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Journal of Applied Physics

Effects of finite junction length on the vortex-flow transistor

James H. Thompson, James B. Beyer, J.E. Nordman

Published Apr 1, 1994• 3 citations
Publisher

Abstract

Volt-ampere characteristics of the Josephson junction vortex-flow transistor were studied through comparison of experiment and numerical simulation. Fine structure in the flux flow branch, which is not predicted by a simple particle model for flux flow, is shown to be quantized in voltage. The quantization results from the interaction of the flux wave with the junction’s cavity resonances. It is shown that the separation of the voltage states is proportional to the maximum operating frequency of the device. In an amplifier circuit, quantization in the output voltage will reduce the dynamic range of the device by adding quantization noise. Hysteretic switching between states produces distortion. Possible device modifications for removing the quantization are discussed.

DOI
10.1063/1.356081

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IEEE Transactions on Magnetics•Mar 1, 1989•11 citations
Volt-ampere characteristics of the Josephson junction vortex-flow transistor were studied through comparison of experiment and numerical simulation. Fine structure in the flux flow branch, which is not predicted by a simple particle model for flux flow, is shown to be quantized in voltage. The quantization results from the interaction of the flux wave with the junction’s cavity resonances. It is shown that the separation of the voltage states is proportional to the maximum operating frequency of the device. In an amplifier circuit, quantization in the output voltage will reduce the dynamic range of the device by adding quantization noise. Hysteretic switching between states produces distortion. Possible device modifications for removing the quantization are discussed.
Transistors based on one dimensional flux flow in hysteretic long Josephson junctions (LJJ) and deriving output from the spatial average over the length can be modeled as a section of transmission line. Time domain analyses made on these transmission line sections with both the ends terminated with typical resistance values reveal a fundamental gain-frequency relationship unique to all devices whose output voltage is equal to the spatial average over the length of the junction. The maximum frequency of a transistor depends on the transit time of carriers and the parasitic elements. The analysis shows that the frequency response of a flux flow transistor is related but not equal to the inverse of the transit time of fluxons. Secondly the step response of these transmission line sections varies linearly with time, which indicates a slew rate limitation. The slew rate that affects the high frequency response of a flux flow device is shown to be a function of the critical current density of the junction and material parameters. Analysis made on niobium-lead junctions indicates that the slew rate peaks at an optimum current density level. The results of these analyses and performance comparisons are presented.
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.
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.
Discrete array of small Josephson junctions can exhibit flux-flow behavior, which is drastically different from that of long Josephson junctions. In particular, the flux-flow voltage does not monotonically increase with magnetic field; rather, it decreases after a threshold until it reaches zero. Here, using simulation and experimental results, we show that the flux-flow voltage switches abruptly to zero if a magnetic field larger than the threshold is applied to a part of the array. This provides for a digital NOT gate in which the flux-flow voltage corresponds to digital state 1 and zero voltage denotes digital state 0. For a device size of 230 μm, a switching frequency of 15 GHz is evaluated, which can be improved by downsizing the device. Since this device provides gain, its input and output are isolated and has unlimited fan-out, designing and implementing various digital systems is easily accomplished.
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.
A niobium-based superconducting amplifier consisting of a series array of 60 vortex flow transistors (VFT) was fabricated and tested. Each VFT consisted of a long Josephson junction biased in the flux flow state, magnetically coupled to a current based on a distributed amplifier configuration. The transresistance of the amplifier in a 50- Omega environment for frequencies up to 1 MHz was measured. The transresistance of the array was less than the expected value calculated from measurements obtained on a single VFT. The difference was probably due to nonuniformities in the fabrication process, which caused some of the junctions not to be biased in the flux flow state or receive the same amount of magnetic field from the control line. However, measurements of the array showed that a large number of VFTs could be connected in series and simultaneously be biased into the flux flow state.>