  Circuit design based on Quantum-dots Cellular Automata technology offers
power-efficiency and nano-size circuits. It is an attractive alternative to
CMOS technology. The XOR gate is a widely used building element in arithmetic
circuits. An efficient XOR gate in QCA computational circuits can significantly
improve efficiency. This paper proposes two different approaches for designing
3-input QCA XOR gates with 10 and 8 cells. They require two clock phases to
create output. They have efficient and scalable structures. To demonstrate the
functionality of these structures, we design QCA full adders using the
suggested gates and compare the results with existing designs. The proposed QCA
full adder has only 12 cells and is the best compared to all the existing
counterparts. We simulated and verified the proposed structures. We proved the
functionality of the proposed QCA full adder and the suggested QCA XOR
structures. Additionally, QCAPro is used to estimate the energy dissipation of
the proposed XOR and Full-adder. The results demonstrated that the proposed
designs have the desired performance based on the number of cells, occupied
area, and latency.
