 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 rectifier than 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 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 is, 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 to the power minus 2 to 10 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, semiconductors have moderate conductivity and moderate resistivity. So their resistivity is of the order of 10 to the power minus 5 till 10 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 to the power 5 to 10 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, right? Resistivity will be very high and conductivity will be very less. So the value of resistivity is of the order of 10 to the power 11 till 10 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 to the power minus 2. And the resistivity of the nonmetal, it starts from 10 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'll 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 poly anthracines. So it is doped. And organic polymers 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.