Suppression of Ambipolar Conduction in Schottky Barrier Carbon Nanotube Field Effect Transistors: Modeling, Optimization Using Particle Swarm Intelligence, and Fabrication

摘要

A mathematical model and experimental analysis of the impact of oxide thickness on the ambipolar conduction in Schottky Barrier Carbon Nanotubes (CNTs) Field Effect Transistor (SB CNTFETs) is presented. Suppression of ambipolar conduction in SB CNTFETs is imperative in order to establish them as the future of IC technology. The ambipolar nature of SB CNTFETs leads to a great amount of leakage current. Employing a gate oxide dielectric of thickness, t(ox)similar to 50 nm suppresses the ambipolar behavior. In an SB CNTFET, it is the electric field at the source/drain contacts that control the conductance and the band bending length at the contacts is defined by t(ox). Therefore, tox is the prime parameter that influences the width of the Schottky barrier and the current in the subthreshold region. Due to the wide SB, there is a loss in on-current due to tunneling, but the current due to thermionic emission is increased by employing a high-kappa dielectric such as Zirconium dioxide (ZrO2). This work proposes an approach to suppress ambipolar behavior in SB CNTFETs without decreasing the on current. The thickness and dielectric constant of the gate oxide are optimized using the particle swarm optimization (PSO) algorithm to achieve suppression of ambipolar conduction without any loss in on-current. The proposed SB CNTFET was modeled using Verilog-A. Experimental demonstration of the suppression of ambipolar property is also presented. Two SB CNTFETs are fabricated using high-kappa dielectric such as ZrO2 with different thickness. A device with thin (similar to 5 nm) gate oxide and another device with thick (similar to 50 nm) gate oxide were fabricated. From the experimental results, it is observed that the device with the thin gate oxide exhibited ambipolar characteristics and the device with the thick gate oxide did not exhibit ambipolar characteristics. The increase in thickness, tox, ensures suppression of ambipolar behavior.