1,721,019 research outputs found
Self-Scattering in Monte Carlo Simulation of Quantum Transport
No full textIn this letter, we shall present a generalization to quantum transport of the self-scattering technique commonly used in semi-classical transport simulations. The proposed Quantum Monte Carlo method originates from a perturbative expansion of the Liouville-von Neumann equation. Following the lines of Chambers' approach to semi-classical transport, an approximate imaginary self-energy hBAR/tau-0 is introduced which plays a role analogous to that of the maximum scattering rate in the traditional Monte Carlo method. At each perturbative order, the exact correction to 1/tau-0 is evaluated. The corresponding Monte Carlo algorithm for the study of quantum transport results to be very similar to the traditional one: by means of this <<quantum self-scattering>> technique, we again deal with the generation of <<free flights>> and <<interaction processes>>, as in the semi-classical case. In particular, these similarities are found to be very useful for the analysis of quantum phenomena which give rise to deviations from semi-classical results
Enhancement of drift-velocity overshoot in silicon due to the intracollisional field effect
No full textA simulation of charge quantum transport in silicon is presented. The authors discuss how effects such as energy nonconserving transitions (or collisional broadening) and intracollisional field effect influence transient transport and, in particular, drift-velocity overshoot. The analysis is based on an improved version of the quantum Monte Carlo method developed by the authors during the last few years. Results show that, for the case of silicon, the intracollisional field effect plays the dominant role in determining deviations from the semiclassical results
A quantum description of drift velocity overshoot at high electric fields in semiconductors
No full textA quantum description of transient charge transport in semiconductors is presented. In particular, we discuss the drift velocity overshoot in GaAs. The paper is intended to show how typical quantum features, such as intracollisional field effect and multiple collisions tend to modify the transient behavior of the system as predicted by semiclassical transport. This analysis has been performed by means of a quantum Monte Carlo procedure which takes into account the GaAs band structure through a many-valley model. The results of the quantum simulation, as regards drift velocity and upper valley population, have been compared with those of the classical theory and this comparison shows that in the case of GaAs quantum features are not relevant. For a better understanding, a semiconductor model, characterized by a very strong electron-phonon coupling constant, has been considered where quantum effects are appreciable and from this analysis it is possible to identify physical systems for which a full quantum treatment is required
A quantum description of impact ionization in semiconductors
No full textThe authors have studied impact ionization with a quantum-mechanical approach beyond the Boltzmann equation. The theoretical background is a two-band density-matrix formalism where, in an electron-hole picture, particle conservation means that only the difference of electrons and holes remains constant. A quantum Monte Carlo procedure has been extended, in the second-quantization formalism, to include variable numbers of electrons and holes. The second-order correction to the number of electrons as a function of time has been evaluated. For delta-like initial distribution functions, quantum-mechanical and semiclassical results are compared. In contrast to a semiclassical treatment, nonconserving energy transitions at short times and the intracollisional field effect influence impact ionization above and below threshold
A Monte-Carlo Solution of the Wigner Transport-Equation
No full textA generalized Monte Carlo method for the solution of the Wigner transport equation in semiconductor devices is proposed. The theoretical approach is based on the Wigner transport equation describing the time evolution of our electron-phonon system, and the Monte Carlo procedure is based on an iterative expansion of such an equation in powers of the various interaction coupling constants. In addition to a fully coherent description of the electron dynamics, the proposed approach allows us, in principle, to introduce in a quantum framework all the various interaction processes, such as electron-phonon, electron-impurity, and electron-electron scattering. Furthermore, boundary conditions, and therefore open systems, can be considered, as required for the analysis of semiconductor devices. Some numerical results are presented for a biased double-barrier structure with electron-phonon interaction
Weighted Monte Carlo approach to electron transport in semiconductors
No full textThe theory of electron transport in semiconductors is traditionally formulated in terms of the semiclassical Boltzmann equation. In nonlinear transport such an equation must be solved without linearization with respect to the external driving fields. This task is practically impossible by analytical means, but for many years a Monte Carlo numerical technique has been successfully applied to all sorts of problems in semiconductor electron transport. In this paper a new approach to Monte Carlo simulation of electron transport in semiconductors, which has recently appeared in the literature, is reviewed. In the traditional Monte Carlo approach a direct simulation of the electron motion is realized, where all possible events (scattering processes) occur with the same probability as in the 'real' world. On the contrary, in the new approach, called the weighted Monte Carlo technique, events occur with arbitrary probabilities, and the weight of the particle in the simulation is accordingly modified in such a way as to maintain an unbiased result. In this way it is possible to emphasize, during the simulation, the analysis of the effect of rare events that in standard simulations would occur too rarely. The traditional Monte Carlo approach is recovered as a special case of this new more general technique. Applications of the weighted Monte Carlo technique to the evaluation of high-energy tails of the distribution function and of the electron current through a potential barrier are presented. A generalization of the method to quantum electron transport is also reviewed
The role of band-tail states on the electric properties of amorphous chalcogenides: A simulative approach
Full text linkBand-tail states, i.e., charge-carrier energy states located in the bandgap at the valence and conduction band edges of amorphous materials, even though not delocalized, exhibit nonzero mobility; thus, they are expected to contribute to the charge-conduction process. A microscopic model based on hydrodynamic transport equations for unipolar conduction, including trap, band-tail, and band states, and coupled to the Poisson equation is presented here. The equations are self-consistently solved by means of a numerical procedure, and the results provide qualitative and quantitative estimates of the influence of band-tail states (namely, of their energy distribution, density, and mobility) on the carrier heating, precursor of the Ovonic threshold switch
Quantum theory of transient transport in semiconductors: A Monte Carlo approach
Full text linkA new Monte Carlo method is presented for the evaluation of the density matrix from the solution of the Liouville–von Neumann equation for an ensemble of noninteracting electrons in a semiconductor crystal. The method is applied to the study of the electron transient response to a high external electric field in Si and to the relaxation of photoexcited electrons in GaAs in absence of external electric fields. The phonon population is always assumed at equilibrium, but no assumptions are made about the strength of the electron-phonon interaction. Results show that typical quantum features such as energy-nonconserving transitions, intracollisional field effect, and multiple collisions change the very first transient of the system with respect to a semiclassical description
Monte Carlo simulation of electron transport in (formula presented) superlattices: Vertical transport enhanced by a parallel field
No full textConsiderable effort is presently devoted to develop Si quantum structures for microelectronics and nanoelectronics. In particular, well-defined (formula presented) superlattices and quantum wells are under study. We investigate here the transport properties of a (formula presented) superlattice with a multiband one-particle Monte Carlo simulator. The band structure is obtained with an analytical model and the scattering mechanisms introduced in the simulator are confined optical phonons, both polar and nonpolar. Owing to the very flat shapes of the bands along the growth direction, very low drift velocities are obtained for vertical transport. However, the simulation shows that, for oblique fields, the transport properties along the vertical direction are strongly enhanced by the in-plane component of the electric field, consequently higher vertical drift velocities can be easily obtained. © 2002 The American Physical Society
Dynamics of electrons in a 2D region coming from a point-contact
No full textBallistic and quasi-ballistic transport in mesoscopic systems is, nowadays, a fundamental tool for the investigation of electronic processes in semiconductors. In this work we present some results concerning a numerical simulation of electrons entering a 2D mesoscopic region from a point contact; a magnetic field is applied perpendicular to the structure and influences the electron dynamics. The simulation is performed through a numerical solution of the Schroedinger equation in a finite-difference scheme. It includes a magnetic field and an arbitrary potential V(r). In this way, the quantum effects of impurities on the conductance of the system have been analysed. The resul shows that each impurity configuration characterizes, in a particular way, the transport properties
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