Nanodevices with Normal Metal—Insulator—Superconductor Tunnel Junctions

Normal metal–insulator–superconductor (NIS) tunnel junctions, as well as SIS junctions, are the main building blocks for superconducting electronics. Single NIS junctions and arrays are used in microwave bolometers, cryogenic thermometers, electron coolers, radiation detector. In such devices, junctions should fit different parameters for area, transparency, material properties and thermal characteristics. This leads to various fabrication methods and technique for measurements. Estimations made for equilibrium measurement conditions can be controversial in the case of microwave bolometers, ultra-low temperature thermometers, electron coolers. In terahertz bolometers, as well as in electron coolers, the energy distribution of electrons becomes different from Fermi distribution. In bolometers illuminated with terahertz radiation, the density of electrons will increase at higher energies, and in electron coolers, the distribution will be with reduced density at higher energies. Andreev reflection and proximity effect at the superconductor-normal metal interface induce changes in IV curve compared to simple SIN model. In this review, we present two levels of description for practical applications; first approximation is conventional with single-electron tunneling and equilibrium electron energy distribution, and the second taking into account non-Fermi distribution, Andreev currents, high-energy phonon creation and phonon escape.