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IEEE Transactions on Applied Superconductivity

Optimization of high-T/sub c/ Josephson fluxon-antifluxon transistors based on numerical simulation

Y.M. Zhang, Esin Terzioglu, M. R. Beasley

Published Jun 1, 1997• 0 citations
Publisher

Abstract

A Josephson fluxon-antifluxon transistor (JFAT) can be constructed utilizing high-T/sub c/ bicrystal or SNS long junctions with a control line on top of the junction, This device can be modeled numerically by solving the perturbed sine-Gordon equation. A JFAT has a higher current gain and a faster transient response compared to a conventional Josephson flux-flow transistor which has a U-shaped control line on top of the long junction. Dependences of the control characteristics on the width and the location of the control line, as well as on the junction length are calculated. The influence of a superconducting ground plane on device characteristics are also discussed. These results are important to the design of high-T/sub c/ digital circuits based on JFATs.

DOI
10.1109/77.621774

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A Josephson fluxon-antifluxon transistor (JFAT) can be constructed utilizing high-T/sub c/ bicrystal or SNS long junctions with a control line on top of the junction, This device can be modeled numerically by solving the perturbed sine-Gordon equation. A JFAT has a higher current gain and a faster transient response compared to a conventional Josephson flux-flow transistor which has a U-shaped control line on top of the long junction. Dependences of the control characteristics on the width and the location of the control line, as well as on the junction length are calculated. The influence of a superconducting ground plane on device characteristics are also discussed. These results are important to the design of high-T/sub c/ digital circuits based on JFATs.
We have realized the Josephson fluxon–antifluxon transistor (JFAT) in high temperature superconductivity using an asymmetric control line on top of either bicrystal junctions or Co-doped YBa2Cu3Ox superconductor–normal–superconductor (SNS) junctions. We have measured current gains as high as 6 for 30 μm-wide bicrystal JFATs (30 K) and as high as 3 for Co-doped SNS JFATs (50 K). An improvement in gain over the Josephson vortex flow transistor, due to improved coupling efficiency, is demonstrated. There is also a reduction of control line inductance that should lead to an improvement in gate speed.
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.
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 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.
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.
With a slight modification to the control line and its bias configuration, a long Josephson junction vortex flow transistor structure can become a so-called fluxonic diode containing fluxon and antifluxon "doped" regions in analogy to a pn junction. Such structures have been made with Nb-Pb tunnel junctions allowing testing of the same device with a number of different doping configurations. This paper compares experimental results with a one dimensional numerical simulation. Different configurations for the diode have been modeled with Stewart-McCumber loss parameters ranging between 50 and 1400. The simulation clarifies the origin of a number of experimental observations, including a peculiar dependence of resonance induced step behavior on loss parameter, and a limit to the value of the gradient of doping in the transition region below which an asymmetric I-V curve representing forward and reverse biases is obtained. In forward bias, the I-V curve above a threshold current is roughly linear and devoid of resonance structure, provided injection of carriers occurs from one region to the other. Injection is a function of loss and the asymmetry of doping.>