With the MOSFET scaling practice, the performance of IC devices is improved tremendously as we experienced in the last decades. However, the small semiconductor devices also bring some drawbacks among which the high gate leakage current is becoming increasingly serious. This thesis work is focused on the of gate leakage current reduction in thin oxide semiconductor devices. The method being studied is the Phonon Energy Coupling Enhancement (PECE) effect induced by Rapid Thermal Processing (RTP). The basic MOS capacitors are used to check improvements of leakage current reduction after appropriate RTP process. Through sets of experiments, it is found that after RTP in Helium environment could bring about four orders reduction in gate leakage current of MOS capacitors.

For effective operation of microgrid systems (MGSs), it is important to understand the major types of power grid failures and how to deal with them. Detecting the fault, locating it, and isolating the faulty line are important to avoid damaging components and interrupting the service for customers. This will also improve the reliability and protection level of the system during fault conditions. Among the most successful protection methods to limit fault currents in power systems is the fault current limiter (FCL). The FCL improves the reliability of the system, voltage stability, and the fault current reduction. However, limited researches consider its applications inMGSs. The location and impedance size of the FCL play a major role in limiting fault currents in the system. Several studies concluded that installing FCLs near all generators, transformers, or loads in the system enhanced the performance of the system during fault conditions. However, increasing the number of FCLs in the system leads to an increase in cost. This dissertation proposes several effective approaches to specify the optimal locations and impedance values of the required number of installed FCLs in a grid-connected MGS. These FCLs improve the reliability and the protection level of the system by limiting fault currents during fault conditions. The goal is to reduce the required number of installed FCLs in the system. These installed FCLs must be able to reduce fault currents under the interrupting ratings of circuit breakers in the system. This goal will lead to lower the cost of installed protection devices in the system. In order to achieve this goal, this dissertation presents a novel fault management approach, sensitivity analysis, and an optimization model to find the optimal solutions. The study of this dissertation is meant to be used during the planning stage of power distribution system design. The results of this dissertation prove the robustness of the proposed approaches. This enhances the system’s performance while minimizing the required number of installed FCLs. Their sizes limit fault currents within safe ranges. Thus, the FCL significantly improves the reliability and protection scheme of the grid-connected MGS.

Three-phase voltage source converters(VSCs) have become the converter of choice in many ac medium and high power applications due to their many advantages, including low harmonics, high power factor, and high efficiency. Modular VSCs have also been a popular choice as building blocks to achieve even higher power, primarily through converter paralleling. In addition to high power ratings, paralleling converters can also provide system redundancy through the so-called (N+1) configuration for improved availability, as well as allow easy implementation of converter power management. Interleaving can further improve the benefit of paralleling VSCs by reducing system harmonic currents, which potentially can increase system power density. There are many challenges to implement interleaving in paralleled VSCs system due to the complicated relationships in a three-phase power converter system. In addition, to maximize the benefit of interleaving, current knowledge of symmetric interleaving is not enough. More insightful understanding of this PWM technology is necessary before implement interleaving in a real paralleled VSCs system. In this dissertation, a systematic methodology to analyze and design a paralleled three-phase voltage source converters with interleaving is developed. All the analysis and proposed control methods are investigated with the goal of maximizing the benefit of interleaving based on system requirement. The dissertation is divided into five sections. Firstly, a complete analysis studying the impact of interleaving on harmonic currents in ac and dc side passive components for paralleled VSCs is presented. The analysis performed considers the effects of modulation index, pulse-width-modulation (PWM) schemes, interleaving angle and displacement angle. Based on the analysis the method to optimize interleaving angle is proposed. Secondly, the control methods for the common mode (CM) circulating current of paralleled three-phase VSCs with discontinuous space-vector modulation (DPWM) and interleaving are proposed. With the control methods, DPWM and interleaving, which is a desirable combination, but not considered possible, can be implemented together. In addition, the total flux of integrated inter-phase inductor to limit circulating current can be minimized. Thirdly, a 15 kW three phase ac-dc rectifier is built with SiC devices. With the technologies presented in this dissertation, the specific power density can be pushed more than 2kW/lb. Fourthly, the converter system with low switching frequency is studied. Special issues such as beat phenomenon and system unbalance due to non-triplen carrier ratio is explained and solved by control methods. Other than that, an improved asymmetric space vector modulation is proposed, which can significantly reduce output current total harmonic distortion (THD) for single and interleaved VSCs system. Finally, the method to protect a system with paralleled VSCs under the occurrence of internal faults is studied. After the internal fault is detected and isolated, the paralleled VSCs system can continue work. So system reliability can be increased. / Ph. D.

No description available.

Radiation from Computed Tomography (CT) is now the major contributor to population radiation dose. Despite controversy around the dose-effect relationship of radiation from CT, the linear non-threshold (LNT) theory is endorsed by many authorities, and constitutes the basis of cancer risk estimates. The purpose of this study was (1) a literature review of radiobiological theories, and methods of dose saving stategies in CT; (2) to highlight the importance of dose saving in CT, and to demonstrate how dose can be saved in a radiology department: Following a 40% reduction in reference X-ray tube current for a CT of the urinary tract, the effecitve dose and estimated lifetime attributable risk of incident cancer due to this CT in a group (n=103) were reduced by 37% and 38% in an age and sex-matched group respectively. The literature review showed that the public health implications of CT radiation exposure remain uncertain. / Health Studies / M.A. (Public Health)

Radiation from Computed Tomography (CT) is now the major contributor to population radiation dose. Despite controversy around the dose-effect relationship of radiation from CT, the linear non-threshold (LNT) theory is endorsed by many authorities, and constitutes the basis of cancer risk estimates. The purpose of this study was (1) a literature review of radiobiological theories, and methods of dose saving stategies in CT; (2) to highlight the importance of dose saving in CT, and to demonstrate how dose can be saved in a radiology department: Following a 40% reduction in reference X-ray tube current for a CT of the urinary tract, the effecitve dose and estimated lifetime attributable risk of incident cancer due to this CT in a group (n=103) were reduced by 37% and 38% in an age and sex-matched group respectively. The literature review showed that the public health implications of CT radiation exposure remain uncertain. / Health Studies / M.A. (Public Health)

In the last 10-15 years there has been a renewed interest in amorphous Se (a-Se) and its alloys due to their application as photoconductor materials in the new fully digital direct conversion flat panel x-ray medical image detectors. For a number of reasons, the a-Se photoconductor layer in such x-ray detectors has to be operated at very high electric fields (up to 10 Volts per micron) and one of the most difficult problems related to such applications of a Se is the problem of the dark current (the current in the absence of any radiation) minimization in the photoconductor layer. <p>This PhD work has been devoted to researching the possibilities for dark current minimization in a-Se x-ray photoconductors devices through a systematic study of the charge transport (carrier mobility and carrier lifetimes) and dark currents in single and multilayered a-Se devices as a function of alloying, doping, deposition condition and other fabrication factors. The results of the studies are extensively discussed in the thesis. We have proposed a new technological method for dark current reduction in single and multilayered a-Se based photoconductor for x-ray detector applications. The new technology is based on original experimental findings which demonstrate that both hole transport and the dark currents in a-Se films are a very strong function of the substrate temperature (Tsubstrate) during the film deposition process. We have shown that the new technique reduces the dark currents to approximately the same levels as achievable with the previously existing methods for dark current reduction. However, the new method is simpler to implement, and offers some potential advantages, especially in cases when a very high image resolution (20 cycles/mm) and/or fast pixel readout (more than 30 times per second) are needed. <p>Using the new technology we have fabricated simple single and double (ni-like) photoconductor layers on prototype x-ray image detectors with CCD (Charge Coupled Device) readout circuits. Dark currents in the a-Se photoconductor layer were not a problem for detector operation at all tested electric fields. Compared to the currently available commercial systems for mammography, the prototype detectors have demonstrated an excellent imaging performance, in particular superior spatial resolution (20 cycles/mm). Thus, the newly proposed technology for dark current reduction has shown a potential for commercialization.

In the last 10-15 years there has been a renewed interest in amorphous Se (a-Se) and its alloys due to their application as photoconductor materials in the new fully digital direct conversion flat panel x-ray medical image detectors. For a number of reasons, the a-Se photoconductor layer in such x-ray detectors has to be operated at very high electric fields (up to 10 Volts per micron) and one of the most difficult problems related to such applications of a Se is the problem of the dark current (the current in the absence of any radiation) minimization in the photoconductor layer. <p>This PhD work has been devoted to researching the possibilities for dark current minimization in a-Se x-ray photoconductors devices through a systematic study of the charge transport (carrier mobility and carrier lifetimes) and dark currents in single and multilayered a-Se devices as a function of alloying, doping, deposition condition and other fabrication factors. The results of the studies are extensively discussed in the thesis. We have proposed a new technological method for dark current reduction in single and multilayered a-Se based photoconductor for x-ray detector applications. The new technology is based on original experimental findings which demonstrate that both hole transport and the dark currents in a-Se films are a very strong function of the substrate temperature (Tsubstrate) during the film deposition process. We have shown that the new technique reduces the dark currents to approximately the same levels as achievable with the previously existing methods for dark current reduction. However, the new method is simpler to implement, and offers some potential advantages, especially in cases when a very high image resolution (20 cycles/mm) and/or fast pixel readout (more than 30 times per second) are needed. <p>Using the new technology we have fabricated simple single and double (ni-like) photoconductor layers on prototype x-ray image detectors with CCD (Charge Coupled Device) readout circuits. Dark currents in the a-Se photoconductor layer were not a problem for detector operation at all tested electric fields. Compared to the currently available commercial systems for mammography, the prototype detectors have demonstrated an excellent imaging performance, in particular superior spatial resolution (20 cycles/mm). Thus, the newly proposed technology for dark current reduction has shown a potential for commercialization.