EEV buried channel charge coupled devices (BC CDs) with technological variations have been studied with respect to their response to 70kVp X-rays. Process variations considered are the conventional BCCD, scintillator coated BCCDs (Gadox(Eu) and Csl(Tl)) and the inversion mode device. The work was made necessary by the use of these CCDs for dental X-ray imaging. Effects investigated include changes in device operating voltages and dark current. The dark current buildup has been characterised in terms of a prompt component seen immediately following irradiation, and a time dependent component which occurs gradually. A major part of this work was the determination of the location and concentration of the energy states responsible for this dark current buildup. Also a novel aspect of the work was the derivation of an expression describing the time dependent component as a function of time and temperature. Effects associated with the bias dependence of the BCCD have also been considered, with particular regard to the effect of a negative substrate bias, and the theoretical explanation has been developed. The findings of this work have demonstrated the suitability of these devices for the commercial application of imaging X-rays for dentistry.

In this thesis, we study the transport of charged particles in mesoscopic systems where the long-range electrostatic interaction plays a key role. In particular, we focus on the statistical properties of currents which satisfy the relation known as fluctuation theorem. A stochastic approach is presented in consistency with the laws of electricity, thermodynamics, and microreversibility. In this approach, densities of charged particles are ruled by stochastic partial differential equations and the electric field generated by the charges is determined with the Poisson equation.We start by proposing a coarse-grained model to describe the long-time behavior of particle transport in nonequilibrium systems in contact with several reservoirs. In this model, the particle exchanges between the reservoirs can be determined after a long enough time by the first and second cumulants of the probability distribution of particle transfers, thus enabling the numerical evaluation of the driving forces. It is proved that, close to equilibrium, the coarse-grained model is applicable to any system in nonequilibrium steady state. Moreover, the studies with specific examples show that the range of application of the coarse-grained model can be extended to the regime arbitrarily far from equilibrium if the detailed transition rates have linear dependence on the local particle concentration. In addition, the finite-time fluctuation theorem is established for systems with linear rates.Then, we use our stochastic approach to study charge transport in diodes and transistors. The counting statistics of the carrier current and the measured total current including the contribution of the displacement current are performed. The fluctuation theorem is shown to hold for both currents. The convergence of the finite-time affinities towards their asymptotic values over long-time scales is tested using our proposed coarse-grained model. Accurate agreement between the numerical affinities and the theoretical predictions is found when affinities take moderate values. This brings further numerical support to the fluctuation theorem for the currents in these nonlinear electronic devices. For diodes, the current-voltage characteristics is obtained, which can be well fit by Shockley curve under the extreme condition where the concentration of majority charge carriers is overwhelmingly larger than that of minority charge carriers. For transistors, the signal amplifying effect is realized under their working conditions. Moreover, the Onsager reciprocal relations and their generalizations to nonlinear transport properties deduced from the fluctuation theorem are numerically shown to be satisfied.Finally, we proceed to investigate the charge transport in a system of three tunneling junctions coupled together through a conductive island or quantum dot. The cumulant generating function is obtained by numerically solving the eigenvalue problem regarding the modified evolution operator including the counting parameters. The symmetry relation implied by the fluctuation theorem is verified, and the Onsager reciprocal relations together with their generalizations are again shown to be satisfied. Furthermore, under certain conditions, the current-voltage characteristics shows the staircase pattern due to the Coulomb blockade effect. / Le transport de particules charg´ees est ´etudi´e dans les syst`emes m´esoscopiques o`u l’interaction ´electrostatique`a longue port´ee joue un rˆole central. En particulier, l’´etude porte sur les propri´et´es statistiques des courants quisatisfont la relation appel´ee th´eor`eme de fluctuations. Une approche stochastique est pr´esent´ee en accord avec leslois de l’´electricit´e, la thermodynamique et la micror´eversibilit´e. Dans cette approche, les densit´es des particlescharg´ees sont gouvern´ees par des ´equations stochastiques aux d´eriv´ees partielles et le champ ´electrique est d´etermin´epar l’´equation de Poisson. Tout d’abord, un mod`ele stochastique r´eduit est propos´e pour d´ecrire le transport departicules dans les syst`emes de non-´equilibre en contact avec plusieurs r´eservoirs. Dans ce mod`ele, les ´echanges departicules entre les r´eservoirs sont d´etermin´es aux temps longs en termes des premiers et seconds cumulants de ladistribution de probabilit´e des transferts de particules, ce qui permet l’´evaluation num´erique des forces d’entraˆınement.Il est prouv´e que, pr`es de l’´equilibre, ce mod`ele peut s’appliquer `a tout syst`eme dans un ´etat stationnaire de non-´equilibre. Par ailleurs, l’´etude d’exemples sp´ecifiques montre que le domaine d’application de ce mod`ele s’´etend `a desr´egimes arbitrairement loin de l’´equilibre si les taux de transition ont des d´ependances lin´eaires en la concentrationde particules. En plus, le th´eor`eme de fluctuations `a temps fini est ´etabli pour ces syst`emes avec des taux lin´eaires.Ensuite, l’approche stochastique est utilis´ee pour ´etudier le transport de charges dans les diodes et les transistors.La statistique de comptage est effectu´ee pour le courant des porteurs de charge et le courant total mesur´e incluantla contribution du courant de d´eplacement. Le th´eor`eme de fluctuations est d´emontr´e pour les deux courants. Laconvergence des affinit´es `a temps fini vers leur valeur asymptotique est test´ee en utilisant le mod`ele stochastiquer´eduit. Un accord est observ´e entre les affinit´es obtenues num´eriquement et les pr´edictions th´eoriques si les affinit´esprennent des valeurs mod´er´ees. Tous ces r´esultats apportent des v´erifications num´eriques du th´eor`eme de fluctuationsdes courants dans les circuits ´electroniques non-lin´eaires. Pour les diodes, la courbe caract´eristique courant-tension estobtenue et celle-ci peut ˆetre ajust´ee par la courbe de Shockley si la concentration des porteurs de charge majoritairesest beaucoup plus grande que celle des porteurs minoritaires. Pour les transistors, l’effet d’amplification du signal estmis en ´evidence dans les conditions de fonctionnement normal. De plus, il est montr´e num´eriquement que les relationsde r´eciprocit´e d’Onsager sont satisfaites, ainsi que certaines de leurs g´en´eralisations aux propri´et´es non-lin´eaires detransport d´eduites du th´eor`eme de fluctuations. Finalement, le transport de charges est ´etudi´e dans un syst`eme form´ede trois jonctions `a effet tunnel coupl´ees par un plot conducteur (quantique). La fonction g´en´eratrice des cumulantsest alors obtenue par r´esolution num´erique du probl`eme aux valeurs propres de l’op´erateur d’´evolution modifi´e pourinclure les param`etres de comptage. La relation de sym´etrie d´eduite du th´eor`eme de fluctuations est v´erifi´ee, ainsique les relations de r´eciprocit´e d’Onsager et leurs g´en´eralisations. Par ailleurs, sous certaines conditions, les courbescaract´eristiques courant-tension montrent une structure en marches d’escalier due `a l’effet de blocage coulombien. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

This Ph.D. thesis carries out extensive and in-depth research on the packaging technology of silicon carbide (SiC) power modules, including new packaging structures, multi-physics modeling and optimal design methods for half-bridge power modules, manufacturing processes, and experimental validations. A new packaging scheme, the Silver-Sintered Molybdenum (SSM) packaging, is proposed in this thesis. It contains a molybdenum (Mo) -based insulated-metal-substrate (IMS) structure, nano-silver sintering die-attachments, and planar interconnections. This technology has the potential to increase the operating temperature of SiC power modules to above 200 degrees, and can greatly improve their lifetime. These advantages are verified by active power cycling and passive temperature cycling simulations. Analytical modeling methods for half-bridge power modules with the SSM packaging are also studied. A decoupled Fourier-based thermal model is introduced. This model considers the decoupling effect between different heat source regions and can give a three-dimensional analytical solution for the temperature field of a simplified half-bridge power module structure. In addition, based on the partial inductance model for rectangular busbars, an analytical stray inductance model for half-bridge power modules is also proposed. The accuracy of these two models is estimated by both numerical simulations and experiments. With the proposed analytical models, an optimal design method for half-bridge power modules with the SSM packaging is proposed in this study, which uses the particle swarm optimization algorithm. This method is successfully applied in the design of a prototype power module and is able to minimize the stray inductance and volume while maintaining desired junction temperatures. This thesis also introduces the manufacturing process of the prototype power module. Several new processes are proposed and validated, including a pressure-less nano-silver sintering process to bond SiC dies on Mo substrates, the formation of the Mo-based IMS structure, and the re-metallization of SiC dies. / Thesis / Doctor of Philosophy (PhD)

The Emitter Controlled Thyristor (ECT) is a new MOS-Gated Thyristor (MGT) that combines the ease of a MOS gate control with the superior current carrying capability of a thyristor structure for high-power applications. An ECT is composed of an emitter switch in series with the thyristor, an emitter-short switch in parallel with the emitter junction of the thyristor, a turn-on FET and the main thyristor structure. Numerical analysis shows that the ECT also offers superior high voltage current saturation capability even for high breakdown voltage ratings. Two different ECT structures are investigated in this research from numerical simulations to experimental fabrications. A novel ECT structure that utilizes IGBT compatible fabrication process was proposed. The emitter short FET, emitter switch FET and turn-on FET are all integrated with a high voltage thyristor. Numerical simulation results show that the ECT has a better conductivity modulation than that of the IGBT and at the same time exhibits superior high voltage current saturation capability, superior FBSOA and RBSOA. The technology trade-off between turn-off energy loss and forward voltage drop of the ECT is also better than that of the IGBT because of the stronger conductivity modulation. A novel self-aligned process is developed to fabricate the device. Experimental characteristics of the fabricated ECT devices show that the ECT achieves lower forward voltage drop and superior high voltage current saturation capability. A Hybrid ECT (HECT) structure was also developed in this research work. The HECT uses an external FET to realize the emitter switching function, hence a complicated fabrication issue was separated into two simple one. The cost of the fabrication decreases and the yield increases due to the hybrid integration. Numerical simulations demonstrate the superior on-state voltage drop and high voltage current saturation capability. A novel seven-mask process was developed to fabricate the HECT. Experimental results show that the HECT could achieve the lower forward voltage drop and superior current saturation capability. The resistive switching test was carried out to demonstrate the switching characteristics of the HECT. / Master of Science

The Emitter Controlled Thyristor (ECT) is a new MOS-Gated Thyristor (MGT) that combines the ease of a MOS gate control with the superior current carrying capability of a thyristor structure for high-power applications. An ECT is composed of an emitter switch in series with the thyristor, an emitter-short switch in parallel with the emitter junction of the thyristor, a turn-on FET and the main thyristor structure. Numerical analysis shows that the ECT also offers superior high voltage current saturation capability even for high breakdown voltage ratings. Two different ECT structures are investigated in this research from numerical simulations to experimental fabrications. A novel ECT structure that utilizes IGBT compatible fabrication process was proposed. The emitter short FET, emitter switch FET and turn-on FET are all integrated with a high voltage thyristor. Numerical simulation results show that the ECT has a better conductivity modulation than that of the IGBT and at the same time exhibits superior high voltage current saturation capability, superior FBSOA and RBSOA. The technology trade-off between turn-off energy loss and forward voltage drop of the ECT is also better than that of the IGBT because of the stronger conductivity modulation. A novel self-aligned process is developed to fabricate the device. Experimental characteristics of the fabricated ECT devices show that the ECT achieves lower forward voltage drop and superior high voltage current saturation capability. A Hybrid ECT (HECT) structure was also developed in this research work. The HECT uses an external FET to realize the emitter switching function, hence a complicated fabrication issue was separated into two simple one. The cost of the fabrication decreases and the yield increases due to the hybrid integration. Numerical simulations demonstrate the superior on-state voltage drop and high voltage current saturation capability. A novel seven-mask process was developed to fabricate the HECT. Experimental results show that the HECT could achieve the lower forward voltage drop and superior current saturation capability. The resistive switching test was carried out to demonstrate the switching characteristics of the HECT. / Master of Science

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No description available.

Optoelectronic characterisation techniques are assessed in their application to three solar cell systems. Charge injection barriers are found in PbS/ZnO colloidal quantum dot solar cells through the use of temperature dependent current-voltage and capacitance-voltage measurements. The injection barriers are shown to complicate the Mott-Schottky capacitance analysis which determines built-in bias and doping density. A model that incorporates depletion capacitance and a constant capacitance arising from the injection barriers is given to explain the Mott-Schottky plots. The junction mechanism at the PbS/ZnO interface is found to transition from excitonic to p-n behaviour based on the amount of UV photodoping the cell has received. External quantum efficiency analysis at different photodoping times reveals a growing charge collection region within the material, demonstrating the shift to p-n behaviour. This is further supported by the observance of depletion capacitance behaviour after, but not before, UV photodoping. Defects within GaAs cells containing InAs quantum dots are found to enhance the sub-bandgap performance of the cell using external quantum efficiency analysis. This is verified by illuminated current-voltage analysis using a 1000 nm high pass optical filter to block photons of larger energy than the bandgap. Using capacitance-voltage analysis, high temperature rapid thermal annealing is shown to induce defects in dilute nitride cells, which explains the drop in open circuit voltage compared to lower temperature annealed cells. The doping level of polymer solar cells exposed to air is found to increase with continued exposure using Mott-Schottky capacitance analysis. Current-voltage measurements show the formation of an Al2O3 barrier layer at the polymer/aluminium interface. The usefulness of capacitance-voltage measurements to probe the polymer/fullerene interface is investigated in thermally evaporated thiophene/C60 cells.

This thesis studies parameters influencing both the performance and stability of solid state sensitized solar cells (ssSSCs). ssSSCs benefit from their low materials and manufacturing processing costs, a consequence of using solution processed materials. However, solution processed materials are often structurally and electronically disordered. By characterizing fully operational ssSSCs and their charge transport properties, this thesis elucidates the factors limiting charge transport and proposes routes towards both improved photovoltaic conversion efficiency and long-term stability. Chapter 2 provides an explanation of the operation of ssSSCs, while Chapter 3 discusses the basic methods used in this thesis. Having set this background, Chapter 4 explores the interaction between atmospheric oxygen and charge doping mechanisms in the organic semiconductors used in ssSSCs. To understand the implications of the findings presented in Chapter 4, a new technique, “transient mobility spectroscopy”, was developed to understand the evolution of balanced charge transport behaviour of disordered semiconductors at different operating conditions in ssSSCs. This technique is presented in full in Chapter 5. The understanding gained in Chapters 4 and 5 suggest that alternative light absorbers with higher extinction coefficients may be beneficial to improving the performance of ssSSCs. Chapter 6 discusses the use of an organometal trihalide perovskite, as light absorber in ssSSCs. Using time resolved techniques, the charge transport and recombination mechanisms in various device architectures are explored, allowing suggestions to be made towards future improvements. Chapter 7 uses the technique presented in Chapter 5 to understand a rapid degradation mechanism of working ssSSCs. Particular focus is placed on the titanium dioxide charge-transporting layer. Building on this newfound understanding, two methods for attaining stable photovoltaic performance are provided, a great step forward for this technology.

Solid-state dye-sensitized solar cells (ssDSCs) offer the possibility of high power conversion efficiencies (PCEs) of over 20%. However, after more than a decade of research, devices still barely reach over 7% PCEs. In this thesis, limitations to device performance are studied in detail, and solutions for future advancement are put forward. In the first part of the thesis, factors limiting charge generation are explored by studying the crystallization environment of mesoporous TiO2 self-assembled through block copolymers. It was found that the density and distribution of sub band gap states are a function of the synthesis conditions and critically affect the performance characteristics of the self-assembled titania used in ssDSCs. As a result, the self-assembled mesoporous oxide system presented in this thesis outperforms for the first time the conventional nanoparticle based electrodes fabricated and tested under the same conditions, with demonstrated PCEs of over 5%. In chapters 6, 7, and 8, the factors limiting the diffusion length and hence, the thickness of the fabricated devices, are carefully examined. Previous literature points towards insufficient pore-filling of the hole transporting material (HTM) as the main limiting factor. In chapter 6, a pore-filling study is shown where a new technique to evaluate the pore-filling fraction of the HTM in the conventional mesoporous metal oxide electrode is also presented and conclude that sufficient pore-filling of thick films can easily be achieved. Another usual strategy to extend the electron lifetime in the devices and thus, the charge diffusion length, involving thin film coatings of insulating metal oxides is examined in chapter 7, with satisfactory results for SnO2-based ssDSCs. The diffusion length can also be extended if the factors limiting the diffusion of charges through the device are identified and removed, as presented in chapter 8. Finally, a study on the stability of the ssDSC is presented in chapter 9. The developments achieved enable long term stability to be effectively targeted, and represent a key milestone towards commercial realization of ssDSCs.