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IEEE Transactions on Magnetics

A soliton device

T. Rajeevakamar

Published Jan 1, 1981• 25 citations
Publisher

Abstract

A simple, novel superconducting tunneling device has been investigated experimentally as well as theoretically. The device can be used to perform logic by the steering of Josephson solitons. It may be used as the basis for wide margin, high density, high speed circuits. The concept was demonstrated experimentally through the operation of a chain of 10 devices fabricated with a 5 μm Pb-alloy technology.

DOI
10.1109/tmag.1981.1060933

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A simple, novel superconducting tunneling device has been investigated experimentally as well as theoretically. The device can be used to perform logic by the steering of Josephson solitons. It may be used as the basis for wide margin, high density, high speed circuits. The concept was demonstrated experimentally through the operation of a chain of 10 devices fabricated with a 5 μm Pb-alloy technology.
The potential of a new technology, based on the superconducting Josephson tunneling effect, has been assessed for use in high-speed computer systems. Josephson tunneling circuits for memory and logic functions can be switched at sub-nanosecond delay, do not require standby power, and dissipate extremely low energy, typically less than 10-13joule during fast switching operation. High-speed circuits with high packing densities, not limited by power dissipation problems, can therefore be expected. Low thermal noise at cryogenic temperatures ensures reliable operation despite the low energy switching signal. Practically lossless signal transmission via superconducting lines can be exploited for high-speed operation even when sizeable memory and logic modules are assembled. Two random access memory modules have been designed on the basis of single device experiments. Operational characteristics leading to the conclusion that 30 Mbit capacity, less than 1 watt refrigeration requirement (at 3.6° K) and cycle times of 40 ns (in one case) and 15 ns (in the second case), can be achieved are presented.
The behavior of simple superconducting circuits in the picosecond regime is described in a comprehensive way, with primary emphasis being given to the step function and pulse responses of these circuits. Topics receiving detailed discussion include Josephson-junction modeling with both the microscopic and shunted-junction models. Limitations of the shunted-junction model are explored by comparing it with experimental results and with the microscopic model. An approximate evaluation is given of the important dynamical properties of junctions made with the dominant fabrication technology (Pb-alloy systems), as a function of tunneling barrier thickness. Rounding out the device aspects of the discussion, we describe in detail the properties of superconducting microstrip transmission lines, with an emphasis on their high-speed behavior. Turning to simple circuits we review experimental results on the measurement of picosecond regime transient signals. The concept of turn-on delay is analyzed anew, providing simplified and extended results. Details of concepts for pulse height and pulsewidth measurements are explored, leading to the conclusion that the time resolution of superconducting circuits is limited to approximately the period of one plasma oscillation. With present Pb-alloy fabrication technology this limit is 2 ps.
The design and experimental investigation of a simple logic circuit employing Josephson tunnel junctions as the switching elements is described. The design utilized matched transmission lines as signal carriers. The physical layout was simulated on a computer with a model consisting of 3 Josephson gates and 28 transmission line segments. Results of both quasi-static and high speed testing were found to be in essential agreement with theoretical expectations; only dynamic results are presented here in detail. The experimental circuit had gates with 165 ps risetimes, and in the high speed investigation, was operated with bursts of 700 ps risetime pulses tightly spaced so as to simulate a repetition rate of ∼1 GHz.
Superconducting devices have intrinsically superior characteristics to those of semiconductor devices. Presently, we can fabricate more than twenty thousand junctions on one chip using niobium technology. We have demonstrated the operation of a network system with a superconducting interconnection chip using voltage-state logic. Single flux quantum devices are promising for future superconducting applications because the clock frequency of SFQ logic is higher than that of voltage-state. We have proposed a high-end switch based on hybrid architecture using optical devices, semiconductors and SFQ devices. To demonstrate the high-speed operation of SFQ circuits, we developed an arbiter circuit that uses SFQ components and the arbiter circuit operates at 60 GHz. We also have developed a high Tc superconducting (HTS) SFQ sampler system for observing ultra-fast signal waveforms. In addition, we will discuss the prospects of future superconducting devices based on the fabrication technologies we developed.
Experimental superconducting Josephson devices being investigated for use in digital logic and memory circuits have been demonstrated to switch in the 10 to 100 picosecond (1 picosecond = 10-12 sec) range. Projections based on the operation of individual logic circuits indicate that they may surpass semiconductor circuits in very high performance CPU's. This potential is based on the fact that these circuits dissipate extremely little energy, on the order of 10 to 1000 attojoules (attojoule = 10-18 joule), while operating with subnanosecond delays. It should, therefore, be possible to package Josephson devices very densely and interconnect them by properly terminated superconducting transmission lines so that the fast switching speed of individual circuits is retained throughout large logic networks.
Experimental superconducting Josephson devices being investigated for use in digital logic and memory circuits have been demonstrated to switch in the 10 to 100 picosecond (1 picosecond = 10-12 sec) range. Projections based on the operation of individual logic circuits indicate that they may surpass semiconductor circuits in very high performance CPU's. This potential is based on the fact that these circuits dissipate extremely little energy, on the order of 10 to 1000 attojoules (attojoule = 10-18 joule), while operating with subnanosecond delays. It should, therefore, be possible to package Josephson devices very densely and interconnect them by properly terminated superconducting transmission lines so that the fast switching speed of individual circuits is retained throughout large logic networks.