An Overview of Full Adders in QCA Technology

Quantum dot Cellular Automata (QCA) is a branch in nano-electron ic for design the logical gates and circuits. The QCA logic gates composed from basic elements which are named as QCA cells. A QCA cell is a nanometer structure that constructed from nanometer semiconductors and capacitors. In QCA scope, we just have majority and inverter logic gates because of this fact that QCA cells perform their logical functions based on columbic forces between them and other QCA logic gates can be constructed by majority and inverter gates and QCA wires. Clock signals controls the QCA cells activities and states and the clocked QCA cells perform the delays and other features for design the conventional and sequential logic circuits. We demonstrate the clocking in QCA circuits using some examples in this paper. Since basic computing elements, in particular full adders are used in different parts of processing and computing circuits such as Arithmetic Logic Unit (ALU), Creating address for caches and main memories they are counted as major and fundamental part of processors. They extremely affect the speed, occupied space and power consumed of processors. In this paper at first step, we present the fundamentals of QCA technology including concepts, QCA logical gates. In the following, we present the famous designs for a 1-bit QCA full adder. In the last part we compare all of these 1-bit full adders using QCA DESIGNER simulator .We compare the cell count and delay time and occupied area and robustness against the temperature features. After review the QCA full adders we find this point that a good full adder have an efficient and compromised logical function using low number of majority and inverter gates. Also this point is important to know that most efficient full adders use the crossover and multilayer connections for reducing the occupied area and delay time and noises.