 Let's get into the more meaningful type of classification. 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. This classification is with respect to energy banks. 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. Sir, I think either your voice or I lagged out a bit. Yeah, I am asking that what makes a conductor special? What is so special about conductor? Easily electricity can pass through it. 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. First electron 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 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 zero. 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 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 is a plot between the energy of the electron on the x-axis and the energy of the electron on the y-axis. And x-axis is just the extension of the space so that I can accommodate. I will 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 will 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, suppose this is the energy required for them to become free. So we can say that in case of metals, there are 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 atom's 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. So that makes the metal little bit special. This is the diagram or energy diagram for metals. Any doubt on this? No sir. Now let us see semiconductors. See with respect to metal, if you see, let us say you draw 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 does not 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. 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, where the conduction band will start from? A 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 am 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. It cannot 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. So the minimum energy gap between the valence and the conduction band is called EG. It is simply represented as energy gap. So this EG is less than 3 electron volt if I talk about semiconductors. So minimum amount of energy electrons required to break free is around 3 electron volt in semiconductor. If 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 am talking about. I am not taking electron out of the surface. I am just giving it sufficient energy so that it leaves a nucleus and remains still inside the material itself. 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 valence band and this is the conduction band. 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. So this I think gives you more insight with respect to how semiconductor behaves.