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PN junction Diode Explained | Forward Bias and Reverse Bias - YouTube
Channel: ALL ABOUT ELECTRONICS
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hey friends welcome to the YouTube
channel all about electronics in the
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previous video we have seen that what is
p-type and n-type semiconductors. and we
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have seen that in a case of a p-type
semiconductor the holes are the majority
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carriers and electrons are minority
carriers. while in case of the n-type
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semiconductor the electrons are majority
carriers and holes are minority carriers.
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now by itself this p-type and n-type
semiconductors acts like a resistor, but by
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doping a one side of silicon crystal
with a p-type impurity and the other
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side by the n-type impurity, we can
convert the silicon crystal into the PN
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Junction. And here this Junction is the
border where the p-type and n-type
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region meets. And this silicon crystal
with p and n-type impurity is also known
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as the diode because here the p-type and
n-type region acts like a two electrodes. so
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understanding this PN Junction we can
understand all kinds of semiconductor
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devices. so in this p-type region each
circled negative sign represents the
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trivalent atom and the positive sign
represents the hole. similarly in the
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n-type region each circled positive sign
represents the pentavalent atom and each
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minus sign represents the electron. And
here for the simplicity in both regions
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the minority carriers are not shown but
in very small quantity they are also
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present in both regions. now due to the
doping there is a large number of
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electrons in the N side but very few
electrons on the P side, because on the P
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side the electrons are minority carriers
and whenever this P and N regions are
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joined then the electrons from the N
side starts diffusing towards the P side.
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So whenever the electron enters the
p-type region then it becomes the
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minority carrier and being surrounded by
the so many holes its lifetime will be
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very short and due to that it will get
recombined with these holes.
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So whenever the electron diffuses across
the junction then it creates the two
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ions. so first whenever it leaves this
n-type region then it leaves behind this
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pentavalent atom which is just one short
of electron. so due to that it will
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become a positive ion and whenever the
electron enters this p-type region then
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it will get recommend with the hole of
this trivalent atom. so after capturing
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the electron this traivalent atom will
become a negative ion. so every time the
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electron diffuses to the P side from the
N side then it creates one positive and
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the negative ion near the junction. now
these ions are the immobile ions and
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unlike the free charge carriers like
electrons and the holes they cannot move
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so due to the recombination of this
holes and electrons near this Junction
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there is hardly any free charge carriers
near this injunction. or we can say that
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due to the recombination this Junction
is depleted from the free charge
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carriers and due to that this region is
also known as the depletion region. so
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this depletion region contains both
positive and the negative ions, which are
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immobile in the nature and apart from
that it also contains a few charge
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carriers which are thermally generated
in this depletion region. now in this
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depletion region these ions sets up the
electric field. And this electric field
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points from the positively charged ions
towards the negatively charged ions and
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due to this electric field only few
electrons from the N side are able to
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cross this depletion region and the same
is true for the holes in this p-side
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region or we can say that these immobile
ions creates the barrier potential for
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this majority carriers, so that they are
not able to diffuse from one side to the
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other side. so this barrier
potential is also known as the built-in
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potential and for the silicon this
built-in potential is equal to 0.7 volt
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while for the germanium it is roughly
around 0.3 volt. so the free charge
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carriers hard to be precise the majority
carriers in both regions has to overcome
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this barrier potential to cross this
depletion region. And only few majority
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carriers are able to cross this barrier
whenever there is a no external biasing
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but due to this electric field, the
minority carriers in the both regions
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will be able to cross this depletion
region. For example, the holes which are
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minority carriers in this n-type region
will get swept from the n-type region
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towards the pre type region and they
will become majority carriers in this
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p-type region
similarly the electrons which are a
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minority carriers in this p-type region
will get swept due to this electric
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field in this n-type region and once
they enter this n-type region they will
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become the majority carriers. so due to
this built in electric field the
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majority carriers are not able to cross
this Junction but still the minority
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carriers will be able to go from one
region to the another region. But when
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there is a no external biasing is
applied then the flow of electron due to
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the electric field and due to the
diffusion will get canceled out to each
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other or in other words we can say that
the flow of minority charge carriers and
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the majority charge carrier will cancel
out each other, so that the overall
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current in the circuit will be zero. And
the same thing is also applicable for
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the holes. so whenever this PN Junction
is not biased then the overall current
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in the circuit will be zero, so if this
majority charge carriers on both N and P
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side wants to cross this depletion
region then they require the external
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biasing voltage. so if we apply the
external field in the same direction as
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the built-in electric field, in that case
this depletion region will provide a
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more resistance to the majority carriers.
on the other end
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if the applied external field is in the
opposite direction to the built-in
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electric field in that case the
resistance which is offered by the
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depletion region will reduce. so based on
that let us see the two types of biasing
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for this PN Junction. And first let us
talk about the forward bias PN Junction.
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so in case of a forward bias, the
positive terminal of the source is
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connected to the P side and the negative
terminal is connected to the N side. And
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in this forward bias condition the
external electric field is in the
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opposite direction to the built-in
electric field, and due to that the
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effective electric field at the junction
will reduce. so in the forward bias
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condition the electrons in the n-type
region and the holes in the p-type
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region will get pushed towards the
junction. so due to that the width of the
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depletion region will reduce. so the
effective resistance which is offered by
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the depletion region will also reduce. so
if you further increase the external
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applied voltage then the width of the
depletion region will further reduce and
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when the applied voltage is more than
the barrier potential of this PN
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Junction then the resistance that is
offered by the depletion region is
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negligible. so for example for the
silicon crystal if the applied external
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voltage is more than the 0.7 volt, in
that case, the resistance that is offered
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by the depletion region will be
negligible and in this condition the
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electrons from the N side can cross this
depletion region. And once they cross
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this depletion region then they will get
attracted towards the positive terminal
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of the battery. so once they come into
the p-side region then just by passing
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through the holes in the p-type region
they can reach the positive terminal of
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the battery. And similarly, the holes will
be pushed towards the depletion region
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and once they enter the n-side region
then they will get attracted towards the
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negative terminal of the battery. so in
this way we get a flow of current due to
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the movement of holes as well as
electrons. And as we increase the
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externally applied biasing voltage the
more and more electrons and the holes
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will be able to cross this depletion
region. And due to that they will
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contribute in the flow of current. so as
we increase the externally applied
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voltage then the flow of current in the
circuit will increase. Now before we go
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ahead let me just clear one thing. The
hole is nothing but the absence of
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electron at the particular location.
so whenever the electrons are moving
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from one place to the other place or
here from right to the left then in a
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way we can say that the hole is also
moving from left to the right so due to
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the movement of electrons we also get
the moment of holes. And in this way, we
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get a flow of current due to the
electrons as well as holes. So now
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similarly let us see when the PN
Junction is reversed biased. so in the
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reverse bias condition, the negative
terminal of the battery is connected to
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the P side and the positive terminal of
the battery is connected to the N side
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so in this condition, the electrons which
are majority carriers in this n-type
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region will get attracted towards the
positive terminal of the battery and
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similarly the holes on the P side region
will get attracted towards the negative
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terminal of this battery. And due to that,
the more ions will get created near the
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junction so we can say that the width of
the depletion region will increase. so in
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the reverse bias condition, as we
increase the reverse bias voltage the
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width of the depletion region will
further increase and due to that now
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this depletion region offers a more
resistance to these majority carriers.
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and due to that virtually there is a no
flow of current due to the majority
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carriers but in this case due to the
built-in electric field, minority
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carriers in the both regions will be
able to cross this depletion region. so
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in this reverse bias condition the holes
which are minority carriers in the
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n-type region will cross this barrier
and
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we'll reach to the p-side region and
once they reach this p-side region then
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they will get attracted towards the
negative terminal of the battery
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similarly the electrons in the p-type
region which are a minority carriers
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will be able to cross this depletion
region and after crossing this depletion
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region they will reach to the n-side
region. And once they reach this inside
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region then they will get attracted
towards the positive terminal of the
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battery so in this way in this reverse
bias condition we will get a flow of
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current due to the movement of these
minority carriers. But as the minority
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carriers are very less in comparison to
the majority carriers, the magnitude of
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the current in this reverse bias
condition will be very low compared to
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the forward bias condition. So this flow
of current which exists in this reverse
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bias condition is known as the reverse
saturation current and that the term
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saturation comes from the fact that it
reaches the maximum level very quickly
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and it does not change significantly
whenever we increase this reverse bias
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voltage. now typically these reverse
saturation current used to be the range
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of micro amperes but nowadays due to the
advancement in the technology for the
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silicon devices this reverse saturation
current used to be in the range of nano
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amperes and here let's denote these
reverse saturation current as Is. So this
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reverse saturation current is a function
of temperature. so as the temperature
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rises, the thermally generated electron
hole pair in the silicon crystal will
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increase or we can say that the minority
carrier charges in the silicon crystal
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will increase and due to that this
reverse saturation current will also
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increase. So for silicon, this reverse
saturation current almost gets doubled
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for the every 10 degree rise in the
temperature or we can say that for the 1
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degree rise in the temperature this
reverse saturation current increases by
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7 percent. so let's say for a one PN
Junction
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if this reverse saturation current is 20 nano
ampere at 25 degree centigrade then at
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35 degree centigrade it will be roughly
around 40 nano ampere. so after every 10
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degree rise in the temperature these
reverse saturation current almost gets
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double. now as I said earlier this
reverse saturation current does not
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change much even if we increase the
reverse bias voltage. but there is a
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limit on the maximum reverse voltage
which can be applied to this PN Junction
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so if we continuously increase this
reverse voltage then we will reach a
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point which is known as the breakdown
voltage. So once the breakdown voltage is
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reached for the diode then suddenly a
lot of minority carriers will appear at
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the depletion region and suddenly the
diode conducts very heavily and we will
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see in detail in the next video that
what happens to this PN Junction diode
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whenever it is used in this breakdown
region
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so whenever the diode is operated in the
reverse bias condition then the applied
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voltage should be less than this
breakdown voltage and in detail we will
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talk about it in the next video. so I
hope in this video you understood what
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is PN Junction and how it can be
operated in the forward and reverse bias
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condition. so if you have any question or
suggestion do let me know here in the
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comment section below. if you like this
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