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Electronics Letters

Asymmetric 2-Josephson-junction interferometer as a logic gate

H. Beha

Published Mar 31, 1977• 19 citations
Publisher

Abstract

For an interferometer with two unequal Josephson point junctions, the maximum Josephson current, as well as the corresponding control current, is given in closed form as a function of one junction phase. In contrast to 3-Josephson-junction interferometers, with maximum Josephson currents I01=I02/2=I03, 2-junction interferometers, with maximum Josephson currents I01=5I02, can have larger current gain, slope and less device area.

DOI
10.1049/el:19770158

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For an interferometer with two unequal Josephson point junctions, the maximum Josephson current, as well as the corresponding control current, is given in closed form as a function of one junction phase. In contrast to 3-Josephson-junction interferometers, with maximum Josephson currents I01=I02/2=I03, 2-junction interferometers, with maximum Josephson currents I01=5I02, can have larger current gain, slope and less device area.
A 1:2:1 Josephson interferometer has been fabricated with small-area, high–current-density tunnel junctions. The short intrinsic RC times of these junctions result in device I-V characteristics with reduced hysteresis suitable for use in nonlatching logic circuits. The control current for this interferometer was fed directly, and asymmetrically, into the base electrode. Critical current versus control current measurements indicate that this device has a current gain greater than unity. The interferometer described represents an exploratory study of circuit design incorporating low-hysteresis junctions.
For a given level of overdrive, there exists a minimum-width control-current pulse that must be applied to switch a Josephson logic gate. Determination of this minimum pulsewidth is useful in setting limits on the speed of various Josephson circuit configurations and also in finding the narrowest pulse that a Josephson-junction gate can detect when it is employed as a pulse detector. The minimum control-current pulsewidth is a strong function of the overdrive factor of the control current. It scales as(C/I_{m})^{1/2}, where Imis the junction critical current, andCis the capacitance, and is on the order of 10 ps for Josephson quantum interferometers in 5-µm technology. Computer simulations have been done to find the dependence of minimum control-current pulsewidths on overdrive magnitude for a single-junction gate and for two- and three-junction interferometers. Analytic expressions of the minimum control-current pulsewidth that are in good agreement with simulations have been obtained for these three types of Josephson logic gates.
This paper proposes a two-junction interferometer called Variable Threshold Logic. It has both magnetically-coupled control-lines and current-injection terminals. The threshold curve of this gate can be shifted by the applied currents. This property enables its use for both OR and AND function with the same structure by only changing the wiring. Operating margins of ±12 % for OR and ± 29 % for AND are obtained with the optimized device parameters. Any logic circuit can be obtained by Josephson master-slice LSI of this gate with a dual-rail logic system.