Measurement of Ballistic Mean-Free-Path of Hot Quasiparticles Injected in Superconductor-Base Hot-Electron Transistors

A mean-free-path (L B) measurement was made for hot quasiparticles injected into the Nb-base film of superconductor-base hot-electron transistors. The measurement was analogous to that used for a conventional hot-electron transistor, and gave L B for the quasiparticles travelling in the direction normal to the Nb-base film surface. An L B(4.2 K) value in excess of 1000 Å was attained for films thicker than 800 Å, whereas a marked reduction in L B was seen in thin films, owing to the coarse crystalline quality of Nb film grown at the beginning of the sputter deposition.