 Hi, I'm Zor. Welcome to Inizor Education. Today we continue talking about using semiconductors in electronics, and we will talk about transistors. Yes, that's the name of this lecture. If you remember when we were talking about electronics in the very beginning, well, historically vacuum tubes were used to create diodes and triodes. And from these devices, we then built some logical or logical and logical not, etc. Now, with advance of semiconductors, we would like to implement the same functionality in semiconductors because it's a better technology. Well, we were talking about diodes before in a previous lecture using semiconductors. Just a regular PN junction was used to implement the functionality of diodes. And now we will talk about the implementation of functionality we are familiar from triodes in semiconductors. Well, in semiconductors it usually called transistors, not triodes, for different historical reasons. It doesn't really matter. But the functionality is very similar. And this lecture is about how exactly the transistor is built based on semiconductors. Well, basically based on PN junctions, obviously. And well, we're not talking about technology of this. We're talking commonly about functionality and ideas because there are many, many details. The whole industry of semiconductors was advancing very, very rapidly and for a substantial amount of time. So we're not actually going through this. We'll talk about functionality, about idea, about the principles of how the transistors are working. In particular, we will talk about only one type of transistors, which is called bipolar junction transistors. And I'll talk about why it's called bipolar. Now, other transistors also have some similar functionality, similar principles of how they work. But our purpose is just to explain the principles, not to cover the whole business of semiconductors. I would like you to understand how we can implement certain functionality which we need in electronics in one particular case of one particular transistor. Okay. Now, this lecture is part of the course called Physics for Teens. It's implemented on Unisor.com. It's implemented as a set of lectures like this one. Every lecture has very detailed notes on this website. So if you found this lecture on YouTube or somewhere else, I do recommend you to go to Unisor.com because all these lectures are combined into a course. Course has certain logical sequence, interdependencies, et cetera. So I would rather encourage you to take the course. Now, there is a prerequisite course called Math for Teens on the same site with the same structure, the same type of lecture and details in writing. Then there are many problems solved, exams which you can take. And the beauty of it is it's completely free. There are no ads, no strings attached. If you don't want to sign in, don't sign in. There is certain advantage functionality if you sign in, but you don't have to. Okay. So let's get back to business. Now, to properly understand how transistors are working, but I would like to present the way how it was built, at least in my mind. I'm not sure that people who built it really were thinking in exactly the same fashion, but I think if I introduce you gradually how transistors can be built, what kind of functionality we have, what we don't have and how to improve to get that more functionality. That's, I believe, is the right explanation which would help you to understand better how transistors are working. So we will start with a simple thing. This is our N-type semiconductor and I will connect it to some kind of source of electricity battery. So this is minus, this is plus. Now this is N-type. Well, as an example, we will take silicon with phosphorus atoms additives. It's called doped, actually. So the phosphorus is doping for silicon. Okay. So phosphorus has five electrons on the valence orbit. Silicon has four. So when silicon atoms are attached to each other, they're attached in the crystalline structure with a pair of adjacent atoms sharing electrons. So each atom has eight, four of its own, and four which are borrowed from other atoms around it. This is another atom. This is another atom. This is another atoms. So these are paired electrons. It's called covalent bonds, if you remember. And this is a structure, crystalline structure of silicon. Well, not in a flat plane, obviously. It's a 3D structure, but it's convenient to present it in this way. Now if you have a phosphorus atom, it has fifth electron which is not connected using the covalent bonds to anything. So there is definitely attraction between this electron and nucleus because nucleus also has a certain amount of protons which corresponds to the amount of neutrons, electrons, so the whole atom is neutral. So there is an electrostatic attraction between this electron and the nucleus, but there are no covalent bonds it's connected to. Or other electrons which belong to atoms of silicon. They are doubly linked first through electrostatic attraction to corresponding nuclei and also pairs of electrons are connected together using covalent bonds. That makes this particular electron freer to move around because sometimes the electrostatic attraction is not sufficient to keep it on the orbit. If we have some kind of a excitement from outside, that electron would move. And this is an excitement. This is a battery. So there is an additional plus and minus here, plus attracts electrons, minus repels them and some electrons which are not connected to covalent bonds like this one from a phosphorus atom. So this is a silicon and this is a phosphorus and this is a silicon again. So these atoms are leaving their orbits around nucleus of phosphorus and they are attracted to positive electrode. Now this becomes positively charged because it used to be neutral when the number of protons was equal to the number of electrons in phosphorus. Now one electron is lost. So from here this is a negative. Negative means excess of electrons. Electrons from here actually migrate to fill up this particular hole. And then it goes again, leaves it because there is this attraction from the plus. So there is a small electrical current basically based on these electrons which are going to the positive and then from the negative go the next, the new ones. Now obviously the electric potential of this battery, the voltage between these two electrodes should be sufficient enough to overcome the electrostatic forces but not sufficient enough to overcome both electrostatic and covalent bonds. So there is a relatively narrow range of voltage which we can apply for this whole thing to work. And obviously it took a lot of time for people to understand this type of thing and what kind of a voltage, what kind of material is used, what's the density of phosphorus atoms among silicon because the more doping we introduce, the more additives we introduce, the more free electrons and the more current can be and maybe the voltage should be different. So these are all the technical details which we are not talking about. So we are assuming that the voltage is exactly in this range. It's greater than needed to overcome only electrostatic in this case like phosphorus atom but not sufficient to rip all other electrons from their orbits and break the covalent bonds. So that's very important. Okay now this is basically just the current will be. Obviously our functionality which we are thinking about, the triode. Remember what triode was doing? It was amplifying the signal and it was actually, it could be used as an on switch. If you will put into this base the middle part between the two electrodes, between the cathode and anode, if you will put something, some voltage in between it was like a net or something like a base, then electrons will not go from one to another. So we are talking about this functionality. Okay, next step. Now we are building actually transistors, right? So we have to introduce steps by step. So this is the first step and obviously it has some functionality but it's not what we need. So what we do, we will do the following step. We will break it into halves and put p-type in between. Now p-type if you remember is when you have a deficiency of valent electrons. So it's not phosphorus, it's let's say boron. It has three valent electrons. So these three are connected through covalent bonds but there is no fourth one. So this one becomes only connected to its own nucleus using electrostatic. Now so what happens is in this particular case if we will apply again some excitement then some electrons might actually jump from one covalent bond to another. So this is like a hole and the hole will move. So electrons again will move towards one side and holes will be moving to another side. Obviously electrons will be moving closer to positive and holes will be moving to the negative. And in this case we are talking about holes more than electrons because it's the excess of holes in this particular case. If in case of m-type of semiconductor we had excess of electrons in this case we have excess of holes and it's always convenient to talk about something which we have the excess of not deficiency of. So excess of holes is moving towards negative and excess of and not excess just deficiency of electrons. Electrons are moving towards this part. But anyway we will you know what p-type semiconductor is. So we will insert this type of semiconductor here and see what happens. So what happens? Well first of all I would like to say that we did have some kind of a current when there was no p-type in between. Now the current will stop and let's think about why it happens. Well first of all you do remember from the previous lectures about p-n junction or n-p junction. So let's talk about this n-p junction. Now this is minus, this is plus. So this has excess of electrons. Well what happens in the junction is if you have an n-type which has excess of electrons they migrate through this border through this junction line onto the p-side. It's just because these electrons free electrons from from phosphorus let's say they are not really very well connected and if there is some kind of a excitement like a voltage they're moving. So they will move to the p-type. Okay what happens at p-type these electrons will take places inside the hole of the p-type. So the covalent bond will be filled. Well that's good. Now what's bad? Because now this atom becomes negatively charged. Well once it's negatively charged these electrons especially in the borderline right near here will actually accumulate and more electrons are accumulated and are taking place inside the covalent bond structure. The more barrier they make they repel all other electrons and very soon the current will stop. The migration of free electrons from here will stop migrating here because this p-type will be saturated. Same thing with here but here it's even worse because from here electrons are pushed from this side towards the boundary and eventually they will accumulate enough electrons to prevent any other movement. In this case electrons are actually gravitating to this so these free electrons which this m-type has they will migrate to plus and they will actually disappear even and well that's it there are no more free electrons and the potential the voltage is not sufficient to break silicon covalent bonds which are reinforced by electrostatic. So we have as soon as all the free electrons from phosphorus are migrated here the rest is not actually the rest of electrons is not actually moving because it's connected with both electrostatic and covalent bonds and the voltage is not sufficient to break that pair of forces. So the current stops that's what happens. Okay now the next step is what I will do is I will do this I will introduce another battery here. Now technically you can connect these two it's not two different connections it's obviously one wire but it doesn't really matter. So what happens in this case well this is plus this is minus but we had if you remember access of electrons here which have migrated took position inside the atoms of boron for instance in this case filled the holes basically so restored the covalent bonds but now these electrons are connected only with covalent bonds not with electrostatic so again it's only one force rather than two and since it's a one force they can be attracted by relatively weak voltage here plus this is plus so these electrons will flow here and they will weaken the barrier the more electrons will go here the weaker the barrier and the sooner the barrier is broken the electrons start moving here okay the barrier is broken now it all depends on how strong these things are if it's strong enough that these electrons are partially going here and most part going further from P to N these electrons the extra electrons which are here these electrons have already left the phosphorus three electrons is left so what's remaining is in atoms of phosphorus they become positively charged as well right becomes positive because the phosphorus is losing if this is a phosphorus and it has an extra electrons and it's moving here then it becomes positively charged so these electrons which are coming through the barrier are actually attracted attracted here and they go and that's what actually make this current again working so this current is working as well because we are having new electrons supplied to the end side but what's important we're consuming electrons which are making a barrier here we're consuming it here in this place and those new electrons are coming here they are actually going through because this part becomes positively charged since we are losing electrons it might be a little confusing N stands for negative P stands for positive but that's only if there is no current if there is a current and these negative things it's because extra electrons the atoms are neutral phosphorus atoms are neutral but as soon as they lose electron they become positively charged so it might be a little confusing but anyway so that's how the functionality of the whole thing actually is is is working now we can actually regulate using different potentials here and there even the smaller potentials here because don't need really strong thing because to to get these electrons down here all we have to do is we have to break only the covalent bond not both covalent and electrostatic right in this case we are only breaking the electrostatic bonds from the atoms of phosphorus the three electrons they're kept only electrostatically not covalent bonds so again so in both cases in this case we are actually breaking the electrostatic bonds that's why this is usually a stronger voltage and this one is breaking the covalent bonds it's a weaker voltage but still a voltage and again it took a lot of time for people to understand what kind of voltage works and what kind of situations but now what happens is with a smaller voltage here we can actually control the bigger voltage there because if we will increase this for instance this voltage the flow of electrons which migrated through this border will increase which makes the border freer for these electrons to penetrate by the way this usually is a really narrow kind of a sink narrow layer between two different n-type so the electrons will will penetrate to a greater degree if we will suck down more electrons from the border area around NP junction so without this this border area will be saturated with electrons very fast but with this battery attached it will actually take these electrons out from the border and free the flow of electrons through this one to a p-type and then it becomes positively charged so electrons will go even further again freeing a hole and new electrons will fill that hole and then the positive charge will attract them further etc so that's how the whole thing is moving so that's how by changing this we can change this flow and again our research and experiments show how to using a smaller change in this you can achieve greater change in this current and that's how amplifier is working right because it's obviously going in sync as we are changing signal here let's say it's some kind of analog signal like a speech electronically expressed in the current for instance then this current will be in sync exactly with this with these signals and it will be stronger or we can also implement a switch on off switch by basically changing this to a switch or or putting a switch here somewhere for instance if you will break this well the flow will stop so if we will have just a switch on and off it will convert it into a switch here and again using a switch with a smaller kind of current which goes through this you know switch can be actually very sophisticated because if it's a big current if you remember we were talking about lots of different things when you're switching things on and off then we can control switching on it often a much much more stronger current okay now in reality if you will take a look at different pictures you will have different kind of pictures we will have resistors here for instance resistors here it's all details which are important for practical aspects of using the semiconductor what we are trying to address right now is just the principles how the p-n junction and p or p-n junction can actually assist in achieving the same function analogy we were learning as far as when we were talking about electronics and diodes and triodes etc so now after we have basically familiarized with ideas of how semiconductor based diodes and triodes or transistors are working then you understand that using exactly the same techniques as we were talking about before addressing the logic like end operation or operation not operation etc we can implement exactly the same thing in semiconductors so that's where this silicon piece of silicon were used because then there is some technology which will add certain additives in one place one additive in another place another additive and that's how the whole integrated circuits are arranged it's a huge it's billions of little details like this on a small piece of silicon it's extremely difficult technology extremely sophisticated but the principle is this exactly the same and if you wish I can say that's how computers are built okay I suggest you to read the notes for this lecture on unison.com it's in physics routine course the part is called electromagnetism and then if you will open it there is semiconductors and electronics part which contains a few lectures like the previous one about diodes and this one about transistors and by the way the name of the transistor which I'm talking about the real name is bipolar junction transistor well you understand why it's junction now bipolar because we have two different n types now in this particular case it's not just bipolar it's NPN bipolar junction transistor because it's NPN now there is a PNP type transistors then it's slightly different arrangement but again ideas is exactly the same something gets through the border the border gets saturated and then when some new source of electricity is attached we are weakening the barrier between a different type of semiconductors and the current actually goes okay that's it for today thank you very much and good luck