Transport Performance Projection of Emerging Nanoscale Devices

Abstract

Emerging devices, such as those based on carbon (in the form of graphene or nanotubes) or III-V compound semiconductors, are constructed on an atomic scale, where the transport is governed by the Schrödinger equation of quantum mechanics or the Boltzmann transport equation (BTE) of semi-classical mechanics. The solutions of the Schrödinger equation and the BTE offer an opportunity not only to explore and understand the rich physics of small-scale devices, but also to predict their performance potential. The Schrödinger equation can generally be tackled by the method of nonequilibrium Green’s functions (NEGF), and the BTE can be solved with the aid of commercial numerical software, such as COMSOL. In this doctoral work, we utilize these state-of-the-art transport approaches to study the performance of emerging nanoscale transistors, namely, III-V high-electron-mobility transistors (HEMTs) and carbon-nanotube transistors (CNFETs). In the first stage of work, we use the NEGF approach to show how quantum-mechanical transport impacts the cutoff frequencies of III-V HEMTs as the gate length is shrunk. We demonstrate that the cutoff frequencies tend to saturate as the gate length is scaled down, i.e., that they attain a maximum value that ceases to improve with further scaling, and we tie this behavior to the low effective mass of electrons in III-V materials, which is a transport property. In the second stage of work, we examine the impact of electron scattering on the performance of CNFETs via the BTE. We show that the collisions of electrons with substrate phonons (arising from lattice vibrations within the substrate on which the CNFET resides) is critical to their performance, and we thereby identify the best and worst choices of substrate for optimum performance. For future work, we propose the creation of a tool that captures the transport of electrons in quantum-dot solar cells. The tool would utilize NEGF to account for quantum-mechanical transport in the presence of light, and its aim would be to facilitate the systematic understanding of cell operation and hence optimal cell design.