Fabrication and characterization of single-electron tunneling transistors in the superconducting state

Electron-beam lithography was used to fabricate single electron charging effect devices with ultrasmall capacitance Al/Al/sub 2/O/sub 3//Al tunnel junctions. The single electron transistor is a three-terminal device composed of two series tunnel junctions and a gate electrode capacitively coupled to the island between them. Typical junctions are of area 60 nm*60 nm with a capacitance of 190 aF. The authors outline the fabrication procedures, discuss operational properties, and give sample handling considerations. These devices exhibit a highly nonlinear I-V characteristic which is modulated by the gate voltage, as expected for the Coulomb blockage. In the superconducting state, the superconducting gap in the quasiparticle density of states leads to transistor action above 1.3 K, a temperature easily reached with pumped liquid /sup 4/He refrigeration. The authors also discuss the observation of an intermittent intrinsic switching noise in the offset charge of the central island.<>