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 light when a forward current is allowed to pass through it. The following figure shows the logic symbol of an LED.

LED Symbol

How Does a PN Junction Diode Emit Light?

LEDs are not made from Silicon or Germanium and elements like Gallium Arsenide (GaAs) and Gallium Phosphide (GaP). These materials are deliberately used as they emit light. 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 light energy.

If LEDs are made from Silicon 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 Gallium Arsenide (GaAs) and Gallium Phosphide (GaP) possess enough photons that are sufficient to produce visible light.

  • If LEDs are made from gallium arsenide, they produce red light.
  • If LEDs are made from Gallium Phosphide, then such LEDs emit green light.

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 applied to this circuit, one LED will operate at a time and due to this circuit action, it emits a different light 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 life expectancy.
  • Construction procedure permits manufacturing in different shapes and patterns.

Applications 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.