Theses
http://hdl.handle.net/10012/6
2021-08-01T00:13:05ZImproving the Performance of Nanoscale Field-Effect Transistors Through Electrostatic Engineering
http://hdl.handle.net/10012/17179
Improving the Performance of Nanoscale Field-Effect Transistors Through Electrostatic Engineering
Bennett, Robert
The continued scaling of field-effect transistors (FETs) requires that nearly every aspect of these devices be optimized to ensure that they can continue to meet practical performance requirements. However, scaling the channel lengths of FETs naturally enhances electrostatic and quantum mechanical short-channel effects, thereby increasing leakage currents in the OFF-state, reducing driving currents in the ON-state, and making it difficult for FETs attain optimal switching behaviours. To mitigate these detrimental effects, it is imperative to (i) thoroughly understand the electrostatic operation of nanoscale FETs and (ii) establish novel design strategies to mitigate short-channel effects.
In this thesis, I address these two challenges by studying the electrostatic operation of nanoscale FETs using simulation techniques. In particular, I use the non-equilibrium Green's function method, an atomistic quantum transport simulation technique, to study the electrostatic operation of MOSFETs and to assess the utility of novel electrostatic design strategies for nanoscale FETs.
The body of this thesis is divided into three main works. In the first, I study how individual elements of a metal-oxide-semiconductor FET's (MOSFET's) semiconductor's anisotropic permittivity affect device performance, and I establish electrostatic-based guidelines for selecting optimal semiconductors for future MOSFETs. Next, I study how replacing an FET's conventional isotropic insulators (i.e. gate insulator and spacers) with anisotropic insulators can improve the performance of both conventional MOSFETs and tunnel FETs, and I propose novel insulator architectures to further optimize the performance of these devices. Finally, in my third study, I examine how fringe-induced barrier lowering, an electrostatic short-channel effect created by implementing high-κ gate insulators, can be exploited to suppress quantum mechanical short-channel effects (source-to-drain tunneling) to improve the overall performance of nanoscale MOSFETs. The operating principles and design rules established in these three works extend the current picture of the electrostatic operation and design rules for nanoscale FETs to help device designers continue to scale FETs while meeting essential performance benchmarks.
2021-07-30T00:00:00ZInformation Retrieval Evaluation Measures Based on Preference Graphs
http://hdl.handle.net/10012/17178
Information Retrieval Evaluation Measures Based on Preference Graphs
Luo, Chengxi
Offline evaluation for web search has used mostly graded judgments to evaluate the performance of information retrieval systems. While graded judgments suffer several known problems, preference judgments simply judge one item over another, which avoids the problem of complex definition of relevance scores. Previous research about evaluation measures for preference judgments focuses on translating preferences into relevance scores applied in the traditional evaluation measures, or weighting and counting the number of agreements between actual ranking from users’ preferences and ideal ranking generated by systems. However, these measures lack clear theoretical foundations and their values have no obvious interpretation. On the other hand, although preference judgments for general web search have been studied extensively, there is limited research on investigating preference judgments application for web image search.
This thesis addresses exactly these questions, which proposes a preference-based evaluation measure to compute the maximum similarity between an actual ranking from users’ preferences and an ideal ranking generated by systems. Specifically, this measure constructs a directed multigraph and computes the ordering of vertices, which we call the ideal ranking, that has maximum similarity to actual ranking calculated by the rank similarity measure. This measure is able to take any arbitrary collection of preferences that might include the property of conflicts, redundancies, incompleteness, and diverse type results (documents or images). Our results show that Greedy PGC matches or exceeds the performance of evaluation measures proposed in previous research.
2021-07-30T00:00:00ZLaminar Separation Bubble Dynamics on a Finite Wing
http://hdl.handle.net/10012/17177
Laminar Separation Bubble Dynamics on a Finite Wing
Toppings, Connor
Laminar separation bubbles substantially influence the performance of finite wings at low
chord Reynolds numbers. The objective of this study is to explore the influence of wingtip
effects on three-dimensional laminar separation bubble topology and dynamics on a finite
wing. An experimental investigation is conducted on a laminar separation bubble forming on
the suction surface of a cantilevered rectangular NACA 0018 wing with a semi-aspect ratio
of 2.5 at a chord Reynolds number of 125 000 and an angle of attack of 6 degrees. Surface pressure
and particle image velocimetry measurements are employed to investigate the separation
bubble flowfield. Using a two-dimensional airfoil of the same profile, the separation bubble
on the wing is compared to a nominally two-dimensional separation bubble at similar
effective angles of attack. On the portion of the wing where laminar boundary layer
separation occurs, the separated shear layer rolls up into spanwise uniform vortices which
develop similarly to the vortices observed on the two-dimensional airfoil, despite spanwise
changes to the mean separation bubble structure along the wingspan. Whereas a decrease in
the angle of attack of the two-dimensional airfoil causes a downstream shift in the locations
of separation and reattachment and a reduction in the frequency of shear layer vortex
shedding, spanwise variations of these parameters on the wing are much smaller than the
variations expected due to the reduction in effective angle of attack near the wingtip. On the
inboard portion of the wing, the location and vortex shedding frequency of the separation
bubble are analogous to the separation bubble on the two-dimensional airfoil at the effective
angle of attack of the wing root. Downwash from the wingtip vortex inhibits boundary layer
separation in proximity to the wingtip, suppressing shear layer vortex shedding and causing
a delay in transition near the wingtip. Unlike a canonical two-dimensional separation bubble,
the separation bubble on the wing becomes an open separation near the wingtip, where
the spanwise pressure gradient causes fluid to enter into the separation bubble, producing
a substantial spanwise flow within recirculation region. A comparison with the results of
previous studies suggests a similar bubble topology across different wing geometries and
experimental conditions. The results of this investigation quantify the influence of wingtip
effects on a laminar separation bubble, elucidating the three-dimensional changes to the
bubble’s mean structure and dynamics along the wingspan.
2021-07-30T00:00:00ZPragmatic Groundwater-Surface Water Model Coupling with Unstructured Grids
http://hdl.handle.net/10012/17176
Pragmatic Groundwater-Surface Water Model Coupling with Unstructured Grids
Scantlebury, Leland
Faced with an array of water issues exacerbated by a rapidly changing climate, hydrologists and hydrogeologists have increasingly found themselves needing to simultaneously model the groundwater and surface water domains together. Historically, for convenience and due to computational limitations, they have been modeled separately, with tools evolving based upon the different needs and questions driving researchers and practitioners in each domain. The tools emerging to solve these new problems range from highly complex, fully coupled, parallelized software solutions requiring enormous computational resources, to comparatively simple combinations of existing models sharing fluxes between the domains. Both groups generally have utilized relatively inflexible representations of the surface-water domain, often with a fixed level of complexity that prevents explorations of model structural uncertainty and process algorithmic skill. In this thesis, a loosely coupled groundwater-surface water modelling framework is presented that allows for adjustable model complexity in both domains. This is accomplished through pairing MODFLOW-USG, a recent version of the industry-standard MODFLOW family of modular groundwater modelling codes that allows for unstructured model grids, with Raven, a state-of-the-art surface water modelling framework supports flexible representations of hydrologic processes, forcing interpolation, and spatial discretization schemes. The resulting software, compiled into a single executable, is aimed at modelling watersheds at the regional scale. Recharge estimated by Raven is directly entered into the MODFLOW-USG flow solution. River-groundwater interactions are handled through a novel sub-grid river package added to MODFLOW-USG, called the polyline boundary junction (PBJ) package. The PBJ method evaluates boundary conditions along individual segment locations within a grid’s dual Delaunay triangulation and geometrically distributes the resultant fluxes to the appropriate Voronoi and/or rectangular cells. Groundwater heads are interpolated along the segment to handle head-dependent flux calculations. The resulting river fluxes are added or subtracted from the Raven river channel water balance, allowing for a closed simulation of the hydrologic cycle. The new coupled Raven framework is demonstrated on the Alder Creek watershed in Southern Ontario and shown to produce physically realistic flows between the surface and subsurface domains.
2021-07-29T00:00:00Z