Module 7—Introduction to Solid-State Devices and Power Supplies
i - ix
, 1-1 to 1-10
1-11 to 1-20
, 1-21 to 1-30
1-31 to 1-40
, 1-41 to 1-47
2-1 to 2-10
, 2-11 to 2-20
2-21 to 2-30
2-31 to 2-40
, 2-41 to 2-50
2-51 to 2-54
, 3-1 to 3-10
3-11 to 3-20
, 3-21 to 3-30
3-31 to 3-40
, 3-41 to 3-50
3-51 to 3-54
4-1 to 4-10
, 4-11 to 4-20
4-21 to 4-30
, 4-31 to 4-40
4-41 to 4-50
, 4-51 to 4-62
Upon completion of this chapter, you should be able to do the following:
1. Define the term
transistor and give a brief description of its construction and operation.
2. Explain how the
transistor can be used to amplify a signal.
3. Name the four classes of amplifiers and give an
explanation for each.
4. List the three different transistor circuit configurations and explain their
5. Identify the different types of transistors by their symbology and alphanumerical
6. List the precautions to be taken when working with transistors and describe ways to
7. Explain the meaning of the expression "integrated circuits."
8. Give a brief
description on how integrated circuits are constructed and the advantages they offer over conventional transistor
9. Name the two types of circuit boards.
10. State the purpose and function of modular
INTRODUCTION TO TRANSISTORS
The discovery of the first transistor in 1948 by a team of physicists at the Bell Telephone Laboratories
sparked an interest in solid-state research that spread rapidly. The transistor, which began as a simple
laboratory oddity, was rapidly developed into a semiconductor device of major importance. The transistor
demonstrated for the first time in history that amplification in solids was possible. Before the transistor,
amplification was achieved only with electron tubes. Transistors now perform numerous electronic tasks with new
and improved transistor designs being continually put on the market. In many cases, transistors are more desirable
than tubes because they are small, rugged, require no filament power, and operate at low voltages with
comparatively high efficiency. The development of a family of transistors has even made possible the
miniaturization of electronic circuits. Figure 2-1 shows a sample of the many different types of transistors you
may encounter when working with electronic equipment.
Figure 2-1.—An assortment of different types of transistors.
Transistors have infiltrated virtually every area of science and industry, from the family car to
satellites. Even the military depends heavily on transistors. The ever increasing uses for transistors have
created an urgent need for sound and basic information regarding their operation.
From your study of the
PN-junction diode in the preceding chapter, you now have the basic knowledge to grasp the principles of transistor
operation. In this chapter you will first become acquainted with the basic types of transistors, their
construction, and their theory of operation. You will also find out just how and why transistors amplify. Once
this basic information is understood, transistor terminology, capabilities, limitations, and identification will
be discussed. Last, we will talk about transistor maintenance, integrated circuits, circuit boards, and modular
The first solid-state device discussed was the two-element semiconductor diode. The next device on our
list is even more unique. It not only has one more element than the diode but it can amplify as well.
Semiconductor devices that have-three or more elements are called TRANSISTORS. The term transistor was derived
from the words
TRANSfer and resISTOR. This term was adopted because it
best describes the operation of the transistor - the transfer of an input signal current from a low-resistance
circuit to a high- resistance circuit. Basically, the transistor is a solid-state device that amplifies by
controlling the flow of current carriers through its semiconductor materials.
There are many different
types of transistors, but their basic theory of operation is all the same. As a matter of fact, the theory we will
be using to explain the operation of a transistor is the same theory used earlier with the PN-junction diode
except that now two such junctions are required to form the three elements of a transistor. The three elements of
the two-junction transistor are (1) the EMITTER, which gives off, or emits," current carriers (electrons or
holes); (2) the BASE, which controls the flow of current carriers; and (3) the COLLECTOR, which collects the
Transistors are classified as either NPN or PNP according to the arrangement of their N and P materials. Their
basic construction and chemical treatment is implied by their names, "NPN" or "PNP." That
is, an NPN transistor is formed by introducing a thin region of P-type material between two regions of
N-type material. On the other hand, a PNP transistor is formed by introducing a thin region of N-type material
between two regions of P-type material. Transistors constructed in this manner have two PN junctions, as shown in
figure 2-2. One PN junction is between the emitter and the base; the other PN junction is between the collector
and the base. The two junctions share one section of semiconductor material so that the transistor actually
consists of three elements.
Figure 2-2.—Transistor block diagrams.
Since the majority and minority current carriers are different for N and P materials, it stands to
reason that the internal operation of the NPN and PNP transistors will also be different. The theory of operation
of the NPN and PNP transistors will be discussed separately in the next few paragraphs. Any additional information
about the PN junction will be given as the theory of transistor operation is developed.
To prepare you
for the forthcoming information, the two basic types of transistors along with their circuit symbols are shown in
figure 2-3. It should be noted that the two symbols are different. The horizontal line represents the base, the
angular line with the arrow on it represents the emitter, and the other angular line represents the collector. The
direction of the arrow on the emitter distinguishes the NPN from the PNP transistor. If the arrow points in, (Points
iN) the transistor is a PNP. On the other hand if the arrow points out, the
transistor is an NPN (Not Pointing iN).
Figure 2-3.—Transistor representations.
Another point you should keep in mind is that the arrow always points in the direction of hole flow, or
from the P to N sections, no matter whether the P section is the emitter or base. On the other hand, electron flow
is always toward or against the arrow, just like in the junction diode.
The very first transistors were known as point-contact transistors. Their construction is similar to the
construction of the point-contact diode covered in chapter 1. The difference, of course, is that the point-contact
transistor has two P or N regions formed instead of one. Each of the two regions constitutes an electrode
(element) of the transistor. One is named the emitter and the other is named the collector, as shown in figure
2-4, view A.
Figure 2-4.—Transistor constructions.
Point-contact transistors are now practically obsolete. They have been replaced by junction transistors,
which are superior to point-contact transistors in nearly all respects. The junction transistor generates less
noise, handles more power, provides higher current and voltage gains, and can be mass-produced more cheaply than
the point-contact transistor. Junction transistors are manufactured in much the same manner as the PN junction
diode discussed earlier. However, when the PNP or NPN material is grown (view B), the impurity mixing process must
be reversed twice to obtain the two junctions required in a transistor. Likewise, when the alloy-junction (view C)
or the diffused-junction (view D) process is used, two junctions must also be created within the crystal.
Although there are numerous ways to manufacture transistors, one of the most important parts of any manufacturing
process is quality control. Without good quality control, many transistors would prove unreliable because the
construction and processing of a transistor govern its thermal ratings, stability, and electrical characteristics.
Even though there are many variations in the transistor manufacturing processes, certain structural techniques,
which yield good reliability and long life, are common to all processes: (1) Wire leads are connected to each
semiconductor electrode; (2) the crystal is specially mounted to protect it against mechanical damage; and (3) the
unit is sealed to prevent harmful contamination of the crystal.
Q1. What is the name given to the
semiconductor device that has three or more elements?
Q2. What electronic function made the transistor famous?
Q3. In which direction does the arrow point
on an NPN transistor?
Q4. What was the name of the very first transistor?
Q5. What is one of the most important parts of any transistor manufacturing process?
You should recall from an earlier discussion that a forward-biased
PN junction is comparable to a low- resistance circuit element because it passes a high current for a given
voltage. In turn, a reverse-biased PN junction is comparable to a high-resistance circuit element. By using the
Ohm's law formula for power
(P = I2R) and assuming current is held constant, you can conclude that
the power developed across a high resistance is greater than that developed across a low resistance. Thus, if a
crystal were to contain two PN junctions (one forward-biased and the other reverse-biased), a low-power signal
could be injected into the forward-biased junction and produce a high-power signal at the reverse-biased junction.
In this manner, a power gain would be obtained across the crystal. This concept, which is merely an extension of
the material covered in chapter 1, is the basic theory behind how the transistor amplifies. With this information
fresh in your mind, let's proceed directly to the NPN transistor.
NPN Transistor Operation
Just as in the case of the PN junction diode, the N material comprising the two end sections of the
NP N transistor contains a number of free electrons,
while the center P section contains an excess number of holes. The action at each junction between these sections
is the same as that previously described for the diode; that is, depletion regions develop and the junction
barrier appears. To use the transistor as an amplifier, each of these junctions must be modified by some external
bias voltage. For the transistor to function in this capacity, the first PN junction (emitter-base junction) is
biased in the forward, or low-resistance, direction. At the same time the second PN junction (base-collector
junction) is biased in the reverse, or high- resistance, direction. A simple way to remember how to properly bias
a transistor is to observe the NPN or PNP elements that make up the transistor. The letters of these elements
indicate what polarity voltage to use for correct bias. For instance, notice the NPN transistor below:
1. The emitter, which is the first letter in the NPN sequence, is connected to the negative side of
the battery while the base, which is the second letter (NPN), is connected to the positive side.
2. However, since the second PN junction is required to be reverse biased for proper transistor
operation, the collector must be connected to an opposite polarity voltage (positive) than that indicated by its
letter designation(NPN). The voltage on the collector must also be more positive than the base, as shown below:
We now have a properly biased NPN transistor.
In summary, the base of the NPN transistor must
be positive with respect to the emitter, and the collector must be more positive than the base.
NPN FORWARD-BIASED JUNCTION.—An important point to bring out at this time, which was not
necessarily mentioned during the explanation of the diode, is the fact that the N material on one side of the
forward-biased junction is more heavily doped than the P material. This results in more current being carried
across the junction by the majority carrier electrons from the N material than the majority carrier holes from the
P material. Therefore, conduction through the forward-biased junction, as shown in figure 2-5, is mainly by
majority carrier electrons from the N material (emitter).
Figure 2-5.—The forward-biased junction in an NPN transistor.
With the emitter-to-base junction in the figure biased in the forward direction, electrons leave the
negative terminal of the battery and enter the N material (emitter). Since electrons are majority current carriers
in the N material, they pass easily through the emitter, cross over the junction, and combine with holes in the P
material (base). For each electron that fills a hole in the P material, another electron will leave the P material
(creating a new hole) and enter the positive terminal of the battery.
NPN REVERSE-BIASED JUNCTION.—The
second PN junction (base-to-collector), or reverse- biased junction as it is called (fig. 2-6), blocks the
majority current carriers from crossing the junction. However, there is a very small current, mentioned earlier,
that does pass through this junction. This current is called minority current, or reverse current. As you recall,
this current was produced by the electron-hole pairs. The minority carriers for the reverse-biased PN junction are
the electrons in the P material and the holes in the N material. These minority carriers actually conduct the
current for the reverse-biased junction when electrons from the P material enter the N material, and the holes
from the N material enter the P material. However, the minority current electrons (as you will see later) play the
most important part in the operation of the NPN transistor.
Figure 2-6.—The reverse-biased junction in an NPN transistor.
At this point you may wonder why the second PN junction (base-to-collector) is not forward biased like
the first PN junction (emitter-to-base). If both junctions were forward biased, the electrons would have a
tendency to flow from each end section of the N P N transistor (emitter and collector) to the center P section
(base). In essence, we would have two junction diodes possessing a common base, thus eliminating any amplification
and defeating the purpose of the transistor. A word of caution is in order at this time. If you should mistakenly
bias the second PN junction in the forward direction, the excessive current could develop enough heat to destroy
the junctions, making the transistor useless. Therefore, be sure your bias voltage polarities are correct before
making any electrical connections.
NPN JUNCTION INTERACTION.—We are now ready to see what happens when we place the two junctions of
the NPN transistor in operation at the same time. For a better understanding of just how the two junctions work
together, refer to figure 2-7 during the discussion.
Figure 2-7.—NPN transistor operation.
The bias batteries in this figure have been labeled V CC for the collector voltage supply, and VBB
for the base voltage supply. Also notice the base supply battery is quite small, as indicated by the number of
cells in the battery, usually 1 volt or less. However, the collector supply is generally much higher than the base
supply, normally around 6 volts. As you will see later, this difference in supply voltages is necessary to have
current flow from the emitter to the collector.
As stated earlier, the current flow in the external
circuit is always due to the movement of free electrons. Therefore, electrons flow from the negative terminals of
the supply batteries to the N-type emitter. This combined movement of electrons is known as emitter current (IE).
Since electrons are the majority carriers in the N material, they will move through the N material emitter to the
emitter-base junction. With this junction forward biased, electrons continue on into the base region. Once the
electrons are in the base, which is a P-type material, they become minority carriers. Some of the electrons that
move into the base recombine with available holes. For each electron that recombines, another electron moves out
through the base lead as base current IB (creating a new hole for eventual combination) and returns to
the base supply battery VBB. The electrons that recombine are lost as far as the collector is
concerned. Therefore, to make the transistor more efficient, the base region is made very thin and lightly doped.
This reduces the opportunity for an electron to recombine with a hole and be lost. Thus, most of the electrons
that move into the base region come under the influence of the large collector reverse bias. This bias acts as
forward bias for the minority carriers (electrons) in the base and, as such, accelerates them through the
base-collector junction and on into the collector region. Since the collector is made of an N-type material, the
electrons that reach the collector again become majority
current carriers. Once in the collector, the electrons move easily through the N material and return to the
positive terminal of the collector supply battery VCC as collector current (IC).
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics