What is LED

Definition of LED :

Nowadays the light is collected from the edge of the full form of an LED, in order to reduce the losses caused by absorption in the active layer and to make the beam more directional. Such a device is known as an edge emitting LED or LED.

Light emitting diode (LED) is a 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. LED has two types of LED structure called has Heterojunction and double heterojunction diode.

One of the major characteristics of an LED is a different color. Initially, various types of LED colors were restricted only red LEDs were available but after semiconductor processes were improved and new research was undertaken to investigate in direction of new LEDs many different colors were available.

Features of LED : 

  • A linear relationship between optical output and current. 
  • Spectral width is 25 to 40 nm or lambda equal to around 0.8 - 0.9 µm.
  • Modulation bandwidth is much large. 
  • Not affected by catastrophic gradation mechanisms hence LEDs are more reliable. 
  • Better coupling efficiency than the surface emitter.
  • Less temperature was sensitive. 
Usage :
  • It is most commonly used for short-range narrow and medium bandwidth links. 
  • Long distance analog links.
  • Suitable for digital systems up to 140 Mb/sec.
Light source material :

In light source material the spontaneous emission due to carrier recombination is called carrier recombination electroluminescence.

To encourage electroluminescence, so it is necessary to select an appropriate semiconductor material. The semiconductors depending on energy bandgap can be categorized into the following way,
  • Direct bandgap semiconductors 
  • Indirect bandgap semiconductors. 
  • In the direct bandgap semiconductors, the electrons, and holes on either side of band gap have the same value of crystal momentum. Hence, direct recombination is ban possible
  • In the direct gap of semiconductors, the maximum and minimum energies occur at different values of crystal momentum. The recombination in these semiconductors is quite slow.
  • The active layer semiconductor material must have a direct band gap to shown in the figure. In the direct bandgap semiconductor, electrons and holes recombine directly without need of the third particle to conserve momentum.
  • In these materials, the optical radiation is sufficiently too much high. 
  • The peak output power is obtained around 810 nm. The width of emission at half power (0.5) is referred to as (full-width half maximum- FWHM) spectral width. For the given LED FWHM is 36 nm.
The fundamental quantum mechanical relationship between gap energy E and the frequency of  v is given as a formula in this form;

 E = hv

 E = hc / ɣ

 ɣ = hc / E

Where energy E is in joules and wavelength is measured in a meter. Expressing the gap energy in electron volts and wavelength in micrometers for this formula.

Types of LEDs according to color:
  • Gallium Arsenide (GaAs) - Infrared
  • Gallium Arsenide  photo-side (GAAsP) - Red to inferred, orange
  • Zine selenide (ZnSe) - Blue
  • Aluminium Gallium  Nitride (ALGaN) - Ultraviolet
  • Silicon Carbide (SiC) - Blue as a substrate
  • Aluminium Gallium Phosphide (AlGaP) - Green
  • Gallium Indium Nitride (GalnN) - Near ultraviolet, bluish-green and blue
  • Zinc Selenide (ZnSe) - Blue
  • Gallium Phosphide (GaP) - Red, yellow, and green 
  • Multicolor LEDs
LED can be used in a variety of application like automotive, mobile devices, Backlighting and projection, General illumination etc. This articles also gives information about various types of application in LED to know more details about LED.

1. Automotive :
  • Headlights
  • Instrument panel
  • Infotainment backlighting
  • Interior lighting
  • RCL
  • General lighting
  • Camera flashes
  • Lighted wallpaper
  • traffic signal
2. Mobile devices :
  • Display back-lighting
  • Mobile phones and display messages
  • Camera flash
3. Back-lighting and projection :
  • The large format TV display
  • Laptop
  • Pocket and data projector
4. General illumination :
  • Residential
  • Industrial
  • Medical data display board
  • Retail display
  • Signs and channel lettering