Extrinsic Semiconductors Video Lecture - Physics Class 12 - NEET

hello friends this video on
semiconductors part 5 is brought to you
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please make sure that you have watched
all the videos in part 4 before going
ahead with part 5 now we will talk about
the second type of semiconductor which
is the extrinsic semiconductor can you
guess what would be helens extrinsic
semiconductor intrinsic semiconductor
was the pure semiconductor so obviously
extrinsic semiconductor would be
something which is impure so what was
the disadvantage what disadvantage did
you feel was there in the intrinsic
semiconductor the disadvantage was that
I am Telling unless you increase the
temperature or until and unless you are
thermally excite the semiconductor it is
same as insulating in extrinsic
semiconductor small amount of a suitable
impurity was added to it what was the
purpose of adding this impurity so that
it can conduct without supplying some
external energy that sounds interesting
right because if you use an extra means
an extrinsic semiconductor will always
be conducting without application of any
external energy sounds good now this
impurity is added in extremely small
amount say a few parts per million
so this ppm is a kind of measuring unit
with safe parts per million that means
when you add something in extremely
small amounts I mean just imagine maybe
you add one impurity out of 10 to the
power 6 impurity and you add one
impurity out of 10 to the power 6 atoms
so that means it is added in extremely
small quantity impurity is added to
increase the conductivity as I told you
because conductivity was the
disadvantage is that with the intrinsic
semiconductor the process of
deliberately adding impurity
is known as doping so remember these new
terminologies and the impurity is known
as dope ain't that so because we are at
we are knowingly adding impurity to a
semiconductor so this process is known
as doping doping is done to increase the
conductivity therefore extrinsic
semiconductor is also sometimes called
as a dope semiconductor that means
basically an intrinsic semiconductor
with addition of some impurity is
becomes an extrinsic semiconductor right
okay so extrinsic semiconductor is more
useful because it has the conducting
properties so this can be put into
several practical applications now let
us look at the impurities or what kind
of impurities do we actually add to the
semiconductor so we will talk about the
dope and sale that means the impurities
which we add a suitable dopant is one
which has the same size as that of the
semiconductor atom same size is a very
crucial thing here
why that's because as I mentioned before
also that here we are talking about
crystal lattices we are talking about
crystals where the atoms and basically
your solid-state physics will talk code
about crystals because it only only
talks about substances which have
periodic arrangement of atoms right so
now let us suppose if I want to sum add
some impurity to a crystal I should add
an impurity in such a way that it should
not affect the structure of the crystal
as a whole now let us suppose if I am
adding a dopant in a silicon crystal so
that silicon crystal consists of
thousands and thousands of silicon atoms
now let us suppose if I remove that
silicon atom and place a very atom an
atom of extremely huge size there so
will that huge atom fit into that space
firstly it will not fit into that space
and secondly if you forcefully try to
fit it into that space it will distort
the structure of the
similarly if you place an extreme small
atom instead of a silicon atom what will
happen there will be lot of empty space
around it correct so that means a
suitable dopant is something which will
have comparable size more or less it
should be of the same size as that of
the semiconductor atom so that is a very
important criteria to select a suitable
dopant now if you look at the periodic
table where does our semiconductor slide
see if you see silicon and germanium lie
in Group four so if you look at this
silicon and germanium line in four of
the periodic table right so if we want
the dopant should have approximately
same size as these of as that of these
semiconductor atoms it the impurity
should lie somewhere nearby so that
means there is one possibility that the
impurity can lie in this group that is
free or there is another possibility
that it can lie in fine because as far
as atomic size is concerned as we go
down the group the atomic size increases
whereas as we move along the period from
left to right the atomic size does not
decrease that much
so it remains almost comparable so that
is why if you compare the atomic size of
boron aluminium or gallium with silicon
or germanium the difference will not be
much so we can replace either with a
group three impurity or a group five in
purity so now we have two possibilities
so if we dope it with a group five
impurity that extrinsic semiconductor is
known as n-type semiconductor and if we
dope it with a group three impurity it
is known as a p-type semiconductor that
means X
jaesik semiconductor is further divided
into two types depending upon the dopant
which is used depending upon what kind
of impurity is added to the
semiconductor
so if Group five impurity is added it is
called n-type if Group three impurity is
added it is called as a p-type
semiconductor so what we basically do
this is how the crystal structure
ancient looks like right and goes wrong
in the in the last few slides right it
will have a valence bonding with the
neighboring atoms now here if you see
these are all silicon atoms but here
just one silicon atom has been replaced
with a pentavalent atom that is a group
five impurity it can be anything it can
be false for us it can be arsenic it can
be antimony so it is replaced with this
atom right so that is how we dope and
extrinsic semiconductor now we will see
how does doping increase the
conductivity what changes take place
when we add this impurity to the
semiconductor thank you please visit
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