Current collapse and device degradation in AlGaN/GaN heterostructure field effect transistors

A spectrum of phenomena related to the reliability of AlGaN/GaN HEMTs are investigated in this thesis using numerical simulations. The focus is on trap related phenomena that lead to decrease in the power output and failure of devices, i.e. the current collapse and the device degradation. The current collapse phenomenon has been largely suppressed using SiN passivation, but there are gaps in the understanding of the process leading to this effect. Device degradation, on the other side, is a pending problem of current devices and an obstacle to wide penetration of the market. Calibration of I-V measurements of two devices is performed with high accuracy to provide a trustworthy starting point for modelling the phenomena of interest. Traditionally, in simulations of nitride based HEMTs, only direct piezoelectric effect is taken into account and the resulting interface charge is thence independent of the electric field. In this work, the impact of the electric field via the converse piezoelectric effect is taken into account and its impact on the bound charge and the drain current is studied, as a refinement of the simulation methodology. It is widely believed that the current collapse is caused by a virtual gate, i.e. electrons leaked to the surface of the device. We have found a charge distribution that reproduced the I-V measurement that shows current collapse, hence validating the concept of the virtual gate. While it was previously shown that the virtual gate has a similar impact on the I-V curve as is observed during the current collapse, we believe that this is for the first time that a wide range of gate and drain voltages was calibrated. High gate/drain voltage leading to permanent degradation was also investigated. The hypothesis that stress induced defects and dislocations might be responsible for the degradation was tested but not fully confirmed. Finally, the leakage of electrons thought to be responsible for formation of the virtual gate and the current collapse due to the Poole-Frenkel emission, is simulated in order to explain the surface charge distribution responsible for the current collapse and deduced in Chapter 5

A new phylodynamic model of Mycobacterium bovis transmission in a multi-host system uncovers the role of the unobserved reservoir

Multi-host pathogens are particularly difficult to control, especially when at least one of the hosts acts as a hidden reservoir. Deep sequencing of densely sampled pathogens has the potential to transform this understanding, but requires analytical approaches that jointly consider epidemiological and genetic data to best address this problem. While there has been considerable success in analyses of single species systems, the hidden reservoir problem is relatively under-studied. A well-known exemplar of this problem is bovine Tuberculosis, a disease found in British and Irish cattle caused by Mycobacterium bovis, where the Eurasian badger has long been believed to act as a reservoir but remains of poorly quantified importance except in very specific locations. As a result, the effort that should be directed at controlling disease in badgers is unclear. Here, we analyse densely collected epidemiological and genetic data from a cattle population but do not explicitly consider any data from badgers. We use a simulation modelling approach to show that, in our system, a model that exploits available cattle demographic and herd-to-herd movement data, but only considers the ability of a hidden reservoir to generate pathogen diversity, can be used to choose between different epidemiological scenarios. In our analysis, a model where the reservoir does not generate any diversity but contributes to new infections at a local farm scale are significantly preferred over models which generate diversity and/or spread disease at broader spatial scales. While we cannot directly attribute the role of the reservoir to badgers based on this analysis alone, the result supports the hypothesis that under current cattle control regimes, infected cattle alone cannot sustain M. bovis circulation. Given the observed close phylogenetic relationship for the bacteria taken from cattle and badgers sampled near to each other, the most parsimonious hypothesis is that the reservoir is the infected badger population. More broadly, our approach demonstrates that carefully constructed bespoke models can exploit the combination of genetic and epidemiological data to overcome issues of extreme data bias, and uncover important general characteristics of transmission in multi-host pathogen systems

The impact of university rankings in the spanish press

La proliferación de rankings de universidades en las últimas décadas y su papel emergente en la evaluación de los sistemas universitarios justifican el estudio de estos productos en los medios de masas. El presente trabajo pretende estudiar la repercusión que han tenido los ranking de universidades en la prensa española durante 160 meses, periodo comprendido entre el 1 de enero de 1998 hasta el 30 de abril de 2012, para ello se ha utilizado MyNewsOnline®. Se analiza la producción temporal, territorial y por cabeceras. Igualmente se estudian las noticias que han generado los principales rankings de universidades. Los resultados demuestran el creciente interés de los medios por estas herramientas científicas, especialmente por parte de la prensa de ámbito nacional. En las cabeceras de cobertura nacional destacan los rankings ARWU y Times, mientras que en la prensa local lo hace el Scimago. Las cabeceras más productivas son El País, La Vanguardia y ABC.The proliferation of university rankings in the last few decades and their increasing role in the evaluation of university systems justify the study of their impact on the media. The present work aims to measure the impact of university rankings in the Spanish press over 160 months, between January 1, 1998 and April 30, 2012. Using the news database MyNewsOnline as the main source of information, we analysed the production of news according to the following variables: timing, geographical distribution, newspaper and rankings. Results show an increasing interest in these tools. The Shanghai Ranking and Times Higher Education University Ranking stand out in the national press, while in the local press the Scimago Institutions Ranking draws the most attention from media. The newspapers with the most news about rankings are “El País”, “La Vanguardia” and “ABC”