Light Emitting Diode and its working:

What is Light Emitting Diode?

Light Emitting Diode
Light Emitting Diode

Light emitting diode or LED is similar to the semiconductor PN Junction diode. When it is forward biased the holes from P type and electrons from N type combine with each other and it emits energy in the form of light and when reverse biased the current does not flow.  

The color of the LED depends on the nature of the semiconductor material used in the manufacturing of the diode. Available LED colors are red, green , blue , yellow , amber and white. 

Symbol of LED:

Symbol of LED
Symbol of LED

Looks like tiny bulb. LED’s are directional devices and it is connected  in the specified direction only. It has two terminals Anode and Cathode. It is shown in the figure above .

Construction of LED:

Construction of LED
Construction of LED

The structure and the construction of LED differs from the normal semiconductor diodes. PN junction is formed by material which have low energy band gap like gallium antimonide, gallium arsenide,  indium antimonide and indium arsenide. The PN junction is then covered with hemispherical shaped shell body made up of transparent solid plastic epoxy resin. 

Some are made with rectangular or cylindrical shaped dome also. This protects the LED from atmospheric disturbances , vibration and thermal shock. The LED is constructed in such a way that the light emitted by the photons in the junction is focused at the top of the dome.
The P type material is the surface of LED. The anode is deposited at the edge of the P type material. Below the P type material N type material is placed and the cathode is made of gold film which is placed below the N type material. Gold film is used for better reflection.

Working of LED:

Working of LED
Working of LED
When it is forward biased, that is when P type is connected with positive terminal and when N type is connected to negative terminal the current starts to flow. So the majority carriers and minority carriers combine each other and it neutralizes the charge carriers in the depletion region which is the junction of P and N type semiconductors.

 When the energy of electrons decreases from higher level to lower level it emits energy in the form of photons. So the movement of majority and minority charge carriers releases some amount of photons in the form of monochromatic light. Its wavelength is in nm which resembles the color of the LED. 

The LED colors depends on the materials used, so depending on the application where it is used the wavelength specifications are customized. This is because of the different energy gaps of different semiconductor materials and the amount of photons emitted with varying frequencies. LED needs very low voltage of about 0.3v to turn on the device.

When reverse biased current does not flow. When the applied external voltage is increased, it permanently damages the device.

V-I characteristics of LED:

V-I Characteristics of LED
V-I Characteristics of LED

Initially only red color LED was used. After some research the different color  LED was formed by using different types of semiconductor material. Each color have particular wavelength.
The following table shows the color ,its wavelength and the semiconductor material used.

Table of colours, wavelength and material used
Table of colours, wavelength and material used


  • Long life time
  • Energy efficient
  • It is made up of solid material, no filament
  • It is environment friendly, does not consist of hazardous materials
  • The brightness and the color can be controlled
  • Low power consumption
  • It does not produce heat


  • LEDs are expensive
  • The color of the object look different than in the sunlight
  • High temperature and over usage can results in permanent device failure
  • Blue light hazard from LED may affect the eyes

Application of LED:

  • Used in general lighting as bulbs
  • Used in traffic light signal
  • Used in vehicles used to dim the light
  • Used in remote controls
  • Used in camera flashes
  • Used in lighted wallpaper
  • Used in horticulture grow lamps

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