The objectives of this study are to assess the utility of the high‐Tc superconductor, yttrium barium copper oxide (YBCO), as a gate material in two‐ and three‐terminal superconductor‐insulator‐semiconductor (SulS) devices, and to study the electrical properties of the insulator and the insulator/Si interface. The YBCO and yttria‐stabilised‐zirconia (YSZ) layers were epitaxially grown on Si by pulsed‐laser deposition. The SulS diodes were fabricated using standard lithographic techniques, with evaporated gold providing the gate and substrate contacts. Electrical characterisation of these superconducting devices is performed using current vs. voltage and capacitance vs. voltage (C‐V) measurements under bias‐temperature cycling. It is found that deposition of thicker YBCO films (≥ 1500 A) minimises the leakage current of the devices, resulting in electrically stable capacitors, especially at superconducting temperatures. A thermally activated process in the temperature range 80–295 K, as determined from flat‐band shifts of C‐V curves, is attributed to trapping/detrapping mechanisms in the SiOx interfacial layer between YSZ and Si. The mobile ions present in YSZ, which affect the room‐temperature C‐V behaviour, give rise to adjustable threshold voltages at superconducting temperatures. These findings will have a significant impact on future transistors using this capacitor as the gate structure.