28-11-2012, 04:19 PM
Introduction to Transistors
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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.
TRANSISTOR FUNDAMENTALS
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.
CONSTRUCTION
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.
TRANSISTOR THEORY
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.