light emitting diode

9:50 PM
Let's discuss kind of p-n junction diode which is light emittingdiode. Let's first discuss about a general p-n junction diode how current flows. We can say that in, a p-n junction diode. In a normal p-n junction we can say in forward and holes from p-side. Combine, and constitute current and this is the reason why in reverse biased no current flows because electrons and holes are drifted.
light emitting diode
light emitting diode

Away from the depletion layer we can also state. That when electrons and holes recombine in depletion layer photons of energy equal to delta e-g this is the fermi energy gap. is released. we already discussed that. A p-n junction, remains in thermal equilibrium because of generation and recombination of electron hole pairs. but specifically in. forward biasing of a p-n junction. When current flows and electrons from conduction band drops to

 


Valance band and combines with the holes. In that situation, these electrons will release a photon which is of energy equal to delta e-g. And the number of electron hole recombinations will be equal to number of photon emitted as Corresponding to each combination, of electrons with the hole, a one photon is generated and for a specific element if, delta e-g. Is given of a specific value in electron volt correspondingly the wavelength is emitted like, if we talk for silicon. We know the value of delta e-g is equal to 1 point 1 4 electron-volt. That implies if we talk about the, wavelength of photon emitted. This wavelength can be written as. H-c by delta e-g because we know that photon energy is equal to h c by lambda

So wavelength we can calculate by using, the expression of energy. So here if we substitute the values of constant h-c and if delta e-g is given electron volt it can be written as. 1 2 4 2 3 by 1 point 1 4 electron volt which is equal to 10898 angstrom and this is the wavelength which lies in infrared region. So this is not visible.

Because we know that the range of visible light is approximately from 4000 to 8000 angstrom. So here if the wavelength emitted is in infrared region only. But say if we talk about a specific semiconductor

This is not in your syllabus but, just for, an example I am taking up. For a. gallium-arsenide phasphide semiconductor if we talk. Or for a gallium-arsenide phasphide semiconductor if we construct a diode then we can say for this particular semiconductor the value of Fermi energy gap is approximately 1 point 9 electron volt. And corresponding to this particular energy when photons are emitted



The wavelength of emitted photon can be given as h c by delta e-g. And in this situation the value will be. 1 2 4 2 3 divided by 1 point 9. On calculation it’ll give the value approximately. 6 5 3 8 angstrom and this is the wavelength matching to. Green color of light in the vibg yor this is in the region of the middle range of the visible colors. So here we can say when a diode is constructed by, gallium-arsenide-phasphide-semiconductor as an intrinsic substrate. And in this situation when current flows through the forward biasing zone. Here we can state.

The wavelength of green color will be emitted from the. Junction and this color is emitted. So such kind of diode which emits light wavelength from the depletion layer these are called, light emitting diodes. On the next sheet we’ll see. A realistic situation how a light emitting diode works? Dear students lets discuss a realistic situation. how a light emitting diode works. Here this picture shows you. a light emitting diode. And it is basically a semiconductor junction we can zoom it and see.

Which is enveloped by a transparent polymer shell and, when it is connected through battery in forward biased mode and here you can see a realistic l e d is. 2 terminals 1 is, short in length and other is large in length, the shorter one is, connected to the n side of the junction and. the longer one is connected to the p side of the junction. Now when the switch is closed you can see it starts blowing with green light. And if we just zoom, into the led we can see.

That there is a p-n junction from whom through its depletion layer continuously green light is being emitted and. it is because of continuous recombination of electrons with the holes in the depletion layer. As in forward biasing, this recombination’s continuously take place. And corresponding to each combination of an electron with the whole one photon of, green light is emitted, so here we can say continuous emission of green light takes place.

Similarly such semiconductors can be used to make l e d’s. In which, the Fermi energy gap is. Of the order of specific energy which corresponds to red light of wavelength or yellow light of wavelength. Or blue light of wavelength. And corresponding l e d’s will glow in various colors. So this how light emitting diode works.