Module 7 − Introduction to Solid−State Devices and Power Supplies
Pages i ,
4−1 to 4−10,
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
4. List the three different transistor circuit configurations
and explain their operation.
5. Identify the different types of transistors by their symbology
and alphanumerical designations.
6. List the precautions to be taken when working with transistors
and describe ways to test them.
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 circuits.
9. Name the two types of circuit boards.
10. State the purpose and function of modular circuitry.
Introduction to Transistors
Figure 2-1 - An assortment of different types of 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.
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 circuitry.
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 current carriers.
Figure 2-2 - Transistor block diagrams.
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.
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
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.
Figure 2-3 - Transistor representations.
Figure 2-4 - Transistor constructions.
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.
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
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
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.
Just as in the case of the PN junction diode, the N material comprising the two
end sections of the NPN 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 to the right.
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
Figure 2-5 - The forward-biased junction in an NPN transistor.
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).
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
Figure 2-6 - The reverse-biased junction in an NPN transistor.
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.
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
Figure 2-7 - NPN transistor operation.
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.
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).
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