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IEEE Transactions on Applied Superconductivity

Resistive states of superconducting hot-electron bolometer mixers: charge-imbalance vs. hotspot

D. Wilms Floet, E. Miedema, J. J. A. Baselmans, T. M. Klapwijk, J. R. Gao

Published Jun 1, 1999• 6 citations
Publisher

Abstract

We present a study of the resistive behavior of hot-electron bolometers in two different regimes. At bath temperatures near the critical temperature and under the condition of a low current density, the behavior can be understood in terms of charge-imbalance, Andreev reflection, and the superconducting proximity effect. At lower temperatures, a high current density and the application of LO power result in the formation of a resistive electronic hotspot. We propose to interpret heterodyne downconversion as the result of the length oscillation of the hotspot at the intermediate frequency.

DOI
10.1109/77.783843

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We present a study of the resistive behavior of hot-electron bolometers in two different regimes. At bath temperatures near the critical temperature and under the condition of a low current density, the behavior can be understood in terms of charge-imbalance, Andreev reflection, and the superconducting proximity effect. At lower temperatures, a high current density and the application of LO power result in the formation of a resistive electronic hotspot. We propose to interpret heterodyne downconversion as the result of the length oscillation of the hotspot at the intermediate frequency.
We present a model for the description of the resistive transition in hot-electron bolometer mixers. We show that the transition is a property of a superconducting microbridge connected to normal conducting cooling pads. Using the concepts of the superconducting proximity effect, charge-imbalance generation, and Andreev reflection, we have calculated the resistance versus temperature of the device and demonstrate its dependence on the length of the microbridge, both theoretically and experimentally. The analysis reopens the question of the relationship between the resistive transition and the situation in which the device is optimally operated as a heterodyne mixer.
We present the hotspot mixing as a new principle of heterodyne mixing in a superconducting hot-electron bolometer. Applying DC and LO powers to a superconducting bridge generates a hotspot within the bridge. The size of the hotspot, and thus the resistance of the bridge, oscillates with the heating power at intermediate frequency. By solving heat-conducting equations, we can predict the mixer performances, such as the conversion efficiency, as a function of dc bias voltage, LO power, or the bath temperature.
Abstract — A hot spot mixer model for phonon cooled hot electron bolometers is set up to accurately determine the dependence of the bolometer resistance on bias and absorbed LO power. An one-dimensional power balance for the nonequilibrium electrons containing an electron heat conduction term along the film and a loss term due to phonon escape to the substrate is solved. In the device's operating region of interest the absorbed heating power is usually large enough to heat a part of the bolometer bridge above the critical temperature forming a normal conducting hot spot. In this model heating due to the LO is assumed to be uniform whereas bias heating takes place in the hot spot region only. Solving the nonlinear power balance for the hot spot length allows to predict current-voltage characteristics, required heating powers and regions of optimal operation. Applying a small signal model in a given operating point yields an estimate for the intrinsic conversion gain. I. LARGE SIGNAL MODEL Existing device models for hot electron bolometers [1,2] treat the bolometer as a lumped element. It is assigned an effective temperature for the phonons and another for the electrons at a given bath temperature. In terms of heating bias power and RF power are equivalent and exchangeable. From this a method [3,4] to calculate the absorbed RF power is derived. This method is referred to as "isotherm " method: Operating points with identical device resistance must have identical temperatures since R(To„,,(,,, ) is a single valued monotonous function. Identical temperature implies identical power losses and identical 592
Superconducting hot electron bolometer (HEB) mixers are the most sensitive heterodyne detectors at frequencies beyond 1 THz, and are used for many astronomical observations. HEB, consisting of a narrow and short superconducting thin film patch connected to an antenna via contact pads, is brought into a highly sensitive operating regime by applying a DC current and a certain amount of local-oscillator power. Potentially, superconducting HEB mixers are capable of exhibiting high sensitivity for frequencies up to several tens of THz, as their output noise and conversion gain are nearly frequency independent according to the mixing mechanism. Moreover, the understanding of mixing mechanism has evolved over time from lumped-element model to distributed hot-spot model. In this review, we provide an overview of theoretical and experimental studies on HEBs and their applications, especially in astronomical observations on ground, airborne and space telescopes.
Hot-electron bolometer devices, used successfully in low noise heterodyne mixing at frequencies up to 2.5 THz, have been analyzed. A distributed temperature numerical model of the NbN bridge, based on a local electron and a phonon temperature, is used to model pumped IV curves and understand the physical conditions during the mixing process. We argue that the mixing is predominantly due to the strongly temperature dependent local resistivity of the NbN. Experimentally we identify the origins of different transition temperatures in a real HEB device, suggesting the importance of the intrinsic resistive transition of the superconducting bridge in the modeling.