L. Donetti

Entangled Networks, Synchronization, and Optimal Network Topology

A new family of graphs, entangled networks, with optimal properties in many respects, is introduced. By definition, their topology is such that it optimizes synchronizability for many dynamical processes. These networks are shown to have an extremely homogeneous structure: degree, node distance, betweenness, and loop distributions are all very narrow. Also, they are characterized by a very interwoven (entangled) structure with short average distances, large loops, and no well-defined community structure. This family of nets exhibits an excellent performance with respect to other flow properties such as robustness against errors and attacks, minimal first-passage time of random walks, efficient communication, etc. These remarkable features convert entangled networks in a useful concept, optimal or almost optimal in many senses, and with plenty of potential applications in computer science or neuroscience.

A Comprehensive Study of the Corner Effects in Pi-Gate MOSFETs Including Quantum Effects

In this paper, simulation-based research on the electrostatics of Pi-gate silicon-on-insulator (SOI) MOSFETs is carried out. To do so, a 2-D self-consistent Schrodinger-Poisson solver has been implemented. The inclusion of the quantum effects has been demonstrated to be necessary for the accurate simulation of these devices in the nanometer range. Specifically, this paper is focused on the corner effects in multiple-gate SOI MOSFETs, defined as the formation of independent channels with different threshold voltages. Corner effects are studied as a function of different parameters, such as the doping density, silicon-fin dimensions, corner rounding, and gate oxide thickness. Finally, the relation between corner effects and the transition from a fully to a partially depleted body is analyzed.

Acoustic phonon confinement in silicon nanolayers: Effect on electron mobility

We demonstrate the confinement of acoustic phonons in ultrathin silicon layers and study its effect on electron mobility. We develop a model for confined acoustic phonons in an ideal single-layer structure and in a more realistic three-layer structure. Phonon quantization is recovered, and the dispersion relations for distinct phonon modes are computed. This allows us to obtain the confined phonon scattering rates and, using Monte Carlo simulations, to compute the electron mobility in ultrathin silicon on insulator inversion layers. Thus, comparing the results with those obtained using the bulk phonon model, we are able to conclude that it is very important to include confined acoustic phonon models in the electron transport simulations of ultrathin devices, if we want to reproduce the actual behavior of electron transport in silicon layers of nanometric thickness.

Improved spectral algorithm for the detection of network communities

We review and improve a recently introduced method for the detection of communities in complex networks. This method combines spectral properties of some matrices encoding the network topology, with well known hierarchical clustering techniques, and the use of the modularity parameter to quantify the goodness of any possible community subdivision. This provides one of the best available methods for the detection of community structures in complex systems.

Influence of acoustic phonon confinement on electron mobility in ultrathin silicon on insulator layers

We show the importance of acoustic phonon confinement in ultrathin silicon-on-insulator inversion layers by comparing electron mobility calculated by the Monte Carlo method assuming a bulk acoustic phonon model (the usual procedure) with that obtained by using a confined acoustic phonon model developed in this work. Both freestanding and rigid boundary conditions are taken into account for the evaluation of the confined phonon dispersion in a three-layer structure. Mobility reductions of 30% are observed for silicon thicknesses of around 5–10nm when the confined acoustic phonon model is used.

Simulation of hole mobility in two-dimensional systems

We develop a fully self-consistent solver for the six-band k ⋅ p Schrodinger and Poisson equations to compute the valence-band structure of Si and Ge devices with arbitrary substrate orientation and uniaxial or biaxial strain. This allows us to compute the potential, charge distribution and subband energy dispersion relation for hole inversion layers in different devices and, using a simplex Monte Carlo simulator, to evaluate the low-field mobility. New procedures have been developed to calculate the scattering rates. The results obtained in the case of a (0 0 1) Si MOSFET device are compared with experimental mobility curves and a very good agreement is found. Then, hole mobility curves for different structures and crystallographic orientations both with strained and unstrained materials are evaluated.

Hole effective mass in silicon inversion layers with different substrate orientations and channel directions

We explore the possibility to define an effective mass parameter to describe hole transport in inversion layers in bulk MOSFETs and silicon-on-insulator devices. To do so, we employ an accurate and computationally efficient self-consistent simulator based on the six-band k·p model. The valence band structure is computed for different substrate orientations and silicon layer thicknesses and is then characterized through the calculation of different effective masses taking account of the channel direction. The effective masses for quantization and density of states are extracted from the computed energy levels and subband populations, respectively. For the transport mass, a weighted averaging procedure is introduced and justified by comparing the results with hole mobility from experiments and simulations.

Equivalent Oxide Thickness of Trigate SOI MOSFETs With High-

The evolution of traditional metal-oxide-semiconductor field-effect transistors (MOSFETs) from planar single-gate devices into 3-D ones with multiple gates and high-kappa insulators imposes the use of new electrical models that accurately reproduce their behavior. This paper demonstrates that the typical expression of equivalent oxide thickness (EOT) for planar devices with high- kappa gate insulators becomes useless for nonplanar ones such as triple-gate (trigate) silicon-on-insulator MOSFETs. An alternative expression of the EOT for these trigate devices has been developed through a semianalytical approach to the gate-insulator capacitance. The proposed model correctly reproduces the total electron density in a wide range of device dimensions and applied biases.

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We show that the effect of phonon confinement in ultrathin double-gate silicon-on-insulator (DGSOI) transistors on hole mobility is weaker than that predicted for electron mobility. To do so, confined phonon modes in SOI devices are computed, employing an elastic continuum model of acoustic phonons in a three-layer structure. A self-consistent k middot p-Poisson solver has been developed for the valence-band structure calculation, and Kubo-Greenwood formalism is used to compute the hole mobility in ultrathin DGSOI transistors under the combined effect of confined phonon and surface-roughness scattering.

Insulators

A new model for calculating Coulomb perturbation potentials in bidimensional semiconductor structures is proposed. The main advantage of this model is that it can be applied for an arbitrary number of layers with different permittivities. As an example of how it could be used, we studied the influence on Coulomb scattering of high-κ materials used as gate insulators in silicon-on-insulator structures. This study was carried out with insulators of different physical and effective oxide thicknesses. The results show that when a silicon dioxide is replaced by a high-κ dielectric with the same thickness, Coulomb scattering is reduced. However, the strength of this beneficial effect might be diminished in actual devices for two reasons. The first is that an interfacial layer of silicon dioxide is usually placed between the silicon slab and the high-κ dielectric, lessening its influence. Second, a gate high-κ dielectric is normally wider than its silicon dioxide counterpart. As a consequence, the metal or polys...