Perturbation analysis of the MNA equations coupled with the drift-diffusion equations

We perform a perturbation analysis on a coupled system modeling an integrated circuit. The modified nodal analysis equations are coupled with the drift-diffusion equations. An index 1 estimate is proven under the assumptions that the network graph contains neither loops of voltage sources and capacitors nor cutsets of current sources and inductors. Additionally, it is assumed that the Dirichlet boundaries of the drift-diffusion modeled region are connected by capacitive paths. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Semiclassical Limit in the Quantum Drift-Diffusion Equations with Isentropic Pressure

The semiclassical limit in the transient quantum drift-diffusion equations with isentropic pressure in one space dimension is rigorously proved. The equations are supplemented with homogeneous Neumann boundary conditions. It is shown that the semiclassical limit of this solution solves the classical drift-diffusion model. In the meanwhile, the global existence of weak solutions is proved.     

Volterra Integral Equation Models for Semiconductor Devices

Van Roosbroeck's bipolar drift diffusion equations cover the qualitative behaviour of many semiconductor devices. The complexity of the model equations however prevents efficient implementations needed in circuit simulations. Under close-to-thermal-equilibrium biasing conditions (zero space charge assumption, low injection limit) the van Roosbroeck system can be replaced by a system of coupled non-linear Volterra integral equations of the second kind. Involving only the macroscopic quantities current, applied voltage and serial resistance this Volterra system can be handled with comparably little effort. Volterra integral equations models are formulated for a large class of semiconductor devices with abrupt pn-junctions. The model equations are made explicit for diodes, transistors and thyristors. A survey on various results concerning Volterra models describing the switching behaviour of pn-diodes is given. The integral equation model allows to recover all relevant properties of the voltage–current characteristics.     

Diffusive semiconductor moment equations using Fermi�CDirac statistics

Diffusive moment equations with an arbitrary number of moments are formally derived from the semiconductor Boltzmann equation employing a moment method and a Chapman?CEnskog expansion. The moment equations are closed by employing a generalized Fermi?CDirac distribution function obtained from entropy maximization. The current densities allow for a drift-diffusion-type formulation or a ??symmetrized?? formulation, using dual-entropy variables from nonequilibrium thermodynamics. Furthermore, drift-diffusion and new energy-transport equations based on Fermi?CDirac statistics are obtained and their degeneracy limit is studied.     

The relaxation-time limit in the quantum hydrodynamic equations for semiconductors

The relaxation-time limit from the quantum hydrodynamic model to the quantum drift-diffusion equations in R³ is shown for solutions which are small perturbations of the steady state. The quantum hydrodynamic equations consist of the isentropic Euler equations for the particle density and current density including the quantum Bohm potential and a momentum relaxation term. The momentum equation is highly nonlinear and contains a dispersive term with third-order derivatives. The equations are self-consistently coupled to the Poisson equation for the electrostatic potential. The relaxation-time limit is performed both in the stationary and the transient model. The main assumptions are that the steady-state velocity is irrotational, that the variations of the doping profile and the velocity at infinity are sufficiently small and, in the transient case, that the initial data are sufficiently close to the steady state. As a by-product, the existence of global-in-time solutions to the quantum drift-diffusion model in R³ close to the steady-state is obtained.     

A finite volume scheme for convection–diffusion equations with nonlinear diffusion derived from the Scharfetter–Gummel scheme

We propose a finite volume scheme for convection–diffusion equations with nonlinear diffusion. Such equations arise in numerous physical contexts. We will particularly focus on the drift-diffusion system for semiconductors and the porous media equation. In these two cases, it is shown that the transient solution converges to a steady-state solution as t tends to infinity. The introduced scheme is an extension of the Scharfetter–Gummel scheme for nonlinear diffusion. It remains valid in the degenerate case and preserves steady-states. We prove the convergence of the scheme in the nondegenerate case. Finally, we present some numerical simulations applied to the two physical models introduced and we underline the efficiency of the scheme to preserve long-time behavior of the solutions.     

Two-dimensional high-resolution schemes and their application in the modeling of ionizing waves in gas discharges

The method for constructing upwind high-resolution schemes is proposed in application to the modeling of ionizing waves in gas discharges. The flux-limiting criterion for continuity equations is derived using the proposed partial monotony property of a finite difference scheme. For two-dimensional extension, the cone transport upwind approach for constructing genuinely two-dimensional difference schemes is used. It is shown that when calculating rotations of symmetric profiles by using this scheme, a circular form of isolines is not distorted in a distinct from the coordinate splitting method. The conservative second order finite-difference scheme is proposed for solving the equations system of the drift-diffusion model of electric discharge; this scheme implies finite-difference conservation laws of electric charge and full electric current (fully conservative scheme). Computations demonstrate absence of numeric oscillations and good resolution of two-dimensional ionizing fronts in simulations of streamer and barrier discharges     

Existence and Uniqueness of Solutions for a Semiconductor Device Model with Current Dependent Generation-recombination Term

A drift-diffusion model for a semiconductor device is studied. It is assumed that mobilities saturate for large densities of current carriers. The model includes the generation-recombination term of Shockley–Read–Hall and Auger as well as the avalanche term. The existence of weak solutions is proved for space dimensions=1, 2, 3 and uniqueness of solutions is showed in the case of one space dimension.     

Numerical Simulation of Thermal Effects in Electric Circuits via Energy Transport equations

In this work we present the coupling of stationary energy-transport (ET) equations with Modified Nodal Analysis (MNA)-equations to model electric circuits containing semiconductor devices. The one-dimensional ET-equations are discretised in space by an exponential fitting mixed hybrid finite element approach to ensure current continuity and positivity of charge carriers. The discretised ET-equations are coupled to MNA-equations and the resulting system is solved with backwarddifference formulas. Numerical examples are shown for a test circuit containing a pn-diode, and the results are compared to those achieved using the drift-diffusion model to describe the semiconductor devices in the circuit. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relaxation limit and initial layer to hydrodynamic models for semiconductors

We study a relaxation limit of a solution to the initial-boundary value problem for a hydrodynamic model to a drift-diffusion model over a one-dimensional bounded domain. It is shown that the solution for the hydrodynamic model converges to that for the drift-diffusion model globally in time as a physical parameter, called a relaxation time, tends to zero. It is also shown that the solutions to the both models converge to the corresponding stationary solutions as time tends to infinity, respectively. Here, the initial data of electron density for the hydrodynamic model can be taken arbitrarily large in the suitable Sobolev space provided that the relaxation time is sufficiently small because the drift-diffusion model is a coupled system of a uniformly parabolic equation and the Poisson equation. Since the initial data for the hydrodynamic model is not necessarily in “momentum equilibrium”, an initial layer should occur. However, it is shown that the layer decays exponentially fast as a time variable tends to infinity and/or the relaxation time tends to zero. These results are proven by the decay estimates of solutions, which are derived through energy methods.     

Metodi misti di exponential fitting per le equazioni di continuita' della corrente

We present an overview of results obtained for the numerical treatment of the current continuity equations arising in the drift-diffusion model for semiconductor devices. In particular, two mixed finite element schemes are discussed. Together with the good features of already known mixed schemes (current preservation and good approximation of sharp shapes) they provideM-matrices, even when a zero order term is present in the equations.     

Weak solutions to the stationary quantum drift-diffusion model

The existence of weak solutions to the stationary quantum drift-diffusion equations for semiconductor devices is investigated. The proof is based on minimization procedure of non-linear functional and Schauder fixed-point theorem. Furthermore, the semiclassical limit ε→0 from the quantum drift-diffusion model to the classical drift-diffusion model is discussed.     

A Finite Difference Equivalent Circuit Approach to Secondary Current Modelling in Annular Porous Electrodes

An equivalent circuit for an annular porous electrode is derived by reinterpreting the differential equations that approximate the distribution of voltage and current in such electrodes. The equivalent circuit is shown to provide useful physical interpretations of the secondary current distributions. Multi-loop circuit techniques are employed to obtain the current distributions within the circuit. The solutions are shown to compare well with the exact solutions of the model equations. In addition, the equivalent circuit approach is used to investigate the effect of curvature on the degree of polarization of the electrode.     

Analysis of the SG discretizatioin scheme on a multi-dimensional domain — application to impact ionization

An analysis is carried out of the multi-dimensional extension of the Scharfetter-Gummel method. It shown that, within the frame of the drift-diffusion model of semiconductor devices, the assumption of constant electric field and current densities over suitable subelements of the discretization grid preserves the necessary degrees of freedom of the problem and, at the same time, provides some information about the current-density field. These informations are exploited for treating a case in which the dependence of the model's coefficients on the current density is strong, namely, impact ionization.     

Structural properties of the one-dimensional drift-diffusion models for semiconductors

This paper is devoted to the analysis of the one-dimensional current and voltage drift-diffusion models for arbitrary types of semiconductor devices and under the assumption of vanishing generation recombination. We show in the course of this paper, that these models satisfy structural properties, which are due to the particular form of the coupling of the involved systems. These structural properties allow us to prove an existence and uniqueness result for the solutions of the current driven model together with monotonicity properties with respect to the total current , of the electron and hole current densities and of the electric field at the contacts. We also prove analytic dependence of the solutions on . These results allow us to establish several qualitative properties of the current voltage characteristic. In particular, we give the nature of the (possible) bifurcation points of this curve, we show that the voltage function is an analytic function of the total current and we characterize the asymptotic behavior of the currents for large voltages. As a consequence, we show that the currents never saturate as the voltage goes to , contrary to what was predicted by numerical simulations by M. S. Mock (Compel. 1 (1982), pp. 165--174). We also analyze the drift-diffusion models under the assumption of quasi-neutral approximation. We show, in particular, that the reduced current driven model has at most one solution, but that it does not always have a solution. Then, we compare the full and the reduced voltage driven models and we show that, in general, the quasi-neutral approximation is not accurate for large voltages, even if no saturation phenomenon occurs. Finally, we prove a local existence and uniqueness result for the current driven model in the case of small generation recombination terms.

     

Relaxation-time limit of the multidimensional bipolar hydrodynamic model in Besov space

In this paper, we study a multidimensional bipolar hydrodynamic model for semiconductors or plasmas. This system takes the form of the bipolar Euler-Poisson model with electric field and frictional damping added to the momentum equations. In the framework of the Besov space theory, we establish the global existence of smooth solutions for Cauchy problems when the initial data are sufficiently close to the constant equilibrium. Next, based on the special structure of the nonlinear system, we also show the uniform estimate of solutions with respect to the relaxation time by the high- and low-frequency decomposition methods. Finally we discuss the relaxation-time limit by compact arguments. That is, it is shown that the scaled classical solution strongly converges towards that of the corresponding bipolar drift-diffusion model, as the relaxation time tends to zero.     

The transient behaviour of multidimensional PN-diodes in low injection

An initial boundary value problem modelling the transient behaviour of a pn-junction semiconductor diode is simplified by formal asymptotics, where the scaled minimal Debye length and the scaled intrinsic number are considered as small parameters. The implicit assumption on the biasing situation is that of low injection. The simplified model is shown to be equivalent to an integral relation between the evolution of the current and of the contact voltage. A simple switching application is governed by a non-linear Volterra integral equation for the current. This equation is shown to possess a unique solution globally in time converging to the unique steady state.     

Hydrodynamic Models with Quantum Corrections

We derive a quantum-corrected hydrodynamic and drift-diffusion model for the out-of-equilibrium particle dynamics in the presence of particle collisions, modeled by a BGK collision term. The quantum mechanical corrections are obtained within the Liouville formalism and are expressed by an effective nonlinear force. The Boltzmann and Fermi-Dirac statistics are included.     

The diffusive relaxation limit of non-isentropic Euler-Maxwell equations for plasmas

This paper concerns the non-isentropic Euler-Maxwell equations for plasmas with short momentum relaxation time. With the help of the Maxwell-type iteration, it is obtained that, as the relaxation time tends to zero, periodic initial-value problem of certain scaled non-isentropic Euler-Maxwell equations has unique smooth solutions existing in the time interval where the corresponding classical drift-diffusion model has smooth solutions. Meanwhile, we justify a formal derivation of the corresponding drift-diffusion model from the non-isentropic Euler-Maxwell equations.     

Drift-dominated current flow in forward-biased semiconductor devices

The drift-diffusion equations of semiconductor physics, allowingfor field-dependent drift velocities, are analysed by the methodof matched asymptotic expansions for one-dimensional PN andPNPN forward-biased structures. The analysis is relevant todescribing the structure of the solutions to the drift-diffusionequations for large electric fields when drift velocity saturationeffects become significant. In this high-field limit, the boundarylayer structure for the solutions to the drift-diffusion equationsis seen to differ substantially from that near equilibrium.In particular, boundary layers for the carrier concentrationscan occur near the contacts. The asymptotic solutions and thecurrent-voltage relations, constructed in the high-field limit,are found to agree well with direct numerical solutions to thedrift-diffusion equations.