in this phone there are nearly a hundred
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
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 neighbors
hidey-ho they right so it forms a
tetrahedral crystal but since all these
electrons are stuck in bonds if you 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
semiconductors 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
five valence electrons like phosphorus
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 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 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 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 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 there 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 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
blowing 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 we want to have let's say 300 of
those qubits then you have like 2 to the
300 classical things which is as many
particles as there are in universe