1,779 research outputs found
Simulation of hot carriers in semiconductor devices
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1995.Includes bibliographical references (p. 109-113).by Khalid Rahmat.Ph.D
Simulation of hot carriers in semiconductor devices
Includes bibliographical references (p. 109-113).Supported by the U.S. Navy. N00174-93-C-0035Khalid Rahmat
Angular adaptivity with spherical harmonics for Boltzmann transport
This paper describes an angular adaptivity algorithm for Boltzmann transport
applications which uses Pn and filtered Pn expansions, allowing for different
expansion orders across space/energy. Our spatial discretisation is
specifically designed to use less memory than competing DG schemes and also
gives us direct access to the amount of stabilisation applied at each node. For
filtered Pn expansions, we then use our adaptive process in combination with
this net amount of stabilisation to compute a spatially dependent filter
strength that does not depend on a priori spatial information. This applies
heavy filtering only where discontinuities are present, allowing the filtered
Pn expansion to retain high-order convergence where possible. Regular and
goal-based error metrics are shown and both the adapted Pn and adapted filtered
Pn methods show significant reductions in DOFs and runtime. The adapted
filtered Pn with our spatially dependent filter shows close to fixed iteration
counts and up to high-order is even competitive with P0 discretisations in
problems with heavy advection.Comment: arXiv admin note: text overlap with arXiv:1901.0492
Synergies between Numerical Methods for Kinetic Equations and Neural Networks
The overarching theme of this work is the efficient computation of large-scale systems. Here we deal with two types of mathematical challenges, which are quite different at first glance but offer similar opportunities and challenges upon closer examination.
Physical descriptions of phenomena and their mathematical modeling are performed on diverse scales, ranging from nano-scale interactions of single atoms to the macroscopic dynamics of the earth\u27s atmosphere. We consider such systems of interacting particles and explore methods to simulate them efficiently and accurately, with a focus on the kinetic and macroscopic description of interacting particle systems.
Macroscopic governing equations describe the time evolution of a system in time and space, whereas the more fine-grained kinetic description additionally takes the particle velocity into account.
The study of discretizing kinetic equations that depend on space, time, and velocity variables is a challenge due to the need to preserve physical solution bounds, e.g. positivity, avoiding spurious artifacts and computational efficiency.
In the pursuit of overcoming the challenge of computability in both kinetic and multi-scale modeling, a wide variety of approximative methods have been established in the realm of reduced order and surrogate modeling, and model compression. For kinetic models, this may manifest in hybrid numerical solvers, that switch between macroscopic and mesoscopic simulation, asymptotic preserving schemes, that bridge the gap between both physical resolution levels, or surrogate models that operate on a kinetic level but replace computationally heavy operations of the simulation by fast approximations.
Thus, for the simulation of kinetic and multi-scale systems with a high spatial resolution and long temporal horizon, the quote by Paul Dirac is as relevant as it was almost a century ago.
The first goal of the dissertation is therefore the development of acceleration strategies for kinetic discretization methods, that preserve the structure of their governing equations. Particularly, we investigate the use of convex neural networks, to accelerate the minimal entropy closure method. Further, we develop a neural network-based hybrid solver for multi-scale systems, where kinetic and macroscopic methods are chosen based on local flow conditions.
Furthermore, we deal with the compression and efficient computation of neural networks. In the meantime, neural networks are successfully used in different forms in countless scientific works and technical systems, with well-known applications in image recognition, and computer-aided language translation, but also as surrogate models for numerical mathematics.
Although the first neural networks were already presented in the 1950s, the scientific discipline has enjoyed increasing popularity mainly during the last 15 years, since only now sufficient computing capacity is available. Remarkably, the increasing availability of computing resources is accompanied by a hunger for larger models, fueled by the common conception of machine learning practitioners and researchers that more trainable parameters equal higher performance and better generalization capabilities. The increase in model size exceeds the
growth of available computing resources by orders of magnitude. Since , the computational resources used in the largest neural network models doubled every months\footnote{\url{https://openai.com/blog/ai-and-compute/}}, opposed to Moore\u27s Law that proposes a -year doubling period in available computing power.
To some extent, Dirac\u27s statement also applies to the recent computational challenges in the machine-learning community. The desire to evaluate and train on resource-limited devices sparked interest in model compression, where neural networks are sparsified or factorized, typically after training. The second goal of this dissertation is thus a low-rank method, originating from numerical methods for kinetic equations, to compress neural networks already during training by low-rank factorization.
This dissertation thus considers synergies between kinetic models, neural networks, and numerical methods in both disciplines to develop time-, memory- and energy-efficient computational methods for both research areas
Multidisciplinary benchmarks of a conservative spectral solver for the nonlinear Boltzmann equation
The Boltzmann equation describes the evolution of the phase-space probability
distribution of classical particles under binary collisions. Approximations to
it underlie the basis for several scholarly fields, including aerodynamics and
plasma physics. While these approximations are appropriate in their respective
domains, they can be violated in niche but diverse applications which require
direct numerical solution of the original nonlinear Boltzmann equation. An
expanded implementation of the Galerkin-Petrov conservative spectral algorithm
is employed to study a wide variety of physical problems. Enabled by
distributed precomputation, solutions of the spatially homogeneous Boltzmann
equation can be achieved in seconds on modern personal hardware, while
spatially-inhomogeneous problems are solvable in minutes. Several benchmarks
against both analytic theoretical predictions and comparisons to other
Boltzmann solvers are presented in the context of several domains including
weakly ionized plasma, gaseous fluids, and atomic-plasma interaction.Comment: 17 pages, 5 figures, 1 tabl
๋ฌดํํ ์๊ด๊ธธ์ด๋ฅผ ๊ฐ์ง๋ ์์ค๋ ์์ก ๋ฐฉ์ ์์ ๊ฒฐ์ ๋ก ์ ํด
ํ์๋
ผ๋ฌธ(๋ฐ์ฌ) -- ์์ธ๋ํ๊ต๋ํ์ : ๊ณต๊ณผ๋ํ ์ ๊ธฐยท์ ๋ณด๊ณตํ๋ถ, 2022. 8. ์ต์ฐ์.We propose a new discrete formulation of the Wigner transport equation (WTE) with infinite correlation length of potentials. Since the maximum correlation length is not limited to a finite value, there is no uncertainty in the simulation results, and Wigner-Weyl transformation is unitary in our expression. For general and efficient simulation, the WTE is solved self-consistently with the Poisson equation through the finite volume method and the fully coupled Newton-Raphson scheme. By applying the proposed model to resonant tunneling diodes and double gate MOSFET, transient and steady-state simulation results including scattering effects are shown.๋ณธ ์ฐ๊ตฌ์์๋ ๋ฌดํํ ์๊ด ๊ธธ์ด๋ฅผ ๊ฐ์ง๋ ์๊ทธ๋ ์์ก ๋ฐฉ์ ์์ ์๋ก์ด ์์นํด์์ ํ์ด๋ฒ์ ์ ์ํ์๋ค. ์ต๋ ์๊ด ๊ธธ์ด๊ฐ ํ์ ๋ ๊ฐ์ผ๋ก ์ ํ๋์ง ์๊ธฐ ๋๋ฌธ์, ์๋ฎฌ๋ ์ด์
๊ฒฐ๊ณผ์ ๋ถํ์ค์ฑ์ด ๋ฐ์ํ์ง ์์ผ๋ฉฐ, ์ ์๋ ํํ๋ฒ์์๋ Wigner-Weyl ๋ณํ์ด unitaryํ๋ค. ์ผ๋ฐ์ ์ด๊ณ ํจ์จ์ ์ธ ์๋ฎฌ๋ ์ด์
์ ์ํด, ์๊ทธ๋ ์์ก ๋ฐฉ์ ์์ ํธ์์ก ๋ฐฉ์ ์๊ณผ ์ ํ ์ฒด์ ๋ฒ๊ณผ ๋ดํด-๋ฉ์จ ๋ฐฉ์์ ํตํด self-consistentํ๊ฒ ํ์๋ค. ์ ์๋ ๋ชจ๋ธ์ resonant tunneling diode์ double gate MOSFET์ ์ ์ฉํ์ฌ, ์ฐ๋ํจ๊ณผ๋ฅผ ๊ณ ๋ คํ ๋์ ๊ทธ๋ฆฌ๊ณ ์ ์ ์๋ฎฌ๋ ์ด์
๊ฒฐ๊ณผ๋ฅผ ๋ณด์ฌ์ฃผ์๋ค.Chapter 1 Introduction 1
1-1 Various models for device simulation 1
1-2 Numerical problems in solving WTE 5
Chapter 2 Simulation methods 10
2-1 WTE with infinite correlation length 10
2-2 Numerical Methods 13
2-3 Multi-dimensional Simulation Methods 23
Chapter 3 Simulation methods 26
2-1 Simulation results according to the correlation length 26
2-2 Simulation for resonant tunneling diode 30
2-3 Simulation for double gate MOSFET 51
Chapter 4 Conclusion 70
Appendix 72
A-1 Numerical integration method of the nonlocal potential terms 72
A-2 2D electron density and electric potential results 75
A-3 Wigner function for each subband 78
References 85
Abstract 92๋ฐ
Trinity: A Unified Treatment of Turbulence, Transport, and Heating in Magnetized Plasmas
To faithfully simulate ITER and other modern fusion devices, one must resolve
electron and ion fluctuation scales in a five-dimensional phase space and time.
Simultaneously, one must account for the interaction of this turbulence with
the slow evolution of the large-scale plasma profiles. Because of the enormous
range of scales involved and the high dimensionality of the problem, resolved
first-principles global simulations are very challenging using conventional
(brute force) techniques. In this thesis, the problem of resolving turbulence
is addressed by developing velocity space resolution diagnostics and an
adaptive collisionality that allow for the confident simulation of velocity
space dynamics using the approximate minimal necessary dissipation. With regard
to the wide range of scales, a new approach has been developed in which
turbulence calculations from multiple gyrokinetic flux tube simulations are
coupled together using transport equations to obtain self-consistent,
steady-state background profiles and corresponding turbulent fluxes and
heating. This approach is embodied in a new code, Trinity, which is capable of
evolving equilibrium profiles for multiple species, including electromagnetic
effects and realistic magnetic geometry, at a fraction of the cost of
conventional global simulations. Furthermore, an advanced model physical
collision operator for gyrokinetics has been derived and implemented, allowing
for the study of collisional turbulent heating, which has not been extensively
studied. To demonstrate the utility of the coupled flux tube approach,
preliminary results from Trinity simulations of the core of an ITER plasma are
presented.Comment: 187 pages, 53 figures, Ph.D. thesis in physics at University of
Maryland, single-space versio
Numerical methods for drift-diffusion models
The van Roosbroeck system describes the semi-classical transport of free electrons and holes in a self-consistent electric field using a drift-diffusion approximation. It became the standard model to describe the current flow in semiconductor devices at macroscopic scale. Typical devices modeled by these equations range from diodes, transistors, LEDs, solar cells and lasers to quantum nanostructures and organic semiconductors. The report provides an introduction into numerical methods for the van Roosbroeck system. The main focus lies on the Scharfetter-Gummel finite volume discretization scheme and recent efforts to generalize this approach to general statistical distribution functions
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