 In our last video, we talked about p-type semiconductors and n-type semiconductors. So we have the p-type, which is this guy right here. It has, as you see, an excess of holes, so it has a positive charge. On this side, we have the n-type semiconductor. It's got an excess of electrons, so it's got a negative charge. Now on their own, these things are pretty useless, but we find that they can become very useful, though. When we take this p-type semiconductor, this fella right here, and we put him right beside this guy right here. So if we take him, and we move him over there, we take this guy, and we move him over there, and we go, oh, get closer, oh, oh, oh, and boom. We got them right next to each other. What's going to happen is we've got what's called a p-enjunction diode. And this diode acts like an electrical valve, and it allows current to flow one direction, not to be confused with the band, but not another. So we will see that as we go through this, that you're going to have current is able to flow this way, but it is not able to flow this way, so it acts like a valve. So let's see what happens when we actually put them next to each other. Now what happens is the electrons in the n-type, so all these guys right here, they see the holes in the p-type. And the p-type guys, these guys see the electrons over here. Now everybody's mission in life is electrons like to fill holes. You can insert your innuendo joke right there. And holes like to be filled by electrons. So they kind of meet up in this middle area here. And let's take a look. Now once we put these p-type and n-type semiconductors next to each other, what will end up happening is you've got this excess of holes over here, this excess of electrons over here. So the electrons are going to make a run for this side, the holes are going to make a run for this side, and they're going to fill each other up. So it's kind of like a school dance where you've got the girls on one side over here, the guys on one side over here, the music starts, they all make a run for it. Some are left over here, some are left over here, but these, this is the dance happening in the center here. They're kind of bouncing, the electrons are bouncing from hole to hole to hole, not much is happening. This creates a region that has both holes and electrons called the party town. And we actually have a name for that, it's not called party town. We call this region the depletion region. Now like I said, not much is happening in this region right now, except for electrons are kind of bouncing from all around the area here from hole to hole to hole, not much direction going on. It's like this random current flow. Now things are going to change though when we apply an outside force to this. Now we're going to apply this outside force, we're going to call it a battery because that's what it is. And we're going to see what happens when we hook it up one direction. So if we have say positive on this side and negative on this side, or if we have the negative terminal of the battery here and the positive terminal of the battery there, that's our next goal. That is the terms reverse bias and forward bias. Now we've got it hooked up to a battery. I've got my negative terminal right here, my positive terminal here. Now remembering that likes attract, sorry likes repel and opposites attract, we're going to see that the holes that were once in the depletion region here, they're going to be attracted to this negative terminal here. So they're going to kind of stretch out to this side of the P type. The electrons, they're attracted to the positive terminal, they move over here. So our depletion region, which used to be just right in this area, has now become huge. It's gotten over this area here, there's no electrons, there's nothing happening in this area. Current will not flow. It makes this diode here act like an insulator. Now there's something that's to be said, as long as the, this guy has a rating that can handle this voltage. If this voltage is too great, these electrons will break free, but we'll talk about that in another video. At this point, these holes that were in the depletion region pull over here, the electrons pull over here and no current is flowing at this point. That is reverse bias. Now let's see what happens when we put this thing into forward bias. Now I place this guy into forward bias, which means that my N type, which is this side here, which has the excess of electrons is attached to the negative side of the battery. The P type is attached to the positive side of the battery. Again likes repel, right? So all these electrons here are going to push this guy through and any electrons that are here, it's just going to bump, bump, bump, and it's going to get pushed through the holes until we get to this point there. Once it gets through the holes, this guy is just going to travel on. And on down through here, the electrons get attracted there. Boom. And again, the whole series continues again. Current starts to flow in the circuit. So that is forward bias. We get some current flowing, whereas before when we're in reverse bias, it acted like an insulator. The holes just went this way, the electrons went that way, nothing happened. Now when that happens, we get a minimal, we have a depletion region still. Before it was like this big, then when we had reverse bias, it was huge. And now when we're in forest bias, forward bias, it's still got a little bit of a depletion region here. There is a little bit of potential difference. Now we need to discuss that because it's going to come up in calculations in the next video. With a silicon diode, when it is in forward bias, it will have a forward bias voltage of 0.7 volts. Again, this is only when it is in forward bias. When it is in reverse bias, it doesn't have a forward voltage. It ends up having to be equal to the voltage across the diode from this point to this point in reverse bias will be equal to this voltage here because those holes and those electrons balance out. It hits an equilibrium and it equals the voltage. We'll fool around with that in the calculation video. Now that's silicon. Another one that we deal with is called germanium and a germanium has a voltage, forward voltage rating of 0.3 volts, which is to say that the volt drop across here will be 0.3 if that's a germanium diode. For all of the calculations we're going to do, you can basically assume that we're going to use silicon. It's the most common type out there. So for all our calculations, let's assume 0.7. I will bring that up in the next video. So that's our PN junction diode. So they don't look like these two blocks where I've got a nice glowing red block here and a nice glowing blue block here. We have a schematic for them and it looks like this. So this is what our schematic roughly looks like of a diode. Now our N type side is going to be this side here. This is the N type. This will have the surplus of electrons. This is our P side, which has got the surplus of holes. They each have their own specific name. We're not going to call it the N side and the P side. What we're going to do is we're going to call this side here the cathode and we're going to call this side here the anode. And in the next video we're going to start pulling around with some calculations here. Just so we know that the cathode, this is going to be your N side. So this is going to be negative. This will be positive. In order for this guy to conduct, we have to have this side be more negative than this side, which means we can pass through it. Now I'll get more into that when we talk about diode characteristics in the next video.