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Applied Physics Letters

A novel current injection Josephson logic gate with high gain

K. Hohkawa, Masaru Okada, Akira Ishida

Published Oct 15, 1981• 31 citations
Publisher

Abstract

A novel Josephson logic gate achieving high gain, employing current injection, and having a noninterferometer configuration has been proposed and studied. The gate includes three small junctions for input sensing, output driving, and I-O isolation. The junctions are appropriately biased by source resistors to obtain high current gain. Computer simulation and preliminary experiment have demonstrated that a current gain higher than 2 can easily be obtained through the proposed configuration.

DOI
10.1063/1.92808

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A novel Josephson logic gate achieving high gain, employing current injection, and having a noninterferometer configuration has been proposed and studied. The gate includes three small junctions for input sensing, output driving, and I-O isolation. The junctions are appropriately biased by source resistors to obtain high current gain. Computer simulation and preliminary experiment have demonstrated that a current gain higher than 2 can easily be obtained through the proposed configuration.
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A versatile current-switched gate is described which consists of two Josephson tunnel junctions and a small resistor in a loop. The directly combined bias and control currents flow through one junction in the zero-voltage state, causing the switching. The second junction and the resistor provide isolation between input and output after switching. The switching speed of a few tens of picoseconds and the microwatt power dissipation are comparable to those of other present designs. The design is particularly simple, compact, and easy to fabricate.
A new 3-Josephson-junction interferometer is presented as a high-tolerance nonlinear current-injection 2-input AND gate. In Josephson LSI circuits, the tolerances are an important issue. Thus, in our new logic device emphasis has been placed on its static and dynamic tolerance properties. The new HTCID offers considerably better input-signal tolerances than known 2-junction current-injection devices (CID) for identical fabrication tolerance assumptions. The HTCID in this example has a Josephson current ratio of 1:3:1 and a characteristic phase λ = 0.72π. Including circuit parameter variations, this new 3-junction CID allows attractive, high input-signal tolerances of ± 33.3%. Therefore, an almost threefold tolerance improvement over the published 2-junction CID (λ = 0.66π) could be achieved. Trade-off between signal tolerances and device speed is discussed. Computer simulation indicates a worst-case delay of less than 47 ps.
We report experimental and theoretical results on the current gain in superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively]. The Josephson critical current density jcaof the acceptor (SISF) junction in the devices is above 9 kA/cm2, which is higher than that in our previous devices. The critical current gain is defined as |δIca|/|δIi|, where a change |δIi| in the injector (SFIFS junction) current produces a change |δIca| in the acceptor maximum Josephson current. We observed a small-signal gain as high as 7.8 and a large-signal current gain of about 2.2. In addition, we found that the Josephson current of the acceptor junction is sensitive to the state of the ferromagnetic layers. A theoretical model is proposed to describe the nonequilibrium processes in the SFT devices, which agrees with the experimental observations. The calculation shows that, in the present device configuration, the dominant contribution to the gap suppression in the middle Nb electrode is due to the quasiparticle injection; spin injection plays a secondary role. We demonstrate that proper device engineering allows one to efficiently control the maximum Josephson current in the SISF acceptor junction using the quasiparticle injection. We conclude that SFT devices can be used as input/output isolators and amplifiers for memory, digital, and RF applications.