Avionics News April 2019 - 57

will be so narrow that it can no longer act as a barrier. Current will
flow as electrons jump the barrier, trying to fill the holes in the Ptype silicon. Of course, these electrons will never be able to fill the
holes, because the P-type silicon is stable in an ionized state. The
electrons just keep flowing through and back into the battery. We
call this forward biasing.
When the battery is connected the other way, with the positive terminal connected to the cathode and the negative terminal
connected to the anode as shown in Figure 2b, the holes are attracted toward the negative battery terminal, looking for electrons.
Simultaneously, atoms in the N-type silicon are drawn toward the
positive battery terminal. As a result, the depletion zone widens
and blocks current flow. It's too far to make the jump. We call
this reverse biasing. The characteristics described above makes the
diode a useful one-way street for current, like a check valve in
hydraulics. We can use diodes to create DC from AC (rectifier), as
a switch, and in many other ways.
Technicians must be aware of the characteristics and limitations
of diode design. When forward biased, a diode will drop a constant
0.6 volts across the junction. This voltage drop is an indication a

diode is working properly. If, when forward biased, the voltage is
lower or higher, then the diode is either shorted or open. Since a diode is a semiconductor and not a conductor, a voltage drop is created, and heat is developed as current flows. Diodes will be rated
for their current carrying capability. Furthermore, a wide depletion
zone can block current, but only to a point. If one were to apply a
large enough voltage in the reverse bias condition, then the high
voltage pressure will overcome the depletion zone and current will
flow with bad results, often including smoke. Always be aware of
a diode's peak inverse voltage rating.
Special diodes
There are a variety of diodes available, allowing engineers to
design a great variety of useful electronic devices.
Unless you've lived life in a remote desert or mountain location,
you are aware of light emitting diodes. These diodes glow when
forward biased. Engineers can design LEDs for a variety of colors
and brightness. Not only do we see LEDs used in televisions, but
also as a replacement for incandescent light bulbs.
Zener diodes are useful as regulating elements. Unlike rectifier
diodes, zener diodes have a low PIV, called the zener voltage. In a
similar manner to the forward biased voltage drop, the zener voltage will remain constant, even as the current through the diode
changes. This constant voltage characteristic allows zener diodes
to be used as voltage regulators.
All diodes have some inherent capacitance, with the P-type and
N-type materials acting as plates and the depletion zone acting as the
dielectric. Varactor diodes are constructed to maximize this characteristic. Installed in a reverse biased condition, the depletion zone
and the capacitance change as the reverse bias voltage is changed.
As a result, varactor diodes are used in frequency synthesizers, filters and in any other circuit designed to be adjusted automatically.
Continued on page 61

Figure 2a: Current flows in a forward biased diode;
Figure 2b: No current will flow in a reverse biased diode.

A few common diodes. The line on the case indicates the cathode side.
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Avionics News April 2019

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