We report a systematic, comprehensive set of measurements on the dynamics and noise
processes in diffusion and phonon-cooled superconducting hot-electron bolometer mixers
which will serve as ultralow noise detectors in THz heterodyne receivers. The conversion
efficiency and output noise of devices of varying lengths were measured with radio
frequency between 8 and 40 GHz. The devices studied consist of 100-Å-thin film Nb bridges
connected to thick (1000 Å), high conductivity normal metal (Au) leads. The lengths of the …