Engineers coveted the tunnel diode for its extremely fast switching times—tens of picoseconds—at a time when transistors loped along at milliseconds. But it never found commercial success, though it was occasionally used as a very fast switch. As a two-terminal device, the diode could not readily be designed for amplification, unlike a three-terminal transistor, whose circuit applications were then growing astronomically. Nevertheless, the tunnel diode was a seminal invention. It provided the first physical evidence that the phenomenon of tunneling, a key postulate of quantum mechanics, was more than an intriguing theory.

Quantum mechanics, the foundation of modern physics, is an elaborate conceptual framework that predicts the behavior of matter and radiation at the atomic level. One of its most fundamental notions is that the exchange of energy at the subatomic level is constrained to certain levels, or quantities—in a word, quantized.

Many of the core concepts and phenomena of quantum mechanics are almost completely counterintuitive. For example, consider a piece of semiconductor joined to an insulator. From the point of view of classical physics theory, the electrons in the semiconductor are like rubber balls, and the insulator is like a low garden wall. An electron would have no chance of getting over the barrier unless its energy were higher than the barrier's. But according to quantum mechanics, the phenomenon of tunneling ensures that for certain conditions an electron with less energy than the barrier's will not bounce off the wall but will instead tunnel right through it.

Ever since the late 1920s, physicists had debated about whether tunneling really occurred in solids. The tunnel diode offered the first compelling experimental evidence that it did.

When Esaki, then a 49-year-old semiconductor research scientist at IBM Corp., won his Nobel Prize in 1973, neither he nor the Nobel committee had any idea about Noyce's work. Esaki had made a tunnel diode and measured its current versus voltage behavior 16 years earlier, when he was working at the company now called Sony Corp. in his native Japan. The Nobel committee, in fact, dated Esaki's discovery from 1957, roughly contemporaneous with Noyce's recollected work in the same field. Stig Lundqvist of the Swedish Royal Academy of Sciences used the "electrons as balls against the wall" analogy in his speech presenting the 1973 Nobel Prize in physics to Esaki; Ivar Giaever and Brian David Josephson shared the award for discovering different aspects of the tunneling phenomenon in solids.