Eighth International Symposium on Space Terahertz Technology, Harvard University, March 1997 Superconducting Transition and Heterodyne Performance at 730 GHz of a Diffusion-cooled Nb Hot-electron Bolometer Mixer J.R. Gao a.5 , M.E. Glastra a , R.H. Heeres a , W. Hulshoff h , D. Wilms Floeta, H. van de Stadt b , T.M. Klapwijk a and Th. de Graauwb a Department of Applied Physics and Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands 5 Space Research Organization of the Netherlands, PO Box 800, 9700 AV Groningen The Netherlands Abstract We report two typical experimental results of waveguide diffusion-cooled Nb hot- electron bolometer (HEB) mixers. The device is a thin (10 nm) Nb bridge with a length of 0.3 Am and a width of 0.9 pm and is defined using photolithography. In the first experiment, the resistance of a HEB is studied as a function of temperature. We observe two critical temperatures, which correspond to the 71. of the Nb bridge and the T, of the Nb under the Au pads, respectively. These two 71. define a superconducting transition width AT of a HEB. The reduced T. of the Nb under the Au pads is explained by the superconducting proximity effect. In the second experiment, we measure relative conversion efficiency vs intermediate frequency (IF) by using two coherent sources, a carcinotron and a Gunn oscillator, and determine an IF roll-off (/F, 011 , ff ) between 0.6 and 1.2 GHz, in agreement with a value estimated in terms of the diffusion-cooled model. 1. Introduction Recent work on diffusion-cooled Nb hot-electron bolometer ( HEB) mixers reported by the Jet Propulsion Laboratory and other groups' has demonstrated that they are promising heterodyne detectors in the THz frequency range. It has been shown that HEB mixers have low noise and a reasonable wide intermediate frequency roll-off (/F roll ,, ff ), but no clear upper limit of operating frequency and no requirement of high local oscillator (LO) power. HEB mixers are expected to compete with NbN SIS mixers around 1.5 THz, but to be superior at much higher frequencies. It is known that coupling a signal at radio frequency (RF) and a signal from a LO to a superconducting HEB can generate a response at IF frequency, provided that the superconductor is biased near the middle point of the transition due to the heating of LO and DC power. So a superconducting HEB can be operated as a mixer. For a practical HEB mixer, it is required that an /F roll.. „ ff , defined as the IF frequency at which the relative conversion efficiency decreases by 3 dB, should be 1.5 Gliz. A diffusion-cooled Nb HEB suggested by Prober' can satisfy this requirement because the hot electron are cooled via out-diffusion, but not via electron-phonon process. This new cooling process can result in a shorter thermal response time and thus a higher /F roll _ off . A typical diffusion-cooled HEB consists of a thin ( — 10 nm) and narrow ( 100 am) superconducting Nb bridge, which is attached to two Au pads serving as heat sinks. The separation of the two pads defines the bridge. A short bridge ( — 200 nm) is needed to ensure a fast out-diffusion 36