this year's picture of A DI owed these air some different transistors and and this is actually an eye seven processor that has over a 1,000,000,000 transistors inside of it.
And, you know, inside all of these are especially made silicon crystals and actually understand how this stuff works.
We need to understand what's going on at the molecular level inside the silicon.
So if we look at a silicon Adam, it's gonna have four electrons and it's in its outer, most like it's it's Ah Vaillant Shell.
And so when it bonds to another silicon, Adam, you're gonna have this this sort of co Vaillant bond between the two and so in a in a whole lattice or like a crystal of silicon.
All of these electrons are gonna be tied up in these co Vaillant bonds on, And so none of the electrons are going to be free to move around, at least not any kind of normal temperature that we would find this that and since in order to conduct electricity, we need to have electrons that could move around Perfectly pure silicon like this actually doesn't conduct electricity.
And so actually the silicon that we used to make die odes, transistors, microprocessors and all.
That sort of stuff actually has some impurities introduced, which is something we call doping.
Um, and these impurities allow it to conduct electricity in a particular way.
And this doped silicon is what we call semiconductor material, since it's not really a regular conductor like like copper wire.
But it's also not a complete insulator, like like something like glass.
So one of the impurities that's commonly added to silicon is his phosphorus and phosphorous has five electrons, and so has one extra electron has one extra proton.
Um, you know beyond what silicon has.
And so if we replace this this Adam here with a phosphorous Adam, you know what's still bond to the four adjacent?
Ah, silicon atoms just like this.
But it would have this extra this extra electron in there as well.
And maybe if we replace this with phosphorus, it would have an extra electron there.
And if this were a phosphorous, it would have an extra electron there.
And so we have these extra electrons that were there were introducing into the silicon crystal.
So we've got the same basic structure, but now we have these electrons that that can actually start to wander around a little bit.
You know, they're not gonna go too far from a phosphorous.
But as long as we've got enough phosphorus in here, you know, these electrons might wander around to this electron here might actually kind of wander over here, for example.
You know, this electron here might hopes this electron here might wander over here and so these electrons can move, can move around.
And so because the electrons can move around, this thing is actually ah, kind of conductor.
So we hooked a battery up like like this here, we'd be able to get current flow right, because electrons that come out of the negative terminal, the battery could could enter this this crystal structure and move around.
And you know, there's going to be free electrons here that can be pulled out of this crystal structure and attracted to the positive terminal, the battery over here.
And so just by doping, this the silicon crystal with a few phosphorus atoms, we can get an electrical current to flow through it.
We can we can turn this into a conductor, and so that's one kind of doping.
But there's another kind of doping where we use boron and boron is basically the same a silicon, except that it has one fewer electron and one fewer proton.
And so, if we were to replace some of the things in this crystal structure with with boron, we replaced this Adam with boron.
Um, we have one fewer electron there.
And if this were boron over here, we might have one fewer electron there.
And if this were boron down here, we might have one fewer electron there.
And so that leaves these sort of the sort of holes here.
Where were these electrons that would normally be part of these co Vaillant bonds are kind of missing.
And now what might happen is that now that these holes are here is like other electrons might actually kind of jump out of their their place and go fill in one of these nearby holes.
And so you can almost think of that as if the whole has kind of moved around.
And so, just like we had electrons, that they were free to kind of move around in here when we had phosphorus with the boron doping, we have holes where electrons would be that are actually kind of free to move around.
You know, in reality, it's the electrons that are sort of jumping into the holes, but but I like to think of it as if as if the holes are moving.
And so what happens if we hook a battery up up to this?
So if we hook a battery up to this, electrons are gonna be attracted to the positive terminal.
The battery you negatively charged electrons can be tracked to the positive charge on the battery there.
And so you know this electron here that's maybe close to that battery terminal.
Well, go get attracted to the battery and leave a hole behind.
But at the same time, electrons are going to be coming from the negative terminal, the battery and maybe filling in some of the holes that are that are closer to this side on.
Then these holes over here will then start migrating over towards the negative side.
So the holes are gonna kind of flow this way or, you know, really, it's the electrons that are filling the holes and kind of jumping along and going this way, and so we have an electric current again because these holes allow the charge to be carried through here.
And so neither of these cases, whether we have extra electrons that air that air flowing through here or if we have, ah, an absence of electrons that leave these holes that are allowed to move around.
You know both of these scenarios we have.
We're able to conduct an electric current.
Okay, so there's actually a little bit of ah terminology here.
And so both of these cases you have seen two kinds of semiconductor material here, one that's dope with phosphorus and one that's dope with boron.
And both were able to conduct electricity because they have.
They have what are called charge carriers.
In the case of phosphorus, the charge carriers are the extra electrons, and in the case of boron, the extra the charge carriers are the are the holes where the electrons would be.
And since electrons are negatively charged, we call this this type of material and end type and type semiconductor, and for negatively charged electrons are negatively charged.
And we call this with the boron recall.
This P type There's a P type semiconductor because holes air sort of positively charged.
I guess you wouldn't think of it that way.
Remember that the n and the P returned refer to the charge carrier whether the charge carriers negative or positive, neither of these materials is actually has an overall electrical charge.
Because even though we have extra electrons over here, for example, the phosphorus atoms have have an extra proton as well, saying here with the boron we have each boron, uh, Adam has won fewer proton.
So overall, we have the same number of electrons and protons and each of these, um and so neither of these is is negatively charged, positively charged.
So the end type in the P type just refer to the type of charge carrier that allows us to conduct, you know, an electrical current through these things.
Okay, so with all that out of the way, let's look at how this is actually useful.
Where this gets interesting is if we have a single silicon crystal here, where half of it is is doped with with n tight material, and half of it is p type material doped with the boron over here.
In the end type half.
We've got negative charge carriers with extra electrons.
They're free to wander around.
And over here in the in the p type half, we've got a positive charge carriers, which are the holes that are free to wander around.
But things get really interesting here at this boundary.
Um, this boundary recall the PN junction, the PM junction, and this is where things get really interesting.
So if we zoom in on this on this PM junction, Yeah, this is kind of what things look like at that at that p injunction.
If we if we zoom in.
So let's take a closer look here.
Remember, these are mostly gonna be silicon atoms, but we're gonna have a few extra phosphorus atoms over here and a few extra boron Adams over here.
And so we got free electrons that air that air.
They're free to wander around over here, and we've got some holes that are free to kind of wander around over here.
But right at this boundary, things get really interesting because you've got this electron sitting very close to this hole, and it's really tempting for that electron toe.
Wanna go and jump into that hole In fact, that is exactly what will happen is this Electron will find it really tempting to go over here and fill that hole.
And same thing here.
These electrons here are gonna find it really tempting to go jump over and fill in thes nearby holes.
You know, and even, you know, some of these electrons over here might find it kind of tempting for them to go jump into some of these into these holes, you know, because they're kind of close enough that they can wander around and find those holes.
And so a couple interesting things are gonna happen here, you know, first, you know this this kind of region of n tight material here is gonna, you know, take on a little bit of ah, positive charge is gonna be kind of a positive charge in this region because the electrons have have kind of wandered off.
But the's philosophers Adam's still have have their extra protons.
And then this.
This region here is going to take on a little bit of a negative charge because some extra protons have shown up.
Some extra electrons have shown up andare filling in those extra holes, so there's a little bit of a slight positive charge here, a little bit of slight negative charge here.
But otherwise, you know, the rest of this and type material over here is gonna be This is n Titan material over here is gonna be kind of normal.
Everything just like we described.
And the rest of this p type material over on this side is gonna be just like we talked about before.
But right at this boundary, you know, a couple interesting things going on.
You know, not only do we have these slight charges separating, but because all of the sort of electrons near the boundary have filled in all the holes near the boundary.
We have this this region, like right in here where there are no charge carriers, there's no extra electrons.
There's no no extra holes and no holes over here.
And so there's no charge carriers in here.
And so we call this the Depletion Region depletion region.
And we call it that because it's depleted of charge carriers.
And so, without charge carriers, this this thin region in here is actually is potentially a bit of an insulator.
So that is it might actually prevent current from flowing from one side of this to the other because we don't have any charge carries in here that could carry that current.
So let's take a look at how that might work.
So if we have cement type material on some P type material, you've got the extra electrons in here.
We've got the holes in here on.
We have this this depletion region here in the middle.
So if we hook this up to a battery like this where this is the negative side, this is the positive side.
Electrons are gonna enter this end region over here, So there's gonna be some extra electrons showing up here, and some of their money gonna wander over towards this.
Ah, this depletion region in the middle.
But when they get to the depletion region, there's there's no charge carriers here.
There's no way for them to kind of move across here and so you won't get a current flowing, you know.
But in reality, actually, if if this battery is really of any decent size, um, there's gonna be enough electrons going in that actually they are going to start to get closer and closer to this depletion, you know, or to the to the boundary, the P M junction.
And at the same time, there's gonna be electrons over on the P side.
They're gonna be attracted to the positive terminal.
The battery, right?
The negatively charged electrons are gonna be attracted to the positive charge here.
And so those electrons are gonna leave, and they're gonna leave holes in here, and these holes are going to kind of move around.
And again, if this battery is got any kind of strength to it, there's gonna be, you know, quite a number of holes that get introduced here, and those holes are going to start to move in this direction towards towards the negative terminal.
And so, if this battery is actually turns out, if it's greater than about 0.6 volts, um, this depletion region will actually just collapse altogether.
And you'll start to get charges that can flow all the way through because you won't have the depletion region in the middle where there's no charge carriers.
There will be charged carriers all the way through on DSO.
You'll get, um, a current flow all the way through this thing.
But now what happens if we hook the battery up the other way?
So what happens if we have the positive terminal over here and the negative terminal over here?
So what's gonna happen is some of these, uh, these electrons here gonna be attracted towards the positive terminal, the battery.
And so some of these electrons will start to will start to leave, and they'll be attracted to the positive terminal, the battery.
At the same time, there's gonna be electrons that they come out of the negative terminal, the battery here and are going to start filling in some of these holes.
These electrons are gonna come fill in some of these holes, um, on this side.
And so really, what ends up happening is in this scenario, the depletion region will actually get well, actually get bigger or get wider, you know, because these electrons are gonna be leaving the material and there's gonna be electrons that are gonna they're gonna come into the P material is this becomes a little bit negatively charged because of the negative terminal, the battery, and fills in these holes.
And so the depletion region ends up, ends up actually getting wider and course because there's no charge carriers in this whole region here, this area is gonna act like like an insulator, and no current will flow here.
And so this Pian junction here has is really interesting property, right that it it'll allow current to flow in one direction but prevent current from flowing in the other direction.
And so maybe maybe you've heard of a DI owed.
But a diet is this electrical component that allows current to flow in one direction but not the other on.
And this is exactly how it's built and exactly how it works.
And this is the electrical schematic for a day I owed.
And, uh, it's a little bit confusing because you have this arrow that points in this direction here, Um, and that's because the electrical engineers tend to prefer to think about current flowing in a positive direction.
So from positive to negative when in reality it's the electrons that air flowing from the negative side to the positive side.
So in a dialled like this, even though the arrow is sort of pointing in this direction, the electrons are flowing in this direction, and it's a little bit confusing, but the, uh I guess the discoverers of electricity didn't didn't know about electrons.
So I guess Benjamin Franklin got it wrong.
You know when when he when he was saying that the charges flow from positive to negative, but it actually turns out that it doesn't really make a difference which way the current is really flowing a CZ long as we use one convention consistently and stick to it, whether it's positive or negative or negative to positive, because the direction of current flow doesn't really affect what the current does.
So a lot of engineers prefer to think of current flowing, Um, just this way from positive to negative, even though, really, the electrons are going from negative to positive.
If it helps, you can think of like the holes flowing in this direction.
Um, but But again, it's, there's there's two conventions that air that are completely opposite, and you know, as long as you're consistently using one, it doesn't really matter.