acase@tulane.edu

Plant parasitic nematodes have long been known to cause extensive crop losses through reduced yields, shortened productive life, or lowered value of produce. The rood knot nematode, Heterodera marioni, is cosmopolitan. It destroys crops and causes economic losses equal to those of any other plant parasitic speies. in 1948 this nematode caused an estimated loss to the carrot raising industry in Iron County, Utah of approximately $15,000. No estimates for the years prior to 1948 are available; however, reports of the county agent (14) for Iron County show that carrot production in the Cedar Valley is decreasing, and is possibly due to this nematode. The importance of the root knot nematode to the carrot industry in the Cedar Valley was Recognized by farmers at harvest time in 1948. Experimental applications of soil fumigants were tried on many farms with varying success in the spring of 1949. (See figure 1 page 2). One of these experimental plots showed spectacular results. When carrots were harvested in the fall those grown in fumigated soil were found to be of the highest quality and were growing in excellent stands; wheras, the carrots grown in unfumigated soil produced poorer stands of poor quality carrots. Before any conclusions as to the practical value of soil fumigants in carrot raising can be reached, additional tests must be conducted in the area. The fact remains that the root knot nematode is present in the area and causes damage to carrots. If carrots, or any other susceptible crops, are to be grown on economical method of controlling this nematode must be found.

Ces travaux s'inscrivent dans le cadre de la qualification des technologies GaN de UMS et plus particulièrement celle de la technologie GH25, et a pour objectif d’apporter un soutien direct au développement des technologies UMS à base de GaN. Le premier chapitre traite des généralités sur les HEMTs AlGaN/GaN. Le deuxième chapitre est consacré à la description des technologies GH50 et GH25 de UMS. Les éléments passifs de la technologie GH25 ont été caractérisés électriquement et thermiquement, puis des mesures de claquage utilisant une technique d’injection de courant de drain ont été mises en oeuvre sur des HEMTs de la technologie GH50 afin d’évaluer l'aire de sécurité de fonctionnement. Le troisième chapitre est dédié à l'étude des effets parasites rencontrés sur les deux technologies GH50 et GH25.Chacun des effets parasites est décrit puis caractérisé de façon approfondie. Le dernier chapitre se concentre sur l'étude de la fiabilité de la technologie GH25. Après avoir présenté les différentes variantes technologiques, les résultats des tests de vieillissement accéléré mis en oeuvre à UMS sont analysés afin d'évaluer leur impact sur la fiabilité de la technologieGH25 et d’identifier les mécanismes de dégradation et les effets parasites. / This work is incorporated within the framework of the qualification of UMS GaNtechnologies and more particularly of the GH25 technology, and aims to support thedevelopment of UMS GaN based technologies. The first part of this report deals withAlGaN/GaN HEMT generality. The second part is dedicated to the description of the UMStechnologies GH50 and GH25. Passive components of GH25 technology have beencharacterized through electrical and thermal measurement, and then breakdown measurementsusing a drain current injection technique have been carried out on GH50 HEMT in order toevaluate the safe operating area of these devices. The third part is dedicated to the study of theparasitic effects observed on the two technologies GH50 and GH25. Each of these parasiticeffects is described and fully characterized. The last part of this work focuses on the study ofthe GH25 technology. Technological variations are first introduced, and then results ofaccelerated aging test carried out at UMS are analyzed to evaluate their impact on thereliability of the GH25 technology and to identify wear out mechanism and parasitic effects.

Les travaux de these s’inscrivent dans le cadre de deux projets: ReAGaN et ExtremeGaN avec des industriels (UMS, Serma Technologies, Thales TRT) et des laboratoires derecherche (LEPMI, LAAS et l’université de Bristol).Les deux technologies GaN (GH50 et GH25) étudiées dans cette thèse sont fournies parla société United Monolithic Semiconductors (UMS) et elles ont été qualifiées durant cettethèse. Plusieurs composants ont subi des tests de vieillissement accéléré en températureréalisés par UMS, ensuite une campagne de caractérisation électrique approfondie a étéréalisée au laboratoire IMS afin d’étudier les effets parasites et les mécanismes de dégradationqui limitent la fiabilté de cette filière.Le premier chapitre traite les bases du transistor HEMT à base de GaN. Les avantagesdu matériau nitrure de gallium ainsi que les substrats adaptés au HEMT à base de GaN sontprésentés. Une brève description du marché europeen des composants GaN est donnée.Ensuite, la structure ainsi que le fonctionnement du HEMT AlGaN/GaN sont décrit ainsi queles deux technologies d’UMS.Le deuxième chapitre présente les tests de vieillissement utilisés pour l’analyse defiabilité. Ensuite, un état de l’art des effets parasites et des mécanismes de dégradation desHEMTs AlGaN/GaN est donné. Le projet ReAGaN est décrit et les différentes techniques decaractérisation utilisées durant les travaux de cette thèse sont présentées.Le troisième chapitre est divisé en quatre études de cas ; les trois premières sont dans lecadre du projet ReAGaN et la quatrième dans le cadre du projet Extreme GaN. Dans lapremière étude de cas, les mécanismes de conduction qui augmentent les courants de fuitesdes HEMTs AlGaN/GaN issus de la technologie GH50 ont été étudiés. La deuxième étude decas est dédiée à l’étude d’un effet parasite électrique qui apparait après un vieillissementaccéléré en température sur la caractéristique de la diode Schottky en polarisation directe.Dans la troisième étude de cas, l’influence de la variation de la fraction molaire des HEMTsAlGaN/GaN sur les paramètres électriques a été analysée. La dernière étude de cas consiste enla détermination des limites de fonctionnement et l’aire de sécurité de la technologie GH25d’UMS en réalisant les mesures des lieux de claquage en mode diode et en mode transistor. / This thesis is in the framework of two projects: ReAGaN and Extreme GaN withindustrials (UMS, Serma Technologies, Thales TRT) and academics (LEPMI, LAAS andUniversity of Bristol).The studied AlGaN/GaN HEMTs are provided by the society United MonolithicSemiconductors (UMS) from the GH50 and GH25 GaN processes that were qualified duringthis thesis. Many devices were submitted to high temperature accelerated life tests by UMSand characterized at IMS laboratory to study the parasitic effects and degradationsmechanisms that are limiting the electrical reliability of GaN based HEMTs technology.The first chapter gives an overview of the basics of GaN based high electron mobilitytransistors (HEMTs). Gallium Nitride material features are reviewed as well as substratessuited for GaN based devices. GaN market in Europe and the main industrial actors are listed.Furthermore, the structure and operation of GaN based HEMTs are described. In the last part,the two UMS GaN processes are described.The second chapter presents the life tests that are used for reliability studies. State of theart of parasitic effects and degradation mechanisms of AlGaN/GaN HEMTs is given.Furthermore, the ReAGaN project in which the main part of this thesis is involved isdescribed. The electrical characterization techniques used at IMS during this thesis arepresented.The third chapter is divided into four case studies; three case studies are in theframework of ReAGaN project and the fourth one in the Extreme GaN project. In the firstcase study, we investigate the conduction mechanisms inducing the leakage current inAlGaN/GaN HEMTs issued from GH50 process. The second case study is dedicated to thestudy of an electrical parasitic effect that appears on the Schottky diode forward characteristicafter temperature accelerated life tests. In the third case study, we study the influence of Almole fraction on the DC electrical parameters of AlGaN/GaN HEMTs. The last case studyconsists in the determination of the limits and safe operating area (SOA) of UMS GH25 GaNHEMTs by carrying out the two and three terminal breakdown voltages measurements.

No description available.

No description available.

Abstract not available

Most organisms are rarely infected with just a single species of parasite and are usually simultaneously infected with a range of species. Thus, the parasite fauna of a host represents an entire community composed of multiple individuals from many different species. In nature, it is within the host that parasites can encounter conspecifics and individuals from other species. As in any ecosystem, while such interactions between parasites can be antagonistic due to competition or conflicting interests, association between different species can also be beneficial. In this thesis, I investigated patterns of associations between parasites in the New Zealand cockle Austrovenus stutchburyi through a combination of descriptive and experimental studies employing both standard ecological field techniques and molecular biology methods. It was found that the presence and infection intensity of various parasites species are not independent of each other. Among cockles, an association was found between two trematode taxa, i.e. between the infection intensity by foot-encysting echinostomes and the metacercariae of Gymnophallus sp. It was also found that the presence of the parasitic copepod Pseudomyicola spinosus was associated with greater infection intensity by the echinostomes but not Gymnophallus sp. While it was postulated that the positive association between the echinostomes and Gymnophallus sp. was due to the latter�s preference to infect cockles that are stranded on the sediment surface as a result of heavy echinostome metacercariae burden in their foot, a field experiment found that Gymnophallus cercariae did not preferentially infect cockles that have been forced to remain above the sediment surface as opposed to those that were forced to remain buried. Meanwhile, the two species of echinostomes known to encyst in the cockle�s foot, Acanthoparyphium sp. and Curtuteria australis, were found to represent cryptic species complexes. The presence of such cryptic species means that it is possible that some potential interspecific interactions are overlooked. A study of the population structure of Gymnophallus sp. found that each cockle contains multiple genetically distinct individuals and that clonal individuals rarely co-occur in the same cockle. This adds to the growing body of evidence suggesting that in addition to acting as a means of reaching the definitive host, the second intermediate host also acts to promote genetic diversity by accumulating cercariae shed by multiple first intermediate hosts in the environment. An experimental infection study conducted with Curtuteria australis cercariae deriving from singly-infected first intermediate hosts revealed that different clonal lineages varied with respect to their contribution to host manipulation. It was found that while certain lineages have a preference for host manipulation, others tend to adopt a "hitch-hiker"-type life history strategy. However, this genetic predisposition was also found to be a phenotypically plastic trait, as the presence of a higher number of manipulators seems to encourage newly-arrived cercariae to become manipulators, regardless of clonal lineage. This thesis provides evidence that patterns of interactions can affect various aspects of parasite life history. Apart from host condition, parasites can also be affected by other parasites. Studying the dynamics of mixed infections can provide informative insights for evolutionary and ecological research.

Bibliography: leaves 113-128 / xii, 128, [63] leaves, [6] leaves of plates : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Microbiology and Immunology, 1984

Bibliography: leaves 218-251.