Principles of LED

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Diodes


Wonders of LED:

A Light Emitting Diode (LED) is one of the latest inventions and is extensively used these days. From your cell phone to the large advertising display boards, the wide range of applications of these magical light bulbs can be witnessed almost everywhere. Today their popularity and applications are increasing rapidly due to some remarkable properties they have. Specifically, LEDs are very small in size and consume very little power. The magnificent, beautiful, dazzling colors involved with LEDs may be quite picturesque, but do you really know how these effects are actually created in them or rather how do LED light bulbs work?

Red, green and blue LEDs

 

What is it??

As is evident from its name, LED (Light Emitting Diode) is basically a small light emitting device that comes under “active” semiconductor electronic components. It’s quite comparable to the normal general purpose diode, with the only big difference being its capability to emit light in different colors. The two terminals (anode and cathode) of a LED when connected to a voltage source in the correct polarity, may produce lights of different colors, as per the semiconductor substance used inside it.

Working Principle: 

A light-emitting diode is a two-lead semiconductor light source. It is a p–n junction diode that emits light when activated. When a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.

Working in a nutshell:

  • The material used in LEDs is basically aluminum-gallium-arsenide (AlGaAs). In its original state, the atoms of this material are strongly bonded. Without free electrons, conduction of electricity becomes impossible here.
  • By adding an impurity, which is known as doping, extra atoms are introduced, effectively disturbing the balance of the material.
  • These impurities in the form of additional atoms are able either to provide free electrons (N-type) into the system or suck out some of the already existing electrons from the atoms (P-Type) creating “holes” in the atomic orbits. In both ways the material is rendered more conductive. Thus in the influence of an electric current in N-type of material, the electrons are able to travel from anode (positive) to the cathode (negative) and vice versa in the P-type of material. Due to the virtue of the semiconductor property, current will never travel in opposite directions in the respective cases.
  • From the above explanation, it’s clear that the intensity of light emitted from a source (LED in this case) will depend on the energy level of the emitted photons which in turn will depend on the energy released by the electrons jumping in between the atomic orbits of the semiconductor material.
  • We know that to make an electron shoot from lower orbital to higher orbital its energy level is required to be lifted. Conversely, if the electrons are made to fall from the higher to the lower orbitals, logically energy should be released in the process.
  • In LEDs, the above phenomena is well exploited. In response to the P-type of doping, electrons in LEDs move by falling from the higher orbitals to the lower ones releasing energy in the form of photons i.e. light. The farther these orbitals are apart from each other, the greater the intensity of the emitted light.

Different wavelengths involved in the process determine the different colors produced from the LEDs. Hence, light emitted by the device depends on the type of semiconductor material used.
Infrared light is produced by using Gallium Arsenide (GaAs) as a semiconductor. Red or yellow light is produced by using Gallium-Arsenide-Phosphorus (GaAsP) as a semiconductor. Red or green light is produced by using Gallium-Phosphorus (GaP) as a semiconductor.

Advantages of LEDs:

1. Very low voltage and current are enough to drive the LED.

Voltage range – 1 to 2 volts. Current – 5 to 20 milliamperes.

2. Total power output will be less than 150 milliwatts.

3. The response time is very less – only about 10 nanoseconds.

4. The device does not need any heating and warm up time.

5. Miniature in size and hence lightweight.

6. Have a rugged construction and hence can withstand shock and vibrations.

7. An LED has a lifespan of more than 20 years. 

Disadvantages:

1. A slight excess of voltage or current can damage the device.

2. The device is known to have a much wider bandwidth compared to the laser.

3. The temperature depends on the radiant output power and wavelength.

To learn more on electricity and telecommunication read here!

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