Semiconductors

Semiconductors have had a monumental impact on our society.
You find semiconductors at the heart of microprocessor chips
as well as transistors. Anything that's computerized or uses
radio waves depends on semiconductors.

Today, most semiconductor chips and transistors are created
with silicon. You may have heard expressions like "Silicon
Valley" and the "silicon economy," and that's why -- silicon
is the heart of any electronic device.

A diode is the simplest possible semiconductor device, and is
therefore an excellent beginning point if you want to
understand how semiconductors work. In this article, you'll
learn what a semiconductor is, how doping works and how
a diode can be created using semiconductors. But first, let's
take a close look at silicon.

Silicon is a very common element -- for example, it is the
main element in sand and quartz. If you look "silicon" up in
the periodic table, you will find that it sits next to
aluminum, below carbon and above germanium.

Carbon, silicon and germanium (germanium, like silicon, is
also a semiconductor) have a unique property in their
electron structure -- each has four electrons in its outer
orbital. This allows them to form nice crystals. The four
electrons form perfect covalent bonds with four neighboring
atoms, creating a lattice. In carbon, we know the crystalline
form as diamond. In silicon, the crystalline form is
a silvery, metallic-looking substance.

Metals tend to be good conductors of electricity because they
usually have "free electrons" that can move easily between
atoms, and electricity involves the flow of electrons. While
silicon crystals look metallic, they are not, in fact, metals.
All of the outer electrons in a silicon crystal are involved
in perfect covalent bonds, so they can't move around. A pure
silicon crystal is nearly an insulator -- very little
electricity will flow through it.

But you can change all this through a process called doping.

Doping Silicon

You can change the behavior of silicon and turn it into
a conductor by doping it. In doping, you mix a small amount
of an impurity into the silicon crystal.

There are two types of impurities:

* N-type - In N-type doping, phosphorus or arsenic is
added to the silicon in small quantities. Phosphorus and
arsenic each have five outer electrons, so they're out of
place when they get into the silicon lattice. The fifth
electron has nothing to bond to, so it's free to move around.
It takes only a very small quantity of the impurity to create
enough free electrons to allow an electric current to flow
through the silicon. N-type silicon is a good conductor.
Electrons have a negative charge, hence the name N-type.

* P-type - In P-type doping, boron or gallium is the
dopant. Boron and gallium each have only three outer
electrons. When mixed into the silicon lattice, they form
"holes" in the lattice where a silicon electron has nothing
to bond to. The absence of an electron creates the effect of
a positive charge, hence the name P-type. Holes can conduct
current. A hole happily accepts an electron from a neighbor,
moving the hole over a space. P-type silicon is a good
conductor.

A minute amount of either N-type or P-type doping turns
a silicon crystal from a good insulator into a viable (but
not great) conductor -- hence the name "semiconductor."

N-type and P-type silicon are not that amazing by themselves;
but when you put them together, you get some very interesting
behavior at the junction. That's what happens in a diode.

A diode is the simplest possible semiconductor device.
A diode allows current to flow in one direction but not the
other. You may have seen turnstiles at a stadium or a subway
station that let people go through in only one direction.
A diode is a one-way turnstile for electrons.

When you put N-type and P-type silicon together as shown in
this diagram, you get a very interesting phenomenon that
gives a diode its unique properties.

Even though N-type silicon by itself is a conductor, and
P-type silicon by itself is also a conductor, the combination
shown in the diagram does not conduct any electricity. The
negative electrons in the N-type silicon get attracted to the
positive terminal of the battery. The positive holes in the
P-type silicon get attracted to the negative terminal of the
battery. No current flows across the junction because the
holes and the electrons are each moving in the wrong
direction.

If you flip the battery around, the diode conducts
electricity just fine. The free electrons in the N-type
silicon are repelled by the negative terminal of the battery.
The holes in the P-type silicon are repelled by the positive
terminal. At the junction between the N-type and P-type
silicon, holes and free electrons meet. The electrons fill
the holes. Those holes and free electrons cease to exist, and
new holes and electrons spring up to take their place. The
effect is that current flows through the junction.