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IEEE Transactions on Applied Superconductivity

Thermal time constant of Nb diffusion-cooled superconducting hot-electron bolometer mixers

D. Wilms Floet, J. R. Gao, T. M. Klapwijk, P.A.J. de Eorte

Published Mar 1, 2001• 2 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 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.

DOI
10.1109/77.919316

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andwidth of diffusion-cooled hot-electron bolometers

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noise results have already been acheived[2] (TR~~~~~~,. = 650 K DSB at 530 GHz). The intermediate frequency (IF) guan bandwidth was a limitation for past bolometric mixers. We recently showed that the IF gain bandwidth can be as large as 6 GHz when the device length is short enough that diffusion-cooling of hot electrons out the ends dominates over phonon-emission as the cooling mechanism[3]. (NbN phonon-cooled mixers can operate with an IF to a few GHz[4] .) The IF nozse bandwidth has not been extensively studied in diffusion cooled mixers; in this paper we present a study for one such device. The main limitation for any bolometric mixer is that the IF guan bandwzdth (defined as the IF frequency at which the conversion efficiency drops by 3dB from its low frequency value) is limited by the thermal time-constant 7th. The conversion efficiency obeys the functional form where the 3dB gain bandwidth is given by fgain3dB = 1/2n~,h[5]. The dominant intrinsic noise source for a well optimized superconducting bolometric mixer is predicted

•Jan 1, 1997•0 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 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.
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.
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 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.
noise results have already been acheived[2] (TR~~~~~~,. = 650 K DSB at 530 GHz). The intermediate frequency (IF) guan bandwidth was a limitation for past bolometric mixers. We recently showed that the IF gain bandwidth can be as large as 6 GHz when the device length is short enough that diffusion-cooling of hot electrons out the ends dominates over phonon-emission as the cooling mechanism[3]. (NbN phonon-cooled mixers can operate with an IF to a few GHz[4] .) The IF nozse bandwidth has not been extensively studied in diffusion cooled mixers; in this paper we present a study for one such device. The main limitation for any bolometric mixer is that the IF guan bandwzdth (defined as the IF frequency at which the conversion efficiency drops by 3dB from its low frequency value) is limited by the thermal time-constant 7th. The conversion efficiency obeys the functional form where the 3dB gain bandwidth is given by fgain3dB = 1/2n~,h[5]. The dominant intrinsic noise source for a well optimized superconducting bolometric mixer is predicted