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Gao, J. R., Hajenius, M., Yang, Z. Q., Baselmans, J. J. A., Khosropanah, P., Barends, R., et al. (2007). Terahertz superconducting hot electron bolometer heterodyne receivers. IEEE Trans. Appl. Supercond., 17(2), 252–258.
Abstract: We highlight the progress on NbN hot electron bolometer (HEB) mixers achieved through fruitful collaboration between SRON Netherlands Institute for Space Research and Delft University of Technology, the Netherlands. This includes the best receiver noise temperatures of 700 K at 1.63 THz using a twin-slot antenna mixer and 1050 K at 2.84 THz using a spiral antenna coupled HEB mixer. The mixers are based on thin NbN films on Si and fabricated with a new contact-process and-structure. By reducing their areas HEB mixers have shown an LO power requirement as low as 30 nW. Those small HEB mixers have demonstrated equivalent sensitivity as those with large areas provided the direct detection effect due to broadband radiation is removed. To manifest that a HEB based heterodyne receiver can in practice be used at arbitrary frequencies above 2 THz, we demonstrate a 2.8 THz receiver using a THz quantum cascade laser (QCL) as local oscillator.
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Baselmans, J. J. A., Baryshev, A., Reker, S. F., Hajenius, M., Gao, J. R., Klapwijk, T. M., et al. (2005). Direct detection effect in small volume hot electron bolometer mixers. Appl. Phys. Lett., 86(16), 163503 (1 to 3).
Abstract: We measure the direct detection effect in a small volume (0.15μm×1μm×3.5nm)(0.15μm×1μm×3.5nm) quasioptical NbN phonon cooled hot electronbolometermixer at 1.6THz1.6THz. We find that the small signal sensitivity of the receiver is underestimated by 35% due to the direct detection effect and that the optimal operating point is shifted to higher bias voltages when using calibration loads of 300K300K and 77K77K. Using a 200GHz200GHzbandpass filter at 4.2K4.2K the direct detection effect virtually disappears. This has important implications for the calibration procedure of these receivers in real telescope systems.
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Barends, R., Hajenius, M., Gao, J. R., & Klapwijk, T. M. (2005). Current-induced vortex unbinding in bolometer mixers. Appl. Phys. Lett., 87, 263506 (1 to 3).
Abstract: We present a description of the current-voltage characteristics of hot electron bolometers in terms of the current-dependent intrinsic resistive transition of NbN films. We find that, by including this current dependence, we can correctly predict the complete current-voltage characteristics, showing excellent agreement with measurements for both low and high bias and for small as well as large devices. It is assumed that the current dependence is due to vortex-antivortex unbinding as described in the Berezinskii–Kosterlitz–Thouless theory. The presented approach will be useful in guiding device optimization for noise and bandwidth.
Keywords: HEB mixer numerical model, HEB model, IV-curves, vortex-antivortex, Berezinskii–Kosterlitz–Thouless theory, diffusion cooling channel, diffusion channel, distributed HEB model, distributed model, self-heating effect, temperature profile
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Baselmans, J. J. A., Baryshev, A., Hajenius, M., Gao, J. R., Klapwijk, T. M., Voronov, B., et al. (2006). Influence of the direct response on the heterodyne sensitivity of hot electron bolometer mixers. In Proc. 17th Int. Symp. Space Terahertz Technol. (81).
Abstract: We present a detailed experimental study of the direct detection effect in a small volume (0.15pm x lpm) NbN hot electron bolometer mixer. It is a quasioptical mixer with a twin slot antenna designed for 700 GHz and the measurement was done at a LO frequency of 670 GHz. The direct detection effect is characterized by a change in the mixer bias current when switching broadband radiation from a 300 K hot load to a 77 K cold load in a standard Y factor measurement. The result is, depending on the receiver under study, an increase or decrease in the receiver noise temperature. We find that the small signal noise temperature, which is the noise temperature that would be observed without the presence of the direct detection effect, and thus the one that is relevant for an astronomical observation, is 20% lower than the noise temperature obtained using 300 K and 77 K calibration loads. Thus, in our case the direct detection effect reduces the mixer sensitivity. These results are in good agreement with previous measurement at THz frequencies [1]. Other experiments report an increase in mixer sensitivity [2]. To analyze this discrepancy we have designed a separate set of experiments to find out the physical origin of the direct detection effect. Possible candidates are the bias current dependence of the mixer gain and the bias current dependence of the IF match. We measured directly the change in mixer IF match and receiver gain due to the direct detection effect. From these measurements we conclude that the direct detection effect is caused by a combination of bias current reduction when switching form the 77 K to the 300 K load in combination with the bias current dependence of the receiver gain. The bias current dependence of the receiver gain is shown to be mainly caused by the current dependence of the mixer gain. We also find that an increase in receiver sensitivity due to the direct detection effect is only possible if the noise temperature change due to the direct detection is dominated by the mixer-amplifier IF match. [1] J.J.A. Baselmans, A. Baryshev, S.F. Reker, M. Hajenius, J.R. Gao, T.M. Klapwijk, Yu.Vachtomin, S. Maslennikov, S. Antipov, B. Voronov, and G. Gol'tsman., Appl. Phys. Lett. 86, 163503 (2005). [2] S. Svechnokov, A. Verevkin, B. Voronov, E. Menschikov. E. Gershenzon, G. Gol'tsman, 9th Int. Symp. On Space THz. Techn., 45, (1999).
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Baselmans, J. J. A., Baryshev, A., Reker, S. F., Hajenius, M., Gao, J. R., Klapwijk, T. M., et al. (2006). Influence of the direct response on the heterodyne sensitivity of hot electron bolometer mixers. J. Appl. Phys., 100(8), 084510 (1 to 7).
Abstract: We present a detailed experimental study of the direct detection effect in a small volume (0.15μm×1μm×3.5nm) quasioptical NbN phonon cooled hot electron bolometer mixer at 673GHz. We find that the small signal noise temperature, relevant for an astronomical observation, is 20% lower than the noise temperature obtained using 300 and 77K calibration loads. In a separate set of experiments we show that the direct detection effect is caused by a combination of bias current reduction when switching from the 77 to the 300K
load in combination with the bias current dependence of the receiver gain. The bias current dependence of the receiver gain is shown to be mainly caused by the current dependence of the mixer gain.
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