A SUPERCONDUCTING TRANSISTOR-LIKE DEVICE Ihas been fabricated and tested by an Oxford-Naples group. The benefits of superconducting electronics are high switching speed, compatibility with sensors that operate at very low temperatures (where conventional transistors perform poorly), and high sensitivity to incoming electromagnetic radiation (useful in astronomical applications). The chief obstacle to superconducting electronics has been the lack of a 3-terminal device which can switch and amplify current.

Now, Norman Booth of Oxford (n.booth1@physics.ox.ac.uk, 011-44-1235-833486 or 01-204-348- 7219 in Canada for the summer) and Antonio Barone of the University of Naples (011-39-081-768-2416, barone@na.infn.it) and their colleagues have overcome this problem by placing together two tunnel junctions, to make a device consisting of three metallic layers separated by two insulating barriers (see figure at Physics News Graphics). A signal current or voltage applied to the first junction breaks some of the bound electron (Cooper) pairs in the superconducting injector layer, and one of the resulting two electronic excitations, called quasiparticles, tunnels into the superconducting part of the middle layer. If now the quasiparticle diffuses into the normal metal part of the middle layer it can give up its energy (which it got from the breakup of the Cooper pair) to many of the free electrons in the normal metal and these in turn can then tunnel through the second junction. This represents, from one side of the device to the other, a gain in current (as high as a factor of 70 in the Oxford-Naples experiments) and power (a factor as high as 1000).

Furthermore, the device operates (at a temperature of 4 K) on a power of about 1 microwatt and voltages of millivolts, in comparison to a power of 1-10 milliwatt and voltages of several volts for conventional transistors. This quasiparticle trapping transistor, which the researchers call a "quatratran," is the superconducting analog of both the pnp and npn transistors and can be batch produced with comparable fabrication techniques. (Pepe et al., Applied Physics Letters, 17 July /pnu/2000/; Select Articles.)