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Applied Physics Letters

Bias-dependence of the thermal time constant in diffusion-cooled superconducting hot-electron bolometer mixers

D. Wilms Floet, T. M. Klapwijk, J. R. Gao, Piet A. J. de Korte

Published Sep 11, 2000• 9 citations
Publisher

Abstract

We present an experimental study of the intermediate frequency bandwidth of a Nb diffusion-cooled hot-electron bolometer mixer for different bias voltages. The measurements show that the bandwidth increases with increasing voltage. Analysis of the data reveals that this effect is mainly caused by a decrease of the intrinsic thermal time of the mixer and that the effect of electrothermal feedback through the intermediate frequency circuit is small. The results are understood using a qualitative model, which takes into account the different effective diffusion constants in the normal and superconducting domains.

DOI
10.1063/1.1287909

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IEEE Transactions on Applied Superconductivity•Mar 1, 2001•22 citations
We present an experimental study of the intermediate frequency bandwidth of a Nb diffusion-cooled hot-electron bolometer mixer for different bias voltages. The measurements show that the bandwidth increases with increasing voltage. Analysis of the data reveals that this effect is mainly caused by a decrease of the intrinsic thermal time of the mixer and that the effect of electrothermal feedback through the intermediate frequency circuit is small. The results are understood using a qualitative model, which takes into account the different effective diffusion constants in the normal and superconducting domains.
We present an experimental study of the intermediate frequency bandwidth of a Nb diffusion-cooled hot-electron bolometer mixer for different bias voltages. The measurements show that the bandwidth increases with increasing voltage. Analysis of the data reveals that this effect is mainly caused by a decrease of the intrinsic thermal time constant of the mixer and that the effect of electrothermal feedback through the intermediate frequency circuit is small. The results are understood using a qualitative model, which takes into account the different effective diffusion constants in the normal and superconducting domains.
Response time of a Superconducting Hot Electron Bolometer (HEB) mixers is a critical parameter, and significantly affects the intermediate frequency (IF) gain bandwidth. In this study, we investigated the effects of temperature and bias voltage on the response time. Response time of superconducting niobium nitride (NbN) HEB mixers were measured by using the pumpprobe technique at temperatures of 5 K, 7 K, and 9 K with different bias voltages. The measured response time ranged from 50 to 80 ps and decreased with increasing temperature and bias voltage. This approach provides a more comprehensive understanding of the thermal transport mechanisms in superconducting HEB mixers, which can ultimately lead to improved performance and properties of the HEB mixers.
A new theoretical model is introduced to describe heterodyne mixer conversion efficiency and noise (from thermal fluctuation effects) in diffusion-cooled superconducting hot-electron bolometers. The model takes into account the non uniform internal electron temperature distribution generated by Wiedemann Franz heat conduction, and accepts for input an arbitrary (analytical or experimental) superconducting resistance-versus-temperature curve. A non-linear large signal solution is solved iteratively to calculate the temperature distribution, and a linear frequency-domain small-signal formulation is used to calculate conversion efficiency and noise. In the small-signal solution the device is discretized into segments, and matrix algebra is used to relate the heating modulation in the segments to temperature and resistance modulations. Matrix expressions are derived that allow single-sideband mixer conversion efficiency and coupled noise power to be directly calculated. The model accounts for self-heating and electrothermal feedback from the surrounding bias circuit.
A theoretical analysis for the noise temperature of hot—electron superconducting mixer has been presented. The contributions of both Johnson noise and electron temperature fluctuations have been evaluated. A set of criteria ensuring low noise performance of the mixer has been stated and a simple analytic expression for the noise temperature of the mixer device has been suggested. It has been shown that an improvement of the mixer sensitivity does not necessarily follow by a decrease of the bandwidth. An SSB noise temperature limit due to the intrinsic noise mechanisms has been estimated to be as low as 40-90 K for a mixer device made from Nb or NbN thin film. Furthermore, the conversion gain bandwidth can be as wide as is allowed by the intrinsic electron temperature relaxation time if an appropriate choice of the mixer resistance has been made. The intrinsic mixer noise bandwidth is of 3 GHz for Nb device and of 5 GHz for NbN device. An additional improvement of the theory has been made when a distinction between the impedance measured at high intermediate frequency (larger than the mixer bandwidth) and the mixer ohmic resistance has been taken into account. Recently obtained experimental data on Nb and NbN bolometer mixer devices are viewed in connection with the theoretical predictions. The noise temperature limit has also been specified for the mixer device where an outdiffusion cooling mechanism rather than the electron—phonon energy relaxation determines the mixer bandwidth. A consideration of the noise performance of a bolometer mixer made from YBaCuO film utilizing a hot—electron effect has been done.
The hotspot model has been used to simulate the intermediate frequency bandwidth of a lattice cooled NbN hot-electron bolometer mixer. The results of simulation suggest that with the increase of the bias current, the bandwidth decreases, passes the minimum, and recovers again. The appearance of the bandwidth minimum correlates with experimentally observed noise maximum at the mixer output that drastically reduces the sensitivity of a relevant receiver. At large bias currents the bandwidth exceeds the reciprocal electron cooling time. Since the noise temperature also increases with the bias current, a compromise between the desirable bandwidth and acceptable noise temperature can be reached by varying the bias current.