Breakdown Voltage in Thin III-V Avalanche Photodiodes

The dead-space multiplication theory of Hayat et al. [Journal of Lightwave Technology 10, 1415 (1992)], in conjunction with the multiplication-width-independent ionization-coefficient model developed by Saleh et al. [IEEE Trans. Electron Devices 47, 625 (2000)], are shown to accurately predict breakdown voltages for thin avalanche photodiodes of GaAs, InP, In0:52Al0:48As, and Al0:2Ga0:8As, over a broad range of device widths. The breakdown voltage is determined from the analytical expression for the impulse-response-function decay rate

Prediction of water table in an alluvial aquifer using modflow

Groundwater is the main source of water in the Kingdom of Saudi Arabia (KSA). A larger part of groundwater is founded in alluvial (unconfined) aquifers. Prediction of water table elevations in unconfined aquifers is very useful in water resources planning and management. During the last two decades, many aquifers in different regions of the KSA experienced significant groundwater decline. The declines in these aquifers raised concerns over the quantity and quality of groundwater, as well as concerns over the planning and management policies used in KSA. The main objective of this study was to predict water table fluctuations and to estimate the annual change in water table at an alluvial aquifer at wadi Hada Al Sham near Makkah, KSA. The methodology was achieved using numerical groundwater model (MODFLOW). The model was calibrated and then used to predict water table elevations due to pumping for a period of 5 years. The output of the model was found to be in agreement with the previous records. Moreover, the simulation results also show reasonable declination of water table elevations in the study area during the study period

Impact-ionization and noise characteristics of thin III-V avalanche photodiodes

It is, by now, well known that McIntyre\u27s localized carrier-multiplication theory cannot explain the suppression of excess noise factor observed in avalanche photodiodes (APDs) that make use of thin multiplication regions. We demonstrate that a carrier multiplication model that incorporates the effects of dead space, as developed earlier by Hayat et al. provides excellent agreement with the impact-ionization and noise characteristics of thin InP, In/sub 0.52/Al/sub 0.48/As, GaAs, and Al/sub 0.2/Ga/sub 0.8/As APDs, with multiplication regions of different widths. We outline a general technique that facilitates the calculation of ionization coefficients for carriers that have traveled a distance exceeding the dead space (enabled carriers), directly from experimental excess-noise-factor data. These coefficients depend on the electric field in exponential fashion and are independent of multiplication width, as expected on physical grounds. The procedure for obtaining the ionization coefficients is used in conjunction with the dead-space-multiplication theory (DSMT) to predict excess noise factor versus mean-gain curves that are in excellent accord with experimental data for thin III-V APDs, for all multiplication-region widths

A study on small business enterprises in Bangladesh : searching for growth factors and obstacles

Small businesses are the heart of the market-based economy with their business operations in rural and urban areas of developed and developing countries. In Bangladesh, small business enterprises are playing a significant role by contributing to the production and services, employmnet and thereby to GDP. But these are found to face servere competition and different types of constraints. As a result, these have not achieved substantial growth. In view of this, the present study is primarily aimed at identifying factors and obstacles that influence the growth of SBEs. Finally, the paper suggests some policy measures which are expected to excel the growth of SBEs

Nonlinear Parameter Estimation for Multiple Site-Type Polyolefin Catalysts Using an Integrated Microstructure Deconvolution Methodology

The microstructure of polyolefins determines their macroscopic properties. Consequently, it is essential to predict how polymerization conditions will affect polyolefin microstructure. The most important microstructural distributions of ethylene/alfa-olefin copolymers made with coordination catalysts are their molecular weight (MWD), chemical composition (CCD), and comonomer sequence length (CSLD). Several mathematical models have been developed to predict these microstructural distributions; reliable techniques to estimate parameters for these models, however, are still poorly developed, especially for catalysts that have multiple site types, such as heterogeneous Ziegler-Natta complexes. Most commercial polyolefins are made with heterogeneous Ziegler-Natta catalysts, which make polyolefins with broad MWD, CCD, and CSLD. This behavior is attributed to the presence of several active site types, leading to a final product that can be seen as a blend of polymers made on the different catalyst site types. The main objective of this project is to develop a methodology to estimate the most important parameters needed to describe the microstructure of ethylene/alfa-olefin copolymers made with these multiple site-type catalysts. To accomplish this objective, we developed the Integrated Deconvolution Estimation Model (IDEM). IDEM estimates ethylene/alf-olefin reactivity ratios for each site type in two-steps. In the first step, the copolymer MWD, measured by high-temperature gel permeation chromatography, is deconvoluted into several Flory’s most probable distributions to determine the number of site types and the weight fractions of copolymer made on each of them. In the second estimation step, the model uses the MWD deconvolution information to fit the copolymer triad distributions measured by 13C NMR and estimate the reactivity ratios per site type. This is the first time that MWD and triad distribution information is integrated to estimate the reactivity ratio per site type of multiple site-type catalysts used to make ethylene/alfa-olefin copolymers. IDEM was applied to two sets of ethylene-co-1-butene copolymers made with a commercial Ziegler-Natta catalyst, covering a wide range of 1-butene fractions. In the first set of samples (EBH), hydrogen was used as a chain transfer agent, whereas it was absent in the second set (EB). Comparison of the reactivity ratio estimates for the sets of samples permitted the quantification of the hydrogen effect on the reactivity ratios of the different site types present in the Ziegler-Natta catalyst used in this thesis. Since 13C NMR it is an essential analytical step in IDEM, triad distributions for the EB and EBH copolymers were measured in two different laboratories (Department of Chemistry at the University of Waterloo, and Dow Chemical Research Center at Freeport, Texas). IDEM was applied to both set of triad measurements to find out the effect of interlaboratory 13C NMR analysis on reactivity ratio estimation

MODELING OLEFIN POLYMERIZATION USING MONTE CARLO SIMULATION: DETAILED COMONOMER DISTRIBUTION

In recent years there have been many efforts to develop and expand the ability of mathematical models capable of describing polymerization systems. Models can provide a key competitive advantage for the industry and research in terms of production and technology development. As new resins are continuously produced to meet the requirement of final applications and processability, it is imperative to pursue strong polymer characterization with special attention to detailed analysis of polymer microstructure. The microstructure of polyolefin is defined by its distribution of molecular weight, chemical composition, branching topology, and stereoregularity. In this work, a Monte Carlo simulation model was developed to describe the polymerization mechanisms of olefin homopolymerization and copolymerization using single-site coordination catalyst. The mathematical model is meant to describe molecular weight and chemical composition distribution in copolymerization system. More specifically, this research work gives a detailed study of the molecular structure for ethylene- alfa-olefin copolymer. The chemical and physical properties of copolymers are influenced not only by their average composition, but also by the monomer sequence distribution along the polymer chains. Predicting the molecular weight and comonomer distributions can lead to a better understanding of the possible morphology in solid stated because they are considered to be the main structural parameters that affect the crystallinity of polymeric materials. As a consequence, final physical properties such as the tensile properties of a copolymer could be controlled by the ratio of crystalline species in the polymer. This work is considered to be a useful tool that enables us to understand and explore specific polymerization catalytic system. Being able to describe the short chain branching and the monomer sequence distribution as a function of chain length enables us to have a better control over semi-batch polymerization reactors

Evaluation of regulatory T cells in obese asthmatic children

Background: Asthma is an airway inflammatory disorder with functional and structural changes. Regulatory T (T reg) cells are important in controlling immune responses. The study was aimed to investigate the frequency of Treg cells in obese asthmatic children, in comparison to non-obese asthmatics and healthy matched controls .Methods: In addition to anthropometric and body mass index (BMI) assessment, peripheral blood samples from healthy control subjects (n = 30) and asthmatic obese (n=30) and asthmatic non-obese children (n=30) were examined for serum IgE, eosinophils counts, and flowcytometric measurement of CD4+CD25+CD127 low/neg- T cells. Pulmonary function testing was performed for asthmatic children . Results: Obese asthmatics showed significantly higher levels of serum IgE and CD4+CD25+CD127 low/neg- T cells as compared to healthy controls (p<0.001, 0.001, respectively) while comparable numbers of T reg cells were found among obese and non-obese asthmatic children. Asthmatics receiving inhaled corticosteroids (ICS) showed higher percentages of CD4+CD25+CD127 low/neg- T cells than the non-receivers (median 11.8% vs 8.8%, p <0.001). No significant correlations were found between Treg and age, eosinophil percentage, total serum IgE, pulmonary functions, or BMI and its Z score . Conclusion: Our study demonstrates an increased frequency of Treg cells in asthmatic children compared to controls with possible association with the use of ICS but not with obesity. Small sample size and lack of obese non-asthmatic group are the main points of limitation in our study

Environmental accounting as perspective for hotels of Aqaba special economic zone authority (ASEZA)

This study aims at pointing out and explaining the major factors that can help in implementing environmental accounting in the hotels of Aqaba Special Economic Zone, the study population is going to be the classified and rated hotels of the Aqaba Special Economic Zone Authority, which is 33 hotels in total. This study will basically focus on two tools for gathering the data and information required for achieving the objectives of the study, the financial reports published by the investment department of Aqaba and the questionnaire. The study shows that the hotels in Aqaba Special Economic Zone Authority face difficulties and obstacles in the implementation of environmental accounting because of factors related to legislation, level of awareness about environmental accounting, factors related to the hotels and the qualification of human resource

Multimodal Graph-Theoretic Approach for Segmentation of the Internal Limiting Membrane at the Optic Nerve Head

In this work, we present a multimodal multiresolution graph-based method to segment the top surface of the retina called the internal limiting membrane (ILM) within optic-nerve-head-centered spectral-domain optical coherence tomography (SD-OCT) volumes. Having a precise ILM surface is crucial as this surface is utilized for measuring several structural parameters such as Bruch’s membrane opening-minimum rim width (BMO-MRW) and cup volume. The proposed method addresses the common current segmentation errors due to the presence of retinal blood vessels, deep cupping, or a very steep slope of the ILM. In order to resolve these issues, the volume is resampled using a set of gradient vector flow (GVF) based columns. The GVF field is computed according to an initial surface segmentation which is obtained through a multiresolution framework. The retinal blood vessel information (obtained from corresponding registered fundus photographs) along with shape prior information are incorporated in a graph-theoretic approach to compute the ILM segmentation. The method is tested on the SD-OCT volumes from 44 glaucoma subjects and significantly smaller errors were obtained than that from current approaches