Direct Measurement of Superconducting Tunnel Junction Capacitance

Superconductor-insulator-superconductor (SIS) junction is the key component for millimeter and submillimeter mixers for radio astronomy and environmental science. The capacitance of the SIS mixer determines both the RF and IF performance. Previously, measurements of this capacitance has had high uncertainty. Herein, we determine the SIS junction capacitance at cryogenic temperature ( ~ 4 K) by direct measurement of the SIS junction impedance at microwave frequencies. The proposed calibration method uses only one short-circuit reference. The SIS junction capacitance measurement is realized by biasing the junction at the different parts of its current-voltage characteristic, thus eliminating a separate measurement of short-circuit standard. In order to verify the measurement results, thin-film capacitors with known capacitance were also measured. The capacitance of four SIS junctions with various areas were measured. The absolute uncertainty of the proposed measurement method was found to vary from 5 to 6.8% amongst different junction areas.