So this right here is a bipolar junction Transistor which is a pretty common kind of transistor and this particular one is an nPN transistor and so this transistor is made up of three layers of Silicon like this and maybe maybe you can guess what it means for it to be an nPN transistor these two layers of Silicon are N-Type and This layer here is P-type sand p.m. And if you haven't watched my previous video where I where I go into quite a bit of detail about what these n-type and P-type Silicon is you should definitely go [back] and watch that otherwise the rest of this isn't going to make a whole time sense and So this is an NPN transistor, and they make Pnp transistors as well But [well] we'll just talk about nPN for now, and so this here is the circuit diagram for a transistor And it's got the three leads coming off of it. There's a Emitter a base and a collector, and if you look at the actual physical transistor there's three leads coming off And this is the emitter the middle one is the base and this is the collector although That's not really standardized, but in this particular case. That's how it how it works and What a transistor does is it is a current controlled switch And so what that means is that if we establish a a small current from the base to the emitter in this direction? The transistor will switch on and it will allow a very large current to flow from the Collector to the emitter And sometimes you can also use this as an amplifier because [the] small current is amplified into a large current flow here And so first off there's there's actually something kind of confusing about the way this this diagram is drawn with this arrow like this which is that in A circuit diagram and a lot of times a lot of engineers like to think about positive current flow which [is] current flowing from positive to negative when in reality it's the electrons that flow from negative to positive and and it doesn't really matter which which convention we use because You know as long as there's current flowing in [one] direction the other the current will have the same effect But when we when we start to actually look at what's going on inside the transistor It's it's easier to think of this in terms of the direction the electrons are flowing instead of the conventional Direction And so so really what's going on in here even though the arrows pointing this way And we think of current flowing that way in reality the electrons are flowing from the emitter to the base And so it'll [just] turn all this around for for this discussion here [and] so To kind [of] restate what it is that the transistor does? In terms of electron flow is that a small flow of Electrons from the emitter to the Base will turn the Transistor on and allow a large flow of Electrons from the emitter to the collector so if we look at it in in this in this scenario This piece of N-type material might be might be the emitter this middle P-type is the base and then this this N-type material over here is the collector and So what were what we're going to do with the transistor is that? We're going to try to put a bunch of electrons in over on this side And we want to get them out over on this side But if the transistor is off Then we're not going to get that current flow and in order to turn the transistor on What we want to do is get a small current flowing through the emitter and out the base like this and once we get that That small current flowing then the transistor will turn on will have a large amount of current able to flow all the way through it So let's redraw this so we can we can kind of get a closer look at what's really going on in here So if we take a closer look at what's going on in here. We've got the emitter here Which is N-Type material and so we have these extra electrons that are in here that are free to kind of move around We've got the base. Which is P-type material, so we have these these holes that are in here that are free to move around And then the collector is is also n-type and so there's electrons that are free to move around But at each of these Pn junctions remember we we have this depletion region Which [I've] kind of exaggerated here. We have this depletion region because the Electrons from the N-Type Material are going to fill in some of the holes and the P-type material and kind of neutralize each other So there are no charge carriers in these in these depletion regions in here, and so what happens is if [we] want to Try to get a current to flow through this thing you know maybe we'll hook up Let me let me move up here if we hook up a [battery] here just across The the whole transistor from the emitter to the collector And so this is the negative terminal this is the positive terminal and we're going to be thinking about Electron flow here The negative terminal of the battery is going to kind of inject some some electrons into here [you] know this is going to become a little bit more or a little bit negatively charged So there's going to be a few more electrons that enter this area and some of them might even kind of creep into this Depletion region a bit if we get enough electrons to come in here, but [it] but we don't have a current flow here We don't we're [not] drawing We're not we're not you know drawing electrons out of the base and adding more holes here so we're not we're not going to see that Diode action here because we don't have a you know that point six volt differential between the Emitter and the base yet So so we might have some [more] electrons coming in here But nothing nothing crazy is going to happen here at the same time you know some of the electrons that are in the collector are going to be attracted towards the positive terminal of battery and so [we'll] Lose some [of] these Electrons that are going to get kind of attracted to the battery so really nothing nothing too exciting has happened here yet No, no current is flowing that's for sure, but now what happens when we try to turn on the transistor So we try to turn on the transistor what we're going to do is we're going to try to apply a small current from the emitter to the base and so we might have a you know a small battery down here that we connect to the across the emitter and the base and So now if we [just] look at the emitter and the base it looks like we have we have a diode and we've we were kind of Trying to get current to flow through it in the in the forward direction what we call forward biasing this diode And so what's going to happen is the same thing that we saw when we looked at the the diode in in the previous video Electrons are going to are going to come into the emitter here And we're also going to draw electrons out of the base and so we draw electrons out of the base We're adding holes here [too] to the base And as long as we get about 0.6 or 0.7 volts of current flowing in this [direction] so if this is greater than about [0.7] volts, I think for for most transistors [then] we're going to start actually getting getting current flow and So these electrons are going to get you know close enough here that this depletion this depletion region will will Will shrink to the point where we now have charge carriers all the way through here And so we can carry a charge all the way through so now this is this is where things get really interesting So we've got a bunch of things going on [here] right so first we've got lots of electrons entering the the emitter [over] here from You know from this battery here, but also we've got electrons entering from this battery here second thing that's going on is the the emitter part of the of the Transistor the way, they manufacture these is is the emitter is really heavily doped So there's there's a relatively higher concentration of those phosphorous atoms here So there's there's a lot of those extra electrons that are free to move around anyway So there's there's actually [a] lot of electrons in here. Just just to start with Just just the way they manufacture the the transistor and so all of this means that once this emitter to base Current gets going this this current this way gets going we've got [a] lot of Electrons here that are just going to start wandering into this base or I think the technical term is diffusing Um so we're going to have a lot of electrons that are going to start diffusing into this base and some of them are going to fill in these holes and [a] few of them are going to are going to come out down here But there's there's something really interesting which is this base is really thin That's the key [if] this base were really thick then you know they would all just flow down this way But because the base is really thin they're kind of close to to this air this area in here and if you remember when we looked at the the way the depletion region Forms the way the depletion region forms is there were electrons? Were over here that went over here to fill in holes, and so this area because the electrons have left is Positively charged so there's this positive charge that happens here because we've got those Those phosphorous atoms that were that were here you know they used to have an extra electron, and they still have an extra proton But that extra electron has gone over here, so there's this positive charge here And there's there's a negative charge here as well to kind [of] offset that and that's where the electrons went But we have [two] all these electrons that are just diffusing into the base so lots of electrons are coming in kind of this way And all these electrons that are bunched up they're going to get they're going to get just attracted to this positive charge and so a lot of these electrons will actually just sort of You know scoot right through this depletion region [and] get attracted here, and then of course once they enter the collector You know they're there? they're free to wander around just because you know just the same way as [any] of these other electrons are free to wander around and of course There's this positive terminal of this battery that's going to attract electrons out here And so you get this this action where you know you've got all the electrons bunched up over here That are being drawn into the base by this little current and once those electrons end up in the base There's this really strong positive charge. It's really close because the base is thin I mean the key here Is [that] the basis is really really thin the way they manufacture it and [so] these [electrons] get drawn to this positive charge You know they're already kind of moving this way and they just get drawn right across and then once they're in the collector They're perfectly happy to wander around here and they and they continue on and So with a really thin base most of the electrons are actually going to make that make that [lead] through this depletion region And and complete this circuit And so just the small amount of current going this way just enough to kind of get rid of this depletion region in here combined with the fact that you've got lots of electrons coming in from here lots of electrons coming in from here and the emitter the Way they manufacture it is heavily doped. So there's already a lot of electrons in here You know as soon as they as soon as you close this depletion [region], and they start start moving into the diffusing into this base You know because it's so thin because you've got this positive charge here most of them 99% of these electrons we are going to get swept into the collector and complete this circuit so just a small current here results in a very large current going through the transistor and And that's how the transistor works as a current controlled switch, so the small current flow here Turns on the transistor and allows a very large Current flow from the Collector to from the emitter to the collector
