So today, we're gonna be talking about a material that is ubiquitous in your life. It's called silicon. Now, to begin with, we have to ask ourselves, what in the world is silicon? Silicon is a group four element. It falls right underneath carbon in the periodic table. All right, and so you're probably very familiar with it.

You've heard of things like the Silicon Valley, and we're living in the silicon age. And so what is this material and why do we use it so much? So first off, let's take a look at the properties. So what are some of the properties of silicon? Well, this is a piece of silicon, all right?

It's basically sand, silicon dioxide without the oxygen. So it just becomes Si. Now this is a very, very pure material and we're gonna see why that's important in a few minutes. But the most important thing you can see from this stuff is that it's very shiny. It's very reflective.

It's very brittle, all right? And very hard material. And it has a very high melting point. You wouldn't know that from looking at it. But and what's unusual is about it is it has a bandgap. And we'll talk about what a bandgap is in a minute. But anyway so these are some of the properties of silicon, so why do we care about silicon so much?

All right, well the reason we care about silicon is because if you make it very, very pure, all right, so let's think about purity for a second. I'm talking about making it nine or ten nines pure. So that means I'm making it 99.99999999% silicon, all right? That means that I have nothing in there, I have less than 1 part per billion of an impurity, if I do that, this material is a very good insulator.

It does not conduct electricity at all. So you would think, all right, great, so I have an insulator. I have glass, I have lots of insulators out there, why would I care about silicon? Well it turns out that what's weird about this material is that then if I add a small amount of an impurity, like boron or arsenic, all right, boron is a group three element and arsenic is a group five element.

When I add that, you can actually increase the conductivity of the material, all right? The conductivity ten trillion times, with less than 1% of an impurity. So that's a stunning property of a semiconductor, all right? So let's think about why that works. To understand why that works the first thing you have to do is you have to understand that this material has, because it's a group four element, it's going to wind up having four covalent bonds.

So it's gonna basically bond tetrahedrally to its neighbors, all right? And then when you put something like other silicons around it, they all bond tetrahedrally to all their silicons and so every free electron is tied up into a bond. And that's why this material is so insulating. Now then if I add an impurity like arsenic into the material, and it goes onto the substitutional site, so it sits where the silicon is, then it's gonna form its four bonds, okay?

But arsenic has five electrons. So it has an extra one left over, right? And so that's the challenge. If you have that extra electron left over, it can suddenly conduct. And that's how you take something that is an insulator, and you turn it into something that is a conductor, all right?

So, why is it useful? Why is this a useful property? Well, first you have to go back and look at computers. What computers were when they first started, the first computer was probably the ENIAC computer. It's composed of 18,000 vacuum tubes, and weighed 30,000 tons. So, huge, huge machine.

And if you think about it, the way it worked was, it was basically a collection of switches, all right? And these switches enabled us to do manipulations of both letters and numbers. And so, the ENIAC computer could do 300 calculations per second. But the key was that you needed to have a switch.

Why is a switch important? Because you can represent numbers either in base ten like we do, one, two, three, four, five, six, or you can represent them in base two, or binary, such as one, one, zero, one, one, one, zero, zero, etc. And so if you represent them in binary then that means that each one could be a switch that's on, and a zero could be a switch that's off.

So the key is to be able to make switches. Now then, how does semiconductors work as a switch? Well they really didn't work very well until in 1947, the group at Bell Labs, John Bardeen and Brattain and Shockley developed an understanding of how to make a point contact transistor.

So they used germanium, and it was the first transistor that was a solid stay transistor, and that basically meant the we could now have a solid state switch. So let's think about how that works. In order to work as a switch, you have to understand a little bit about how silicon can be made into a transistor.

All right, so the key to making silicon into a transistor is that a transistor is a three tonal device. All right, you're going to take silicon, like this, and you're going to add an impurity to that silicon, and that impurity is going to wind up being something like arsenic or boron.

And if you do that in one region and another region, you'll actually create two contacts. So you'll actually create a region that's metallic here and a region that's metallic there. If in the middle, I put some oxide on the surface, and then I put another metal, then when I try to pass electricity from one side to the other side, through these two metalized regions where the doping is, right, the current won't flow.

However, if I apply a bias to this third contact I can actually pull electrons up to the surface and create a conducting pathway. And when I do that, suddenly current flows from one side to the other. So in effect, if I'm applying a bias I can turn it on, and if I take off the bias I can turn it off.

And so I have a switch that I can turn on and off, and that's how a transistor works, all right? So, early on they used geranium, however they quickly discovered that silicon was not only less expensive, but silicon formed a fabulous dielectric, SiO2, silicone dioxide on the surface.

And that left you with a much better transistor. And so quickly silicon replaced germanium in the 50s, all right? So now you know what a transistor is. A transistor can also act as an amplifier, by the way. So if I modulate the voltage on that middle contact, what we called the gate, a little bit, then I'll get a big amplification on the drain side, that third contact.

And that's how your transistor radio worked, as it would pick up a very weak radio signal and it would amplify it. And so you suddenly have, you could hear the music you're trying to find on the radio station. Okay, so now we know how transistor works, what is the brief history of all of this?

Well so in the 50s we were making discrete transistors for things like radios, and then towards the end of the 50s, a man named Jack Kilby and Texas Instruments and Bob Robert Noyce at Fairchild came up with the idea of integrating these onto a single wafer. The problem they were getting is if they had two transistors and they wanted to connect them together, they had to put a wire between them.

And that wire was delaying the speed at which those two could communicate. So they thought, well I could put them onto the same piece of silicon. And so Kilby had the idea of putting a bunch of different discrete devices on the same silicon piece, and Noyce had the clever idea of actually doing that but also adding metal lines on there to connect all the pieces together.

And so ultimately they awarded him the patent. But they settled their differences, and in 2000 they were rewarded the Nobel Prize in Physics for discovery of the micro-electronic device. So in 1968, Noyce and another guy named Gordon Moore left Fairchild to start a new company and that company became Intel.

It was Intel. And so Intel, of course, is ubiquitous today as one of the leading manufacturers of microelectronic devices. One of the things Gordon Moore noticed when 1965 was that there was ten devices on a chip. And by 1969, it had grown to a thousand devices on a single chip.

So what was amazing was that we had been doubling the number of devices on these chips every two years since 1965. All right now, there was some interesting predictions that were made at that time. Ken Olsen, President of Decks, famously said in 1977 that there's no reason anyone would want a computer in their home.

So people did not always see where this was leading to. But what you can see down here is just that we started to figure out how to grow these bools of silicon, all right, and to process this silicon wafer. And it's been evolving ever since then. So let's talk about how you actually process silicon.

To begin with, you have to take sand and you have to convert it to silicon. Now metallic grade silicon, as that is called, is a tremendous business. 90% of all the silicon that is made today goes into the metals industry. It's actually used for alloying aluminum and iron.

So if only a fraction of all the silicon that is reduced from sand actually goes into electronic grade silicon. The next step is that you have to actually purify that silicon, and that requires you to first turn the silicon into silicon tetrachloride, a solution that you distill. That gets it pretty pure.

And then you do a bunch of zone refining, which is a way of basically sweeping out the impurities using a melt to further refine the material. Once you got it very pure, you're gonna put it into a crucible and you're gonna melt it. And then you're going to stick a single crystal seed into that and then pull it out very slowly while you're turning it.

And that will grow a bool, a very long cylinder of the silicon out of the melt. Now you're gonna take that cylinder, lay it down, and slice it. And that's why you have wafers. Now, what you can see here is that when we first started growing these things, the wafers were very small.

This is because the bools were very small, and this is what we actually processed. So as time has gone on, we've gone from one inch, to two inch, to four inch, to six inch, to eight inch. We're all the way up to 12 inch wafers now. So this is the size of the wafer that's used currently in the microelectronics industry.

There is talk about going to 450 millimeter wafers, which would be 16 inch but probably won't happen until 2022 or later. Now once I have this wafer, I have to turn around and figure out how I'm gonna make a chip out of this, right? I'm gonna make a whole bunch of transistors on this thing.

The way you do that is you're actually gonna start by cleaning the wafer, everything has to be very, very clean, this is why it has to be done in a clean room. And then you're going to spin on a photo resist. A photo resist is a thin plastic layer that goes over the surface that's photo sensitive.

And then I will put it into a system in which I can expose that photo resist and open holes in that photo resist. That allows me to do things like etch, or add impurities through ion implantation, or other processing steps where I'm gonna actually go about making the transistor.

Then what I'll do is, I'll wipe off that photo resist, or I'll burn it off, ash it off, and then I'll do another layer, and another layer, and you keep doing this over and over. Eventually you're gonna get your transistor built, then you have to connect all the transistors.

So you're gonna start laying down all these layers of metal, eight, nine layers of metal, on top of it. Each one separated by a dielectric. So it takes about a month and a half to actually process that. At the end of that month and a half, however, what you'll have is a wafer that looks something like this, all right?

This is a wafer from Intel that's been processed, and each one of those four squares represents one of your Pentium processors. So this is effectively 350 laptop computers, right? Now these have to be chopped up and then put into a packing, hermetically sealed into a package, so that they can then function in your computer.

So this will be shipped off to Singapore, diced up, put into packaging, sent back and then Apple or whomever your buying your computer will then stick these chips into your computer. All right, so now then, as I mentioned, Moore's Law has been in effect since 1965. And so right now we're up to, in this particular generation of devices, we're currently at two billion transistors in these four squares.

In another year, we'll be at four billion transistors. Two years after that, we'll be to eight billion transistors. So it continues to double. This takes a tremendous amount of effort because if you think about it, the size of these transistors is getting smaller and smaller. The packing is getting tighter and tighter, right?

And the power requirements are going up and up. And so, this becomes a very, very challenging problem to stay on Moore's Law, all right? So this requires billions and billions of dollars in investment. Now what's fascinating about this is that if you actually look at the calculations that you can do per dollar, this has also been growing enormously.

In the graph you can see, and so you can tell right now that we're getting closer and closer in terms of calculations to that of the power of the brain eventually maybe. So we've got a long ways to go. We've got some really, really important challenges. And you're gonna see this in your other video, cuz one of the challenges that we're running into is that as these devices get smaller and smaller, the amount of power it takes to run them, because you're running all of them, they start to leak a little bit more.

They require more power and so the electricity requirements go up. And so we're constantly trying to drive that electricity requirement down. So we're making lower and lower power devices, and so we have to come up with ways in which we can make these, and we may ultimately have to switch materials away from silicon to possibly something else.

Materials they're considering are things like molydisulfide, which is a two dimensional material that maintains its mobility, ease of the electron flow, even when you get it down to very, very small sizes, only a few atom layers thick. So, this is the challenge. Now when you think about it, what is the social impact of silicon, all right?

So you could sit down and try to think of all the things that you could have that wouldn't exist without silicon. So all of your cellphones, your computers, your calculators, your watches, your everything requires silicon, right? Even my FitBit requires silicon. So it's become very ubiquitous in your life and it will continue to be a challenge going forward.

And so, as a society, we have to figure out not only what are we gonna do with it today, but how are we gonna use it in the future? Especially when we start to invent things like flexible electronics, all right? And so, then it's gonna become more and more a part of your life.

It has some negative connotations as well. So for example, there's been a tremendous increase in myopia in your eyes because of the fact that everybody is staring at very small screens. And so there are some potential downsides to this. There's also some social considerations, and so you'll hear more about those when you do your exercise, and you consider the idea that that maybe we have more and more friends today because we have access to all this technology, but we have fewer and fewer close friends.

So it's an interesting thought. So I don't know where the future's gonna go. I do know that the electronics industry is probably gonna stay on Moore's law for a while. We're gonna continue to pack more and more transistors in there, we're gonna find alternative materials to possibly use when we have to, and it's gonna continue to be part of our lives.

And I think the challenge is gonna be to figure out how we use this and how we make the best integration of this into our lives in the future.