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R.W. Kelsall

Bio: R.W. Kelsall is an academic researcher from University of Leeds. The author has contributed to research in topics: Dynamic Monte Carlo method & Monte Carlo method. The author has an hindex of 1, co-authored 1 publications receiving 425 citations.

Papers
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Proceedings ArticleDOI
R.W. Kelsall1
03 Apr 1995
TL;DR: If the authority ascribed to Monte Carlo models of devices at 1/spl mu/m feature size is to be maintained, modelling of the fundamental physics must be further improved, and the device model must be made more realistic.
Abstract: There can be little doubt that the Monte Carlo method for semiconductor device simulation has enormous power as a research tool. It represents a detailed physical model of the semiconductor material(s), and provides a high degree of insight into the microscopic transport processes. However, if the authority ascribed to Monte Carlo models of devices at 1/spl mu/m feature size is to be maintained for devices below O.1/spl mu/m, modelling of the fundamental physics must be further improved. And if the Monte Carlo method is to be successful as a semiconductor device design tool, the device model must be made more realistic. Success in the industrial sector depends on this, but also on achieving fast run-times optimisation - where the scope and need for ingenuity is now greatest.

436 citations


Cited by
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Journal ArticleDOI
TL;DR: This review article aims at providing a compendium of the available methods, their theoretical foundations, and their ranges of validity for organic molecular crystals, and illustrates these through applications found in the literature.
Abstract: The booming field of molecular electronics has fostered a surge of computational research on electronic properties of organic molecular solids. In particular, with respect to a microscopic understanding of transport and loss mechanisms, theoretical studies assume an ever-increasing role. Owing to the tremendous diversity of organic molecular materials, a great number of computational methods have been put forward to suit every possible charge transport regime, material, and need for accuracy. With this review article we aim at providing a compendium of the available methods, their theoretical foundations, and their ranges of validity. We illustrate these through applications found in the literature. The focus is on methods available for organic molecular crystals, but mention is made wherever techniques are suitable for use in other related materials such as disordered or polymeric systems.

313 citations

Journal ArticleDOI
03 Sep 2008
TL;DR: An introduction to the nonequilibrium Green's function (NEGF) approach, which is a powerful conceptual tool and a practical analysis method to treat nanoscale electronic devices with quantum mechanical and atomistic effects.
Abstract: We aim to provide engineers with an introduction to the nonequilibrium Green's function (NEGF) approach, which is a powerful conceptual tool and a practical analysis method to treat nanoscale electronic devices with quantum mechanical and atomistic effects. We first review the basis for the traditional, semiclassical description of carriers that has served device engineers for more than 50 years. We then describe why this traditional approach loses validity at the nanoscale. Next, we describe semiclassical ballistic transport and the Landauer-Buttiker approach to phase-coherent quantum transport. Realistic devices include interactions that break quantum mechanical phase and also cause energy relaxation. As a result, transport in nanodevices is between diffusive and phase coherent. We introduce the NEGF approach, which can be used to model devices all the way from ballistic to diffusive limits. This is followed by a summary of equations that are used to model a large class of structures such as nanotransistors, carbon nanotubes, and nanowires. Applications of the NEGF method in the ballistic and scattering limits to silicon nanotransistors are discussed.

303 citations

Journal ArticleDOI
TL;DR: A variational method EnVarA (energy variational analysis) is used that combines Hamilton's least action and Rayleigh's dissipation principles to create a variational field theory that includes flow, friction, and complex structure with physical boundary conditions of ions next to a charged wall.
Abstract: Ionic solutions are mixtures of interacting anions and cations. They hardly resemble dilute gases of uncharged noninteracting point particles described in elementary textbooks. Biological and electrochemical solutions have many components that interact strongly as they flow in concentrated environments near electrodes, ion channels, or active sites of enzymes. Interactions in concentrated environments help determine the characteristic properties of electrodes, enzymes, and ion channels. Flows are driven by a combination of electrical and chemical potentials that depend on the charges, concentrations, and sizes of all ions, not just the same type of ion. We use a variational method EnVarA (energy variational analysis) that combines Hamilton’s least action and Rayleigh’s dissipation principles to create a variational field theory that includes flow, friction, and complex structure with physical boundary conditions. EnVarA optimizes both the action integral functional of classical mechanics and the dissipation functional. These functionals can include entropy and dissipation as well as potential energy. The stationary point of the action is determined with respect to the trajectory of particles. The stationary point of the dissipation is determined with respect to rate functions (such as velocity). Both variations are written in one Eulerian (laboratory) framework. In variational analysis, an “extra layer” of mathematics is used to derive partial differential equations. Energies and dissipations of different components are combined in EnVarA and Euler–Lagrange equations are then derived. These partial differential equations are the unique consequence of the contributions of individual components. The form and parameters of the partial differential equations are determined by algebra without additional physical content or assumptions. The partial differential equations of mixtures automatically combine physical properties of individual (unmixed) components. If a new component is added to the energy or dissipation, the Euler–Lagrange equations change form and interaction terms appear without additional adjustable parameters. EnVarA has previously been used to compute properties of liquid crystals, polymer fluids, and electrorheological fluids containing solid balls and charged oil droplets that fission and fuse. Here we apply EnVarA to the primitive model of electrolytes in which ions are spheres in a frictional dielectric. The resulting Euler–Lagrange equations include electrostatics and diffusion and friction. They are a time dependent generalization of the Poisson–Nernst–Planck equations of semiconductors, electrochemistry, and molecular biophysics. They include the finite diameter of ions. The EnVarA treatment is applied to ions next to a charged wall, where layering is observed. Applied to an ion channel, EnVarA calculates a quick transient pile-up of electric charge, transient and steady flow through the channel, stationary “binding” in the channel, and the eventual accumulation of salts in “unstirred layers” near channels. EnVarA treats electrolytes in a unified way as complex rather than simple fluids. Ad hoc descriptions of interactions and flow have been used in many areas of science to deal with the nonideal properties of electrolytes. It seems likely that the variational treatment can simplify, unify, and perhaps derive and improve those descriptions.

264 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present results of the numerical simulation of the transient behavior of shallow junction single photon avalanche diodes (SPADs) and develop a bidimensional model for above breakdown simulations and show that the initially photogenerated charge density builds up locally by an avalanche multiplication process and then spreads over the entire detector area by a diffusion-assisted process.
Abstract: We present results of the numerical simulation of the transient behavior of shallow junction single photon avalanche diodes (SPAD's). We developed a bidimensional model for above breakdown simulations and show that the initially photogenerated charge density builds up locally by an avalanche multiplication process and then spreads over the entire detector area by a diffusion-assisted process. To model real geometries, we developed a simplified model based on the obtained results. The importance of the photon-assisted spreading mechanism is evaluated and compared with the diffusive one. The contribution of the photon-assisted mechanism is minor in these geometries. The model is compared with the experimental data on the avalanche leading edge and the timing resolution; the agreement is good. We conclude that the model can be considered to be a useful tool for the design of improved structures.

203 citations

Journal ArticleDOI
TL;DR: In this article, the performance, scalability, and resilience of Si SOI FinFETs and gate-all-around (GAA) nanowires (NWs) are studied using in-house-built 3D simulation tools.
Abstract: Performance, scalability, and resilience to variability of Si SOI FinFETs and gate-all-around (GAA) nanowires (NWs) are studied using in-house-built 3-D simulation tools. Two experimentally based devices, a 25-nm gate length FinFET and a 22-nm GAA NW are modeled and then scaled down to 10.7- and 10-nm gate lengths, respectively. A TiN metal gate work-function granularity (MGG) and line edge roughness (LER) induced variability affecting OFF and ON characteristics are investigated and compared. In the OFF-region, the FinFETs have over an order of magnitude larger OFF-current that those of the equivalent GAA NWs. In the ON-region, the 25/10.7-nm gate length FinFETs deliver 20/58% larger ON-current than the 22/10-nm gate length GAA NWs. The FinFETs are more resilient to the MGG and LER variability in the subthreshold compared to the GAA NWs. However, the MGG ON-current variability is larger for the 10.7-nm FinFET than that for the 10-nm GAA NW. The LER ON-current variability depends largely on the RMS height; whereas a 0.6-nm RMS height yields a similar variability for both FinFETs and GAA NWs. Finally, the industry preferred 〈110〉 channel orientation is more resilient to the MGG and LER variability in both architectures.

140 citations