Di means two and odes means
terminals. Thus PN junction diode is a device who has a junction made up of P and N type materials with two terminals.
The P type material consists of a lot of positive charges and N type material
consists of negative charges.The junction doesn’t means a sandwich of the two
materials and there is a procedure to make this PN junction diode.
A pure semiconducting material is
taken and from one side trivalent impurity and from other side a pentavalent
impunity has been added. At the junction there is a region formed depletion
region. At this region some holes from the P material will shift towards the N
side. It is simply because there is a repulsion from the P-side and there is attraction from the other N side
material.
This kind of the transfer will happen only with the first batch of the charges and it is not going to happen further. The logic behind this is simple. The first batch of the charges who have transferred to the opposite side will oppose the flow of the further transfer of the charges.
This kind of the transfer will happen only with the first batch of the charges and it is not going to happen further. The logic behind this is simple. The first batch of the charges who have transferred to the opposite side will oppose the flow of the further transfer of the charges.
Thus there is
a region formed which is a neutral region and the region is called as depletion
region. This region obstruct the further flow of any of the charges from one
site to another side hence it is going to behave like insulator.
There is a potential difference developed at the junction which is opposing the flow of the current because of this depletion region and the potential difference can also be called as barrier potential. The value of the barrier potential depends on the nature of the material.For silicon it is around 0. 5 to 0.7 volts whereas germanium it is around 0.2 V.
There is a potential difference developed at the junction which is opposing the flow of the current because of this depletion region and the potential difference can also be called as barrier potential. The value of the barrier potential depends on the nature of the material.For silicon it is around 0. 5 to 0.7 volts whereas germanium it is around 0.2 V.
Forward Bias
When you don’t connect a
battery there is no possibility for the flow of the charges from one side to
another side and it behaves like insulators.Not only this device, every
electronic device behaves like a insulating material when you don’t connect a
battery because batteries the driving force who can make the current flow from
one place to another place.
When you are connecting the
battery how to options.The positive plate of the battery can be connected to
the P type material of the diode and the negative plate connected to the N type
material. This way of connection is called as forward biased connection and
this enables the flow of the charges from one site to another side. It is
simply because the positive plate of the battery a positive charge starts.It starts
repelling the positive charges of the holes and push the width of the depletion
layer to a smaller value. The same will happen from the other side also.
Simultaneously from the negative plate of the battery electron starts and they start repelling the electrons of the N type material.Hence the thickness of the depletion layer will decrease significantly.Thus there is a convenient environment for the flow of the charges and hence the device is going to behave like a conducting material.
Simultaneously from the negative plate of the battery electron starts and they start repelling the electrons of the N type material.Hence the thickness of the depletion layer will decrease significantly.Thus there is a convenient environment for the flow of the charges and hence the device is going to behave like a conducting material.
In forward biased connection there
will be flow of current in the devise only when the applied voltage is more
than the barrier potential.Until then the applied voltage is not sufficient to
overcome the barrier potential and hence there is no flow of current. That
means when the applied voltage is less than barrier potential the device is
going to behave like insulating material.
Once if the applied voltage is more than the barrier potential with the increase of the applied voltage the charges will get more energy to flow from one side to another side and hence the current flow will be automatically increasing.The corresponding graph is as shown.
Once if the applied voltage is more than the barrier potential with the increase of the applied voltage the charges will get more energy to flow from one side to another side and hence the current flow will be automatically increasing.The corresponding graph is as shown.
Reverse Bias
If the positive plate of the
batteries connected to the N type material and a negative plate of the batteries
connected to the P type material that kind of the connection is called as
reverse biased connection. In reverse biased connection the charges the battery
will attract the charges of the P and N type materials back towards the plates
and hence the width of the depletion layer increases further.This makes
the flow of the current further very
difficult and hence it is going to behave like insulating material.
Break Down Potential
Therefore the PN junction diode
in forward bias acts like a conducting body and in reverse bias it behaves
like insulating material.Though it is behaving like insulating body during the
reverse biased connection, with the increase of the voltage in the reverse
biased connection the width of the depletion level further increase and makes
the flow of the correctness even more difficult.
This will be keep on happening until the depletion layer width has reached its maximum. Once if this is happened, there is no further scope for the increase of the depletion layer and if the charges were given enough energy to cross that larger depletion layer, we can make flow of current from one side another side. This is possible only when you apply a very high-voltage the reverse biased connection and that high-voltage is called as break down potential.
This will be keep on happening until the depletion layer width has reached its maximum. Once if this is happened, there is no further scope for the increase of the depletion layer and if the charges were given enough energy to cross that larger depletion layer, we can make flow of current from one side another side. This is possible only when you apply a very high-voltage the reverse biased connection and that high-voltage is called as break down potential.
Once if the break down potential
is applied all the charges at both the sides will get enough energy to cross
from one side to another side and hence the current flow increases abnormally
and there is no need of the increase of the voltage further.
Thus if the breakdown voltage is applied,there is an abnormal increase in the current and the charges doesn’t need any
further voltage therefore.Hence the voltage further remains constant and there is a
big increase in the flow of the current. This can be noticed in the graph drawn
that after reaching the breakdown voltage the graph is just right line parallel
to y-axis along which the current is shown.
There is one practical problem
with the reverse biased connection of the PN junction diode. Though it is
theoretically possible that it can flow the current in the reverse bias, when
the applied voltage is more than the breakdown voltage.But practically that is not
possible to reach.The PN junction diode is made with such a sort of material that
it cannot withhold the breakdown voltage. Even before you apply the breakdown
voltage the diode will damage therefore we can never operate in the reverse
Bias and for all practical purposes can operate it only in the forward bias.
Distance and displacement comparison
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