emits a different pght when one LED is forward biased and the other is reverse biased or vice versa.

Advantages of LEDs

LEDs offer the following advantages −

Quite small in size.

Very fast switching.

Can be operated with very low voltage.

A very long pfe expectancy.

Construction procedure permits manufacturing in different shapes and patterns.

Apppcations of LEDs

LEDs are mostly used in numeric displays indicating the numbers 0 through 9. They are also used in seven-segment display found in digital meters, clocks, calculators, etc.