Unveiling the microscopic mechanism of superconducting metallic transistors

Transistors are the basis for microchips and the whole electronic industry. The invention of transistors, by Bardeen and Brattain in 1947, awarded with a Nobel prize, is regarded as one of the most important discoveries of the 20th century.

Traditional transistors are based on modulating an electric current under an electric field, which is possible only using semiconductor materials. In semiconductors, there are fewer free charge carriers compared to metals, and the Fermi level (which is the thermodynamic work required to add one electron to the system) sits in an energy band gap, which implies that electrons are more difficult to excite.

By doping semiconductors, one can create a certain number of free carriers, e.g., in an empty band, which now can be excited to larger momenta and therefore can carry electric current through the material.

With semiconductors, a controlled flow of electrons from a source to a sink is possible under application of an electric field. Since the current-voltage characteristic of the material is strongly nonlinear, an electric signal can thus be amplified or suppressed, as in a p–n junction diode.

Why are transistors made of semiconductors and not, for example, metals? With metal conductors, it is not possible to make transistors due to the large number of free (extremely mobile) electrons, which completely screen the electric field inside the material.

In practice, as soon as you switch on an electric field across the conducting sample, all the electrons almost instantaneously move inside the sample and redistribute internally such that their new spatial distribution creates an electric field that exactly cancels the externally applied electric field.

This phenomenon thus prevents the possibility of controlling the flow of electricity (microscopically, the flow of free electrons) when an external electric field is switched on across the conductor.

Recently, metallic superconductors only a few nanometers thick have been used experimentally to realize a new electric field effect as a viable route toward metallic transistors. Superconducting materials are metals, which, if they are cooled down below a certain critical temperature, can support the flow of electrons with no resistance. In other words, they are ideal conductors where electricity can be carried through with no dissipation or resistance.

The reason for this seemingly magic behavior resides in the formation of electron pairs kept together by a "glue" provided by lattice thermal motions. These pairs obey quantum statistics (Bose-Einstein statistics), which allows for a huge number of particles (glued electron pairs, in this case) to occupy the lowest energy state or ground state.

The ground state then forms a coherent quantum wavefunction which is immune from scattering processes that generate resistivity, and thus, the electrons can flow freely through the material and carry electricity with no energy dissipation.

Working with these superconducting metal devices, an experimental team led by Francesco Giazotto at the Italian Centro Nazionale delle Ricerche (CNR) observed that an external electric field of sufficient amplitude can suppress the electric current. This phenomenon thus enables the use of the superconducting thin film as a diode, since now, we can control the electric current through the metal by tuning the external electric field.