Review and Characterization of Gallium Nitride Power Devices

Gallium Nitride (GaN) power devices are an emerging technology that have only recently become available commercially. This new technology enables the design of converters at higher frequencies and efficiencies than those achievable with conventional Si devices. This thesis reviews the characteristics and commercial status of both vertical and lateral GaN power devices from the user perspective, providing the background necessary to understand the significance of these recent developments. Additionally, the challenges encountered in GaN-based converter design are considered, such as the consequences of faster switching on gate driver design and board layout. Other issues include the unique reverse conduction behavior, dynamic on-resistance, breakdown mechanisms, thermal design, device availability, and reliability qualification. Static and dynamic characterization was then performed across the full current, voltage, and temperature range of this device to enable effective GaN-based converter design. Static testing was performed with a curve tracer and precision impedance analyzer. A double pulse test setup was constructed and used to measure switching loss and time at the fastest achievable switching speed, and the subsequent overvoltages due to the fast switching were characterized. The results were also analyzed to characterize the effects of cross-talk in the active and synchronous devices of a phase-leg topology with enhancement-mode GaN HFETs. Based on these results and analysis, an accurate loss model was developed for the device under test. Based on analysis of these characterization results, a simplified model was developed to describe the overall switching behavior and some unique features of the device. The consequences of the Miller effect during the turn-on transient were studied to show that no Miller plateau occurs, but rather a decreased gate voltage slope, followed by a sharp drop. The significance of this distinction is derived and explained. GaN performance at elevated temperature was also studied, because turn-on time increases significantly with temperature, and turn-on losses increase as a result. Based on this relationship, a temperature-dependent turn-on model and a linear scaling factor was proposed for estimating turn-on loss in e-mode GaN HFETs

Characterization Methodology, Modeling, and Converter Design for 600 V Enhancement-Mode GaN FETs

Gallium Nitride (GaN) power devices are an emerging technology that have only become available commercially in the past few years. This new technology enables the design of converters at higher frequencies and efficiencies than those achievable with conventional Si devices. This dissertation reviews the unique characteristics, commercial status, and design challenges that surround GaN FETs, in order to provide sufficient background to potential GaN-based converter designers.Methodology for experimentally characterizing a GaN FET was also presented, including static characterization with a curve tracer and impedance analyzer, as well as dynamic characterization in a double pulse test setup. This methodology was supplemented by additional tests to determine losses caused by Miller-induced cross talk, and the tradeoff between these losses and overlap losses was studied for one example device.Based on analysis of characterization results, a simplified model was developed to describe the overall switching behavior and some unique features of the device. The impact of the Miller effect during the turn-on transient was studied, as well as the dynamic performance of GaN at elevated temperature.Furthermore, solutions were proposed for several key design challenges in GaN-based converters. First, a driver-integrated overcurrent and short-circuit protection scheme was developed, based on the relationship between gate voltage and drain current in GaN gate injection transistors. Second, the limitations on maximum utilization of current and voltage in a GaN FET were studied, particularly the voltage overshoots following turn-on and turn-off switching transients, and the effective cooling of GaN FETs in higher power operation. A thermal design was developed for heat extraction from bottom-cooled surface-mount devices. These solutions were verified in a GaN-based full-bridge single-phase inverter

Dynamo action in the ABC flows using symmetries

This is an Accepted Manuscript of an article published by Taylor & Francis in Geophysical and Astrophysical Fluid Dynamics, Volume 108, Issue 1, 2014 available online 25 Sep 2013: http://wwww.tandfonline.com/10.1080/03091929.2013.832762.This paper concerns kinematic dynamo action by the 1:1:1 ABC flow, in the highly conducting limit of large magnetic Reynolds number Rm. The flow possesses 24 symmetries, with a symmetry group isomorphic to the group O24 of orientation preserving transformations of a cube. This can be exploited to break up the linear eigenvalue problem into five distinct symmetry classes, or irreducible representations, which we label I–V. The paper discusses how to reduce the scale of the numerical problem to a subset of Fourier modes for a magnetic field in each representation, which then may be solved independently to obtain distinct branches of eigenvalues and magnetic field eigenfunctions. Two numerical methods are employed: the first is to time step a magnetic field in a given symmetry class and obtain the growth rate and frequency by measuring the magnetic energy as a function of time. The second method involves a more direct determination of the eigenvalue using the eigenvalue solver ARPACK for sparse matrix systems, which employs an implicitly restarted Arnoldi method. The two methods are checked against each other, and compared for efficiency and reliability. Eigenvalue branches for each symmetry class are obtained for magnetic Reynolds numbers Rm up to 104 together with spectra and magnetic field visualisations. A sequence of branches emerges as Rm increases and the magnetic field structures in the different branches are discussed and compared. In a parallel development,results are presented for the corresponding fluid stability problem as a function of the Reynolds number Re.Leverhulme Trus

The Potential Uses of Operational Earthquake Forecasting

This article reports on a workshop held to explore the potential uses of operational earthquake forecasting (OEF). We discuss the current status of OEF in the United States and elsewhere, the types of products that could be generated, the various potential users and uses of OEF, and the need for carefully crafted communication protocols. Although operationalization challenges remain, there was clear consensus among the stakeholders at the workshop that OEF could be useful

CNC Application and Design

Machining is an important manufacturing process that is used in wide range of applications. We acquired primary machine shop skills that provide us an opportunity to mill and drill a class of components to specified dimensions and tolerances. For each component, we created a detailed engineering working drawing that helps to shape and construct all the operations and procedures that must be undertaken, and controlled, to attain component machining without any breakdown or failure. Through hands-on machining, we discovered many different factors involved in milling, drilling, and the effects they exhibit on the tolerance and surface finish of a part

Submillimetre observations of WISE-selected high-redshift, luminous, dusty galaxies

We present SCUBA-2 850um submillimetre (submm) observations of the fields of 10 dusty, luminous galaxies at z ~ 1.7 - 4.6, detected at 12um and/or 22um by the WISE all-sky survey, but faint or undetected at 3.4um and 4.6um; dubbed hot, dust-obscured galaxies (Hot DOGs). The six detected targets all have total infrared luminosities greater than 10^13 L_sun, with one greater than 10^14 L_sun. Their spectral energy distributions (SEDs) are very blue from mid-infrared to submm wavelengths and not well fitted by standard AGN SED templates, without adding extra dust extinction to fit the WISE 3.4um and 4.6um data. The SCUBA-2 850um observations confirm that the Hot DOGs have less cold and/or more warm dust emission than standard AGN templates, and limit an underlying extended spiral or ULIRG-type galaxy to contribute less than about 2% or 55% of the typical total Hot DOG IR luminosity, respectively. The two most distant and luminous targets have similar observed submm to mid-infrared ratios to the rest, and thus appear to have even hotter SEDs. The number of serendipitous submm galaxies (SMGs) detected in the 1.5-arcmin-radius SCUBA-2 850um maps indicates there is a significant over-density of serendipitous sources around Hot DOGs. These submm observations confirm that the WISE-selected ultra-luminous galaxies have very blue mid-infrared to submm SEDs, suggesting that they contain very powerful AGN, and are apparently located in unusual arcmin-scale overdensities of very luminous dusty galaxies.Comment: 12 pages, 8 figures, 3 tables, accepted for publication in MNRA

The Roots of Diversity: Below Ground Species Richness and Rooting Distributions in a Tropical Forest Revealed by DNA Barcodes and Inverse Modeling

F. Andrew Jones is with the Smithsonian Tropical Research Institute, David L. Erickson is with the Smithsonian Institution, Moises A. Bernal is with the Smithsonian Tropical Research Institute and UT Austin, Eldredge Bermingham is with the Smithsonian Tropical Research Institute, W. John Kress is with the Smithsonian Institution, Edward Allen Herre is with the Smithsonian Tropical Research Institute, Helene C. Muller-Landau is with the Smithsonian Tropical Research Institute, Benjamin L. Turner is with the Smithsonian Tropical Research Institute.Background -- Plants interact with each other, nutrients, and microbial communities in soils through extensive root networks. Understanding these below ground interactions has been difficult in natural systems, particularly those with high plant species diversity where morphological identification of fine roots is difficult. We combine DNA-based root identification with a DNA barcode database and above ground stem locations in a floristically diverse lowland tropical wet forest on Barro Colorado Island, Panama, where all trees and lianas >1 cm diameter have been mapped to investigate richness patterns below ground and model rooting distributions. Methodology/Principal Findings -- DNA barcode loci, particularly the cpDNA locus trnH-psba, can be used to identify fine and small coarse roots to species. We recovered 33 species of roots from 117 fragments sequenced from 12 soil cores. Despite limited sampling, we recovered a high proportion of the known species in the focal hectare, representing approximately 14% of the measured woody plant richness. This high value is emphasized by the fact that we would need to sample on average 13 m2 at the seedling layer and 45 m2 for woody plants >1 cm diameter to obtain the same number of species above ground. Results from inverse models parameterized with the locations and sizes of adults and the species identifications of roots and sampling locations indicates a high potential for distal underground interactions among plants. Conclusions -- DNA barcoding techniques coupled with modeling approaches should be broadly applicable to studying root distributions in any mapped vegetation plot. We discuss the implications of our results and outline how second-generation sequencing technology and environmental sampling can be combined to increase our understanding of how root distributions influence the potential for plant interactions in natural ecosystems.FAJ acknowledges the support of a Tupper postdoctoral fellowship in tropical biology and the National Science Foundation (DEB 0453665). Funding was provided by the Smithsonian Institution Global Earth Observatory, the Smithsonian Tropical Research Institute/Center for Tropical Forest Sciences endowment fund, and the Smithsonian Tropical Research Institute/Frank Levinson fund. We would like to thank Autoridad Nacional del Ambiente and the Smithsonian Tropical Research Institute for processing research permits. We thank S. Hubbell and R. Condit for access to plot data, S. Schnitzer for liana census data (NSF DEB 0613666), and L. Comita and S. Hubbell for access to seedling data (NSF DEB 0075102 and DEB 0823728). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Marine Scienc

Combination antiretroviral therapy in population affected by conflict: outcomes from large cohort in northern Uganda

Objective To measure the clinical and immunological outcomes of HIV positive adult patients receiving combination antiretroviral therapy in conflict affected northern Uganda

The presence and degradation of residual permafrost plateaus on the western Kenai Peninsula Lowlands, southcentral Alaska

Permafrost influences roughly 80% of the Alaskan landscape (Jorgenson et al. 2008). Permafrost presence is determined by a complex interaction of climatic, topographic, and ecological conditions operating over long time scales such that it may persist in regions with a mean annual air temperature (MAAT) that is currently above 0 °C (Jorgenson et al. 2010). Ecosystem-protected permafrost may be found in these regions with present day climatic conditions that are no longer conducive to its formation (Shur and Jorgenson, 2007). The perennial frozen deposits typically occur as isolated patches that are highly susceptible to degradation. Press disturbances associated with climate change and pulse disturbances, such as fire or human activities, can lead to immediate and irrevocable permafrost thaw and ecosystem modification in these regions. In this study, we document the presence of residual permafrost plateaus on the western Kenai Peninsula lowlands of southcentral Alaska (Figure 1a), a region with a MAAT of 1.5±1 °C (1981 to 2010). In September 2012, field studies conducted at a number of black spruce plateaus located within herbaceous wetland complexes documented frozen ground extending from 1.4 to 6.1 m below the ground surface, with thaw depth measurements ranging from 0.49 to >1.00 m. Ground penetrating radar surveys conducted in the summer and the winter provided additional information on the geometry of the frozen ground below the forested plateaus. Continuous ground temperature measurements between September 2012 and September 2015, using thermistor strings calibrated at 0 °C in an ice bath before deployment, documented the presence of permafrost. The permafrost (1 m depth) on the Kenai Peninsula is extremely warm with mean annual ground temperatures that range from -0.05 to -0.11 °C. To better understand decadal-scale changes in the residual permafrost plateaus on the Kenai Peninsula, we analyzed historic aerial photography and highresolution satellite imagery from ca. 1950, ca. 1980, 1996, and ca. 2010. Forested permafrost plateaus were mapped manually in the image time series based on our field observations of characteristic landforms with sharply defined scalloped edges, marginal thermokarst moats, and collapse-scar depressions on their summits. Our preliminary analysis of the image time series indicates that in 1950, permafrost plateaus covered 20% of the wetland complexes analyzed in the four change detection study areas, but during the past six decades there has been a 50% reduction in permafrost plateau extent in the study area. The loss of permafrost has resulted in the transition of forested plateaus to herbaceous wetlands. The degradation of ecosystem-protected permafrost on the Kenai Peninsula likely results from a combination of press and pulse disturbances. MAAT has increased by 0.4 °C/decade since 1950, which could be causing top down permafrost thaw in the region. Tectonic activity associated with the Great Alaska Earthquake of 1964 caused the western Kenai Peninsula to lower in elevation by 0.7 to 2.3 m (Plafker 1969), potentially altering groundwater flow paths and influencing lateral as well as bottom up permafrost degradation. Wildfires have burned large portions of the Kenai Peninsula lowlands since 1940 and the rapid loss of permafrost at one site between 1996 and 2011 was in response to fires that occurred in 1996 and 2005. Better understanding the resilience and vulnerability of the Kenai Peninsula ecosystem-protected permafrost to degradation is of importance for mapping and predicting permafrost extent across colder permafrost regions that are currently warming