Estimation of Cotunneling in Single-Electron Logic and Its Suppression

It is known that cotunneling causes a serious decline in the reliability of single-electron logic circuits. We report the effects of cotunneling on single-electron logic circuits studied using computer simulation in which both two-electron cotunneling and normal single-electron tunneling are taken into account. It is proved that the cotunneling reduces the operation accuracy of single-electron logic circuits considerably even at absolute zero. At finite temperatures both normal tunneling and cotunneling are enhanced by thermal fluctuation. Temperature dependence of tunneling rates of both processes is also discussed. In order to suppress cotunneling, we consider the tunnel resistance of junctions as one of the design parameters and show that the cotunneling rate can be reduced by optimizing the resistance. In addition, we present an inverter circuit that exhibits a better tolerance of the cotunneling effect.