Quantum Transport Simulation of the DOS function, Self-Consistent Fields and Mobility in MOS Inversion Layers

We describe a simulation of the self-consistent fields and mobility in (100) Si-inversion layers for arbitrary inversion charge densities and temperatures. A nonequilibrium Green's functions formalism is employed for the state broadening and conductivity. The subband structure of the inversion layer electrons is calculated self-consistently by simultaneously solving the Schrödinger, Poisson and Dyson equations. The self-energy contributions from the various scattering mechanisms are calculated within the self-consistent Born approximation. Screening is treated within RPA. Simulation results suggest that the proposed theoretical model gives mobilities which are in excellent agreement with the experimental data

Diffusive Transport in Quasi-2D and Quasi-1D Electron Systems

Quantum-confined semiconductor structures are the cornerstone of modern-day electronics. Spatial confinement in these structures leads to formation of discrete low-dimensional subbands. At room temperature, carriers transfer among different states due to efficient scattering with phonons, charged impurities, surface roughness and other electrons, so transport is scattering-limited (diffusive) and well described by the Boltzmann transport equation. In this review, we present the theoretical framework used for the description and simulation of diffusive electron transport in quasi-two-dimensional and quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs and nanowires is presented in detail.Comment: Review article, to appear in Journal of Computational and Theoretical Nanoscienc

The Role of the Collisional Broadening of the States on the Low-Field Mobility in Silicon Inversion Layers

abstract: Scaling of the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) towards shorter channel lengths, has lead to an increasing importance of quantum effects on the device performance. Until now, a semi-classical model based on Monte Carlo method for instance, has been sufficient to address these issues in silicon, and arrive at a reasonably good fit to experimental mobility data. But as the semiconductor world moves towards 10nm technology, many of the basic assumptions in this method, namely the very fundamental Fermi’s golden rule come into question. The derivation of the Fermi’s golden rule assumes that the scattering is infrequent (therefore the long time limit) and the collision duration time is zero. This thesis overcomes some of the limitations of the above approach by successfully developing a quantum mechanical simulator that can model the low-field inversion layer mobility in silicon MOS capacitors and other inversion layers as well. It solves for the scattering induced collisional broadening of the states by accounting for the various scattering mechanisms present in silicon through the non-equilibrium based near-equilibrium Green’s Functions approach, which shall be referred to as near-equilibrium Green’s Function (nEGF) in this work. It adopts a two-loop approach, where the outer loop solves for the self-consistency between the potential and the subband sheet charge density by solving the Poisson and the Schrödinger equations self-consistently. The inner loop solves for the nEGF (renormalization of the spectrum and the broadening of the states), self-consistently using the self-consistent Born approximation, which is then used to compute the mobility using the Green-Kubo Formalism.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Numerical simulation of advanced CMOS and beyond CMOS devices

Co-supervisore: Marco PalaopenLo scaling dei dispositivi elettronici e l'introduzione di nuove opzioni tecnologiche per l'aumento delle prestazioni richiede un costante supporto dal punto di vista della simulazione numerica. Questa tesi si inquadra in tale ambito ed in particolare si prefigge lo scopo di sviluppare due tool software completi basati su tecniche avanzate al fine di predire le prestazioni di dipositivi nano-elettronici progettati per i futuri nodi tecnologiciDottorato di ricerca in Ingegneria industriale e dell'informazioneembargoed_20131103Conzatti, Francesc

Novel effects of strains in graphene and other two dimensional materials

The analysis of the electronic properties of strained or lattice deformed graphene combines ideas from classical condensed matter physics, soft matter, and geometrical aspects of quantum field theory (QFT) in curved spaces. Recent theoretical and experimental work shows the influence of strains in many properties of graphene not considered before, such as electronic transport, spin-orbit coupling, the formation of Moir\'e patterns, optics, ... There is also significant evidence of anharmonic effects, which can modify the structural properties of graphene. These phenomena are not restricted to graphene, and they are being intensively studied in other two dimensional materials, such as the metallic dichalcogenides. We review here recent developments related to the role of strains in the structural and electronic properties of graphene and other two dimensional compounds.Comment: 75 pages, 15 figures, review articl

Quantum corrected full-band semiclassical Monte Carlo simulation research of charge transport in Si, stressed-Si, and SiGe MOSFETs

This Ph.D. research is centered around a full-band Monte Carlo device simulator (“Monte Carlo at the University of Texas”, MCUT) with quantum corrections (based on one-dimensional Schrödinger equation solver). The code itself was based on a solid infrastructure of a Monte Carlo simulator, “MoCa” from the University of Illinois at Urbana-Champaign. To that there were added new methods and features during my Ph.D. program, including strained band structures, alternative (to conventional 100 ) surface orientations, full-band scattering mechanisms, and valley-dependent quantum correction. These features enable “MCUT” to be used to model various strained and/or alloyed silicon MOSFETs, as well as the MOSFETs composed of alternative materials such as Ge, in sub-100 nm regime. Monte Carlo simulation, itself, handles short channel effects and hot carriers in ultra small device well; full-band structure replaces the inaccurate and unknown (for new/strained materials) analytical formulae; and the quantum corrections approximate quantum-confinement effects on device performance. The goal is to understand and predict the device behavior of the so called “non-classical” CMOS ― beyond bulk Si based CMOS ― in the sub-100 nm regime.Electrical and Computer Engineerin

Full 3D Quantum Transport Simulation of Atomistic Interface Roughness in Silicon Nanowire FETs

The influence of interface roughness scattering (IRS) on the performances of silicon nanowire field-effect transistors (NWFETs) is numerically investigated using a full 3D quantum transport simulator based on the atomistic sp3d5s* tight-binding model. The interface between the silicon and the silicon dioxide layers is generated in a real-space atomistic representation using an experimentally derived autocovariance function (ACVF). The oxide layer is modeled in the virtual crystal approximation (VCA) using fictitious SiO2 atoms. -oriented nanowires with different diameters and randomly generated surface configurations are studied. The experimentally observed ON-current and the threshold voltage is quantitatively captured by the simulation model. The mobility reduction due to IRS is studied through a qualitative comparison of the simulation results with the experimental results

ANALYTICAL MODELS AND ELECTRICAL CHARACTERISATION OF ADVANCED MOSFETS IN THE QUASI BALLISTIC REGIME

International audienceThe quasi-ballistic nature of transport in end of the roadmap MOSFETs device is expected to lead to significant on state current enhancement. The current understanding of such mechanism of transport is carefully reviewed in this chapter, underlining the derivation and limits of corresponding analytical models. In a second part, different strategies to compare these models to experiments are discussed, trying to estimate the "degree of ballisticity" achieved in advanced technologies

Electronic Structure of SiC/SiO2 by Density Functional Theory

Silicon carbide (SiC) is a promising semiconductor material with desirable properties for many applications. SiC-based electronic devices and circuits are being developed for use in high-temperature, high-power, and high-radiation conditions under which conventional semiconductors cannot function. Additionally, it has the advantage of growing a native oxide, SiO2, by simple thermal oxidation. Despite all desirable properties, SiC-based devices still face major challenges. The main problem of SiC-based devices is the great density of imperfections at the SiC/SiO2 interface, which not only degrades the device performance but also causes reliability problems coming from the extreme operating conditions. The quality of the interface affects the channel mobility of MOSFETs, which is the most critical parameter of devices. In this work a hybrid functional density functional theory framework is employed to model the (0001)4H-SiC/SiO2 abrupt interface. Using this, defect energy levels in the bandgap have been calculated through the total and projected density of states. There is experimental evidence for improvement of the quality of the interface after passivation, However the atomic mechanisms of the improvement are not yet clear., Thus, the impact of various passivations on the potential defects has also been studied. Since the interface of SiC/SiO2 is not perfectly abrupt, several atomic configurations for (0001)4H-SiC/SiO2 transition layers have also been modeled, and their effect on the bandgap, and the near interface trap density has been studied. A DFT-based Monte Carlo carrier transport simulation technique is employed to compute the average velocities, phonon-limited and ionized-impurity-limited mobilities of the most probable transition layer structures. Finally, since low frequency noise calculation is a powerful tool to diagnose quality and reliability of semiconductor devices, a DFT-based method is presented to calculate the current spectral noise density of the (0001)4H-SiC/SiO2 transition layers