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.
Clockwise from top: A chip, an LED and
a transistor are all made from semiconductor
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 edition of
you'll learn what a semiconductor is, how doping works and how
a diode can be created using semiconductors.
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.
Silicon sits next to aluminum and below
carbon in the periodic
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,
In a silicon lattice, all silicon atoms bond
perfectly to four neighbors, leaving no free electrons
to conduct electric current. This makes a silicon
crystal an insulator rather than a
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.
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:
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
- N-type - In N-type doping, phosphorus
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
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
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.
Creating 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
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.
A device that blocks current in one direction while letting
current flow in another direction is called a diode.
Diodes can be used in a number of ways. For example, a device
that uses batteries often contains a diode that protects the
device if you insert the batteries backward. The diode simply
blocks any current from leaving the battery if it is reversed
-- this protects the sensitive electronics in the device.
A semiconductor diode's behavior is not perfect, as shown
in this graph:
When reverse-biased, an ideal diode would block all
current. A real diode lets perhaps 10 microamps
through -- not a lot, but still not perfect. And if you apply
enough reverse voltage
(V), the junction breaks down and lets current through.
Usually, the breakdown voltage is a lot more voltage than the
circuit will ever see, so it is irrelevant.
When forward-biased, there is a small amount of
voltage necessary to get the diode going. In silicon, this
voltage is about 0.7 volts. This voltage is needed to start
the hole-electron combination process at the junction.
Transistors and Chips
A transistor is
created by using three layers rather than the two
layers used in a diode. You can create either an NPN or a PNP
sandwich. A transistor can act as a switch or an amplifier.
A transistor looks like two diodes back-to-back. You'd
imagine that no current could flow through a transistor
because back-to-back diodes would block current both ways. And
this is true. However, when you apply a small current to the
center layer of the sandwich, a much larger current can
flow through the sandwich as a whole. This gives a transistor
its switching behavior. A small current can turn a
larger current on and off.
A silicon chip is a piece of silicon that can hold
thousands of transistors. With transistors acting as switches,
you can create Boolean
gates, and with Boolean gates you can create microprocessor
The natural progression from silicon to doped silicon to
transistors to chips is what has made microprocessors and
other electronic devices so inexpensive and ubiquitous in
today's society. The fundamental principles are surprisingly
simple. The miracle is the constant refinement of those
principles to the point where, today, tens of millions of
transistors can be inexpensively formed onto a single chip.
For more information on semiconductors, diodes, chips and
more, check out the links on the next page.
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