 Hi, I'm Zor. Welcome to a new Zor education. I would like to have a few lectures dedicated to semiconductors. Well Semiconductors are very important and partially I did touch this when talking about solar cells and how we generate energy from Sun. So I did explain at the time basically what semiconductors are and today I will partially repeat it, partially explain something about atomic structure. So basically you will not learn a lot of new information today relative to whatever was in that lecture about using Sun to generate electricity. However, it's very important because this will actually gradually bring us to how transistors are made using semiconductors and that's very very important because before in electronics we were using basically old models of diodes, triodes, etc. Using vacuum tubes and heated elements, etc. Contemporary electronics is built on semiconductors and that's why it's very important to get into this particular theory because it approaches us to how transistors are made. Okay, now this lecture is part of the course called Physics for Teens. It's presented on Unisor.com. This is the course, actually. So if you find this lecture somewhere on YouTube, on any other source, you should understand it's only one lecture, but the website Unisor.com contains the whole course Physics for Teens and well, it has basically the whole curriculum and it's a set of lectures which are logically connected to each other. I'm usually going always forward and using something which I have already explained in future lectures. So I do recommend you to use the website. Besides, the course has a prerequisite course called Math for Teens. You can't approach physics without math, so your math knowledge should be really good and if it's not, use the Math for Teens on the same website. Everything on this website is completely free. There are no ads, no strings attached. You don't even have to sign in, but you might actually choose if you would like to retain your exam results, for instance. Okay, so let's go into theory of semiconductors. Well, first of all, let's talk about conductivity and why certain elements have more conductivity and less resistance than others. Well, here we have to go to atomic structure. Now, I will probably again talk about atomic structure in more details in other set of lectures dedicated to what actually atom is, but for now you basically remember this orbital model of atom when you have protons and neutrons comprising a nucleus of the atom and certain... So this is a nucleus which contains protons and neutrons and then certain number of orbits where electrons are circulating around. Well, you can actually say circulating, but it's not really a real like circulating of a satellite around the planet, although this is a model and this model satisfies our experimental data. So let's just think right now that these are orbits. Now, in more precise terminology, each orbit, consider you have the three-dimensional things, right? So orbit can be in this plane or can be in that plane, even if it's on the same distance from the nucleus. So we usually should talk about the bend. So this orbit usually contains... It's not really like a fixed orbit. It's a bend of orbits, if you wish. So what's interesting is that according to certain theories, which I'm not going to mention exactly, there are only fixed reduces, if you wish, of those bends. So if electron becomes excited, it can actually jump to the next bend. It cannot be in between the bends. So at certain level of energy, electrons is on this orbit, but if we were excited, like heating, for instance, or put the sunlight, for instance, on the surface, the electrons which are getting a little bit more energy becomes more excited, and that energy should really result in something. And in this particular case, the electron can jump from one bend to another. Now, if the distance between these bends is smaller, it's easier for electron to jump from one to another, and then to another, and then to another, and then all the way out, maybe, from the atom and become a free electron. If this distance is relatively large, well, relatively, obviously, on an atomic level, then it becomes more difficult, which means we have to give more energy to electron to jump to a higher orbit, to a higher bend. So those elements which have a very small distance between these bends, and I'm not going to talk about why, I just don't know, but those elements have a better conductivity because electrons are easier to excite. So you don't really have to have enough heat or light or something else, or electric potential, if you wish, to excite them, and they're easier going out. It's like energy later, if you wish. It's more difficult to jump from level to level in this case than into this case. So even a smaller amount of energy allows electrons to go up this ladder of energy, but if the step is very large, then you need more energy for electron to jump. And that's what actually determines the conductivity. Those elements with smaller distance between the bends, for instance, metals, they have a better conductivity. You don't need a lot of, let's say, electric potential on both sides of the piece of metal to push electrons to go from one place to another. But if it's a glass or a diamond, where the distance between these bends is very, very large, relatively atomic dimensions, then you really cannot push electrons from one place to another if they are connected with a diamond or a glass or something, which is really insulator. So that's why we have conductors. Those are elements with smaller distance between these bends. And we have insulators where this distance is very large, relatively very large. Well, there are in-between cases. It's not really very, very large, but it's not really small either, which means that under normal conditions, these elements are insulators. But if you will just heat it a little bit or maybe do something else, and we will talk about what exactly that something else is, they become conductors. So the perfect example of this is silicon. And if you will take a piece of silicon and you will put some electric potential here, under normal condition, the electricity will not be going through. Electrons will not move through this silicon. But if you will heat it, then electricity will start going. Typical experiment, you just put a source of energy here and a lamp here. Under normal condition, the lamp will not build it up. But if you will heat it, then you will see that the lamp lights up. So, silicon is one of those elements which have this distance between the bends, not very large, but not very small. So, under normal circumstances like that, it's not a conductor, but you will have to do something like heating, for instance, and it becomes a conductor. Okay, so that's the conductivity and that's what exactly semiconductors are. These are elements which have this distance between the bends, not very large, not very small, semiconductors. Example, perfect example, is silicon and that's what's mostly used. And there are some others. We're not talking about basically the practical aspect, that's just enough that silicon is a perfect example of a semiconductor. Now, how to make it a conductor? I just said you can heat it up, for instance. Well, that's not very convenient. What people do, they do a very interesting and smart thing. Now, let's talk about silicon inside. Well, inside, silicon has 14 electrons and 14 protons in a nucleus. That's why it's electrically neutral. Okay, now, these electrons, so if this is a nucleus, then there are two electrons on the first orbit or in the first bend of orbit. Then you have next orbit where you have eight electrons so that's 10. Now we need 14. So the next one has four electrons. Now, I picture this on a flat surface. Obviously, this is a three-dimensional structure. So how these orbits are arranged relatively to each other, one electron can go this way, another can go this way, et cetera. So that's not exactly, but as a model, it fits perfectly our purpose. So the silicon atoms are arranged in some kind of a lattice. And the lattice basically means that each one, you see, these are four electrons and each one is connected to four other atoms. And these electrons actually become connected to other electrons from the other atom. So the pair of electrons here and I will use only the outer orbit. And this is another atom. This is a nucleus. So these two electrons, they're valent electrons. And again, we'll talk about atomic structure in more details later. But right now, for our purpose, they are related to each other and they become, it's called covalent links. So these are linked together. And that's what keeps actually the whole lattice in shape. It connects atoms and that's why silicon has a structure basically inside. It's not like glass, for instance, or water where molecules are not really related to each other. In silicon, you have a really crystallized structure. And that's what these covalent links make the crystallized structure. Now, how it's arranged, again, right now we're not talking, but there is a link and that what keeps the whole ledges together. So that's basically what we have here. Each one is connected and that's and that's what actually keeps the crystallized structure in shape. Now, why silicon actually can be made a conductor if you heat it up? Well, when you heat it up, you excite these electrons and when they are excited, they're not really as much connected to each other. They would like to really do something, move somewhere and they can fly away. If they fly away, they become free electrons and then if you have a potential attached to the whole piece of silicon, these free electrons might actually migrate. Then these electrons will go to a positive and from a negative side new electrons might take their place. So that's why the whole thing is working. It's all in movement. So we are not losing actually the number of electrons, they're just moving gradually to this side because this is generated new ones to compensate for the lost electrons and that's how electricity is moving. But you have to really like heat it up or do something. Can we do something different to force electrons to become free? Yes, we can. And here is a very important trick if you wish. People came up with let's say you have this structure, this is silicon, this is silicon, this is silicon, this is silicon. And we introduce impurity instead of silicon inside of this. We will introduce let's say phosphorus which has 15 atomic weight which means 15 protons and 15 electrons. So on the outer orbit, so inside we have on the first level we have two, next level eight and the next five. So we have this electron. Now this is phosphorus, these are silicones. Now silicones, they're all connected to each other, right? This connected to this, connected to this, they're all connected into structure. But for this particular electron which belongs to this phosphorus atom, there is no pair to link to which means it's easier for this electron to become free. So this mix of a base silicon and some impurities introduced into silicon phosphorus atoms, this new kind of a material has a better conductivity. It becomes really a conductor and that's actually the something which is extremely important for all the electronics. So depending on amount of phosphorus or whatever other characteristics, I'm not really sure exactly which ones, this becomes better or worse but it becomes a conductor. So you might not actually need to heat it up or maybe you need to have a much smaller difference in electric potential to force electrons to move in this particular material. So material which contains base elements, silicon and certain impurities introduced in terms of phosphorus atom and again it might not necessarily be phosphorus but it's just one of the things which are obvious. When you have extra electron which does not have any pair to make a crystallized structure, it doesn't have this covalent link to anything. So basically in this particular case for four atoms of silicon being one atom of phosphorus, right? So that's probably more or less the kind of proportion if you wish. So then you will have one electron available. You will have basically as many electrons available as many atoms of phosphorus are introduced. And now again as I'm saying even a smaller difference in electric potential will force electrons to move to a positive pole. So this particular semi-conductivity it's called n-type where n stands for negative, it's electron. Electron is negative. So that's one type of changing the semiconductor into conductor or almost the conductor by introducing this impurity with extra electron on the orbit. There is another way. Let's instead of this put boron 5. Boron 5 has 5 protons and 5 electrons. 2 on the inner orbit and then 3 on the next one. So we don't have this. On the outer orbit as far as valent electrons there are only three of them. So now this one becomes not connected using the covalent link, right? So again it's easier to basically bump it if you have certain electric potential arranged. So if you have this then this electron will go. Now then what happens next instead some other electron should take its place. So and you have basically an absence of electron in another place. So you have holes basically. So this particular picture is almost equivalent to that one which we have for phosphorus. But then phosphorus electrons are moving here to a positive side. And these are extra electrons, right? So we don't really have any kind of a change of numbers of electrons in every atom of silicon. In this case it's the holes which actually are moving. The electrons, three electrons move here, but the holes which remain move to the opposite direction. The hole basically plays exactly the same role as a positive particle if you wish. So whenever electron moves here, the hole moves here, then something replaces the electron in this particular position from let's say here or there or somewhere else. And here you have a hole. So holes are actually moving to the left and electrons are moving to the right. So that's why what I'm saying is that the holes are kind of equivalent to positive, electrically positively charged particles. So this is called a P type semiconductor. P stands for positive. Hole is positive. So either phosphorus or borous, they do have this kind of better conductivity than silicon by itself. So we have two new materials. We have N type of material and we have P type of material. And these are basic components from which we can start building more complex and interesting devices called transistors, which we will talk about later on. But today I would like to finish with one very interesting case which we can have. What happens if you have two different materials, two different semiconductors, P type and N type connected like two different disks. One is with silicon disk with phosphorus, another silicon disk with boron. So one is a P type, another is an N type and they are connected to each other. This is P type and this is N type. So what happens in the border? That's very interesting actually process. So if this is an N type, we have extra electrons here which are actually, well, they're just by themselves, they are fluctuating because they are not connected to another atom to another electron actually using covalent links. So these are extra electrons which do not fit into a crystallized structure of silicon. So they are moving, randomly moving within the material itself. And some of them might actually penetrate this border. Same thing here, analogously. There are holes here because boron, boron has only three electrons so there is a hole in crystallized structure which can be fit from another. So the holes are actually moving. Electrons are fitting the hole which means the hole moving somewhere else. Now what actually becomes, what's very interesting is, now if there is no movement of these electrons, the whole thing is neutral because the phosphorous which adds this electron to orbit, it has extra proton. So the whole thing is neutral. But if this electron moves here, this becomes positively charged, right? Now if the hole moves here, this becomes negatively charged. So just by diffusion, just by normal process of random walking of these electrons and holes, we have, this becomes negatively charged on the border and this becomes positively charged on the border from all those electrons moving from here to here and holes moving from here to there. Because we need to move these holes to maintain the crystallized structure. That's why we have deficiency of holes here and deficiency of electrons here. So this becomes positively, this becomes negatively around the margin. It's not actually, don't go too far deep. Why? Because as soon as this becomes negative, it prevents other electrons to go there. Because this negative charge will repel other electrons. So this process goes up to a certain extent, which means that there is certain difference in potential between this place and this place. So these two n-type and p-type materials when combined together, basically that's enough to produce certain difference in potential. And that's exactly where the sunlight is important in producing the electricity from solar batteries. Because whenever we are putting sunlight into the n-type, it excites more and these electrons are greater, to a greater quantity, migrate to this. Because the more excited they are, the deeper they go into this. And if you will actually put some kind of a consumer of electricity, then the electricity will move here and the new sunlights will move, new electrons, so the whole thing starts moving by itself and that's how the energy is generated. That's what I was explaining in the lecture about solar batteries. But meanwhile, right now, all I would like to end up this lecture with, that if you have this border between n-type and p-type, there is a certain difference in potential which becomes, because electrons are migrating from here to there and holes are migrated from there to here. And why are they actually doing it? Because they are replacing covalent links missing here and extra here. So they're extra, so they're migrating and they're filling up those holes in the crystallized structure and this becomes negative, this becomes positive basically. But the crystallized structure becomes, so basically to maintain the crystallized structure is important for some reason. I mean, these are holes and extra electrons which are replacing each other. Extra electrons from here replace the holes here and that's what actually makes the crystallized structure. So it's covalent bonds covalent links are filled and the crystallized structure is maintained. So that's probably the whole reason why it's happening. And that's basically the end of this particular lecture. I want to introduce you to the concept of p-type and n-type semiconductors and these are the bases from which transistors are made and that's the subject of future lectures. Okay, thank you very much. I do suggest you to read notes for this lecture on Unizord.com, so you go to Physics 14's course. That's electromagnetism and there you will see the semiconductors on the menu. This is the first lecture in this particular chapter. Thanks very much and good luck.