Logic gates with shaped Josephson junctions

Long (L/λ/SUB j/>5) in-line Josephson junctions, with varying width along the length L of the device, are investigated as logic gates (λ/SUB j/ being the Josephson penetration depth). The devices realized have an asymmetric threshold characteristic with almost suppressed sidelobes, providing good logic gain and permitting logic fan-in with multiple control lines. Optimum conditions are found for junctions with width varying approximately sinusoidally along the device length. The so-called shaped junctions are incorporated in various flip-flop circuits to evaluate the transfer time and transfer efficiency of loop circuits, and in a self-resetting inverter circuit to demonstrate the feasibility of self-resetting logic. The principle of current steering and the relatively large operating currents (I/SUB G/≃6 mA) make the circuits suitable for medium-speed applications such as in the decode and control logic of a main-memory chip. For a fan-out of four, the minimum circuit delay is 300 ps, resulting in a power-delay product in the order of 3×10/SUP -15/ J.