Estimation of background carrier concentration in fully depleted GaN films

Buffer leakage is an important parasitic loss mechanism in AlGaN/GaN HEMTs and hence various methods are employed to grow semi-insulating buffer layers. Quantification of carrier concentration in such buffers using conventional capacitance based profiling techniques is challenging due to their fully depleted nature even at zero bias voltages. We provide a simple and effective model to extract carrier concentrations in fully depleted GaN films using capacitance-voltage (C-V) measurements. Extensive mercury probe C-V profiling has been performed on GaN films of differing thicknesses and doping levels in order to validate this model. Carrier concentrations as extracted from both the conventional C-V technique for partially depleted films having the same doping concentration, and Hall measurements show excellent agreement with those predicted by the proposed model thus establishing the utility of this technique. This model can be readily extended to estimate background carrier concentrations from the depletion region capacitances of HEMT structures and fully depleted films of any class of semiconductor materials.Comment: 16 pages, 6 figure

Evaluation of Satellite-Based Rainfall Estimates in the Lower Mekong River Basin (Southeast Asia)

Satellite-based precipitation is an essential tool for regional water resource applications that requires frequent observations of meteorological forcing, particularly in areas that have sparse rain gauge networks. To fully realize the utility of remotely sensed precipitation products in watershed modeling and decision-making, a thorough evaluation of the accuracy of satellite-based rainfall and regional gauge network estimates is needed. In this study, Tropical Rainfall Measuring Mission (TRMM) Multi-Satellite Precipitation Analysis (TMPA) 3B42 v.7 and Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) daily rainfall estimates were compared with daily rain gauge observations from 2000 to 2014 in the Lower Mekong River Basin (LMRB) in Southeast Asia. Monthly, seasonal, and annual comparisons were performed, which included the calculations of correlation coefficient, coefficient of determination, bias, root mean square error (RMSE), and mean absolute error (MAE). Our validation test showed TMPA to correctly detect precipitation or no-precipitation 64.9% of all days and CHIRPS 66.8% of all days, compared to daily in-situ rainfall measurements. The accuracy of the satellite-based products varied greatly between the wet and dry seasons. Both TMPA and CHIRPS showed higher correlation with in-situ data during the wet season (JuneSeptember) as compared to the dry season (NovemberJanuary). Additionally, both performed better on a monthly than an annual time-scale when compared to in-situ data. The satellite-based products showed wet biases during months that received higher cumulative precipitation. Based on a spatial correlation analysis, the average r-value of CHIRPS was much higher than TMPA across the basin. CHIRPS correlated better than TMPA at lower elevations and for monthly rainfall accumulation less than 500 mm. While both satellite-based products performed well, as compared to rain gauge measurements, the present research shows that CHIRPS might be better at representing precipitation over the LMRB than TMPA

The effect of slurry composition on the microstructure and mechanical properties of open-cell Inconel foams manufactured by the slurry coating technique

Open-cell nickel-based alloy foams are attractive materials for applications such as sound damping and heat exchange, especially those involving exposure to high temperature environments. This study demonstrated the potential of a developed slurry coating technique for manufacturing open-cell Inconel alloy foams, and then investigated the effect of slurry composition on the microstructure and mechanical properties of the foams. It was found that the compressive properties of the foam can be quantitatively related to its relative density using the empirical equations. The deformation behaviour of the foam is bending-dominated; and unit cell struts undergo brittle fracture after the elastic region. Increasing the slurry solid loading leads to a higher average bulk foam density and more non-uniform crush bands in the foam under compression. Compared to other fabrication processes, this slurry coating technique is able to produce open-cell Inconel foams with relatively higher strength-to-weight ratios. This study also revealed that the sound absorption capability of the foam increases when its unit cell size is reduced

Approximation Algorithms for Covering/Packing Integer Programs

Given matrices A and B and vectors a, b, c and d, all with non-negative entries, we consider the problem of computing min {c.x: x in Z^n_+, Ax > a, Bx < b, x < d}. We give a bicriteria-approximation algorithm that, given epsilon in (0, 1], finds a solution of cost O(ln(m)/epsilon^2) times optimal, meeting the covering constraints (Ax > a) and multiplicity constraints (x < d), and satisfying Bx < (1 + epsilon)b + beta, where beta is the vector of row sums beta_i = sum_j B_ij. Here m denotes the number of rows of A. This gives an O(ln m)-approximation algorithm for CIP -- minimum-cost covering integer programs with multiplicity constraints, i.e., the special case when there are no packing constraints Bx < b. The previous best approximation ratio has been O(ln(max_j sum_i A_ij)) since 1982. CIP contains the set cover problem as a special case, so O(ln m)-approximation is the best possible unless P=NP.Comment: Preliminary version appeared in IEEE Symposium on Foundations of Computer Science (2001). To appear in Journal of Computer and System Science

Bosque River Environmental Infrastructure Improvement Plan: Phase I Final Report

The Bosque River and its associated watershed face a myriad of water quality challenges. Previous attempts made to address these concerns have met with limited success due to a relatively narrow, specific problem approach. The goal of this project is to develop a comprehensive plan that considers all aspects of existing issues for collaborators to implement and assist in planning for improved environmental infrastructure. The project set forth will aid in identifying appropriate management practices and structures for rehabilitating and maintaining watershed health from a landscape scale approach. Implementation of an environmental infrastructure program employing a series of best management practices (BMPs) and efforts is desirable for addressing overall watershed health. This report is the first phase of a project that is focused on developing and employing a strategic approach to identifying priority areas in the watershed where field investigations should begin to investigate the need to reduce pollution and in choosing appropriate BMPs for specific areas that are best suited to meet pollution reduction needs both efficiently and economically. There needs to be more in-depth analysis of cost benefits and economic and environmental alternative analysis need to follow in the next phases of this project before any field implementation is undertaken. In-depth analysis using applicable Geographic Information Systems (GIS) data generated specifically for this project identified specific areas of need. Sub-watersheds were evaluated using an impact index that assigns a ranking to each sub-watershed based on three pollution quantifying indices: a concentration impact index, a load per unit area index and a load impact index. The sum of the three index rankings yields the overall ranking for each sub-watershed. A scientific advisory committee developed a list of potential BMPs. The list consists of 22 feasible BMPs that have been assigned a priority index based on potential water quality effects, capital and maintenance costs, and applicability of the practice in the watershed. After establishing the prioritized list, BMPs were evaluated by the Spatial Sciences Lab (SSL) at Texas A&M University using GIS to identify areas within the watershed where implementing these practices would be most effective. Six spatial criteria and six location-specific criteria were used to determine optimum potential locations within the watershed for each BMP to be implemented. This document outlines an effective methodology for determining which locations in the watershed should receive focus when field work begins, and which BMPs would be most effective in specific sub-watersheds. Six steps were identified as an effective process to choose the proper BMP for each sub-watershed in the basin. If these steps are followed, the best BMP(s) for each location should be effectively identified

Visual surface perception: Surface reconstruction from shading and stereo.

Computing shape and depth from stereo vision and shading is one of the important perceptual tasks in early vision. This thesis work is aimed at understanding the computational issues involved in reconstructing a viewed surface in three dimension. In using stereo vision to compute depth, stereo correspondence between points in the left and right images can be reliably achieved only at points of intensity changes. Owing to the need for computing depth at every point in the image, interpolation of this sparse depth data becomes necessary. Since several surfaces can fit a given sparse grid, an appropriate choice comes from imposing additional constraints. In our work, we propose that the interpolant should not only introduce any additional discontinuities (other than those dictated by the intensity changes), but also that they should preserve those discontinuities. A class of interpolants known as Shepard\u27s surfaces, is shown here to satisfy this constraint. The Shepard\u27s interpolants have been implemented here and the result from testing them on Random DOt Stereograms shows that stereo vision can function alone without any additional visual cues. The natural stereo pair shows that even when intensity changes are sparse the reconstruction preserves the shape although, the interpolant exhibits a tendency to consider spurious stereo matches also as a potential data point. Besides depth from stereopsis, shape information also becomes important to reconstruct the surface. An important shape cue is available in the smooth shading that an object renders. From the perspective of obtaining shape description (instead of surface normals alone), we propose a method to compute relative depth, normals and principal curvatures of the surface. Shape information is intrinsic to the surface and is independent of viewer position. Since continuity in normals is ensured through these shape descriptors, the numerical error introduced in the process of reconstruction is shown to be independent of coordinate axes chosen. Our method involves minimization of a global objective function formulated by imposing the following constraints: (i) continuity and integrability of the normals, (ii) minimal deviation from the irradiance values, and (iii) unit normal. Minimizing the objective function with respect to the normals n, relative depth z and shape descriptor A, results in direct computation of all these quantities. In addition, the principal curvatures are shown to be computable from the shape descriptor used here. Shape and depth information may also be available from other visual cues. We show that depth information at arbitrary set of points can be included as additional set constraints in the shape computing algorithm. Also, any known set of normals can also be exploited to improve the convergence of the shape from shading algorithm, besides smoothly incorporating the additional source of information. This work, in essence, has resulted in developing a framework that delivers shape information in the form of local curvatures and depth at every point in the image plane.Dept. of Electrical and Computer Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1990 .R344. Source: Dissertation Abstracts International, Volume: 52-11, Section: B, page: 6006. Co-Supervisors: M. Ahmadi; M. Shridhar. Thesis (Ph.D.)--University of Windsor (Canada), 1990