Light Emitting Diode (LED): How it works?

Light Emitting Diode (LED): How it works?

As the name suggests, it is a diode that emits light is known as the Light Emitting Diode. It abbreviates as LED for general purposes. It releases energy in the form of PHOTONS. This phenomenon is known as electroluminescence. This phenomenon discovered by the H.J. Round in 1907. After the commercial production of LED, it used as a replacement of the incandescent and neon indicator lamps. The LED is a little bit different than the power diode or PN Junction diode. The symbol of the LED is also slightly different than the symbol of a diode.

Symbol

Light Emitting Diode symbol
Light Emitting Diode symbol

There are three regions in LED.

  • P-type region
  • N-type region
  • Active region

The active region placed between the P-type and N-type regions. This region is responsible for the emission of light. It emits light all the way around the layered structure.

This layered structure placed in a tiny reflective cup. Hence, the light gets reflected in the desired direction. The outer structure of the LED is different according to the different types. In below figure shown a cupped reflector LED.

Working principle of Light Emitting Diode

Similar to the diode, LED also works in the forward bias. LED connects in such a way that, the anode made positive with respect to the cathode.

When LED is in forward bias, the electrons in N-region cross the junction and recombine with the holes in P-region.

In conduction band, free electrons are available and in valence band, the holes are available. The energy level of free electrons is higher than the energy level of the holes. Therefore, free electrons move from the conduction band to the valence band.

Light Emitting Diode (LED) Structure
Light Emitting Diode (LED) Structure

The energy level of the valance band is lower than the energy level of the conduction band. While moving, the difference between the energy levels of the conduction band and valence band released by the free electrons.

It appears in the form of light due to the special material used in the LED. The energy released depends on the forbidden gap energy which determines the wavelength and the color of the emitted light.

There are different types of materials used according to the application of phonon and the color of light.

Material

Photon and Color

Gallium Arsenide (GaAs) Infrared
Aluminum Gallium Arsenide Phosphide (AlGaAsP) High-brightness red, orange-red, orange, and yellow
Aluminum Gallium Phosphide (AlGaP) Green
Gallium Phosphide (GaP) Red, yellow and green
Zinc Selenide (ZnSe) Blue
Gallium Indium Nitride (GaInN) Near-ultraviolet, bluish-green and blue
Silicon Carbide (SiC) Blue as a substrate
Gallium Nitride (GaN) Green, emerald green
Aluminum Gallium Nitride (AlGaN) Ultraviolet
Gallium Arsenide Phosphide (GaAsP) Red to infrared, orange

Advantages

  • Small in size. Hence, it is easy to use and place in PCB.
  • Very low voltage and current required to drive an LED.
  • The total power output is very less.
  • Very fast in operation
  • Long life
  • Very less response time
  • Cheap in cost
  • Light in weight
  • Available in various color
  • Easy to interface with various other electronics circuits

Disadvantages

  • Temperature-dependent characteristic
  • It is a current sensitive device. A slight change in current and voltage can damage the LED.
  • LED known to have a much wider bandwidth compared to the LASER.

Application

  • It used in optical devices such as optocouplers.
  • It used as an ON-OFF indicator in various electronics circuits.
  • Some LED’ radiated infrared light which is invisible. But this kind of LED is useful in Remote control and burger alarms.
  • It uses in displays (seven-segment display and alphanumeric display)
  • It used in Watches and calculators

Difference between Light Emitting Diode (LED) and PN junction Diode

LED

PN Junction Diode

In forward bias, it emits light in the form of photons. It cannot emit the lights.
On state voltage is in the range of 1.2V to 2V. On state voltage is in the range of 0.7V for silicon and 0.3V for germanium.
The reverse breakdown voltage is low. It is in the range of 3V to 10V. The reverse breakdown voltage is high. It is in the range of 50V and more.
Needs large power for operation. Needs less power for the operation.
It uses gallium arsenide phosphide and gallium phosphide. It uses a material like silicon and germanium.
Draws more current compared to the PN diode. Draws less current compared to the LED.
It uses in optocouplers, seven-segment displays, alphanumeric displays, and indoor-outdoor lights. It uses in rectifiers, clippers, clampers, voltage multipliers, and many other electronic circuits.

 

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