Quantum simulation of carbon nanotube field-effect transistors

Abstract

In recent years, a lot of progress in understanding the device physics of carbon nanotube field-effect transistors and in finding their potential applications has been made. In a carbon nanotube, the transport is nearly ballistic across distances of hundreds of nanometers since the scattering in such a ID structure is quite weak. Moreover, the perfect atomic structure allows deposition of high- $\kappa$ gate insulators, and the direct band gap also enables optical emission. Due to these attractive points, carbon nanotube field-effect transistors, where the carbon nanotube is used as a conducting channel, are being considered as one of candidates for future electronic devices. In this thesis, a quantum mechanical simulator for carbon nanotube transistors using the nonequilibrium Green``s function (NEGF) formalism to treat ballistic transport with self-consistent electrostatics has been developed. To understand how the NEGF formalism can be applied to simulate nanoscale devices and how the Schottky Barrier (SB) transistors operate, the Schottky Barrier tunnel transistors are first investigated, and then three types of carbon nanotube electronic devices, Schottky Barrier, MOSFET-like, and p-i-n carbon nanotube transistors, are explored. The Schottky Barrier tunnel transistors were successfully simulated using the NEGF formalism, and it was suggested that the channel length of the Schottky Barrier tunnel transistors can be reduced to about 10 nm without substantial loss in performance in terms of threshold voltage and transistor gain. The carbon nanotube field-effect transistors (CNTFETs) with Schottky contact between the source/drain and the nanotube channel (SB CNTFETs) were examined to show that the ambipolar conduction which leads to large leakage currents is found to be an important factor that determines the device characteristics. Several techniques to suppress the ambipolar conduction in the SB CNTFETs were suggested, and the scaling behaviors of SB C...