Charge Sensitivity of Superconducting Single-Electron Transistor

It is shown that the noise-limited charge sensitivity of a single-electron transistor using superconductors (of either $SISIS$ or $NISIN$ type) operating near the threshold of quasiparticle tunneling, can be considerably higher than that of a similar transistor made of normal metals or semiconductors. The reason is that the superconducting energy gap, in contrast to the Coulomb blockade, is not smeared by the finite temperature. We discuss also the increase of the maximum operation temperature due to superconductivity and a new peak-like feature on the $I-V$ curve of $SISIS$ structures.