Hot-wire chemical vapour deposition (HWCVD) is a promising technique that permits polycrystalline silicon films with grain size of nanometers to be obtained at high deposition rates and low substrate temperatures. This material is expected to have better electronic properties than the commonly used amorphous hydrogenated silicon (a-Si:H).In this work, thin-film transistors (TFTs) were fabricated using nanocrystalline hydrogenated silicon film (nc-Si:H), deposited by HWCVD over thermally oxidized silicon wafer. The employed substrate temperature during the deposition process permits inexpensive materials as glasses or plastics to be used for various applications in large-area electronics. The deposition rate was about one order of magnitude higher than in other conventionally employed techniques. The deposited nc-Si:H films show good uniformity and reproducibility. The films consist of vertically grown columnar grains surrounded by amorphous phase. The columnar grains are thinner at the bottom (near the oxide interface) and thicker at the top of the film. Chromium layer was evaporated over the nc-Si:H in order to form drain and source contacts. Using photolithography techniques, two types of samples were fabricated. The first type (simplified) was with the chromium contacts directly deposited over the intrinsic nc-Si:H layer. No dry etching was involved in the fabrication process of this sample. The transistors on the wafer were not electrically separated from each other. Doped n+ layer was incorporated at the drain and source contacts in the second type of samples (complete samples). Dry etching was employed to eliminate the nc-Si:H between the TFTs and to isolate them electrically from each other.The electrical characteristics of both types of nc-Si:H TFTs were similar to a-Si:H based TFTs. Nevertheless, some significant differences were observed in the characteristics of the two types of samples. The increasing of the off-current in the simplified structure was eliminated by the n+ layer in the second type of samples. This led to the improving of the on/off ratio. The n+ layer also eliminated current crowding of the output characteristics. On the other hand, the subthreshold slope, the threshold voltage and the density of states were slightly deteriorated in the samples with incorporated n+ layer. Surface states created by the dry etching could be a possible reason. Other cause could be a bad quality of the nc-Si:H/SiO2 interface. The TFTs with incorporated n+ contact layer and electrically separated on the wafer were used in the further studies of stability and device modelling.The nc-Si:H TFTs were submitted under prolonged positive and negative gate bias stress in order to study their stability. We studied the influence of the stressing time and voltage on the transfer characteristics, threshold voltage, activation energy and density of states. The threshold voltage increased under positive gate bias stress and decreased under negative gate bias stress. After both positive and negative stresses, the threshold voltage recovered its initial values without annealing. This behaviour indicated that temporary charge trapping in the channel/gate insulator interface is the responsible process for the device performance under stress. Measurements of space-charge limited current confirmed that bulk states were not affected by the positive nor by negative stress.Analysis of the activation energy and the density of states gave more detailed information about the physical processes taking place during the stress. Typical drawback of the nc-Si:H films grown by HWCVD with tungsten (W) filament is the bad quality of the bottom, initially grown, interfacial layer. It is normally amorphous and porous. We assume that this property of the nc-Si:H film is determining for charge trapping and the consecutive temporary changes of the TFT's characteristics. On the other hand, the absence of defect-state creation during the gate bias stress demonstrates that the nc-Si:H films did not suffer degradation under the applied stress conditions. The electrical characteristics and the operational regimes of the nc-Si:H TFTs were studied in details in order to obtain the best possible fit using the Spice models for a-Si:H and poly-Si TFTs existing until now. The analysis of the transconductance gm showed behaviour typical for a-Si:H TFTs at low gate voltages. In contrast, at high gate voltages unexpected increasing of gm was observed, as in poly-Si TFTs. Therefore, it was impossible to fit the transfer and output characteristics with the a-Si:H TFT model neither with poly-Si TFT model.We performed numerical simulations using the Silvaco's Atlas simulator of semiconductor devices in order to understand the physical parameters, responsible for the device behaviour. The simulations showed that the reason for this behaviour is the density of acceptor-like states, which situates the properties of nc-Si:H TFTs between the amorphous and the polycrystalline transistors. Taking into account this result, we performed analysis of the concentrations of the free and the trapped carriers in nc-Si:H layer. It was found that nc-Si:H operates in transitional regime between above-threshold and crystalline-like regimes. This transitional regime was predicted earlier, but not experimentally observed until now. Finally, we introduced new equations and three new parameters into the existing a-Si TFTs model in order to account for the transitional regime. The new proposed model permits the shapes of the transconductance, the transfer and the output characteristics to be modelled accurately.

Many methods for EMI suppression have been developed in the last fifty years, most of them, showing a hardly change in its implementation. Traditional tools for EMI suppression are related to the use of filters, shielding techniques and new methods for layout improvement. These hardware techniques are normally supported with waveform shapes having themselves a lower spectral content. This kind of signals makes part of a different concept of EMI suppression that consists of limiting the spectral content in the signal itself. When possible, just waveforms with a lower spectral content should be used, this way making easier, simpler and cheaper the use of filters and other suppression means. In this line, EMI-reduction techniques such a Spread Spectrum Clock Generation (SSCG) are contributing to eliminate or limit the problem at the root, that is, at the signal itself.This thesis is developed in several parts, corresponding to different chapters. A summary of these chapters is presented onwards:After introduction in chapter 1, a wide theoretical development of the modulation and related concepts are presented in chapter 2. It is explained generically all aspects related to the modulation and particularly, to the frequency modulation. Main parameters of frequency modulation are presented and explained in detail and how practical considerations may affect to the theoretical behaviour of these parameters. Because the theoretical part of this thesis is completely based on the fundamentals of Fourier Transform, a sufficient explanation was thought to include for its right understanding . Finally, all this knowledge is summarized in a computational algorithm (MATLAB environment), capable of generating any frequency modulation of a sinusoidal carrier and the corresponding spectral components resulting from the modulation process.Chapter 3 takes profit of the results obtained in Chapter 2 where it is possible to obtain the theoretical behaviour of the different modulation profiles of interest: sinusoidal, triangular, exponential and mixed waveforms. This way, chapter 3 is intended to completely understand and analyze the theoretical behaviour of these modulation profiles and be quantified according to several significant measure parameters. Afterwards, a comparison of these modulation profiles is carried out by means of the measure parameters defined previously. A proposal of control for a real power converter and theoretical considerations to apply a certain SSCG method to switching power converters are also included in this chapter. After all aspects of frequency modulation by means of SSCG methods have been theoretically developed, it is mandatory the verification of the theoretical conclusions through an experimental test plant. Chapter 4 starts with the description, theoretical calculation and physical implementation of this test plant. Most practical considerations are here dealt with, like the influence of the Spectrum Analyzer's Resolution Bandwidth (RBW) on the measured EMI, a proposal of a practical method to select a valuable SSCG technique applied to Switching Power Converters, comparative measurements of conducted EMI within the range of conducted emissions (0 Hz 30 MHz) and a proposal about SSCG as a method to avoid interfering a certain signal.Chapter 5 summarizes the whole conclusions gathered through the previous chapters and, finally, chapter 6 lists references related to the thesis, separated into different thematic groups.

La exitosa implantación de la telefonía móvil a escala mundial presenta una muy importante oportunidad para la expansión del comercio electrónico sobre entornos inalámbricos. El comercio electrónico para móviles, m-Commerce, implica tres aspectos básicos: 1) la negociación y el servicio en la vecindad de cliente, 2) información oportuna y georeferenciada mientras el usuario esta en movimiento, 3) la posibilidad para completar una transacción en cualquier sitio y momento. El usuario debe tener las facilidades siguientes: la negociación y la entrega inmediata, métodos rápidos de micro y de macro pago, y facilidad de uso en el ambiente de móvil. Una de las novedades del comercio móvil es la posibilidad de atraer a clientes en el vecindario hacia un centro de venta y/o servicios proporcionándoles la información apropiada. Existen, sin embargo, una serie de factores que dificultan la implantación y el desarrollo del comercio móvil respecto al comercio electrónico. Esos inconvenientes se relacionan con las características del ambiente inalámbrico: normalmente menor ancho de banda, latencia más baja, menos estabilidad de conexiones, la disponibilidad menos previsible. Y las limitaciones en los equipos móviles: la unidad de procesamiento central menos potente, menos memoria, limitaciones en el consumo de potencia, formato reducido de pantalla, y otras más.La seguridad extremo a extremo entre el servidor de Internet y el terminal móvil es indispensable para aplicaciones de comercio electrónico. La especificación de la capa de seguridad WTLS (Wireless Transport Layer Security de WAP) no proporciona este nivel de seguridad. El uso de WTLS y la seguridad a nivel de la capa de transporte TLS (Transport Layer Security) permite la privacidad en los canales inalámbrico e Internet, pero la seguridad alcanzada no es suficiente para aplicaciones de comercio electrónico; por ello se precisan mecanismos de seguridad extremo a extremo. En esta tesis se propone una alternativa a la arquitectura de la seguridad de WAP para resolver el problema mencionado, basada en la implementación de una capa de seguridad nueva dentro de la capa WAE (Wireless Application Environment). Esto hace posible la seguridad extremo a extremo, compatibilidad con TLS, transparencia ante el usuario, y se evita la traducción y descompresión en la pasarela de WAP. Varios estudios sobre usabilidad de los dispositivos con capacidades de WAP indican que los usuarios se desconectaron debido a tiempos de respuesta lentos y la falta de comodidad en el uso (interfaces no agradables, servicios costosos, . ). Los estudios coincidieron en que las velocidades más rápidas y el uso extendido de equipos móviles de datos promueven el comercio móvil, y que el número de usuarios familiarizados con equipos móviles sube constantemente, particularmente entre usuarios de WAP, de modo que estos usuarios comienzan a ver sus teléfonos móviles como algo más que meros teléfonos. La movilidad del usuario contribuye a hacer las redes inalámbricas más complejas, y constituyen los nuevos paradigmas en el intercambio de información. Las posibilidades que se abren para la Internet inalámbrica constituyen una oportunidad importante para el comercio electrónico. El futuro e impacto social de las tecnologías utilizadas, WAP, GPRS, UMTS, . son cuestionadas habitualmente. En esta tesis se analiza la posibilidad de realizar operaciones de m-Commerce sobre WAP. Por ello, se presentan los mecanismos de seguridad de WTLS (Wireless Transport Layer Security) y se analiza la seguridad extremo a extremo en este protocolo. En redes inalámbricas, existe la necesidad de acelerar la respuesta al usuario, facilitarle el uso en ambientes ruidosos, con desconexión no previsible, sobre dispositivos con formato limitado. Para paliar los problemas relacionados a la entrega de información, facilitar el uso y personalizar el servicio con el cliente, se presenta una propuesta basada en un sistema intermediario. Los objetivos de dicho intermediario son a) reducir el tráfico de datos en el canal inalámbrico delegando tareas tales como la búsqueda, interacción y filtrado de la información al Intermediario aún estando fuera de línea; b) Reutilizar contenidos: reutilizar la información residente en el almacén intermedio mediante un sistema distribuido de caches y proxies; c) Personalizar la información: conociendo previamente el perfil del usuario se puede filtrar y distribuir información en forma predictiva y d) Gestionar la información del estado de los certificados con OCSP (On-line Certificate Status Protocol).

En aquesta tesi doctoral es presenta un mètode sistemàtic per a la síntesi de giradors de potència. A partir d'aquest mètode s'han generat i classificat diverses estructures giradores. Cadascun d'aquests giradors, que poden tenir característiques diferents, pot ser útil en diferents aplicacions.Des d'un punt de vista circuital, es tracta d'una estructura de dos ports que es caracteritza per algun d'aquests dos grups d'equacions: 1) I1=gV2, I2=gV1 , 2) V1=rI2, V2=rI1, on I1, V1, i I2, V2 són els valors en contínua corresponents als valors de tensió i corrent als ports d'entrada i sortida respectivament, essent g (r) la conductància (resistència) del girador.En aquesta tesi, les estructures giradores de potència s'han classificat en funció de com transformen una font d'excitació al port d'entrada en la seva representació dual al port de sortida. Segons aquesta classificació es poden distingir tres tipus de giradors: 1) girador de potència de tipus G, 2) girador de potència de tipus G amb corrent d'entrada controlada i 3) giradors de potència de tipus R. Les categories 1 i 2 són les dues possibles solucions de síntesi de les equacions (1), mentre que la categoria 3 correspon a la solució de síntesi de les equacions (2).A més a més, no existeixen estudis sistemàtics on basant-se en les equacions de definició s'arribi finalment a una verificació experimental. En aquesta tesi es presenta el disseny i anàlisi dels giradors que s'han presentat. L'anàlisi cobreix exhaustivament l'estudi tant del comportament dinàmic com estàtic dels giradors presentats. Aquests giradors es poden considerar com estructures canòniques per al processat de potència.A més a més, es presenten algunes funcions bàsiques del processat de potència realitzades amb giradors de potència. Com per exemple: conversió tensió-corrent, corrent-tensió, adaptació d'impedàncies i regulació de tensió.Les característiques de cada girador depenen no només de la topologia convertidora sinó també del funcionament del control del convertidor. S'han investigat dos tècniques de control: el control en mode lliscant i el control no lineal basat en dinàmica zero. Per tant, les estructures giradores proposades poden treballar tant a freqüència constant com a freqüència variable.Finalment s'han verificat les previsions teòriques mitjançant simulació i verificació experimental. / In this thesis, a systematic approach to the synthesis of power gyrators is presented. Based on this approach, several gyrator structures can be generated and classified. Each of these gyrators has its own features and is suitable of different applications.From a circuit standpoint, a power gyrator is a two-port structure characterized by any of the following two set of equations: 1) I1=gV2, I2=gV1 , 2) V1=rI2, V2=rI1, where I1, V1, and I2, V2 are DC values of current and voltage at input and output ports respectively and g ( r ) is the gyrator conductance ( resistance ).In this thesis, power gyrator structures are classified by the manner they transform an excitation source at the input port into its dual representation at the output port. Based on this classification, there exist three types of power gyrators: 1) power gyrators of type G, 2) power gyrators of type G with controlled input current and 3) power gyrators of type R. Categories 1 and 2 are the two possible synthesis solutions to the set of equations ( 1 ) while category 3 corresponds to the synthesis solution of ( 2 ).Thus far, no systematic works have been done starting at the definition equations and ending at the experimental verification. In this thesis, the analysis and design for the disclosed power gyrators are presented. The analysis covers exhaustingly the study of both static and dynamic behavior of the reported power gyrators. These power gyrators presented can be considered as canonical structures for power processing.Thus, some basic power processing functions done by the presented power gyrators are reported. Namely, voltage to current conversion, current to voltage conversion, impedance matching and voltage regulation.The performance characteristics of a power gyrator depend not only on the circuit topology but also depend on the converter control operation.Hence, two main control schemes are investigated, namely, sliding-mode control schemes and zero-dynamics-based PWM nonlinear control. Therefore, the proposed gyrator structures can operate indistinctly at constant or at variable switching frequency.In addition, experimental and computer simulation results of the power gyrators presented are given in order to verify the theoretical predictions.

El sistema de acondicionamiento de potencia es clave para el funcionamiento correcto del resto de sistemas eléctricos de un satélite. Los reguladores conmutados continua - continua son las piezas que enlazan los paneles solares y las baterías con el bus principal de alimentación. La estabilidad, precisión y robustez del sistema de control son fundamentales en el diseño de esos convertidores.Desde el punto de vista de los especialistas en control, los convertidores conmutados son sistemas de estructura variable, en los cuales se aplica de forma natural el control en modo deslizante. Sin embargo, no es fácil encontrar un ejemplo en el que se haya aplicado con éxito en los sistemas de alimentación de satélites. Con esta tesis se pretende cubrir el vacío existente entre la teoría de control en modo deslizante y la práctica de diseño de controles de convertidores conmutados continua - continua que tienen aplicación en los sistemas de alimentación de satélites.La tesis comienza introduciendo las características básicas del sistema de alimentación y presentando algunos reguladores cargadores y descargadores de batería que han sido utilizados en satélites concretos. El resto de la tesis es un estudio teórico con el fin de diseñar reguladores controlados en modo deslizante para esa aplicación.Se ha comenzado obteniendo un conjunto finito de convertidores básicos, a partir de los circuitos con dos interruptores, aplicando un método de síntesis basado en reglas topológicas y se han seleccionado los convertidores con estrés mínimo en los interruptores. Se han escogido, de entre estos, los convertidores con la corriente de salida continua, para utilizar como descargadores de batería, y los que tienen la corriente de entrada continua como cargadores de batería.Se ha encontrado que todos los convertidores seleccionados (tomando como salida del sistema el error de la corriente que se debe controlar) tienen grado relativo uno y su dinámica cero es asintóticamente estable, si se cumplen ciertas relaciones entre los parámetros que se han obtenido explícitamente para cada convertidor. En algunos casos es necesario acoplar magnéticamente los dos inductores o añadir una red R-C serie en paralelo con el condensador.Se han proporcionado criterios de diseño del lazo de corriente y del lazo de tensión, utilizando comparadores con histéresis para generar la señal de control en modo deslizante. Se han simulado varios de estos reguladores, encontrando que cumplen las especificaciones restrictivas de impedancia en el bus y al mismo tiempo tienen un comportamiento robusto frente a perturbaciones de gran señal cuando los parámetros toman sus valores extremos.Los reguladores encontrados pueden servir para mejorar las prestaciones y reducir el coste de los sistemas de alimentación de satélites y tienen muchas otras aplicaciones fuera de la industria aerospacial. / Power conditioning system is key to correct operation of the rest of electrical systems of satellites. Dc-to-dc switching converters link solar panels and batteries with the main power bus. Control system stability, accuracy and robustness is basic in the design of those converters.Switching converters are variable structure systems from the point of view of control specialist. In those systems, sliding-mode control can be applied in a natural way. Nevertheless it is difficult to find a successful example in power conditioning systems of satellites. This thesis try to fill the gap between sliding-mode control theory and control design practice of dc-to-dc switching converters for power conditioning systems of satellites.Thesis begin introducing basic features of power conditioning system and some battery charge and discharge regulators which have been actually used in satellites. The rest of the thesis is a theoretical study with the purpose of design sliding-mode controlled regulators for that application.A finite set of basic converters has been obtained from two switch circuits, applying a synthesis method based on topological rules. Converters with minimum switch stress have been selected from that set. Among them, converters with continuous output current have been chosen for battery discharge regulators and converters with continuous input current for battery charge regulators.The analysis of selected converters show that all of them (taking as system output the error of current that must be controlled) have a relative degree of one and an asymptotically stable zero dynamics if certain conditions on the parameters are fulfilled. Those conditions have been found individually for each converter. In some cases, inductors must be coupled and a series R-C network must be added in parallel with a capacitor.A comparator with hysteresis is used to get sliding-mode control signal. Design criteria for the parameters of current and voltage loops are given. Some of those regulators have been simulated finding that they meet restrictive bus impedance specifications. In addition, they have a robust behavior in simulations of large signal perturbations where parameters take their extreme values.The found regulators can be used to improve performance and reduce cost of power conditioning systems of satellites. They also have a lot of others applications outside aerospace industry.