A new superconducting device with transistor-like properties

A superconducting transistor with large current gain and bandwidth at low t emperatures would have many applications. We have fabricated and tested a n ew three-terminal superconducting device with transistor-like properties at an operating temperature of 4.2 K, It is based on a stacked double tunnel junction structure where the intermediate film is a bilayer of superconduct ing Nb and an Al quasiparticle trap which can work either in the supercondu cting or in the normal metal state. Current amplification factors of up to 2.0 are observed at a temperature of 4.2 K when the Al is superconducting, while large current gains of more than 50 are observed when the Al is in th e normal state. The device shows a high degree of unidirectionality. The re sults can be explained on the basis of the recently proposed QUAsiparticle TRApping TRANsistor, which should have wide applications in detection syste ms operating at low temperatures.