Transistor Story: Groping in the Dark

The road to the solid-state switches was a long and difficult one. It began with the discovery that certain materials behave strangely in the presence of electricity - not the way the theories that existed at that time had predicted. This was followed by a story about how in the 20th century technology became more and more scientific and institutional discipline. Amateurs, novices and professional inventors, without any scientific education, made serious contributions to the development of telegraph, telephony and radio. But, as we will see, almost all advances in the history of solid-state electronics occurred thanks to scientists who studied at universities (and usually have a Ph.D. in physics) and worked at universities or corporate research laboratories.

Anyone with workshop access and basic materials skills can assemble relays from wires, metal, and wood. The creation of electron tubes requires more specialized tools that can create a glass flask and pump air out of it. The solid-state devices disappeared into the rabbit hole, from which the digital switch never returned, and plunged deeper into worlds that only abstract math understood and accessible only with the help of insanely expensive equipment.

Galena

In 1874, Ferdinand Brown , 24-year-old physicist from the School of St.. Thomas in Leipzig, published the first of many important scientific papers in his long career. The work “On the passage of electric currents through the sulphides of metals” was accepted in the journal Pogendorff's Annalen, a prestigious journal dedicated to the physical sciences. Despite the boring headline, Brown's work described several surprising and mysterious experimental results.
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Ferdinand Brown

Brown was intrigued by sulphides - mineral crystals consisting of sulfur compounds with metals - thanks to the work of Johann Wilhelm Hitthorf . Back in 1833, Michael Faraday noted that the conductivity of silver sulfide increases with temperature, which is completely opposite to the behavior of metal conductors. Hittorf compiled a thorough quantitative report on the measurements of this effect in the 1850s, for both silver and copper sulfides. Now, using a clever experimental setup, Braun pressed the metal wire to the sulfide crystal with a spring to ensure good contact, discovered something even more strange. The conductivity of the crystals depended on the direction — for example, the current could flow well in one direction, but when the polarity of the battery was reversed, the current could suddenly drop sharply. Crystals in one direction worked more like conductors (like normal metals), and in the other, more like insulators (like glass or rubber). This property became known as straightening due to the ability to straighten the "tortuous" alternating current, turning it into a "flat" constant.

At about the same time, researchers discovered other strange properties of materials such as, for example, selenium, which could be smelted from certain sulfide ores of metals. Under the influence of light, selenium increased conductivity and even began to generate electricity, and it could also be used for straightening. Was there any connection with sulphide crystals? Without theoretical models capable of explaining what was happening, confusion reigned in this area.

However, the lack of theory did not stop the attempts of practical application of the results. In the late 1890s, Brown became a professor at the University of Strasbourg, recently annexed by France during the Franco-Prussian War and renamed Kaiser Wilhelm University. There, he was sucked into an exciting new world of radio telegraphy. He agreed to the proposal of a group of businessmen to jointly create a wireless communication system based on the transmission of radio waves through water. However, they and their accomplices soon abandoned the original idea in favor of the aerial signaling that Marconi and others used.

Among the aspects of the radio that Brown’s group sought to improve was the standard receiver at the time, the coherer . It was based on the fact that radio waves forced metal filings to gather into a ball, which allowed current from the battery to pass to the signaling device. It worked, but the system responded only to relatively strong signals, and for breaking a bundle of sawdust it was necessary to constantly hit the device. Brown remembered his old experiments with sulfide crystals, and in 1899 he recreated his old experimental setup with a new goal - to serve as a detector of wireless signals. He used the rectification effect to convert a tiny oscillating current generated by passing radio waves into a direct current that could be powered by a small speaker that produced audible clicks for each dot or dash. This device later became known as the " cat's whisker detector " because of the appearance of the wire, which easily touched the top of the crystal. In British India (where Bangladesh is today), the scientist and inventor Jagadish Bose built a similar device, perhaps even in 1894. The rest soon began to make similar detectors based on silicon and carborundum (silicon carbide).

However, it was galena , lead sulfide, which was smelted to obtain lead from ancient times, became the preferred material for crystalline detectors. They were easy to manufacture and cheap, and as a result they became extremely popular among the early generation of radio amateurs. Moreover, in contrast to the binary coherer (with sawdust, which either hid in a lump or not), the crystal rectifier could reproduce a continuous signal. Therefore, he could produce audible ear voice and music, and not just Morse code with its points and dashes.


Detector "cat mustache" based on galena. A small piece of wire on the left is a mustache, and a piece of silver material below is a galena crystal.

However, as soon as disgusted radio amateurs were established, it could take minutes or even hours to search for a magic point on the surface of the crystal that would give a good straightening. And the signals without amplification were weak and had a metallic overtone. By the 1920s, electronic-tube receivers with triode amplifiers virtually deactivated crystal detectors almost everywhere. Their attractive feature was only cheap.

This brief appearance in the arena of radio receivers seemed to be the limit of practical application of the strange electrical properties of the material discovered by Brown and others.

Copper oxide

Then, in the 1920s, another physicist named Lars Grondal discovered something strange with his experimental setup. Grondal, the first of a chain of intelligent and restless husbands from the history of the American West, was the son of a civil engineer. His father, who emigrated from Norway in 1880, worked for several decades on railways in California, Oregon and Washington. At first, Grondal seemed to decide to leave his father’s engineering world behind, and went to the Johns Hopkins Institute to receive a doctoral degree in physics in order to take an academic path. But then he got involved in the railway business and settled on the position of research director at Union Switch and Signal, a division of industrial giant Westinghouse , which supplied equipment for the railway industry.

Various sources indicate the controversial reasons that motivated Grondal for his research, but whatever it was, he began experimenting with copper discs heated on one side to create an oxidized layer. Working with them, he paid attention to the asymmetry of the current - the resistance in one direction was three times greater than in the other. A disc made of copper and copper oxide straightened the current, just like a sulfide crystal.


Copper oxide rectifier circuit

For the next six years, Grondal developed a commercial rectifier, ready for use, based on this phenomenon, enlisting the help of another US researcher, Paul Geiger, and then sent a patent application and announced his discovery in the American Physical Society in 1926. The device immediately became a commercial hit. Due to the absence of brittle filaments, it was much more reliable than a rectifier on electron tubes, based on the valve principle of Fleming, and was cheap to manufacture. In contrast to Brownian rectifier crystals, it worked on the first attempt, and due to the larger contact area of ​​metal and oxide, it worked with a large range of currents and voltages. He could charge batteries, detect signals in various electrical systems, work as a safety shunt in powerful generators. When used as a photocell, the disks could work as light meters, and were particularly useful in photography. Other researchers at about the same time developed rectifiers from selenium, which found similar applications.


Copper oxide based pack. The assembly of several disks increased the reverse resistance, which made it possible to use them with high voltage.

A few years later, two physicists from Bell Laboratories, Joseph Becker and Walter Brattein , decided to study the principle of operation of a copper rectifier - they were interested in finding out how it works and how it can be used in the Bell System company.


Bratteyn in old age - approx. 1950

Brattein came from the same places as Grondal, from the Pacific Northwest, where he grew up on a farm located a few kilometers from the Canadian border. In high school, he became interested in physics, he showed abilities in this area, and eventually received his doctorate at the University of Minnesota in the late 1920s, and got a job at Bell Lab in 1929. Among other things, he studied the latest theoretical physics at the university which was gaining popularity in Europe, and was known as quantum mechanics (its curator was John Hazbruck Van Vleck , who also instructed John Atanasoff).

Quantum revolution

The new theoretical platform has been slowly evolving over the past three decades, and at one time it will be able to explain all the strange phenomena that have been observed in such materials as galena, selenium and copper oxide for many years. A whole cohort of predominantly young physicists, mainly from Germany and neighboring countries, caused a quantum revolution in physics. Everywhere, wherever you look, they found not a smooth and uninterrupted world, which they were taught, but strange discrete lumps.

It all started in the 1890s. Max Planck, a well-known professor at the University of Berlin, decided to work with a well-known unsolved problem: how does an “ absolutely black body ” (an ideal substance absorbing all energy and not reflecting it) emit radiation in the electromagnetic spectrum? Different models were tried, none of which did not coincide with the experimental results — they could not cope either at one or the other end of the spectrum. Planck discovered that if we assume that energy is emitted by the body in small “batches” of discrete magnitude, then we can write a simple law of the relationship between frequency and energy, ideally coinciding with empirical results.

Soon after, Einstein discovered that the same is done with light absorption (the first hint of photons), and JJ Thomson showed that electricity is also not transferred by a continuous fluid or wave, but by discrete particles — electrons. Then Niels Bohr created a model that explained how excited atoms emit radiation, assigning to the electrons separate orbits in the atom, each of which has its own energy. However, this name is misleading, since they behave in no way similar to the orbits of the planets - in the Bohr model, electrons instantly passed from one orbit, or energy level, to another, without passing through an intermediate state. And finally, in the 1920s, Erwin Schrödinger, Werner Heisenberg, Max Born, and others created a generalized mathematical platform, known as quantum mechanics, which included all of the special quantum models created in the previous twenty years.

By this time, physicists were already convinced that materials such as selenium and galena, which exhibit photovoltaic and rectifying properties, belong to a separate class of materials, which they called semiconductors. The classification took so much time for several reasons. First of all, the categories “conductors” and “insulators” themselves were quite extensive. T.N. “Conductors” were extremely different in conductivity, the same (to a lesser extent) was also characteristic of insulators, and it was not clear how any particular conductor could be attributed to any of these classes. Moreover, until the middle of the 20th century it was impossible to obtain or create very pure substances, and any oddities in the conductivity of natural materials could always be attributed to pollution.

Now physicists have appeared both mathematical tools of quantum mechanics and a new class of materials to which they could be applied. British theorist Alan Wilson was the first to put it all together and built a general model of semiconductors and the principle of their work in 1931.

At first, Wilson argued that conductive materials differ from dielectrics by the state of their energy bands. Quantum mechanics argues that electrons can exist at a limited number of energy levels inherent in the shells, or the orbitals of individual atoms. If we squeeze these atoms together in the structure of a material, then it would be more correct to imagine continuous energy zones passing through it. There are free places in the conductors in the high energy zones, and the electric field can freely move electrons there. In insulators, zones are filled, and to a higher, conducting zone, on which electricity is easier to go, climb quite far.

This led him to the conclusion that impurities — alien atoms in the structure of a material — should contribute to its semiconductor properties. They can either supply extra electrons that easily go into the conduction band, or holes - the absence of electrons compared to the rest of the material - which creates empty energy spaces where free electrons can move. The first option was later called n-type semiconductors (or electronic) for an excessive negative charge, and the latter for p-type, or hole ones for an excessive positive charge.

Finally, Wilson suggested that the current rectification by semiconductors can be explained in terms of the quantum tunnel effect , the sudden jump of electrons through a thin electrical barrier in the material. The theory looked plausible, but predicted that the current in the rectifier should flow from oxide to copper, although in reality it was the opposite.

So, despite all the Wilson breakthroughs, semiconductors remained difficult to explain. As it gradually became clear, microscopic changes in the crystal structure and concentration of impurities disproportionately strongly influenced their macroscopic electrical behavior. Ignoring the lack of understanding - since no one could explain the experimental observations made by Brown 60 years earlier - Brattein and Becker developed an efficient production process of copper-oxide rectifiers for their employer. The Bell System quickly began to replace rectifiers on electronic tubes throughout the system with a new device, which their engineers called a varistor , as its resistance changed depending on the direction.

Golden medal

Mervyn Kelly, a physicist and former head of the Bella Labs Laboratories Department, was very interested in this achievement. For a couple of decades, electron tubes have served Bella with an invaluable service, and could perform functions inaccessible to the previous generation of mechanical and electromechanical components. But they were very hot, regularly overheated, consumed a lot of energy and were difficult to maintain. Kelly was going to rebuild the Bell system based on more reliable and durable solid-state electronic components, such as a varistor, which did not require either hermetic housings, filled with gas or empty, or red-hot filaments. In 1936, he became the head of the research department at Bell Laboratories, and began to redirect the organization to a new path.

Having obtained a solid-state rectifier, the next obvious step was to create a solid-state amplifier. Naturally, like a tube amplifier, such a device could work as a digital switch. Bell was particularly interested in this, since a huge number of electromechanical digital switches still operated in telephone switches. The company was looking for a more reliable, compact, energy-efficient and cold-fitting electronic lamp in telephone systems, radio receivers, radars and other analog equipment, where they were used to amplify weak signals to the level accessible to the human ear.

In 1936, Bell Labs finally lifted the prohibition on hiring personnel during the Great Depression . Kelly immediately began hiring experts in quantum mechanics to help launch his research program in the field of solid-state devices, among which was William Shockley , another native of the West Coast, from Palo Alto (California). The topic of his dissertation, which was recently drawn up at MIT, perfectly suited Kelly’s needs: “Electronic zones in sodium chloride”.

Brattein and Becker continued their research on a copper oxide rectifier at this time, seeking to obtain an improved solid state amplifier. The most obvious way to make it was to go by analogy with an electronic lamp. Just as Lee de Forest took a tube amplifier and placed an electrical grid between the cathode and the anode, so Brattein and Becker presented how to insert the grid into the contact between copper and copper oxide, where it was supposed to be straightened. However, due to the small thickness of the layer, they found it impossible to do this, and did not succeed in this.

Meanwhile, other developments have shown that Bell Labs was not the only company interested in solid state electronics. In 1938, Rudolf Hilsch and Robert Paul published the results of experiments conducted at the University of Göttingen on a solid-state amplifier, created through the introduction of a grid into a potassium bromide crystal. This laboratory device was of no practical value - mainly because it operated at a frequency of not more than 1 Hz. And yet this achievement could not please everyone interested in solid-state electronics. In the same year, Kelly identified Shockley as a new independent research team on solid-state devices and gave him and his colleagues, Foster Nix and Dean Woolridge, a blank check to study their capabilities.

At least two more inventors were able to create solid state amplifiers before World War II. In 1922, the Soviet physicist and inventor Oleg Vladimirovich Losev published the results of successful experiments with zinc-based semiconductors, but his work went unnoticed by the Western community; In 1926, American inventor Julius Lillenfield filed a patent for a solid state amplifier, but there is no evidence of the performance of his invention.

Shockley's first major insight in the new position happened while reading the work of the British physicist Neville Mota "Theory of Crystal Rectifiers" from 1938, where, finally, the principle of the Grondal rectifier on copper oxide was explained. Mott used the mathematics of quantum mechanics to describe the formation of an electric field at the junction of a conductive metal and a semiconducting oxide, and how electrons “jump over” this electric barrier instead of tunneling as Wilson suggested. The current flows more easily from metal to semiconductor than vice versa, since the metal has much more free electrons.

This led Shockley to exactly the same idea that Brattein and Becker had considered and rejected many years before — making a solid-state amplifier by inserting a grid of copper oxide into the gap between copper and copper oxide. He hoped that the current flowing through the grid would increase the barrier that limits the current going from copper to oxide, creating an inverted, amplified version of the signal on the grid. His first rough attempt failed completely, so he turned to a man who had more refined laboratory skills and was well acquainted with rectifiers - Walter Bratteyn. And, although he had no doubts about the outcome, Brattein agreed to satisfy Shockley's curiosity, and created a more complex version of the “grid” amplifier. She also refused to work.

Then the war intervened, leaving Kelly's new research program in disarray. Kelly became the head of the Radar Working Group at Bell Laboratories, which was supported by the main US radar research center at MIT. Brattein did not work for a long time with him, and then turned to research on the magnetic detection of submarines commissioned by the navy. Woolridge worked on fire control systems, the Knicks worked on the diffusion of gases for the Manhattan project, and Shockley went into operational research and first dealt with the fight against submarines in the Atlantic, and then with strategic bombings in the Pacific Ocean.

But, despite this intervention, the war did not stop the development of solid-state electronics. On the contrary, she organized a massive infusion of resources into this area, and led to a concentration of research on two materials: germanium and silicon.

What else to read

Ernest Bruan and Stuart MacDonald, Revolution in Miniature (1978)

Friedrich Kurylo and Charles Susskind, Ferdinand Braun (1981)

GL Pearson and WH Brattain, “History of Semiconductor Research,” Proceedings of the IRE (December 1955).

Michael Riordan and Lillian Hoddeson, Crystal Fire (1997)