| Summary: | 碩士 === 國立交通大學 === 電子物理系所 === 92 === Diverse device structures have been recently proposed explored, and found better characteristics than that of conventional used single-gate (SG) metal-oxide-semiconductor field-effect-transistors (MOSFETs). Among them, the double-gate (DG) MOSFETs and gate-all-around MOSFETs (GAA) have been paid of great interest in recent years. Comparing with the SG silicon-on-insulator (SOI), these structures suppress short channel effects, have high transconductance, and sustain subthreshold swing. They have a superior ability in channel control that drain induced channel barrier height lowing (DIBL), threshold voltage roll off, and off state leakage are greatly improved.
We compare the electrical characteristics for the three structures by using the quantum-mechanical (QM) simulation. In this comparison, the different gate voltage, various oxide thickness, and different channel doping concentration are designed. A comprehensive comparison leads to a conclusion that MOSFETs with a gate oxide thinner than 3nm should be carefully corrected with QM model when modeling their physical transport phenomena. The three nanoscale structures SG, DG, and GAA are explored numerically with various theoretical approaches that have been considered to study the quantum confinement effects. Those are full quantum mechanical model (e.g. nonequilibrium Green’s function) and quantum corrections to the Boltzmann transport and classical drift-diffusion (DD) and hydrodynamic (HD) transport models. In the beginning of this work, a set of Schrödinger-Poisson equations has been applied to study the quantum effect in the inversion layers, but it is a time-consuming task in the application to realistic device two-dimensional (2D) and three-dimensional (3D) simulations. In considering with the computational efficiency and simplicity, other quantum correction models are more attractive and greatly used in industrial application.
In comparison, the different quantum correction models such as the Hänsch, the modified local density approximation (MLDA), the density gradient (DG), and the effective potential (EP) model are introduced and studied. Comparing of the 2D electrical characteristics for the three structures, we use the DG method to calculate them. It is easy to converge and can be demonstrated good accuracy in physical characteristics. Finally, we get GAA has the best quality, high mobility, low threshold voltage, superior channel controllability.
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