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Current time:0:00Total duration:10:49

Diffusion, drift & barrier voltage

Video transcript

to charge a battery of a mobile phone you need a current to flow through the battery only in one direction but the current that you get from the socket from the wall is an alternating current the current is always changing its directions all the time then how is it that we can charge your phone by connecting it to the socket well the adapter must be somehow converting this alternating current into a one directional current and the secret to how an adapter does that is in the PN Junction so let's understand the PN Junction in a previous video we saw that a PN Junction consists of a lot of holes on one side a lot of electrons on the other side and there are two motions that are happening one is the diffusion the holes from the P are diffusing into the N and the electrons from the N are diffusing into the P and this is due to the difference in the concentration but also because of the recombination effect there is a charged space that has been created over here and that is slowing down diffusion but that's also allowing a second kind of motion where the minority charge carrying swept across now notice that the holes from the N R's being swept into the P and the electrons from the P are being swept into the N and are equilibrium for every hole that gets diffused into the n1 hole sweeps back and for every electron that gets diffused into the p1 electron gets swept back and the equilibrium is maintained in this video we're going to talk about the currents that are involved due to these two kinds of motion in a PN Junction and you also see that a voltage gets set up across the junction since holes and electrons are charged particles and holes are treated like charged particles even though they're not and motion of charged particles create current constitute current there are currents in this PN Junction and as we're going to talk about now the first kind of current would be diffusion the current due to the diffusion let's talk a little bit about that what do you think is the direction of the diffusion current think about this recall that the direction of the current is the same as a direction in which a positive charge is moving and if there are negatives is moving the direction of the current is in the opposite direction so just pause this video for a while and we can even pause the animation we should be able to do that without the animation and think about what direction is the diffusion current is it from P to M or n to P all right let's figure this out notice that for diffusion holes will diffuse towards the right because there are a lot of holes here there are less number of holes over here and that constitutes a current towards the right so if you write that down here that uses a current towards the right and notice electrons are diffusing towards the left because more electrons here and less electrons over here but electrons are negatively charged particles so since they're moving towards the left that also constitutes the current towards the right so notice that both the motion both the motion of diffusion were holes and electrons constitute the current in the same direction they don't cancel out they produce they add up and give the current in the same direction and therefore the diffusion current which we'll call it as I diffusion and this is caused due to the majority charge carriers I'm just gonna call it as major majority charge carriers Falls from P to n that is from P to n and there's a second type of current that is due to the minority carriers remember that this hoe for example when it wanders all the way till here it gets swept across look at the direction of the motion the hole is moving from n to P that constitutes a current from n to P similarly a minority charge carriers like an electron when that electron wanders and comes still here it gets swept across because of this charges over here again that causes the current to in the opposite direction n to P so notice both these motion constitute another current in the opposite direction and we call that current as drift current so that's called as drift I drift we'll call it and that's called that is caused due to the minority charge carriers and that is from n to P so this is from n to P and you might be like wondering why is it called drift current what's this whole drifting thing well that's because well if you look at this region where this is charge charged part of the semiconductor then notice you may have learned before that the whenever you have charges they create an electric field and the electric field runs always from positive to negative that's our convention that's a convention that we choose so the electric field lowers here if you were to draw let me choose let's say you choose yellow the electric field if you draw runs this way this is the direction of the electric field alright this is the electric field and notice the minority charge carriers are being swept across due to this electric field and whenever you have a motion of charge carriers in an electric field we usually call that motion as drifting motion you may have studied about that in in electricity chapter but anyways that's the reason it's called as drift motion a drift current and AB equilibrium the two currents must be equal to each other we saw that for every hole that diffuses one hole drifts back and as a result the total current is zero and a one technical detail notice that if you look at this region we could separate our PN Junction into three regions actually three regions you have this region over here which is neutral because all the charges the negative charge is balanced by the holes so this is neutral neutral you have this n region also neutral this is also neutral because again the positive charge of the phosphorus ions are are balanced by the electrons over here but notice this region in between the in the near the junction it's not neutral it's charged and that's where you have the electric field and why is it charged or because there are no electrons and holes to cancel out the charge of the ions its depleted of our mobile charge carriers it's depleted of them and as a result this region which is depleted of charge carriers or we call it the depletion region for for obvious reasons it's depleted of charge carriers depletion region we call it so depletion region just means there are no charge carriers over here and of course if you look at the animation one more time of course it I'll just keep moving diffusion and drift is continuously happening in the two are cancelling each other but it's continuously happening but they're not staying over here that's the key point that's why it's called depletion region over here note is pretty a holes a pretty much staying over here electrons are pretty much staying over there that's the whole idea behind this and if you look carefully because of this electric field in the depletion region there is a wall tidge developed across this junction there is a potential difference developed and here's a way to think about why that happens let's first define what potential even means what is the meaning of word voltage well in electricity voltage V can be just thought of as potential energy potential energy of a plus 1 Coulomb charge all right so what I'm saying is that if if there was a plus 1 Coulomb charge and that moon across you would see that it's potential energy would change across this PN Junction and let's understand why by actually imagining that there is a particular 1 Coulomb charge over there so let's imagine a hypothetical in imaginary 1 Coulomb charge let's move it across and see what happens to its potential energy so let's say I give it a kick over here initially and because of this kick it'll have some speed and let's keep track of what happens to its speed based on that we can hat we can see what happens to kinetic energy alright so we've given it a kick let's say and then right now it's in the neutral zone notice that in the neutral region because there are no charges it is neutral it doesn't experience any attraction or repulsion due to charges and as a result its speed will not change so in the neutral zone its kinetic energy will pretty much remain a constant and if the kinetic energy is a constant its potential energy must also be a constant because the total energy must always remain constant remember consideration of mechanical energy it is its connecticut is a constant so it's potential energy is a constant so the voltage over here is pretty much a constant whatever that number is it's a constant but now this charge will enter into the depletion region notice that once it enters over here because there is an electric field it experiences a force in the opposite direction can you see that it gets pushed backwards now think about what will happen to the speed the charge is moving towards the right it's being pushed backwards so it's speed will reduce right as a result when it goes from here to here its kinetic energy drops it slows down it becomes slower and slower because it's being pushed back this is very similar to what happens when you throw a ball up the gravity slows it down it's kinetic energy decreases but we say that it's potential energy increases right that's the whole idea being potential energy well the same thing happens over here it's kind of thick energy decreases so let me show that one more time it's coming from here it slows down slows down its kinetic energy decreasing its potential energy increases from here to here and once it reaches into this end zone the end type now it's again neutral now its speed will again remain a constant and so it's it's voltage will remain a constant so if you were to make a quick drawing of what that voltage looks like let's let's draw that over here let's say here we plot the voltage V alright then initially over here notice this ax is the voltage initially there was some voltage I don't know what it was it doesn't matter but that voltage was a constant until we reach the depletion region that voltage is a constant so until this point that voltage is a constant but then as you go from here to here the potential energy increases because it's speed decreases right so the voltage increases till here the voltage increases and then after that the voltage again remains a constant I hope that makes cents now and so you can clearly see that there is a potential difference difference in the world age between P and n n is at a higher voltage than P and if you do the math one can do the math and figure this out and people have already done that it turns out that if you take silicon at room temperature this potential difference is about 0.7 volt and it's this voltage that's actually acting like a barrier for diffusion that's what diffusion is finding it's so hard this is what we call as the barrier potential we also call this as VB the barrier potential and understanding this potential is the key to understanding what happens when we are going to attach a battery to it but long story short in a PN Junction at equilibrium when there are no batteries attached there's an already inbuilt potential difference between between the across the junction the n-type is at a higher voltage compared to the p-type