Superconductor insulator superconductor (SIS) mixers are used in heterodyne receivers for radio-astronomical observations. The high frequency resolution of the heterodyne technique combined with the high sensitivity of SIS mixers enables high resolution spectroscopy of the interstellar medium. This work focuses on the development of a novel superconductor insulator superconductor (SIS) mixer technology for 800 GHz that could in the future be used up to 1.1 THz. This is an important part of the THz spectrum that is still accessible for astronomy from ground based observatories. Compared to the existing mixers in this frequency regime the sensitivity shall be improved by the use of fully superconducting radio-frequency (RF) matching circuits. The main innovation in this work is the inclusion of an energy relaxation layer made of gold that enables the use of these fully superconducting RF circuits in combination with the mature niobium SIS junction technology. The energy relaxation layer prevents heating of the SIS junction due to Andreev re¿ection at the interface between the junction electrode and the matching circuit transmission line which was observed in earlier developments. This work includes the development of a fabrication process as well as extensive DC and RF measurements and their interpretation.