Drift-diffusion

The asymmetrically mounted flat plasma actuator is investigated using a self-consistent two-dimensional fluid model at atmospheric pressure. The computational model assumes the drift-diffusion approximation and uses a simple plasma kinetic model. It investigated the electrical and kinetic properties of the plasma, calculated the charged species concentrations, surface charge density, electrohydrodynamic forces, and gas speed. The present computational model contributes to understand the main physical mechanisms, and suggests ways to improve its performance.

The critical assumptions in the drift-diffusion model are the local force approximation and the use of the Einstein relation under nonequilibrium conditions. The validity of these two approximations is investigated by full-band Monte Carlo simulation for a SiGe-HBT. It is found that neither the local force approximation nor the Einstein relation holds. Even Einstein relations generalized with the local temperature fail under quasiballistic transport conditions, indicating that the energy transport and hydrodynamic approach are also problematic.

A genuinely two-dimensional discretization of general drift-diffusion (including incompressible
Navier-Stokes) equations is proposed. Its numerical fluxes are derived by computing
the radial derivatives of “bubbles” which are deduced from available discrete data by exploiting the
stationary Dirichlet-Green function of the convection-diffusion operator. These fluxes are reminiscent
of Scharfetter-Gummel’s in the sense that they contain modified Bessel functions which allow to
pass smoothly from diffusive to drift-dominating regimes. For certain flows, monotonicity properties
are established in the vanishing viscosity limit (“asymptotic monotony”) along with second-order
accuracy when the grid is refined. Practical benchmarks are displayed to assess the feasibility of the
scheme, including the “western currents” with a Navier-Stokes-Coriolis model of ocean circulation.

A discrete drift-diffusion model is derived from a microscopic sequential tunneling model of charge transport in weakly coupled superlattices provided temperatures are low or high enough. Realistic transport coefficients and novel contact current--field characteristic curves are calculated from microscopic expressions, knowing the design parameters of the superlattice. Boundary conditions clarify when possible self-sustained oscillations of the current are due to monopole or dipole recycling.

A Wigner–Poisson kinetic equation describing charge transport in doped semiconductor superlattices is proposed. Electrons are assumed to occupy the lowest miniband, exchange of lateral momentum is ignored and the electron–electron interaction is treated in the Hartree approximation. There are elastic collisions with impurities and inelastic collisions with phonons, imperfections, etc. The latter are described by a modified BGK (Bhatnagar–Gross–Krook) collision model that allows for energy dissipation while yielding charge continuity. In the hyperbolic limit, nonlocal drift-diffusion equations are derived systematically from the kinetic Wigner–Poisson–BGK system by means of the Chapman–Enskog method. The nonlocality of the original quantum kinetic model equations implies that the derived drift-diffusion equations contain spatial averages over one or more superlattice periods. Numerical solutions of the latter equations show self-sustained oscillations of the current through a voltage...

ABSTRACT In this work we investigate the performance of double-gate and cylindrical nanowire FETs with high-kappa gate dielectrics at their extreme miniaturization limits. The model fully accounts for quantum electrostatics; current transport is simulated by an improved quantum drift-diffusion approach supported by a new thickness-dependent mobility model which nicely fits the available measurements for both SiO2 and HfO2 gate dielectrics. The on-current is simulated using both the quantum drift-diffusion model and a full-quantum transport approach based on the quantum transmitting boundary method, which assumes a purely ballistic transport. The performance comparison between SiO2 and HfO2 insulated-gate FETs with the same electrical oxide thickness demonstrates that the latter provides a slight degradation of the short-channel effect compared with the former but, at the same time, gives an improved on-current due to lateral capacitive-coupling effects, despite the inherent degradation of the low-field mobility

Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be described neither by drift-diffusion, nor by purely ballistic models. In carbonbased nanotransistors, source and drain contacts are often characterized by the formation of Schottky Barriers (SBs), with strong influence on transport. Here we present a model for onedimensional field-effect transistors (FETs), taking into account on equal footing both SB contacts and FFE transport regime. Intermediate transport is introduced within the Buttiker probe approach to dissipative transport, in which a non-ballistic transistor is seen as a suitable series of individually ballistic channels. Our model permits the study of the interplay of SBs and ambipolar FFE transport, and in particular of the transition between SB-limited and dissipation-limited transport.

Positron motion in an electric field is studied experimentally by measuring the drift length of positrons in the space-charge region of a Au-Si surface-barrier diode. An electric field of the order of 104 V/cm gives rise to a drift length from 2 to 3 mum. The drift-diffusion approximation can explain positron transport up to an electric-field strength of 3×104 V/cm.

This paper describes the high current behavior of a lateral, n-channel, high-voltage transistor. The starting points are TCAD experiments where the phenomenological behavior is analyzed. Based on these results a transistor high current model is derived, which is based on the vertical integrated free carrier concentration in the drift region. The important model parameter is the gate voltage, which defines

We determine the induced voltage generated by spatial and temporal magnetisation textures (inhomogeneities) in metallic ferromagnets due to the spin diffusion of non-equilibrium electrons. Using time dependent semi-classical theory as formulated in Zhang and Li [1] and the drift-diffusion model of transport it is shown that the voltage generated depends critically on the difference in the diffusion constants of up and down spins. Including spin relaxation results in a crucial contribution to the induced voltage. We also show that the presence of magnetisation textures results in the modification of the conductivity of the system. As an illustration, we calculate the voltage generated due to a time dependent field driven helimagnet by solving the Landau–Lifshitz equation with Gilbert damping and explicitly calculate the dependence on the relaxation and damping parameters.

A two-dimensional small bias model has been developed for a patterned metal current collector $|$ mixed oxygen ion and electronic conductor (MIEC) $|$ patterned metal current collector electrochemical cell in a symmetric gas environment. Specifically, we compute the electrochemical potential distributions of oxygen vacancies and electrons in the bulk and near the surface for $\text{Pt} | \text{Sm}_{0.15}\text{Ce}_{0.85}\text{O}_{1.925} | \text{Pt}$ symmetric cell in a $\text{H}_2-\text{H}_2\text{O}-\text{Ar}$ (reducing) atmosphere from 500 to $650^o C$. Using a two-dimensional finite-element model, we show that two types of electronic current exist within the cell: an in-plane drift-diffusion current that flows between the gas $|$ ceria chemical reaction site and the metal current collector, and a cross-plane current that flows between the two metal electrodes on the opposite side of the cell. By fitting the surface reaction constant $\tilde k_f^0$ to experimental electrode resistan...

A two-dimensional small bias model has been developed for a patterned metal current collector $|$ mixed oxygen ion and electronic conductor (MIEC) $|$ patterned metal current collector electrochemical cell in a symmetric gas environment. Specifically, we compute the electrochemical potential distributions of oxygen vacancies and electrons in the bulk and near the surface for $\text{Pt} | \text{Sm}_{0.15}\text{Ce}_{0.85}\text{O}_{1.925} | \text{Pt}$ symmetric cell in a $\text{H}_2-\text{H}_2\text{O}-\text{Ar}$ (reducing) atmosphere from 500 to $650^o C$. Using a two-dimensional finite-element model, we show that two types of electronic current exist within the cell: an in-plane drift-diffusion current that flows between the gas $|$ ceria chemical reaction site and the metal current collector, and a cross-plane current that flows between the two metal electrodes on the opposite side of the cell. By fitting the surface reaction constant $\tilde k_f^0$ to experimental electrode resistance values while fixing material properties such as bulk ionic and electronic equilibrium defect concentrations and mobilities, we are able to separate the electrode polarization into the surface reaction component and the in-plane electron drift-diffusion component. We show that for mixed conductors with a low electronic conductivity (a function of oxygen partial pressure) or a high surface reaction rate constant, the in-plane electron drift-diffusion resistance can become rate-limiting in the electrode reaction.

Abstract: A two-dimensional small bias model has been developed for a patterned metal current collector $| $ mixed oxygen ion and electronic conductor (MIEC) $| $ patterned metal current collector electrochemical cell in a symmetric gas environment. Specifically, we compute the electrochemical potential distributions of oxygen vacancies and electrons in the bulk and near the surface for $\ text {Pt}|\ text {Sm} _ {0.15}\ text {Ce} _ {0.85}\ text {O} _ {1.925}|\ text {Pt} $ symmetric cell in a $\ text {H} _2-\ text {H} _2\ text {O}-\ text {Ar} $( ...

The morphological evolution of hillocks at the unpassivated sidewalls of the single crystal metallic thin films is investigated via computer simulations by using the free-moving boundary value problem. The effects of the drift-diffusion anisotropy on the development of surface topographical scenarios is fully explored under the action of electromigration and capillary forces, utilizing numerous combination of the surface texture, the drift-diffusion anisotropy and the direction of the applied electric field. The present simulation studies yield very rich and technologically imported information, in regards to the critical texture of the single crystal thin film surfaces, and the intensity and the orientation of the applied electric field, as far as the device reliability is concerned.

With the scaling of field-effect transistors to the nanometre scale, it is well recognised that TCAD simulations of such devices need to account for quantum mechanical confinement effects. The most widely used method to incorporate quantum effects within classical and semi-classical simulators is via density gradient quantum corrections. Here we present our methodologies for including the density gradient method within our Drift-Diffusion and Monte Carlo simulators and highlight some of the additional benefits that this provides when dealing with the charge associated with random discrete dopants. KeywordsDensity gradient-Quantum corrections-Drift-Diffusion-Monte Carlo-Simulation-MOSFETs

The effect of interface state trap density, D it , on the device characteristics of n-type, enhancement-mode, implant-free (IF) In 0.3 Ga 0.7 As MOSFETs [1] and [2] has been investigated using a commercial drift-diffusion (DD) device simulation tool. Methodology has been ...