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.
As a self-taught amateur electronics hobbyist, who's struggled for years with understanding the physics behind how transistors work, I found this video incredibly easy to follow.
Minor nitpick: "How does a Field Effect Transistor work". He could at least have mentioned FETs/BJTs and which one he was describing.
But how does the gate turn on and off?
Eh. He never points out a central concept: silicon with no mobile charges is an insulator. And, if voltage-fields should attract some mobile charges into an insulator, it turns conductive.
That's the real magic: an insulating solid which contains a conductive 'object' which can move around inside, even while the solid stays still. The big metal Frankenstein switch is still there, its just made of waves of conductive silicon "moving" inside the insulating silicon.
Big insight: ask why we cant do the same with copper or aluminun? Why all this mucking about in semiconductor-space? Its because doped Si has incredibly few mobile electrons, as if a compressible gas is flowing inside. Metals could only be turned insulating if we applied megavolts. "Compressible electricity" is in transistors, while only incompressible "electric fluid" is found in the wires.
Well, he doesn't explain why the electrons don't just go to the gate, which is a positive charge and thus should attract the electrons.
This is the part that I never understood and I'd be thankful if somebody could explain that.
Thanks, this was nice.
Veritasium is the tits. I don't think there's a video on there that I didn't thoroughly enjoy.