Performance evaluation of an ultra-high speed adder based on quantum-dot cellular automata

Abstract

Quantum-dot cellular automata (QCA) is among the most promising nanotechnologies as the substitution for the current metal oxide semiconductor field effect transistor based devices. Therefore, lots of attention have been paid to different aspects to improve the efficiency of QCA circuits. In this way, the adder circuits are widely investigated since their performance can directly affect the whole digital system performance. In this paper, a new ultra-high speed QCA full adder cell is proposed based on multi-layer structures. The proposed full adder cell is simple in design using 3-input Exclusive-OR (TIEO), which computes the Sum bits and Majority gate, which computes the Carry bits. To verify the efficacy of the presented full adder cell, it is considered, the main constructing block in 4-bit ripple carry adder circuit. Hence, significant improvements in terms of area and cell count have been achieved. Particularly simulation results show 20% and 1.8% reduction respectively in the area and cell count overhead. Detailed performance evaluation and structural analysis are performed in different aspects to authenticate the proposed circuits (one-bit and 4-bit) having superb performance in comparison to previously reported works. QCADesigner CAD tool has been used to verify the correct functionality of the proposed architectures.

Appendix

In below, equations of Delay, Area, and Cell Count are described [37].

$$Delay = a_{0} + \mathop \sum \limits_{k = 5}^{i = n - 1} \left( {\frac{{\left( {2^{k - 1} + 2 } \right)}}{16 \times 4}} \right) + 2^{n - 1}$$

The equation of Delay consists of three important parts. The first and second parts are propagation delays of Rst wire through fast 4-bit (0.25) and 5th to an N − 1 bit of full-adder, respectively; moreover, the third part is a delay of n^thbit counter (2ⁿ⁻¹) [37].

$$Area = 18 \times \left( {\left( {\mathop \sum \limits_{k = 5}^{k = n} \left( {2^{k - 1} + 2} \right)} \right) + 33} \right) \times 400$$

The height of the full-adder is 18 QCA cells. The width is divided into two parts. The first and the second parts are the first 4-bit (33) and least n-4 bits of full-adder, respectively, center to center of the Cell is 20 nm, as the results each QCA cell area is 400 nm² [37].