Many 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.
By Ed Bukstein
Northwestern Television and Electronics Institute Minneapolis, Minnesota
For a true understanding of semiconductors, you must be able "to speak the language".
Here is how.
The 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 transistor physics.
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 natural state.
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 diode.
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 output current.
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.
BBecause 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.
The collector 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
Posted November 11, 2019