Until now, no one other than Moore and Shockley had seen Robert Noyce's 1956 description of a tunnel diode. But Noyce copied his work and saved it. How he managed to copy these pages is unclear--photocopy technology was in its infancy in the late 1950s, and Noyce never made note of going back to his Shockley notebooks later in life--but that the pages are legitimate is indisputable. Leslie Berlin, one of this article's authors, found them in January 2001 tucked in one of Noyce's Fairchild notebooks stored in Santa Clara, Calif., at a company that prefers not to be identified.
Berlin compared these copied pages to the only surviving notebook from Shockley Lab: the book belonging to William Shockley housed in the Special Collections of Stanford University, in California. The pages on which Noyce's ideas are written are clearly from the same type of lab book that Shockley issued to his staff, and the handwriting is undoubtedly Noyce's. This, along with the date of Noyce's work (which correlates with his 1976 comments about it), and Moore's recollections of the event, further validate their authenticity.
A quick comparison of Noyce's notebook pages with Esaki's seminal paper, "New Phenomenon in Narrow Germanium p-n Junctions," published in Physical Review in January 1958 (and received by that journal in October 1957), shows striking parallels. Both men used an energy-band diagram that represents the electron and hole energies on the y (vertical) axis versus their position in the p-n junction on the x (horizontal) axis [see sidebar, "The Noyce Diode," two pages from Noyce's notebook].
Noyce's "energy-level" diagram, which is now called an energy-band diagram [on the left-hand page], shows where the electrons and holes are located. It also illustrates the conditions necessary for tunneling current. The upper solid line in the diagram represents the bottom of the semiconductor's conduction band; in this band electrons can move freely as a result of the donor atoms. The lower solid line represents the top of the valence band, where acceptor impurities allow holes to move freely. The separation between the conduction and valence bands is the energy gap, or E g , and is the range in energy where no electrons or holes are permitted. For this reason, E g is sometimes called the forbidden gap.
In Noyce's diagram, the Fermi energy, or E f , represents the energy boundary for most of the holes in the p-type semiconductor and most of the free electrons in the n-type. For a highly doped, or degenerate, semiconductor, E f falls below the edge of the valence band and rises above the edge of the conduction band. Electrons "sink" so they fill the lowest energy levels in the conduction band, while holes "float" and fill the highest levels of the valence band. Therefore, it is the holes between the top of the valence band and E f and the free electrons between E f and the bottom of the conduction band that are significant for tunneling.
The region between the p and the n sides where the valence and conduction band edges bend is called the depletion region; this is where the potential barrier exists. This region narrows for large donor and acceptor concentrations and would be less than 10 nanometers for a tunnel diode.
Note that without an applied bias, the holes on the p side are at a higher energy than the electrons on the n side. For tunneling to occur, there must be holes at the same energy as the free electrons. But a forward bias (a positive voltage connected to the p side), raises E f and the conduction-band electrons on the n side with respect to E f on the p side by the amount of the bias voltage. Now there are free electrons at the same energy as the holes, and the electrons can tunnel through the potential barrier to holes on the p side, resulting in a current. As the forward bias is increased, more free electrons and holes are at the same energy and the tunneling current increases.
Both Noyce and Esaki recognized that as the bias increased further, E f on the n side would be raised further with respect to E f on the p side and the concentration of free electrons at the same energy as holes would diminish and result in a reduced tunneling current, as shown in Noyce's current (I) vs. voltage (V) plot. At a larger bias, the normal diode current would flow at the voltage E g in Noyce's plot.
This plot [on the right-hand page] is very similar to the measured I-V plot that Esaki shows. This phenomenon of decreasing current with increasing voltage is negative resistance, a characteristic that has been exploited to build oscillators.
But there was one important difference between Noyce's and Esaki's work. Noyce only predicted the drop in current (the evidence of tunneling) would occur. Esaki, who actually built a device to demonstrate his ideas, showed that it would. This difference is crucial--many good ideas die en route from the mind to the lab bench.
Noyce's failure to implement his brilliant idea was almost certainly a direct result of Shockley's discouraging comments to him in 1956. Noyce was an experimentalist at heart. ("He admired people who did things," his friend Maurice Newstein explained to one of the authors in 2003.) Noyce would later prove a bit of an iconoclast as well, joining a covert effort to build silicon transistors at Shockley whenever the boss, who had decided the lab should focus its attention on an obscure device he had invented called a four-layer diode, was away. But in August 1956, Noyce was 29 years old and not yet six months into his job at Shockley Lab. If his boss told him to drop an idea, at that point he would have done it.
Noyce no longer worked for Shockley when he read Leo Esaki's Physical Review article in 1958. In September 1957, he, Moore, and six other Shockley employees--more than half the senior technical staff--had left their temperamental boss to start their own transistor company, Fairchild Semiconductor. Shockley's business venture, meanwhile, withered (he joined the Stanford faculty in 1963), and he suffered the additional indignity of watching his proteges' new company achieve phenomenal success. In less than a decade, Fairchild--under Noyce's leadership--grew to employ 11 000 people and generate more than US $12 million in profits.
The publication of Esaki's article caused quite a sensation in the electronics community. At an international physics conference in 1958, the audience for Esaki's presentation was overflowing. Interestingly, Esaki credits William Shockley, who explicitly mentioned Esaki's work in his keynote address earlier to the conference, with the large attendance at his presentation.
We can never know why Shockley changed his mind about the importance of the diode, but there are several possible explanations. Shockley was infamous for his swings of opinion--one person who worked for him said he was regularly "jerking the company back and forth." Similar remarks from other colleagues indicate that Shockley may have changed his mind in this case. Moreover, the grudge he bore against the eight Fairchild founders was still fresh in 1958.
After reading Esaki's article, Noyce brought his copy of Physical Review to Moore and laid it on his desk. Noyce could not mask the irritation he felt with William Shockley and, even more, with himself for not pursuing his ideas after Shockley dismissed them. "If I had gone one step further," he told Moore, who would go on to found Intel with him in 1968, "I would have done it."
About the Author
Leslie Berlin is a visiting scholar in the Program in the History and Philosophy of Science and Technology at Stanford University, in California. Her biography of Robert Noyce, The Man Behind the Microchip: Robert Noyce and the Invention of Silicon Valley, will be published by Oxford University Press on 1 June.
H. Craig Casey Jr. is Professor Emeritus at Duke University, in Durham, N.C. He is a life fellow of the IEEE and a past president of the Electron Devices Society.