Design and characterization of minority gate as a universal logic for quantum-dot cellular automata

This paper presents the design and characterization of minority gate as a universal logic for quantum-dot cellular automata (QCA) based circuits. In the looming roadblock of the present CMOS technology of VLSI design in near future, quantum-dot cellular automata is considered to be a promising nano-scale technology for future generation ICs. Traditionally, OCA circuits are built around the majority voter constructed with four cross-shaped cells. However, majority voter is not a universal logic because the logical NOT operation cannot be realized with it. Designers have to use separate cell structures for the NOT gate. In this paper we propose a structure, composed of four plus-shaped QCA cells, realizing the minority gate given by the Boolean expression M(x(1), x(2), x(3)) = x(1)' (.) x(2)' + x(2)' (.) x(3)' + x(3)' (.) x(1)'. The functional completeness of minority gate is proved and minority gatew oriented design principles are provided. Two novel minority gate oriented QCA circuit for XOR gate and a full adder are presented as illustrative examples. Simulation results show that the proposed designs are effective in achieving the desired functionality. Moreover, the defect characterization of the minority gate is provided, followed by a QCA structure realizing a fault tolerant minority gate.