Chapter 3
Special Devices
LEARNING OBJECTIVES
Learning objectives serve as a preview of the information you are expected to learn in this chapter.
The comprehensive check-on-learning questions, found at the end of the chapter, are based on the
objectives. Upon completion of this chapter, you will be able to perform the following learning
objectives:
Explain the basic operation and the major applications of the Zener diode.
Describe the basic operation of the tunnel diode and the varactor.
Explain the basic operation of the SCR and the triac and compare the
advantages and disadvantages of each.
List the five most commonly used optoelectronic devices and explain the uses
of each.
Describe the basic operation, applications, and major advantages of the
unijunction transistor.
Describe the basic operation, applications, and major advantages of the field-
effect transistor and the metal-oxide semiconductor field-effect transistor.
INTRODUCTION TO SPECIAL DEVICES
3-1.
If you consider the sensitive nature and the various interacting properties of
semiconductors, it should not surprise you that solid state devices can be designed for
many different purposes. In fact, there are many devices with special features and new
designs are so frequently introduced that it would be beyond the scope of this chapter to
describe all of the devices in use today. Therefore, this chapter will include a variety of
representative devices that are used extensively in Army equipment to give you an idea of
the diversity and versatility that has been made possible. These devices have been grouped
into three categories (diodes, optoelectronic devices, and transistors). In this chapter we
will cover and describe each device and its basic operation.
DIODES
3-2.
Diodes are two-terminal semiconductors of various types that are used in
seemingly endless applications. The operation of normal PN junction diodes has already
been covered. However, there are a number of diodes with special properties with which
you should be familiar. To cover all of the developments in the diode field would be
impossible, so some of the more commonly used special diodes have been selected for
explanation. These include Zener diodes, tunnel diodes, varactors, SCRs, and triacs.
Zener Diodes
3-3.
When a PN junction diode is reverse biased, the majority carriers (holes in the
P-material and electrons in the N-material) move away from the junction. The barrier or
depletion region becomes wider (see Figure 3-1) and majority carrier current flow becomes
very difficult across the high resistance of the wide depletion region. The presence of
23 June 2005
TC 9-62
3-1
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