Mixing and noise in diffusion and phonon cooled superconducting hot-electron bolometers

We report a systematic, comprehensive set of measurements on the dynamics a nd noise processes in diffusion and phonon-cooled superconducting hot-elect ron bolometer mixers which will serve as ultralow noise detectors in THz he terodyne receivers. The conversion efficiency and output noise of devices o f varying lengths were measured with radio frequency between 8 and 40 GHz. The devices studied consist of 100-Angstrom-thin film Nb bridges connected to thick (1000 Angstrom), high conductivity normal metal (Au) leads. The le ngths of the devices studied range from 0.08 to 3 mm. For devices longer th an the electron-phonon interaction length Le-ph = root D tau(e-ph), with D the diffusion constant and tau(e-ph)(-1) the electron-phonon interaction ra te, the hot electrons are cooled dominantly by the electron-phonon interact ion, which in Nb is too slow for practical applications. If the device leng th is less than pi Le-ph (approximate to 1 mu m at 4.2 K), then out diffusi on of heat into the high conductivity leads dominates the cooling process. In this limit, the intermediate frequency (IF) bandwidth is found to vary a s L-2, with L the bridge length, as expected for diffusion cooling. The sho rtest device has an IF bandwidth greater than 6 GHz, the largest reported f or a low-T-c superconducting bolometric mixer. The dominant component of th e output noise decreases with frequency in the same manner as the conversio n efficiency, consistent with a model based on thermal fluctuations. The no ise bandwidth is larger than the gain bandwidth, and the mixer noise is low , ranging from 100 to 530 K (double sideband). The crossover from phonon do minated to diffusion dominated behavior is also demonstrated using noise th ermometry measurements in the normal state. Scalar measurements of the devi ce differential impedance in the intermediate state agree with a theoretica l model which takes into account the thermal and electrical dynamics. We al so present detailed comparisons with theoretical predictions of the output noise and conversion efficiency. (C) 1999 American Institute of Physics. [S 0021-8979(99)08602-8].