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Influence Of Junction-Quality And Current Density On HIFI Band 2 Mixer Performance

Authors:
R. Teipen, M. Justen, T. Tils, S. Glenz, C. E. Honingh, K. Jacobs
Abstract:
The current density of Nb/Al203/Nb SIS junctions that are used as sub-millimeter wave mixers is a determining parameter in the device design. Integrated tuning circuits for single junction designs typically have a fractional bandwidth of 1/[omega]RC, with [omega] the angular frequency, R the junction normal state resistance and C the junction capacitance. As RC~1/jc (jc is the current density), for higher frequencies and/or larges bandwidths one would like to use a devices with higher current density. Admittedly large bandwidth can also be achieved by parallel arrays of junctions with a lower current density. However a higher jc, will also improve the coupling for tuning circuits with normal metal lines that have some of loss, as are used at frequencies above the gap frequency of niobium (700 GHz). In addition at higher frequencies and for certain applications like array- or satellite mixers, one wants to avoid unnecessary complexity in the operation of the mixer, for example in tuning the magnetic field for an array of junctions. For higher current densities it is observed that junctions start to exhibit a lower quality, expressed in the ratio of the sub-gap resistance (Rsub_gap) and the normal state resistance (RN). This is attributed to an increasing number of imperfections in the barrier (pinholes). Dieleman has shown that the additional sub-gap current through the pin-holes leads to a disproportionately high additional shot-noise generation in the SIS junction, because the charge transport through the pinholes is multiple Andreev reflections (MAR). The increase in current density at the expense of quality leads to a trade off between improved coupling and higher junction noise in the optimization of a mixer design. For future design decisions it is important to obtain mixer data to verify the predictions of performance by the Quantum theory of mixing (QTM) that take into account the MAR in the barrier.
Categories:
SIS Mixers
Year:
2003
Session:
2
Full-text:
Download a PDF of this paper.
Page Number(s):
55-62