Conversion efficiency and noise properties of high-temperature superconductive mixers

Electronic devices with nonlinear current-voltage characteristics can be used as mixers in receivers for frequency downconversion. Mixers based onsuperconductors show high conversion efficiencies and low noise. They are widely used especially in receivers for the millimeter and submillimeter wavelengthrange at frequencies larger than 100 GHz to detect very weak signals, e.g. in the field of radioastronomy or molecular spectroscopy. Up-to-datesuperconducting mixer devices are mainly made from conventional, metallic superconductors like niobium. This material requires cooling to temperaturesaround 4 K, while, in contrast, mixers based on high-temperature superconductors (HTS) promise operation at much higher operating temperatures.This thesis reports on mixers based on the high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{7_x}$ (YBCO). The first mixer type utilizes the Josephsoneffects between two weakly coupled superconducting electrodes. The second type of mixer utilizes bolometric effects for frequency downconversion. Thisthesis describes the fabrication of HTS mixers, their DC and RF characterization, and the modeling of their physical properties.For the Josephson mixers, single step-edge and bicrystal junctions on MgO and sapphire substrates were fabricated, and junctions integrated withplanar antenna were characterized at frequencies between 90 GHz and 550 GHz. The measured noise temperatures at around 550 GHz were comparableto those of semiconductor Schottky mixers, but at much lower local oscillator power levels. The results show that the Josephson mixer noise is about twoto four times higher than models predicts. The second part of this thesis is devoted to the hot-electron bolometer (HEB) mixer. In HEB mixers, the electron gas of a thin, superconductingfilm is utilized as the absorber, and the superconductine phase transition serves as source of nonlinearity. This mixer type is especially attractive fordetection of signals in the THz frequency range. First, the thermal relaxation times in thin HTS films on substrates withhigh thermal conductivity like MgO and sapphire were determined. Then, their film-thickness, frequency and temperature dependences were measuredand compared with predictions based on the two-temperature model. It was shown that no effective phonon diffusion to the normal metal electrodestakes place and that the dominant relaxation process was the escape of the phonons through the film/substrate interface into the substrate. Additionally,unexpected effects of the operating temperature; the bias current, and the operating temperature, the bias current, and the operating frequency on the effective relaxation time were observed