How Does a Transistor Work?
In this phone, there are nearly 100 million
transistors, in this computer there’s over 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 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.