Modeling heating effects in nanoscale devices: the present and the future

In this review paper we give an overview on the present state of the art in modeling heat transport in nanoscale devices and what issues we need to address for better and more successful modeling of future devices. We begin with a brief overview of the heat transport in materials and explain why the simple Fourier law fails in nanoscale devices. Then we elaborate on attempts to model heat transport in nanostructures from both perspectives: nanomaterials (the work of Narumanchi and co-workers) and nanodevices (the work of Majumdar, Pop, Goodson and recently Vasileska, Raleva and Goodnick). We use our own simulation results which we have used to examine heat transport in nanoscaling devices to point out some important issues such as the fact that thermal degradation does not increase as we decrease feature size due to the more pronounced non-stationary transport and ballistic transport effects in nanoscale devices. We also point out that instead of using SOI, if one uses Silicon on Diamond technology there is much less heat degradation and better spread of the heat in the Diamond material. We also point out that tools for thermal modeling of nanoscale devices need to be improved from the present state of the art as 3D tools are needed, for example, to simulate heat transport and electrical transport in a FinFET device. Better models than the energy balance equations for the acoustic and optical phonons what we presently use in our simulators are also welcomed. The ultimate goal is to design the tool that can be efficient enough but at the same time can simulate most accurately both electrons and phonons within the particle pictures by solving their corresponding Boltzmann transport equations self-consistently. Investigations in integration of Peltier coolers with CMOS technology are also welcomed and much needed to reduce the problem of heat dissipation in nanoscale devices and interconnects.     

Confined phonon scattering in multivalley Monte Carlo simulation of quantum cascade lasers

We present a detailed investigation of the effects that optical phonon confinement has on the electronic transport properties of GaAs-based multiple-quantum-well (MQW) quantum cascade laser (QCL) structures. Both confined and interface phonon modes are included based on the macroscopic dielectric continuum model. Interface phonon dispersions are obtained using the transfer matrix method with periodic boundary conditions. Scattering rates of both Γ- and X-valley electrons by all the interface and confined phonon modes are calculated and fully incorporated in the multivalley Monte Carlo simulation of a deep-active-well 6.7 μm GaAs-based MQW QCL. We find that the inclusion of phonon confinement enhances the electron-phonon scattering rates and output current to a relatively small extent with respect to the bulk phonon approximation.     

Full band Monte Carlo simulation of dislocation effects on 250 nm AlGaN-GaN HEMT

In this work, we present a full band Monte Carlo simulation of the effects of dislocation scattering on the performance of a 0.25 μm AlGaN/GaN HEMT (high electron mobility transistor). We performed a full characterization of the device and validated the simulation results with experimental data (Lee et al. in IEEE Electron. Dev. Lett. 24:613–615, [2003]). Here we show a study of the DC device performance as a function of the density of thread dislocations.     

660 MW机组脱硝 SCR分区喷氨控制技术改造

针对燃煤机组中选择性催化还原脱硝系统(selectivecatalyticreduction,SCR)存在喷氨量过高、NOX浓度波动大和空预器堵塞等问题,采用SCR分区喷氨控制技术改造某660MW燃煤机组。结果表明,相同运行负荷条件下,SCR分区喷氨控制系统的引入可有效改善脱硝效率、空预器阻力和SCR出口NOX浓度分布均匀性。     

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.     

A full 3D non-equilibrium Green functions study of a stray charge in a nanowire MOS transistor

The effect of the location of a negative stray charge associated with an acceptor type defect state in the channel of a nanowire transistor has been investigated using a Non Equilibrium Green’s Function Formalism in the effective mass approximation. Due to the fact that the nanowire cross-section is 2.2×2.2 nm², we have calculated the effective masses using Tight Binding (TB) calculations. A third neighbor sp ³ TB model has been used. We have found that the on current is two time smaller when the charge is located in the source end as compared to its location in the drain end. We have also studied the effect on the current of the spatial distribution of the acceptor charge. The calculations show that when the charge is more distributed (de-localized) the effect of the blocking of the current is less efficient, so the current is higher.     

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.     

Boundary conditions for Density Gradient corrections in 3D Monte Carlo simulations

Monte Carlo remains an effective simulations methodology for the study of MOSFET devices well into the decananometre regime as it captures non-equilibrium and quasi-ballistic transport. The inclusion of quantum corrections further extends the usefulness of this technique without adding significant computational cost. In this paper we examine the impact of boundary conditions at the Ohmic contacts when Density Gradient based quantum corrections are implemented in a 3D Monte Carlo simulator. We show that Neumann boundary conditions lead to more stable and physically correct simulation results compared to the traditional use of Dirichlet boundary conditions.     

An extended Hückel theory based atomistic model for graphene nanoelectronics

An atomistic model based on the spin-restricted extended Hückel theory (EHT) is presented for simulating electronic structure and I–V characteristics of graphene devices. The model is applied to zigzag and armchair graphene nano-ribbons (GNR) with and without hydrogen passivation, as well as for bilayer graphene. Further calculations are presented for electric fields in the nano-ribbon width direction and in the bilayer direction to show electronic structure modification. Finally, the EHT Hamiltonian and NEGF (Nonequilibrium Green’s function) formalism are used for a paramagnetic zigzag GNR to show 2e ²/h quantum conductance.     

An advanced description of oxide traps in MOS transistors and its relation to DFT

Recently, an advanced model for defects in the insulating regions of semiconductor devices has been suggested, which can explain the removable component of the negative bias temperature instability (NBTI) and recoverable random telegraph/flicker noise. We give a brief introduction to the atomic scale physics behind the model and show how model parameters can be extracted from density functional theory (DFT) calculations. The central link between DFT calculations and device simulation is the carrier energy dependent part of the capture cross section, the line shape function. Calculations of the line shape functions of model defect structures using a simple harmonic approximation are presented. The calculations show a considerable shift in the oscillator frequency upon charge state transitions for the defects investigated.     

Ex-vivo cellular MRI with b-SSFP: quantitative benefits of 3 T over 1.5 T

INTRODUCTION: The use of MRI with iron-based magnetic nanoparticles for imaging cells is a rapidly growing field of research. We have recently reported that single iron-labeled cells could be detected, as signal voids, in vivo in mouse brains using a balanced steady-state free precession imaging sequence (b-SSFP) and a customized microimaging system at 1.5 T. METHODS: In the current study we assess the benefits, and challenges, of using a higher magnetic field strength for imaging iron-labeled cells with b-SSFP, using ex vivo mouse brain specimens imaged with near identical systems at 1.5 and 3.0 T. RESULTS: The substantial banding artifact that appears in 3 T b-SSFP images was readily minimized with RF phase cycling, allowing for banding-free b-SSFP images to be compared between the two field strengths. This study revealed that with an optimal 3 T b-SSFP imaging protocol, more than twice as many signal voids were detected as with 1.5 T. CONCLUSION: There are several factors that contributed to this important result. First, a greater-than-linear SNR gain was achieved in mouse brain images at 3 T. Second, a reduction in the bandwidth, and the associated increase in repetition time and SNR, produced a dramatic increase in the contrast generated by iron-labeled cells.     

Surface roughness induced device variability: 3D <Emphasis Type="Italic">ab initio</Emphasis> Monte Carlo simulation study

It is expected that published results from drift diffusion simulation of oxide thickness fluctuations in nano-scale devices underestimates the true intrinsic device parameter variation by neglecting local variations in surface roughness scattering. We present initial results from 3D ‘bulk’ Monte Carlo simulation including an ab initio treatment of surface roughness scattering capable of capturing such transport variation. The scattering is included directly through the real space propagation of carriers in the fluctuating potential associated with a randomly generated interface. We apply this approach to simulate inversion layer mobility in order to validate the model before its possible application in device variability simulations. Qualitative agreement is found with universal mobility data and avenues for possible calibration of surface and simulation parameters are highlighted.     

Advance Compensated Mho Relay Algorithm for a Transmission System with Shunt Flexible AC Transmission System Device

Abstract—The presence of shunt flexible AC transmission system devices adversely affect the performance of distance relay and create security and reliability issues. This article introduces a noble compensated Mho relay algorithm for the protection of transmission line employing shunt flexible AC transmission system devices, such as a static VAR compensator and static synchronous compensator. A detailed model of transmission system employing a shunt flexible AC transmission system device is explained. Then compensated impedance inserted by a shunt device in the transmission line is calculated, and finally, a compensated Mho relay algorithm is proposed to protect zone one of the transmission line. Simulation work is carried out in PSCAD/EMTP software. Results show that the proposed relay is secure, accurate, and reliable under the wide variation in power system parameters, such as load angle, fault resistance, fault location, and compensation level.     

Discrete space vector modulation and optimized switching sequence model predictive control for three-level voltage source inverters

This paper proposes a discrete space vector modulation and optimized switching sequence model predictive controller for three-level neutral-point-clamped inverters in grid-connected applications. The proposed strategy is based on cascaded model predictive control (MPC) for controlling the grid current while maintaining the capacitor voltage balanced without weighting factor. To enhance the closed-loop performance, the external MPC evaluates 19 basic and 138 virtual vectors (VV) of the proposed space vector method. The optimal control voltage is then selected using an extended deadbeat method to reduce the execution time of the proposed control algorithm. By using the discrete space vector modulation principle, the VV are synthesized based on switching sequence (SS) and are divided into negative and positive SSs considering their impact on the neutral point (NP) potential. The inner MPC evaluates both types of SSs and selects the one that keeps the capacitor voltage balanced. Various controllers are evaluated and compared against the proposed control strategy. The results show that the proposed strategy improves performance without weighting factor, while maintaining a total harmonic distortion of current to be less than 2%. Compared to the modulated MPC which provides the same fxed switching frequency, the proposed controller reduces the computational burden by over 50% while also providing better NP voltage balance accuracy     

Analytical and self-consistent quantum mechanical model for a surrounding gate MOS nanowire operated in JFET mode

We derive an analytical model for the electrostatics and the drive current in a silicon nanowire operating in JFET mode. We show that there exists a range of nanowire radii and doping densities for which the nanowire JFET satisfies reasonable device characteristics. For thin nanowires we have developed a self-consistent quantum mechanical model to obtain the electronic structure.     

Simulation of resonant tunneling heterostructures: numerical comparison of a complete Schrödinger-Poisson system and a reduced nonlinear model

Two different models are compared for the simulation of the transverse electronic transport through a heterostructure: a 1D self-consistent Schrödinger-Poisson model with a numerically heavy treatment of resonant states and a reduced model derived from an accurate asymptotic nonlinear analysis. After checking the agreement at the qualitative and quantitative level on quite well understood bifurcation diagrams, the reduced model is used to tune double well configurations for which nonlinearly interacting resonant states actually occur in the complete self-consistent model.     

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.     

Positive contrast with therapeutic iron nanoparticles at 4.7 T

Object  

The purpose of the study was to show the feasibility of a positive contrast technique GRadient echo Acquisition for Superparamagnetic particles with Positive contrast (GRASP), for a specific type of magnetic particles, designed for tumor treatment under MRI monitoring.