[MUSIC PLAYING] [SINGING] Science. Out loud. Here, I have a circuit with a
battery, a light bulb, and gap. If I fill this gap with a metal,
[DING], the light comes on. If I fill this gap with
glass, the light stays off. [BUZZ] You probably already know
this, because the metal is an electric conductor,
and glass is an insulator. But what happens when I fill
this gap with a silicon wafer? The light stays off,
so you might think that silicon's an insulator. But what if I heat it up. Thank you. It lights up. The silicon is insulating
at room temperature, but conducts electricity
when it's very hot. It's a semiconductor,
whose conductivity changes based on the environment. This special ability
makes semiconductors the perfect brains for
electronic devices. Circuits of small semiconductor
switches, called transistors, are the heart of computer
chips, and enable them to do math and run programs. Semiconductors have
enabled electronics to become smaller,
faster, and more reliable. But what is it exactly
about semiconductors that allow them to either
conduct or insulate? In a single atom, electrons can
occupy specific energy levels. When multiple atoms
bond, the electrons are shared between them. But because the atoms
are now interacting, the energy levels shift around. In a solid, trillions
and trillions of atoms interact with each other. Their individual energy levels
smear into energy bands. For a material to
conduct, the electrons must be able to jump from lower
energy states to higher ones. The spacing of
these energy levels and how they're
filled with electrons determines if the material
is a conductor, insulator, or semiconductor. If there's a huge gap
between the lower energy levels and the higher ones,
it's hard for electrons to jump to the higher ones,
so a current can't flow, and it's an insulator
like this glass. Metals have no gap at all. Electrons can move to the higher
energy levels with no problem. Current can flow. Semiconductors fall
somewhere in the middle. They have a
medium-sized band gap. So technically, I could make
this glass conduct electricity if I added enough
energy through heat to push the electrons
into a higher band. But that amount of heat would
either melt or break the glass before it actually conducts. This is true of most insulators. The amount of energy
needed to make them conduct is just too hot. But in a semiconductor,
the band gap is small enough that electrons
can jump into the higher energy band so that current can flow. The amount of heat
we apply determines how many electrons jump
into the higher band, and how much current flows. And heat isn't the only way
to change the connectivity in a semiconductor. We can also use light, electric
currents, and in a computer, electric fields. As I've said,
computers are made up of semiconductors,
which is called transistors, that switch between
conducting and insulating. Computers use electric
fields because heat is slow and would burn too much energy. We can turn this wafer
into a computer chip by printing a circuit
of transistors on it, using a process called
photolithography. Here, in the photo
room, we cover the wafer with a light sensitive
material and expose it to light that we shine
through a patterned mask. Then, we develop
the wafer, like film in photography, which
leaves behind a pattern that becomes the circuit. Printing the
transistors at once lets you make circuits that are
smaller and cheaper than if you built them from
individual parts. Transistors make up the
logic elements, the memory components, and the
communication modules that let computers talk to each other. With semiconductors, you
can cheaply add transistors to almost any
device you can think of, from spaceships, to serves,
to maybe even your toaster. Semiconductors have enabled
the technological revolution, the internet, the computer,
and the cellphone. No semiconductors,
no information age. I'm Jamie, and thanks
for watching this episode of Science Out Loud. Be sure to check out
some of our other videos, including mine on how
computers compute. Check on out our website
for more information. The end. Ugh. I'm just like.
