Electronic refrigerators: Optimization studies

Electric current flowing through a normal metal-insulator-superconductor (N IS) junction accompanied by the heat transfer from normal metal to supercon ductor is able to cool electrons in the normal metal electrode below the la ttice temperature under certain conditions. The tunneling electric current dependence on biasing voltage at different n ormal metal electrode electron temperatures is calculated. Refrigerators wi th one and with two NIS junctions are investigated. Further, the cooling power of SNIS and SINIS refrigerators, its dependence on biasing voltage and on the temperature of normal metal electrode electro ns are investigated. With reduced insulator layer thickness the resistivity of a NIS junction de creases. Thus, the cooling power of a refrigerator increases. But simultane ously the probability for Andreev reflections which diminish the efficiency of the refrigerator increases. Therefore, the optimal thickness of the ins ulator layer needs to be found. A method for calculating an optimal insulat or layer thickness based on the introduction of a new function that needs t o be minimized is described. Geometrical limits on the influence of Andreev reflection are determined.