A Study On Quantum-dot Cellular Automata Based Digital Logic Circuit Design

Over the past half-century, a series of significant achievements in IC technology have been gained following Moore's low. Downscaling of device feature sizes is a main approach to increase ICs'integration density and improve their performance. However, with device scaling into nanometer regime, quantum effects begin to be dominant and may cause device failure. As a mainstream technology for ICs, CMOS technology is approaching its physical limits and facing serious challenges especially in power consumption and interconnection. Therefore, a great deal of attention has been paid to overcome these obstacles by amending CMOS theory and improving its fabrication processes. Meanwhile extensive research on nanometre alternatives to CMOS has also been carried out. One of the alternatives is known as Quantum-Dot Cellular Automata (QCA), which implements a unique computational paradigm and overcomes some limitations of current technologies, with the potential for faster speed, smaller size and lower power consumption than CMOS.Since introduction of QCA, a number of interesting QCA-based logic circuits have been proposed. Howeve, there is still much work to be done to the design of QCA-based logic circuits. Based on investigation of QCA's basic principles, this thesis focuses on the study of QCA-based digital logic circuit design and proposes some novel QCA-based logic circuits. The detailed contents and main innovations in this work are summerized as follows.1. Design and application of QCA-based optimal universal logic gate (ULG) and universal threshold logic gate (UTLG). As novel fundamental logic elements for future ICs, two novel QCA-based ULGs and one UTLG have been proposed. As examples, full adder/subtraction, full comparator and 4-to-1 multiplexer are implemented with proposed modular ULG.2. Compared with traditional design based on majority gates and inverters, their performance has been improved dramatically. Then, examples showing how to realize three-variable logic functions with a QCA-based ULG.3 or UTLG is given, respectively. All 256 three-variable logic functions can be realized using tabular design method. Simulation results obtained by using QCADesigner tool for all proposed QCA-based circuits show that the proposed circuits have correct logic function.2. Design of QCA-based dual-edge triggered (DET) flip-flops. Flip-flops are critical for the performance of digital systems. QCA is limited in its sequential circuit design with high performance flip-flops while a number of combinational logic circuits have been presented. Based on a brief introduction of DET flip-flop, two original QCA-based D and JK DET flip-flops are proposed, offering the same data throughput of corresponding single-edge triggered (SET) flip-flops at half of the clock pulse frequency. The logic functionality of the two proposed flip-flops is verified with QCADesigner tool. All the proposed QCA-based DET flip-flops show higher performance than their SET counterparts in terms of data throughput. By using lower clock pulse frequency, the proposed DET flip-flops are promising for constructing QCA-based sequential circuits and systems with high performance.3. Simulation of ternary quantum-dot cellular automata (tQCA) basic logic circuits using Matlab. The use of binary logic in computer structures was historically necessary only due to technological limitations. The benefits of using multi-valued processing are obvious and range from higher data storage capabilities to faster and more sophisticated processing. Based on an introduction of ternary QCA cell which allows for the representation of three logic values, a simulation algorithm with tQCA's semiclassical model is presented. Then simulations about three tQCA basic logic circuits such as tQCA wire, inverter and majority voter followed with corresponding counterparts have been performed by using Matlab. The simulations show that the behavior of above tQCA-based structures agrees with the ternary truth table raised by ALukasiewicz except the majority gate. The majority gate structure does not behave as intended. So it should be noted that this initial work represents only a step forward in developing a more basic circuits of tQCA-based ternary logic circuits.