Download "Physics Semiconductors part 8 (PN P-N Junction) CBSE class 12 XII"
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Russian
00:00:00
hello friends this video on00:00:02
semiconductors part eight is brought to00:00:05
you by exam feel calm no more fear from00:00:08
exam please make sure that you have00:00:10
watched all the videos tuned pad 700:00:12
before going ahead with part 8 we will00:00:15
now talk about a PN Junction formation00:00:18
so what is a PN Junction so gradually00:00:22
you if you see we started with the very00:00:24
basics so now we will gradually get into00:00:28
complex test however this is not complex00:00:31
but now we till now we understood what00:00:33
is p-type semiconductor what does n-type00:00:36
semiconductor now what we will see is if00:00:39
we combine a p-type semiconductor with00:00:41
and n-type semiconductor what will00:00:44
happen PN Junction will be formed that00:00:47
means there will be a junction which00:00:49
will be formed in between let us look at00:00:52
what is a PN Junction PN Junction is00:00:55
like the building block of semiconductor00:00:58
devices so the speak you should00:01:01
understand this concept of PN Junction00:01:03
very clearly because this will be the00:01:06
same concept on this concept itself you00:01:08
build everything else when we talk of00:01:11
diodes when we talk of transistors I00:01:13
mean those were the things which I00:01:15
showed you in the slide for introduction00:01:17
that why are we studying this lesson so00:01:19
if you understand this PN Junction you00:01:21
will be able to understand those things00:01:22
very easily so p-type and n-type00:01:25
semiconductor00:01:27
connected back to back so that gives00:01:30
rise to a PN Junction now what happens00:01:33
in this case we know that in case of a00:01:36
p-type semiconductor number of holes is00:01:39
very high whereas in case of an n-type00:01:43
semiconductor number of electrons is00:01:46
very high right so that means if you00:01:50
look at it here in this region holes are00:01:54
too many whereas in this region there00:01:57
are too many electrons that means there00:01:59
is a difference in concentration in the00:02:02
two regions if we think of number of00:02:04
holes holes are very high in00:02:07
concentration in P region than in n00:02:10
region now due to the00:02:12
difference and concentration in P region00:02:14
and n region what happens diffusion00:02:17
takes place you know diffusion right00:02:20
what does diffusion diffusion means00:02:22
movement of particles from one region of00:02:25
higher concentration towards a region of00:02:28
lower concentration00:02:29
so here diffusion takes place because P00:02:32
region has high concentration of holes00:02:35
and n region has low concentration of00:02:37
holes so what happens holes start moving00:02:40
from how will the holes diffuse holes00:02:46
will diffuse from which region to which00:02:49
region from region of higher00:02:51
concentration that means from region of00:02:53
higher concentration of holes so that is00:02:56
P so it will move from P to n whereas00:02:59
electrons will diffuse from region of00:03:02
higher concentration of electrons that00:03:04
is from n to P so this diffusion this00:03:08
electron diffusion and hold the fusion00:03:10
with starts taking place right whenever00:03:15
you connect a P and n type semiconductor00:03:17
back to back electron diffusion and hole00:03:21
diffusion will start taking place now00:03:23
what will happen as these diffusion00:03:26
takes place now as a whole diffuse from00:03:29
P region to n region as soon as a whole00:03:32
diffuse from here to here it leaves00:03:34
behind a negative immobile charge00:03:37
similarly when an electron diffuse from00:03:40
n region to P region it leaves behind a00:03:43
negative immobile charge I am sorry00:03:46
a positive a mobile charge somewhat like00:03:48
this let us suppose this is P region and00:03:50
this is n region now electrons from here00:03:54
are moving towards P so when the00:03:57
electron moves what is left behind some00:04:00
positive charge is left behind similarly00:04:02
when a hole moves from P to n where hole00:04:06
moves what is left behind a negative00:04:09
charge peg so that means as this00:04:11
diffusion takes place some negative00:04:14
charge are left behind in P region and00:04:16
some positive charge are left behind00:04:19
in n-region and these charges are00:04:21
immobile because they are bounded to the00:04:25
neighboring atoms so they cannot move00:04:27
right and these charges give rise to a00:04:32
region for example as electron diffuses00:04:37
when electron diffuse it leaves behind00:04:44
positive immobile charge so these00:04:52
positive emails are j-- gives rise to00:04:56
positive space charge region on n side00:05:05
that means let us suppose this was the00:05:10
separation between P and n-type region00:05:13
now as the electrons move from n region00:05:16
the positive charges were left behind in00:05:19
the N region so these positive charges00:05:21
in the end on the N side is known as the00:05:25
positive space charge region on n side00:05:27
similarly as the poles diffused00:05:31
similarly has holes diffuse they leave00:05:34
behind negative in mobile charge and00:05:36
these gives rise to negative space00:05:39
charge region on the fill cycle now00:05:42
these positive space charge region and00:05:44
negative space charge region together is00:05:47
known as the depletion region so why is00:05:53
this moon as the depletion region00:05:55
because inside this region this region00:05:57
consists thought all in mobile charges00:06:01
that means they cannot conduct current00:06:03
that is why it is called depletion00:06:04
region that means it is depleted of00:06:07
conduction so this depleted region is00:06:10
made up of the immobile positive and00:06:13
negative charges so that means diffusion00:06:18
why does diffusion happen diffusion00:06:20
happen when p-type and n-type00:06:23
conductors are connected to each other00:06:25
as a result of diffusion what happens00:06:29
depletion region is formed right okay so00:06:35
what did we study on this slide we00:06:37
talked about depletion region that is00:06:39
the space charge region on either side00:06:42
of the junction diffusion of majority00:06:46
charge carriers give rise to depletion00:06:49
region okay another important thing to00:06:51
note here is diffusion always happens00:06:54
with majority charge carriers because as00:06:56
I mentioned you before diffusion will00:06:59
always take place from region of higher00:07:01
concentration to lower concentration so00:07:03
if I am saying that electron diffusion00:07:06
is taking place that means electron is00:07:08
moving from a region where electron00:07:10
concentration is higher that means n00:07:12
region so n region for n region electron00:07:16
is the majority carriers right so00:07:19
diffusion always takes place of majority00:07:22
charge carriers so diffusion of these00:07:24
majority charge carriers give rise to00:07:27
depletion region fine so are you00:07:31
understanding what will happen if you00:07:33
connect a p-type and n-type together00:07:35
first of all diffusion of majority00:07:38
charge carriers will start taking place00:07:39
so the majority charge carriers will00:07:42
start flowing across the junction as a00:07:44
result at the junction you will have00:07:46
some positive charges and negative00:07:48
charges which are together which will00:07:50
together form the depletion region now00:07:53
what will happen as a result of them00:07:56
depletion region if you look at it this00:08:01
depletion region also consists of00:08:03
positive charges on one side and00:08:05
negative charges on the other side so as00:08:08
a result an electric field will be00:08:11
established for this depletion region00:08:15
there will be an electric field00:08:18
established right and because of this00:08:22
electric field it will give rise to00:08:25
drift of minority charge carriers that00:08:28
means this will be the direction of00:08:30
electric field now what will happen00:08:36
than negative charges which are present00:08:38
on the p-side those negative charges00:08:41
will start moving in a direction00:08:43
opposite to the direction of the00:08:45
electric field that means the electrons00:08:49
which are present in the P region these00:08:52
electrons will move in a direction00:08:54
opposite to this electric field so that00:08:55
electron drift will take place along00:08:58
this direction similarly the holes which00:09:02
are present here they are positive00:09:04
charges so they will move along the00:09:06
direction of electric field so the for00:09:08
drift will take place in this direction00:09:11
so what do we see we see that diffusion00:09:15
gives rise to depletion region and00:09:18
depletion region gives rise to drift so00:09:22
drift of minority charge carriers find00:09:27
is it clear to you because if you see00:09:30
the majority charge carriers are busy in00:09:33
diffusion so all the majority charge00:09:35
carriers are moving across the junction00:09:38
now as this diffusion keep taking place00:09:41
the width of these depletion region keep00:09:44
increasing because as more and more00:09:47
diffusion takes place more and more00:09:49
eMobile charges will be created00:09:51
therefore the depletion region will00:09:53
increase in width now as the width of00:09:56
the depletion region increases the00:09:58
strength of the electric field will also00:10:00
increase now as the strength of the00:10:02
electric field increases the drift of00:10:05
the minority charge carrier will also00:10:07
increase is that correct00:10:09
oh so that means let me note it down00:10:15
whatever I spoke so diffusion happens00:10:19
whenever we connect a p-type and n-type00:10:21
together because there is a different in00:10:24
concentration of electrons and holes so00:10:26
diffusion gives rise to depletion region00:10:33
so that depletion region width as00:10:36
diffusion increases depletion region00:10:39
width increases as a result the electric00:10:42
field strength also increases right and00:10:48
as a result00:10:50
then drift current also increases so if00:10:56
you see when a PN Junction is formed00:10:58
that means when you join in a p-type and00:11:00
an n-type semiconductor there are two00:11:03
categories of currents involved one is00:11:05
diffusion current one is drift current00:11:08
diffusion current is due to the majority00:11:11
charge carriers and drift current is due00:11:13
to the minority charge carriers right so00:11:18
what do we see initiating diffusion00:11:21
current is large and drift current is00:11:23
Swan that's because initially there was00:11:26
no depletion region as soon as you join00:11:28
a p-type and n-type you will not get a00:11:30
depletion region as the diffusion keeps00:11:33
taking place the depletion region will00:11:35
gradually form right so initially your00:11:38
drift current is very small because00:11:40
there is no depletion region but the00:11:42
diffusion current is large because as00:11:44
soon as you join p-type and n-type due00:11:46
to the difference in concentration00:11:47
electron diffusion and hole diffusion00:11:49
will start but later what happens00:11:52
diffusion current diffusion current00:11:56
drift current keeps increasing and it00:11:59
reaches a stage when the drift current00:12:00
becomes equal to the diffusion current00:12:02
and at this stage we see that a PN00:12:06
Junction is under equilibrium so a PN00:12:09
Junction is under equilibrium when the00:12:12
width of the depletion region is such00:12:14
that the electric field strength is just00:12:18
enough to overcome the diffusion current00:12:22
because here you can see that the00:12:24
direction of diffusion current and drift00:12:26
current are opposite to each other so if00:12:28
the electric field strength increases in00:12:31
such a way that it reaches a point when00:12:33
this electron depth is exactly equal and00:12:36
opposite to the electron diffusion then00:12:38
what will happen00:12:39
there will be no net current because00:12:42
diffusion current and drift current00:12:44
being equal and opposite will cancel00:12:46
each other right so you understood this00:12:51
concept of diffusion current and drift00:12:54
current okay so now let us talk about00:12:57
that PN Junction and the equilibrium00:13:00
when00:13:01
the injunction is at equilibrium what00:13:03
happens well now at equilibrium what00:13:10
will happen the net current through the00:13:14
PN Junction will be equal to zero that's00:13:17
because at equilibrium that electron00:13:20
current the electron diffusion current00:13:23
will be equal to the electron drift00:13:24
current00:13:25
similarly the pole diffusion current00:13:27
will be equal to the whole trip track so00:13:30
therefore the net current will be equal00:13:32
to zero however there will be a00:13:35
potential difference between P and n00:13:39
however a potential difference exists00:13:43
between N and P but why does a potential00:13:49
difference exists between N and P that's00:13:51
because the electrons moved from n to P00:13:57
right the major right now I'm talking00:14:00
about the diffusion of electrons the00:14:02
majority charge carriers so the majority00:14:04
charge carriers moved from n to P so00:14:07
what happens to n n became positive that00:14:11
means n side region became positive00:14:14
right holes moved from P to n since so00:14:21
many holes moved from P to n what00:14:24
happened inside becomes negative so that00:14:28
means n region is positive so n is00:14:32
positive with respect to P so this00:14:39
results in a potential difference00:14:41
between N and P right so this potential00:14:45
difference restricts further flow of00:14:47
electron from n to P is that right00:14:51
why does I mean try to understand it in00:14:54
this way as soon as you join a p-type00:14:57
and n-type due to the difference in00:14:59
concentration diffusion starts that is00:15:02
majority charge carriers start flowing00:15:04
from n region and P region right as a00:15:07
result of this depletion region is00:15:10
formed now as this diffusion continuing00:15:13
the depletion region increases in width00:15:15
at the same time that is one part at the00:15:20
same time as diffusion continues the00:15:22
potential difference between N and B00:15:26
also increases so at a certain point the00:15:29
width of the depletion region is such00:15:31
and the potential difference between N00:15:34
and B is such that it does not allow for00:15:38
the diffusion of majority charge00:15:41
carriers right so at that point we see00:15:44
that the PN Junction is at equilibrium00:15:49
right so this potential difference at00:15:52
equilibrium between P and n is known as00:15:55
barrier potential so how do we define00:15:58
barrier why do we call it barrier00:16:00
potential because this potential00:16:03
difference between N and P restricts00:16:06
further flow of majority charge carriers00:16:08
so it is the potential difference00:16:10
between n region and P region which00:16:13
restricts further flow of majority00:16:15
charge carriers across the junction00:16:18
barrier potential is also sometimes00:16:20
known as barrier height right so that00:16:24
means whenever we talk about this00:16:26
depletion region we talk about two00:16:29
things one is the width of the depletion00:16:31
region and the other one is the barrier00:16:35
height so barrier height is nothing but00:16:37
the potential difference between n00:16:39
region and P region so these two00:16:41
parameters actually decide when the00:16:45
diffusion will stop right so you00:16:49
understood the concept of a PN Junction00:16:52
C please understand it very clearly00:16:54
because this is going to help you as you00:16:56
as we go ahead and start studying other00:16:59
things thank you please visit exam fee00:17:03
or comm to watch free educational videos00:17:06
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thank you once againDescription:
Physics Semiconductors part 8 (PN P-N Junction) CBSE class 12 XII
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