Welcome back to Electronics. This is Dr. Ferri. We're starting module three, on diodes. The first lesson is an introduction to PN Junctions. We're going to demonstrate the physics of semiconductors, which is the basis for a lot of our electronic devices. And in particular, we're going to introduce PN junctions and examine, particularly, their physical behavior. Let's start with something simple, a metal, a conductor, copper. Now copper, if you look at the structure of the copper atom, it's got one electron in its outer valence shell. And, in this particular case, it's very easy, it takes very little energy, to strip that electron away and have it free. So that it can flow through metal wire for example, and conduct electricity. Now, there's another set of elements called Semiconductors, which do not naturally have that tendency. It takes more energy to release their electrons. For example, silicon, atom shown here has four electrons in its outer shell. Well, the outer shell to be full would have eight, so it's only got four, and it just takes too much energy to release those as it is. But we can change the properties of semiconductors, and at certain times, make it more conductive. So, let's look at silicon in particular, one of the most common semiconductors. If I put two silicon atoms together, they tend to form a bond, because they would each like to have eight electrons in their outer valence shell. And so they tend to share electrons, and to make those, that eight. So here, I'm going to actually redraw this, emphasizing not the, the valence shell, here, but the inter-connections there, the bonding. Now when I put a lot of these together, I can form a lattice structure, where I've got electrons that are near each other sharing these, these electrons in their outer shell. And so by sharing them, they all tend to have eight electrons, even though they're shared. Now Doping is a procedure where we add impurities, such as boron or phosphorus to the semiconductor. And, when we do that, we can change the way it conducts. We can make it more conductive. For example, boron has three electrons in its outer shell, valence shell, and phosphorous has five electrons. If I look at the example of what happens when I dope it with boron. In this lattice structure, I replace some of the silicon atoms with boron atoms. Now boron, having only three electrons, we end up with a hole, here, where there's not an electron in the outer shell. There's only seven instead of eight. And I'm going to represent that in this diagram as an actual hole. So, this represents a hole there, where there's a spot for an extra electron to fill out the valence shell. We have two types of semiconductors, P-type and N-type. P-type are ones that are doped with elements that give it extra holes. So like boron, we've got the extra holes in here. The N-type semiconductors are semiconductors that are doped with extra, with elements that give it extra electrons. So, phosphorous, again, had five electrons in its outer shell, and so it contributes. When it contributes five, and these surrounding silicon atoms contribute four, well there's an extra one. And that extra one is, takes very little energy to have it move. And that means that we can have electricity flow through that. So again, by doping these semiconductors with either, something like boron or phosphorus, with these impurities, we can make it more conductive. A PN junction is formed by just putting these together, physically. The P-type and the N-type of, of semiconductors. And what happens at the interface, at the junction here, that there's a diffusion happens. That we've got these free electrons here, and they will diffuse over and fill the free holes that are on the other side of the junction. And what, why do they do that? Well, it, they do it because there's a little bit of energy or heat. So it might be ambient temperature, for example, in a room temperature. That's all it takes to do that. So, it gives it just enough energy to jump over and fill those, those holes. What happens, what's left over, is that. This used to be neutrally charged. And in fact, everything over, in this side here, was neutrally charged. But, now, it's lost an electron. Which means it's now a positively charged ion. Well, on the left side, where we've got the P-Type, this was all neutrally charged. But now, the ones that are near the junction here, they've got this extra electrons. And that makes them negatively charged. Not, next, I'm going to redraw this picture, but I'm going to take away this lattice structure. I want to show you the lattice structure you could, so you can see where these electrons are coming from or the holes they're coming from, but it becomes a little bit confusing in the diagrams and what I'm going to show you next. So, I'm just going to replace it with a diagram that just shows the extra free electrons that are due to the impurities and the extra free holes that are in the P-side. And again, all the free electrons near the junction migrate over to fuse over to fill the holes near the junction. And what's left, is a positively charged region over here, and a negatively charged region over here. Because remember, these elect, these atoms lost their electrons, and these atoms gained electrons. And this region is called the Depletion Region. We call it Depletion Region because it is depleted of any free carriers. So this positive lid, positive charge here and the negative charge here, creates an electric field in this direction. And this electric field causes this to form a kind of a, steady state size of this depletion region. Because, this, these electrons are going to want to Are going to want to move at, they're going to be attracted to these charges right here. The positive charges here, but because of this electric field electrons want to oppose this. So, electrons want to go in this direction because of the electric field. And the locally, the electrons want to go this direction because of the positive charge, and they balance each other. These two forces balance each other. And so we reach a steady state size of the depletion region. Now let's see what happens when we put the PN Junction into a circuit. Now first I put some metal contacts on there. On the P-type semiconductor side we'll, we'll call it an anode, and on the N-type side we'll call it a cathode. And then I, I'm going to put a battery here. This is a, a battery. It's got a, so we've got a potential across here. We've got the positive side here, which means the anode will be positively charged, and the cathode, negatively charged. What happens there? Because of the positive charge here, and the negative charge here. And assuming this battery is fairly large, has a fairly large voltage across it. We've got a very large electric field generated here, in this directions, which opposes this one. This one due to depletion region. This one's much smaller, so this one kind of swamps it. So these electrons are going to want to flow in this direction. What happens because of this large electric field, is that these electrons are going to want to flow in this direction. And as a result, the depletion region actually becomes smaller. And anything, any of the electrons that are close to the depletion region will jump over it, and the other electrons will just start drifting to the left. So, let's just show that drift. And then, we have new electrons coming in from the,from the cathode side, replacing any that were pushed to the left. So, we're going to have a steady flow of electrons from the cathode. And we're also going to have a steady drift to the left. So, let's show that. 'Kay, now at the same time, we're going to have a drifting of the holes to the right, towards the cathode. And again, they'll be jumping over the depletion region. And what happens is, that we've got a general flow, a steady flow, of positive charge carriers to the right. And, by definition of current, that means we've got a current flowing to the right, it's a, the flow of the positive charge. This is a PN Junction when it's conducting. We've got current flowing. Now, what happens when I replace this, I switch this around, I turn the battery around basically. So I've turn the battery around. I now have a positive charge here and a negative charge here. I've reversed the direction of my electric field. Now it's actually same direction as my depletion region electric field. And, my electrons are going to be attracted to the cathode. So I'm going to have the electrons actually moving to the right. And the same time my holes are going to be attracted to my anode. So there's this general drifting away from the depletion region. And what happens is all the free character, carriers, the electrons here, have left this region, and all the free holes have left this region. So, I've expanded the depletion region. It's becomes, since it's become very large, it becomes very difficult for the electrons on the anode side to jump over all the way over here, to the cathode side. And we reach a steady state where we've blocked the current. It's just too hard for these electrons to move across here. We've blocked the current. And so that's why we call it nonconducting. And that was achieved by reversing the polarity, on the battery here. To summarize the behavior, when it's conducting, we've got a positive charge on the anode and a negative charge on the cathode, and current flows. When we reverse the polarity, and we have a positive side on the cathode, negative on the anode. Current does not flow, and this is the not conducting region. Well, a PN Junction is the basic component of a diode, and that is really just essentially a PN Junction. I'm showing, and a diode is one of our most common electronic devices. Now, I've reshown it here, rather than showing it as a battery. I'm showing it as a voltage source that could vary. It could be positive or negative. And this is it, the, common IV characteristics of a diode, IV meaning, the current versus voltage. The current, through the diode, and the voltage across the diode, when that voltage is positive. So, when this is positive, it's conducting. The current is positive, the current is flowing. So this is a conducting region, out this way. And, in our later modules, we will have a module on diodes, we will call that the forward bias. So I'm just introducing the term here. Over here, when the voltage is negative, that's when I had, for example, reversed the direction of the battery, it was not conducting. And later, in our later modules, we will call that the reverse biode region. The reverse bias region. Now over here is when it breaks down. And that's beyond the, the model that we've looked at so far in examining the diode. The physics of the diode behavior. There are diodes that are purposely designed to work in this region, but it's beyond again, beyond the scope of the model that we've looked at so far. We just looked at the model where we, we can explain non-conducting and conducting behavior. Now this type of behavior is really important for building electronic devices and designing circuits. To have, just think about it, to have an element where you can block current if the voltage across it is in one direction, and it allows current to flow in another direction where we can, we can really control the behavior circuits that way. So in summary, we've said that semiconductors become better conductors when they're doped with impurities, either the P-type or N-type of impurities, and also when they're, exposed to heat or light. In other words added energy to it. We looked in particular on the physics of PN Junctions, which are the basic components of diodes. And to summarize the behavior of the PN Junctions, is right here, just showing the IV characteristics of this, of a diode. This is like standard characteristics for any diode. And it all depends on the, these voltage that's across the diode, if it's positive or negative. Now in the remainder of module three, on diodes. We will look at circuit analysis with simple diode models. And then we'll go through some applications. These are very common circuit models and circuits that you can use in a lot of different applications. Like rectifiers and detectors, LEDs, voltage limiters, voltage regulators and AC to DC conversion. Now please go to the forum and ask and answer questions. Thank you.
