1,720,966 research outputs found
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
Quantum analysis of Zener-assisted impact ionization for transient regimes and strong electric fields
No full textQuantum theory of impact ionization in coherent high-field semiconductor transport
Full text linkGeneration of carriers in semiconductors by impact ionization is studied under the influence of a constant, arbibrarily high electric field. Using the density-matrix approach a system of equations for the coherent dynamics of electrons and holes in the presence of impact ionization and Auger recombination is derived, which extends the semiconductor Bloch equations by the inclusion of impact-ionization density-correlation functions as additional dynamic variables. From these equations we recover the pure (Zener) and the photon-induced (Franz-Keldysh) carrier tunneling rate and derive an expression for the field-assisted impact-ionization scattering rate. Different levels of approximation of the kinetic equations are discussed. It is shown that in contrast to the semiclassical treatment in the presence of an electric field, a fixed impact-ionization threshold does no longer exist, and the impact-ionization scattering rate is drastically enhanced around the semiclassical threshold by the intracollisional field effect. The close connection of field-assisted impact ionization to the Franz-Keldysh effect is emphasized
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
MONTE-CARLO SIMULATIONS OF HIGH-ENERGY ELECTRONS AND HOLES IN SI-N-MOSFETS
No full textMonte Carlo (MC) device simulations of high energy electrons and holes in Si-n-MOSFET's are presented. Key features of this work include the use of a suitable silicon model for carrier transport at high electric fields, an original impact ionization model, and sophisticated numerical techniques to speed up the calculation. The case of submicrometer Si-n-MOSFET's is considered as a relevant application
AN IMPROVED IMPACT-IONIZATION MODEL FOR HIGH-ENERGY ELECTRON-TRANSPORT IN SI WITH MONTE-CARLO SIMULATION
No full textA new model for impact ionization in Si is presented, which goes beyond the limitations of the Keldysh formula and is based on a more realistic scheme developed starting from a first-order perturbation theory. This scattering mechanism is modeled by an extended band structure which includes many bands for electrons and one band for holes in a finite Brillouin zone. Some processes have been identified to bring the dominant contribution to the scattering probability, in the present approach, for electron energies ranging up to 3 eV. Expressions for the differential and integrated scattering probabilities have been obtained which are consistent with the band model and can be included in a Monte Carlo simulation of the electron gas. Results for transport quantities are shown for a bulk material in presence of homogeneous and static electric fields under physical conditions where impact ionization influences the carrier dynamics. A comparison with theoretical and experimental data from the literature is also given
A MULTIBAND MODEL FOR HOLE TRANSPORT IN SILICON AT HIGH-ENERGIES
No full textA new microscopic silicon model for hole transport at high electric fields featuring two valence bands in a finite Brillouin zone is presented. The band parameters and the electron-phonon coupling constants were determined by best fitting the density of states and the experimental and theoretical results for transport properties in the low and intermediate field-strength range. Hole impact ionization has been introduced following a new scheme that goes beyond the limitations contained in the Keldysh formula. The present model, coupled to an analogous model already developed for electrons, allows study of bipolar transport in silicon devices. Applications to bulk Si and Si p-MOSFETs are presented.A new microscopic silicon model for hole transport at high electric fields featuring two valence bands in a finite Brillouin zone is presented. The band parameters and the electron-phonon coupling constants were determined by best fitting the density of states and the experimental and theoretical results for transport properties in the low and intermediate field-strength range. Hole impact ionization has been introduced following a new scheme that goes beyond the limitations contained in the Keldysh formula. The present model, coupled to an analogous model already developed for electrons, allows study of bipolar transport silicon devices. Applications to bulk Si and Si p-MOSFETs are presented
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