Είναι γνωστό ότι ο πιο σημαντικός παράγοντας για την προστασία και τη σωστή λειτουργία ενός εξοπλισμού υψηλής τάσης είναι η μόνωση του. Η πιο διαδεδομένη μορφή μόνωσης είναι η χρήση υγρών διηλεκτρικών, και συγκεκριμένα ελαίων. Αρχικά, χρησιμοποιούσαν μόνο ορυκτά έλαια ως μονωτικό υλικό διοτί ήταν εύκολη η διάθεση τους. Τα τελευταία όμως χρόνια, ήθισται να προτιμούνται φυτικά έλαια έναντι των ορυκτών, χάρη των ιδιοτήτων και πλεονεκτημάτων που παρουσιάζουν. Τα βασικά πλεονεκτήματα είναι η φιλικότητα τόσο προς το περιβάλλον όσο και στον άνθρωπο, η μη τοξικότητα τους, το υψηλό σημείο ανάφλεξης, η βιοδιάσπαση τους, καθώς και η βελτίωση της απόδοσης του μετασχηματιστή. Σκοπός της παρούσας διπλωματικής εργασίας είναι η διερεύνηση της συμπεριφοράς του φυτικού ελαίου FR3 κατά τη διάρκεια καταπόνησης του με εναλλασσόμενη τάση (AC) και η σύγκριση του με το ορυκτό έλαιο. Ακόμη, στόχος μας είναι να παρατηρήσουμε τη συμπεριφορά του FR3 σε συνδυασμό με μονωτικό χαρτί, αφού γνωρίζουμε ότι τα τυλίγματα μετασχηματιστών υψηλής τάσης μονώνονται και από τα 2 υλικά. Για το λόγο αυτό χρησιμοποιήθηκε η συσκευη BAUR oil tester DTA 100C που αποτελεί κομμάτι του ηλεκτρολογικού εξοπλισμού του εργαστηρίου των υψηλών τάσεων. Η παραγωγή γραμμικά αυξανόμενης εναλλασσόμενης (AC) τάσης με σταθερό ρυθμό 2kVrms/s και οι μετρήσεις της τάσης διάσπασης έγιναν από τη συσκευή αυτή. Τα διάκενα στα οποία καταπονήθηκε το φυτικό έλαιο είναι: rogowski 2mm, ακίδα-πλάκα 2mm, rogowski-πλάκα 2.5mm.Στη συνέχεια, πραγματοποιήσαμε μετρήσεις με διάταξη rogowski 2.5mm σε φυτικό λάδι χωρίς ανάδευση και σε καταπονημένο σε κρουστική τάση. Οι επόμενες μετρήσεις αφορούσαν το σύνθετο διάκενο χαρτί –λάδι και χρησιμοποιήσαμε 2 τύπους χαρτιού: DPP PAPER και KRAFT PAPER σε διάταξη rogowski 2.5mm. Τέλος, χρησιμοποιήσαμε ορυκτό έλαιο σε διάκενα rogowski 2.5mm και Ακίδα πλάκα 2,5 mm Επίσης, για να είμαστε άρτιοι στη σύγκριση των δύο τύπων ελαίων, πραγματοποιήσαμε αναλυτική στατιστική έρευνα ώστε να διαπιστώσουμε αν η κατανομή των πειραματικών αποτελεσμάτων προσεγγίζει την κανονική. Συγκεκριμένα, χρησιμοποιήσαμε το διάγραμμα διασποράς για την εκτίμηση της κατανομής. Έπειτα, με τη χρήση της συνάρτησης πυκνότητας πιθανότητας παρατηρήσαμε εποπτικά αν τα πειραματικά αποτελέσματα ακολουθούν κανονική κατανομή και υπολογίσαμε την ασσυμμετρία και την κυρτωση αυτών. Tέλος, χρησιμοποιήθηκαν τρία κριτήρια κανονικότητας, το Shapiro-Wilk, το Kolmogrov-Smirnov, και το Anderson-Darling καθώς και το διάγραμμα Normal Plot. Κρίνοντας από τα αποτελέσματα, προκύπτει ότι η τάσης διάπασης για το φυτικό έλαιο είναι μεγαλύτερη σε σχέση με το ορυκτό. Επίσης, είναι φανερό ότι η ύπαρξη σύνθετου διακένου (έλαιο – χαρτί) μειώνει την τάση διάσπασης του FR3. Τέλος, φαίνεται ότι σχεδόν για όλες τις διατάξεις, τα πειραματικά αποτελέσματα ακολουθούν την κανονική κατανομή. / It is known that the most important factor for the protection and proper operation of high voltage equipment is the insulation achievements. The most widespread form of insulation is the use of liquid dielectrics, manely dielectric oils. Initially, only mineral oil were used as an insulating material because it was easily available. However, the last decades vegetable oils are preferred to fossil thanks to the properties and advantages they present. The main advantages are the friendliness of both the environment and humans, non- toxicity, high flash point, biodegradation, as well as improving the efficiency of the transformer. The purpose of this thesis is to investigate the behavior of natural oil FR3 during stress with alternating current (AC) and compared with the mineral oil. Furthermore, our aim is to observe the behavior of this oil in combination with insulating paper, since we know that the high voltage transformer windings are insulated from the two materials. For this reason, using the device BAUR oil tester DTA 100C which is part of the electrical equipment of High Voltage laboratory. The production increased linearly alternating (ac) voltage steadily 2kVrms/s, and measurements of the breakdown voltage were made of this device.The gaps that we were measured are: rogowski 2mm, point-plane 2mm, rogowski-plane 2mm. Then, we measured the breakdown voltage for rogowski 2,5mm without stirring and for an overworked oil. Next measurments concered the combination of paper-oil, where we used two types of insulating paper:DPP and KRAFT PAPER (rogowski 2.5mm).finally we measured mineral oil in gaps: rogowski 2.5 mm and point-plane 2.5 mm Also, being correct to compare the two types of oils, we conducted a detailed survey about whether the experimental results follow the normal distribution. Specifically, we used the scatter plot to estimate the distribution of our experimental results. Then, using the probability density function we observed if the experimental results follow a normal distribution and calculated the coefficients of skewness and kurtosis. Finally, we used two hypothesis tests, the Shapiro – Wilk test,the Kolomogrov-Smirnov test and the Anderson-Darling, as well as the Normal Probability Plot. Αccording to the results, we observe that the breakdown voltage of natural ester oil is greater than mineral’s oil. Also, it is obvious that the existence of a complex gap (oil-paper) reduces the breakdown voltage. Finally, it seems that for almost all the provisions, the experimental results follow the normal distribution.

Σκοπός της παρούσης διπλωματικής εργασίας είναι η μελέτη στοχαστικών μοντέλων για τη μελέτη της τάσης διάσπασης διακένων με σύνθετη μόνωση. Αρχικά, περιγράψαμε τη συμπεριφορά των υγρών σε ισχυρά ηλεκτρικά πεδία. Μιλήσαμε για την εξάρτηση της πίεσης από ένα ηλεκτρικό πεδίο και την ηλεκτρική αγωγιμότητα σε ένα ισχυρό ηλεκτρικό πεδίο. Ακόμα, αναφέραμε κάποιες πληροφορίες για την κατανομή της πίεσης σε ένα διηλεκτρικό υγρό. Μετά περιγράψαμε τη συμπεριφορά της αέριας φάσης των υγρών σε ισχυρά ηλεκτρικά πεδία. Μιλήσαμε για το ρόλο των φυσαλίδων στην κατάσταση του διηλεκτρικού υγρού και για τα φαινόμενα που συμβαίνουν σ' αυτές. Αργότερα, δώσαμε κάποιες πληροφορίες για τη διάδοση της εκφόρτισης σε μικρά διάκενα και αναφέραμε την εξάρτησή της από την πίεση. Επιπλέον, αναλύσαμε τη διάδοση της εκφόρτισης σε διάκενα με ανομοιογενές πεδίο. Περιγράψαμε την κατασκευή μιας εκφόρτισης και την επίδραση του στερεού διηλεκτρικού στην τάση διάσπασης. Ακόμη, δώσαμε πληροφορίες για την περιοχή ιονισμού και το κανάλι leader. Στη συνέχεια, αναλύσαμε την εξάρτηση της διηλεκτρικής αντοχής από ορισμένους παράγοντες. Οι παράγοντες στους οποίους αναφερθήκαμε είναι: η διάρκεια, η μορφή και η συχνότητα των παλμών τάσης, η πολικότητα της τάσης, η πίεση, η θερμοκρασία, η χημική φύση και σύνθεση των υγρών και η γεωμετρία του διακένου. Επίσης, αναλύσαμε τους φυσικούς μηχανισμούς έναρξης εκφόρτισης. Περιγράψαμε το μηχανισμό έναρξης εκφόρτισης με τη βοήθεια φυσαλίδων και κάποιους άλλους μηχανισμούς όπως τους μηχανισμούς έναρξης εκφόρτισης με ιονισμούς. Αργότερα, αναφέραμε τους μηχανισμούς διάδοσης εκφόρτισης. Έγινε περιγραφή της διαδικασίας διάδοσης του γρήγορου καναλιού και του αργού καναλιού. Ακόμα, μιλήσαμε για τη μετατροπή του αρχικού καναλιού σε leader. Στη συνέχεια, κάναμε μια εκτενή εισαγωγή στα στοχαστικά μοντέλα και αναφερθήκαμε στη χρησιμότητα τους, στις διάφορες κατηγορίες τους και στην εφαρμογή τους στον τομέα των ηλεκτρικών διασπάσεων. Για το λόγο αυτό, περιγράψαμε τις βασικές αρχές για το μοντέλο FFC και το μοντέλο του Biller. Μετά από όλα αυτά, αναλύσαμε τη διαδικασία που ακολουθήσαμε κατά τις εξομοιώσεις. Κάναμε λεπτομερή περιγραφή του προγράμματος που χρησιμοποιήσαμε για τις εξομοιώσεις. Οι εξομοιώσεις έγιναν για 2 διάκενα 2,5mm ομοιογενούς και ανομοιογενούς πεδίου και 2 διάκενα 1,5mm ομοιογενούς και ανομοιογενούς πεδίου. Κατά τις εξομοιώσεις το πάχος του φράγματος θεωρήθηκε σταθερό, ενώ μεταβάλαμε τη θέση του και το συντελεστή γήρανσης του λαδιού. Για τα διάκενα ομοιογενούς πεδίου, τοποθετήσαμε το φράγμα σε 3 διαφορετικές θέσεις και για τα διάκενα ανομοιογενούς διακένου τοποθετήσαμε το φράγμα σε 5 διαφορετικές θέσεις. Οι εξομοιώσεις πραγματοποιήθηκαν σε υπολογιστή του Εργαστηρίου Υψηλών Τάσεων του τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας Υπολογιστών του Πανεπιστημίου Πατρών. Επιπλέον, παραθέσαμε σε πίνακες τα αποτελέσματα που προέκυψαν από τις εξομοιώσεις. Στη συνέχεια κατασκευάσαμε διαγράμματα, για να καταλήξουμε σε κάποια συμπεράσματα. Για τα διάκενα ομοιογενούς πεδίου, σχεδιάσαμε την τάση V συναρτήσει του gi για τις διάφορες θέσεις του φράγματος. Για τα διάκενα ανομοιογενούς πεδίου, σχεδιάσαμε την τάση V συναρτήσει της σχετικής απόστασης (d1/d) του φράγματος από το πάνω ηλεκτρόδιο για διάφορες τιμές του gi. Έγινε σύγκριση μεταξύ των καμπυλών που προέκυψαν για τα διάκενα ανομοιογενούς πεδίου, με καμπύλη που προέκυψε από πειραματικά δεδομένα και βρίσκεται στο “Barrier Effect on the Dielectric Strength of Oil Gaps under DC Voltage” των A. Zouaghi και Α. Beroual. Στη συνέχεια, σχεδιάσαμε την τάση V συναρτήσει της σχετικής απόστασης (d1/d) του φράγματος από το πάνω ηλεκτρόδιο για διάφορες τιμές του gi, για τα διάκενα ομοιογενούς και ανομοιογενούς πεδίου μαζί. Τέλος, μετά τις συγκρίσεις διαπιστώσαμε ότι τα αποτελέσματά μας συμφωνούν μόνο σε κάποιες περιπτώσεις με τα πειραματικά δεδομένα. Αυτό συμβαίνει λόγω της απλότητας των μοντέλων και ίσως μηχανισμών που δεν έχουν συνυπολογιστεί. Οπότε, συμπεράναμε ότι το μοντέλο που χρησιμοποιήσαμε, απαιτεί βελτίωση. / The purpose of this diploma thesis is the study of stochastic models about the breakdown voltage of complex insulation gaps. Initially, we described the behavior of liquids in strong electric fields. We mentioned the dependence of the pressure on an electric field and electric conductivity in strong electric fields. We reported some information about the pressure distribution in a dielectric liquid. Afterwards, we described the behavior of gaseous phase in liquids in strong electric fields. We reported the influence of the bubbles in the state in the dielectric liquids and the phenomena at them. Later, we gave some information about the discharge propagation in short gaps and we refered its dependence on the pressure. Moreover, we analyzed the discharge propagation in gaps with a non uniform field. We described the structure of a discharge and the influence of the solid dielectric on the breakdown voltage. Also, we gave information about the ionization region and the leader channel. Afterwards, we analyzed the dependence of the dielectric strength on certain factors. The factors in which we were reported are: the duration, the shape and the frequency of voltage pulses, the voltage polarity, the pressure, the temperature, the chemical nature of liquids and the gap geometry. Also, we analyzed the physical discharge initiation mechanisms. We described the bubble discharge initiation mechanism and some other mechanisms like the ionization discharge initiation mechanism.Later, we mentioned the mechanisms of discharge propagation. We described the propagation process of the fast and the slow channel. Also, we were reported in the conversion of the initial channel into leader channel. Afterwards, we did an extensive import in the stochastic models and were reported in their usefulness, in their various categories and in their application in the sector of electric breakdown. For this reason, we described the basic elements of the FFC and the Biller model. After all these we analyzed the process that we followed at the simulations. We did a detailed description of the program used for the simulations. We did simulations for 2 gaps 2,5mm uniform and non uniform field and 2 gaps 1,5mm uniform and non uniform gaps. During the simulations, the barrier thickness was considered stable, while we were changing the position and the factor of ageing of the oil. For the uniform field gaps, we placed the barrier in 3 different positions and for the non uniform gaps, we placed the barrier in 5 different positions. The simulations were realised at computer at High Voltage Laboratory of the department of Electrical and Computer Engineering of the University of Patras, Greece. Moreover, we mentioned in tables the results that resulted from the simulations. Then, we drew diagrams to make some conclusions. For the uniform field gaps, we drew the voltage V as a function of the factor of ageing of the oil gi for the various positions of the barrier. For the non uniform field gaps, we drew the voltage V as a function of the relative distance (d1/d) of the barrier from the upper electrode for the various numbers of gi. We did comparison between the curves that resulted for the non uniform gaps and the curve tht resulted for experimental data and is found in “ Barrier Effect on the Dielectric Strength of Oil Gaps under DC Voltage” of A. Zouaghi and Α. Beroual. Afterwards, we drew the voltage V as a function of the relative distance (d1/d) of the barrier from the upper electrode for the various numbers of gi, for both uniform and non uniform gaps. Finally, after the comparisons we realised that the results agree only in some cases with the experimental data. This may happened because of the simplicity of the models and perhaps mechanisms that have not been included. So, we concluded that the model that we used, need improvement.

abstract: The U.S. Navy is interested in evaluating the dielectric performance of SF6 at 30 kHz in order to develop optimal bushing designs and to ensure reliable operation for the Very Low Frequency/ Low Frequency (VLF/LF) transmitting stations. The breakdown experiments of compressed SF6 at 30 kHz in the pressure range of 1-5 atm were conducted in both the uniform field (plane-plane gap) and the non-uniform field (rod-plane gap). To understand the impact of pressure on the breakdown voltage of SF6 at VLF/LF, empirical models of the dielectric strength of SF6 were derived based on the experimental data and regression analysis. The pressure correction factors that present the correlation between the breakdown voltage of SF6 at VLF/LF and that of air at 50/60 Hz were calculated. These empirical models provide an effective way to use the extensively documented breakdown voltage data of air at 60 Hz to evaluate the dielectric performance of SF6 for the design of VLF/LF high voltage equipment. In addition, several breakdown experiments and similar regression analysis of air at 30 kHz were conducted as well. A ratio of the breakdown voltage of SF6 to that of air at VLF/LF was calculated, from which a significant difference between the uniform gap and the non-uniform gap was observed. All the models and values provide useful information to evaluate and predict the performance of the bushings in practice. / Dissertation/Thesis / M.S. Electrical Engineering 2010

Thermal spray coatings offer new alternatives in the production of electronic power modules that use alumina ceramic as an isolation layer. Current processes use direct bond copper (DBC) soldered to a nickel plated copper heat spreader. A coefficient of thermal expansion (CTE) mismatch exists between copper and alumina and leads to reliability issues that arise due to product failure during thermal cycling and lifetime operation. The substitution of an AlSiC metal matrix composite (MMC) heat spreader baseplate addresses the problem of CTE mismatch and will reduce the number of product failures related to cracking and delamination caused by this pronounced mismatch in the thermal expansion coefficient.. The substitution of an AlSiC (MMC) heat spreader baseplate also allows the production process to be achieved with a fewer number of metallization layers. Thermal spray can apply alumina ceramic coatings directly to the AlSiC (MMC) baseplates. A reduction in process steps will lead to a reduction in manufacturing costs, the main driving objective in Microelectronics Industries. Thermal spray coatings have a major problem since they have a porous microstructure which can trap undesired moisture. The moisture basically causes the coatings to have a lower dielectric breakdown voltage and a higher leakage current at normal operating voltages. This problem can be eliminated by manufacturing the electronic power modules in a controlled environment and packaging the devices in a hermetically sealed package. This thesis analyzes the data obtained from direct-voltage dielectric breakdown and direct-voltage leakage current tests conducted on coupons manufactured using the thermal plasma spray coating process and the thermal high-velocity oxyfuel (HVOF) coating process. ASTM specifications defining appropriate testing procedures are used in testing the dielectric strength of these coupons. Issues relating to the dielectric strength and dielectric leakage current are evaluated and validated at the Microelectronics Laboratory at Virginia Polytechnic Institute & State University. The objective to conduct this research study using plasma and HVOF alumina coatings as dielectric isolation layers is to support the Microelectronics Industries in developing a product with increased reliability at a lower manufacturing cost. / Master of Science

The superior characteristics of silicon carbide, compared with silicon, have suggested considering this material for the next generation of power semiconductor devices. Among the different power switches, the bipolar junction transistor (BJT) can provide a very low forward voltage drop, a high current capability and a fast switching speed. However, in order to compete on the market, it is crucial to a have high current gain and a breakdown voltage close to ideal. Moreover, the absence of conductivity modulation and long-term stability has to be solved. In this thesis, these topics are investigated comparing simulations and measurements. Initially, an efficient etched JTE has been simulated and fabricated. In agreement with the simulations, the fabricated diodes exhibit the highest BV of around 4.3 kV when a two-zone JTE is implemented. Furthermore, the simulations and measurements demonstrate a good agreement between the electric field distribution inside the device and the optical luminescence measured at breakdown. Additionally, an accurate model to simulate the forward characteristics of 4H-SiC BJTs is presented. In order to validate the model, the simulated current gains are compared with measurements at different temperatures and different base-emitter geometries. Moreover, the simulations and measurements of the on-resistance are compared at different base currents and different temperatures. This comparison, coupled with a detailed analysis of the carrier concentration inside the BJT, indicates that internal forward biasing of the base-collector junction limits the BJT to operate at high current density and low forward voltage drop simultaneously. In agreement with the measurements, a design with a highly-doped extrinsic base is proposed to alleviate this problem. In addition to the static characteristics, the comparison of measured and simulated switching waveforms demonstrates that the SiC BJT can provide fast switching speed when it acts as a unipolar device. This is crucial to have low power losses during transient. Finally, the long-term stability is investigated. It is observed that the electrical stress of the base-emitter diode produces current gain degradation; however, the degradation mechanisms are still unclear. In fact, the analysis of the measured Gummel plot suggests that the reduction of the carrier lifetime in the base-emitter region might be only one of the causes of this degradation. In addition, the current gain degradation due to ionizing radiation is investigated comparing the simulations and measurements. The simulations suggest that the creation of positive charge in the passivation layer can increase the base current; this increase is also observed in the electrical measurements. / QC 20120522

It is common practice to perform accelerated aging with 60 Hz ac to determine the lifetime characteristics of insulation used in the machine. Comparable breakdown measurements are performed at different voltage levels and temperatures for the polyimide insulated machine winding under normal operating conditions of 60 Hz ac. The result shows that the time to failure can be represented by the inverse power law and the Arrhenius equation with respect to test voltage and temperature respectively. However, the use of power electronic devices causes harmonics, and spikes that electrically degrade the machine winding insulation. Applied high frequency pulse voltages can be used to study the impact of electrical degradation of the machine winding insulation that exists in electrical machines. Evaluation of change in dielectric strength, partial discharge and breakdown voltage is monitored versus aging caused by high frequency pulse voltage at 90% of operating temperature.

No description available.

97% des articles publiés sur les études climatiques racontent que le réchauffement climatique est entièrement causé par les activités humaines. Les gaz émis lors de la production d'énergie électrique ainsi que d'autres gaz rejetés par les voitures ont un réel impact sur l'atmosphère. Une solution consiste à mettre au point des composants présentant des pertes de conduction plus faibles et des caractéristiques de claquage plus élevées qui pourraient être utilisés dans des centrales nucléaires, des cellules de commutation à haute puissance, des voitures hybrides (électriques), etc.De nos jours, les composants à base de silicium contrôlent environ 95% des dispositifs électroniques. Le carbure de silicium SiC et le nitrure de gallium GaN sont actuellement à l’étape de R&D, et commencent à être intégrés dans certains circuits électroniques. D'autres matériaux tels que Ga2O3, AlN ou le diamant sont encore à l’étape de recherche. Les derniers sont connus sous le nom de matériaux à bande ultra large et semblent être la solution requise pour les faibles pertes de puissance. Le diamant est reconnu comme le matériau ultime pour la prochaine génération de composants de puissance en raison de ses propriétés physiques exceptionnelles telles qu'un champ de claquage élevé (>10 MV/cm) permettant d'utiliser le dispositif pour une commande de puissance élevée, une mobilité de porteurs élevée (2 000 cm^2/V.s pour les trous), une vitesse de saturation élevée, une conductivité thermique élevée (22 W/cm.K) pour une parfaite dissipation de chaleur et une faible constante diélectrique. Théoriquement, le diamant est le semi-conducteur offrant le meilleur compromis entre résistance à l'état passant et tension de claquage. En particulier, en raison de l'ionisation incomplète des dopants, il est encore plus efficace à haute température. Diverses diodes Schottky en diamant (SBD) avec de bonnes performances à l’état passant et bloqué (7,7 MV/cm) ont été rapportées. En plus des SBDs, des transistors à effet de champ (FET) ont également été étudiés à travers des oxyde-métal semi-conducteur FETs (MOSFETs) utilisant une surface hydrogénée avec des densités de courant élevées à l'état passant ou des surface oxygéné avec de bonnes caractéristiques de blocage. Pour les composants de haute-tension, il est nécessaire de changer l’architecture de l’électrode afin d’éviter un claquage prématuré due à l’encombrement du champ électrique aux bords. Dans ce but, les techniques de terminaison de bord sont utilisées pour atteindre les caractéristiques idéales. La tâche évidente avant toute fabrication de composant est la partie simulation qui prédit l’optimisation de l’architecture et les caractéristiques attendues. Une bonne prédiction nécessite la connaissance des paramètres du matériau. Les paramètres importants pour le claquage sont les coefficients d'ionisation par impact. Plusieurs coefficients ont été publiés pour le diamant. Toutefois, ils ont été extraits en « fittant » des structures non optimisées, d'où un manque de précision.Dans cette étude, deux structures de terminaisons de bord pour des diodes Schottky, appelées plaque de champ et anneaux à champ flottant, ont été étudiées. Leur efficacité de distribution du champ de surface par analyse de courant induit par faisceau d'électrons (EBIC) a été observée. De plus, des FETs ont été fabriqués et caractérisés, un MESFET et un RB-MESFET. Les FETs présentent un claquage élevé, jusqu’à 3 kV et une faible résistance. Le développement des transistors est indissociable de la diode Schottky, car ils sont tous deux nécessaires à la fabrication de cellules de commutation. Et enfin, les coefficients d'ionisation par impact pour les électrons ont été mesurés à l'aide d’EBIC pour un champ >0,5 MV/cm dans une région sans défaut. Les valeurs mesurées sont (sous l’equation de Chynoweth) an = 971 /cm et bn=2,39x10^6 V/cm. Ces valeurs sont proches des coefficients mesurés expérimentalement et rapportés dans la littérature. / 97% of the published climate studies articles agree with the fact that recent global warming is entirely caused by human activities. The gases emitted to produce electrical energy plus other gases rejected by cars impact considerably on the atmosphere by greenhouse effect (without referring other factors). A solution to this problem is the development of components with lower power conduction losses and higher breakdown characteristics that could be used in nuclear power plants, high power commutation cells, hybrid (electric) cars and so on.The choice of the material to reach low power conduction losses and higher breakdown is of great importance. Nowadays, silicon-based devices control about 95% of all electronic components. Silicon carbide SiC and gallium nitride GaN are at present under research and development and start to be integrated into some electronic circuits. Other materials like Ga2O3, AlN or diamond are under research for power electronic application. The last ones are known as ultra wide bandgap materials and they seem to be the required solution to low power losses. Diamond is recognized as the ultimate material for the next next-generation of power devices owing to its exceptional physical properties such as high breakdown field (>10 MV/cm) to use the device for high power control, high carrier mobility (2000 cm^2/V.s for holes) for fast switching and high frequency devices, high saturation velocity, high thermal conductivity (22 W/cm.K) for a perfect heat dissipation and low dielectric constant. Theoretically, diamond is the best semiconducting material showing the best trade-off between on-resistance and breakdown voltage. Especially, due to the incomplete ionization of the dopant, it is even more efficient at high temperature. Various diamond Schottky barrier diodes (SBDs) with good forward and reverse performances (7.7 MV/cm) were reported. In addition to SBDs, switches diamond field effect transistors (FETs) were also investigated through metal-oxide-semiconductor FETs (MOSFETs) using either an H-terminated diamond surface with high current densities in on-state or an O-terminated one with high blocking characteristics. For the high blocking voltage devices, one needs to properly terminate the edge of the electrode at the surface in order to avoid premature breakdown of the devices due to electric field crowding at the borders. In that aim, edge termination (ET) techniques are used to push the limit of the devices and reach ideal features. The obvious task before any device fabrication if the simulation part that predicts the device optimization and expected characteristics. A good device prediction requires knowledge of the material parameters. Important parameters for device breakdown in the off-state are the impact ionization coefficients. At present, several ionization coefficients were reported for diamond, however, they were extracted by fitting non-optimized structures and hence there is a lack of accuracy.In this study, two edge terminations structures for Schottky barrier diodes called field plate (FP) oxide and floating field rings were investigated. Their effectiveness in surface field distribution via electron beam induced current (EBIC) analysis was observed. In addition, normally-on FETs were fabricated and characterized, a MESFET and a reverse blocking (RB)-MESFET. The FETs exhibited a high BV, up to 3 kV and a low on-resistance. The development of transistors is inseparable from the Schottky diode since both are required to fabricate commutation cells. And finally, impact ionization coefficients for electrons were measured using EBIC for a field >0.5 MV/cm in a defect-free region. The measured values are (in a Chynoweth form) an = 971 /cm and bn = 2.39x10^6 V/cm. These values are close to the experimentally measured coefficients reported in the literature.

Silicon carbide bipolar junction transistors (BJTs) are attractive power switching devices because of the unique material properties of SiC with high breakdown electric field, high thermal conductivity and high saturated drift velocity of electrons. The SiC BJT has potential for very low specific on-resistances and this together with high temperature operation makes it very suitable for applications with high power densities. For SiC BJTs the common emitter current gain (β), the specific on-resistance (RSP_ON), and the breakdown voltage are important to optimize for competition with silicon based power devices. In this thesis, power SiC BJTs with high current gain β ≈ 60 , low on-resistance RSP_ON ≈ 5 mΩcm2, and high breakdown voltage BVCEO ≈ 1200 V have been demonstrated. The 1200 V SiC BJT that has been demonstrated has about 80 % lower on-state power losses compared to a typical 1200 V Si IGBT chip. A continuous epitaxial growth of the base-emitter layers has been used to reduce interface defects and thus improve the current gain. A significant influence of surface recombination on the current gain was identified by comparing the experiments with device simulations. In order to reduce the surface recombination, different passivation layers were investigated in SiC BJTs, and thermal oxidation in N2O ambient was identified as an efficient passivation method to increase the current gain. To obtain a low contact resistance, especially to the p-type base contact, is one critical issue to fabricate SiC power BJTs with low on-resistance. Low temperature anneal (~ 800 oC) of a p-type Ni/Ti/Al contact on 4H-SiC has been demonstrated. The contact resistivity on the ion implanted base region of the BJT was 1.3 × 10-4 Ωcm2 after annealing. The Ni/Ti/Al p-type ohmic contact was adapted to 4H-SiC BJTs fabrication indicating that the base contact plays a role for achieving a low on-resistance of SiC BJTs. To achieve a high breakdown voltage, optimized junction termination is important in a power device. A guard ring assisted Junction Termination Extension (JTE) structure was used to improve the breakdown voltage of the SiC BJTs. The highest breakdown voltage of the fabricated SiC BJTs was obtained for devices with guard ring assisted JTE using the base contact implant step for a simultaneous formation of guard rings. As a new approach to fabricate SiC BJTs, epitaxial regrowth of an extrinsic base layer was demonstrated. SiC BJTs without any ion implantation were successfully demonstrated using epitaxial regrowth of a highly doped p-type region and an etched JTE using the epitaxial base. A maximum current gain of 42 was measured for a 1.8 mm × 1.8 mm BJT with a stable and reproducible open base breakdown voltage of 1800 V. / QC 20100819

This thesis describes the design, fabrication and characterization of AlGaN/GaN Heterostructure Field E ect Transistors (HFETs) grown by a Metal Organic Chemical Vapor Deposition (MOCVD) on sapphire substrates. The objective of this research is to develop AlGaN/GaN power devices with high breakdown voltage (greater than 1 kV) and low turn-on resistance. Various characteristics such as current drive (Idss), transconductance (gm) and threshold voltage (Vth) have also been measured and the results have been discussed. Two major challenges with the development of high breakdown voltage AlGaN/GaN HFETs in the past have been high material defect density and non-optimized fabrication technologies which gives rise to bu er leakage and surface leakage, respectively. In this thesis, mesa isolation, ohmic and gate metal contacts, and passivation techniques, have been discussed to improve the performance of these power transistors in terms of low contact resistance and low gate leakage. The relationship between breakdown voltage and Rds(ON)A with respect to the gate-drain length (Lgd) is also discussed. First, unit cell devices were designed (two-fingered cells with Wg = 100, 300, 400 m) and characterized, and then they were extended to form large area devices (upto Wg = 40 mm). The design goals were classied into three parts: - High Breakdown Voltage: This was achieved by designing devices with variations in Lgd, - Low turn-on resistance: This was achieved by optimizing the annealing temperatures as well as incorporating additional thick metal pads, as well as optimizing the passivation etch recipe, - Low Gate Leakage: The gate leakage was reduced signicantly by using a gate metal with a larger barrier height. All devices with Lgd larger than 10 m exhibited excellent breakdown voltage characteristics of over 800 V, and it progressed as the Lgd increased. The turn-on resistance was also reduced signicantly below 20 m-cm2, for devices with Lgd = 15, 25, and 20 m. The gate leakage was measured for all devices upto 200 V, and was in the range of 10-100 nA, which is one of the best values reported for multi-ngered devices with Lgd in the range of 2.4-5 mm. Some of the key challenges faced in fabrication were determining a better gate metal layer to reduce gate leakage, optimizing the passivation via etch recipe, and reducing surface leakage.