Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
In April of 1952 when this article appeared in Radio & Television
News magazine, the bipolar junction transistor (BJT) had only
made it out of the experimental laboratory of Messrs. Bardeen,
Shockley, and Brattain at Bell Labs a mere three years earlier
in December of 1948. It did not take long for commercial production
to begin. Along with being a great primer for anyone new to
transistors, herein is also some background on how the now ubiquitous
BJT schematic symbol was created. Interestingly, only Dr. William
Shockley is mentioned, making me wonder whether the contributions
of Dr. John Bardeen, and Dr. Walter H. Brattain was
not widely publicized early on. Not to worry, though, because
all three were duly recognized as recipients of the
1956 Nobel Prize in Physics.
The Junction Transistor
H. S. Renne
Technical Editor Radio & Television News
Comparative size of the new junction transistor
and a standard miniature type 6AK5, which is 3/4" in dia.
Details on Bell Laboratories' newly-developed unit and a
discussion of some potential uses.
There was a tremendous amount of excitement among electronic
engineers when the first point-contact transistor was announced
by Bell Telephone Laboratories some four years ago. This initial
enthusiasm, however, was soon replaced by a great deal of hard
work at the laboratories where efforts were made to iron out
the "bugs" existing in the original design, and to ready the
device for mass production. These goals have now been reached
and reliable, uniform point-contact transistors are in limited
The original interest in transistors was given a new stimulus
last July with the announcement of the junction transistor.
Based on theoretical work at Bell Laboratories by Dr. William
Shockley, this new device possessed qualities which seemed to
indicate that in many respects it would prove to be superior
to the point-contact type of transistor. Intensive research
and development work is now in progress all over the country
to determine its limitations and capabilities and to prepare
it for mass production.
Transistors are constructed of a metal called germanium,
which is classed as a semiconductor. Conduction of electricity
through germanium can take place either by a .flow of electrons
or by a flow of so-called "holes" which are, in effect, an absence
of electrons. The result is similar to what would occur if an
electron were replaced by a positive charge. The type of conduction
is determined by very slight amounts of impurities in the germanium.
If these impurities are such that conduction takes place by
means of electrons, the germanium is of the "N" type. If conduction
is by means of "holes," the germanium is known as the "P" type.
The basic construction of the junction transistor is shown
in Fig. 1. It consists essentially of a very thin wafer of "P"
type germanium cut from a single crystal and placed between
two tiny bars of "N" type germanium, also cut from a single
crystal. Because of this construction, the junction transistor
is often referred to as the "NPN" transistor.
This new device has many characteristics which entitle it
to be called the first serious rival to the vacuum tube. Its
amplifying properties are in many respects superior to conventional
tubes, and it is far more rugged with respect to shock and vibration
than any known tube. It is much smaller (about half the size
of a pea), and has an expected service life greatly exceeding
that conventional tubes.
Fig. 1 - Basic construction of the junction
transistor. Two types of germanium are used.
Fig. 3 shows the form of presentation which has been adopted
for the junction transistor. The base is the center or "P" type
section of germanium; the collector and emitter are the "N"
Power amplifications as high as 51 db (100,000 times) have
been achieve! with specific units, and it is reasonable to assume
that this figure can be approached or even exceeded in production
units. The actual power output rating of the transistor itself
is rather small, depending primarily on the cross-sectional
area of the germanium at the junction. Most of the experimental
units use a germanium rod about a sixty-fourth of an inch in
diameter, and have an output rating on the order of 50 milliwatts,
heat dissipation within the unit being the limiting factor.
One transistor has been assembled with a cross-sectional area
of a square centimeter which has a rating of two watts, and
still higher-powered units are possible.
A variety of input and output impedances may be achieved
by connecting the transistor in different ways. Fig. 2 shows
the three possible connections, together with a corresponding
practical circuit for each.
The arrangement in Fig. 2A is called the grounded-base circuit,
characterized by a low input impedance and a high output impedance.
Typical values would be from about 50 to 250 ohms for the former,
and from 1.5 to 13.5 megohms for the latter. These values depend
to quite a large extent on the construction of the transistor,
as well as the amount and kinds of impurities present in the
germanium. Power gains of 40 to 50 db can readily be achieved
with this arrangement when impedances are matched, and appreciable
gains can be obtained with a load resistance of only a few thousand
ohms. One advantage of the latter arrangement is that the gain
is almost completely independent of those transistor properties
which vary from unit to unit. When amplifier stages of this
type are cascaded, a matching transformer should be used.
Fig. 2 - How the input and output impedances
may be varied by connecting the junction transistor different
ways. In each case. the connection is shown along with a corresponding
practical circuit. (A) Grounded-base circuit. (B) Grounded-emitter
circuit. (C) Grounded-collector hookup. See article for a complete
description of their possible applications.
The second type of connection, illustrated in Fig. 2B, is known
as the grounded-emitter circuit. This is the most desirable
circuit for many applications. Input impedance is higher and
output impedance much lower than for the grounded-base arrangement,
typical values being 250 to 1500 ohms for the input impedance,
and 250,000 ohms to 1.5 megohms for the output impedance. Maximum
available gain is over 50 db. In the practical circuit of Fig.
2B, the base will float at a certain potential. If a different
potential is desired, various arrangements for biasing may be
used. A two-stage amplifier utilizing this circuit and having
a power gain of about 90 db is shown in Fig. 4. This amplifier
is pictured at the right of Fig. 5.
The power gain available from the grounded-collector stage,
shown in Fig. 2C, is rather low, about 15 to 20 db, but the
circuit has certain other advantages which make it desirable.
Input impedance is high although varying widely with the load
resistance. Output impedance likewise depends upon input loading,
but in general is quite low. Thus, this circuit arrangement
takes on the characteristics of a cathode follower, and if the
source impedance is on the order of a few thousand ohms, the
output impedance may be 25 ohms or less. Audio enthusiasts would
undoubtedly like to use this device to drive a voice coil direct,
without using an output transformer!
Fig. 3 - Symbol which has been adopted to
designate the junction transistor.
The junction transistor is especially suited for use at very
low power levels. Its efficiency is exceptionally high, closely
approaching the theoretical maximum of 50% for Class A and 100%
for Class C amplification. Thus, the input battery power required
can be extremely small. For example, an audio oscillator (shown
at left, Fig. 5) has been built which operates satisfactorily
with an input power of only 0.08 microwatt, consisting of 50
millivolts at 1.5 microamperes! To demonstrate the small power
required, Mr. R. L. Wallace, of Bell Laboratories, powered this
oscillator with the output of a photovoltaic cell exposed to
room illumination. In another experiment, he formed a battery
by wrapping a dime in a piece of paper which he had previously
moistened by chewing on it. The moist paper became one electrode
of the battery, and the dime the other. In both cases, the output
was sufficient to be audible from an ordinary headphone.
Mr. Wallace also performed some interesting calculations
to show that this device could really be called a "flea-power"
device. Assuming a dog flea weighing one milligram and jumping
to a height of 50 centimeters, both of which are reasonable
figures, he calculated that the flea, in making one such jump
every minute, would use approximately the same amount of energy
as the minimum required to keep the oscillator functioning!
It is logical to assume, therefore, that satisfactory amplifiers
can be built which would operate on comparably small amounts
of input power.
It should not be supposed from the previous discussion that
this transistor is a cure-all for everything. There are disadvantages
which mayor may not be overcome in future development work.
One is the problem of frequency response. Maximum gain of the
junction transistor is attained at frequencies on the order
of a few kilocycles, with the gain falling off rapidly as the
frequency increases. (Usable gains up to one megacycle and more
have been realized, however.) One factor affecting frequency
response is the transit time of the electrons and "holes" through
the "P" layer. Making this layer thinner should increase the
frequency response, but it would also increase the capacity
between the two "N" electrodes. A thickness of about a thousandth
of an inch has been used for this "P" layer.
Fig. 4 - A grounded-emitter type circuit
used as a two-stage amplifier which has a power gain of approximately
Another problem is that of the mass production of these items.
The amount of impurities in the germanium must be controlled
very carefully, as variations of as little as one part in one
hundred million can change the properties of the transistor.
Also, there is the mechanical problem of fastening leads to
such small pieces of germanium. How this is done on present
samples has not been revealed. The units must be made to have
uniform characteristics in production, so that a transistor
in a piece of equipment may be replaced by another transistor
without re-engineering the whole circuit. Samples which have
been tested have been found to be very temperature-sensitive,
although operation is completely satisfactory over normal temperature
These problems are gradually being overcome and it is expected
that production samples on a limited basis will be available
not too many months hence.
Here is a device which most certainly will give the vacuum
tube a run for its money - if not completely replace it - in
the majority of applications. It has practically no heat loss,
making. it useful in applications where very large numbers are
required, such as electronic computers of various kinds. It
is extremely rugged, reliable, and has an almost indefinite
life. A typical value of transconductance is 33,000 micromhos
per milliampere, and of amplification factor, 39,000. Since
the output impedance of a cathode follower is 1/Gm,
an output impedance of 5 ohms is possible with a current of
5 ma. The Gm is inversely proportional to the absolute
temperature, so that fantastically large values can be achieved
by supercooling. Its noise figure is better than some vacuum
tubes, being as low as 10 db and less on some of the units which
have been tested.
Fig. 5 - (Left) Audio oscillator which operates
with an input power of 0.08 micro watt and (Right) a two-stage
audio amplifier which was built using the circuit of Fig. 4.
These characteristics are certainly sufficient to justify
the excitement and anticipation brought about by the announcement
that such a device had been developed. Lest anyone be misled,
it would be well to emphasize that this device is not commercially
available at the present time, but commercial production is
definitely planned for the near future.
Newsreel from 1956, showing William Shockley,
Walter Brattain and John Bardeen receiving the Nobel Prize for