a billion. The transistor is in virtually every electronic device we use: TV's, radios,

Tamagotchis.

But how does it work?

Well the basic principle is actually incredibly simple. It works just like this switch, so

it controls the flow of electric current.

It can be off, so you could call that the zero state or it could be on, the one state.

And this is how all of our information is now stored and processed, in zeros and ones,

little bits of electric current. But unlike this switch, a transistor doesn't have any

moving parts. And it also doesn't require a human controller. Furthermore, it can be

switched on and off much more quickly than I can flick this switch. And finally, and

most importantly it is incredibly tiny. Well this is all thanks to the miracle of semiconductors

or rather I should say the science of semiconductors.

Pure silicon is a semiconductor, which means it conducts electric current better than insulators

but not as well as metals. This is because an atom of silicon has four

electrons in its outermost or valence shell. This allows it to form bonds with its four

nearest neighbours,

Hidey ho there! G'day

Wasaaaaap!?

So it forms a tetrahedral crystal.

But since all these electrons are stuck in bonds, few ever get enough energy to escape

their bonds and travel through the lattice. So having a small number of mobile charges

is what makes silicon a semi-conductor.

Now this wouldn't be all that useful without a semiconductor's secret weapon -- doping.

You've probably heard of doping, it's when you inject a foreign substance in order to

improve performance.

Yeah it's actually just like that, except on the atomic level.

There are two types of doping called n-type and p-type. To make n-type semiconductor,

you take pure silicon and inject a small amount of an element with 5 valence electrons,

like Phosphorous.

This is useful because Phosphorous is similar enough to silicon that it can fit into the

lattice, but it brings with it an extra electron. So this means now the semiconductor has more

mobile charges and so it conducts current better.

In p-type doping, an element with only three valence electrons is added to the lattice.

Like Boron. Now this creates a 'hole' - a place where there should be an electron, but

there isn't. But this still increases the conductivity

of the silicon because electrons can move into it.

Now although it is electrons that are moving, we like to talk about the holes moving around

-- because there's far fewer of them. Now since the hole is the lack of an electron,

it actually acts as a positive charge. And this is why p-type semiconductor is actually

called p-type. The p stands for positive - it's positive charges, these holes, which are moving

and conducting the current.

Now it's a common misconception that n-type semiconductors are negatively charged and

p-type semiconductors are positively charged. That's not true, they are both neutral because

they have the same number of electrons and protons inside them.

The n and the p actually just refer to the sign of charge that can move within them.

So in n-type, it's negative electrons which can move, and in p-type

it's a positive hole that moves. But they're both neutral!

A transistor is made with both n-type and p-type semiconductors. A common configuration

has n on the ends with p in the middle. Just like a switch a transistor has an electrical

contact at each end and these are called the source and the drain. But instead of a mechanical

switch, there is a third electrical contact called the gate, which is insulated from the

semiconductor by an oxide layer.

When a transistor is made, the n and p-types don't keep to themselves -- electrons actually

diffuse from the n-type, where there are more of them into the p-type

to fill the holes.

This creates something called the depletion layer. What's been depleted? Charges that

can't move. There are no more free electrons in the n-type

-- why? Because they've filled the holes in the p-type.

Now this makes the p-type negative thanks to the added electrons. And this is important

because the p-type will now repel any electrons that try to come across from the n-type.

So the depletion layer actually acts as a barrier, preventing the flow of electric current

through the transistor. So right now the transistor is off, it's like an open switch, it's in

the zero state.

To turn it on, you have to apply a small positive voltage to the gate. This attracts the electrons

over and overcomes that repulsion from the depletion. It actually shrinks the depletion

layer so that electrons can move through and form a conducting channel.

So the transistor is now on, it's in the one state.

This is remarkable because just by exploiting the properties of a crystal we've been able

to create a switch that doesn't have any moving parts, that can be turned on and off very

quickly just with a voltage, and most importantly it can be made tiny.

Transistors today are only about 22nm wide, which means they are only about 50 atoms across.

But to keep up with Moore's law, they're going to have to keep getting smaller. Moore's Law

states that every two years the number of transistors on a chip should double.

And there is a limit, as those terminals get closer and closer together, quantum effects

become more significant and electrons can actually tunnel from one side to the other.

So you may not be able to make a barrier high enough to stop them from flowing.

Now this will be a real problem for the future of transistors, but we'll probably only face

that another ten years down the track. So until then transistors, the way we know them,

are going to keep getting better.

Once you have let's say three hundred of these qubits, then you have like two to the three

hundred classical bits. Which is as many particles as there are in

the universe.