Silicon is the main material used in electronics. The evolution of power electronics and the need for more power efficient semiconductor devices led silicon to its limits. Silicon carbide is a promising material for electronic applications with a wide band-gap, high critical electric field, high thermal conductivity and saturation velocity. Except from its superiority to silicon, silicon carbide comes with a drawback of about two orders of magnitude more interface traps in the SiC/SiO2 interface compared with silicon. A result of this drawback is a flat-band voltage shift when applying a stress to the gate of MOS capacitors and power MOSFETs. In order to study the pure characteristics of the SiC/SiO2 interface, two stress methods, a current pulse stress and gate voltage upsweep, have been applied on 4H-SiC capacitors with nitrided thermal oxides at room temperature and at higher temperatures. The flat-band voltage recovery was examined. The flat-band voltage could be restored at room temperature with a gate voltage downsweep while a restoration is not needed at higher temperatures. The maximum voltage (initial voltage) and the voltage rate of the downsweep were investigated and higher initial voltages and lower voltage rates showed to lead to better VFB restoration. A 200 millisecond long current pulse stress was implemented and it had almost similar effects as the voltage upsweep which lasts 50 seconds. / Kisel är det viktigaste materialet som används i elektronik. Utvecklingen av kraftelektronik och behovet av mer energieffektiva halvledarkomponenter ledde kisel till sina gränser. Kiselkarbid är ett lovande material för elektroniska applikationer med ett brett bandgap, högt kritiskt elektriskt fält, hög värmeledningsförmåga och hög mättningshastighet. Förutom dess överlägsenhet gentemot kisel, kommer kiselkarbid med en nackdel med cirka två storleksordningar fler gränssnittsfällor i SiC / SiO2-gränssnittet jämfört med kisel. Ett resultat av denna nackdel är en förskjutning av flatbands-spänningen, VFB, när man applicerar en spänning på gaten till MOS-kondensatorer och kraft- MOSFETar. För att studera de rena egenskaperna hos SiC/SiO2-gränssnittet har två spänningsmetoder, en strömpulsstress och ett uppåtriktat gate-spänningssvep, applicerats på 4H-SiC- kondensatorer med nitriderade termiska oxider vid rumstemperatur och vid högre temperaturer. Återställning av VFB undersöktes. VFB kan återställas vid rumstemperatur med ett nedåtriktat gate-spänningssvep medan en återställning inte behövs vid högre temperaturer. Den maximala spänningen (initialspänningem) och svephastigheten för det nedåtriktade svepet undersöktes och högre initialspänningar och lägre svephastigheter visade sig leda till bättre VFB-återställning. En 200 millisekund lång strömpuls-stress implementerades och den hade nästan samma effekter som ett uppåtriktat spänningssvep

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The design of series-loaded resonant converters using the state of the art SiC power transistorsis investigated in the thesis. SiC devices are chosen as they offer lower switching losses comparedto conventional Si based devices A very detailed study about the working and differentmodes of operation of the resonant converter is carried out. The thesis further explains how thehigh speed switching capabilities of the SiC devices remain untapped owing to the presence ofstray inductances in the switch-snubber layout. A comparison of all the commercially availableSiC devices are carried out to find the most suitable switch for the resonant converter. Thethesis also carries out a very detailed step by step design of the circuit and the PCB layout forthe resonant converter. Two different layouts are proposed and then compared for their strayinductance and power losses. Finally, based on the experiments the thesis validates the suitabilityof using discrete SiC power transistors in place of power modules. / Utformningen av serie-belastade resonansomvandlare med hjälp av toppmoderna SiC högeffekttransistorernaundersöks i denna avhandlingen. SiC-enheter väljs eftersom de erbjuderlägre switch-förluster jämfört med konventionella Si-baserade enheter.En mycket detaljeradstudie om funktionen och de olika operationella tillstånden hos resonansomvandlare utförs.Avhandlingen förklarar vidare hur förmågan till högfrekvent switchning hos SiC-enheterförblir ofullständigt utnyttjad på grund av förekomsten av ströinduktanser i switch-snubberlayouten.En jämförelse av alla kommersiellt tillgängliga SiC-enheter genomförs för att hittaden mest lämpliga switchen för resonansomvandlaren. Avhandlingen utförs också en mycketdetaljerad steg-för-steg-utformning av resonansomvandlaren kretsschema och kretskortlayout.Två olika layouter föreslås och jämförs därefter utifrån deras ströinduktanser ocheffektförluster. Slutligen, baserat på experimentella resultat bekräftar avhandlingen.Lämpligheten att använda diskreta SiC-effekttransistorer istället för effektmoduler medintegrerade drivarsteg för styrelektroderna.

Quantum information technologies include secure quantum communications and ultra precise quantum sensing that are significantly more efficient than their classical counterparts. To enable such technologies, we need a scalable quantum platform in which qubits are con trollable. Color centers provide controllable optically-active spin qubits within the coherence time limit. Moreover, the nearby nuclear spins have long coherence times suitable for quantum memories. In this thesis, I present a theoretical understanding of and control protocols for various color centers. Using group theory, I explore the wave functions and laser pumping-induced dynamics of VSi color centers in silicon carbide. I also provide dynamical decoupling-based high-fidelity control of nuclear spins around the color center. I also present a control technique that combines holonomic control and dynamically corrected control to tolerate simultaneous errors from various sources. The work described here includes a theoretical understanding and control techniques of color center spin qubits and nuclear spin quantum memories, as well as a new platform-independent control formalism towards robust qubit control. / Doctor of Philosophy / Quantum information technologies promise to offer efficient computations of certain algorithms and secure communications beyond the reach of their classical counterparts. To achieve such technologies, we must find a suitable quantum platform to manipulate the quantum information units (qubits). Color centers host spin qubits that can enable such technologies. However, it is challenging due to our incomplete understanding of their physical properties and, more importantly, the controllability and scalability of such spin qubits. In this thesis, I present a theoretical understanding of and control protocols for various color centers. By using group theory that describes the symmetry of color centers, I give a phenomenological model of spin qubit dynamics under optical control of VSi color centers in silicon carbide. I also provide an improved technique for controlling nuclear spin qubits with higher precision. Moreover, I propose a new qubit control technique that combines two methods - holonomic control and dynamical corrected control - to provide further robust qubit control in the presence of multiple noise sources. The works in this thesis provide knowledge of color center spin qubits and concrete control methods towards quantum information technologies with color center spin qubits.

High-frequency and large amplitude current is a driving requirement for applications such as induction heating, wireless power transfer, power amplifier for magnetic resonant imaging, electronic ballasts, and ozone generators. Voltage-fed resonant inverters are normally employed, however, current-fed (CF) resonant inverters are a competitive alternative when the quality factor of the load is significantly high. The input current of a CF resonant inverter is considerably smaller than the output current, which benefits efficiency. A simple, parallel resonant tank is sufficient to create a high-power sinusoidal signal at the output. Additionally, input current is limited at the no-load condition, providing safe operation of the system. Drawbacks of the CF resonant inverter are associated with the implementation of the equivalent current source. A large input inductor is required to create an equivalent dc current source, to reduce power density and the bandwidth of the system. For safety, a switching stage is implemented using bidirectional voltage-blocking switches, which consist of a series connection of a diode and a transistor. The series diode experiences significant conduction loss because of large on-state voltage. The control of the output current amplitude for constant-frequency inverters requires a pre-regulation stage, typically implemented as a cascaded hard-switched dc/dc buck converter. The pre-regulation also reduces the efficiency. In this dissertation, a variety of CF resonant inverters with two input inductors and two grounded switches are investigated for an inductive-load driver with loaded quality factor larger than ten, constant and high-frequency (~500 kHz) operation, high reactive output power (~14 kVA), high bandwidth (~100 kHz), and high efficiency (over 95 %). The implementation of such system required to question the fundamental operation of the CF resonant inverter. The input inductance is reduced by around an order of magnitude, ensuring sufficient bandwidth, and allowing rich harmonic content in the input current. Of particular importance are fundamental and second harmonic components since they influence synchronization of the zero-crossing of the output voltage and the turn-on of the switches. The synchronization occurs at a particular frequency, termed synchronous frequency, and it allows for zero switching loss in the switches, which greatly boosts efficiency. The synchronous conditions were not know prior this work, and the dependence among circuit parameters, input current harmonics, and synchronous frequency are derived for the first time. The series diode of the bidirectional switch can reduce the efficiency of the system to below 90 %, and has to be removed from the system. The detrimental current-spikes can occur if the inverter is not operated in synchronous condition, such as in transients, or during parametric variations of the load coil. The resistance of the load coil has a wide variance, five times or more, while the inductance changes as well by a few percent. To accommodate for non-synchronous conditions, a low-loss current snubber is proposed as a safety measure to replace lossy diodes. The center-piece of the dissertation is the proposal of a two-phase zero-voltage switching buck pre-regulator, as it enables fixed frequency and synchronous operation of the inverter under wide parametric variations of the load. The synchronous operation is controlled by phase-shifting the switching functions of the pre-regulator and inverter. The pre-regulator reduces the dc current in the input inductors, which is a main contributor to current stress and conduction losses in the inverter switches. Total loss of the inverter switches is minimized since no switching loss is present and minimal conduction losses are allowed. The dc current in the input inductors, once seen as a means to transfer power to load, is now contradictory perceived as parasitic, and the power is transferred to the load using a fundamental frequency harmonic! The input current to the resonant tank, previously designed to be a square-wave, now resembles a sine-wave with very rich harmonic content. Additionally, the efficiency of the pre-regulator at heavy-load condition is improved by ensuring ZVS for with an additional inductive tank. The dissertation includes five chapters. The first chapter is an introduction to current-fed resonant inverters, applications, and state-of-the-art means to ensure constant frequency operation under load's parametric variations. The second chapter is dedicated to the optimization of the CF resonant inverter topology with a dc input voltage, two input inductors, and two MOSFETs. The topology is termed as a boost amplifier. If the amplifier operates away from the synchronous frequency, detrimental current spikes will flow though the switches since the series diodes are eliminated. Current spikes reduce the efficiency up to few percent and can create false functioning of the system. Operation at the synchronous frequency is achieved with large, bulky, input inductors, typically around 1-2 mH or higher, when the synchronous frequency follows the resonant frequency of the tank at 500 kHz. The input inductance cannot be reduced arbitrarily to meet the system bandwidth requirement, since the synchronous frequency is increased based on the inductance value. The relationship between the two (input inductance and the synchronous frequency) was unknown prior this work. The synchronous frequency is determined to be a complicated mathematical function of harmonic currents through the input inductors, and it is found using the harmonic decomposition method. As a safety feature, a current snubber is implemented in series with the resonant tank. Snubber utilizes a series inductance of cable connection between the tank and the switching stage, and it is more efficient than the previously employed series diodes. Topology optimization and detailed design procedure are provided with respect to efficiency and system dynamics. The mathematics is verified by a prototype rated at 14 kVA and 1.25 kW. The input inductance is reduced by around an order of magnitude, with the synchronous frequency increase of 2 %. The efficiency of the power amplifier reached 98.5 % and might be improved further with additional optimization. Silicon carbide MOSFETs are employed for their capability to operate efficiently at high frequency, and high temperature. The third chapter is dedicated to the development of the boost amplifier's large signal model using the Generalized State-space Averaging (GSSA) method. The model accurately predicts amplifier's transient and steady-state operation for any type of input voltage source (dc, dc with sinusoidal ripple, pulse-width modulated), and for either synchronous or non-synchronous operating frequency. It overcomes the limitation of the low-frequency model, which works well only for dc voltage-source input and at synchronous frequency. As the measure of accuracy, the zero-crossing of the resonant voltage is predicted with an error less than 2° over a period of synchronous operation, and for a range of interest for input inductance (25 μH – 1000 μH) and loaded-quality factor (10 – 50). The model is validated both in simulation and hardware for start-up transient and steady-state operation. It is then used in the synthesis of modulated output waveforms, including Hann-function and trapezoidal-function envelopes of the output voltage/current. In the fourth chapter, the GSSA model is employed in development of the PWM compensation method that ensures synchronous operation at constant frequency for the wide variation of the load. The boost amplifier is extended with a cascaded pre-regulator whose main purpose is to control the output resonant voltage. The pre-regulator is implemented as two switching half-bridges with same duty-cycle and phase-shift of 180°. The behavior of the cascaded structure is the same as of the buck converter, so the half-bridges are named buck pre-regulators. ZVS operation is ensured by putting an inductive tank between the half-bridges. Each output of half-bridges is connected to each of input inductors of the boost to provide the PWM excitation. Using the GSSA model, the synchronous condition and control laws are derived for the amplifier. Properties of the current harmonics in the input inductors are well examined. It is discovered that the dc harmonic, once used to transfer power, is unwanted (parasitic) since it increases conduction loss in switches of the boost. A better idea is to use the fundamental harmonic for power transfer, since it does not create loss in the switches. Complete elimination of the dc current is not feasible for constant frequency operation of the amplifier since the dc current depends on the load coil's resistance. However, significant mitigation of around 55 % is easily achievable. The proposed method improves significantly the efficiency of both the buck pre-regulator and the boost. Synchronous operation is demonstrated in hardware for fixed switching frequency of 480 kHz, power level up to 750 W, input voltage change from 300 V to 600 V, load coil's resistance change of three times, and load coil's inductance change of 3.5 %. Measured efficiency is around 95 %, with a great room for improvements. Chapter five summarizes key contributions and concludes the dissertation. / Ph. D.

For decades, the Silicon-based semiconductors have been the solution for power electronics applications. However, these semiconductors have approached their limits of operation in blocking voltage, working temperature and switching frequency. Due to material superiority, the relatively-new wide-bandgap semiconductors such as Silicon-Carbide (SiC) MOSFETs enable higher voltages, switching frequencies and operating temperatures when compared to Silicon technology, resulting in improved converter specifications. The current study tries to investigate the impact of emerging medium-voltage SiC MOSFETs on industrial motor drive application, where over a quarter of the total electricity in the world is being consumed. Firstly, non-commercial SiC MOSFETs at 3.3 kV and 400 A rating are characterized to enable converter design and simulation based on them. In order to feature the best performance out of the devices under test, an intelligent high-performance gate driver is designed embedding required functionalities and protections. Secondly, total of three converters are targeted for industrial motor drive application at medium-voltage and high-power range. For this purpose the cascaded H-bridge, the modular multilevel converter and the 5-L active neutral point clamped converters are designed at 4.16-, 6.9- and 13.8 kV voltage ratings and 3- and 5 MVA power ratings. Selection of different voltage and power levels is done to elucidate variation of different parameters within the converters versus operating point. Later, comparisons are done between the surveyed topologies designed at different operating points based on Si IGBTs and SiC MOSFETs. The comparison includes different aspects such as efficiency, power density, semiconductor utilization, energy stored in converter structure, fault containment, low-speed operation capability and parts count (for a measure of reliability). Having the comparisons done based on simulation data, an H-bridge cell is implemented using 3.3 kV 400 A SiC MOSFETs to evaluate validity of the conducted simulations. Finally, a novel method is proposed for series-connecting individual SiC MOSFETs to reach higher voltage devices. Considering the fact that currently the SiC MOSFETs are not commercially available at voltages higher above 1.7 kV, this will enable implementation of converters using medium-voltage SiC MOSFETs that are achieved by stacking commercially-available 1.7 kV MOSFETs. The proposed method is specifically developed for SiC MOSFETs with high dv/dt rates, while majority of the existing solutions could only work merely with slow Si-based semiconductors. / Ph. D.

This work investigates the voltage scaling feasibility of several low voltage SiC MOSFET modules operated as a single series-connected switch using active gate control. Both multilevel and two-level topologies are capable of achieving higher blocking voltages in high-power converter applications. Compared to multilevel topologies, two-level switching topologies are of interest due to less complex circuitry, higher density, and simpler control techniques. In this work, to balance the voltage between series-connected MOSFETs, device turn-off speeds are dynamically controlled on active gate-drivers using active gate control. The implementation of the active gate control technique (specifically, turn-off dv/dt control) is described in this thesis. Experimental results of the voltage balancing behavior across eight 1.7 kV rated SiC MOSFET devices in series (6 kV total dc bus voltage) with the selected active dv/dt control scheme are demonstrated. Finally, the voltage balancing performance and switching behavior of series-connected SiC MOSFET devices are discussed. / Master of Science / According to ABB, 40% of the world's power demand is supplied by electrical energy. Specifically, in 2018, the world's electrical demand has grown by 4% since 2010. The growing need for electric energy makes it increasingly essential for systems that can efficiently and reliably convert and control energy levels for various end applications, such as electric motors, electric vehicles, data centers, and renewable energy systems. Power electronics are systems by which electrical energy is converted to different levels of power (voltage and current) depending on the end application. The use of power electronics systems is critical for controlling the flow of electrical energy in all applications of electric energy generation, transmission, and distribution. Advances in power electronics technologies, such as new control techniques and manufacturability of power semiconductor devices, are enabling improvements to the overall performance of electrical energy conversion systems. Power semiconductor devices, which are used as switches or rectifiers in various power electronic converters, are a critical building block of power electronic systems. In order to enable higher output power capability for converter systems, power semiconductor switches are required to sustain higher levels of voltage and current. Wide bandgap semiconductor devices are a particular new category of power semiconductors that have superior material properties compared to traditional devices such as Silicon (Si) Insulated-Gate Bipolar Junction Transistors (IGBTs). In particular, wide bandgap devices such as Silicon Carbide (SiC) Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) have better ruggedness and thermal capabilities. These properties provide wide bandgap semiconductor devices to operate at higher temperatures and switching frequencies, which is beneficial for maximizing the overall efficiency and volume of power electronic converters. This work investigates a method of scaling up voltage in particular for medium-voltage power conversion, which can be applied for a variety of application areas. SiC MOSFET devices are becoming more attractive for utilization in medium-voltage high-power converter systems due to the need to further improve the efficiency and density of these systems. Rather than using individual high voltage rated semiconductor devices, this thesis demonstrates the effectiveness of using several low voltage rated semiconductor devices connected in series in order to operate them as a single switch. Using low voltage devices as a single series-connected switch rather than a using single high voltage switch can lead to achieving a lower total on-state resistance, expectedly maximizing the overall efficiency of converter systems for which the series-connected semiconductor switches would be applied. In particular, this thesis focuses on the implementation of a newer approach of compensating for the natural unbalance in voltage between series-connected devices. An active gate control method is used for monitoring and regulating the switching speed of several devices operated in series in this work. The objective of this thesis is to investigate the feasibility of this method in order to achieve up to 6 kV total dc bus voltage using eight series-connected SiC MOSFET devices.

<p dir="ltr">Electronic structure calculations, specifically plane wave Density functional theory (DFT) in conjunction with data science resources on the nanoHUB platform, are a powerful tool for the study of materials at extreme conditions. nanoHUB’s resources for FAIR (Findable, Accessible, Interoperable and Reproducible) data and workflows accelerate research and sharing of results. With these combined resources we studied the high-pressure properties and stability of various ceramic materials, such as the polytypes of silicon carbides (SiC).</p><p dir="ltr">Silicon carbides are of interest for their exceptional mechanical strength, thermal stability and chemical resistance making them attractive for applications at extreme conditions. To understand the temperature- and pressure-induced phase transitions we studied the electronic structure, phonon dispersion and elastic constants of major synthesizable SiC polytypes as a function of pressure. In addition, we report on the elastic constants as a function of pressure for the main polymorphs. This DFT workflows are published online, not only enhances the reproducibility of findings, but can also accelerates the discovery and development of new material properties.</p>

L’augmentation du nombre d’interfaces dans une matrice céramique permet d’améliorer sa ténacité. L’étude de la structure feuilletée de la nacre a démontré que cette ténacité pouvait être accrue par la présence de pontages entre les couches. Dans la première partie, le modèle de décohésion proposé par Pompidou et al. a été utilisé pour choisir un bicouche dont l’interface est naturellement favorable aux décohésions. Compte tenu du contexte de l’étude, cette analyse a permis de choisir le couple SiC/pyC comme bi-couche de base pour l’étude des interfaces. Par la suite, des matrices multicouches modèles (SiC/pyC)n (SiC, carbure de silicium issu du mélange CH3SiCl3/H2 – pyC, pyrocarbone à partir du propane) ont été élaborées par dépôt chimique en phase vapeur (CVD). Deux voies de pontage ont été abordées. La première met en œuvre une discontinuité entre les couches : les conditions d’élaboration ont été optimisées de façon à contrôler la croissance de couches minces massives et le développement de particules de surface (submicroniques) faisant office de pontage. La deuxième est basée sur un gradient de composition entre les couches de SiC grâce au développement d’une couche de SiC riche en co-dépôt de carbone, une interphase mixte est créée. Le pontage est assuré par la présence simultanée dans les couches à gradient de composition de grains de SiC et d’une phase carbonée. Les propriétés physico-chimiques et structurales des différents éléments des matrices ont été analysées et les différents comportements des fissures dans chacune des matrices ont été observés à la suite d’essais mécaniques. / The improvement of ceramic matrix toughness may be achieved through the presence of interfaces. Moreover, studies on a mother of pearl structure have shown the usefulness of mineral bridges between the layers. On the first part of this work, the Pompidou model was used for the selection of a bi-layered ceramic with an interface which is naturally favorable to crack deflection. SiC/pyC was taken as basic material for the interfaces study. Then, multilayered ceramic matrices (SiC/pyC)n (silicon carbide from CH3SiCl3/H2 mixture – pyC from propane) were fabricated using chemical vapor deposition (CVD). In the study, two bypass ways were proposed. On the one hand, a physical discontinuity exists between the different layers: elaboration parameters were optimized in order to develop both bulk layers and submicronic surface particles, acting as ceramic bypass. On the other hand, composition gradient films were developed between each SiC layers: by realizing carbon rich SiC layers, a mixed interphase was created. The presence of both SiC grains and carbon phases ensures the bypass structure. Physico-chemical and structural properties of multilayered ceramic matrices were analyzed and the crack propagation in each of them was observed following mechanical tests.

This work presents a 6.5-kV 1-kA SiC bipolar diode module for megawatt-range medium voltage converters. The study comprises a review of SiC devices and bipolar diodes, a description of the die and module technology, device characterization and modelling and benchmark of the device at converter level. The effects of current change rate, temperature variation, and different insulated-gate bipolar transistor (IGBT) modules for the switching cell, as well as parasitic oscillations are discussed. A comparison of the results with a commercial Si diode (6.5 kV and 1.2 kA) is included. The benchmark consists of an estimation of maximum converter output power, maximum switching frequency, losses and efficiency in a three level (3L) neutral point clamped (NPC) voltage-source converter (VSC) operating with SiC and Si diodes. The use of a model predictive control (MPC) algorithm to achieve higher efficiency levels is also discussed. The analysed diode module exhibits a very good performance regarding switching loss reduction, which allows an increase of at least 10 % in the output power of a 6-MVA converter. Alternatively, the switching frequency can be increased by 41 %.