Adiabatic quantum-flux-parametron (AQFP) circuits are known as ultra-low-power superconducting circuits. To realize large scale AQFP integrated circuits, a lot of studies are conducted up to present. One problem of the AQFP circuits is the decrease of the signal current amplitude in a long gate-to-gate interconnection. Fan-out also reduces the signal current amplitude. The reduction of the signal amplitude affects the bit-error rate of the circuits. To operate AQFP logic circuit with a low enough bit-error rate, the wire length and the number of fan-out have to be restricted. The limitations of the wire length and the number of fan-outs are currently 0.8 mm and two, respectively. This limitation deteriorate the flexibility in the AQFP circuit design.

 To enable longer wiring and large fan-out, we propose two types of AQFP current boosters, a booster 1 and a booster 2, as shown in Fig. 1. The booster 1 can be used to increase the wire length, while the booster 2 can be used for large fan-out. The advantage of the proposed boosters is that they can be clocked by a single excitation current, and hence their latency is just one clock phase. We simulated the wire-length limit and found that the wire-length limitation is increased to 1.7 mm in the booster 1. The wire-length limitation after fan-out 2 is also improved by using the booster 2. We also evaluated the energy dissipation of the boosters and found that it is about 12 times of that of the conventional AQFP buffer.

Keywords: AQFP, booster, LSI, wire-length