Physicist Leo Esaki provided the first unequivocal experimental demonstration of electron tunneling in solids, stimulating research and development involving other tunneling phenomena.
On October 11, 1957, the journal Physical Review received a brief article, submitted as a letter to the editor, entitled “New Phenomenon in Narrow Germanium p-n Junctions.” The author was Leo Esaki, a Japanese graduate student enrolled in a doctoral program in physics at the University of Tokyo and working simultaneously in research and development at Sony Corporation
“Tunneling” refers to the ability of a particle to pass through a region forbidden to it by a classical mechanical model. The theory of tunneling is a natural corollary to the models of Louis de Broglie
Semiconductor tunnel diodes (sometimes called Esaki tunnel diodes after Leo Esaki) and transistors are similar in form and composition, and the development of the tunnel diode followed directly from the invention of the transistor
The operation of transistors and semiconductor tunnel diodes depends on the electrical properties of semiconductors that have been “doped” with impurities. The distribution of energy states of electrons within a crystalline solid is analogous to the distribution of energy states of electrons in an isolated atom; in the solid, the electron shells merge to form a continuous valence band. Conduction of electricity within a solid depends on the existence of unfilled electron valence bands. In a conductor (such as copper), the outer electron shell is unfilled and electrons move freely; in an insulator, the outer shell is filled. Pure (intrinsic) semiconductors such as silicon have a filled outer valence band, but they conduct electricity because the energy gap between the filled valence band and the next-higher empty band is low relative to the gap in good insulators, and some electrons are raised to the higher energy level by thermal excitation.
Leo Esaki.
The conducting capacity of an intrinsic semiconductor can be increased by the addition of impurities in the form of an element that has either one fewer or one more outer valence electron than the parent semiconductor crystal. Crystalline silicon, an intrinsic semiconductor with a valence of +4, can be doped with a material with a 3 valence, giving rise to an unsatisfied bond with one of the valence electrons and producing what is known as a p-type (for positive) semiconductor, or with a material with a +5 valence, resulting in an extra electron and producing an n-type (for negative) semiconductor. A p-n junction
When a p-type and an n-type semiconductor are joined, a potential barrier exists in a narrow region surrounding the junction because holes and electrons pair up; in the classic transistor, current flows across the junction only when the externally applied voltage increases to the point that it is sufficient to surmount the potential barrier. A transistor consists of three elements: a base, a collector, and an emitter. A weak current flowing into the base and out the emitter controls a much stronger current between the collector and emitter. The weak current acts as a switch, turning the semiconductor junction on and off, and thus controlling the flow of the stronger current across the junction. As a simple on/off switch, transistors control the flow of current in logic circuits in electronic computing devices. In a radio, the transistor serves as an amplifier, imparting the frequency of amplitude modulation of the incoming signal to a stronger current, which generates the audio output.
Esaki’s 1957 contribution to transistor technology began with the development of techniques for improving the precision of impurity doping, in terms of both the concentration of impurities and the distance across the p-n contact. The experiments which Wilson, Frankel and Joffe, and Nordheim had performed in attempting to demonstrate tunneling in metal-semiconductor junctions had failed because the existing technology was inadequate to produce a sufficiently thin junction. Esaki observed that current flow as a function of voltage across a heavily doped germanium p-n junction did not follow the usual pattern of zero flow over the range below the potential barrier and abrupt rise at the potential barrier (the simple on/off switching property of the classic transistor). The flow did, however, show two maxima, one below the voltage of the potential barrier the other at the barrier.
Esaki concluded that the lower peak could be ascribed to electrons “tunneling”—that is, crossing the barrier without reaching the higher energy level. This phenomenon of tunneling by electrons had been predicted on theoretical grounds based on the wave properties of particles. In the 1930’s, C. Zener
The Esaki tunnel diode exhibits some advantages and some disadvantages when compared with a nontunneling transistor. It is capable of operating at higher switching speeds and over a wider temperature range, it is more radiation resistant, it requires two rather than three terminals, it operates at microwave frequencies, it is hard to damage with overload current, and it is relatively insensitive to atmospheric gases. Its disadvantages include gain only in alternating current and a small usable voltage swing. In practical terms, the Esaki tunnel diode was most important as a first step in the discovery of electronic devices employing tunneling phenomena. Tunnel diodes are used together with transistors in computers and have been found to be particularly useful in sensitive microwave detectors, serving as an inexpensive alternative to masers in microwave amplifiers.
Demonstration of the tunneling phenomenon in p-n semiconductor junctions stimulated investigation in other areas, notably in the field of superconductivity. In 1960, Ivar Giaver applied tunneling theory to the current-voltage curve between superconducting metals separated by a thin insulating oxidizing film. The results confirmed the microscopic theory of superconductivity postulated by Bardeen, Leon N. Cooper, and John Robert Schrieffer and were sensitive enough to measure the energy gap that forms when the electrons condense into correlated bond pairs in a super conductor.
The work of Esaki and Giaver stimulated a blossoming of research into tunneling effects in solids. Brian D. Josephson, who in the early 1960’s was a graduate student at the University of Cambridge, formulated a theory of tunnel junctions between superconductors that predicted a supercurrent associated with the tunneling of bonded electron pairs across the junction. Two effects were predicted: a D-C Josephson effect in which a supercurrent flows in the absence of a voltage drop across the junction, and an A-C Josephson effect in which current oscillates at a frequency that is dependent on the voltage drop across the junction. Both predictions were greeted initially with skepticism until demonstrated experimentally in the laboratory. The A-C and D-C Josephson effects gave rise to a whole new generation of instruments of unprecedented sensitivity. Esaki, Josephson, and Giaver shared the 1973 Nobel Prize in Physics
In 1960, Esaki left Sony Corporation to join the staff of International Business Machines
Although an American resident, Esaki retained his Japanese citizenship and continued to be an active member of the Japanese scientific and business community, serving on the board of directors of IBM Japan and acting as a kind of unofficial diplomat in technological contacts between the two countries. He assumed a stance of polite neutrality when both countries claimed the honor of his Nobel Prize.
The significance of the demonstration of tunneling in solids can be divided conveniently into three areas: microelectronic engineering problems of broad applicability, specialized scientific instrumentation, and implications for theoretical physics. Esaki tunnel diodes are a common component of the ubiquitous silicon chip, which revolutionized communications and information processing in the last quarter of the twentieth century.
In practical engineering terms, the diode itself (or its cousin, the transistor) is a microscopic or near-microscopic region that has been selectively doped with impurities and isolated from the surrounding silicon crystal by a thin layer of insulating oxide. The entire chip (to paraphrase the introduction to Microelectronics and Society) contains perhaps 100,000 of such elements photographically stamped on a device the size of a cornflake and equivalent to a room full of the tubes and wire circuits that powered the earliest computers. The minuteness and low r cost of the switching elements were critical to the development of accessible computer technology. Tunnel diodes, with their unique current-voltage response characteristics, introduced greater flexibility into the engineering process.
Discovery of the A-C and D-C Josephson effects provided physicists with the basis for powerful tools to study the superconducting state, in which substances cooled to near absolute zero behave as if they were enormous atoms. A superconducting interferometer based on the Josephson effect enabled physicists to verify earlier theories of the superconducting wave function and to demonstrate its long-range quantum phase coherence.
Supercurrents across Josephson junctions are orders of magnitude more sensitive to applied electrical and magnetic fields than the most sensitive transistors. The unprecedented sensitivity of these devices made possible previously impossible experiments in many fields. Superconducting quantum interference devices
In terms of computer technology, this translates into vastly increased speed of operation and greater memory accessibility, with an equivalent degree of miniaturization of components. Because of the low temperatures required to achieve the superconducting state, supercomputers and electronic instrumentation employing Josephson effects are the province of large research facilities and government and military installations.
The A-C Josephson effect is used to define the volt in standards laboratories, replacing the formerly used standard cell. There has been a general tendency in later years to define measurements in terms of quantum effects, which are both more precise and more reproducible than the physical objects formerly used as standards.
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Braun, Ernest, and Stuart Macdonald. Revolution in Miniature: The History and Impact of Semiconductor Electronics. New York: Cambridge University Press, 1978. A historical survey of the development and diffusion of transistor technology and the progress of integrated circuits from sophisticated military and scientific instrumentation to mass-produced applications. Esaki’s research is not mentioned specifically, and the book as a whole downplays the contributions of the Japanese. -
Esaki, Leo. “Long Journey into Tunneling.” Science 183 (March 22, 1974): 1149-1155. This article is the text of Esaki’s Nobel lecture, delivered on December 12, 1973. The theoretical and technical antecedents to Esaki’s tunneling discoveries are explained, as well as the sequence of events that took place in his laboratory. The explanations of tunnel diodes, metal-oxide semiconductor junctions, and resonant transmission are highly technical. -
Friedrichs, Günter, and Adam Schaff. Microelectronics and Society: For Better or for Worse, a Report to the Club of Rome. New York: Pergamon Press, 1982. Provides clear, diagrammatic, nontechnical descriptions of how microelectronic semiconductor devices work. Helps readers to visualize transistors as isolated devices and as subunits in the complex circuitry of computers. -
Gentile, Sylvester P. Basic Theory and Application of Tunnel Diodes. Princeton, N.J.: D. Van Nostrand, 1962. A general textbook and manual aimed at electronics engineers. The first two chapters contain clear explanations of the workings of transistors and tunnel diodes in diagrammatic, nonmathematical terms. The remainder of the book consists of descriptions of specific circuit types employing tunnel diodes. -
Langenberg, D. N. “The 1973 Nobel Prize for Physics.” Science 182 (November 16, 1973): 70 1-704. A relatively nontechnical overview of the subject of tunneling, describing the separate contributions of Esaki, Giaver, and Josephson and the way in which they complement one another. Assesses the significance of tunneling phenomena to the infrastructure of science and technology. -
Orton, John. The Story of Semiconductors. New York: Oxford University Press, 2004. History of the impact of semiconductors upon electronics and human culture. Bibliographic references and index. -
Riordan, Michael, and Lillian Hoddeson. Crystal Fire: The Invention of the Transistor and the Birth of the Information Age. New York: Norton, 1998. Account of the scientific and industrial developments behind the invention of the transistor and its subsequent applications in computing and other information-based technologies. Bibliographic references and index. -
Scientific American Editors. Microelectronics. San Francisco: W. H. Freeman, 1977. A compendium of illustrated, semipopular articles that appeared in the September, 1977, issue of Scientific American, reissued in book form. Chapter 2, on microelectronic circuit elements, gives clear descriptions of the theory of p-n junctions and the structure and functions of semiconductor transistors, though without specific reference to tunneling diodes. Josephson’s low-temperature superconductor tunneling phenomena are discussed later in the context of high-level memory storage systems.
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