years have passed since I sat in a college classroom to learn about
transistor fundamentals. The industry had long moved past germanium
transistors and was solidly into silicon. Having been formally introduced
to transistors in the USAF, I was familiar with their functionality
from a technician's perspective of checking for gain, proper bias
(as indicated on "educated" schematics), and determining go-no-go
health by performing a front-to-back resistance measurement using
an ohmmeter. Holes, energy bands, gate widths, and doping levels
were first encountered in solid state physics class, however. This
article does a nice job of introducing the terms and concepts at
a layman's level.
I actually found the vacuum tube circuits
in our radar unit easier to troubleshoot than transistor circuits,
partially because I had a little experience with them prior to enlistment
and also because the point-to-point component mounting made it easy
to isolate or remove components from the rest of the circuit. It
was rare to destroy a vacuum tube by shorting out or improperly
installing another component in the circuit. Today's field techs
typically swap out circuit board assemblies after the system's built-in-test
(BIT) advises corrective action on a computer monitor.
Northwestern Television and Electronics Institute
For a true understanding of semiconductors,
you must be able "to speak the language". Here is how.
transistor has now ceased to be an experimental device and has become,
instead, a commercial reality. The technician who fails to recognize
this fact and does not prepare himself accordingly, faces a future
as limited as that of an automobile mechanic who does not acquaint
himself with automatic transmissions. In his attempts to read up
on transistors, the technician too often finds himself engulfed
in a fog of unfamiliar terminology. Such terms as injected carriers,
intrinsic semiconductor, trivalent impurity, etc. are from the vocabulary
of the semiconductor physicist and to the average technician are
as meaningless as Sanskrit. It is the purpose of this article to
define, in terms familiar to the technician, the terminology of
In some substances, the atoms are arranged
in neat, orderly geometric patterns, like oranges in a newly packed
crate. These are known as crystalline substances in order to distinguish
them from materials in which the atoms, like grains of sand on a
beach, have no regular pattern of arrangement. Because of the geometric
orientation of their atoms, crystalline substances have characteristic
shapes. A familiar example is the six-sided rod of quartz in its
Fig. 1. Hexagonal tiles fit together perfectly. Each
tile is associated with six adjacent tiles. Atoms of germanium
fit together perfectly. The four outer orbit electrons of
each atom are associated with outer orbit electrons of adjacent
atoms. Such associations are called "valence" bonds.
Fig. 2. Since boron has only three valence electrons,
one of the valence bonds is left unsatisfied when an atom
of boron is added to germanium. The "missing" electron creates
a hole in the crystal structure. When arsenic is added to
germanium, each atom contributes a surplus electron.
Germanium is a crystalline substance whose electrical resistance
is too great to permit ·its use as a conductor and too low to be
used as an insulator. For this reason, germanium is classified as
a semiconductor. In each atom of germanium, 32 electrons revolve
around the nucleus. These planet-like electrons are located in four
orbits or rings. The outer orbit, known as the valence ring, contains
four of the electrons. In a germanium crystal, the four valence
ring electrons of each atom are associated with the valence ring
electrons of adjacent atoms. These associations or partnerships
of outer orbit electrons are known as valence bonds or covalent
bonds. To illustrate the concept of valence bonds with a purely
mechanical analogy, consider a floor made up of hexagonal tiles
as shown in Fig. 1. It is apparent that tiles of this particular
shape will fit together perfectly, and that each tile is associated
with six adjacent tiles. An analogous relationship exists in the
atomic structure of a germanium crystal. As shown in Fig. 1, each
outer orbit electron is associated with an outer orbit electron
of an adjacent atom. Since no outer orbit electron is without a
mate, the atoms (like the tiles) fit together perfectly. The chemist
describes this situation by saying that all of the valence bonds
are satisfied. As is common practice, only the outer orbit or valence
electrons are shown in the drawing of Fig. 1 shown below.
Consider now the consequence of replacing one of the tiles of
Fig. 1 with a tile having a different number of sides. It is immediately
apparent that such a tile will not fit. Either it will, overlap
adjacent tiles or will leave empty spaces. These structural defects
have analogies in the transistor. If one of the atoms of Fig. 1
is replaced with an atom having either too many or too few outer
orbit electrons, this impurity atom will not properly fit into the
structure of the crystal. Either there will be an overlap (extra
electron) or an empty spot (hole) in the structure, depending upon
the number of valence electrons in the impurity atom. Under these
conditions, all of the outer orbit electrons do not have mates.
In the language of the chemist, all of the valence bonds are not
satisfied. In transistor physics, such structural imperfections
are known as lattice defects and are intentionally created by introducing
impurity atoms into the semiconductor material. By definition, an
impurity atom is one having either more or less valence electrons
than the atoms of the semiconductor to which it is added.
When the chemical impurity added to a semiconductor material
has fewer valence electrons than the atoms of the semiconductor,
the impurity is known as an acceptor. For example, boron is an acceptor
when added to germanium because it has only three outer orbit electrons
as compared to four for germanium. As a consequence, each boron
atom will rob or accept a valence electron from an atom of germanium.
The site formerly occupied by the stolen electron is known as a
hole. Because it is the consequence of a missing electron, the hole
possesses the properties of a positively charged particle. This
condition is shown schematically in Fig. 2. Since its atoms contain
three valence electrons, boron is known as a trivalent impurity.
Indium, gallium, and aluminum are also trivalent and therefore are
acceptors with respect to germanium. Germanium to which acceptor
impurities have been added is characterized by an abundance of holes
and is therefore known as positive or p-type germanium.
When the chemical impurity added to the semiconductor material has
more valence electrons than the atoms of the semiconductor, the
impurity is known as a donor. For example, arsenic is a donor when
added to germanium because it has five outer orbit electrons as
compared to four for germanium. As a result, each arsenic atom provides
one extra electron which is not in valence bond and therefore free
to act as a current carrier. This condition is shown schematically
in Fig. 2. Since its atoms contain five valence electrons, arsenic
is known as a pentavalent impurity. Germanium, to which donor impurities
have been added, is characterized by an abundance of electrons and
is therefore known as negative or n-type germanium.
Since its atoms contain four valence electrons, germanium is known
as a tetravalent element. When the germanium is pure or when it
contains equal amounts of donor and acceptor impurities, it is referred
to as intrinsic germanium.
A junction diode is made up of
n-type and p-type germanium as shown in Fig. 3. When the negative
terminal of a battery is connected to the n-type (electron-rich)
layer of the junction, and the positive terminal is connected to
the p-type (hole-rich) layer, the diode is biased in the forward
direction. Under these conditions, the electrons in the n-type layer
are repelled by the negative terminal of the battery and move toward
the junction. At the same time, the holes in the p-type layer are
repelled by the positive terminal of the battery and also move toward
the junction. At the junction, the electrons and the holes effectively
neutralize each other and permit current flow. This represents the
low resistance direction of the junction diode. When the polarity
of the battery is reversed, the electrons in the n-type layer and
the holes in the p-type layer move away from the junction. Very
little current can now flow across the junction because there are
few current carriers in this region. This is referred to as reverse
bias and represents the high resistance direction of the junction
The junction transistor consists of two back-to-back
junction diodes with the center layer (known as the base) participating
in both junctions. The input junction (emitter and base) is biased
in the forward direction, and the output junction (collector and
base) is biased in the reverse direction. The transistor shown in
Fig. 3C consists of a layer of p-type germanium between two layers
of n-type germanium. This is known as an n-p-n transistor. An opposite
arrangement is used in the p-n-p transistor and the battery polarities
must be opposite those shown in Fig. 3C.
Fig. 3. The base-collector junction of the transistor
is biased in the reverse direction, but current flow is
increased by the presence of carriers injected by emitter.
Input current therefore controls output current.
Fig. 4. Resistor R determines the magnitude of the
d.c. bias current. The input signal swings this current
around the operating point, causing related variations in
In some respects, the emitter of a transistor is comparable to the
cathode of a vacuum tube since both emit or inject the current carriers.
In the transistor, the injected carriers may be either electrons
or holes. In the operation of an n-p-n transistor, a negative potential
is applied to the emitter and electrons are repelled from emitter
to base. The emitter has thus injected carriers into the base region.
The emitter of a p-n-p transistor is made positive with respect
to base. Each electron attracted towards the positive terminal of
the battery leaves a hole at its former location. The hole then
captures an electron from an adjacent atom, creating another hole
farther back toward the base. In effect, the emitter has injected
holes into the base region.
The collector-base junction
is biased in the reverse direction and the current flow is therefore
relatively small. This current, however, is increased by the presence
of the additional carriers injected by the emitter. When an input
signal is applied to the transistor, it varies the number of carriers
injected into the base region and therefore varies the collector
current. The ratio of the change of collector current to the change
of emitter current (with collector voltage held constant) is known
as the alpha of the transistor. Since it specifies the ratio of
the output to input current, alpha is the current gain of the transistor.
Alpha is defined with respect to the common base circuit, a configuration
in which the base is common to both the input arid the output circuit.
The alpha of a junction transistor is less than unity because some
of the carriers injected by the emitter are neutralized in the base
region and therefore do not reach the collector. For example, some
of the electrons injected by the emitter of an n-p-n transistor
are neutralized in the hole-rich base region. In the p-n-p transistor,
the injected carriers are holes, and some of them are neutralized
in the electron-rich base. It is for this reason that the output
current is less than the input current. The alpha of junction transistors
commercially available is in the range of 0.80 to 0.99. The higher
values of alpha (approaching unity) are obtained when the base layer
of the transistor is made very thin. The injected carriers then
pass through the base in less time and fewer of them are neutralized.
Because the alpha of a transistor is less than one does
not mean that it is incapable of producing voltage gain. The feature
of the transistor that makes voltage gain possible is the high output
resistance as compared to the input resistance. Even though the
output current is slightly less than the input current, it flows
through a higher value of resistance and therefore produces a signal
voltage of greater magnitude than that of the input signal. For
the same reason, the transistor is capable of power gain.
When a transistor is connected in a common emitter circuit,
the input signal is applied to the base and the output is taken
from the collector. With this configuration, a current gain greater
than unity can be achieved. This base-to-collector current gain
is known as beta, and values of 30 to 40 are common for commercially
available transistors, The beta of a transistor is related to its
alpha as follows: beta = alpha/(1 - alpha). From this relationship,
it is apparent that the beta of a transistor becomes greater as
its alpha approaches unity.
The vacuum tube is a voltage-operated
device and bias voltage is used to establish the desired operating
point. The input signal then swings the grid around this operating
point, The transistor, however, is a current-operated device. A
steady d. c. bias current is used to establish the initial condition
and the input signal then swings this current around the operating
point. In the common emitter circuit, the input signal is applied
to the base of the transistor, The steady bias current, upon which
the signal current is superimposed, is known as the base bias current.
In the common emitter circuit shown in Fig. 4A, battery B1
supplies the base bias current, and battery B2
is used to bias the collector circuit. Fig. 4B is a circuit arrangement
which uses a single battery for biasing both input and output circuits.
Resistor R determines the magnitude of the base bias current and
therefore establishes the operating point.
of a transistor is biased in the reverse (high-resistance) direction.
Consequently, current in the collector circuit is relatively small.
This current, however, is increased by the presence of injected
carriers and therefore varies in accordance with the variations
of input signal. Even with the input current reduced to zero, some
small amount of current will flow in the collector circuit. This
is known as collector current cut-off. It is not a true cut-off
condition such as can be obtained in a vacuum tube because the collector
draws some current even with reverse bias and with no injected carriers.
The current, however, is sufficiently small to justify the use of
the term cut-off.
The amount of power that can be dissipated
in the collector of a transistor is limited by the possibility of
damage or serious change of characteristics as a result of overheating.
Except for specially designed power transistors, collector dissipation
is usually in the range of 50 to 150 milliwatts. Numerically, collector
dissipation is equal to the product of collector current and collector
voltage. The transistor must be so operated that this product does
not exceed the maximum dissipation rating. For example, if the maximum
collector dissipation of a transistor is 100 milliwatts and the
collector voltage is 25, the collector current must not exceed 4
ma. If the collector voltage is reduced to 20 volts, the permissible
collector current will be 5 ma. Naturally, the operating range of
the transistor should be so limited that at no point will the rated
maximums of collector voltage and collector current be exceeded.