A new design for a superconducting transistor could revive the dream of circuits that would operate without electrical resistance. Such circuits might run much faster than conventional electronics and fit into a smaller space without overheating. The new design, which will be published in Applied Physics Letters, has a key advantage over the other candidate design, called the Josephson junction: It can not only act as an on-off switch, but can also perform the other function of normal transistors--amplifying an incoming current.
Josephson junctions consist of two layers of superconductor sandwiching an insulating layer; when a voltage bias switches the circuit "on," electrons "tunnel" through the insulating layer from one superconductor--usually a metallic, low-temperature superconducting material--to the other. Researchers from Groningen University in the Netherlands replaced the insulating layer with a 0.1-micrometer-wide gold layer. When the circuit is "on," electrons are simply conducted through the gold layer from one superconductor to the other. But the gold layer enables the circuit to act as something more than an on-off switch.
A conventional current that flows perpendicularly through the gold layer controls the supercurrent. Because it is so thin, the gold layer behaves as a so-called "quantum well," confining electrons in a layer so narrow as to affect their quantum-mechanical properties. This forces them to reside only in specific energy levels. When a small current flows along the gold layer, it "heats" the electrons, which fill up many of the available energy levels and impede the current. The effect can shut off the supercurrent entirely. But variations in the current flow through the gold can also regulate the supercurrent without switching it off completely, allowing the device to act as an amplifier, says team member Teun Klapwijk.
The circuit still needs to be optimized before it can be put to use, team members say. But Konstantin Likharev of the State University of New York, Stony Brook, who is pursuing his own approach to superconducting electronics based on Josephson junctions, says that a superconducting circuit "should provide enormous speed advantages. I don't see it here." Still, the golden-hearted transistor has won some admirers. "It's nice work," says Michel Devoret of France's Atomic Energy Commission in Saclay, noting that any transistor that can function at extremely low temperatures also has the advantage of low inherent noise.