Other articles in the series:
- History of the relay
- History of electronic computers
- History of the transistor
- Internet history
The road to solid state switches has been a long and difficult one. It began with the discovery that certain materials behave strangely in the presence of electricity, not in the way the theories then predicted. What followed was a story of how technology became an increasingly scientific and institutional discipline in the XNUMXth century. Amateurs, novices and professional inventors, with virtually no scientific education, made serious contributions to the development of the telegraph, telephony and radio. But, as we shall see, almost all the advances in the history of solid-state electronics have been due to scientists who studied at universities (and usually have a Ph.D. in physics) and worked at universities or corporate research laboratories.
Anyone with access to a workshop and basic material handling skills can assemble relays from wire, metal, and wood. Creating vacuum tubes requires more specialized tools that can create a glass bulb and pump air out of it. Solid-state devices disappeared down a rabbit hole from which the digital switch never returned, and plunged deeper and deeper into worlds understood only by abstract mathematics and accessible only with the help of insanely expensive equipment.
Galena
In 1874 year , a 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 paper "On the Passage of Electric Currents through Metal Sulfides" was accepted by Pogendorff's Annalen, a prestigious journal dedicated to the physical sciences. Despite the boring title, Brown's paper described several surprising and puzzling experimental results.
Ferdinand Brown
Brown became intrigued by sulfides—mineral crystals made up of compounds of sulfur with metals—thanks to the work . As early as 1833, Michael Faraday noted that the conductivity of silver sulfide increases with temperature, which is completely opposite to the behavior of metallic conductors. Gittorff compiled a careful quantitative report of measurements of this effect in the 1850s, for both silver and copper sulfides. Now Brown, using an ingenious experimental setup that pressed a metal wire against a sulfide crystal with a spring to ensure good contact, discovered something even stranger. 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. The crystals worked more like conductors in one direction (like normal metals) and more like insulators in the other (like glass or rubber). This property became known as rectification, due to the ability to straighten "tortuous" alternating current, turning it into a "flat" direct current.
Around the same time, researchers also discovered other strange properties of materials such as selenium, which could be smelted from certain metal sulfide ores. When exposed to light, selenium increased conductivity and even began to generate electricity, and it could also be used for rectification. Was there any connection to the sulfide crystals? Without theoretical models capable of explaining what was happening, confusion reigned in this area.
However, the lack of theory did not stop attempts to apply the results in practice. In the late 1890s, Brown became a professor at the University of Strasbourg - recently annexed from France in the course of and renamed Kaiser Wilhelm University. There he was sucked into the exciting new world of radiotelegraphy. He agreed to the proposal of a group of entrepreneurs to jointly create a wireless communication system based on the transmission of radio waves through water. However, he and his accomplices soon abandoned the original idea in favor of the aerial signaling used by Marconi and others.
Among the aspects of radio that Brown's group sought to improve was the then-standard receiver, . It relied on the fact that radio waves caused the metal filings to clump together, allowing current from the battery to flow to the alarm device. It worked, but the system only responded to relatively strong signals, and breaking the sawdust wad required constant tapping on the device. Brown remembered his old experiments with sulfide crystals, and in 1899 recreated his old experimental setup with the new purpose of serving as a wireless signal detector. He used the rectification effect to convert the tiny oscillating current generated by passing radio waves into a direct current that could power a small speaker that produced audible clicks for each dot or dash. This device later became known as "» due to 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, possibly as early as 1894. Others soon began making similar detectors based on silicon and carborundum (silicon carbide).
However, it is , lead sulfide, which has been smelted to produce lead since ancient times, has become the material of choice for crystal detectors. They were easy to make and cheap, and as a result became wildly popular with the early generation of radio amateurs. Moreover, unlike a binary coherer (with sawdust that either clumped together or not), a crystal rectifier could reproduce a continuous signal. Therefore, he was able to produce voice and music transmissions audible to the ear, and not just Morse code with its dots and dashes.
Cat's whisker detector based on galena. The small piece of wire on the left is the mustache, and the piece of silvery material below is the galena crystal.
However, as frustrated radio amateurs soon discovered, it could take minutes or even hours to find the magic point on the crystal surface that would give good rectification. And the signals without amplification were weak and had a metallic overtone. By the 1920s, vacuum tube receivers with triode amplifiers had virtually eliminated crystal detectors almost everywhere. Their attractive feature was only cheapness.
This brief advent of the radio receiver seemed to be the limit of the 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 men in the history of the American West, was the son of a civil engineer. His father, who emigrated from Norway in 1880, worked for decades on the railroads in California, Oregon, and Washington. At first, Grondal seemed determined to leave his father's engineering world behind, and went to Johns Hopkins for a doctorate in physics to pursue an academic path. But then he got involved in the railroad business and took a position as director of research for Union Switch and Signal, a division of the industrial giant. , which supplied equipment for the railway industry.
Various sources indicate conflicting reasons that motivated Grondal for his research, but be that as it may, he began to experiment with copper disks heated on one side to create an oxidized layer. Working with them, he drew attention to the asymmetry of the current - the resistance in one direction was three times greater than in the other. A disc of copper and copper oxide rectified the current, just like a sulfide crystal.
Copper oxide rectifier circuit
For the next six years, Grondal developed a ready-to-use commercial rectifier based on this phenomenon, enlisting the help of another US researcher, Paul Geiger, and then submitted 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 the vacuum tube rectifier based on the Fleming valve principle, and was cheap to manufacture. Unlike Brownian crystal rectifiers, it worked on the first try, and due to the larger metal-oxide contact area, it worked with a wide range of currents and voltages. It could charge batteries, detect signals in various electrical systems, work as a safety shunt in powerful generators. When used as a photoelectric cell, the disks could work as meters for the amount of light, and were especially useful in photography. Other researchers around the same time developed selenium rectifiers that found similar applications.
A pack of rectifiers based on copper oxide. An assembly of several disks increased the reverse resistance, which allowed them to be used with high voltage.
A few years later, two Bell Labs physicists, Joseph Becker and , decided to study the principle of operation of a copper rectifier - they were interested in learning how it works and how it can be used in the Bell System.
Brattain in old age - approx. 1950
Brattain was from the same area as Grondal, in the Pacific Northwest, where he grew up on a farm a few miles from the Canadian border. In high school, he became interested in physics, he showed ability in this area, and eventually received a doctorate from the University of Minnesota in the late 1920s, and took a job at Bell Laboratories in 1929. Among other things, at the university he studied the latest theoretical physics , which was gaining popularity in Europe, and known as quantum mechanics (its curator was , who also mentored John Atanasoff).
Quantum revolution
A new theoretical platform has been slowly developed over the past three decades, and in time it will be able to explain all the strange phenomena that have been observed for many years in materials such as galena, selenium and copper oxide. A whole cohort of predominantly young physicists, mostly from Germany and neighboring countries, caused a quantum revolution in physics. Everywhere they looked, they found not the smooth and continuous world that they had been taught, but strange discrete lumps.
It all started in the 1890s. Max Planck, the famous professor at the University of Berlin, decided to work with a well-known unsolved problem: how ""(an ideal substance that absorbs all energy and does not reflect it) emits radiation in the electromagnetic spectrum? Various models were tried, none of which matched the experimental results—they failed either at one or the other end of the spectrum. Planck discovered that if one assumes that energy is emitted by a body in small "packets" of a discrete value, then one can write down a simple law of the relationship between frequency and energy, which coincides perfectly with empirical results.
Shortly thereafter, Einstein discovered that the same was true for the absorption of light (the first hint of photons), and J. J. Thomson showed that electricity was also carried not by a continuous fluid or wave, but by discrete particles - electrons. Niels Bohr then created a model to explain how excited atoms emit radiation by assigning electrons to separate orbits in the atom, each with its own energy. However, this name is misleading, because they behave in no way like the orbits of the planets - in Bohr's model, electrons instantly jumped 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 the special quantum models that had been created over the previous twenty years.
By this time, physicists were already convinced that materials such as selenium and galena, which exhibit photovoltaic and rectifying properties, belonged to a separate class of materials, which they called semiconductors. The classification took so long for several reasons. Firstly, the categories "conductors" and "insulators" themselves were quite extensive. so-called. "conductors" varied greatly in conductivity, as did insulators (to a lesser extent), and it was not obvious how any particular conductor could be assigned to any of these classes. Moreover, until the middle of the XNUMXth 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.
Physicists now had both the mathematical tools of quantum mechanics and a new class of materials to which they could be applied. British theorist first put it all together and built a general model of semiconductors and how they work in 1931.
Wilson first argued that conductive materials differ from dielectrics in the state of their energy bands. Quantum mechanics states that electrons can exist in a limited number of energy levels inherent in the shells, or orbitals, of individual atoms. If you squeeze these atoms together in the structure of any material, then it would be more correct to imagine continuous energy bands passing through it. There are free places in conductors in high energy zones, and the electric field is free to move electrons there. In the insulators, the zones are filled, and the higher, conductive zone, through which it is easier for electricity to go, is quite a long way to climb.
This led him to conclude that impurities - foreign atoms in the structure of a material - must contribute to its semiconductor properties. They can either supply extra electrons that easily escape into the conduction band, or holes—the absence of electrons compared to the rest of the material—which creates empty energy spots where free electrons can move. The first option was later called n-type semiconductors (or electronic) - for an excessive negative charge, and the second - p-type, or hole - for an excessive positive charge.
Finally, Wilson proposed that the rectification of current by semiconductors could be explained in terms of quantum , the sudden jump of electrons through a thin electrical barrier in a material. The theory looked plausible, but predicted that in the rectifier the current should flow from the oxide to the copper, when in reality it was the other way around.
So, despite all of Wilson's breakthroughs, semiconductors remained difficult to explain. As it gradually became clear, microscopic changes in the crystal structure and impurity concentration disproportionately affected their macroscopic electrical behavior. Ignoring the lack of understanding—because no one could explain Brown's experimental observations 60 years earlier—Brattain and Becker developed an efficient manufacturing process for copper oxide rectifiers for their employer. The Bell System quickly began replacing vacuum tube rectifiers throughout the system with a new device that their engineers called , since its resistance varied depending on the direction.
gold medal
Mervyn Kelly, a physicist and former head of the vacuum tube department at Bell Labs, was very interested in this achievement. For a couple of decades, vacuum tubes served Bell invaluably, and were able to perform functions that were not available to the previous generation of mechanical and electromechanical components. But they got very hot, regularly overheated, consumed a lot of energy and were difficult to maintain. Kelly was going to rebuild Bell's system with more reliable and durable solid-state electronic components, such as a varistor, which did not require either hermetically sealed gas-filled or empty cases, or hot filaments. In 1936 he became head of the research department at Bell Laboratories, and began to redirect the organization in a new direction.
With the solid state rectifier in place, the obvious next step was to build a solid state amplifier. Naturally, like a tube amplifier, such a device could also work as a digital switch. This was of particular interest to the Bell firm, since telephone switches still operated a huge number of electromechanical digital switches. The company was looking for a more reliable, compact, energy efficient and cool replacement for the vacuum tube in telephone systems, radios, radars and other analog equipment, where they were used to amplify weak signals to a level accessible to the human ear.
In 1936, Bell Laboratories finally lifted the ban on hiring personnel imposed during . Kelly immediately began hiring experts in quantum mechanics to help launch his solid-state research program, which included , another native of the West Coast, from Palo Alto (California). The topic of his recent dissertation at MIT suited Kelly's needs perfectly: "Electronic Zones in Sodium Chloride."
Brattain and Becker continued their research into the copper oxide rectifier at this time, aiming for an improved solid-state amplifier. The most obvious way to make it was to go by analogy with a vacuum tube. Just like Lee de Forest took a tube amp and between the cathode and the anode, and Brattain and Becker imagined how a grid could be inserted at the point where the copper and copper oxide met, where the rectification was supposed to take place. 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 showed that Bell Labs wasn't the only company interested in solid-state electronics. In 1938, Rudolf Hilsch and Robert Pohl published the results of experiments carried out at the University of Göttingen on a working solid-state amplifier created by embedding a grid in a potassium bromide crystal. This laboratory device was of no practical value, mainly because it operated at a frequency of no more than 1 Hz. Nevertheless, this achievement could not but please all those interested in solid-state electronics. In the same year, Kelly assigned Shockley to the new independent solid-state device research group and gave him and his colleagues Foster Nix and Dean Woolridge carte blanche to explore their capabilities.
At least two other inventors managed to create solid-state amplifiers before World War II. In 1922, the Soviet physicist and inventor published the results of successful experiments with zincite semiconductors, but his work went unnoticed by the Western community; In 1926, the American inventor Julius Lilenfield applied for a patent for a solid state amplifier, but there is no evidence that his invention worked.
Shockley's first major insight in his new position came while reading the work of the British physicist Neville Mot "The Theory of Crystalline Rectifiers" from 1938, which finally explained the principle of operation of the Grondal copper oxide rectifier. Mott used the mathematics of quantum mechanics to describe the formation of an electric field at the junction of a conducting metal and a semiconducting oxide, and how electrons "jump" over this electrical barrier instead of tunneling as Wilson suggested. Current flows more easily from metal to semiconductor than vice versa, because the metal has many more free electrons.
This led Shockley to exactly the same idea that Brattain and Becker had considered and rejected many years earlier - to make a solid-state amplifier by inserting a copper oxide mesh in the gap between copper and copper oxide. He hoped that the current flowing through the grid would increase the current-limiting barrier from the copper to the oxide, creating an inverted, amplified version of the grid signal. His first crude attempt failed completely, so he turned to a man with more polished lab skills and familiarity with rectifiers, Walter Brattain. And, although he had no doubts about the outcome, Brattain 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 head of the Bell Labs Radar Working Group, supported by the major US Radar Research Center at MIT. Brattain worked briefly with him, and then moved on to research on magnetic detection of submarines by order of the navy. Woolridge worked on fire control systems, Nix on gas diffusion for the Manhattan Project, and Shockley went into operations research, first in anti-submarine warfare in the Atlantic and then in strategic bombing in the Pacific.
But despite this intervention, the war did not stop the development of solid-state electronics. On the contrary, it orchestrated 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)
Source: habr.com