THE INCREASING demand for higher computing power, smaller dimensions, and lower power consumption of integrated circuits leads to a pressing need to downscale semiconductor components. However, downscaling of conventional MOSFETs leads to many problems, such as short-channel effects, gate-leakage current, and so forth. Therefore, novel structure and materials such as multiple gate MOSFETs, CNT-FETs, and molecular based transistors, are expected to be introduced to meet the requirements for scaling .
Since the discovery of carbon nanotubes (CNTs) by IIJIMA in 1991 , significant progress has been achieved in both understanding the fundamental properties and exploring possible engineering applications. The possible application for nano-electronic devices has been extensively explored since the demonstration of the first CNT transistors (CNT-FETs) [3,4].
CNTs are attractive for nano-electronic applications due to their excellent electrical properties. The phase space for scattering is severely reduced due to the one-dimensional nature of the density of states. The low scattering probability is responsible for high on-current in semiconducting CNT transistors. Due to the chemical stability and perfection of the CNT structure carrier mobility is not affected by processing and roughness scattering as it is in the conventional semi-conductor channel. The fact that there are no dangling bond states at the surface of CNTs allows for a wide choice of gate insulators. This improves gate control while meeting gate leakage constrains. The purely one-dimensional transport properties of the SW-CNTs should lead to a suppression of short-channel effects in transistor devices . Furthermore, the conduction and valence bands are symmetric, which is advantageous for complementary applications, and finally, the combined impact of transport and electrostatic benefits together with the fact that semiconducting CNTs are, unlike silicon, direct-gap materials, suggests applications in opto-electronics as well [6,7].
Chapter 2 describes the fundamentals of CNTs. It presents a comprehensive overview of electron and phonon properties along with electron-phonon interaction parameters, which are the key points to understand transport phenomena in CNTs. The chapter continues with a brief historical overview of CNT-FETs. The operation of these devices can be explained in terms of SCHOTTKY barriers which are formed at the metal-CNT interfaces. CNT-FETs can operate by modulating the transmission coefficient through these barriers, which results in device characteristics different from that of conventional MOSFETs.
Chapter 3 outlines the theory of the non-equilibrium GREEN's function (NEGF) formalism. Knowledge of the single-particle GREEN's function provides both the complete equilibrium or non-equilibrium properties of the system and the excitation energies of the systems containing one more or one less particle. The many-particle information about the system is cast into self-energies, parts of the equations of motion for the GREEN's functions. GREEN's functions can be expressed as a perturbation expansion, which is the key to approximate the self-energies. GREEN's functions provide a very powerful technique for evaluating properties of many-particle systems both in thermodynamic equilibrium and also in non-equilibrium situations. This formalism has been successfully used to investigate the characteristics of nano-scale transistors [8,9], CNT-FETs [5,10], and molecular transistors .
Chapter 4 discusses the numerical implementation of the NEGF formalism to study quantum transport in CNT-FETs. To solve the transport equations numerically they have to be discretized. The discretization of the transport equations in both the spatial and energy domain are discussed in detail. We employed a tight-binding HAMILTONian and applied a mode-space transformation to reduce the computational cost. The calculation of self-energies due to electron-phonon interactions are also presented in this chapter. Finally, the iterative method for self-consistent simulation and its convergence rate is studied.
In Chapter 5 several applications are discussed. By using the described methodology the physics of CNT-FET has been explored. A comprehensive study of the role of electron-phonon interaction on the performance of CNT-FETs is presented. Scaling of some geometrical parameters is investigated and we show that by appropriately selecting these parameters considerable improved performance can be achieved.
Finally, Chapter 6 briefly summarizes the thesis with some conclusions.M. Pourfath: Numerical Study of Quantum Transport in Carbon Nanotube-Based Transistors