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LED or Light Emitting Diode

What is Light Emitting Diode?

The light emitting diode is a special type of PN junction diodes. The light emitting diode is specially doped and made of special type of semiconductor. The light emitting diode can emit light when it is forward biased.

Working Principle of Light Emitting Diode

To understand the working principle of light emitting diode, we first have to understand a basic of band gap theory of semiconductor. According to this theory, when an electron comes down from its higher energy level to lower energy level, it emits energy. The emitted energy is equal to the gap between these two energy levels. When a pn junction diode is forward biased, current flows through the diode. The 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 the current. Hence during flow of these charge carriers, there will be recombinations. Light Emitting Diode In normal silicon, germanium semiconductor diode the entire radiation of electromagnetic wave during recombinations is in the form of infrared radiation. The wavelengths of the infrared are out of our visible range so we can not see it. The infrared electromagnetic radiation is nothing but heat. Silicon and germanium semiconductors are not direct gap semiconductors rather these are indirect gap semiconductors.

In indirect gap semiconductor, the maximum energy level of the valence band and minimum energy level of conduction band do not occur at same momenta of electrons. Hence during migration of an electron from the conduction band to the valence band, the momentum of the electron would be changed. The energy will mostly be utilized for the electron momentum. In the direct gap semiconductor, the maximum of the valence band and minimum of the conduction band occur at same electron momenta. Hence, there will be no change of momentum of an electron during migration from the conduction band to valence band so the energy emitted due that recombination has not to provide momentum to the electron. There are some special types of specially alloyed direct energy gap semiconductors whose energy gap between condition and valence band is such that the electromagnetic radiation emitted during recombinations has wavelengths within our visible range. In these semiconductors, the forbidden energy gap is equal to the energy radiated from the electron during recombination. Recombination means an electron in condition band jumps down to the valence band. During this jump electron will emit electromagnetic energy equal to forbidden energy gap Eg. direct gap semiconductor Again according to quantum theory, the energy emitted in form of photons. The energy of a photon is the product of the 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 the velocity of light as f = c / λ. Where λ is the wavelength of the electromagnetic radiation. So we have seen that wavelength of electromagnetic radiation is inversely proportional to the forbidden energy gap. In light emitting diodes, the semiconductors have such forbidden energy gaps that when recombinations between electrons and holes take place, the wavelengths of the emitted energy will be within visible range. This is how light emitting diodes work.

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