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Analysis of the Noise Performance of a Hot-Electron Superconducting Bolometer Mixer

Boris S. Karasik, A. I. Elant’ev

Published Mar 1, 1995• 29 citations
OpenAlex

Abstract

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.

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Noise and conversion efficiency of aluminum superconducting hot-electron bolometer mixer

Irfan A. Siddiqi, A. Verevkin, D. E. Prober +5 more

We report on microwave measurements of superconducting aluminum hot-electron bolometers (Al HEBs). Diffusion-cooled Al HEB mixers are good candidates for space-borne applications in the Terahertz frequency range since they are predicted to have small local oscillator (LO) power requirements, intermediate frequency (IF) bandwidths /spl gsim/10 GHz, and a noise temperature lower than that of Nb and NbN HEB mixers. Mixer measurements were made at an LO frequency /spl sim/30 GHz, with an IF in the range 0.1 to 7.3 GHz. For T<0.8 K, a magnetic field H=0.1-0.3 T was applied to suppress the superconductivity in the contact pads, and partly in the bridge. For a 0.6 /spl mu/m long device, we measure an IF bandwidth of 4 GHz, a conversion efficiency /spl eta/=-8 dB, and a mixer noise temperature T/sub M//spl gsim/4 K, DSB (T/sub mixer/=T/sub output noise//2/spl eta/). These results are shown to be in quantitative agreement with simple theoretical predictions.

IEEE Transactions on Applied Superconductivity•Mar 1, 2001•17 citations
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.
A theoretical analysis for the noise temperature of a low-temperature hot-electron superconducting mixer has been presented. The contribution of both Johnson noise and electron temperature fluctuations has been evaluated. The theoretical limit of the single-side band noise temperature of the device due to the intrinsic noise mechanisms has been estimated to be as low as 20–40 K, depending on the film material. The corresponding noise bandwidth can be as large as 2–3 GHz, for a device with the electron-phonon cooling mechanism. An improvement of the sensitivity is not necessarily followed by a decrease of the mixer conversion gain bandwidth.
We present theoretical analysis of the performance of a superconducting hot-electron bolometer mixer with electron cooling due to electron phonon interaction. The analysis is based on a complete set of thermal balance equations for phonons and electrons. A non-thermal action of a dc current has been considered phenomenologically using an analytical fit of isothermal current-voltage characteristics of the mixer. The operation point of the mixer has been determined as a result of the heating by both the power of a local oscillator and the power dissipated by the current. Electron temperature fluctuations, Nyquist noise, and shot noise have been supposed to only contribute to the mixer noise. The conversion gain and the noise temperature of the mixer have been evaluated taking into account an electro-thermal feedback between the mixer and an intermediate frequency circuit with an arbitrary impedance. We have obtained general expressions for the conversion gain, noise temperature and impedance which enabled us to estimate ultimate performance of a superconducting hot-electron bolometer mixer. Theoretical predictions are in good agreement with experimental results obtained for a NbN hot-electron bolometer mixer at a radiation frequency of 2.5 THz.
We report a systematic, comprehensive set of measurements on the dynamics and noise processes in diffusion and phonon-cooled superconducting hot-electron bolometer mixers which will serve as ultralow noise detectors in THz heterodyne receivers. The conversion efficiency and output noise of devices of varying lengths were measured with radio frequency between 8 and 40 GHz. The devices studied consist of 100-Å-thin film Nb bridges connected to thick (1000 Å), high conductivity normal metal (Au) leads. The lengths of the devices studied range from 0.08 to 3 μm. For devices longer than the electron–phonon interaction length Le–ph≡Dτe–ph, with D the diffusion constant and τe–ph−1 the electron–phonon interaction rate, the hot electrons are cooled dominantly by the electron–phonon interaction, which in Nb is too slow for practical applications. If the device length is less than πLe–ph(≈1 μm at 4.2 K), then out diffusion of heat into the high conductivity leads dominates the cooling process. In this limit, the intermediate frequency (IF) bandwidth is found to vary as L−2, with L the bridge length, as expected for diffusion cooling. The shortest device has an IF bandwidth greater than 6 GHz, the largest reported for a low-Tc superconducting bolometric mixer. The dominant component of the output noise decreases with frequency in the same manner as the conversion efficiency, consistent with a model based on thermal fluctuations. The noise 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 dominated to diffusion dominated behavior is also demonstrated using noise thermometry measurements in the normal state. Scalar measurements of the device differential impedance in the intermediate state agree with a theoretical model which takes into account the thermal and electrical dynamics. We also present detailed comparisons with theoretical predictions of the output noise and conversion efficiency.
A systematic study of the intermediate frequency noise bandwidth of Nb thin-film superconducting hot-electron bolometers is presented. We have measured the spectrum of the output noise as well as the conversion efficiency over a very broad intermediate frequency range (from 0.1 to 7.5 GHz) for devices varying in length from 0.08 μm to 3 μm. Local oscillator and rf signals from 8 to 40 GHz were used. For a device of a given length, the spectrum of the output noise and the conversion efficiency behave similarly for intermediate frequencies less than the gain bandwidth, in accordance with a simple thermal model for both the mixing and thermal fluctuation noise. For higher intermediate frequencies the conversion efficiency decreases; in contrast, the noise decreases but has a second contribution which dominates at higher frequency. The noise bandwidth is larger than the gain bandwidth, and the mixer noise is low, between 120 and 530 K (double side band).
An experimental study is presented of the output noise in hot-electron superconducting NbN bolometer mixers. Fluctuations of the electron temperature have been shown to be the main source of electrical noise. The rolloff frequency of the spectral noise density corresponds well to the inverse electron temperature relaxation time (≊230 ps). Equivalent noise data have been obtained for the device at a physical temperature within superconducting transition region, and at a somewhat lower physical temperature with the device pumped by 350 GHz radiation to reach the same operating point. The single sideband (SSB) mixer noise temperature due to intrinsic noise mechanisms for the NbN device is estimated to 360 K independent of radiation frequency. A larger critical current density may promote a further reduction of the intrinsic conversion loss of the mixer device.
We report on microwave measurements of superconducting aluminum hot-electron bolometers (Al HEBs). Diffusion-cooled Al HEB mixers are good candidates for space-borne applications in the Terahertz frequency range since they are predicted to have small local oscillator (LO) power requirements, intermediate frequency (IF) bandwidths /spl gsim/10 GHz, and a noise temperature lower than that of Nb and NbN HEB mixers. Mixer measurements were made at an LO frequency /spl sim/30 GHz, with an IF in the range 0.1 to 7.3 GHz. For T<0.8 K, a magnetic field H=0.1-0.3 T was applied to suppress the superconductivity in the contact pads, and partly in the bridge. For a 0.6 /spl mu/m long device, we measure an IF bandwidth of 4 GHz, a conversion efficiency /spl eta/=-8 dB, and a mixer noise temperature T/sub M//spl gsim/4 K, DSB (T/sub mixer/=T/sub output noise//2/spl eta/). These results are shown to be in quantitative agreement with simple theoretical predictions.