Thermometry with a nearly temperature independent sensitivity using a normal-superconducting tunnel diode biased close to the superconducting gap

We analyze the temperature dependence of the tunneling current in a normal-insulator–superconductor (NIS) tunnel junction and find that such a junction can serve as a primary thermometer below the superconducting critical temperature Tc with a nearly temperature independent sensitivity, provided the junction is suitably biased close to the superconducting gap. Deviations from expected behavior when the superconducting electrode does not follow most simple s-wave BCS density of states, and possible applications are briefly discussed.