Use of Small Tunnel Junctions Operating at T=0.3K

Small tunnel junctions, two conducting electrodes separated by a thin insulator, can be successfully used for the fabrication of small planar devices operating at the temperature typical of a helium-3 refrigerator. Junctions can be classified on the base of the electrode material. When the top electrode is a normal-metal and the base electrode is a superconductor we have a NIS junction. With this type of small junctions (overlap area smaller than 0.1 ?m2 high sensitive thermometry can be performed and, in particular the temperature change of a normal-metal electron gas of a hot electron bolometer (NHEB)1,2 can be detected. When both electrodes are superconducting we have SIS junctions that allow the fabrication of SSETs (Superconducting Single Electron Transistor), a small superconducting island (0.2-0.4?m2), weakly coupled to two superconducting leads (Cooper pair reservoirs) by means of two tunnel barriers. With a SSET one can realize the most sensitive detector of electric charge: it is generally used to measure the charge stored in small conducting islands 3.