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# LED or Light Emitting Diode

## What is light emitting diode?

The pn junction diode, which is specially doped and made of special type of semiconductor, emits light when it is forward biased is called light emitting diode.

## Working principle of light emitting diode

To understand the working principle of light emitting diode, we first have to understand a basic of quantum theory. According to this theory, when an electron comes down from its higher energy level to lower energy level, it emits energy in form of a photon. The energy of this photon is equal to the energy gap between these two energy levels. When a pn junction diode is forward biased, current flows through the diode. Flow of current through the semiconductor is caused by both flow of free electrons in opposite direction of current and flow of holes in the direction of current. Hence during flow of these charge carriers, there will be recombinations. Recombination mean electrons in condition band jump down to the valence band. During this jump electron will emit electromagnetic energy in form of photons whose energy is equal to forbidden energy gap Eg. Again according to quantum theory, energy of a photon is the product of frequency of electromagnetic radiation and Planck constant. Where h is Planck constant. Again velocity of electromagnetic radiation is fixed and it is equal to the speed of light i.e. c. The frequency of radiation f is related to velocity of light as f = c / λ. Where λ is wavelength of the electromagnetic radiation. Hence from equation (1) So we have seen that wavelength of electromagnetic radiation is inversely proportional to the forbidden energy gap. In normal silicon, germanium semiconductor this forbidden energy gaps between condition and valence band are such that entire radiation of electromagnetic wave during recombinations is in the form of inferred radiation. The wavelengths of the inferred are out of our visible range so we can not see it. Inferred electromagnetic radiation is nothing but heat. This is because, silicon and germanium semiconductor are not direct gap semiconductor rather these are indirect gap semiconductor. In indirect gap semiconductor the maximum energy level of valence band and minimum energy level of conduction band do not occur at same momenta of electrons. Hence during recombinations of electrons and holes that is migration of electrons from conduction band to valence band the momentum of electrons would be changed. The photons originated from these electrons will be mostly utilized for the electron momentum. In direct gap semiconductor the maximum of valence band and minimum of conduction band occur at same electron momenta. Hence, there will be no change of momentum of electrons during migration from conduction band to valence band so the photons originated due that migration have not to provide momentum to electrons. As a result, the photons are emitted from the surface of semiconductor crystal. There are some special type of specially alloyed direct energy gap semiconductors whose energy gap between condition and valence band are such that the electromagnetic radiation emitted during recombinations has wavelengths within our visible range. That means in these special semiconductors when recombinations between electrons and holes occur, there will be emissions of light. This is how a light emitting diode works.

The wave length of output optical signals depends upon the band gap energy. The output wave length can be engineered within certain limits by using compound semiconductors, so that a particular color can be observed, provided the output is in visible range.

## Application of LED or Light Emitting Diode

Today almost everywhere LEDs lights are used and the application of LED is huge. First we are going to see through the list, then we will categorize the application of these.
• In motorcycle and bicycle lights.
• In traffic lights and signals.
• In message displaying boards.
• In light bulbs and many more.
Now, practically if we sit to list all the applications it will be a non-ending list. So, here we are classifying the use in to some parts.
1. Indicators and Signs:- These are mainly used in traffic signals, exit signs, light weight message, displaying box etc
2. Lighting:- Light Emitting Diode lamps have become highly popular and as the energy consumption is very low for them, they are also being made by LED s. In 2001, the Italian village Torraca was the first place to convert all its lighting to LED. In television and computer/laptop displaying, LEDs are used.
3. Non Visual Application:- Communication, sensor are the main area of non visual application of LEDs.

## Advantages of LED or Light Emitting Diode

If anybody compares LEDs to other illumination methods present in the market now days it will be found that LED lighting in by far the most saving solution. In modern era of technology, there is an up gradation from analog to digital. You can say LED is digital light which has huge advantages over conventional analog lights. The main advantages are briefly described below.
1. Size :- Sizes of Light Emitting Diodes are from 3 mm to 8 mm long. The small size allows them to be used in small spaces where tube lights cannot be used. Because of its small size, various designs can be made very simply.
2. Larger lifetime :- This is the number one benefit of LEDs lights. As an example a high power white LEDs life time is projected to be 35,000 to 50,000 hours. Where as an incandescent bulbs life time is 750 to 2,000 hours. For compact fluorescent bulbs, the life time is 8,000 to 10,000 hours. Actually unlike standard lighting LEDs do not burn out. They just gradually fade.
3. Lower Temperature :- LED's mechanism does not consists of any step to produce heat. In conventional lights, the production of heat are very common fact. They waste most of their energy as heat. They remain cool.
4. Energy Efficiency :- Light Emitting Diode is today’s most energy efficient way of lighting its energy efficiency is nearly 80% to 90% whereas traditional lights have 20% energy efficiency, 80% is lost, as heat. More over the quality of lighting is very good.
5. Design Flexibility :- LEDs can be merged in any shape or combination. They can be used in singly as an irony. Single LED can be operated, resulting in a dynamic control of light. Superb lighting effects of different colors can be achieved by well designed LED illumination system.
6. Ecologically Friendly:- LED lights do not contain any toxic chemical. They do not leave any toxic material and 100% recyclable. Their illuminations are close to no UV emission. The solid package of it can be designed to focus its light also.
7. Color:- LEDs can be emit light of intended color this is done by charging the compositions of the solid state materials doping without using any color filter.
8. On/Off Time:- Light Emitting Diodes can be operated very quickly. They can be used in frequent on/off operation in communication devices.

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