1,720,972 research outputs found
Modeling and Optimization of Single Photon Avalanche Photodiodes for X-Ray Detection
Get PDFAvalanche Photodiodes (APDs) are electronic devices that transduce a photon flux into an electrical current and provide internal amplification of this current exploiting the impact ionization mechanism. APDs are used as receivers in optical fiber communication links as well as detectors in physics experiments and medical imaging. According to the needs of the target application, they can either be operated below (Linear mode) or above (Geiger mode) their breakdown voltage.
For X-ray detection, APDs fabricated in III-V compound semiconductors, such as GaAs, offer an interesting alternative to SiAPDs, thanks to the short attenuation length at high energies offered by these materials. However, to improve the poor noise performance of APDs fabricated in III-V compounds given by similar electron’s and hole’s impact ionization coefficients, structures alternative to p-i-n APDs have to be employed. A possible solution is the use of staircase APDs, where heterojunctions between III-V compound semiconductors and their alloys with metals are exploited to enhance the electron to hole impact ionization probability by creating an artificial superlattice.
This thesis aims at proposing models to compute the figures of merit of APDs fabricated in III-V compound semiconductors and operating in Linear mode for the detection of X-rays. Since accurate modeling of impact ionization is key to obtain reliable data from simulations, we present the development of a suite of simulations tools that includes finite difference and a Random Path Length algorithm implementation of a newly derived nonlocal history dependent impact ionization model and a Full Band Monte Carlo transport simulator. All these models have been validated against experimental results and are thus powerful tools in support of the interpretation of single photon APDs electrical measurements and for the optimization of their performance. These simulation tools have been used to compute the gain, the excess noise factor, the response time, the bandwidth and the jitter of different APD structures, including staircase APDs.
In addition, the Full Band Monte Carlo transport simulator has been employed to assess the basic assumptions, identify the limitations and improve
the calibration of nonlocal history dependent impact ionization models. We have found that, even though nonlocal history dependent models give results that are in a satisfactory agreement with experiments, they neglect that after an impact ionization event secondary carriers are generated with non null kinetic energy and that carrier-phonon scattering may lead to electrons and holes that travel for few free flights with velocities that are opposite to the direction of the electric fields. These aspects may become relevant and yield misleading results, in particular for short devices
Modeling Low and High Field Uniform Transport in Monolayer MoS2
No full textWe have developed a multi-valley Monte Carlo simulator to study uniform transport in MoS 2 monolayers. At low electric field, our solver is in excellent mutual agreement with a numerical solution of the linearized Boltzmann Transport Equation. We have then explored high field transport and analyzed the influence of different scattering mechanisms on the electron saturation velocity. Although scattering with neutral defects and Coulomb centers strongly affects the mobility, the effect on the saturation velocity is only modest. On the other hand, scattering with surface optical phonons has a significant influence on the saturation velocity, which we physically interpreted by inspecting the energy and momentum distributions of carriers
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Modeling charge collection in x-ray imagers
No full textWe present a TCAD simulation-based investigation of charge transport and charge collection mechanisms of a pinned photodiode pixel for X-ray imaging. Following static simulations to determine the electric-field and potential profiles, charge collection was analyzed with transient numerical simulations. A new, simple and computationally affordable calculation based on the extension of Ramo's theorem to account for carrier diffusion is proposed to estimate the trajectories of the generated carriers and the corresponding currents at the device terminals. The new method is validated and successfully compared to the time-dependent TCAD simulations of a p-i-n diode and a template 3D pinned photodiode
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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