Hierarchical simulation of transport in silicon nanowire transistors

We propose a very fast hierarchical simulator to study the transport properties of silicon nanowire FETs. We obtain the transverse wave functions and the longitudinal effective masses and band-edges of the lowest conduction bands from a nearest-neighbor sp ³ d ⁵ s ^* tight-binding study of an infinite nanowire with null external potential. Then we plug these parameters into a self-consistent Poisson-Schrödinger solver, using an effective mass approach and considering the bands decoupled. We apply this method, which gives quantitatively correct results with notable time savings, for the simulation of transport in two different silicon nanowire FETs.     

Electronic and transport properties of silicon nanowires

The electronic, structural and transport properties of silicon nanowires have been investigated with different approaches. The Empirical Tight-Binding model (ETB) and Linear Combination of Bulk Bands (LCBB) method are used to calculate effect of quantum confinement on electronic energies, bandgap and effective masses in silicon nanowires in function of Si cell size. Both hydrogenated and SiO₂ terminated silicon surfaces are studied. Transport properties of nanowires are obtained by applying the Non-Equilibrium Green Function (NEGF) method. NEGF approach has been used to describe nanoMOSFET devices based on Silicon nanowires.     

Semiclassical transport in silicon nanowire FETs including surface roughness

In this paper we investigate the effect of surface roughness scattering on transport in silicon nanowire FETs using a deterministic Boltzmann equation solver previously developed by the authors. We first solve the coupled Schrödinger-Poisson equations to extract the subband profiles along the channel, and then address the transport problem. Some features of the low-field mobility as a function of the wire diameter and gate bias are discussed and the effect of surface roughness on the I–V characteristics is presented.     

膜孔肥液自由入渗土壤水氮运移转化的数值模拟

本文根据土壤水动力学方程和溶质运移理论,考虑了土壤氮素的挥发、吸附、硝化、反硝化和矿化作用等动力学过程,而忽略作物根系对土壤水分和氮素吸收过程,建立了膜孔肥液自由入渗土壤水氮运移转化的数学模型,运用试验资料确定与模型相关的土壤水分及土壤硝态氮和土壤铵态氮的参数,利用ADI格式结合Gauess-Seidel迭代法对方程进行求解.结果表明:土壤含水率和土壤铵态氮以膜孔为中心沿水平方向和垂直方向逐渐减小,而土壤硝态氮先增大后减小.随分布和运移转化时间的增大土壤含水率和土壤铵态氮逐渐减小,土壤硝态氮逐渐增大.土壤含水率及土壤硝态氮和土壤铵态氮实测值与计算值根方误差的平均值分别为0.7444、0.6518和0.8300,吻合良好,具有变化规律的一致性,则该模型可以反映膜孔肥液自由入渗土壤水氮运移转化规律.     

Numerical simulation of ballistic surface transport in cylindrical nanosystems

We study, by numerical simulations, the effect of topology on coherent electron transport through cylindrical junctions and bent cylindrical surfaces. The dynamics of a particle bound to a curved surface is described by a modified Schrödinger equation depending on two curvilinear coordinates. The modeling approach based on a finite-difference discretization of the latter equation is described, and a comparison with the standard solution of Schrödinger equation for flat surfaces is given. The effect of the geometrical parameters of the structures is reported.     

Non-equilibrium quantum transport theory: current and gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green’s function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al_.15Ga_.85As QCL structure. Phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.     

Numerical simulation of electronic transport in zigzag-edged graphene nano-ribbon devices

We present a numerical study of the current-voltage characteristics in zigzag-edged graphene nano-ribbon (Z-GNR) devices. Our calculations employing the non-equilibrium Green’s function method and the density-functional tight-binding method show that the Z-GNR with transverse symmetry can exhibit remarkable current saturation behavior in spite of the absence of the bandgap. We further demonstrate that the saturation current can be controlled by the additional doping in the channel region. The mechanism of such current saturation can be explained in terms of the symmetry of the wavefunctions corresponding to the conduction and the valence bands in Z-GNR.     

Self-consistent quantum transport theory: Applications and assessment of approximate models

We have implemented a fully self-consistent non-equilibrium Green’s function approach for vertical quantum transport in open quantum devices with contacts and study theoretically quantum well heterostructures, resonant tunneling diodes and quantum cascade laser structures in this formalism. We systematically investigate the role and consequences of several widely used approximations such as decoupling the equations for the scattering states and their occupation, neglect of inelastic scattering, and neglect of nonlocal scattering self-energies.     

Physical modeling of hole mobility in silicon inversion layers under uniaxial stress

A physical model for hole mobility under either biaxial or uniaxial stress has been developed. The six-band k ? p theory is used to obtain the bandstructure through stress-dependent Hamiltonian. The hole mobility in the silicon inversion layer is studied in details using Monte Carlo method. A numerically robust method has been applied to achieve self-consistent solution of Poisson’s and Schrödinger equations.     

Modeling and simulation of silicon neuron-to-ISFET junction

Modeling and simulation of the silicon neuron-to-ISFET junction is presented. The neuronal electrical activity, extracellularly recorded by the ISFET, was simulated as a function of the neuro-electronic junction parameters such as the seal resistance, double-layer capacitance, and general adhesion conditions. This goal was achieved with a configuration consisting of “silicon neurons” i.e., assemblies of CMOS circuits that mimic the generation of the equivalents of the ionic currents and of the action potentials of real (biological) neurons; “silicon synapses” whose performances simulate those of their biological counterpart; depletion-mode MOSFET-based ISFETs that simulate the signal recording devices; passive component-based circuits that model the neuro-electronic junction. The models of the neuron, synapse, coupling interface, and ISFET were implemented in HSPICE and used to simulate the behavior of the junction between stimulated neurons (described by the compartmental model) and ISFETs.     

Joule heating and phonon transport in silicon MOSFETs

This work examines the generation of heat in silicon MOSFETs using self-consistent Monte Carlo device simulation with full electron bandstructure and a full phonon dispersion computed from the Adiabatic Bond Charge model. We devise an efficient algorithm for the inclusion of full phonon dispersion in order to account for anisotropy and details of heat transport with great accuracy. We compute the density-of-states (DOS) and the lattice thermal energy numerically and use them to generate maps of local temperatures in a representative small-channel MOSFET device. Our results show that most heat is dissipated in the form of optical g-type phonons in a small region in the drain, and that the heat flows in a preferred direction aligned with the flow of the electron current. We also show that the distribution of generated phonons in energy closely follows the phonon DOS.     

Numerical simulation of small silicon partially insulated MOSFETs

The scaling of conventional MOS bulk transistors with gate lengths below 100 nm seems to be difficult due to short channel effects. Especially the adjustment of the threshold voltage V _th is difficult because of the rapid drop down at shorter gate lengths. For low power consumption and high speed applications SOI technologies have been developed, but floating body effects, increasing leakage currents, kink phenomena and decreased heat dissipation occur in SOI-FETs. To combine the benefits of conventional and SOI-MOSFETs and to avoid the disadvantages, partially insulated FETs (Pi-FETs) with oxide regions under source and drain are candidates for scaling down the gate length into the deep submicron area [1–3, 5, 6]. We present the results of several numerical simulations to compare conventional bulk transistors, SOI-FETs and Pi-FETs in their static and dynamic behaviour.     

Brownian simulation of charge transport in α-Haemolysin

In this paper we present the results of self-consistent Brownian Dynamics simulations of the ion Channel alpha-Haemolysin. We show that with simple scaling, excellent agreement with experimental measurement of the current voltage characteristics of this molecule.     

Transport in silicon nanowires: role of radial dopant profile

We consider the electronic transport properties of phosphorus (P) doped silicon nanowires (SiNWs). By combining ab initio density functional theory (DFT) calculations with a recursive Green’s function method, we calculate the conductance distribution of up to 200 nm long SiNWs with different distributions of P dopant impurities. We find that the radial distribution of the dopants influences the conductance properties significantly: surface doped wires have longer mean-free paths and smaller sample-to-sample fluctuations in the cross-over from ballistic to diffusive transport. These findings can be quantitatively predicted in terms of the scattering properties of the single dopant atoms, implying that relatively simple calculations are sufficient in practical device modeling.     

Numerical simulation of electrical characteristics in nanoscale Si/GaAs MOSFETs

In this paper, electrical characteristics of metal-oxide-semiconductor field effect transistor (MOSFET) with silicon/gallium-arsenic (Si/GaAs) stacked film are numerically studied. By calculating several important device characteristics, such as the on-state current, the subthreshold swing, the drain induced barrier lowering, the threshold voltage, the threshold voltage roll-off, and the output resistance, a 50 nm Si/GaAs MOSFET is simulated with respect to different thicknesses of Si/GaAs film. Compared with the results of pure Si MOSFET, Si/GaAs MOSFET shows promising characteristics after properly selecting the thickness of Si/GaAs film. Among Si, germanium (Ge), and Si/Ge MOSFETs, Si/GaAs MOSFET relatively exhibits a higher driving capability due to higher carrier mobility within the Si/GaAs film. However, quantitatively accurate estimation of device characteristics will depend upon more precise calculation of band structure of the stacked film.     

Scaling dependence of electron transport in nano-scale Schottky barrier MOSFETs

The scaling dependence of electron transport in the double-gated Schottky barrier MOSFET (DG-SBT) below 10 nm is investigated in the framework of quantum transport theory, using non-equilibrium Green’s function method. Simulation results show that the current-voltage characteristics in ultra-small DG-SBT are characterized by both resonant and direct tunneling effects. The electron potential in the 10-nm-scale DG-SBT surrounded by Schottky barriers acts as a resonant cavity and produce a negative differential resistance due to resonant tunneling effect. While, further scaling shallows the depth of the cavity and makes it difficult to form resonance levels. Hence, at the scaling limit, direct tunneling currents simply dominate the current-voltage characteristics of DG-SBT.     

Self-consistent ion transport simulation in carbon nanotube channels

We propose a method to self-consistently deal with polarisation effects in Monte Carlo particle simulations of charge transport. The systems of interest were membrane structures with a narrow (4–8 Å) carbon nanotube (CNT) channel in an aqueous environment. Due to computational limitations for Molecular Dynamics (MD) simulations, we extended the Transport Monte Carlo known from semiconductor simulations to ionic transport in water as a background medium. This method has been used successfully to compute transport rates of ions in biological channels but polarization effects on protein walls cannot be easily included self-consistently, due to the complexity of the structure. Since CNTs have a regular structure, it is practical to employ a self-consistent scheme that accounts for the charge redistribution on the channel wall when an external bias is applied or when the electrical field of a passing ion is screened out. Previous work has shown that this is necessary and the computationally efficient tight-binding (TB) approach developed there [1] is combined with transport Monte Carlo simulations in this work.     

Numerical methods in the simulation of charge transport in solid dielectrics

This paper introduces some numerical methods pertaining to the simulation of charge transport in solid dielectrics that can be of interest to the electrical engineering community. These methods are generally scattered over numerical analysis and computing literature making difficult for the non-specialist to select the appropriate computing schemes. In the first part we consider an example of a set of physical parameters needed to describe charge transport and we introduce mathematical equations that must be solved. A series of numerical schemes are compared for one-dimensional convection problems in a second part. Numerical results show that a third order upwind scheme named QUICKEST, combined with a flux limiter (ULTIMATE) is competent for the resolution of the transport equation in solid dielectrics, avoiding numerical oscillations and numerical diffusion. The third part deals with numerical resolution of the Poisson's equation comparing two different methods.