John Bardeen
John Bardeen (1908-1991) was an American physicist, the theorist among the three inventors of the transistor and the only person to have won the Nobel Prize in Physics twice. He shared the 1956 prize with William Shockley and Walter Brattain for the transistor, and the 1972 prize with Leon Cooper and J. Robert Schrieffer for the theory of superconductivity. His career is an unusual case of one scientist explaining the central mystery of two different fields, semiconductor surfaces and the vanishing resistance of cold metals, by pairing deep quantum theory with close attention to what experimenters actually measured.
His training combined engineering with mathematical physics. Born in Madison, Wisconsin, Bardeen earned his bachelor's and master's degrees at the University of Wisconsin, in electrical engineering, then worked briefly as a geophysicist for an oil-exploration research laboratory before doing a doctorate at Princeton under Eugene Wigner, with a thesis on the electronic behavior of metals. After fellowships and a faculty post, and wartime work on naval ordnance, he joined Bell Labs in 1945 to work in the new solid-state physics group. That mixture of practical engineering sense and theoretical depth shaped his style, which was to look for the simplest model consistent with experiment.
His key contribution to the transistor was the surface-state theory of 1946-47. Shockley's field-effect devices had failed to show the expected effect, and Bardeen proposed that electrons trapped at the semiconductor surface formed a screening layer that shielded the interior from an applied field. This did not just explain a failure; it told Walter Brattain what to measure and how to change the surface, and it led to the two-contact germanium device that amplified a signal in December 1947. Credit for the invention is shared, with Bardeen supplying the physical picture and Brattain the experimental craft, while Shockley led the group and soon turned to the junction transistor.
In 1951 Bardeen left Bell Labs for the University of Illinois at Urbana-Champaign, where he held a professorship in both electrical engineering and physics. The move is often attributed to tensions at Bell Labs after the transistor, including the growing friction with Shockley, and to his wish to work on superconductivity, a problem he had begun to study earlier. At Illinois he worked with the postdoctoral researcher Leon Cooper and the graduate student J. Robert Schrieffer. Cooper showed that electrons near the Fermi level can form bound pairs through interaction with lattice vibrations, and the trio assembled this into the 1957 theory known by their initials, BCS. In it, paired electrons condense into a collective state that carries current without resistance.
BCS theory and the transistor are conceptually different but share a theme. Both depend on the quantum behavior of electrons in a crystal lattice, and both are about how a solid can be made to behave in a way ordinary classical pictures do not predict. Bardeen's two Nobel Prizes are therefore not a coincidence of fame: the first concerns energy bands and carriers that can be engineered with dopants, the second concerns a ground state with an energy gap that protects a current from scattering. He stayed at Illinois until his later years and died in Boston in 1991.
A frequent mistake is to rank the three transistor inventors by fame, or to cast Bardeen simply as the junior partner. His 1956 prize was shared equally, and the field-effect principle he helped identify, controlling a surface by an electric field, is the working principle of the metal-oxide-semiconductor transistors that fill today's chips. Superconducting circuits, magnets in MRI scanners and quantum-computing hardware use materials understood within the BCS framework or its extensions. Strictly, though, BCS theory describes conventional low-temperature superconductors, and it is not a complete account of every high-temperature superconductor discovered since.
Read John Bardeen as the physicist who explained two hidden layers of matter, the electron states on a semiconductor surface and the paired electrons in a superconductor, and who won a Nobel Prize in Physics for each.