What is light emitting diodes

Light emitting diode (LED) is component its convert the electrical signal into a corresponding light that is injected into the fiber. Basically, the light emitter is a key element in any fiber optic system. Essentially LED is a PN junction diode. 

p-n Junctions 

A basically conventional p-n junction is called as homojunction as the same semiconductor material.

The electron-hole recombination occurs in relatively wide layer = 10 micrometers. As the carriers are not confined the immediate vicinity of a junction, hence high current densities cannot be realized.

The carrier confinement problem can be resolved by sandwiching a thin layer of 0.1 micrometers between the p-type and n-type layers. 

 When the carrier confinement occurs due to bandgap discontinuity of the junction. Such a junction is called heterojunction and its device is called a double heterostructure. 

In an optical communication system when the requirement is an LED is 

1) In LED bit rate of 100-200 Mb/sec.
2) In LED optical power in tens of microwatts. 

LED Structures

Heterojunctions

A (heterojunction) is an interface between two adjoining single crystal semiconductors device with different band gap.


Heterojunctions are of two types, Antisotype ( p-n ) or,  Isotype (n-n or p-p)

Double Hetero-junctions (DH) 

In order to achieve efficient confinement of emitted radiation double hetero-junctions are used in LED structures.



In double hetero-junction, the crosshatched regions represent the energy levels of free charge recombination occurs only inactive InGaAsP layer.

A hetero-junction is a junction formed by dissimilar semiconductors. Double heterojunction ( DH) is formed by two different semiconductors on each side of the active region. The figure shows double heterojunction (DH) light emitter.


Double heterojunction emitter ( DH )
The two materials have different refractive indices and different band gap energies. The changes in band gap energies create the potential barrier for both holes and electronsThe free charges can recombine only in narrow, well defined active layer side.

A double hetero-junction (DH) structure will confine both electrons and holes a narrow active layer. Under forward bias, it will be a large number of carriers injected into the active region where they are efficiently confined. 

Main LED materials or Type :

  • Gallium Arsenide ( GaAs ) - infra-red
  • Gallium Phosphide ( GaP ) - Red, yellow and green
  • Aluminum Gallium Phosphide ( AlGaP ) - Green
  • Gallium Indium Nitride ( GaInN ) - near ultraviolet, bluish-green and blue
  • Aluminium Gallium Arsenide Phosphide ( AlGaAsP ) - High brightness red, orange-red, orange and yellow
  • Silicon Carbide ( SiC ) - Blue as a substrate
  • Gallium Nitride ( GaN ) - green, emerald green
  • Zinc Selenide ( ZnSe ) - Blue
  • Aluminium Gallium Nitride ( AlGaN ) - Ultraviolet
Advantages of LED structure :
  • Carrier recombination occurs in a small region 
  • light emission occurs in an optical waveguide, which serves to narrow the output beam
  • Smaller Size
  • Light in weight
  • Longer lifetime
  • Operate very fast
For detailed information

Application of LED diode :
  • Sensor application
  • Mobile application
  • Sign application
  • Indicator
  • Remote  control
  • Optoisolator
  • LED signal
  • Illumination
  • Automotive uses
For detailed information
Read more >> Application of LED