Towards Multi-Scale Modeling of Carbon Nanotube Transistors

Multiscale simulation approaches are needed in order to address scientific and technological questions in the rapidly developing field of carbon nanotube electronics. In this paper, we describe an effort underway to develop a comprehensive capability for multiscale simulation of carbon nanotube electronics. We focus in this paper on one element of that hierarchy, the simulation of ballistic CNTFETs by self-consistently solving the Poisson and Schrodinger equations using the non-equilibrium Greens function (NEGF) formalism. The NEGF transport equation is solved at two levels: i) a semi-empirical atomistic level using the pz orbitals of carbon atoms as the basis, and ii) an atomistic mode space approach, which only treats a few subbands in the tube-circumferential direction while retaining an atomistic grid along the carrier transport direction. Simulation examples show that these approaches describe quantum transport effects in nanotube transistors. The paper concludes with a brief discussion of how these semi-empirical device level simulations can be connected to ab initio, continuum, and circuit level simulations in the multi-scale hierarchy

Determination of key device parameters for short- and long-channel Schottky-type carbon nanotube field-effect transistors

The Schottky barrier, contact resistance and carrier mobility in carbon nanotube (CNT) field-effect transistors (FETs) are discussed in detail in this thesis. Novel extraction methods and definitions are proposed for these parameters. A technology comparison with other emerging transistor technologies and a performance projection study are also presented. A Schottky barrier height extraction method for CNTFETs considering one-dimensional (1D) conditions is developed. The methodology is applied to simulation and experimental data of CNTFETs feasible for manufacturing. Y-function-based methods (YFMs) have been applied to simulation and experimental data in order to extract a contact resistance for CNTFETs. Both extraction methods are more efficient and accurate than other conventional approaches. Practical mobility expressions are derived for CNTFETs covering the ballistic as well as the non-ballistic transport regime which enable a straightforward evaluation of the transport in CNTs. They have been applied to simulation and experimental data of devices with different channel lengths and Schottky barrier heights. A comparison of fabricated emerging transistors based on similar criteria for various application scenarios reveals CNTFETs as promising candidates to compete with Si-based technologies in low-power static and dynamic applications. A performance projection study is suggested for specific applications in terms of the studied design parameters

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

Semi-analytical model for carbon nanotube and graphene nanoribbon transistors

Carbon nanotubes and graphene provide high carrier mobility for ballistic transport, high carrier velocity for fast switching, and excellent mechanical and thermal conductivity. As a result, they are widely considered as next generation candidate materials for nanoelectronics. In this thesis, I first propose a physics-based semi-analytical model for Schottky-barrier (SB) carbon nanotube (CNT) and graphene nanoribbon (GNR) transistors. The model reduces the computational complexity in the two critical but time-consuming steps, namely the calculation of the tunneling probability and the self-consistent evaluation of the surface potential in the transistor channel. Since SB-type CNT and GNR transistors exhibit ambipolar conduction that is not preferable in digital applications, I further propose a semi-analytical model for the double-gate transistor structure that is able to control the ambipolar conduction in-field. Future directions, including the modeling of new CNT and GNR devices and novel circuits based on the in-field controllability of ambipolar conduction, will also be described

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