CRYOGENIC COOLING USING TUNNELING STRUCTURES WITH SHARP ENERGY FEATURES

Thermoelectric cooling, based upon the extraction of hot electrons and holes from a metallic electron gas, holds unrealized potential for refrigeration at cryogenic temperatures. We discuss the performance of two such electronic refrigerators: the quantum-dot refrigerator (QDR) and the normal-insulator-superconductor (NIS) refrigerator. We obtain the QDR base temperature using a numerical simulation and verify the validity of certain simplifying assumptions which allow refrigerating performance to be summarized on a diagram of ambient temperature versus electronic temperature. In this way, we find that the best refrigeration is obtained with the electronic distribution far from the equilibrium Fermi-Dirac function and the temperature reduction achieved is limited by the rate at which phonons are absorbed. We predict that, with sufficient thermal isolation, electronic devices could be cooled to a small fraction of the ambient temperature using these solid-state refrigerators. The NIS refrigerator should be capable of cooling thin-film devices from above 300 mK to below 100 mK; the QDR will cool macroscopic metallic samples in the mu K or nK range. We also discuss topics related to thermoelectric refrigeration including other cryogenic thermoelectric cooling schemes, the validity of the linear-response theory of thermoelectric effects, the refrigerating efficiency of an optimized thermoelectric refrigerator, and the overall cooling power of thermoelectric refrigeration.