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IEEE Transactions on Applied Superconductivity

Bandwidth of a hot-electron bolometer mixer according to the hotspot model

Alexei D. Semenov, Heinz‐Wilhelm Hübers

Published Mar 1, 2001• 22 citations
Publisher

Abstract

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.

DOI
10.1109/77.919318

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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.
We demonstrate that the performance of NbN lattice cooled hot electron bolometer mixers depends strongly on the interface quality between the bolometer and the contact structure. We show experimentally that both the receiver noise temperature and the gain bandwidth can be improved by more than a factor of 2 by cleaning the interface and adding an additional superconducting interlayer to the contact pad. Using this we obtain a double sideband receiver noise temperature TN,DSB=950 K at 2.5 THz and 4.3 K, uncorrected for losses in the optics. At the same bias point, we obtain an IF gain bandwidth of 6 GHz.
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 report a record double sideband noise temperature of 600 K (5hν/kB) offered by a NbN hot-electron bolometer receiver at 2.5 THz. Allowing for standing wave effects, this value was found to be constant in the intermediate frequency range 1–7 GHz, which indicates that the mixer has an unprecedentedly large noise bandwidth in excess of 7 GHz. The insight into this is provided by gain bandwidth measurements performed at the superconducting transition. They show that the dependence of the bandwidth on the mixer length follows the model for an HEB mixer with diffusion and phonon cooling of the hot electrons.
We demonstrate that the performance of NbN lattice cooled hot electron bolometer mixers depends strongly on the interface quality between the bolometer and the contact structure. Both the receiver noise temperature and the gain bandwidth can be improved by a factor of 2 by cleaning the interface and adding an additional superconducting interlayer to the contact pad. Using this we obtain a double sideband receiver noise temperature of 950 K at 2.5 THz and 4.3 K, using a 0.4/spl times/4 /spl mu/m HEB mixer with a spiral antenna. At the same bias point, we obtain an IF gain bandwidth of 6 GHz. To comply with current demands on THz mixers for use in space based receivers we reduce the device size to 0.15/spl times/1 /spl mu/m and use a twin slot antenna. We report measurements of the noise temperature, LO power requirement, stability and the direct detection effect, using a mixer with a 1.6 THz twin slot antenna and a 1.462 THz solid state LO source with calibrated output power.
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.
In this paper, we investigate the critical temperature dependence of the performance of niobium nitride superconducting hot electron bolometer (HEB) mixers. The input noise temperature and intermediate frequency (IF) noise bandwidth of superconducting HEB mixers of different critical temperatures Tc(between 6 and 12 K) are simulated with a hot spot model. Simulation results show that the mixer input noise temperature is the lowest at Tcapproximately equal to 7-9.5 K and the IF noise bandwidth increases with Tc. We also measure three superconducting HEB mixers of different critical temperatures (i.e., 7.5, 8.8, and 10.3 K) at 0.85 and 1.3 THz. An agreement between measurement and prediction is observed.