- [Instructor] The materials used in electronic devices
are commonly categorized as being either
an electrical conductor or an electrical insulator.
For example, metals like gold and copper
are used as conductors because they allow electrons
to move freely amongst their atoms.
On the other hand, in materials like glass and rubber,
electrons are bound more tightly to the atoms
and have a harder time moving around.
So those materials make good insulators.
In this course, I'll be focusing on devices
that use a third category of materials that exist somewhere
between conductors and insulators, called semiconductors.
Semiconductor materials are able to conduct electricity,
but only partly.
They can conduct current better than an insulator,
but not as well as a conductor.
The most commonly used semiconductor material is silicon,
which is used to manufacture everything
from computer chips to solar panels.
Silicon is popular because it's relatively cheap,
it has good electrical properties and it's easy
for manufacturers to fabricate into electronic devices.
To understand why silicon acts as a semiconductor,
we need to look at its atomic structure,
the number of electrons in a silicon atom,
how those electrons are organized
and how silicon atoms are bound together.
An atom of silicon has four electrons that it can use
to bond with other nearby atoms.
And those are called its valence electrons.
A silicon atom will form bonds with the four atoms
closest to it by sharing its valence electrons with them.
And each of those four atoms will, in turn,
form bonds with their nearest neighbors
by sharing their own four valence electrons.
And so on, which forms a crystal structure of silicon atoms.
Since all of those valence electrons are tied up
in forming bonds with adjacent atoms,
they're not very mobile.
They can't move around easily from atom to atom
to carry electrical current.
So on its own, although pure silicon qualifies
as a semiconductor, electrically,
it's not all that interesting.
To make silicon useful for electronic devices,
a process called doping is used to inject other elements
into the silicon structure to give it more mobile charges.
Semiconductor materials like silicon can be doped
in one of two ways, called n-type and p-type.
The letters N and P refer to the sign of the charge
that can move around within those doped materials.
So in an n-type material, it's the negatively charged
electrons which can easily move around
and in a p-type material, it's the positively charged holes
that move around.
Those holes represent an absence of electrons.
To make an n-type semiconductor, pure silicon
is injected with a small amount of another element
whose atoms have one more valence electron
to use for bonding than silicon atoms.
Phosphorous is commonly used for that purpose
because phosphorous atoms have five valence electrons
and they're about the same size as silicon atoms,
so it's easy for them to replace some of the silicon atoms
in the crystal structure.
The injected phosphorous atoms use some of their electrons
to bond with their four nearest neighbors,
like a silicon atom would, but since the phosphorous atom
has one more valence electron, that spare electron
doesn't get tied up in a bond.
That extra electron from the phosphorous atom
can move around amongst the silicon atoms relatively easily,
which gives the n-type doped silicon more mobile
negative charges to use for conducting current.
To help me conceptualize that movement,
I like to think of the atoms in a silicon crystal
like stacks of coins next to each other.
In pure silicon, each of the stacks in this row
has four coins in it, representing the four
valence electrons in each atom.
If I apply a voltage to the stacks by pushing on one end
with my finger, I can create a current by moving coins,
but it's difficult.
Moving one coin requires the entire row to shift with it.
Now if I dope this row of silicon by replacing
one of the stacks with a phosphorous atom,
which has five valence electrons, with that extra coin
on the stack, I can easily create a current
by sliding the fifth electron from the phosphorous
across the other stacks.
It's not tightly bound to its neighboring atoms.
To make a p-type semiconductor, the pure silicon structure
is injected with another type of element
that has one less electron available to use for bonding,
like boron, which only has three valence electrons.
That lack of an electron creates a hole amongst the bonds
that other electrons can jump into to fill,
making it easier for electrons to move around.
So when a voltage is applied to the p-type material,
which pushes on its electrons, as those electrons
move through the material by shifting into open holes,
they leave behind a hole in the place they just came from.
Even though it's actually the negatively charged electrons
that are moving from atom to atom to carry current
through a p-type semiconductor, we consider the holes
that are moving in the opposite direction
to represent positive charge and that flows
in the direction of conventional current.
I can conceptualize that more easily using my stack of coins
by removing a coin from one of the silicon stacks
to replace it with a boron atom,
which only has three valence electrons.
Now because of the hole created by the boron,
it's easy to move around the top layer of the electrons
by shifting them into that open hole.
Shifting electrons towards the left to fill the hole
causes the open hole to effectively drift towards the right,
which is the direction of conventional current
through this stack of coins.
One common misconception about doped semiconductor materials
is that n-type materials are negatively charged
and p-type materials are positively charged.
In actuality, n-type and p-type semiconductors
are both neutral because the individual atoms of silicon
and phosphorous and boron that make up those materials
are each neutrally charged because they have the same number
of electrons and protons inside of them.
The terms n-type and p-type just refer to the types
of charge that easily move through the material,
either negatively charged electrons
or positively charged holes.
So I've taken a semiconductor like silicon,
which, on its own, can partially conduct electricity
and have injected it with boron or phosphorous
to give it more mobile charges
so it can conduct electricity better.
Why does that matter?
If I just wanted a material that could
conduct electricity well, it would be a lot easier for me
to use a metal like copper or gold,
than to go through all this trouble
to dope a bunch of silicon.
On their own, n-type and p-type silicon
are not all that exciting.
They're just materials that conduct well.
The magic really happens when they're used together.
Semiconductor components like diodes and transistors
have structures that contain both n-type
and p-type materials.
Certain combinations of those materials
produce components that behave in different ways
when certain voltages are applied to them
and can be used to control and direct the flow of current
through devices to create everything
from power amplifiers to digital computers.