L10 Forward biasing of diode - Semiconductors, Physics, Class 12 Video

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ahead with part 10 so let us talk about
the forward biasing of PN Junction diode
in detail so what happens when we fall
with bias a PN Junction type let us
suppose before biasing the diode when no
external voltage was applied across the
diode there was a potential difference
between the P side and the N side and
that potential difference was known as
the barrier potential let me denote that
as V naught or we also call that as
barrier height so let me call that as V
naught right now let us suppose when we
apply an external voltage that is when
external voltage is applied say external
voltages V so what will happen this is
positive side which is connected to P
the negative side connected to n now
when this external voltage V is applied
these positive will repel the holes
present in P because P side will have
more holes and holes are also positively
charged so when it is connected to
positive terminal it will get repelled
from this side and it will start moving
the holes will start moving towards the
depletion region similarly if we look at
the N side the electrons will get
repelled from the side due to this
negative terminal and as a result the
electrons of the N side will also start
moving towards the depletion region now
when the holes reach this depletion
region it will combine or it will
recombine with some of these negative
charges
similarly electrons will combine with
some of these positive charges and as
are the size the width of the depletion
region this width will reduce because
some of these charges will get nullified
by because of these holes and these
electrons right so the width is reduced
and also the barrier height will reduce
that's because earlier the barrier high
was v-not but now due to this movement
and recombination of holes and electrons
the potential difference between these
two ends will also reduce so when
external voltage is applied what will
happen depletion width will reduce and
also the barrier height will reduce so
the barrier height will become earlier
what was the barrier height it was V
naught now it will become V naught minus
V so it will reduce by V right now if
the applied voltage if this value of V
is very small in that case what will
happen the width will reduce little bit
there will be a slight decrease in the
depletion width therefore when the width
reduces little bit little more electrons
will be able to pass the junction little
more majority carriers will flow across
the junction as a result the current
will also increase a little bit now when
the applied voltage is very large in
that case what will happen the decrease
in the potential and the decrease in the
depletion width will be very large so
the depletion width will become very
less and as a result the concurrent will
increase even more so that proves that
as the external voltage is increased the
current will also increase so forward
biasing encourage the majority carriers
flow that means as you increase the
forward biasing as you increase the
voltage external voltage in case of
forward biasing you're more and more
majority carriers will flow across the
junction therefore the current will
increase so that means in case of
forward biasing what is happening
electrons are flowing from n region to B
region and holes are moving from P
region to n region so it is encouraging
the flow of majority carriers so in
another way what is happening so in
another way we can
see that majority carriers flow is
encouraged by forward biasing of a PN
Junction diode okay so the same thing we
can interpret in this way as I mentioned
in the previous slide since electron is
flowing from n to P and holes are moving
from P to n that means what is happening
electrons are basically injected into P
that means what does this mean this
means electrons are injected into P
region similarly holes are injected into
n region now what are electrons in P
region electrons are the minority
carriers in P region so we can say that
minority carriers are injected into the
region so this is also known as minority
carrier injection so what happens when
we forward bias a PN Junction when we
forward bias a PN Junction diode it
encourages the majority carrier flow now
when the majority carriers flow from
their region of high concentration
towards the region of low concentration
it also it in turn injects minority
carriers into a junction so that means
forward biasing causes majority carriers
flow across the junction minority
carrier injection occurs both of these
mean the same right because majority
carriers flow across the junction that
means electron which is majority carrier
in n it flows from n to P so this is
called majority carrier flow because
electron is the majority carrier in n so
it is flowing from n to P but if you
look at it from point of view of P what
is happening for P electron is the
minority carrier and electron is getting
injected into P so that means minority
carriers are getting injected into P
region so this is what you can
see here or in the P region you have the
injected electrons and in the N region
you have the injected holes okay so what
is the total current in this case the
total current will be old diffusion
current plus electron diffusion current
because in this when you forward bias as
I told it encourages the majority
carriers and majority carriers always
diffuse diffusion is always related to
majority carriers and drift is always
related to minority carriers so in
forward biasing forward biasing will
always encourage the flow of majority
carriers and majority carriers always
diffuse across the junction so in this
case total current is equal to whole
diffusion current plus electron
diffusion correct thank you please visit
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