___________________________________________________________________________ Transistors
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 covered in chapter 1. However, when the PNP or NPN material is grown
(see Figure 2-4, view (B)), the impurity mixing process must be reversed twice in order 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.
2-12. Although there are many 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. These techniques include the following:
Wire leads are connected to each semiconductor electrode.
The crystal is specially mounted to protect it against mechanical damage.
The unit is selected to prevent harmful contamination of the crystal.
TRANSISTOR THEORY
2-13. Chapter 1 covered how 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. Therefore, 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.
NPN Transistor Operation
2-14. As in the PN junction diode, the N-type material that makes up the two end
sections of the NPN transistor contains a number of free electrons. 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. Each of these junctions must be modified by some external
bias voltage in order to use the transistor as an amplifier. 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. 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 (see Figure 2-5). Since the second PN junction is required to be reverse biased
for proper transistor operation, then the collector must be connected to an opposite polarity
23 June 2005
TC 9-62
2-5
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