Modeling the effects of applied stress and wafer orientation in silicon devices: from long channel mobility physics to short channel performance

We review our novel simulation approach to model the effects of applied stress and wafer orientation by mapping detailed dependencies of long channel physics onto short channel device conditions in Silicon NMOS and PMOS. We use kp and Monte Carlo methods to show the long channel dependencies of these effects on gate fields, doping levels, extrinsic charges, and homogeneous driving fields. Our model predicts the reduced effect of wafer orientation on short channel linear and saturation current drives due to weak gate confinement, high carrier density, high stress, and high driving field prevalent in scaled devices. This reduces NMOS (110) wafer orientation loss compared to (100), while keeping PMOS (110) gains over (100) surface orientation in current drives in 〈110〉 channels, consistent with data.     

Modeling drive currents and leakage currents: a dynamic approach

The dynamics of electrons and holes propagating through the nano-scaled channels of modern semiconductor devices can be seen as a widespread manifestation of non-equilibrium statistical physics and its ruling principles. In this respect both the devices that are pushing conventional CMOS technology towards the final frontiers of Moore’s law and the upcoming set of alternative, novel nanostructures grounded on entirely new concepts and working principles, provide an almost unlimited playground for assessing physical models and numerical techniques emerging from classical and quantum mechanical non-equilibrium theory. In this paper we revisit the Boltzmann as well as the Wigner–Boltzmann equation which offers a valuable platform to study transport of charge carriers taking part in drive currents. We focus on a numerical procedure that regained attention recently as an alternative tool to solve the time-dependent Boltzmann equation for inhomogeneous systems, such as the channel regions of field-effect transistors, and we discuss its extension to the Wigner–Boltzmann equation. Furthermore, we pay attention to the calculation of tunneling leakage currents. The latter typically occurs in nano-scaled transistors when part of the carrier distribution sustaining the drive current is found to tunnel into the gate due the presence of an ultra-thin insulating barrier separating the gate from the channel region. In particular, we discuss the paradox related to the very existence of leakage currents established by electrons occupying quasi-bound states, while the (real) wave functions of the latter cannot carry net currents. Finally, we describe a simple model to resolve the paradox as well as to estimate gate currents provided the local carrier generation rates largely exceed the tunneling rates.     

Past, present and future of fuel cells

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Mathematical advances and horizons for classical and quantum-perturbed drift-diffusion systems: solid state devices and beyond

The classical drift-diffusion model employed in semi-conductor simulation is now seen as part of a hierarchy of mathematical models designed to capture the intricate patterns of current flow in solid-state devices. These models include those incorporating quantum mechanical effects. Scientific computation has vastly outpaced our mathematical understanding of these models. This article is restricted in its focus, and describes mathematical understanding achieved during the last few decades primarily in terms of Gummel decomposition, as applied to drift-diffusion models and the closely related family of quantum corrected drift-diffusion models. Drift-diffusion models are being employed once again in organic devices, and in bio-chip devices, and a re-examination is now seen as timely, as such studies proceed beyond solid state devices.     

Three-dimensional optimal multi-level Monte–Carlo approximation of the stochastic drift–diffusion–Poisson system in nanoscale devices

The three-dimensional stochastic drift–diffusion–Poisson system is used to model charge transport through nanoscale devices in a random environment. Applications include nanoscale transistors and sensors such as nanowire field-effect bio- and gas sensors. Variations between the devices and uncertainty in the response of the devices arise from the random distributions of dopant atoms, from the diffusion of target molecules near the sensor surface, and from the stochastic association and dissociation processes at the sensor surface. Furthermore, we couple the system of stochastic partial differential equations to a random-walk-based model for the association and dissociation of target molecules. In order to make the computational effort tractable, an optimal multi-level Monte–Carlo method is applied to three-dimensional solutions of the deterministic system. The whole algorithm is optimal in the sense that the total computational cost is minimized for prescribed total errors. This comprehensive and efficient model makes it possible to study the effect of design parameters such as applied voltages and the geometry of the devices on the expected value of the current.     

Nanowire transistor modeling: influence of ionized impurity and correlation effects

This study presents two relevant effects influencing the electronic transport of nanowire transistors. We first focus on the ionized impurity impacts and calculate the current characteristics with a self-consistent three-dimensional (3D) Green’s function approach. The results show the effects of both acceptor and donor impurities on the physical electron properties. In particular, we emphasize that the presence of a donor induces different transport phenomena according to the applied gate bias. In a second part, we report a numerical study of the self-energy correction due to correlation effects from dynamic screening of the moving electron in silicon nanowire transistors. This many-body effect, which is not included in the usual Hartree approximation, is then incorporated self-consistently into a non-equilibrium Green’s function (NEGF) code. The results pinpoint the importance of dielectric confinement whose magnitude can not be neglected compared to its quantum counterpart in ultimate nanowire transistors.     

Down-scaling limitations in CMOS devices: Is there a role for the ferroelectrics?

Abstract

This paper reviews down-scaling limitations in CMOS devices, with a special emphasis on the possible application of ferroelectric materials. Ultimate limit in reduction of the thickness of gate oxide is found as the most important limitation. The possibility of replacing or enhancing the gate oxide by a ferroelectric material is critically considered in view of the desired properties of the silicon - to - gate insulator interface.     

Analysis of Space Harmonic Interactions in Squirrel Cage Induction Machines—Part I: Modeling and the Equivalent Circuit

ABSTRACT

By means of the transition from the symmetrical components to the α, β components regarding the rotor variables it is possible to obtain a linear model of the voltage equations representation. It is shown that, even in this linear formulation the harmonic interactions are not separated. The equivalent circuit which expresses the harmonic interactions and accounts for the stator additional harmonic currents is presented. In the light of this equivalent circuit, the suitable choice of the cage bars number can be performed. It is possible also to make a good base regarding the assessment of the differential leakage.     

Spontaneous and stimulated electron–photon interactions in nanoscale plasmonic near fields

The interplay between free electrons, light, and matter offers unique prospects for space, time, and energy resolved optical material characterization, structur...     

The times they are a changin’: Current and future trends in electricity demand and supply

The increasing penetration of variable renewable electricity generation in Australia’s National Electricity Market over the past decade has led to a sustained change in the shape of electricity demand. In particular, intra-day demand has become more volatile, with demand in some Australian regions increasingly resembling the widely-cited ‘duck curve’ or more appropriately the ‘emu curve’. The changes in demand have, in turn, economically driven out generators whose technical characteristics are ill-suited to supplying this demand profile: high capacity-factor, slow-start plant. In this article, we describe the changes to date in the profile of electricity demand, and draw on other studies to argue that these trends are likely to accelerate going forward based on projected future uptake of variable renewables. In combination with technological and policy developments, these trends imply that flexible plant, such as peaking gas, hydro, and dispatchable storage, are likely to be better suited to the changing profile of demand, in contrast to slow-start and relatively inflexible technologies such as coal and combined-cycle gas plants. These trends are based on the flexibility of different generation technologies and their interaction with emissions considerations.     

Sparking, electrical discharge, and heating in synchronous andinduction machines: can it be controlled?

It is impossible to eliminate all occurrences of sparking and localized heating from the interior of electric machines during the 20-40 years that they are normally in service. One can minimize the probability of sparking and localized heating by exercising precautions which will eliminate or minimize the sources of these conditions in AC machines. Most new machines have a low probability of sparking, but some design steps can be taken which condition the machine as a sparking type or a nonsparking type. This paper discusses the aspects of arcing between rotor and stator in the air gap, sparking between rotor parts, sparking between frame components, sparking due to broken or open bars and endrings, and surface discharge on the stator winding. Also discussed is what action, if any, can be taken in design, application, and maintenance to eliminate or control these problems     

Quantum transport of holes in 1D, 2D, and 3D devices: the?k?p?method

Quantum transport of holes in one-, two- and three-dimensional devices is simulated based on the 6-band k?p method. Detailed numerical aspects for an efficient development of a k?p-based simulator are provided. In particular, real-space and k-space discretization schemes for devices of different dimensionality are described and their effectiveness in numerical implementation is compared. The mode-space approach for drastic reduction of computation time is also described. The capability of the?k?p-based simulator is demonstrated by investigating various aspects of hole transport in devices of different dimensionality.     

The role of the source and drain contacts on self-heating effect in nanowire transistors

We find that self-heating effects are not pronounced in silicon nanowire transistors with channel length 10 nm even in the presence of the wrap-around oxide. We observe a maximum current degradation of 6% for V _G =V _D =1.0 V in a structure in which the metal gates are far away from the channel. The overall small current degradation is attributed to the significant velocity overshoot effect in these structures. The lattice temperature profile shows moderate temperature rise and velocity of the carriers is slightly deteriorated due to self-heating effects when compared to isothermal simulations.     

Analysis of a thermal generator’s participation in the Western Energy Imbalance Market and the resulting effects on overall performance and emissions

The inception of the Western Energy Imbalance Market (EIM) in November of 2014 has had significant impacts on participating balancing authorities (BAs) and individual generators. The EIM serves to balance available energy across a large geographical footprint. Since the market’s creation, the California Independent System Operator (CAISO) has reported that many benefits have been realized by participating BAs, including savings of $502 million and the avoided curtailment of an estimated 734,000 MWh of energy produced by variable renewable energy (VRE). Balancing energy across a larger geographic footprint, in conjunction with other factors (e.g., fluctuating fuel prices, new environmental policies), has also resulted in significant changes in the operation of some thermal generating stations which have traditionally operated as baseload units. The effects of these new operating practices on baseload units have not been characterized. This study analyzes the detailed operational history of a single coal-fired thermal generating station experiencing a large degree of load flexing located within the footprint of the EIM. A dataset comprised of hourly operational data from two and one-half years before and after beginning market participation was analyzed. Changes in operating practices, unit performance, and emission rates from this unit between the two periods were identified from this analysis. It was observed that the operational range of the unit has expanded by 65% and the unit has experienced ramping situations approximately 4.6 times more often since entering the EIM. Continual load changes have contributed to 18 times more turbine cycles, the impact of which on unit health and longevity cannot be fully characterized. The unit has demonstrated a strong adaptive ability in altering operational practices to meet the needs of the new market. Primarily due to lower average load operation, overall efficiency has decreased by 2.7%.     

Monitoring demyelination and remyelination by magnetization transfer imaging in the mouse brain at 9.4 T

OBJECTIVE: The aim of this study was to assess quantitatively structural changes in myelin content occurring during demyelination and remyelination by magnetization transfer imaging (MTI). MATERIALS AND METHODS: In a reversible model of demyelination with no axonal loss, mice intoxicated by cuprizone were studied by MTI in vivo at 9.4 T. MRI data were compared to histopathological examinations. RESULTS: Data revealed that the magnetization transfer ratio (MTR) decreased significantly during demyelination and increased during remyelination with strong correlation to the myelin content (r = 0.79, P = 0.01). CONCLUSIONS: This study demonstrated that MTR is a sensitive and reproducible quantitative marker to assess myelin loss and repair. This may lead to in vivo monitoring of therapeutic strategies promoting remyelination.     

Unified simulation of transport and luminescence in optoelectronic nanostructures

Computer simulation of microscopic transport and light emission in semiconductor nanostructures is often restricted to an isolated system of a single quantum well, wire or dot. In this work we report on the development of a simulator for devices with various kinds of nanostructures which exhibit quantization in different dimensionalities. Our approach is based upon the partition of the carrier densities within each quantization region into bound and unbound populations. A bound carrier is treated fully coherent in the directions of confinement, whereas it is assumed to be totally incoherent with a motion driven by classical drift and diffusion in the remaining directions. Coupling of the populations takes place through electrostatics and carrier capture. We illustrate the applicability of our approach with a well-wire structure.     

3-D self-consistent Schrödinger-Poisson solver: the spectral element method

In this paper, we developed an efficient three-dimensional (3-D) nanoelectronic device simulator based on a self-consistent Schrödinger-Poisson solver to simulate quantum transport. An efficient and fast algorithm, the spectral element method (SEM), is developed in this simulator to achieve spectral accuracy where the error decreases exponentially with the increase in the sampling density and the order of the polynomial basis functions, thus significantly reducing the CPU time and memory usage. Perfectly matched layer (PML) boundary method, as an alternative to the open-boundary conditions in NEGF, is applied in this solver to simplify the numerical implementation. The validity of the Schrödinger and Poisson solvers are illustrated by a multiple-terminal device and a spherical charge example, respectively. The utility of the self-consistent Schrödinger-Poisson solver is illustrated by a nanotube example.     

7 T renal MRI: challenges and promises

The progression to 7 Tesla (7 T) magnetic resonance imaging (MRI) yields promises of substantial increase in signal-to-noise (SNR) ratio. This increase can be traded off to increase image spatial resolution or to decrease acquisition time. However, renal 7 T MRI remains challenging due to inhomogeneity of the radiofrequency field and due to specific absorption rate (SAR) constraints. A number of studies has been published in the field of renal 7 T imaging. While the focus initially was on anatomic imaging and renal MR angiography, later studies have explored renal functional imaging. Although anatomic imaging remains somewhat limited by inhomogeneous excitation and SAR constraints, functional imaging results are promising. The increased SNR at 7 T has been particularly advantageous for blood oxygen level-dependent and arterial spin labelling MRI, as well as sodium MR imaging, thanks to changes in field-strength-dependent magnetic properties. Here, we provide an overview of the currently available literature on renal 7 T MRI. In addition, we provide a brief overview of challenges and opportunities in renal 7 T MR imaging.