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We establish the dependence of the permittivity of oxidized ultra-thin silicon films on the film thickness by means of atomistic simulations within the density-functional-based tight-binding (DFTB) theory. This is of utmost importance for modeling ultra-thin and extremely thin silicon-on-insulator MOSFETs, and for evaluating their scaling potential. We demonstrate that electronic contribution to the dielectric response naturally emerges from the DFTB Hamiltonian when coupled to Poisson equation solved in vacuum, without phenomenological parameters, and obtain good agreement with the available experimental data. Comparison with calculations of H-passivated Si films reveals much weaker dependence of permittivity on film thickness for the SiO2-passivated Si, with less than 18% reduction in the case of 0.9-nm silicon-on-insulator.

In this paper, we present a framework for the simulation of electronic devices based on a multiscale and multiphysics approach. A formal description is provided that includes both multiscale and multiphysics problems and which can be linked to already established multiscale methods. We present a set of simulations of an AlGaN/GaN nanocolumn based on a multiscale coupling between atomistic descriptions and continuous media models, illustrating the application of such a multiscale approach to electronic device simulation.

We report density-functional theory (DFT) atomistic simulations of the nonequilibrium transport properties of carbon nanotube (CNT) field-effect transistors (FETs). Results have been obtained within a self-consistent approach based on the nonequilibrium Green's functions (NEGF) scheme. We show that, as the current modulation mechanism is based on the local screening properties of the nanotube channel, a completely new, negative quantum capacitance regime can be entered by the device. We show how a well-tempered device design can be accomplished in this regime by choosing suitable doping profiles and gate contact parameters. At the same time, we detail the fundamental physical mechanisms underlying the bulk-switching operation, including them in a very practical and accurate model, whose parameters can be easily controlled in order to improve the device performance. The dependence of the nanotube screening properties on the temperature is finally explained by means of a self-consistent temperature analysis

We report atomistic simulations of the transport properties of Si-nanowire (SiNW) field-effect transistors. Results have been obtained within a self-consistent approach based on the nonequilibrium Green's function (NEGF) scheme in the density functional theory framework. We analyze in detail the operation of an ultrascaled SiNW channel device and study the characteristics and transfer characteristics behavior of the device while varying several parameters including doping, gate and oxide lengths, and temperature. We focus our attention to the quantum capacitance of the SiNW and show that a well-tempered device design can be accomplished in this regime by choosing suitable doping profiles and gate contact parameters.

In this paper, we investigate the influence of molecular vibrations on the tunneling of electrons through alkeno-thiolates of varying lengths sandwiched in between two gold contacts. The study is confined to the elastic scattering. The vibrational modes are treated quantum-mechanically and the tunneling current is computed as an ensemble average over the distribution of the atomic configurations, obtained by a suitable approximation of the density matrix for the normal mode oscillators. The quantum-mechanical treatment is necessary in order to correctly include the zero-point fluctuations. The calculations show no temperature dependence for the tunneling current in the regime between 270-350 K.