Osteoporosis is a bone disease associated with fracture risk. Accurate assessments of fracture risk, guidelines to initiate preventive intervention, and monitoring treatment response are of paramount importance in public health. Clinically, osteoporosis is defined by low bone mineral density, which explains 65-75% of the variance in bone stiffness. The remaining variability is due to the cumulative and synergistic effects of various factors, including trabecular bone micro-architecture. Osteoporostic imaging is critically important in identifying fracture risks for planning of therapeutic intervention and monitoring response to treatments. In this work, quantitative analysis of trabecular bone micro-architecture using volumetric imaging techniques and computational biomechanical simulation through finite element modeling (FEM) are applied on in vivo imaging for various human studies. The ability of imaging methods in characterizing trabecular bone micro-architecture was experimentally examined using MRI and multi-row detector CT. They were found suitable for cross-sectional and longitudinal studies in monitoring changes of trabecular micro-architectural quality in clinical research. A framework which consists of robust segmentation of in vivo images and quality mesh generator, was constructed for FEM analysis. The framework was experimentally demonstrated effcient and effective to predict bone strength under limited spatial resolution. The ability of distinguishing bone strengths of different groups were evaluated on various human studies. And the relation between FEM and image-based micro-architectural measures was explored. Quantitative analysis supports the hypothesis that trabecular bone have distinct structural properties in different anatomic sites and the osteoporosis related change of the micro-architecture also varies. It highlight the importance of standardizing the definition of bone scan locations and the segmentation of such well-defined regions. A shape modeling method was proposed to solve the problem and its application in human proximal femur using MRI were presented. The method was compared with manual segmentation and found highly accurate. Together with tools developed for quantitative analysis, this work facilitates future researches of trabecular bone micro-architecture in different anatomic sites.

Cette thèse porte sur l’étude des transistors de puissance à haute mobilité électronique en Nitrure de Gallium à enrichissement récemment apparus sur le marché. Ces travaux se focalisent sur l’analyse physique de la défaillance lors du vieillissement de ces composants sous des profils de mission réels de commutation. Un banc de stress a été conçu et construit, permettant d’imposer des conditions de fonctionnement réel sur le transistor sans utiliser des charges et donc en minimisant la consommation d’énergie. Une autre originalité de ce banc est qu’il contient des transistors témoin qui subissent une partie du stress imposé au transistor sous test, aidant ainsi à la compréhension des dégradations. Les composants vieillis sont caractérisés électriquement pour établir les modes de défaillance. Des méthodes de décapsulation de composants et de préparation d’échantillons mulit-échelle (mm au nm) ont été également développées afin d’accéder à la zone active des transistors. Par l’analyse de la microstructure des dispositifs, avec des techniques de photoémission spectrale, microscopie électronique à balayage et en transmission, les mécanismes de défaillance sont déterminés et la corrélation avec les modes dedéfaillance correspondants est réalisée. / This PhD Thesis is about the reliability of enhancement Gallium Nitride high electron mobility power transistors, recently on the market. This work is focused on physics of failure analysis on aged devices under real mission switching profiles. A novel stress bench able of stablish real operating conditions on a transistor without using a load, so minimizing energy consumption. Also, the circuit has two additional devices that undergo separate current and voltage stresses and are used for better understanding of degradations on main device under test. Electric characterization of aged devices allows determination of failure modes. Decapsulation and sample preparation procedures at different scales were developed to access the active zone of the devices and perform micro-structural analyses by the means of spectral photoemission, scanning and transmission electronic microscopies. Then, failure mechanisms are correlated with the corresponding modes.

La microscopie à émission de photons spectrale (SPEM) est une technique non destructive utilisée comme outil de localisation des défauts et comme indicateur des mécanismes de défaillance. Cette thèse présente un nouveau système de SPEM développé pour étudier la fiabilité des dispositifs de puissance à large bande interdite, notamment les MOSFET SiC et les MEMTs AlGaN/GaN. Un aperçu des différents aspects fondamentaux de l'émission de lumière dans les dispositifs à semi-conducteurs est présenté. L'analyse spectrale en électroluminescence des MOSFET SiC à haute puissance et des HEMTs AlGaN/GaN est rapportée et corrélée avec des analyses électriques et micro-structurales pour localiser les défaillances et identifier l'origine physique de la dérive des performances de ces composants. / Spectroscopic photon emission microscopy (SPEM) is a non-destructive technique used as a defect localizing tool and as an indicator of the failure mechanisms. This thesis presents a new system of SPEM developed to study the reliability of wide band Gap power devices notably SiC MOSFETs and AlGaN/GaN HEMTs. An overview of different fundamental aspects of the light emission defects on semiconductors devices is presented. The electroluminescence spectral analysis of high power stressed SiC MOSFETs and AlGaN/GaN HEMTs is reported and correlated with electrical and micro-structural analysis to localize the failures and identify the physical origin of the performance drift of these components.