Just For Starters
Guest Hosted by Terence Thomas
Basics For Beginners
Just For Starters
A Tutorial on Diodes
Materials that can conduct
current (silver, gold, aluminum, copper, etc.) contain large numbers of loosely held
electrons. Their resistance to the flow
of current is measured in just a few
millionths of an ohm per centimeter.
Insulators — such as glass, rubber,
and plastic — have very few loosely
held electrons and their resistance to
the flow of current is measured in a
few million ohms per centimeter.
of aluminum, gallium, or indium.
P-N Junction
When a p-type junction is made,
the electrical equivalent of “holes” or
an absence of electrons is set up. An
n junction has an excess of electrons.
Combining p and n materials and
applying negative current results in a
diffusing of electrons across the junction is known as diffusion current.
As you can see in Figure 1, a
surplus of electrons from the n
material penetrate the space-charge
region, flow across the junction, and
move through the holes to the
positive lead. This is known as
forward biasing and results in signal
being transferred through the diode.
When the current is reversed, the
free electrons are drawn to the
positive terminal and the holes are
attracted to the negative terminal.
This is known as reverse biasing and
results in no passage of current
through the diode. The more reverse
biasing is applied to it, the more
resistance the diode presents.
threshold voltage is achieved. The
biasing level can vary from one
diode to another, but most diodes
will trigger at about 0.7 volts. Reverse
biasing does not have a threshold
and a diode starts responding at the
first sign of a signal. Since diodes are
mostly given the duty of switching
non-sinusoidal signals, biasing — for
the most part — is not a factor.
Doping
Semiconductors
As the term implies, semiconductors are not as conductive as
metal, nor are they as non-conductive
as insulators. The most commonly
used materials in semiconductors are
germanium and silicon. Germanium,
in its pure form, is rated at 60 ohms
per centimeter, while silicon is rated
at 60,000 ohms per centimeter.
What enables semiconductors to
change their state of conductivity is
the addition of controlled amounts of
impurities. Arsenic and antimony
added to the mix will produce what is
known as an n-type material because
of the negative charge from the excess
of free electrons. A p-type junction can
be produced with the careful addition
Doping is the name given to the
process of adding impurities that
enable the diode to respond to current. Different doping procedures can
produce different types of diodes.
For example, over-doping can produce something called a tunnel diode.
Tunnel Diodes
Biasing
Like bipolar transistors, diodes
do not respond to
signals until a
Figure 1. Combined P and N junctions
A tunnel diode has a high concentration of impurities in both the p and
the n sections of the device. The
space-charge region is so narrow in
this diode that an electrical charge can
pass through the device by tunneling,
a quantum-mechanical action that
produces a negative-resistance region
that has the potential of achieving
amplification. In circuits where signal
intensity is compromised for one reason or another, the tunnel diode can
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Figure 2. Zener diode operation
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SEPTEMBER 2004
