 All right, so write this chapter's name, semi-conductors. So this chapter is, it introduces a unique type of material which is named as semi-conductors. So because of its unique properties, we can utilize this material to create various useful instruments. These instruments could be as simple as rectifier, then solar cells or photo diodes, LEDs, transistors. In fact, the laptop which you have, if semi-conductors would not have been there, we cannot imagine any of the electronics device so as to say. So we will get into the application of semi-conductors towards the later part of the chapter. And since we are getting introduced to this particular substance, which is semi-conductor, so first, we will devote good amount of time understanding what is its property and how it behaves, which leads to some useful application of this particular device, of this particular material. So that is what this chapter is all about. And again, but then that question will come in your mind again and again that what is so special about this particular substance. So before I can elaborate in greater detail, I'll just talk about some basic difference between semi-conductor and so to say metals. Now, if you have a piece of metal, let's say this is a piece of metal, then this metal piece is sort of a dumb device. When I say dumb device, what I mean to say is that it doesn't apply its own thought or its own logic. It just goes by what is the external condition and based on that, the current will move in this particular piece of metal. So if you have a battery like this, the current will go like that. If you reverse a polarity, what will happen to current? Are you guys on mute? So tell me, I'm asking something. So if you reverse a polarity, what will happen to the current? It will go in the opposite direction. Right, so right now it is, let's say clockwise. Now it will become anti-clockwise. So this metal piece, it just goes by whatever is the external condition and based on that, the current will flow. What semi-conductor does is that it will apply its own logic based on which it is not just completely dependent on what is external condition. Internally also, depending on how you're connecting the semiconductor, the flow of the current and voltage and everything can be affected. So basically, semi-conductor gives you a conditional flow of current or conditional voltage. So it sort of give you if then else statement itself. If this happens, then something will happen. If something else happened, then what there was earlier happening will not happen. For example, let's say you have a piece of semiconductor, let's say a P-ingension, which we'll study a little bit later. If you connect a battery like this, current will flow. It will flow. But if you connect the same P-ingension like this, the current doesn't flow. There is no current. And right now, there is a current. So there is a conditional flow of current. It does not just depends on what is a voltage you are applying. And there are so many other useful applications. For example, there can be a device in which if current flow like this, then only a current like this can be supported. So this horizontal line current is possible only when the current flows vertically. So again, this is an intelligent device. So based on the conditions, it changes its behavior. Fine? So we know that entire computer science or entire, so to say, processing itself is nothing but if-then-else statement only, isn't it? So when you write a computer code, you have, based on conditions, you do something. So like that only, semiconductor gives you a conditional output. So hence, it is very useful to build devices, which I'm talking about. OK. So we will first talk about the properties of semiconductor. Let's see what are some of the properties of semiconductor that makes it different from a metal and a non-metal. So what is the best way to understand how it is different? The best way is to just classify it based on the properties. So we have first the classification of the material. When I say material, I'm just talking about three kinds of material here, metals, semiconductors, and non-metals. So we are classifying first all these material based on conductivity. So let's see how they are different based on conductivity. So mind you, you should always understand one thing that this is just a classification. Classification is not a definition of anything. This is the way these substances are different. So just because semiconductors, conductivity let's say something, it doesn't make it special that way. It's just a classification of these three substances. So first one, all of you write down metals. What do you think their conductivity will be? Conductivity will be very high, and resistivity will be very less. So the value of resistivity is of the order of 10 raise to the power minus 2 to 10 raise to the power minus 8 ohm meter. And the value of the conductivity, you know it is inverse of resistivity. You guys know this or not? Yes, sir. OK. Conductivity is just inverse of resistivity. So if resistivity range is 10 to the power minus 2 to 10 to the power minus 8, conductivity range is just inverse of it. The inverse of ohm is represented by letter S. So this is the metal. And if you look at semiconductors, semiconductor have moderate conductivity and moderate resistivity. So their resistivity is of the order of 10 raise to the power minus 5 till 10 raise to the power 6 ohm meter. So you can see that there is an overlap of the values of resistivity for metals and semiconductors. Fine. So that you should keep in mind. And resistivity, of course, it will be inverse. So 10 raise to the power 5 to 10 raise to the power minus 6 like this. Now comes the nonmetal. What do you think nonmetal's resistivity will be? It will be very high. Resistivity will be very high and conductivity will be very less. So the value of resistivity is of the order of 10 raise to the power 11 till 10 raise to the power 19. So you can see how much is the difference between metals, resistivity, and the nonmetal's resistivity. There is a huge gap. Fine. So the resistivity of metals starts from 10 raise to minus 2. And the resistivity of the nonmetal, it starts from 10 raise to the power 11. So there is a big gap. Metals and nonmetals don't overlap when it comes to the values. So the conductivity, of course, should be very less. And it is inverse of resistivity. So it is like this. So these are the classification based on their conductivity and resistivity values. And it is found that the resistivity and conductivity of the semiconductor is in a way in between the metals and nonmetals. So that is the reason why one of the reason you can understand why the name is semiconductors. So since our prime interest is with respect to semiconductors, so we will try to understand semiconductors in greater detail. We have classified first all the materials into different kinds of material. Now what we are going to do is that we will classify the semiconductor itself based on what kind of semiconductor it is. So let's try to classify semiconductors. Again, right now we are just discussing some of the properties of semiconductor or how it is different from others. We haven't yet gotten into detail about how these properties can be used. That will come a little bit later. So be patient. And right now just focus on the properties of the semiconductors. The semiconductor broadly can be divided into two types. One is elemental. This is elemental semiconductor. And the other one, you can say it is compound semiconductors. Now the example of elemental semiconductors is silicon and germanium. Both silicon and germanium, they are from the same family. But the carbon is not semiconductor. So you will understand why others are not semiconductors a little bit later. But then these are the two examples of elemental semiconductors. Now this is compound semiconductor. And the compound semiconductor itself can be further classified into three parts. The first one is inorganic. Inorganic compound semiconductors, then you have organic. And the last compound semiconductor is organic polymers. So when I say organic compounds, I'm excluding organic polymers semiconductors. I'll quickly write down a few of the examples. So inorganic semiconductor could be cadmium sulfate, gallium arsenate, like this. Then organic compound could be doped polyanthrocynes. So it is doped. And organic polymer can be polyperol. So I'm not getting into too much of detail with respect to the compound semiconductor. Because in our syllabus, we are just going to discuss about elemental semiconductor. Compound semiconductor is not in our syllabus. So this is with respect to, let's say, just physical classification or how they are, what they're made up of. Let's get into the more meaningful type of classification. This, while I'm saying this is the more meaningful type of classification, because this type of classification, which we are going to discuss now, will give you more insights on how semiconductor behaves. So this classification is with respect to energy bands. So the basis of this classification now is energy bands. Now, the thing is, Ajah, can you first tell me what makes a conductor a conductor? As in, what is so special about conductors? Ramcharan will enlighten us. Yeah, you can speak. Oh, sir, I think either your voice or my, or like I lagged out a bit. Yeah, I'm asking that what makes a conductor special? What is so special about conductor? So like, easily electricity can pass through it. Rathik is there, Kondanya? Yes, sir. So what makes a metal special is that it has a lot of free electrons. What do we mean by free electrons? Free electrons are the electrons that can jump around. So free electrons can leave one particular atom, and it can jump to another atom, and it can just freely float. So if you take a piece of metal like this, electron can go from here, and it can travel along. So effectively, there is a transfer of charge from one location to other location, and the medium to transfer the charge is electron. So when I say that metal has free electron, it automatically means that the electron has left the nucleus. It is not bound in a particular nucleus. Now tell me one thing, you have supposed to electron. Let's say first electron, it is bounded by the nucleus. And another electron is free. Which has more energy? Bounded electron or the free electron has more energy? Free electron. Free electron has more energy. So if a electron is bounded in the nucleus, then its potential energy is lesser. It needs basically energy to leave the particular orbit and move to the next orbit. And if electron is free, it means that it is at any tending to infinity level. So its energy is 0. But if electron is bounded, then its energy is negative. So electron which is bounded has lesser energy, and electron that is free has more energy. So I can say that there are a large number of electrons in the metal which have higher energy. So if I draw an energy diagram, what I mean to say the energy diagram, it's a plot between the energy of the electron on the x-axis and sorry, the energy of the electron on the y-axis. And x-axis, it just extension of the space so that I can accommodate. I'll just point, suppose I point four electrons. So I get a space to say that the different energy levels these electrons will have. So if I just plot, I take every electron and plot its energy level. So I'll get, suppose these dots. Each dot represents an electron's energy level. Are you getting it? Yes, sir. And suppose there is a minimum energy required, there is a minimum energy required for them to be free. For example, suppose this is the energy required, OK? Suppose this is the energy required for them to become free. So we can say that in case of metals, there are a lot of electrons which are already free because they have energy which is more than the energy required for them to be free. So I can keep on plotting all the electrons energy. So there will be electrons energy like that. So this particular band from here to here, this particular band is called conduction band. Why it is called conduction band? Because these electrons are free and because of these electrons, the metal is conducting. And from here to here, this is called the valence band. Why they are called valence band? Because you can plot the electrons energy which are revolving around the nucleus and you get a plot of a particular atoms valence electrons. So that is why it is called valence band. So you can see that there are few electrons in the valence band which are revolving around the nucleus which already has good amount of energy and they can break free. They can leave a nucleus and jump to another nucleus. Fine. So that makes the metal a little bit special. So this is the diagram or energy diagram for metals. Any doubt on this? No, sir. OK. Now let us see semiconductors. See with respect to metal, if you see, let's say you draw a couple of nucleases like this, these are nucleus. And this yellow line represents the electron that is freely moving. So it can just leave a nucleus and it can freely revolve around. It can jump from one nucleus to other nucleus. It doesn't need extra energy to do all this. It freely floats. But with respect to semiconductor, the electron, although it is bounded in a nucleus, it will not readily leave the nucleus. But then if you give very small amount of energy, it will just leave the nucleus and jump to the adjacent one. And it will start conducting electricity. Fine. So if you draw the valence band and conduction band for the semiconductor, you will get a band like this. Let us say this represents the valence band. Then what do you think the conduction band will start from? If this is the valence band, so where the conduction band will start from? So little bit above. Little bit above. So the conduction band will be like this. And there will not be any electrons in the conduction band. Why I'm drawing this line at the top? Because there is a fixed amount of energy after which the electron will just come out of metal surface itself or semiconductor surface itself. Then it can't conduct electricity anyway. So there is a top limit also up to which electron can have energy in the conduction band. This is conduction band. And this energy is called energy gap, EG, fine? So the minimum energy gap between the valence and the conduction band is called EG, which is simply represented as energy gap. So this EG is less than 3 electron volt if I talk about semiconductors, OK? So minimum amount of energy electrons required to break free is around 3 electron volt in semiconductor. When I say break free, what I mean to say, it becomes free from one nucleus. It can jump to other nucleus. It is not a photoelectric effect I'm talking about. I'm not taking electron out of the surface. I'm just giving it sufficient energy so that it leaves a nucleus and remains still inside the material itself. OK, now let's talk about non-metal. What do you think the non-metal diagram, how it will be? The energy gap will be quite a lot. Quite a lot, good. So this is the, let us say, the valence band. And this is, let us say, conduction band, OK? So all the electrons will be in the valence band and there is a huge gap between conduction and the valence band. So this EG is way above 3 electron volt. Fine, so this, I think, gives you more insight with respect to how semiconductor behaves, OK? Now, since we are talking about elemental semiconductors, so we will see whether I can further classify elemental semiconductors. Because earlier what we have done, we have classified the semiconductor itself, OK? We have classified the type of semiconductor, which are intrinsic semiconductor, extrinsic semiconductor. We have elemental and compound semiconductor, sorry. So compound semiconductor is not something which we are studying in this particular class, OK? So since we are discussing about the elemental semiconductor, let us further classify elemental semiconductor and see whether there are different kinds of elemental semiconductor also, OK? So there are two kinds of elemental semiconductors, write down. The first one is intrinsic and the second one is extrinsic, OK? So when I say intrinsic, what I mean to say a semiconductor which is in purest form, OK? Let us say it is germanium semiconductor. So entire semiconductor is just made up of a single element germanium, OK? What I mean to say when I talk about extrinsic semiconductor is there will be small amount of impurity added to it, OK? For example, arsenic may be added inside the germanium or silicon semiconductor, fine? So these impurity will tremendously increase the conductivity of the semiconductor, OK? That we'll discuss a little bit later, OK? Right now let's talk about intrinsic semiconductor and see how it works, OK? So we are trying to gain more insight with respect to how it works and what happens inside the crystal lattice or how the electron comes out of a particular nucleus and things like that, fine? So since we are taking example of silicon and germanium only, these are group four elements. We know that the nucleus will have plus four charge. So let's say we have silicon and germanium lattice. It will be like this. All of you draw this. So every circle represents a nucleus, OK? And these branches which I am drawing, these branches represent some sort of bond. It's a coal and bond electron sharing is happening. So one line represents one electron. It's plus four, plus four, plus four, OK? So we have one electron here, one electron there. So like this, electron sharing is happening and silicon or germanium complete their octets, fine? So you can see that electrons are tightly held inside a covalent bond, OK? And these electrons cannot just break free on their own. So what will happen is that if you supply energy, OK? Now you can supply energy using different means. You can supply, let's say, you can supply energy using EM wave. You can just give heat, OK? You can heat it up or you can apply a large amount of electric field. So there are multiple ways you can generate energy and give it to the electron. So once, let us say, this electron gets sufficient energy, what will happen? This electron will just leave the bond. So bond is sort of broken. There will be a site which is open here and electron can come over here and sit. So this electron, which was there earlier at this site, becomes free. And this electron can come under the influence of external electric field and can conduct electricity, OK? At the same time, this hole, you can say that this hole is positively charged, which has plus E. Because earlier it was neutral. So if minus E charge comes out, plus E will remain. So this hole will also attract the neighboring electrons. So this electron, suppose, jumps over there, like that. So hole will get created here. So in a way, hole is also moving, getting it. So hole also moves and electron also move. So in the intrinsic semiconductor, there is a current due to electron movement and current due to hole movement. So the total current is due to both of them. And because of the charge conservation, I can say that the amount of electron current will be equal to amount of hole current, OK? Because number of holes that are getting generated per unit time is same as number of free electron getting generated per unit time, OK? So the rate of generation should be same, OK? So like this, you can understand that how an intrinsic semiconductor works. Any doubts, both of you? Anything? No, sir. No. OK, tell me one thing. Why carbon is not a semiconductor? Carbon also has the same crystal lattice. Why carbon is not a semiconductor? But silicon and germanium are the semiconductors. Do you need a lot of energy to remove the electron? Why? See, sir, not too sure. Right. So see, the thing is, when it comes to carbon, the outer most shell electron, they are forming the bonds, and they can only become free and start conducting. The inner shell electron will not come out, OK? But when you compare the outer most shell electron of carbon with outer most shell electron of silicon and germanium, OK? Carbon is much closer to the nucleus, the outer most shell. So because of that, you need a lot of energy. And the energy becomes more than three electron volts. So carbon is classified as non-metal and not the semiconductors. Fine. So this is a brief of intrinsic semiconductors. So we don't use intrinsic semiconductor, OK? All the useful application happens for the extrinsic semiconductor only, OK? So we can discuss extrinsic semiconductor in greater detail now, OK? Now extrinsic semiconductor, like what we discussed earlier, you can put impurity in extrinsic semiconductor. All of you write down extrinsic semiconductor. So it is nothing but you are adding impurity to the intrinsic semiconductor. So it is intrinsic plus a small amount of impurity, fine? Now impurity doesn't mean that you can add anything, OK? So although the name is impurity, but it is very selective kind of material that you have to add. So there are two ways you can add impurity or two types of impurity which you can add in intrinsic semiconductor and make it extrinsic. These are your, you can add trivalent. If you add trivalent impurity on intrinsic, OK? This will become p-type semiconductor, OK? And if you add pentavalent, you add pentavalent impurity to the intrinsic, OK? This will become n-type semiconductor, getting it? So these are the two types of extrinsic semiconductor that we are going to discuss, OK? Let's see first what happens if I add pentavalent impurity to a pure semiconductor, OK? Let's see how it affects the overall conductivity. So we will first talk about n-type semiconductor. So arsenic is one of the example of n-type semiconductor. So the amount of impurity that you add is very less. It is like one out of 1 million atoms. So like this, you add the impurity. But then even though it is just one out of 1 million, there's so many atoms that even this kind of concentration of the impurity can also generate good amount of free electrons that we are requiring for them to conduct the electricity, OK? Let's see how it works. Suppose you have impurity, which is pentavalent. Now, of course, since it is very less, one out of million sort, so a pentavalent nucleus is bigger chance that it will be surrounded by the tetravalent elements. So the impurity will be surrounded by the actual atoms because they are much more in numbers. So now tell me, how many valence electron a pentavalent element will have? Yeah, I was asking you something. So your voice is breaking. I couldn't hear you. You heard what I asked? No, sir. Your voice is breaking. OK, I thought it is pretty clear because you guys are not speaking up. Are you able to hear now? Yes, I can. So tell me, how many valence electron the pentavalent element will have? You're not able to hear me? So they'll have five. You have problem in answering some questions? Kondane, why? I mean, you can't answer the simple question which I'm asking here, huh? Sure. Let's keep it interactive. Let's not keep it like a monologue. Keep answering whenever I'm asking, OK? Otherwise, you can as well watch a recorded lecture. What is the point of taking a live session, isn't it? Yes, sir. Anyways, so pentavalent element will have five valence electrons. So the four electron will get shared by the four neighboring tetravalent elements, OK? What will happen to the fifth valence electron? Will it create a bond? Not too sure. No, it will not create a bond. So it will remain in the orbit of pentavalent element. The only four electron can create a bond, OK? So this electron is not participating in the bond. Now tell me whose energy will be more? This electron which is not participating in the bond or the electron that are participating in the bonds? This one, right? Electron, if it participate in a bond, its energy will go down, OK? So if you draw an energy diagram for this n-type semiconductor, let's say this is energy, all right? This was the normal, so to say, valence band, and this is the normal conduction band, this one, OK? This is conduction band, the upper one, and the lower one is the valence band. Now can you show the energies of the electron? All of you, there will be electrons like this. What are these electrons? These electrons have participated in the bond. These valence electrons have participated in formation of bonds, where the energy level of these electrons will be, which are not participating in the bond formation. It will be in the higher one. Will it be in the conduction band, inside the conduction band? This is conduction band. See, if electron is in the conduction band, it means that the electron is free to jump from one nucleus to other. Is this electron free to jump from one nucleus to other? It is not. It is not, OK? But then its energy is higher than the valence electron, but is not yet in the conduction band. So there will be a layer at this location, OK? But then the good thing about this particular electron level is that it requires very, very less amount of energy for these electrons to jump to conduction band and start conducting. Getting it? So you don't need to now put the valence electron in the conduction band, because you have these electrons, which already have higher energy. You just need to supply this much energy. This energy is approximately 0.01 electron volt. So we have increased the conductivity of this material by doping it or putting impurity of pentavalent element. All of you clear about it? So but for it to go to the conduction band, it should enter like the bonds, right? So for that, one electron should be removed. And for that, you need energy. So that is more than 0.01 electron volts, right? No, this electron, you are not just removing electron from one of the atom. See, if you remove this electron from here, some other electron will get attracted and come here, OK? What you're doing is you're just making it free. It is loosely held to this nucleus. It will go to some other nucleus. And some other electron will come back here at plus 5 site. So like that, you're just increasing the mobility of the electron. You're not creating a charge as such. Or you're not taking electron outside of the material. It remains inside only. But you're increasing its movement, OK? So this is n-type semiconductor. Now let's see what is so special about the p-type semiconductor. Write down. So in p-type semiconductor, we are doping a trivalent element on tetravalent element, OK? So let us say I'm doping boron or aluminum, OK? So this is plus 3, right? So what will happen here is that plus 3 has only 3 valence electrons. So it will create boron like this. And it will have an incomplete octet. So there will be a hole like that. Getting it? Both of you? Yes, sir. Now this hole will try to attract the electron. So electron will leave one of the site and go to the other site. And this will move the hole here. So like that, movement of the charged particles will start. And you can say that electricity is now conducted. Getting it? That's how the p-type semiconductor works. And let us try to build the energy diagram of p-type semiconductors, energy. So let's say this is the normal valence band. And we can say that this is normal conduction band, OK? These are the electrons that have participated in the bond formation. Now, can you guess how we can draw the energy levels further? Technically, you don't need like any, right? Because anyways, there'll be a hole. So by itself, the electrons will start moving. OK, now tell me one thing. If there is a site, the energy will be there with respect to the site also. So this hole which is there, OK? That hole itself will have some energy. And when electron is inside that hole, then it will have a certain energy. Whose energy will be more a site without or a hole without electron or when electron reaches the hole? Which one? The energy is more. That's an octave short. The when the electron reaches the hole, it completes the octet. So energy of the hole goes down. Getting it? So the hole has slightly higher energy if it doesn't have an electron. Fine? So we can say that this is the level for the holes. OK? Now for the electron to start conducting, all electron have to do is to jump from the valence band to where? The conduction band? Yes, sir. No. They have to just jump to the hole. That's what it is happening, right? If this electron leaves this site and jumps onto the hole, and electricity gets conducted. So it doesn't need to go to conduction band to conduct electricity. It can just reach up to the site of the hole and electricity will get conducted. So even this energy is 0.01 electron volt. Are you guys clear? Yes, sir. OK. And one more thing, there is a thermal equilibrium condition. The derivation is not in our syllabus, but then the formula is given. So we'll just take it as it is. Write down thermal equilibrium condition. This tells us that number of electrons per unit volume multiplied by number of holes per unit volume is equal to ni square. OK? So you understand, right? Ni and NH are the electron and hole concentration in a particular type of semiconductor, OK? And Ni is the concentration of charge carriers in the intrinsic semiconductor, OK? Suppose I take silicon semiconductor, so it will have a fixed value of Ni. OK, for silicon, it has a fixed value of number of charge carriers per unit volume. For germanium, fixed value of Ni, OK? But when you add impurity inside it and number of electrons and number of holes, they will get changed. But the product of that will remain a constant, which is Ni square, OK? Yes, sir. Fine. Let us take a numerical. Both of you do this. Suppose the pure silicon crystal has 5 into 10 raise to the power 28 atoms per meter cube. Pure silicon has this much concentration of atoms, OK? It is doped with, doped by one parts per million of pentavalent arsenic. You need to calculate number of electrons and number of holes. What is given is Ni equal to 1.5 into 10 raise to the power 16 per meter cube, OK? Do this. What have we done? Sir, is Ni 5 into 10 to the power 22? Yes. And H will be 4.5 into 10 to the power 9. Yes, sir. You got it? Yes, sir. OK, good. So basically, one part per million is one out of 10 to the power 6. So if you're putting a doping of arsenic like that, so you can count number of arsenic atoms per unit volume. That is nothing but number of free electron that can get generated. Because every arsenic atom will give you one free electron. So Ne will be nothing but number of arsenic atoms per unit volume. This divided by 10 raise to the power 6, 4.22. Now we have Ni and Ne. And Ne into NH is equal to Ni square. From there, you can get the value of NH, right? So numericers like these can get asked in your exams, OK? So let us move forward, OK? 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 whatever application we can think of. But till now, we have got introduced to N-type and P-type semiconductor. Let's see how they will behave. So if you have, let us say, N-type semiconductor, OK? 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, OK? Now if I reverse the battery, will current will flow? No, sir. Why? You're 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'll 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'll behave like metals. There is no special property they have acquired just because they are P-type or N-type, OK? So the first useful device in the semiconductor is not P or N-type, but it is a junction between P and N, OK? So the first useful device in semiconductor that we are going to study is P and junction. So junction, when you join P and N diodes, or sorry, 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, OK? So let's see what is so special about P and junction, OK? Write down P and junction. As the name suggests, P and junction is the junction between P-type and N-type semiconductors, OK? Let's say that this is the junction. This is P-type. This is N-type, OK? Now, N has what? N has a lot of free electrons. And P has what? It has a lot of holes, OK? So naturally, holes will attract the free electrons, isn't it? So from the boundary of the P, from this junction that I'm 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, OK? And holes will move from P to N-side, getting it? So electron, when it reaches the P-side, it can go a little bit inside like this, and holes travels like that. This is, let's say, diffusion, OK? 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, OK? 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, OK? When the diffusion current become equal to the drift current, then sort of equilibrium is attained, OK? Sort of equilibrium is attained and no longer this width of this will increase. They getting it? Yes, sir. This is 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, OK? Point number two, inside junction barrier, there are no free charge carriers. Getting it? So this entire zone, OK? 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, OK? 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, OK? 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 inside 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, OK? 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 electrons that they together move, fine? But in case of the semiconductor junction, PN junction, which is here, electron from inside 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. OK. And just one more thing, point number three, OK? So there is a charge separation at the junction barrier, OK? 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 end side will gain sufficient kinetic energy to jump this potential difference, fine? So these are the few things that you should keep in mind. All right, now let us get into the greater details of how this junction work. This PN 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, OK? So this is negative, positive, OK? So what happens if you connect a battery like this, a potential difference like that? So the electrons from the end 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, OK? So as this junction barrier shrinks, then it becomes closer to the conductor. Are you guys clear about it now? Yes, sir. OK, so this is called forward bias. When you connect P side to the positive potential of the battery and end side to the negative potential of the battery, OK? What happens is the junction barrier shrinks. Now this is called, all of you write down, forward bias, OK? This is forward bias. Now let us try to connect a battery which has negative potential P side and positive potential end 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, OK? But then because of that, what will happen? Because of that, the electron, see, a 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. So now you can see that the flow of current not just depend on the external condition. It depends on what is the material and how it is corrected to. Fine, so let us take a small break. So we'll meet in 10 minutes. Right now it is 3-2. So we will meet. Where is Kondil? I don't know. Do you have schools till when? So not sure. When will you? Are you saying holidays or school clothes? Yes, school clothes, right? When you pre-coach start? Oh, yeah. So probably December, I'm not too sure. How do you guys are placed with respect to preparation for JMNs right now? Vareed? Serve it. Vareed will not help you, right? So you just prepare the list of chapters. You are in a decreasing order of your strength. Suppose loss of motion is very strong for you. Like that, you create a list of chapters and there will be few topics which is like completely pathetic for you guys. So leave it, OK? Work on your strong areas and make them so strong that if a question comes, you should be able to crack it. So do not try to strengthen a chapter which is already very weak. Just leave it, at least for the January JMNs. And then also, there are a few topics, like, for example, modern physics. It is very simple. So don't leave out the easy chapters also. Keep it simple, OK? Don't make any complicated strategies. Just solve, let us say, HCRMF or physics. Then look at the previous questions. And also, when there is a mock test, that will start in the December month. Take all the mock tests. Analyze the question papers. So keep your strategy pretty simple. And work accordingly. And 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. Echo. 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's like I didn't come today because of class only, that's it, because it's online class. And the KRM people come. So how will you teach us also? So that's why I didn't come today. Oh, technology is great nowadays. You can have hybrid. Both online and offline can be together. Anyways, for this chapter, it's all right. There is no problem as such. OK. And you guys are taking KVPY also in November? So I'm writing. So I'm not. But both of you. So stay focused, all right? Anyways, we have learned the behavior of the junction diode or PN junction diode with respect to forward and reverse bias. Now, it is a little strange when you compare it with metals. So how metals or, let's say, a resistor, how it will behave is that they will follow Ohm's law. What is Ohm's law? Potential difference across the 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. But when it comes to semiconductor, things are strange. It doesn't follow Ohm's law. So every time we will be learning an instrument in semiconductor, we'll be talking about the characteristic graph. We'll be seeing how potential difference and current they are plotted against each other, how they are behaving. 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. And since we have just discussed PN junction, so let's try to understand the characteristic graph for PN junction itself right now. This is PN junction diode. Now, in order to draw characteristic graph, we have to measure potential difference and current across a diode and see the different values, measure it, and then plot the graph, simple. So the overall circuit to do that, it's like this. You have a PN junction. By the way, this is how you represent a PN junction in a circuit like this. This flat side, this is P side, and this is N side. So you have a PN junction like that. And in order to measure the potential difference across a PN junction, you connect a voltmeter across a PN. And I also have to measure the current through the diode. Fine. So this is milli emitter. And then it is connected like this. Tell me whether it is forward bias or reverse bias. Forward. Forward. It is forward because P side is connected to the positive terminal of the battery. OK. Now, this potential difference, which is the supply voltage, I can change. So that is why there is an arrow like that. Fine. So I can change this voltage, vary it, and accordingly, I'll get the different values of V and I. So this is forward bias. Now for the reverse bias also, I can have a very similar looking circuit. So we'll have a circuit. Now the reverse bias, negligible amount of current flows. So rather than milli emitter, I will connect micro emitter. So that the slightest of the current that is there should be measured. This is the reverse bias now. And I am connecting a voltmeter across a diode like this. So reverse bias, there should be no current only, no? See, reverse bias, there is a small amount of current that happens. Let me show you what we have discussed earlier. So this is the reverse bias. This one is reverse bias. So if few electrons from this side jump to P side, then there will be a current. Now the electron that could jump from N side to P side, that is because of what? Because of diffusion? Are you getting it? So higher concentration is on the N side. P side has lower concentration. When equilibrium is attained, then this width is constant. So there will be a small amount of diffusion current because of the flow from higher concentration to lower concentration. But then that is negligible. So that is why we say that it is like insulator. But then if you supply tremendous amount of voltage, suppose you supply huge voltage, then breakdown may happen. What do I mean by breakdown is that all these valence electron, which are there in this junction barrier, all the bonds will get broken. And then you have a lot of free charge circuits to carry that electricity. So in a reverse bias also, if I have huge amount of voltage, then at a certain point the entire junction diode will break and a surge of current you will observe. So if I plot current versus voltage using these circuits, treating reverse bias as negative voltage and forward bias as positive voltage, and this is what I'll get. This is current in the milliampere. This is voltage. Now if I say that positive voltage is forward bias and a negative voltage is reverse bias, then since current also changes its direction, I can say that the current in the reverse bias is negative and current in the forward bias is positive. So when it comes to forward bias, you will have a plot in the first quadrant where voltage and current both are positive. And if it is reverse bias, then since voltage and current both are negative, it will come in the third quadrant. Now tell me that if I supply a small amount of voltage in the forward bias, like very, very less, let's say 0.01 volt, what will happen? Will there be any current? I'll get it there. I'll get it there. I'll say yes, sir. Will there be any current? Oh, Amogh is watching from YouTube. I didn't know that. Amogh, you can also answer. Amogh, you went to picnic? You're watching this from your picnic spot? Are you at home? You are at home. Tell me if I have a very small voltage applied, let's say 0.01 volt, will there be any current? Yes or no? No, I'm not sure. See, when you apply a very small amount of current, very small amount of voltage, let's say 0.01 volt, that voltage may not be sufficient for electron to jump over that barrier, which was created earlier. Electron has to jump from n side to p side for electricity to get conducted. And in between there is a potential barrier. So external voltage should be more than that junction barrier, then only current will flow, fine? So typically the junction barrier in forward bias is around 0.6 to 0.7 volt, fine? So if you are applying a battery or voltage less than 0.6, 0.7, then the diode will not get activated. The current will be close to zero only. But if you apply, let's say one volt or three volt, then electrons will gain sufficient kind of energy to overcome the barrier and there will be a good amount of current, okay? And the entire diode will behave like a conductor with a potential difference of whatever is a junction barrier. Let's first draw the diagram, then I'll speak more about it. This is how it will be. It goes like this and then it grows. This is around 0.8 volts, where it starts jumping. This is the knee voltage, fine? This is a forward bias characteristic, all right? So let's say I have a diode, so this is P and this is N and the resistance are, this is connected like that. Let's say this is five volt. Can you guess what will be the current? How much is the current? This PN junction will act like a reverse voltage of 0.7 volt. So the current will be equal to five minus 0.7 which is forward bias junction barrier divided by R, fine? That's how it will work. So can you repeat that once and get that part? See, what I was saying that in forward bias, PN junction just creates a junction barrier, okay? Of let's say around 0.7 volt and after that junction barrier, if you overcome that, it behaves like a metal, okay? So if you supply external voltage of five volt, 0.7 volt, it requires to overcome the barrier and rest of the voltage you can treat as if it is connected to a metal piece, fine? So five minus 0.7 volt is the effective voltage across the resistance R. So in a forward bias, it behaves like this, fine? Now comes the reverse bias. In reverse bias, you have negative potential connected to the P side, fine? So in a forward bias, I'm slow increasing the voltage and current increases very fast. In reverse bias, I can increase the voltage faster. I can say that this 20 volt, this is 40, 60, this is around 80, fine? So if I do that and then measure the current and voltage, then this is what I will get. It goes on like this and then there is sudden surge of current. This is the breakdown that happens. This is breakdown voltage. At this voltage, the entire diode breaks down. What do you mean by breaking down is that at the junction barrier, the bonds get broken off and a lot of free electrons get generated that can start conducting electricity, fine? So there's a surge of free electrons and then the Y axis measures the current and in the reverse bias, I'm not measuring current in milliampere but I'm measuring in microampere, okay? Because it's a very small amount of current up till 80 volts till the breakdown happens, okay? Now, one thing which is noticeable in this graph is that, you know, this particular portion, this portion is flat. Why it is flat? Because after 80 volts, one breakdown happens, then it becomes exactly like a metal piece. There is no junction barrier also and sudden surge of electron comes in and also there is a large amount of external potential that is around 80 volts is already there. So suddenly current will rise and this line could be very flat. You may think that in a forward bias also, this line is flat but then if you notice, on the X axis, the potential is increasing very slowly, 0.1, 0.2 up to 0.8, okay? So a flat looking line here and a flat looking line this side, where the potential is changing rapidly, there is a much, there is a lot of difference, okay? A flat line over here is way flatter or more steeper than this line, okay? Because here this entire axis is shrink and this axis is stretched. So even after stretching the axis, this line remains flat, fine? So I can say that after breakdown, the voltage is sort of constant. Any doubt till now? No, sir. No, sir. Fine, so this is with respect to the characteristic of the PN junction diode. Now, there is no point defining static resistance for this particular, what is static resistance? A ratio between voltage and current. So there is no point talking about static resistance because at every point it changes and usually what happens, we are going to use a PN junction at the varying voltage scenario or varying current scenario. What we do, we measure the dynamic resistance that is change in voltage divided by change in current, okay? This is called dynamic resistance at any point in time, okay? Because the slope of V and I is not a constant. So this is not a straight line. Okay, even that may not be exactly straight line. So there is a curved portion also here. So what we do, we measure delta V by delta I and say that that is the dynamic resistance, fine? So there is a small numerical that's there. Let me project it on the screen. Just give me one moment. You can see this? So in current is 10, what's the voltage? When current is 10, see, it's 0.8 and when current is 20, the voltage is 0.9. Oh, sorry, when the current is 10, it is 0.7. And when current is 20, it is 0.8. So first answer is 10 ohms. From the next picking. Yes, sir. Others, Amogh, what is your answer? Message. Okay, so for the first- So again, 20. See, at 15 milli ampere, I need to get the value of resistance, okay? Now here, it is rapidly changing curve, okay? So that is why I'm trying to find here dynamic resistance, delta V by delta I. So delta V across 15 milli ampere is 20 minus 10, that is 10 milli ampere, or that is delta I, sorry. Delta I is 10 into 10 is so minus three. That is 10 is so minus two. And delta V is 0.8 minus 0.7. That is 0.1, okay? So 0.1 divided by 10 is so minus two. It will come out to be 10 ohms. Fine? Ramcharan, understood? Yes, sir, sir. Okay, now let's see with respect to minus 10 volts. Now for minus 10 volt, you can see that whatever you do, current almost remains minus one micro ampere only, okay? So dynamic resistance delta V by delta I, you will get that to be infinite. And that is in a way true also. It is close to infinite because changing voltage is not changing the current, okay? So rather than finding the dynamic resistance, we can give a rough estimate about how much is a ratio between voltage and current over here, fine? So that will be 10 divided by 10 is so minus six. It will come out to be 10 is for seven ohms. So when it comes to reverse bias in that zone where the curve is flat, you just take a ratio of voltage and current because delta I is close to zero and you can't have delta V by delta I. That becomes indeterminate. All right, so let us move to next topic. So till now we have only learned about how this PN junction actually functions, okay? Now we are going to learn about the application of the PN junction. We are going to learn about some of the instruments that we can make out of the characteristic what we have observed just now, VI characteristics. So this is a very special characteristic and you will slowly understand why the behavior of voltage and current, the way it is shown here is very special, okay? And we can exploit it in many different ways. So first application of PN junction is as a rectifier, fine? Do you know what is rectifier? No idea. So rectifier is a device that converts alternating current into direct current. Are you hearing rectifier term for the first time in your life? No, sir, we learned it in school. Oh, you already learned. Fine, so we are going here some of that's correct. Fine, so first we are going to learn about the half wave rectifier. What is half wave rectifier? Why it is called half wave rectifier? This thing will be clear when we discuss this particular device, okay? Let's first see the construction of half wave rectifier. This is how it will look like. All of you draw it with me. Almost every device which you use at your home, it actually runs on DC only, okay? Your TV, your fridge. What happens inside the TV or fridge or radio, whatever you use, inside it, there will be a rectifier that converts your AC current into DC current, okay? Then only that device runs. So that is why this has a good importance. And also when you put your mobile phone for charging, okay, you use an adapter for computer also, you have an adapter, fine. What does that adapter do? That adapter converts alternating current into direct current, fine? So this is what this rectifier device does. It converts alternating current into direct current, okay? So we are going to just look at the block diagram. Of course, the picture is not as simple as it looks, but then we can at least understand the principle of how it works. Draw it with me. And also one more thing, the transformer is one of the most integral part of any electronic circuit, okay? Because from outside, you supply 220 volt, but then suppose as an output, you just want five volt and two ampere, so there has to be a transformer inside the device, fine? So that is why, you know, I'm showing here this dotted line as a transformer. This is not a transformer that is connected outside your home, okay? This is a transformer of a device only. Small transformer will be there, okay? So this is primary circuit on the left-hand side. This dotted line encloses a transformer, fine? Then this one is a secondary coil, secondary, okay? Then you have point A and point B across which you have connected a load resistance, RL, okay? Now the primary input is nothing but your AC voltage. Okay, so how the AC voltage will be? It will be a sinusoidal curve which comes in your home. It will vary like this, isn't it? This is the primary voltage. Now let's see what this load resistance will feel. Now this load resistance is your device, okay? Suppose you're connecting mobile phone, fine? So mobile phone is the load resistance, fine? So load resistance is an equipment. So I'm going to see what this load resistance will feel. So let's define the sign convention first. According to sign convention, what I'm doing is I'm assuming that whenever there is a positive cycle of the voltage, I'm saying voltage of A is more than voltage of B, okay? And when it is a negative cycle, voltage of B is more than A, fine? So like this I can denote. Now tell me when it is positive cycle, the diode is forward bias or reverse bias? Forward bias. It is forward bias and when it is in a negative cycle diode is in reverse bias, okay? So during the positive cycle, it will conduct electricity, okay? And the current will go from X to Y. But when it comes to negative cycle, the diode becomes reverse bias and it no longer conducts electricity. And I can say that no current to the load resistance, fine? So what I get the voltage across RL is something like this. I'll get this, then the middle section will be out, fine? So I'll get only positive cycle. Negative is gone, okay? And since the current is not reversing its direction, it remains positive only throughout, I can say that it is direct current, fine? But then it is very lossy proposition because half the power is readily lost. Are you guys clear about it? Yes sir, yes sir. Okay, so this is half wave rectifier. Now why to lose so much power? Can we devise an instrument which converts entire cycle into direct current? Okay, so devise that converts entire cycle into direct current, that device is called a full wave rectifier, okay? So in half wave rectifier, we have only one diode. In full wave rectifier, we will have two diodes, okay? Let's see how the construction of full wave rectifier will be, okay? Write down, full wave rectifier. So I'll just draw the circuit diagram first and then we'll discuss about it, okay? You can also draw with me. It has two diodes like this. This is let's say diode one, that is diode two. This dotted line, it encloses the transformer. So there is a center tap that is there in the transformer. So you basically take the feed from the middle of the coil from the secondary, secondary coil of the transformer from the center of it, you take an input. This is center tap, okay? From the center tap goes like this and you are connecting a load resistance like this. This is X and that is Y, fine? Now let's see how it functions. So first I will draw the input waveform for the voltage. This is the input waveform, okay? And let me just draw an axis here where I'll plot the output one. So again, the sine convention remains the same. Positive means forward bias and negative means reverse bias. This positive, negative. Now let's see here, when it is a positive cycle, let's say this is point A, this is point B and that is point C. Can I say that VA is more than VC? Yes or no? For a positive cycle is VA more than VC? Yes sir. Right, that's how we have used sine convention. So for negative cycle, this is for positive cycle and for negative cycle VC is more than VA, okay? This is for negative cycle. Now let's say there's a positive cycle that is going on. Hence diode number one will be forward bias and diode two will be reverse bias, okay? So the current when it goes on like this, this path is closed because that is reverse bias diode. The current will just travel like this and that's how it completes the loop in a positive cycle. And when it comes to negative cycle, diode one becomes reverse bias, okay? And diode two becomes forward bias. In negative cycle, we have VC more than VB. So the current will go like this and it will again go from X to Y only and that's how it completes the loop, fine? The one thing you might have noticed here that no matter whether it is positive cycle or negative cycle, the current always, current always move from X to Y. So what does it mean? Potential of X will be more than potential of Y all the time, okay? So in both the cycles, positive or negative, VX is more than VY, right? So the load is feeling a direct current in a way. Fine? So that is the reason why you will see this kind of voltage across the load. Any doubt? So why do you connect to the ground? This one? Yeah. See, you have to take a reference somewhere. It's like finding potential energy, right? An object at a height H, the potential energy is MGH. So you have assumed that it is zero potential energy, the horizontal line. So like that, I have to assume some potential to be zero in order to find out all the potentials related to it. It's a common practice. It's nothing too great about it. Just it just gives you a convenient way of identifying the voltages, fine? Are you there Ramcharan? Yes, sir, yes, sir. Okay, so load resistance will always feel a positive voltage only. So this is a full wave rectifier because you have captured the entire wave and converted entire wave as direct current. That half a rectifier was incidental rectifier because you have made alternating current into direct current just by removing the negative part of it, but this is actually a full working rectifier where you convert part also to come out to be a direct current voltage, right? So this is how you convert AC into DC using full wave rectifier, but then even though the direction of current or voltage is not changing, this is not something which you desire, okay? What you desire is you need a constant voltage, a constant direct voltage you want, even though it is not fluctuating the voltage, but then the magnitude of the voltage is keep on changing. Direction is not changing, but the magnitude is changing. I want a constant magnitude voltage, right? So in order to achieve that, what we do is we connect a capacitor, fine? We call this a filter capacitor. So first let me draw a capacitor here. So this is how you connect. You connect a capacitor parallel to the load resistance. So write down capacitor connected parallel to load resistance, okay? This capacitor will act like a filter capacitor, fine? Now see what will happen because of this? The capacitor as the voltage increases, okay? As the voltage increases, capacitor will get charged, right? Now, first of all, understand that this plot is the plot for the potential difference between X and Y, okay? So what will happen here is that when the potential across the capacitor is decreasing. So whatever charge capacitor has stored during the charging or when the capacitor voltage was increasing, capacitor will gain the charge, right? So whatever charge it had gained earlier when the potential goes down, the capacitor will discharge and will just throw away the charge out, okay? Because you're decreasing the voltage. So when the capacitor throws away the charge, this current will sort of compensate for any decreasing current in the load resistance, fine? So in a way, the current in the load resistance largely remains constant, fine? So if current in the load resistance remains constant, even the potential will remain almost constant. So every time charging happens, it will slowly get charged and when discharging starts, capacitor discharges and make sure the current through X and Y remains constant and that will create sort of a uniform voltage. Although still a processing is required, but more or less you get, I mean, somewhere close to where you want it to be. All of you clear about it? Any doubt? No, sir. No, sir. Good. Let's now talk about other application of PN-gension. So there are so many applications of PN-gension itself, okay? So we'll take it one by one. So right now we have taken one application of PN-gension that is rectifier. The second application of PN-gension is from a very special purpose PN-gension diode that is Zener diode. Write down, I am going to talk about an application of a special purpose diode. It is called Zener diode. Now, Zener diode, first of all, is represented by this symbol. This is how you represent a Zener diode, okay? A unique thing about Zener diode is that once breakdown happens for the Zener diode, you can reuse it again. Usually what happens when some breakdown happens, the current becomes so high that the diode burns off, okay? Lot of heat gets generated and after that, the diode becomes useless. But Zener diode is not like that. You can reuse it, fine? So I hope you remember the characteristic graph of a diode. Zener diode characteristic graph is also very similar to just like any other diode. The only difference is it can be reused after the breakdown, okay? So we are going to use Zener diode as voltage regulator or voltage stabilizer, fine? Write down Zener diode as voltage stabilizer. Voltage regulator is better word as voltage regulator. So it regulates the voltage and it does not let the voltage increase beyond a point, okay? So let me first draw the circuit diagram of the voltage regulator using Zener diode, then we can discuss it in greater detail. This is how it is. So this is simple representation of the voltage regulator. What happens is that we are connecting load. Load is your device, okay? You're connecting a device parallel to the Zener diode, fine? So when an unregulated voltage comes as an input, okay? Then if you have like a huge amount of reverse bias voltage that is unregulated, then what will happen? Zener diode will break down, okay? And if Zener diode breaks down, lot of current will flow through the Zener diode, okay? And one more unique thing happens that across the Zener diode, the voltage becomes less than the Zener diode. The voltage becomes constant. So beyond this voltage, across the Zener diode, the voltage won't go beyond this, fine? So if you connect a load resistance parallel to the Zener diode, then after break down, it can become a constant voltage across the load, fine? So that is how it works. And there is a safety criteria also here. Safety criteria is like, you know, just to ensure that both load and Zener diode, they are safe. The safety criteria is the current in the Zener diode should be five times the current in the load. This actually ensures that Zener diode doesn't have such huge amount of current that even Zener diode burns off. And also it ensures that whatever is excess current that doesn't goes through the load, but only Zener diode captures it, okay? So I'll again repeat here. We are right now exploiting the fact that after break down the potential difference across the Zener diode becomes constant. We are exploiting that property. And how will you exploit that? You connect a load parallel to the Zener diode, okay? So whatever Zener diode potential is, which becomes constant after a while, the load will feel that resistance. So the load will feel that voltage, fine? So beyond a certain point, the voltage becomes constant no matter what is the input voltage, fine? So this RS, which you connect here, this is called series resistance in a voltage regulator. This series resistance is required, otherwise the current can grow very, very quickly, okay? Current can be very large. So in order to work safely, we have a series resistance that decreases the current little bit. And also do not forget this safety criteria, okay? Fine, so let us solve a numerical based on the voltage regulator. By the way, do you have any doubts? Anyone? No, sir, no, sir. Okay, fine, so here is a question. In a Zener regulated power supply, Zener diode has VZ to be equal to six volt. What does it mean? VZ is six volt. This is the breakdown potential, fine? VZ is six volt. The load current, which is IL, load current is given as four milliampere, okay? And unregulated voltage, that is the input voltage is given as 10 volt, okay? You need to find out the value of series resistance. RS should be equal to what? Okay? Keep in mind the safety criteria also. So this, both of you? So is it 0.6? 0.6 ohms? Yes, sir. Oh, no, no, no, I need to convert it. No, that's not correct. Amov did you? Okay, 300 ohms, Amov is saying, no, that's not correct. So that's 600? Nope. So is it 250? Let me solve this. See, unregulated voltage is 10 volt, fine? Voltage across the diode will be six volt, okay? Because the Zener diode, once the breakdown happens, which will happen in this scenario, because external voltage is 10 volt and Zener diode operates in the reverse bias. So this will be equal to six volt, okay? Now, the remaining voltage will feature here. This much will be how many volts? Four volts, isn't it? So we have load current to be equal to four into 10 raised to the power minus three ampere. So the Zener current should be equal to what? Zener current should be equal to five times the load current, safety criteria. So that is 20 into 10 raised to the power minus three ampere, fine? So now Zener current is this and the load current is this. So the current through RS is what? Current through RS is sum of load current and Zener current, fine? This becomes 24 into 10 raised to the power minus three ampere, isn't it? You can apply junction rule here. Current incoming should be equal to current this side plus current that side. This is IL and this is IZ, fine? So this is the current across RS and voltage across RS is four volt. So resistance RS will be equal to what? That will be equal to four divided by current, V by I, isn't it? So it will come out to be four divided by 2.4 into 10 is four two, okay? So it will be around 167 ohms. All of you clear? Yes, sir. Yes, sir. Where you made an error? So I didn't take the voltage like properly. I took it as the entire unregulated voltage and then I tried using Kirchoff's loop rule. You can use Kirchoff's loop rule, that is correct. You can't use Ohm's law and also see the reason why you guys are not getting it right is because this is different, okay? This, your first of, first thing, you are analyzing a circuit with a diode in it, okay? So that automatically gives you a lower confidence when you solve this particular question, okay? But then when you look at the solution, you know it's straightforward in a way, okay? So that hesitation or that lower confidence will go only when you practice a lot of questions. All right guys, so we have, I think, 10 more minutes. So I can briefly just introduce the next topic. Write down, optoelectronic junction devices, okay? Now, why it is called optoelectronic because light has a major role to play in these devices, okay? So light can be created, if it is getting created because of the junction, it is also an optoelectronic junction device or if light is used for its functioning, then also we call it to be an optoelectronic device, okay? So we are going to talk about these devices which utilize light or create light, okay? Can you think of few devices, optoelectronic junction devices? No? Solar cells? Solar cells, correct. And LEDs, okay? LED is also optoelectronic device, LED is light emitting diode, okay? Then we have photodiodes also. So I'll just quickly discuss them one by one. So photodiode, let's finish this particular topic. It will take, I think, 20 minutes more after 4.30, but then it is fine, okay? At least this will complete one chunk of the chapter which is about diodes, fine? So photodiodes is used to detect light, EM waves, okay? Second, LED. It converts electrical energy into light energy. And third, photovoltaic cells or simply we can say solar cells. This converts light energy into electrical energy. Okay, let us see how photodiode actually works. Photodiode, first of all, works in reverse bias, okay? This is the circuit of the application of photodiode. Ramu, Kondi, you're there, right? Yes, sir. Ramu is not there. So I'm here only. This is the reverse bias micro emitter like this, okay? And there will be a junction or a barrier, okay? This is the barrier zone. And if a light hits this barrier, then current gets created. Let's see first what we are talking about here is, first of all, PN junction is connected to reverse bias. So right now, negligible current or you can say no current is flowing. Why no current is flowing? Because in the barrier, there are no charge carriers, okay? So when the light, whose photons energy is H mu, hits the barrier, okay? And it hits the barrier. There's a chance that electrons from the valence band gain sufficient energy and the bonds get broken away and free electrons get generated, fine? So when this light hits the junction, some charge carriers can get generated. And because of that, current may be visible, fine? So you can detect the radiation using a photodiode. So if you know that what particular frequency or beyond what frequency the radiation should be, so that electron breaks away from a bond, using that knowledge, you can roughly guess what is the frequency of the light you're detecting, okay? Not only just frequency, you can actually detect the intensity of the light also because there is a characteristic curve between intensity, voltage, and current. Let's see how it is. Ajay, while I'm drawing this, can you answer me why it is working under its reverse bias? Why not forward bias? So because very little current will be generated. So reverse bias will be like my cramp, you're right? So that's a theory of reverse bias. No, that's not the reason. See, in reverse bias, almost no current is flowing. So even if slightest of the current flows, then you'll be able to detect. It's like the moon is there when sun is there, right? But it is very difficult to detect moon during daytime because a lot of light is already coming from the sun. But during nighttime, when sun is not there, you can easily detect the moons and stars. It's like that only. So the amount of current that this radiation will generate is so less that you'll be able to detect only when there was no current earlier. And that is why we are having reverse bias. And also in a reverse bias, the junction barrier will be wider. So you have wider space on which radiation can fall and current could get generated. Okay? So your junction should be transparent to the radiation so that radiation falls onto it and then photodiode will work under the reverse bias mode. Fine, any doubt? No, sir. No, sir. Okay, now let's talk about functioning of LEDs. Light emitting diodes. So is this chapter therefore advanced also? It was there, I guess, last year it was there. For mains, definitely a couple of questions come. And for mains, whatever question comes, there will be, you know, it is like, it's like inorganic chemistry. If you do it properly, you will get it right with almost good amount of certainty. Okay, so do this chapter properly because couple of questions will definitely come from this particular topic. Fine, let's talk about LEDs now. You might have seen LEDs and I think you might be using LED at your home also. So this is widely accepted as a device which can efficiently convert electrical energy into light energy. It has very, very high efficiency, okay? So LED works in, so LED works under the forward bias mode. So in a forward bias, what happens is, let's say this is the PN junction, okay? You have this as barrier, okay? So we have discussed that there is a dynamic equilibrium, isn't it? So the electron and holes, they recombine and they separate again and again. So there is a dynamic equilibrium that is happening, fine? Now every time electron combined with a hole at the P side, there's a difference in energy, right? Whenever electron combined, what will happen? It goes to lower energy, okay? And from the external circuit, it gets energy to again come out of the hole. So there is an equilibrium. Now when electron goes from higher energy state to lower energy state, as in when the hole captures it, then the difference in energy, difference in energy is given away as light H mu, okay? And if this light that is coming out, if the frequency of the light is in the visible spectrum, we can say that this is LED, okay? So you'll have red light, then you have blue light, white light, depending on what frequency of the light is coming out, different colors of LEDs will be there, okay? Now LEDs have certain advantages over the conventional lighting. So let's write it down one by one, first one. What do you think the first one will be? Quickly, any advantage you can think of, Fondi? So like for a small amount of current, it can produce a lot of light. Or you mean to say higher efficiency, right? Yeah. Correct, that is given like that's like that comes with it. Any other advantages you can think of? You might have seen that in LED lamps, how many voltage or how many watts typically is written in a box of LED light, have you ever noticed? No, sir. Never. You have never, ever seen the box of LED. Do you have an idea typically what volts, the typically what power LED light consumes? No idea. Anyways, so typically you'll see that for an incoming and decent light, which is around, yes, I'm correct. So let's say 100 watt bulb, which is, I'm talking about the conventional lamp, okay? So 100 watt bulb is like similar to let's say 18 watt CFL and this is similar to around three to five watt of LED, okay? So you can see that such small amount of power generate light which is equivalent to 100 watt of the conventional light. So it is very efficient, okay? So low operational voltage and less power, right Don? Low operational voltage and less power, okay? Then you have fast action. Fast action as in it quickly switched on, okay? No warm-up is required. The frequency which comes out from the LED is near monochromatic, okay? Which is around 100 Armstrong to 500 Armstrong. Typically there are like multiple wavelength that comes out from the light, but with LED since the cause of light coming out is a combination of electron and the whole. So that remains almost constant. So that is a near monochromatic light. This is an advantage if you want a near monochromatic light, okay? Then you have long life and ruggedness, okay? Okay, which is a biggest plus point of the LED light. It will have a long life. So even if you purchase it for a higher amount or a period of time since your electricity bill will be very less, your running cost is very less compared to the conventional lamp and within like a few years you will recover your cost. And then it becomes all the more meaningful to use LED. Then the last advantage is fast on and off, switching capability. Now you might be knowing already that there are some research going on to use LED as a Li-Fi device. So Li-Fi is similar to Wi-Fi. So rather than using radio waves for internet wireless connectivity we are going to use light, okay? So all I have to do is to transmit zero and one signal, right? So if you switch on a light, so that switching on could mean one, switching off could mean zero. So if you have fast on and off switching capability then using sensor you can detect how many times it's switched on and off. So very quickly you can transmit the signal or very quickly you can transmit the entire data. And the kind of speed we are talking about is gigabytes per second. So LEDs can be really useful. In future you can expect Li-Fi to be there. And once it comes this technology will just capture entire space very quickly. That's how things are happening nowadays. Like for example, nobody knew that there will be a solar and Uber couple of years back and suddenly it has captured entire things. So the acceptability of the market when it comes to anything which is better or efficient is very quick nowadays. All right, so this is LEDs. Do you have any doubts till now? Any doubt? No, sir. No, sir. Okay, let's talk about solar cells now. So this is the last topic we are going to discuss today. Okay. Solar cell actually converts light energy into electric energy. What do you think solar cells will be forward bias or reverse bias? Quickly. So reverse bias. Ramcharan? One option. How can it be biased? It itself creates the voltage, right? There is no point connecting battery if it itself is a battery. Okay. So PN junction itself is a battery. You're not connecting anything outside. Fine, so what happens? The light falls at the junction barrier. And again, what we discussed, if the energy of the photon is more than the energy required to break the bonds, then what will happen? The free electron gets generated and the current will start to flow. Okay, why current is flowing? Because there is an electric field here. You can see that once electron get generated here, electron will move this side because positive charge will attract and holes will go that side. Fine, so the current, because of the electron movement is this side and because the whole movement also that side. Fine, hence we have, because of H mu, current flowing in the circuit and current times resistance is the voltage. Okay, if you draw the characteristic graph of the solar cell, okay. It will be on the fourth quadrant because you can see that the battery potential is plus over here but then current flow is opposite, like usually what happens if battery is like that, the current flows like this, isn't it? But right now positive potential is on the right hand side but current is flowing towards the left hand side, okay. So I can say that I can assume voltage to be positive then current I have to assume to be negative because current is flowing in opposite direction of the usual way. That is why it is coming in the fourth quadrant and the kind of graph you get is like that. This is open circuit voltage, current is not there and this is short circuit, I short circuit. As in the resistance once it becomes zero, voltage is also zero close to that and current will be very large, okay. So like this you can understand how solar cell works. Fine. So that's it from my side. You have any doubts in the entire chapter till now? Anything? No, sir. Amog? No, sir. Sir, how do we ensure that light emitted is in the visible spectrum? Amog it is experimental and we may have documented stuff that, okay, if you have a PN junction of this material then light from this frequency will get emitted, okay. And if you know exactly what is the energy difference between a hole and electron. So once they recombine, the difference in energy gets emitted as a photon. So because of that, you will estimate what frequency will get emitted and you know the frequency range of the visible spectrum, okay. Fine, so that's it from my side and guys I know your school may have a lot of work it might be pressurizing, but then trust me if you focus at the right thing you will come out as much more happier person, okay. So many a times you have to take tough calls, fine. So that is your individualistic call. You have to take a call, what is more important, what can affect you more, all right. So according to me, if you're planning to be in India there is, I mean, there is hardly any thought you should give other than just what is coming in January first week, okay. So that's it from my side, we'll meet next week. Thank you. Okay, sir, thank you, sir. Thank you, sir. So tomorrow can we come to this end?