Investigation of DC and AC Characteristics in Gate-All-Around Field-Effect Transistor Using the Non-equilibrium Green’s Function Approach

Abstract

Electron transport simulation plays a pivotal role in understanding and predicting the performance of electronic devices. In both industry and academia, Technology Computer-Aided Design (TCAD) is commonly used for device simulation. Additionally, the Multi-Subband Boltzmann Transport Equation (MSBTE) is often employed to incorporate physical aspects of electron transport. However, as device dimensions continue to scale down, quantum effects become increasingly significant. To address these quantum effects, quantum transport simulations have been widely adopted, with the Non-Equilibrium Green’s Function (NEGF) method serving as a standard approach. Over the years, NEGF has evolved to involve complex band structures, improve computational efficiency, and integrate various physical models. It is now widely used in electron transport simulations and frequently serves as a benchmark for validating other transport models. Despite its advancements, NEGF still faces several limitations. One such limitation is that it has traditionally been restricted to DC simulations, whereas TCAD and MSBTE are already capable of AC and transient analyses. AC simulation is particularly important for extracting parameters such as Y-parameters and cutoff frequency, and it also serves as a prerequisite for noise analysis. Another limitation is that NEGF does not deterministically account for the effect of surface roughness. This thesis aims to address these limitations. Chapter 1 provides a brief introduction to device simulation and the NEGF method. Chapter 2 introduces conventional DC NEGF simulation, outlining the governing equations and typical simulation procedure. Chapter 3 presents a fully coupled scheme in which the Poisson equation and NEGF are combined into a single system matrix and solved simultaneously. As a result, the number of iterations required for convergence can be reduced. Moreover, the proposed fully-coupled scheme provides the capability of AC NEGF simulation. In chapter 4, the implementation of AC NEGF is described, extending NEGF’s capabilities beyond DC analysis. The accuracy of the AC NEGF implementation is validated, and current conservation is examined. Subsequently, AC simulations are performed to extract Y-parameters and cutoff frequencies. Plasma instabilities induced by ungated regions are investigated using the AC NEGF approach. In addition, the AC NEGF simulation is extended to include electron-phonon scattering, and the corresponding results were analyzed. Chapter 5 presents NEGF simulations incorporating the mode coupling effect which may be induced by the surface roughness. Mode coupling is represented as a scattering process via this mathematically derived model. The derivation of the proposed geometric scattering model is introduced. The proposed model is validated through the simulation and discussion is provided.