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IEEE Transactions on Magnetics

Characteristics of superconducting flux-flow transistors

Gert K. G. Hohenwarter, J. Martens, J.H. Thompson, James B. Beyer, J.E. Nordman, David S. Ginley

Published Mar 1, 1991• 23 citations
Publisher

Abstract

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.

DOI
10.1109/20.133917

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Effects of finite junction length on the vortex-flow transistor

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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.

Journal of Applied Physics•Apr 1, 1994•3 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.
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.
Research during the initial year of this grant concentrated on fabrication and modeling 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.
We fabricated Josephson vortex flow transistors (JVFTs) with a parallel array of Josephson junctions that were prepared using c-axis-oriented 100-nm-thick YBa 2 Cu 3 O 7-δ (YBCO) thin films grown on 24° bicrystal MgO (100) substrates. We observed clear modulations of the critical current and the flow voltage with DC current input to the control line that was inductively coupled to the array of junctions. From the results, we estimated the parameters of the device, e.g., the mutual inductance and the self-inductance, and calculated the operation frequency at which the device potentially exhibited these parameters. Moreover, the current gain and the transresistance were evaluated and found to be 0.5 and 0.15 Ω, respectively. In addition, we observed the high-frequency responses of the JVFT to the input AC current of the sine wave or the square pulse wave. A clear oscillation of the output voltage could be observed with a 1 MHz sine wave and 250 kHz square pulse wave. We also discussed the feasibility of higher frequency operation by using it as an input interface for a single flux quantum (SFQ) logic circuit.
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.