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 |
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.