Circuit-level modelling and simulation of carbon nanotube devices

The growing academic interest in carbon nanotubes (CNTs) as a promising novel class of electronic materials has led to significant progress in the understanding of CNT physics including ballistic and non-ballistic electron transport characteristics. Together with the increasing amount of theoretical analysis and experimental studies into the properties of CNT transistors, the need for corresponding modelling techniques has also grown rapidly. This research is focused on the electron transport characteristics of CNT transistors, with the aim to develop efficient techniquesto model and simulate CNT devices for logic circuit analysis.The contributions of this research can be summarised as follows. Firstly, to accelerate the evaluation of the equations that model a CNT transistor, while maintaining high modelling accuracy, three efficient numerical techniques based on piece-wise linear, quadratic polynomial and cubic spline approximation have been developed. The numerical approximation simplifies the solution of the CNT transistor’s self-consistent voltage such that the calculation of the drain-source current is accelerated by at least two orders of magnitude. The numerical approach eliminates complicated calculations in the modelling process and facilitates the development of fast and efficient CNT transistor models for circuit simulation.Secondly, non-ballistic CNT transistors have been considered, and extended circuit-level models which can capture both ballistic and non-ballistic electron transport phenomena, including elastic scattering, phonon scattering, strain and tunnelling effects, have been developed. A salient feature of the developed models is their ability to incorporate both ballistic and non-ballistic transport mechanisms without a significant computational cost. The developed models have been extensively validated against reported transport theories of CNT transistors and experimental results.Thirdly, the proposed carbon nanotube transistor models have been implemented on several platforms. The underlying algorithms have been developed and tested in MATLAB, behaviourallevel models in VHDL-AMS, and improved circuit-level models have been implemented in two versions of the SPICE simulator. As the final contribution of this work, parameter variation analysis has been carried out in SPICE3 to study the performance of the proposed circuit-level CNT transistor models in logic circuit analysis. Typical circuits, including inverters and adders, have been analysed to determine the dependence of the circuit’s correct operation on CNT parameter variation

CNFET-based design ternary logic design and arithmetic circuit simulation using HSPICE

This project report focuses on the multiple-value logic (MVL) or commonly known as ternary logic gates by using carbon nanotube (CNT) FETs devices (CNTFETs). It is shown ternary logic has promising future in CNTFETs when compare to conventional binary logic design, due to its simplicity and energy efficiency in digital design reduced circuit overhead such as chip area and interconnection. In this research, existing CNTFET-based binary inverter and standard ternary inverter with resistive-load (STI-R) for comparison with the other three types of inverter are proposed - Complementary Standard Ternary Inverter (CSTI); Standard Ternary Inverter with 1 resistor and 3 NCNTFET (NSTI-R); Standard Ternary Inverter with 1 resistor and 3 PCNTFET (PSTI-R) to analysis the performance, structure design and application. In addition, the research covers all the basic logic Ternary NAND gate and Ternary NOR gate for further benchmarking. All simulation results using SPICE are obtained and analyzed in the Direct Current (DC) setting and verifed using half adder. Further study behavior of ternary logic includes the implementation of partial binary design into the ternary design and performance benchmarking. The result shows the CSTI have advantage on low power design with low leakage while NSTI-R has advantage on high-speed design inverter. In addition, partial binary design in the arithmetic circuit ternary design with CSTI shows added advantage in a low power design

Design of Ternary Memory Cell Using QDGFET

Ternary logic is a promising option to conventional binary logic because it can handle higher information in less number of gate count. Less number of gates requires less area in a chip which is equivalent to gold in today’s nano scale circuits. A novel design of a ternary memory cell based on QDGFETs is proposed. Memory cell is made of two back to back connected inverters. It is the conventional 6T memory cell design. Main advantage of QDGFET is that it can be used directly by replacing CMOS in the circuit without making any changes. Embedded memory requires the largest share of area in modern high-performance circuit designs. As the technology progresses the demand for high capacity memories also increases. So to fulfil this demand, researchers are trying to come up with new technology and solutions. The use of ternary logic instead of binary logic is a possible solution. So in this paper I have designed a ternary memory cell which stores one bit of ternary logic data

Novel Ternary Logic Gates Design in Nanoelectronics

In this paper, standard ternary logic gates are initially designed to considerably reduce static power consumption. This study proposes novel ternary gates based on two supply voltages in which the direct current is eliminated and the leakage current is reduced considerably. In addition, ST-OR and ST-AND are generated directly instead of ST-NAND and ST-NOR. The proposed gates have a high noise margin near V_(DD)/4. The simulation results indicated that the power consumption and PDP underwent a~sharp decrease and noise margin showed a considerable increase in comparison to both one supply and two supply based designs in previous works. PDP is improved in the proposed OR, as compared to one supply and two supply based previous works about 83% and 63%, respectively. Also, a memory cell is designed using the proposed STI logic gate, which has a considerably lower static power to store logic ‘1’ and the static noise margin, as compared to other designs

Variability and reliability analysis of carbon nanotube technology in the presence of manufacturing imperfections

In 1925, Lilienfeld patented the basic principle of field effect transistor (FET). Thirty-four years later, Kahng and Atalla invented the MOSFET. Since that time, it has become the most widely used type of transistor in Integrated Circuits (ICs) and then the most important device in the electronics industry. Progress in the field for at least the last 40 years has followed an exponential behavior in accordance with Moore¿s Law. That is, in order to achieve higher densities and performance at lower power consumption, MOS devices have been scaled down. But this aggressive scaling down of the physical dimensions of MOSFETs has required the introduction of a wide variety of innovative factors to ensure that they could still be properly manufactured. Transistors have expe- rienced an amazing journey in the last 10 years starting with strained channel CMOS transistors at 90nm, carrying on the introduction of the high-k/metal-gate silicon CMOS transistors at 45nm until the use of the multiple-gate transistor architectures at 22nm and at recently achieved 14nm technology node. But, what technology will be able to produce sub-10nm transistors? Different novel materials and devices are being investigated. As an extension and enhancement to current MOSFETs some promising devices are n-type III-V and p-type Germanium FETs, Nanowire and Tunnel FETs, Graphene FETs and Carbon Nanotube FETs. Also, non-conventional FETs and other charge-based information carrier devices and alternative information processing devices are being studied. This thesis is focused on carbon nanotube technology as a possible option for sub-10nm transistors. In recent years, carbon nanotubes (CNTs) have been attracting considerable attention in the field of nanotechnology. They are considered to be a promising substitute for silicon channel because of their small size, unusual geometry (1D structure), and extraordinary electronic properties, including excellent carrier mobility and quasi-ballistic transport. In the same way, carbon nanotube field-effect transistors (CNFETs) could be potential substitutes for MOSFETs. Ideal CNFETs (meaning all CNTs in the transistor behave as semiconductors, have the same diameter and doping level, and are aligned and well-positioned) are predicted to be 5x faster than silicon CMOS, while consuming the same power. However, nowadays CNFETs are also affected by manufacturing variability, and several significant challenges must be overcome before these benefits can be achieved. Certain CNFET manufacturing imperfections, such as CNT diameter and doping variations, mispositioned and misaligned CNTs, high metal-CNT contact resistance, the presence of metallic CNTs (m-CNTs), and CNT density variations, can affect CNFET performance and reliability and must be addressed. The main objective of this thesis is to analyze the impact of the current CNFET manufacturing challenges on multi-channel CNFET performance from the point of view of variability and reliability and at different levels, device and circuit level. Assuming that CNFETs are not ideal or non-homogeneous because of today CNFET manufacturing imperfections, we propose a methodology of analysis that based on a CNFET ideal compact model is able to simulate heterogeneous or non-ideal CNFETs; that is, transistors with different number of tubes that have different diameters, are not uniformly spaced, have different source/drain doping levels, and, most importantly, are made up not only of semiconducting CNTs but also metallic ones. This method will allow us to analyze how CNT-specific variations affect CNFET device characteristics and parameters and CNFET digital circuit performance. Furthermore, we also derive a CNFET failure model and propose an alternative technique based on fault-tolerant architectures to deal with the presence of m-CNTs, one of the main causes of failure in CNFET circuits

Current-Voltage characteristics of carbon Nanotube field effect transistor considering Non-Ballistic conduction

This thesis report is submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Electrical and Electronic Engineering, 2013.Cataloged from PDF version of thesis report.Includes bibliographical references (page 103-112).The need for technological advancement in the field of electronics has been ever increasing. Till now silicon has been the prime material of choice for meeting the current demands. However, silicon has its own limitations; Silicon based integrated circuits and the scaling of silicon MOSFET design faces complications like tunneling effect, gate oxide thickness effect etc. which has given the scope for new materials to emerge. The growing academic interest in carbon nanotubes (CNT) as a promising novel class of electronic material has led to significant progress in the understanding of CNT physics including ballistic and non-ballistic electron transport characteristics. In a nanotube, low bias transport can be nearly ballistic across distances of several hundred nanometers. Non-ballistic CNT transistors have been considered, and extended circuit-level models which can capture both ballistic and non-ballistic electron transport phenomenon, including elastic, phonon scattering, strain and tunneling effects, have been developed. The purpose of this paper is to establish a comparative analysis of the transport characteristics of ballistic and non-ballistic carbon nanotubes. The simulation is carried out using MATLAB and the main focus is on the changes in the I-V characteristic curves of elastic scattering effect, bandgap strain effect, tunneling effect and the overall combined effect, varying the parameters such as gate oxide thickness, temperature, dielectric constant, and chirality. The obtained results were then compared to their respective ballistic results. We verified our work by further comparison of our findings with other established academic papers published under the same category.Nirjhor Tahmidur RoufAshfaqul Haq DeepRusafa Binte HassanB. Electrical and Electronic Engineerin

Physical Modeling of Graphene Nanoribbon Field Effect Transistor Using Non-Equilibrium Green Function Approach for Integrated Circuit Design

The driving engine for the exponential growth of digital information processing systems is scaling down the transistor dimensions. For decades, this has enhanced the device performance and density. However, the International Technology Roadmap for Semiconductors (ITRS) states the end of Moore’s law in the next decade due to the scaling challenges of silicon-based CMOS electronics, e.g. extremely high power density. The forward-looking solutions are the utilization of emerging materials and devices for integrated circuits. The Ph.D. dissertation focuses on graphene, one atomic layer of carbon sheet, experimentally discovered in 2004. Since fabrication technology of emerging materials is still in early stages, transistor modeling has been playing an important role for evaluating futuristic graphene-based devices and circuits. The GNR FET has been simulated by solving a numerical quantum transport model based on self-consistent solution of the 3D Poisson equation and 1D Schrödinger equations within the non-equilibrium Green’s function (NEGF) formalism. The quantum transport model fully treats short channel-length electrostatic effects and the quantum tunneling effects, leading to the technology exploration of graphene nanoribbon field effect transistors (GNRFETs) for the future. A comprehensive study of static metrics and switching attributes of GNRFET has been presented including the performance dependence of device characteristics to the GNR width and the scaling of its channel length down to 2.5 nanometer. It has been found that increasing the GNR width deteriorate the off-state performance of the GNRFET, such that, narrower armchair GNRs improved the device robustness to short channel effects, leading to better off-state performance considering smaller off-current, larger ION/IOFF ratio, smaller subthreshold swing and smaller drain-induced barrier-lowering. The wider armchair GNRs allow the scaling of channel length and supply voltage resulting in better on-state performance such as higher drive current, smaller intrinsic gate-delay time and smaller power-delay product. In addition, the width-dependent characteristics of GNR FETs is investigated for two GNR semiconducting families (3p,0) and (3p+1,0). It has been found that the GNRs(3p+1,0) demonstrates superior off-state performance, while, on the other hand, GNRs(3p,0) shows superior on-state performance. Thus, GNRs(3p+1,0) are promising for low-power design, while GNRs(3p,0) indicate a more preferable attribute for high frequency applications

Carbon Nanotube based approach on FETs using Ternary Comparator

A Carbon Nanotube Field Effect Transistor (CNFET) is referred to as a FET that utilizes a single Carbon Nanotube or an array of carbon nanotubes as the channel material instead of bulk silicon in the traditional MOSFET structure. New approach of ternary magnitude comparator based on the CNFET ternary logic gates. These gates are promising alternative to conventional logic design because of its energy efficiency, it can accomplish by the reduced circuit on ternary logic. Ternary comparator implementation is based on prefix based design and combines ternary and binary logic gates for optimized implementation. A novel approach of a comparator has been implemented and simulated using SPICE. A design response indicates that the 1-bit comparator consumes less power say 0.65µW and has a delay of 21ps. The simulation results for comparators with versatile lengths of operands