Low-Energy Consumption RSFQ Circuits Driven by Low Voltages

We report successful operations of low-energy consumption rapid single-flux-quantum (RSFQ) circuits applying lowered driving voltages, called LV-RSFQ, using different fabrication processes. In the LV-RSFQ, we feed bias currents to Josephson junctions from lowered constant voltages (less than 1 mV) through small resistors without extra inductors. Both static and dynamic energy consumption are reduced because of suppression of amplitudes of SFQ pulses, in exchange for slower switching speed. We show that the switching speed in LV-RSFQ circuits is improved by increasing critical current density of Josephson junctions. We demonstrated high-speed operations of LV-RSFQ shift registers in a range of 5-40 GHz and 15-90 GHz using the 2.5-kA/cm² and 10-kA/cm² fabrication technologies, respectively. Comparison of the experimental results of the LV-RSFQ circuits fabricated using two different technologies derives the optimum bias voltage in terms of energy-delay product ranged from 0.1 to 1.0 I_CR_S, where I_CR_S is the product of Josephson critical current and shunt resistance.