 Okay, anyways, so we will introduce, we will introduce a new useful element in semiconductor. We have talked about how useful simple PN junction can be, okay. The next useful device is junction transistor, right down junction transistor. The PN junction has two, sorry, PN junction has one junction, but the transistor has two junctions, okay. And you can use only P and N type semiconductors, okay. So if you put one N type like this and one P here and one N over there, you will have two junctions, one there and one here, okay. Similarly, if you put N type between two P type, you will get another type of device which has two junctions, okay. So a device like this is called transistor, getting it? So there are only two types of transistors that can exist, NPN or you can simply say small NPN and PNP, okay. Only these two types of semiconductors can exist, right. So we will just quickly talk about it. In NPN transistor, there are two segments of N type semiconductors, okay. There are two segments of N type, they are separated by one segment, a segment of P type, okay. Similarly you can define PNP transistor, alright, likewise. You guys have to put you yourself on mute, I can still hear noise, okay. So, okay. So now this is just the basic structure of a transistor, we are going to get into detail about the construction of it, okay. We will not get into the usefulness of the transistor yet, we will first discuss how it is arranged and what are the shapes and sizes of N and P type semiconductor used in the transistor and then after that we will learn how that will help us to create a device, okay. So let's first talk about, we will first talk about the configuration, okay. So every transistor have three units, okay. This side is called emitter, this side is called collector, okay. And this one is base, alright. So in NPN transistor, the P type is base, in PNP, N type is the base, okay. But in both the cases, there will be one emitter, one collector and one base, fine. So I am going to in general talk about first the physical aspects of these three elements, okay. The first one, let's discuss about emitter, okay. So emitter, write down, it is a segment on one side of transistor, it is moderate in size and it is heavily doped, fine. I hope you know what doping means, right. You are, this is N type. So you dope a pentavalent element on silicon or germanium to make it N type, okay. So heavily doped means you are putting the pentavalent element in very high concentration, okay. So that's how the emitter is. We will talk about why it is like this and how it helps later on. But right now we will just discuss about the physical aspect. Second is base, okay. So the base is very thin and lightly doped, fine. So you can also make out the way we have drawn NPN or PNP transistor that always I am drawing this middle segment very thin, fine. Third, third one is collector. Collector is the major portion. As in this collector itself could be around 70% of the total transistor size, okay. So the collector is moderately doped and large in size. So this is large and moderately doped, fine. So this is how the physical aspects of a transistor are. And also, see the way this transistor is different from a diode is like this, you know. You have a diode which has just one junction like this. This is P and this is N, okay. There will be only two wires that will come out of it, okay. One could be input and other could be output, fine. Or you can say that the current will enter from one side and just leave from the other side. It's a simple device. It can be, you know, connected in series, fine. But then here when it comes to a transistor, it has three junctions or three points to connect, all right. So this can be connected in different ways when it comes to how it is connected in a circuit, fine. This diode can be simply connected in series, all right. But then here you have three points to connect, right. And also, when you connect a diode in a circuit and you draw a circuit element, you show it like this, fine. So left-hand side is your P and the right-hand side is your N, okay. But when you connect this transistor, since you have three pins coming out, okay, your representation in circuit should also have three pins, all right. So the way you represent a transistor is slightly different, okay. So in a circuit transistor is represented like that. All of you draw this. This is base, emitter and collector, okay. Now this arrow is something which will tell you whether it is PNP or NPN, okay. So when arrow is from base to emitter, it is NPN, okay. The arrow goes from base to emitter. The way I used to remember is the P has holes, majority holes, right. So you can say that holes, majority holes will get diffused onto N. So this shows some in some way direction of current, fine. So like that you can say that when the arrow is pointing from base to emitter, it is like current is flowing from base to emitter and this could be NPN, okay. So if this is NPN, PNP will be simply reverse of it, okay. Everything is same, just that the direction of arrow is like this, this coming in, okay. Now going forward, what I am going to do, I am going to represent base as B, collector as C and emitter as E, so that it is easy for me to write down again and again, okay. Any doubts till now? Is this in clear? Yes, sir. Okay. Fine. So this is how it will be used in a circuit. Now let us further discuss about why it is like the way it is, okay. How it is useful or why and how it functions. Then we will discuss few instruments that can be made out of it, okay. So in order to understand that, I will draw this transistor slightly bigger. The base is middle one, okay. And I am right now drawing NPN transistor, this is N, this is P and this is N, okay. So this is how you are connecting it from here and then it is connected like that, okay. Now the way it is connected is this collector base emitter, okay. Now the way it is connected is, yes tell me, isn't base collector reverse biased? That's how it, yeah it should be correct, I wanted to, yes it should be. Now see here we have always you need to understand one thumb rule is there that emitter base has to be forward bias, okay. Now what is biased? Biased is always a junction, okay. So this junction between base and emitter should be forward biased, okay. And base and collector, this junction, junction between base and collector should be reversed bias, okay. Then only this device will operate. Now let's see why it is like that. So what happens is, you know, since base and emitter is forward bias, so this boundary layer here will be thin, okay. And since base and collector is the reverse bias, so here this barrier or the boundary will be widened up, okay. And if you look closely here, this one, here it will be negatively charged, okay. And this is positive charge, okay. And similarly here it is positive, okay, and this is negative, fine. Now the good thing is that, you know, once the electron goes from emitter to base because it is forward bias, it will not, I mean it will not face lot of resistance. So it will be able to cross emitter base junction, are you getting it? So when it crosses emitter base junction and it reaches here, it reaches here in this zone. What will happen to that electron? Can you guess? It will move forward because the electric field here is like this, fine. But when electron which has negative charge reaches inside this zone, it will feel a force in forward direction, fine. So is reverse bias helping in flow of charge or not? Yes, sir, it is. So it creates some, you know, some effect because of the reverse bias and it is helping the current to flow, okay. Now this is slightly counterintuitive but you have to understand one thing is that what happens in the reverse bias is that there will not be any charge carriers inside this zone but charge carrier can come from outside. So there are no charge carriers that are created in the reverse bias but it is coming from outside and that is being welcomed by the reverse bias and that's how the current is flowing. Are you able to understand now? Sir, but how is it different from a, how is, yeah, how is it different from a normal reverse bias? See. Sir, what do you, what do you mean by charge carriers from outside? Okay. So when you have a reverse bias like this, okay, suppose this is P and this is N, okay, and it is reverse bias, what will happen? A big zone will be created, all right, and current will try to flow like this. Now tell me, current is flowing like this, so electrons will try to go this way, okay, this is, this is negative, okay, and that is positive, fine. Now tell me how it is helping the current to flow that reverse bias, is it helping in any way? Yeah, no, sir. It is not, okay, it's not. In fact, you know, when first of all the P doesn't have lot of electrons which can go like this, are you getting it? So if P has lot of electrons, then this reverse bias will help for it to move forward. But P itself has excess holes, it doesn't have free electrons. So P will not be able to generate any free charge carriers which can be welcomed by the reverse bias. But here in this case, you have a source of electrons sitting here as N type, which is a third thing which is connected this end, getting it. So that is why this electrons which are excess charge carriers in N type will be able to just cross the entire reverse bias, fine. Now we'll follow a few notations. This potential which is connected between emitter and the base, it is referred as VEE, okay. And this one is referred as VCC, fine. So that's how, you know, you make a transistor work and now you can understand one more thing here is that when the current is flowing, let's say this is a current which is emitter current, okay. And this is base current, then this is collector current. That's how the current will flow or not. Electrons are going like this, fine. Electrons are going in this direction, the current will go in reverse direction, fine. And few of the electron will pass through the base down, so current will be up, okay. So if electrons go from emitter to the collector, few electron go from the base outside. So that's how the current direction will be, it will be reverse or it will be opposite of the flow of the direction of the electron, okay. Not getting it? Okay, so there is a relation between the current. So emitter current, if you look at this junction, emitter current comes like this and divides into two. So emitter current will be equal to base current plus collector current, okay. This relation is true, it doesn't matter whether it is PNP or NPN, fine. So I will not now talk about the PNP type because PNP is very similar just that the polarities of the batteries will be reversed and the direction of currents will be reversed, okay.