Kind: captions
Language: en
[Applause]
In this phone, there are nearly a 100
million transistors. In this computer,
there's over a billion. The transistor
is in virtually every electronic device
we use. TVs, 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. And this is
because an atom of silicon has four
electrons in its outermost or veence
shell. This allows it to form bonds with
its four nearest neighbors.
How?
What's up?
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
semiconductor. 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 Nype and
Ptype. To make end type semiconductor,
you take pure silicon and inject a small
amount of an element with five veence
electrons like phosphorus. This is
useful because phosphorus 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
ptype doping an element with only three
veence 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 semiconductor
because electrons can move into it. Now
although it's the 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 ptype
semiconductor is actually called ptype.
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 ntype
semiconductors are negatively charged
and ptype 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
PT type, it's a positive hole that
moves, but they're both neutral. A
transistor is made with both N type and
ptype 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 end type. Why? Because I filled
the holes in the ptype. Now, this makes
the ptype negative thanks to the added
electrons. And this is important because
the ptype will now repel any electrons
that try to come across from the end
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 layer. 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 22 nanometers wide, which means
they're 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 2 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
important 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 10 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, 300 of those cubits,
then you have like two to the 300
classical bits, which is as many
particles as there are in the universe.