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Superconductor Science and Technology

Superconductive amplifying devices using fluxon dynamics

J.E. Nordman

Published Sep 1, 1995• 31 citations
Publisher

Abstract

Control of the creation and the motion of vortices in Josephson junctions and in superconductive thin films is the basis of operation for a class of amplifying devices, usually called flux-flow transistors. We describe and compare electrical characteristics of these structures and some related devices.
DOI
10.1088/0953-2048/8/9/001

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Research during the initial year of this grant concentrated on fabrication and modelling of long Josephson junction structures. A particular effort was made to understand the basic operating mechanisms of a type of superconducting transistor, the vortex flow transistor, and to optimize its circuit performance. Fabrication techniques were developed using niobium and lead superconductors. These techniques allow rapid production of a variety of device shapes. An optimization of geometric configuration was made to produce the most desirable device characteristics. Amplification was observed at low frequencies and initial coupling circuit designs were made to enable high frequency measurement. Development of potentially more versatile fabrication techniques using niobium and niobium nitride was also begun, as was a simulation study aimed primarily at understanding spatially nonuniform long junction structures.

Final Report•Feb 1, 1989•0 citations
The operational characteristics and physics of three superconducting thin-film-based transistor structures are compared. The devices are based on the motion of quantized vortices, either Josephson fluxons in a long tunnel junction of Abrikosov fluxons in a superconducting film. The transistor amplification mechanism is accomplished by controlling magnetic field at the boundaries of the structures. An overview of the present understanding of device mechanisms and measured characteristics, including voltampere relations and small and large signal circuit parameters is provided. Demonstrated applications and anticipated limitations are discussed.
Josephson fluxonic bipolar junction transistor (JFBJT) has been proposed as a superconducting amplifying element. The results on a simple realization of this device are provided to demonstrate its working principle. These experiments demonstrate that JFBJT provides current gain without suffering from the problems associated with the regular long junction vortex flow transistors. In particular, no resonance structure is present in its I-V characteristics and its impedance can be matched to any load without degradation of its characteristics. Suggestion is made on how a practical device can be fabricated.
The dynamics of fluxons in the vortex flow transistor (VFT) has been much studied. The fluxon transit time determines the fundamental speed limit of operation. Since fluxons can travel at the velocity of electromagnetic waves in the junction, the VFT has the potential for operating in high frequency systems ( ≈ 100 GHz). However, owing to the low input and output impedance of the VFT, use of the device in a conventional circuit would be quite limited. A distributed amplifier configuration consisting of many VFTs has been proposed to remedy the problems of low impedance levels. However, the realization of such an amplifier circuit at microwave or millimetre wave frequencies depends on obtaining a circuit model. In this paper, microwave superconducting VFT distributed amplifiers using the balanced control technique is reviewed. This kind of amplifier has the advantage that the capacitive feedthrough effect is decreased to a negligible extent. This is the major limiting factor for high frequency applications. The self-field effect which makes the current step inclined is reduced by injecting bias current only around the region of one end of the junction thus obtaining steeper I- V characteristics. With the asymmetric geometry, the slope of the current step is about one hundred times steeper than those obtained with the conventional overlap geometry. Owing to the diamagnetic behaviour of the Josephson junction, a little of the magnetic field induced by the current in the control line is allowed to penetrate the junction. Thus, the transresistance rm of a VFT is very small. Some methods for maximizing rm and minimizing the output impedance r0 also appear in this review. The feasibility of fabricating VFT distributed amplifiers using low-Tc superconducting material has been demonstrated. The power gain of the amplifier can be as high as 15 dB with a flat frequency response.
We provide two qualitative models for the operation of Josephson fluxon–antifluxon transistor based on the pendulum model and image currents induced in the junction. This device, which is a variant of a Josephson vortex flow transistor, utilizes high temperature superconductor long Josephson junctions with a control line on top of the junction. Our models are consistent with the experimental observation that the coupling efficiency increases with this geometry. We also provide a quantitative model using numerical simulations to confirm the static and dynamic characteristics predicted by the intuitive models.
Superconducting flux flow transistors (SFFTs) have recently been used in a W-band amplifier operating from 91 to 98 GHz and in an invertor configured as an oscillator operating at 86 GHz. These SFFTs consist of three weakened YBaCuO links in parallel. The links are about 1- mu m wide, 3- mu m long, and 20-nm thick (relative to an electrode thickness of about 200 nm). The links are biased above their local critical field so that vortices flow through the links in accordance with the Lorentz-type force supplied by the bias. The 'gate' is a control line located proximate to the link system and supplies a local magnetic field. Above a critical current, flux flow begins and the device enters the active region characterized by a transresistive response to control current and a finite output resistance. A flux flow invertor circuit can consist of 2 SFFTs: both devices are in parallel with the current bias and the input is a control current to one device.
Research during the initial year of this grant concentrated on fabrication and modelling of long Josephson junction structures. A particular effort was made to understand the basic operating mechanisms of a type of superconducting transistor, the vortex flow transistor, and to optimize its circuit performance. Fabrication techniques were developed using niobium and lead superconductors. These techniques allow rapid production of a variety of device shapes. An optimization of geometric configuration was made to produce the most desirable device characteristics. Amplification was observed at low frequencies and initial coupling circuit designs were made to enable high frequency measurement. Development of potentially more versatile fabrication techniques using niobium and niobium nitride was also begun, as was a simulation study aimed primarily at understanding spatially nonuniform long junction structures.