The Quest for the Crystal That Amplifies
June 1968 Popular Electronics
I never really thought about the transistor as a crystal amplifier,
but in fact it was fabricated from a crystal of germanium with two 'cat
whisker' wires pressing on its surface. Before the transistor was the
simple rectifier made of a crystal of selenium or even carborundum with
a point contact. Those were used for turning AC into DC and for detecting
radio wave modulation envelopes. For a really good synopsis of the early
development path of semiconductors, read this story from a 1968 edition
of Popular Electronics that commemorated the 20th anniversary of Bardeen,
Shockley, and Brattain announcing their transistor. Having been written
much closer to the days of discovery, the story has not been filtered
through as many writers' points of view, and contains some information
I'll bet you have never read before. E.g., did you know that semiconductor
dopants were originally referred to as adulterants? Did you know that
Shockley's early research was on field effect devices and, if successful,
would had made FETs the first forms of transistors rather than bipolar
junction types? Did you know that the name 'transistor' (trans-resistor)
was coined after the vacuum tube's amplification action called 'transconductance?'
Read on for more.
June 1968 Popular Electronics
[Table of Contents]People old and young enjoy waxing
nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October
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The Quest for the Crystal That Amplifies
We call it a transistor - but 20 years ago it was and ugly duckling.
by Daniel M. Costigan
The birth of the transistor was not something that happened overnight.
It marked the culmination of many years of dreaming and searching, not
only by scientists, but by a couple of generations of quixotic tinkerers
as well, seeking to extract from a tiny chunk of the mineral galena
some magical energy that might eliminate the need for power-consuming
vacuum tubes in radio receivers. Some of these experimenters actually
produced crystal sets that could operate loudspeakers-at low volumes
- perpetually and without need for external power.
Sir Oliver Lodge (1851-1940) made many important contributions
to wireless transmission and reception, which led to UHF transmission
of the galena mystique thrived for some thirty years on trial-and-error
and wishful thinking. During this time, they were joined by a handful
of scientists who worked unnoticed behind the scenes, taking a more
methodical approach to essentially the same goal. The scientists were
mostly engineers, metallurgists, and physicists who had been recruited
by industry - some of them from university faculties - to help seek
new ways to improve the efficiency of electric power and communications
Finally, in the late 1940's, the quest for the crystal
that amplifies ended in triumph for a trio of distinguished industrial
scientists. The year 1968 marks the 20th anniversary of what has since
become recognized as one of the most momentous events in the annals
of electrical science.
How The Quest Began.
The primordial spark to which this quest can be traced occurred
around the turn of the century when the need for a practical rectifying
device arose almost simultaneously in two budding young industries:
electric power, and radio.
In electric power, the advocates
of alternating current had won their battle against the d.c. interests
and had begun to distribute a.c. power on a wide scale. This meant that
some kind of practical converter, other than a motor-generator, would
be needed to permit the operation of d.c. apparatus - battery chargers,
electroplating equipment, telephonic devices, etc. - on commercial a.c.
The rectifying properties of selenium had been known
for nearly a century, but it wasn't until 1924 that semiconductor rectifiers
became commercially available. By the early thirties, copper-oxide rectifiers
had come into wide use as converters. Selenium, which had at first proved
unsatisfactory for use in power conversion, was gradually improved and
eventually surpassed copper-oxide in popularity.
The names Mott
and Schottky are two that stand out in connection with the early evolution
of rectifier theory. Working independently of one another - Sir Nevill
Mott in England and W. Schottky in Germany - both men concluded that
rectification took place in a thin electrical barrier that formed at
the junction of a metal contact and a semiconductor. Schottky called
this surface barrier an "inversion region" within the thickness of which
a change of conductivity took place. The theory was to play a prominent
role in the reasoning that later led to the invention of the transistor.
Radio's need was for
a practical rectifying detector of received signals, and it arose with
the advent of voice transmission. In its embryonic stage (1894 to about
1906), a radio receiver had at its heart a "coherer," in which metal
filings clung together on exposure to electromagnetic disturbances,
thereby varying the current in a local battery circuit. The disturbances
were set up by a spark transmitter being turned on and off with a telegraph
First used by England's Sir Oliver Lodge in 1894, the coherer*
had been steadily improved in design and had reached a fairly high level
of refinement by 1900 when Professor Reginald Fessenden of the University
of Pittsburgh succeeded in transmitting voice on a continuous wave.
Radio suddenly found itself faced with much the same situation
that had created the need for rectifiers in the electrical industry.
The transmitted radio wave became a "carrier," its cargo a modulation
envelope that was electrically self-cancelling until the alternating
current could be made to flow in one direction only. Radio detection
thus became a matter of rectification.
Fessenden's first detector was an electrolytic device of his own design.
It was highly sensitive, but also critical and unreliable.
Professor Reginald Aubrey Fessenden (1866-1932). a pioneer in
wireless communication, was the first man to voice-modulate
a continuous-wave carrier.
first practical crystal detectors appeared in 1906. The one invented
by G. W. Pickard used silicon and featured a "catwhisker" (fine wire)
contacting arrangement similar to that suggested by a German experimenter
named Braun some 30 years earlier. Another type, invented by H. H. Dunwoody,
an executive of the De Forest Wireless Company, used a small chunk of
carborundum clamped between two electrodes.
The crystal detector reigned supreme until
the early twenties when the vacuum tube began to make inroads. Silicon
had proved to be the most stable crystal, but galena (lead sulphide)
was the most sensitive and therefore the most popular.
everything, however, it takes power to beget power; thus, there were
the inevitable "A" and "B" supplies wherever vacuum tubes were used.
What's more, some of the power had to be wasted in heating the tube
filament - an unfortunate requisite that was to impose serious restrictions
on the tube's useful life and on the design of the tube-using equipment.
Crystal detectors needed no external power supply; they were
simple and compact, and there was no heat problem. But they couldn't
amplify - at least not until a group of scientists at a major industrial
research laboratory had undertaken an intensive investigation into the
mysteries of solid-state semiconductors.
In 1934, the Bell Telephone
Laboratories began to develop fixed semiconductor diodes for use in
microwave experiments. The earlier ones were silicon and germanium point-contact
devices resembling the "fixed" detectors that had been used in some
crystal broadcast receivers. ("Point-contact" is a sophisticated term
for the old familiar catwhisker.) The more advanced junction diodes
were developed during and immediately following World War II.
A New Turn.
Walter H. Brattain had come
to Bell Labs shortly after having received his Ph.D from the University
of Minnesota. His involvement during the thirties in the study of electrical
conductivity in semiconducting materials eventually brought him in contact
with William Shockley, a brilliant young Ph.D who joined the staff in
1936 and soon began to form some ideas of his own on the potentials
In that same year (1936), Dr. Mervyn Kelly
was appointed Director of Research at Bell Labs, and one of his first
acts was to assemble a team of physicists to formally explore the behavior
of electrons in solids. Brattain and Shockley were among those selected.
Since the late thirties, Shockley had been entertaining the
notion that a semiconductor ought somehow to be able to amplify an electric
current. His attempts to achieve "valve action" in a copper-oxide device
were interrupted by World War II. Immediately following the war, he
constructed a special device based on a scheme he had worked out on
paper. But, as is so often the case, what appeared workable on paper
did not work in actuality.
Ironically, the device that had failed
was the forerunner of the field-effect transistor (FET), which was to
re-emerge many years later to be heralded as one of the more important
advances in solid-state technology. Had Shockley's experiment been successful
- if more had been known about the characteristics of semiconductor
materials - the development of solid-state devices might have taken
a completely different course.
It was about the time of Shockley's
"field-effect" experiment that the Bell Labs team was enhanced by the
addition of a new member. He was John Bardeen, a 37-year-old theoretical
physicist and former university professor whom Shockley had personally
recruited. During the preceding decade, Bardeen had done extensive work
in the field of electroconductivity in solids. The fact that Shockley's
experiment had not yielded the expected result interested Bardeen and
set him working on a theory to explain why.
Taking his cue from Mott and Schottky, Bardeen theorized that
surplus electrons gathered at the surface of a semiconductor and became
immobilized so that, in effect, they acted as a sort of barrier to externally
applied currents. To test his "surface states" theory, he and Brattain
performed a series of interesting experiments.
At first they
used a liquid electrolytic as a current-carrying medium between one
side of a power source and the surface of a piece of semiconducting
silicon. They found that by passing a current through the electrolyte,
the surface charge on the silicon could be altered.
Brattain to suggest a slightly modified approach. Germanium was substituted
for silicon, and a thin layer of gold for the electrolyte as the special
contacting interface. Two currents were made to flow in opposite directions
through the germanium, one between the gold contact and a solid connection
at the base of the material, and the other between the base connection
and a cat-whisker contacting the surface - near the gold contact. As
had by now been anticipated, varying the one current produced corresponding
variations, but of greater magnitude, in the other. Amplification had
The technical explanation for the phenomenon was highly complex and
dealt with such things as atomic valences, "holes," "donors," and "defect
conductivity." Stated simply, what had happened was that the tiniest
plus charge at one of the two contact points on the semiconductor surface
had drawn off enough of the material's surplus electrons to create "holes,"
which, in turn, were attracted to the adjacent negative point and therefore
functioned as vehicles by which the lesser current could influence the
greater one. In essence, then, the semiconductor had become a variable
resistance, enabling control of current flow in one circuit by varying
the current in another.
William Shockley, John Bardeen, and Walter H. Brattain, co-discoverers
of the transistor, received the 1956 Nobel Physics Award for
By the close of 1947, experiments had
proved the new device capable of amplifying audio frequency signals.
Bardeen and Brattain quietly announced their discovery via a letter
to the editor of "The Physical Review," published in the July 15, 1948,
The First Transistors.
transistor, by which the device was to become known, was suggested by
another Bell Labs physicist, John R. Pierce. Pierce observed that, where
a vacuum tube amplified by transconductance - the effect of the grid
voltage on plate current - the new device did its amplifying by what
might more aptly be termed "trans-resistance." The name may also be
thought of as suggesting the transfer of signals through a varistor,
a varistor being a semiconductor diode whose electrical resistance decreases
substantially with a moderate increase in applied voltage. (Varistors
are often used as buffers to protect delicate components from voltage
The first transistors produced in quantity in the laboratory
contained a tiny chunk of slightly impure germanium on which two "catwhiskers"
converged, contacting the surface at points less than a hair's breadth
William Shockley, meanwhile, continued to pursue his
own ideas on how best to make a semiconductor amplify. His quest led
to the invention, later in 1948, of the junction transistor in which
transistor action was achieved by the sandwiching together of p-type
(electron deficiency) and n-type (electron surplus) semiconductors.
Shockley's design, although at first more difficult to fabricate, proved
more predictable in its properties and less fragile than its "point
contact" predecessor, and was therefore soon to supersede it. The junction
transistor was introduced early in 1951.
A major obstacle to mass production was
the requirement that semiconductor materials contain carefully controlled
degrees of impurities to insure the proper electrical imbalance. This
meant starting with a nearly pure substance and then adding adulterants
(such as arsenic or gallium) by a carefully controlled doping process.
The introduction of zone refining in 1955 was the first big breakthrough
in high volume production of the basic materials.
of the magnitude of the revolution they had kindled, the transistor's
trio of inventors - Shockley, Brattain, and Bardeen - were awarded the
1956 Nobel Prize in Physics.
But, during the middle fifties,
the widely acclaimed little device still suffered a number of serious
shortcomings. For one thing, it was critically temperature-sensitive
and therefore unable to handle power beyond a fraction of a watt. What's
more, it was noisy and unstable, had ridiculously low input impedance,
and was a sluggish device whose switching speed and frequency response
left much to be desired.
These shortcomings, however, were gradually
overcome as new manufacturing processes were introduced and semiconductor
materials with improved electrical characteristics were developed. By
the late fifties, the reliability of the device had already begun to
exceed that of vacuum tubes.
The year 1957 was the best ever
for the sale of vacuum tubes. Unit for unit, they were still outselling
transistors by thirteen to one. But the gap was rapidly narrowing, with
the turning point due in the early sixties. (In 1959, 77.5 million germanium
transistors were sold for a total of 151.8 million dollars, at an average
price of $1.96 per unit. By 1966, the picture was considerably changed:
368.7 million units were sold for a total of 164.5 million dollars,
an average price of 45 cents each!)
Further Improvements. New
developments followed in rapid succession, and with them came a whole
new electronics vocabulary: p-type, n-type, bipolar, diffused junction,
epitaxial growth. The grown junction gave way to the alloy junction,
which, along with the introduction of the diffusion process, resulted
in improved frequency response and switching speeds. It was now feasible
to use transistors in computers - a marriage which, in turn, was to
enhance the evolution of still faster and more reliable semiconductor
The diffusion process also broke the transistor's power-handling
and temperature barriers by facilitating the use of silicon in place
of germanium. Mesa, planar, and epitaxial devices emerged as some of
the more prominent offshoots of the diffusion technique.
field-effect transistor, which had continued to lie dormant in the laboratories,
seemed to hold the key to some of the improvements still needed - higher
input impedance, for example, and lower levels of noise and distortion.
Of all semiconductor devices, it came closest to a vacuum tube in characteristics.
But the electrical surface properties of the semiconductor material
used in its fabrication were critical, and it was not until recently
that FET's finally became competitive with other semiconductors.
Similarly, the unijunction transistor, with its single p-n junction,
was originally developed in the early fifties, but is just now beginning
to emerge as one of the less expensive, more stable, and temperature-resistant
The late fifties saw the introduction of miniature
circuit modules, silicon-controlled rectifiers (SCR's), and Esaki's
remarkable tunnel diode, with which amplification was possible without
the traditional "third element." SCR's shrunk the gap between tube and
semiconductor capabilities by providing a highly efficient solid-state
replacement for thryatrons and mercury-arc rectifier tubes in power
What Does The Future Hold?
The pace of development of new transistor types has been absolutely
staggering. The list now numbers in the thousands, and it continues
to grow as the mighty midget celebrates its 20th birthday.
from its having reshaped an entire industry and opened many new doors,
perhaps the most fascinating offshoot of the whole solid-state technology
to date has been the subordinate art of micro-electronics. Already in
the making are integrated circuits so minute that an entire amplifier
could hide behind a single transistor.
has been growing and changing at such a dizzying pace that it is difficult
to predict what lies ahead even in the next year or so. Perhaps at this
very moment, somewhere in the world, a small but persistent group of
experimenters is exploring a radically new concept that may someday
render the present technology obsolete.
*For more details, see article
entitled "The 'Coherer' " which appeared in the May, 1961, issue of