Cryogenic readout integrated circuits for submillimeter-wave camera

Development of cryogenic readout circuits for superconducting tunnelling junction (STJ) direct detectors for submillimeter-wave is presented. Because of relatively large impedance of the STJs (∼10 MΩ in a dark condition), it requires readout preamplifier with low current noise. Therefore, we chose readout circuit based on semiconductor field effect transistors for STJ readout circuit rather than superconductive quantum interferences device. A SONY n-channel GaAs junction field effect transistor (JFET) is a candidate for circuit elements of the preamplifier. We measured electrical characteristics of the GaAs JFETs at 0.3 K, and found that the GaAs JFETs work with low power consumption of a few microwatts, and shows good current-voltage characteristics without cryogenic anomalies such as kink phenomena or hysteresis behaviours. Furthermore, measurements at 4.2 K show the input referred noise is as low as 0.5 μV/√HZ at 1 Hz. Based on these results and noise calculations, we estimate that a readout circuit with the GaAs JFETs has low noise and STJ detectors operate below background noise limit.