Development of a 100 milli-kelvin electronic refrigerator based on a normal-insulator-superconductor tunnel junction

We report recent progress on an all-electronic, on-chip, 100 mK refrigerator. This refrigerator utilizes the unique thermal transport properties of a normal-insulator-superconductor (NIS) tunnel junction to preferentially remove electrons whose energy is higher than the Fermi energy from the normal electrode, and thus, to lower the temperature of electrons in the normal electrode. We present the first measurements demonstrating electron cooling in a device having a substantially larger area (∼10⁴ μm²) than typical submicron-scale devices demonstrated previously. The performance of this refrigerator is analyzed by a simple thermal model that includes energy transport through the junction, thermal loading from the environment, recombination of quasiparticles in the superconductor electrode, and non-ideal junction characteristics.