Schottky, Walter Hans
Schottky, Walter Hans
SCHOTTKY, WALTER HANS
(b. Zürich, Switzerland, 23 July 1886; d. Forchheim, Germany, 4 March 1976), physics, vacuum-tube electronics, telecommu nications technology, thermodynamics, defects in crystal lattices, solid-state electronics.
Schottky, although a theorist, was the most important industrial physicist in twentieth-century Germany. Being one of the “outstanding research personalities” of the Siemens company, he was granted scientific freedom to pursue problems of pure physics. Thus his contributions include both technological and scientific achievements: invention of the tetrode electron tube and the superheterodyne principle for receiving wireless signals, discovery of the “small-shot effect” (noise due to the quantum structure of electricity) in electron tubes, theory of crystal defects, and theory of rectification in semiconductor-metal contacts. Unfortunately, Schottky was “a meritorious but unintelligible man,” as he would be referred to by his younger Siemens colleagues in the 1950s, and therefore he needed “translators” of his ideas into language that the ordinary industrial researchers and electrical engineers understood.
Education Walter Hans Schottky was the second son of the German mathematician Friedrich Schottky. After his graduation from the Humanistisches Gymnasium in Berlin-Steglitz in 1904, Walter Schottky enrolled in the Royal Friedrich-Wilhelms-University in Berlin to study physics. Walther Nernst and Max Planck were the teachers who impressed him most. In 1912, Schottky obtained his PhD degree under Planck with an award-winning dissertation on special relativity. Schottky had studied physics twice as long as was usual at the time (sixteen instead of eight semesters), but had acquired an enormously deep and thorough knowledge of physics and physical chemistry. Planck’s and Nernst’s influence on Schottky later became apparent in his thermodynamic and often also physical-chemistry-based approach to both physical and technological problems.
At Planck’s recommendation, Schottky looked for a possibility to be trained in experimental physics as well. He ended up in Jena under Max Wien, inventor of the then-famous Löschfunken (quenched spark) wireless transmitter. Starting out in a lab-work training program for advanced students (experiments on the photoelectric effect), he soon defined his own course of research. During the summer of 1912, he theoretically derived the V3/2 law for the dependence of the current on the voltage in a vacuum tube—independently of Irving Langmuir. On 2 December 1913, Schottky was able to prove the V3/2 law experimentally.
After having returned to his parents’ home in Berlin later in December 1913, Schottky immediately started working up his results for publication. Langmuir’s article “The Effect of Space Charge and Residual Gases on Thermionic Currents in High Vacuum” published in
German translation in Physikalische Zeitschrift (Physical journal) in 1914 (the original article had appeared in Physical Review in 1913) was like a “blow with a club” for Schottky. Nevertheless, he published some of his findings that went beyond Langmuir’s results. In Berlin, he resumed experimental work on electron emission in high vacuum at the Physical Institute of the university and discovered the effect that strong fields increase the emission of electrons from hot filaments—later to be known as the Schottky effect.
Working for Siemens in World War I Early in 1915, Schottky gave a talk on his work on thermionic emission at Berlin University. The Siemens company was strongly interested in maintaining close contact with university research in order to be able to import new ideas and scientists. Ragnar Holm, a Swedish physicist and Siemens employee, immediately realized the importance of Schottky’s work for the problem of electronic amplifier tubes and contacted him after the talk.
When World War I had broken out, Schottky had not been drafted into the army because of his poor health—which he considered to be at least embarrassing, perhaps even a disgrace. By doing research work for Siemens and inventing a war telecommunications device, Schottky wanted “to prove that he, too, could be useful for the fatherland” (Henriette Schottky, 1936); besides, practical work was a chance to escape from the neurotic cage of an extreme introvert in which he lived.
Inspired by Holm, Schottky started working on a vacuum tube that would compensate for the negative effects of space charge. Siemens neither offered Schottky a job nor paid him anything but supported his experimental work at Berlin University by lending him equipment. The company applied for and took out a patent for Schottky’s Raumladegitter-Röhre (space-charge-grid tube). On 1 September 1915, Schottky informed Siemens that both the prototype of the space-charge-grid tube (made at the university) and the theory for the design of new vacuum tubes were ready. Despite Holm’s backing of Schottky, the Siemens management failed to understand the importance of Schottky’s theory of the vacuum tube. The high amplification factor of the crude prototype tube showed, however, that the strange introvert was more than just another unintelligible theorist.
After Schottky had again been declared unfit for military service, he asked Siemens for a job. He was accepted and took up his work as a regular Siemens employee on 2 March 1916. Schottky’s work at Siemens was highly successful. In 1916, he invented the Schutznetz-Röhre (protective-net tube, after further development called screen-grid tetrode). In 1917, he became scientific director of the K-Laboratorium (cable laboratory—the lab where work on Pupin cables, electron tubes, and amplifiers was carried out) of Siemens & Halske. In 1918, he invented the superheterodyne principle for receiving wireless signals and discovered the Schroteffekt (small-shot effect—i.e., noise due to the quantum structure of electricity) in electron tubes.
His work that soon covered all kinds of telecommunications systems brought him into contact with the problem of the crystal detector (point-contact rectifier). The effect of unipolar (i.e., unidirectional, or, as it was later called, rectifying) electricity conduction in metal-on-metal-compound (i.e., metal-semiconductor) contacts had been discovered by Ferdinand Braun as early as 1874. Braun himself had exploited the effect in the crystal detector introduced into wireless telegraphy to replace the unstable coherer. Nevertheless, the physical mechanism responsible for rectification (i.e., conversion of alternating to direct current) was unknown. It was only natural that Schottky’s first approach to this problem was based on the theoretical models that had been successfully used to describe the phenomena of thermionic emission.
In 1919 when war restrictions on publications were no longer in effect, Schottky published his theory of the vacuum-tube amplifier. It was not favorably received by engineers because it was difficult to understand, both because of the too mathematical approach and the use of his own symbols (the ones that he had used in his first draft submitted to Siemens in 1915). It was Heinrich Barkhausen who in his famous textbook series ElektronenRöhren (Electron tubes, first editions of volumes I to III, Barkhausen, 1923–1929) translated Schottky’s ideas into a language that electrical engineers understood.
In 1936, the Royal Society in London awarded Schottky “the Hughes Medal in recognition of your discovery of the Schrot effect in thermionic emission and your invention of the screen-grid tetrode and superheterodyne method of receiving wireless signals” (Royal Society, 1936).
Academic Interlude Still in 1919, Schottky left his Siemens post in Berlin and moved to Würzburg in order to work toward his Habilitation degree (in Germany, the completion of a Habilitation thesis and successfully giving a lecture including a discussion afterward was, and in general still is, the precondition for becoming a university lecturer or professor) which he obtained under Wilhelm Wien in 1920. Schottky then became a Privatdozent (lecturer) at Würzburg University. Unfortunately, he was not good at teaching and consequently not enthusiastic about it. Therefore he concentrated on his own scientific interests. In pursuit of problems he had encountered while working on his Habilitation thesis, he investigated the quantum-theoretical implications of Nernst’s theorem (third law of thermodynamics). This work, together with his interest in practical applications of thermodynamics, eventually led to his textbook Thermodynamik, which was written with the help of coauthors Hermann Ulich and Carl Wagner and published in 1929. This textbook contained several pieces of original work, among them also the roots of Schottky’s 1935 discovery of what Wilhelm Jost shortly afterward called SCHOTTKYsche Fehlordnung(literal translation: SCHOTTKYan disorder; Schottky defect in the English literature) of a crystal lattice: missing atoms without compensating atoms in interstitial positions because of thermodynamic necessity—above absolute zero the entropy of the crystal is larger than zero, and as entropy is also a measure of disorder, there must be imperfections even in the lattice of a pure substance.
On 19 September 1922, Schottky began to work as a salaried assistant at the Physical State Laboratory in Hamburg, on leave from Würzburg for the whole winter semester of 1922–1923. In Hamburg, Schottky summed up his previous investigations on electron emission and the crystal detector and pursued them further. As an external scientific consultant for Siemens, he kept in touch with the company’s technological problems. In 1922, he started working on the problem of arcbacks inside mercury-vapor rectifiers. Siemens soon dropped further development work on these rectifiers since they did not promise big sales at the time. Schottky, however, having been appointed professor of theoretical physics at Rostock University (he assumed this post on 1 January 1923), pursued the problem out of scientific interest. In contrast to Siemens, competitors AEG and BBC continued empirical technological development work because they foresaw future market demands for such rectifiers.
Back to Siemens The emergence of the cuprous-oxide (Cu2 O) junction rectifier on the market shortly after its announcement in 1926 by its inventor Lars O. Grondahl marks the beginning of modern use of semiconductors. Despite the importance of the new device, its underlying physics were not understood at the time.
When the long-planned electrification of the Berlin S-Bahn (suburban commuter railroad system) finally got under way in 1924, the decision of the state railroads to use direct current supplied via a third rail for this service resulted in a great demand for high-power mercury-vapor rectifiers. AEG and BBC were able to offer units for 1,500 amperes and 1,600 amperes, respectively, whereas the biggest Siemens rectifier at the time had an output of only 600 amperes! Such a mistake was not to be repeated; however, the importance of the Cu2 O rectifier for low-power applications was realized immediately. Both patent and production problems made it imperative to know the physical mechanisms that were responsible for rectification in the boundary layer between Cu2 O and Cu.
Despite a promotion within the academic ranks of Rostock University in 1926, Schottky was not happy there. Theoretical physics led a Cinderella existence, there were few and usually not very talented students, and he still found teaching uncongenial. Schottky wanted to be in closer contact with practical work again and longed for collaboration in a larger context. Driven by an “irresistible inner compulsion [to go] to research centers with optimum conditions” as he later remembered (Schottky, 1960), he contacted Siemens for a job in 1927, just at the time when a heavy research program to catch up with competitors got under way. Fully in line with the prevailing research strategy of the company to put almost all its eggs into the one basket of outstanding research personalities, Siemens hired Schottky to work on the theory of both mercury-vapor and Cu2 O rectifiers. On 1 October 1927, Schottky joined the company as a full-time in-house scientific consultant.
Within the framework of the Siemens corporate research strategy and with full company support, Schottky not only solved the arcback problems in high-power mercury-vapor rectifiers but also worked out the theory of rectification in semiconductor-metal contacts between 1927 and 1939. Eberhard Spenke, who had applied for a job with Siemens and been hired in 1929 at Schottky’s recommendation, worked with Schottky to elaborate the new theory mathematically in 1939. The result, titled “Zur quantitativen Durchführung der Raumladungsund Randschichttheorie der Kristallgleichrichter” (On the quantitative elaboration of the space-charge and boundary-layer theory of crystal rectifiers) and published in the same year in Wissenschaftliche Veröffentlichungen aus den Siemens-Werken (Scientific publications from the Siemens-Works), offered for the first time a complete theory of the semiconductor rectifiers known at the time, unraveled “the causes of the most important—wanted and unwanted—properties of the characteristic curves of rectifiers” (Siemens, 1939), and thus gave hints for their improvement. It was a major scientific breakthrough because the effect of rectification in semiconductor-metal contacts had already been discovered in 1874 and had been used in the form of cat's-whisker detectors (i.e., point-contact rectifiers) in early radio and again since the second half of the 1920s in the form of area (in contrast to point-contact) cuprous-oxide and selenium rectifiers (area rectifiers are called junction rectifiers today).
Spenke became the “translator” of Schottky’s ideas, at first only for Siemens research and development (R&D), but after World War II also for the whole semiconductor community in the German-speaking world. Siemens researchers never felt comfortable about developing devices they did not understand—and consequently were not good at it. While competitors had empirically found better production methods than Siemens R&D, a Siemens physicist had worked out the theory of the device and won great renown in the world of physics. Schottky’s “Vereinfachte und erweiterte Theorie der Randschichtgleichrichter“(Simplified and extended theory of boundary-layer rectifiers) of 1941, published in 1942, completed his theoretical edifice.
In World War II, Schottky’s ideas were taken up by Heinrich J. Welker, who worked on the problem of finding a suitable detector for cm-waves (i.e., radar waves) within the framework of “Aviation Research” at Gräfelfing experimental station. During the second half of the war, parts of production and R&D were evacuated to rural regions in order to escape the ever-increasing Allied air raids on the big cities. In 1943, the Siemens-Schuckertwerke Transformer Works in Nuremberg moved their test lab to Pretzfeld Castle in Upper Franconia. Schottky also moved to Pretzfeld in early 1944 after his house in Berlin had been bombed to pieces and research in Berlin had become impossible.
After World War II Immediately after World War II, German industry was in a terrible condition. In addition to the war damage, many factories, especially in the east, were dismantled and removed as reparations. The top scientists of the company had been scattered all over Germany during the final stage of the war or had been captured and “convinced” by the Russians to work in the Soviet Union. Schottky, however, was safe in Pretzfeld in the American zone.
He renewed his affiliation with Siemens in 1946 and also lectured on semiconductor theory at Erlangen University during the winter semester of 1947–1948 and the summer semester of 1948. In early 1947, he received an offer from the U.S. War Department to work in the United States, but turned it down; he was sixty at the time and most probably felt too old to adapt to a new language and new surroundings. Besides, in contrast to his attitude in World War I, he did not want to be involved in military research, a commitment that he had been able to evade even during the Nazi regime and what he called the “most criminal and most dilettantish of all wars” (Schottky, 1945). Nevertheless, he worked as a consultant for the Scientific Research Group of the Office of Military Government for Bavaria and then for the Scientific Research Division of the Office of the U.S. High Commissioner for Germany from 1 July 1948 to 2 August 1952.
Despite some patents, Schottky had no influence on technological developments after 1945. His great contribution in the 1950s was of an organizational nature: still on the Siemens payroll, he played a pivotal role in the establishment of solid-state and especially semiconductor physics as a field of research in its own right in postwar Germany.
A complete list of Schottky’s publications can be found in Serchinger, 2007. Collections of Schottky’s papers can be found in the following archives:NL 100 Schottky. Archives of the Deutsches Museum, Munich, Germany. SAA 11/Lc 75 and SAA 11–40/Lc 166 Schottky. Siemens Archives, Munich, Germany.
WORKS BY SCHOTTKY
“Vakuumverstärkerröhre mit Glühkathode und Hilfselektrode” [Vacuum amplifier tube with glowing cathode and auxiliary electrode]. DRP [German Reich Patent] 300617, filed 1 June 1916. Schottky’s screen-grid tube patent.
“Empfangsanordnung für elektrische Wellensignale” [Receiving array for electric wave signals]. DRP [German Reich Patent] 368937, filed 19 June 1918. Schottky’s superheterodyne patent was filed six months before Edwin H. Armstrong's. Old German patents (up to the end of World War II) like these can be found in the library of the Deutsches Museum, Munich, Germany.
“Über spontane Stromschwankungen in verschiedenen Elektritzitätsleitern” [On spontaneous current fluctuations in various electricity conductors]. Annalen der Physik 57 (1918): 541–567. The original paper on the “small-shot effect” (shot noise, Schottky noise).
“Über Hochvakuumverstärker” [On high-vacuum amplifiers]. Archiv für Elektrotechnik 8 (1920): 1–31, 299–328. Schottky’s complete theory of electron tubes and tube amplifiers.
“Small-Shot Effect and Flicker Effect.” Physical Review 28 (1926): 74–103. Schottky’s only publication in English, translated by the discoverer of the flicker effect, J. B. Johnson of the Bell Telephone Laboratories.
With Hermann Ulich and Carl Wagner. Thermodynamik [Thermodynamics]. Berlin: Verlag von Julius Springer, 1929. Reprinted Berlin: Springer Verlag, 1973. Schottky’s preface gives insights into his ideas about applying theoretical physics to engineering problems.
“Zur Theorie der thermischen Fehlordnung in Kristallen” [On the theory of thermal disorder in crystals]. Die Naturwissenschaften 23 (1935): 656–657. Introduced what shortly afterward was called the Schottky defect.
“Über den Mechanismus der Ionenbewegung in festen Elektrolyten” [On the mechanism of ion movement in solid electrolytes]. Zeitschrift für physikalische Chemie 29 (1935): 333–355. Elaborated the concept of the Schottky defect.
“Halbleitertheorie der Sperrschicht” [Semiconductor theory of the barrier layer]. Die Naturwissenschaften 26 (1938): 843. Short overview published in advance to ensure priority over Nevill F. Mott.
With Eberhard Spenke. “Zur quantitativen Durchführung der Raumladungs- und Randschichttheorie der Kristallgleichrichter” [On the quantitative elaboration of the space-charge and boundary-layer theory of crystal rectifiers]. Wissenschaftliche Veröffentlichungen aus den Siemens-Werken 18 (1939): 1–67. Contains detailed derivations of formulas and calculations.
“Vereinfachte und erweitere Theorie der Randschichtgleich-richter” [Simplified and extended theory of boundary-layer rectifiers]. Received 27 September 1941. Zeitschrift für Physik 118 (1942): 539–592. Final published version of his theory.
Letter to his mother and his sister Lilly, 17 March 1945. Schottky papers. NL 100 Schottky. Archives of the Deutsches Museum, Munich, Germany. Hitler and his regime were still in power at that time.
Letter to the rector of Berlin Technical University, 18 May 1960. Schottky papers. NL 100 Schottky. Archives of the Deutsches Museum, Munich, Germany. Published in Physikalische Blätter 49 (1993): 858–859.
Barkhausen, Heinrich. Elektronen-Röhren, I. Elektronentheoretische Grundlagen, II. Verstärkung schwacher Wechselströme [Electron tubes, I. Electron-rheoretical foundations, II. Amplification of weak alternating currents]. Leipzig, Germany: Verlag von S. Hirzel, 1923. As is usual in textbooks, only the most important original works were cited. Schottky was the author most often cited in the footnotes.
———. Elektronen-Röhren, 2. Bd., Röhrensender [Electron tubes, vol. 2, Tube transmitters]. Leipzig, Germany: Verlag von S. Hirzel, 1925.
———. Elektronen-Röhren, 3. Bd., Empfänger [Electron tubes, vol. 3, Receivers]. Leipzig, Germany: Verlag von S. Hirzel, 1929. In his explanation of the superheterodyne principle, Barkhausen cited neither Schottky nor Edwin H. Armstrong.
Langmuir, Irving. “The Effect of Space Charge and Residual Gases on Thermionic Currents in High Vacuum.” Physical Review 2 (1913): 450–486. German translation in Physikalische Zeitschrift [Physical journal] 15 (1914): 348–353, 516–526.
Rhoderick, E. H. “Obituary. Walter Schottky.” Nature 263 (1976): 263. Schottky’s affiliation with Siemens incorrectly described, but otherwise correct.
Rothe, Horst, Eberhard Spenke, and Carl Wagner. “Zum 65. Geburtstag von Walter Schottky” [On Walter Schottky’s 65th birthday]. Archiv der elektrischen Übertragung 5 (1951): 306–313. An excellent overview (including formulas) of his scientific work, written by three outstanding colleagues.
Royal Society, London, to Walter Schottky, 5 November 1936. Schottky papers. NL 100 Schottky. Archives of the Deutsches Museum, Munich, Germany.
Schottky, Henriette. “Vaters Leben vom 24.7.1851–12.8.1935” [Father’s life from 24 July 1851 to 12 August 1935]. Schottky papers. NL 100 Schottky. Archives of the Deutsches Museum, Munich, Germany. Published in Mathematik in Berlin. Geschichte und Dokumentation [Mathematics in Berlin. History and documentation], edited by Heinrich Begehr. Second half-volume. Aachen, Germany: Shaker Verlag, 1998, pp. 77–102.
Serchinger, Reinhard W. “Walter Schottky und die Forschung bei Siemens” [Walter Schottky and research at Siemens]. In Oszillationen: Naturwissenschaftler und Ingenieure zwischen Forschung und Markt [Oscillations: Scientists and engineers between research and the market], edited by Ivo Schneider, Helmuth Trischler, and Ulrich Wengenroth. Munich, Germany: R. Oldenbourg Verlag, 2000, pp. 167–209. Focus is on how Schottky was embedded in the Siemens corporate strategy.
———. “Wirtschaftswunder in Pretzfeld, Upper Franconia: Interactions between Science, Technology, and Corporate Strategies in Siemens Semiconductor Rectifier Research & Development, 1945–1956.” History and Technology 16 (2000): 335–381. Focus is on the transformation of Schottky’s theory into technological reality (i.e., products) by Eberhard Spenke.
———. Walter Schottky—Atomtheoretiker und Elektrotechniker [Walter Schottky—Atomic theorist and electrical engineer). Stuttgart, Germany: Franz Steiner Verlag, 2007.
Serchinger, Reinhard W., and Martin Schottky. “Schottky, Walter Hans.” In J. C. Poggendorff Biographisch-literarisches Handwörterbuch der exakten Naturwissenschaften, vol. 8, part 3, edited by Hans Wussing. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2004. Contains a comprehensive list of works about Schottky.
Siemens, Zentralstelle für wissenschaftlich-technische Forschungsarbeiten der Siemens-Werke [Central authority over scientific-technological research work of the Siemens-Works]. Foreword to Wissenschaftliche Veröffentlichungen aus den Siemens-Werken 18 (1939).
Teichmann, Jürgen. Zur Geschichte der Festkörperphysik— Farbzentrenforschung bis 1940 [On the history of solid-state physics—Color-center research up to 1940]. Stuttgart, Germany: Franz Steiner Verlag Wiesbaden, 1988. Although focus is on Robert Wichard Pohl and his school, this book covers Schottky’s contributions to color-center research, which are not treated elsewhere.
Reinhard W. Serchinger