 So, let us move forward, so this is with respect to the introduction of semiconductor, we have not yet discussed any usefulness of the semiconductor, how it is used for our whatever application we can think of. But till now we have got introduced to n-type and p-type semiconductor, let us see how they will behave, so if you have let us say n-type semiconductor, if you have n-type semiconductor and if you connect it across a battery, let us say that you connect this like that, current will flow or not, current will flow, okay. Now if I reverse the battery, will current will flow, why, you are telling me that if you connect n-type semiconductor like this, current won't flow, it will still flow, but in opposite direction, fine, it will flow, there is no reason why it will not flow, electrons are there free electron inside this end, they will move whichever direction they get the push. So, in a way a pure n-type or pure p-type, when I say pure what I mean is that only n-type and only p-type if you take, they will behave like metals, there is no special property they have acquired just because they are p-type or n-type, okay. So the first useful device in the semiconductor is not p or n-type, but it is a junction between p and n, okay. So the first useful device in semiconductor that we are going to study, okay is p-n junction. So junction, when you join p and n-type semiconductors, because of the junction it acquires some unique properties that we will exploit to build some equipments, okay. So let's see what is so special about p-n junction, okay, write down p-n junction, as the name suggests p-n junction is the junction between p-type and n-type semiconductors, okay. Let's say that this is the junction, this is p-type, this is n-type, okay. Now n has what, n has lot of free electrons and p has what, it has lot of holes, okay. So naturally holes will attract the free electrons, isn't it? So from the boundary of the p, from this junction which I am talking about, the electron will diffuse like this. Diffusion happens from higher concentration to lower concentration, isn't it? So the diffusion happens from higher to lower concentration, because of that the electrons will move from n-side to p-side, okay and holes will move from p to n-side, getting it? So electron when it reaches the p-side, you know, it can go a little bit inside like this and holes travels like that, this is let's say diffusion, okay. Now will this diffusion go on forever, this diffusion current, will it stop or not stop? Both of you. It will stop. Why? So because there will only be a certain amount of holes and electrons, right? Suppose that is not limited, like there are good amount of supplies there as much as you want. Will it stop then? See what happens is that once electron gets accumulated here and positive charge goes there, there is an electric field that get developed in this direction, okay. So because of this electric field, electron will feel a force in opposite direction of electric field. So a current because of this electric field also starts in opposite direction. This is drift current, this is diffusion current, okay. When the diffusion current become equal to the drift current, then sort of equilibrium is attained, okay, sort of equilibrium is attained and the no longer this width of this will increase. Are you getting it? Yes, sir. This is. Yes, sir. Junction barrier. Fine. Now few more things you need to understand about this junction barrier is this. First of all, this electron doesn't just come here and sits. There is an equilibrium between this side and that side. So it's a dynamic equilibrium. Holes recombine with electron and electron gets created. So like that, there is an equilibrium that is happening at this side and there is a dynamic equilibrium at that side also. Getting it? That is point number one. So point number one is it is dynamic equilibrium both sides of junction barrier, okay. Point number two, inside junction barrier, there are no free charge carriers. Getting it? This entire zone, okay, this entire zone is as if, you know, all the electrons are in the valence band. There is no electron in the conduction band, so it's like a gap, okay. There's a gap, so to say, you know, you can say that it has huge amount of resistance. So there is no charge carrier inside this junction, okay. Because of this, if junction is bigger, if junction is bigger, it will not be able to freely conduct the electricity, fine. So wider the junction, difficult for current to flow. Getting it? Now, if the current has to flow, what should happen? Charge carrier from the n side or charge carrier from the p side, it has to go from all the way from this point to that point like this. It has to travel entire width, okay. But what happens in a conductor, let us say if you have a conductor, all these electrons together they start moving, fine. Like this, they move and this electron need not come here to have a current from this point to that point. There's in series, there's so many electron that they together move, fine. But in case of the semiconductor junction, p-n junction, which is here, electron from n side has to jump to the p side, then only the current will flow, fine. And that is the reason why it, wider the junction, more difficult for current to flow. Are you guys understanding this? Yes sir. Okay. And just one more thing, point number three, okay. So there is a charge separation at the junction barrier, okay. So there will be a junction potential also. So there's a potential difference between this end and that end. There's a potential difference, fine. So junction potential is there, which is also called junction barrier. So external battery potential, which you are applying, external battery potential has to be more than the junction potential, then only electrons from the n side will gain sufficient kinetic energy to jump this potential difference, fine. So these are the few things that you should keep in mind. Now let us get into the greater details of how this junction work. This p-n junction, together it is called semiconductor diode or just diode. So if you connect this semiconductor, let's say you connect it like this, both of you draw this. This is p, this is n, okay. So this is negative, positive, okay. So what happens if you connect a battery like this, a potential difference like that, so the electrons from the n side, they get a push this side, fine. And that is the reason why current is flowing like this, fine. So if electron gets a push this side, then this equilibrium will shift left-hand side because more and more electrons will recombine with this positive, getting it? Because of that, this junction barrier, this barrier itself will shrink to very less size, okay. So as this junction barrier shrinks, then it becomes closer to the conductor. Are you guys clear about it now? Yes, sir. Okay, so this is called forward bias. When you connect p side to the positive potential of the battery and n side to the negative potential of the battery, okay. What happens is the junction barrier shrinks. Now this is called, all of you write down forward bias, okay, this forward bias. Now let us try to connect a battery which has negative potential p side and positive potential n side and see what happens then, this is p, this is n. Now tell me what will happen? Current will flow the other way and the barrier will increase. Current will try to flow in other way, okay. But then because of that, what will happen? Because of that, the electron, see, electron will get pulled this side, getting it? So electron gets pulled this side, the hole gets pulled that side. And if there is any current that has to flow, then there has to be a charge that should get transferred from n side to p side. It should be like electron jumping this side or the hole jumping that side. Both of them not happening here. So the battery is trying to pull electron this side and hole that side. So the equilibrium will shift in such a way that this zone itself will get widened up and no current will flow. Claire, both of you? Yes, sir. Yes, sir. Fine. So this is reverse bias. Fine. So I hope you are getting a feel of now how this device can be selective. So in a forward bias, it behaves like a conductor. In a reverse bias, it behaves like an insulator. So a junction barrier, it actually creates a sort of selective way the current should flow. So it is unfavorable for reverse bias and it is very favorable for the forward bias, okay. Now you can see that the flow of current not just depend on the external condition. It depends on how, what is the material and how it is corrected to. Fine. So let us take a small break. So we will meet in 10 minutes. Right now it is 3-2. So we will meet. Hello. Hmm, where is Kondi? Sir, I don't know. You have schools till when? Sir, not sure. When will you open? Are you saying holidays or like when school closes? Yes, school closes, right? When you people start? Oh, yeah. When? So probably like December. I am not sure. How do you guys are placed with respect to preparation for JMNs right now? What are you? Servant. What are you will not help you, right? So you just prepare a list of chapters. You are, you know, decreasing order of your strength. Like suppose laws of motion is very strong for you. Like that you create a list of chapters and there will be few topics which, which is like completely pathetic for you guys. So just leave it, okay? Work on your strong areas and make them so strong that if a question comes, you should be able to crack it, right? So do not try to strengthen a chapter which is already very weak. Just leave it at least for the January JMNs. And then, and also there are few topics like for example modern physics. It is very simple. Don't leave out the easy chapters also. Keep it simple, okay? Don't make any complicated strategies. Just solve, let us say HCRMF or physics. Then look at the previous year questions. And also when there is a mock test that will start in the December month. Take all the mock test. Analyze the question paper. So keep your strategy pretty simple. And work accordingly. Having a worry about the exam will not make you any better. So just work hard towards it and give your best shot and be happy then whatever you get. Because of that. Yes sir. Fine. And also see you guys having online classes, right? So it is a good idea that you guys come here once in a while and face to face interaction. So I was expecting that you will travel because you could have attended this class physical as well. But anyways. It is like I didn't come today because of class only. Because like it is online class. I am the KRMP people come. So all your teachers also. So that is why I didn't come today. So now it is you can have hybrid. Both online and offline can be together. Anyways. For this chapter. It's alright. There is no problem as such. Okay. And you guys are taking KVPY also in November. So I am writing. So I am not. But both of you. So stay focused. Anyways. We have learned the behavior of the junction diode. Or PN junction diode. Okay. With respect to forward and reverse bias. Now it is little strange when you compare it with metals. So. How metals. Or let's say a register. How it will behave is that. They will follow Ohm's law. What is Ohm's law? Potential difference across resistance. It is proportional to the current flowing through it. And constant of proportionality is resistance R. So you have an Ohm's law. To get a relation between potential difference and current. And because of this you can analyze the entire circuit. Okay. But when it comes to semiconductor. You know things are strange. It doesn't follow Ohm's law. Fine. So every time we will be learning instrument in semiconductor. We will be talking about the characteristic graph. We will be seeing how potential difference and current. They are plotted against each other. How they are behaving. Okay. So there is no direct correlation. There is no formula. So that is the reason why we have to rely on the graph. When it comes to a relation between potential difference and current. Okay. And since we have just discussed PN junction. So let's.