Inverse kinematics of a 6 DoF human upper limb using ANFIS and ANN for anticipatory actuation in ADL-based physical Neurorehabilitation

Objective: This research is focused in the creation and validation of a solution to the inverse kinematics problem for a 6 degrees of freedom human upper limb. This system is intended to work within a realtime dysfunctional motion prediction system that allows anticipatory actuation in physical Neurorehabilitation under the assisted-as-needed paradigm. For this purpose, a multilayer perceptron-based and an ANFIS-based solution to the inverse kinematics problem are evaluated. Materials and methods: Both the multilayer perceptron-based and the ANFIS-based inverse kinematics methods have been trained with three-dimensional Cartesian positions corresponding to the end-effector of healthy human upper limbs that execute two different activities of the daily life: "serving water from a jar" and "picking up a bottle". Validation of the proposed methodologies has been performed by a 10 fold cross-validation procedure. Results: Once trained, the systems are able to map 3D positions of the end-effector to the corresponding healthy biomechanical configurations. A high mean correlation coefficient and a low root mean squared error have been found for both the multilayer perceptron and ANFIS-based methods. Conclusions: The obtained results indicate that both systems effectively solve the inverse kinematics problem, but, due to its low computational load, crucial in real-time applications, along with its high performance, a multilayer perceptron-based solution, consisting in 3 input neurons, 1 hidden layer with 3 neurons and 6 output neurons has been considered the most appropriated for the target application.

Performance analysis of AlGaAs/GaAs tunnel junctions for ultra-high concentration photovoltaics

An n(++)-GaAs/p(++)-AlGaAs tunnel junction with a peak current density of 10 100Acm(-2) is developed. This device is a tunnel junction for multijunction solar cells, grown lattice-matched on standard GaAs or Ge substrates, with the highest peak current density ever reported. The voltage drop for a current density equivalent to the operation of the multijunction solar cell up to 10 000 suns is below 5 mV. Trap-assisted tunnelling is proposed to be behind this performance, which cannot be justified by simple band-to-band tunnelling. The metal-organic vapour-phase epitaxy growth conditions, which are in the limits of the transport-limited regime, and the heavy tellurium doping levels are the proposed origins of the defects enabling trap-assisted tunnelling. The hypothesis of trap-assisted tunnelling is supported by the observed annealing behaviour of the tunnel junctions, which cannot be explained in terms of dopant diffusion or passivation. For the integration of these tunnel junctions into a triple-junction solar cell, AlGaAs barrier layers are introduced to suppress the formation of parasitic junctions, but this is found to significantly degrade the performance of the tunnel junctions. However, the annealed tunnel junctions with barrier layers still exhibit a peak current density higher than 2500Acm(-2) and a voltage drop at 10 000 suns of around 20 mV, which are excellent properties for tunnel junctions and mean they can serve as low-loss interconnections in multijunction solar cells working at ultra-high concentrations.

Envelope amplifier based on switching capacitors for high efficiency RF amplifiers

Modern transmitters usually have to amplify and transmit signals with simultaneous envelope and phase modulation. Due to this property of the transmitted signal, linear power amplifiers (class A, B, or AB) are usually used as a solution for the power amplifier stage. These amplifiers have high linearity, but suffer from low efficiency when the transmitted signal has high peak-to-average power ratio. The Kahn envelope elimination and restoration technique is used to enhance the efficiency of RF transmitters, by combining highly efficient, nonlinear RF amplifier (class E) with a highly efficient envelope amplifier in order to obtain a linear and highly efficient RF amplifier. This paper presents a solution for the envelope amplifier based on a multilevel converter in series with a linear regulator. The multilevel converter is implemented by employing voltage dividers based on switching capacitors. The implemented envelope amplifier can reproduce any signal with a maximum spectral component of 2 MHz and give instantaneous maximum power of 50 W. The efficiency measurements show that when the signals with low average value are transmitted, the implemented prototypes have up to 20% higher efficiency than linear regulators used as a conventional solution.

Three-level cell topology for a multilevel power supply to achieve High

This paper presents an envelope amplifier solution for envelope elimination and restoration (EER), that consists of a series combination of a switch-mode power supply (SMPS), based on three-level voltage cells and a linear regulator. This cell topology offers several advantages over a previously presented envelope amplifier based on a different multilevel topology (two-level voltage cells). The topology of the multilevel converter affects to the whole design of the envelope amplifier and a comparison between both design alternatives regarding the size, complexity and the efficiency of the solution is done. Both envelope amplifier solutions have a bandwidth of 2 MHz with an instantaneous maximum power of 50 W. It is also analyzed the linearity of the three-level cell solution, with critical importance in the EER technique implementation. Additionally, considerations to optimize the design of the envelope amplifier and experimental comparison between both cell topologies are included.

IBPOWER Project, Intermediate band materials and solar cells for photovoltaics with high efficiency and reduced cost

IBPOWER is a Project awarded under the 7th European Framework Programme that aims to advance research on intermediate band solar cells (IBSCs). These are solar cells conceived to absorb below bandgap energy photons by means of an electronic energy band that is located within the semiconductor bandgap, whilst producing photocurrent with output voltage still limited by the total semiconductor bandgap. IBPOWER employs two basic strategies for implementing the IBSC concept. The first is based on the use of quantum dots, the IB arising from the confined energy levels of the electrons in the dots. Quantum dots have led to devices that demonstrate the physical operation principles of the IB concept and have allowed identification of the problems to be solved to achieve actual high efficiencies. The second approach is based on the creation of bulk intermediate band materials by the insertion of an appropriate impurity into a bulk semiconductor. Under this approach it is expected that, when inserted at high densities, these impurities will find it difficult to capture electrons by producing a breathing mode and will cease behaving as non-radiative recombination centres. Towards this end the following systems are being investigated: a) Mn: In1-xGax N; b) transition metals in GaAs and c) thin films.

High Efficiency Envelope Amplifier Using The Slow-Envelope Technique

High efficiency envelope amplifiers are demanded in EER technique for RF transmitters, which benefits low maintaining cost or long battery time. The conventional solution is a dc-dc switching converters. This dc-dc converter should operate at very high frequency to track an envelope in the MHz range to supply the power amplifier. One of the alternative circuits suitable for this application is a hybrid topology composed of a switched converter and a linear regulator in series that work together to adjust the output voltage to track the envelope with accuracy. This topology can take advantage of the reduced slew-rate technique (also called slow-envelope technique) where switching dc-dc converter provides the RF envelope with limited slew rate in order to avoid high switching frequency and high power losses, while the linear regulator performs fine adjustment in order to obtain the exact replica of the RF envelope. The combination of this control technique with this topology is proposed in this paper. Envelopes with different bandwidth will be considered to optimize the efficiency of the dc-dc converter.

Short, high current electrodynamic tether

An electrodynamic tether experiment, to be carried out in the Russian spacecraft Almaz, is proposed. A 10 km tether would be deployed downwards; the lower 8 km would be nonconductive, the upper 2 km would be conductive, bare, and 2.2 mm in diameter, and would act as a thruster, with power supply at the top. This hybrid arrangement allows for other, onelectrodynamic experiments,reducing costs; it also limits the induced electromotive force, reducing the power to be handled. The current-voltage characteristic of contactors would be measured. With the anode switched off, the wire itself should collect a current over 5 A at day conditions, providing a thrust of 0.11 N at a 0.77 kW power.

Transformer-coupled converter for voltage modulation techniques

In high performance digital systems as well as in RF systems, voltage scaling and modulation techniques have been adopted to achieve a more efficient processing of the energy. The implementation of such techniques relies on a power supply that is capable of rapidly adjusting the system supply voltage. In this paper, a pulsewidth modulation multiphase topology with magnetic coupling is proposed for its use in voltage modulation techniques. Since the magnetic coupling in this topology is done with transformers instead of coupled inductors, the energy storage is reduced and very fast voltage changes are achieved. Advantages and drawbacks of this topology have been previously presented in the literature and in this paper, the design criteria for implementing a power supply for the envelope elimination and restoration technique in an RF system are presented along with an implementation of the power supply.

Ripple cancellation technique applied to a synchronous buck converter to achieve very high bandwidth and very high efficiency envelope amplifier

This work presents a single stage converter for a high bandwidth and a high efficiency envelope amplifier. The current ripple cancellation technique is applied to a synchronous buck converter to cancel the output current ripple and to decrease the switching frequency without a reduction in the large signal bandwidth. The converter is modeled and the new design with ripple cancellation circuit is detailed. The advantages of the proposed design are presented and validated experimentally. The transfer function of the output filter of the buck converter with ripple cancellation circuit has been modeled and compared to measurements, showing a good correspondence. Experimental validation is provided at 4MHz of switching frequency for DC and variable output voltage for a sinusoidal and a 64QAM signal. Additional experimental validation of the efficiency improvement is provided, compared to the equivalent design (same bandwidth and output voltage ripple) of the conventional buck converter.

High Efficiency Envelope Amplifier based on a Ripple Cancellation Buck Converter. Design, Optimization and Integration in an EER RFPA

Durante los últimos años la tendencia en el sector de las telecomunicaciones ha sido un aumento y diversificación en la transmisión de voz, video y fundamentalmente de datos. Para conseguir alcanzar las tasas de transmisión requeridas, los nuevos estándares de comunicaciones requieren un mayor ancho de banda y tienen un mayor factor de pico, lo cual influye en el bajo rendimiento del amplificador de radiofrecuencia (RFPA). Otro factor que ha influido en el bajo rendimiento es el diseño del amplificador de radiofrecuencia. Tradicionalmente se han utilizado amplificadores lineales por su buen funcionamiento. Sin embargo, debido al elevado factor de pico de las señales transmitidas, el rendimiento de este tipo de amplificadores es bajo. El bajo rendimiento del sistema conlleva desventajas adicionales como el aumento del coste y del tamaño del sistema de refrigeración, como en el caso de una estación base, o como la reducción del tiempo de uso y un mayor calentamiento del equipo para sistemas portátiles alimentados con baterías. Debido a estos factores, se han desarrollado durante las últimas décadas varias soluciones para aumentar el rendimiento del RFPA como la técnica de Outphasing, combinadores de potencia o la técnica de Doherty. Estas soluciones mejoran las prestaciones del RFPA y en algún caso han sido ampliamente utilizados comercialmente como la técnica de Doherty, que alcanza rendimientos hasta del 50% para el sistema completo para anchos de banda de hasta 20MHz. Pese a las mejoras obtenidas con estas soluciones, los mayores rendimientos del sistema se obtienen para soluciones basadas en la modulación de la tensión de alimentación del amplificador de potencia como “Envelope Tracking” o “EER”. La técnica de seguimiento de envolvente o “Envelope Tracking” está basada en la modulación de la tensión de alimentación de un amplificador lineal de potencia para obtener una mejora en el rendimiento en el sistema comparado a una solución con una tensión de alimentación constante. Para la implementación de esta técnica se necesita una etapa adicional, el amplificador de envolvente, que añade complejidad al amplificador de radiofrecuencia. En un amplificador diseñado con esta técnica, se aumentan las pérdidas debido a la etapa adicional que supone el amplificador de envolvente pero a su vez disminuyen las pérdidas en el amplificador de potencia. Si el diseño se optimiza adecuadamente, puede conseguirse un aumento global en el rendimiento del sistema superior al conseguido con las técnicas mencionadas anteriormente. Esta técnica presenta ventajas en el diseño del amplificador de envolvente, ya que el ancho de banda requerido puede ser menor que el ancho de banda de la señal de envolvente si se optimiza adecuadamente el diseño. Adicionalmente, debido a que la sincronización entre la señal de envolvente y de fase no tiene que ser perfecta, el proceso de integración conlleva ciertas ventajas respecto a otras técnicas como EER. La técnica de eliminación y restauración de envolvente, llamada EER o técnica de Kahn está basada en modulación simultánea de la envolvente y la fase de la señal usando un amplificador de potencia conmutado, no lineal y que permite obtener un elevado rendimiento. Esta solución fue propuesta en el año 1952, pero no ha sido implementada con éxito durante muchos años debido a los exigentes requerimientos en cuanto a la sincronización entre fase y envolvente, a las técnicas de control y de corrección de los errores y no linealidades de cada una de las etapas así como de los equipos para poder implementar estas técnicas, que tienen unos requerimientos exigentes en capacidad de cálculo y procesamiento. Dentro del diseño de un RFPA, el amplificador de envolvente tiene una gran importancia debido a su influencia en el rendimiento y ancho de banda del sistema completo. Adicionalmente, la linealidad y la calidad de la señal de transmitida deben ser elevados para poder cumplir con los diferentes estándares de telecomunicaciones. Esta tesis se centra en el amplificador de envolvente y el objetivo principal es el desarrollo de soluciones que permitan el aumento del rendimiento total del sistema a la vez que satisfagan los requerimientos de ancho de banda, calidad de la señal transmitida y de linealidad. Debido al elevado rendimiento que potencialmente puede alcanzarse con la técnica de EER, esta técnica ha sido objeto de análisis y en el estado del arte pueden encontrarse numerosas referencias que analizan el diseño y proponen diversas implementaciones. En una clasificación de alto nivel, podemos agrupar las soluciones propuestas del amplificador de envolvente según estén compuestas de una o múltiples etapas. Las soluciones para el amplificador de envolvente en una configuración multietapa se basan en la combinación de un convertidor conmutado, de elevado rendimiento con un regulador lineal, de alto ancho de banda, en una combinación serie o paralelo. Estas soluciones, debido a la combinación de las características de ambas etapas, proporcionan un buen compromiso entre rendimiento y buen funcionamiento del amplificador de RF. Por otro lado, la complejidad del sistema aumenta debido al mayor número de componentes y de señales de control necesarias y el aumento de rendimiento que se consigue con estas soluciones es limitado. Una configuración en una etapa tiene las ventajas de una mayor simplicidad, pero debido al elevado ancho de banda necesario, la frecuencia de conmutación debe aumentarse en gran medida. Esto implicará un bajo rendimiento y un peor funcionamiento del amplificador de envolvente. En el estado del arte pueden encontrarse diversas soluciones para un amplificador de envolvente en una etapa, como aumentar la frecuencia de conmutación y realizar la implementación en un circuito integrado, que tendrá mejor funcionamiento a altas frecuencias o utilizar técnicas topológicas y/o filtros de orden elevado, que permiten una reducción de la frecuencia de conmutación. En esta tesis se propone de manera original el uso de la técnica de cancelación de rizado, aplicado al convertidor reductor síncrono, para reducir la frecuencia de conmutación comparado con diseño equivalente del convertidor reductor convencional. Adicionalmente se han desarrollado dos variantes topológicas basadas en esta solución para aumentar la robustez y las prestaciones de la misma. Otro punto de interés en el diseño de un RFPA es la dificultad de poder estimar la influencia de los parámetros de diseño del amplificador de envolvente en el amplificador final integrado. En esta tesis se ha abordado este problema y se ha desarrollado una herramienta de diseño que permite obtener las principales figuras de mérito del amplificador integrado para la técnica de EER a partir del diseño del amplificador de envolvente. Mediante el uso de esta herramienta pueden validarse el efecto del ancho de banda, el rizado de tensión de salida o las no linealidades del diseño del amplificador de envolvente para varias modulaciones digitales. Las principales contribuciones originales de esta tesis son las siguientes: La aplicación de la técnica de cancelación de rizado a un convertidor reductor síncrono para un amplificador de envolvente de alto rendimiento para un RFPA linealizado mediante la técnica de EER. Una reducción del 66% en la frecuencia de conmutación, comparado con el reductor convencional equivalente. Esta reducción se ha validado experimentalmente obteniéndose una mejora en el rendimiento de entre el 12.4% y el 16% para las especificaciones de este trabajo. La topología y el diseño del convertidor reductor con dos redes de cancelación de rizado en cascada para mejorar el funcionamiento y robustez de la solución con una red de cancelación. La combinación de un convertidor redactor multifase con la técnica de cancelación de rizado para obtener una topología que proporciona una reducción del cociente entre frecuencia de conmutación y ancho de banda de la señal. El proceso de optimización del control del amplificador de envolvente en lazo cerrado para mejorar el funcionamiento respecto a la solución en lazo abierto del convertidor reductor con red de cancelación de rizado. Una herramienta de simulación para optimizar el proceso de diseño del amplificador de envolvente mediante la estimación de las figuras de mérito del RFPA, implementado mediante EER, basada en el diseño del amplificador de envolvente. La integración y caracterización del amplificador de envolvente basado en un convertidor reductor con red de cancelación de rizado en el transmisor de radiofrecuencia completo consiguiendo un elevado rendimiento, entre 57% y 70.6% para potencias de salida de 14.4W y 40.7W respectivamente. Esta tesis se divide en seis capítulos. El primer capítulo aborda la introducción enfocada en la aplicación, los amplificadores de potencia de radiofrecuencia, así como los principales problemas, retos y soluciones existentes. En el capítulo dos se desarrolla el estado del arte de amplificadores de potencia de RF, describiéndose las principales técnicas de diseño, las causas de no linealidad y las técnicas de optimización. El capítulo tres está centrado en las soluciones propuestas para el amplificador de envolvente. El modo de control se ha abordado en este capítulo y se ha presentado una optimización del diseño en lazo cerrado para el convertidor reductor convencional y para el convertidor reductor con red de cancelación de rizado. El capítulo cuatro se centra en el proceso de diseño del amplificador de envolvente. Se ha desarrollado una herramienta de diseño para evaluar la influencia del amplificador de envolvente en las figuras de mérito del RFPA. En el capítulo cinco se presenta el proceso de integración realizado y las pruebas realizadas para las diversas modulaciones, así como la completa caracterización y análisis del amplificador de RF. El capítulo seis describe las principales conclusiones de la tesis y las líneas futuras. ABSTRACT The trend in the telecommunications sector during the last years follow a high increase in the transmission rate of voice, video and mainly in data. To achieve the required levels of data rates, the new modulation standards demand higher bandwidths and have a higher peak to average power ratio (PAPR). These specifications have a direct impact in the low efficiency of the RFPA. An additional factor for the low efficiency of the RFPA is in the power amplifier design. Traditionally, linear classes have been used for the implementation of the power amplifier as they comply with the technical requirements. However, they have a low efficiency, especially in the operating range of signals with a high PAPR. The low efficiency of the transmitter has additional disadvantages as an increase in the cost and size as the cooling system needs to be increased for a base station and a temperature increase and a lower use time for portable devices. Several solutions have been proposed in the state of the art to improve the efficiency of the transmitter as Outphasing, power combiners or Doherty technique. However, the highest potential of efficiency improvement can be obtained using a modulated power supply for the power amplifier, as in the Envelope Tracking and EER techniques. The Envelope Tracking technique is based on the modulation of the power supply of a linear power amplifier to improve the overall efficiency compared to a fixed voltage supply. In the implementation of this technique an additional stage is needed, the envelope amplifier, that will increase the complexity of the RFPA. However, the efficiency of the linear power amplifier will increase and, if designed properly, the RFPA efficiency will be improved. The advantages of this technique are that the envelope amplifier design does not require such a high bandwidth as the envelope signal and that in the integration process a perfect synchronization between envelope and phase is not required. The Envelope Elimination and Restoration (EER) technique, known also as Kahn’s technique, is based on the simultaneous modulation of envelope and phase using a high efficiency switched power amplifier. This solution has the highest potential in terms of the efficiency improvement but also has the most challenging specifications. This solution, proposed in 1952, has not been successfully implemented until the last two decades due to the high demanding requirements for each of the stages as well as for the highly demanding processing and computation capabilities needed. At the system level, a very precise synchronization is required between the envelope and phase paths to avoid a linearity decrease of the system. Several techniques are used to compensate the non-linear effects in amplitude and phase and to improve the rejection of the out of band noise as predistortion, feedback and feed-forward. In order to obtain a high bandwidth and efficient RFPA using either ET or EER, the envelope amplifier stage will have a critical importance. The requirements for this stage are very demanding in terms of bandwidth, linearity and quality of the transmitted signal. Additionally the efficiency should be as high as possible, as the envelope amplifier has a direct impact in the efficiency of the overall system. This thesis is focused on the envelope amplifier stage and the main objective will be the development of high efficiency envelope amplifier solutions that comply with the requirements of the RFPA application. The design and optimization of an envelope amplifier for a RFPA application is a highly referenced research topic, and many solutions that address the envelope amplifier and the RFPA design and optimization can be found in the state of the art. From a high level classification, multiple and single stage envelope amplifiers can be identified. Envelope amplifiers for EER based on multiple stage architecture combine a linear assisted stage and a switched-mode stage, either in a series or parallel configuration, to achieve a very high performance RFPA. However, the complexity of the system increases and the efficiency improvement is limited. A single-stage envelope amplifier has the advantage of a lower complexity but in order to achieve the required bandwidth the switching frequency has to be highly increased, and therefore the performance and the efficiency are degraded. Several techniques are used to overcome this limitation, as the design of integrated circuits that are capable of switching at very high rates or the use of topological solutions, high order filters or a combination of both to reduce the switching frequency requirements. In this thesis it is originally proposed the use of the ripple cancellation technique, applied to a synchronous buck converter, to reduce the switching frequency requirements compared to a conventional buck converter for an envelope amplifier application. Three original proposals for the envelope amplifier stage, based on the ripple cancellation technique, are presented and one of the solutions has been experimentally validated and integrated in the complete amplifier, showing a high total efficiency increase compared to other solutions of the state of the art. Additionally, the proposed envelope amplifier has been integrated in the complete RFPA achieving a high total efficiency. The design process optimization has also been analyzed in this thesis. Due to the different figures of merit between the envelope amplifier and the complete RFPA it is very difficult to obtain an optimized design for the envelope amplifier. To reduce the design uncertainties, a design tool has been developed to provide an estimation of the RFPA figures of merit based on the design of the envelope amplifier. The main contributions of this thesis are: The application of the ripple cancellation technique to a synchronous buck converter for an envelope amplifier application to achieve a high efficiency and high bandwidth EER RFPA. A 66% reduction of the switching frequency, validated experimentally, compared to the equivalent conventional buck converter. This reduction has been reflected in an improvement in the efficiency between 12.4% and 16%, validated for the specifications of this work. The synchronous buck converter with two cascaded ripple cancellation networks (RCNs) topology and design to improve the robustness and the performance of the envelope amplifier. The combination of a phase-shifted multi-phase buck converter with the ripple cancellation technique to improve the envelope amplifier switching frequency to signal bandwidth ratio. The optimization of the control loop of an envelope amplifier to improve the performance of the open loop design for the conventional and ripple cancellation buck converter. A simulation tool to optimize the envelope amplifier design process. Using the envelope amplifier design as the input data, the main figures of merit of the complete RFPA for an EER application are obtained for several digital modulations. The successful integration of the envelope amplifier based on a RCN buck converter in the complete RFPA obtaining a high efficiency integrated amplifier. The efficiency obtained is between 57% and 70.6% for an output power of 14.4W and 40.7W respectively. The main figures of merit for the different modulations have been characterized and analyzed. This thesis is organized in six chapters. In Chapter 1 is provided an introduction of the RFPA application, where the main problems, challenges and solutions are described. In Chapter 2 the technical background for radiofrequency power amplifiers (RF) is presented. The main techniques to implement an RFPA are described and analyzed. The state of the art techniques to improve performance of the RFPA are identified as well as the main sources of no-linearities for the RFPA. Chapter 3 is focused on the envelope amplifier stage. The three different solutions proposed originally in this thesis for the envelope amplifier are presented and analyzed. The control stage design is analyzed and an optimization is proposed both for the conventional and the RCN buck converter. Chapter 4 is focused in the design and optimization process of the envelope amplifier and a design tool to evaluate the envelope amplifier design impact in the RFPA is presented. Chapter 5 shows the integration process of the complete amplifier. Chapter 6 addresses the main conclusions of the thesis and the future work.

High-Power High-Efficiency Laser Power Transmission at 100m Using Optimized Multi-Cell GaAs Converter

A high-power high-efficiency laser power transmission system at 100m based on an optimized multi-cell GaAs converter capable of supplying 9.7W of electricity is demonstrated. An I-V testing system integrated with a data acquisition circuit and an analysis software is designed to measure the efficiency and the I-V characteristics of the laser power converter (LPC). The dependencies of the converter’s efficiency with respect to wavelength, laser intensity and temperature are analyzed. A diode laser with 793nm of wavelength and 24W of power is used to test the LPC and the software. The maximum efficiency of the LPC is 48.4% at an input laser power of 8W at room temperature. When the input laser power is 24W (laser intensity of 60000W/m2), the efficiency is 40.4% and the output voltage is 4 V. The overall efficiency from electricity to electricity is 11.6%.

Two-photon photocurrent and voltage up-conversion in a quantum dot intermediate band solar cell

It has been proposed that the use of self-assembled quantum dot (QD) arrays can break the Shockley-Queisser efficiency limit by extending the absorption of solar cells into the low-energy photon range while preserving their output voltage. This would be possible if the infrared photons are absorbed in the two sub-bandgap QD transitions simultaneously and the energy of two photons is added up to produce one single electron-hole pair, as described by the intermediate band model. Here, we present an InAs/Al 0.25Ga 0.75As QD solar cell that exhibits such electrical up-conversion of low-energy photons. When the device is monochromatically illuminated with 1.32 eV photons, open-circuit voltages as high as 1.58 V are measured (for a total gap of 1.8 eV). Moreover, the photocurrent produced by illumination with photons exciting the valence band to intermediate band (VB-IB) and the intermediate band to conduction band (IB-CB) transitions can be both spectrally resolved. The first corresponds to the QD inter-band transition and is observable for photons of energy mayor que 1 eV, and the later corresponds to the QD intra-band transition and peaks around 0.5 eV. The voltage up-conversion process reported here for the first time is the key to the use of the low-energy end of the solar spectrum to increase the conversion efficiency, and not only the photocurrent, of single-junction photovoltaic devices. In spite of the low absorption threshold measured in our devices - 0.25 eV - we report open-circuit voltages at room temperature as high as 1.12 V under concentrated broadband illumination.

Technology and characterization of GaN-HEMTs devices: high temperature and trapping effects

Los transistores de alta movilidad electrónica basados en GaN han sido objeto de una extensa investigación ya que tanto el GaN como sus aleaciones presentan unas excelentes propiedades eléctricas (alta movilidad, elevada concentración de portadores y campo eléctrico crítico alto). Aunque recientemente se han incluido en algunas aplicaciones comerciales, su expansión en el mercado está condicionada a la mejora de varios asuntos relacionados con su rendimiento y habilidad. Durante esta tesis se han abordado algunos de estos aspectos relevantes; por ejemplo, la fabricación de enhancement mode HEMTs, su funcionamiento a alta temperatura, el auto calentamiento y el atrapamiento de carga. Los HEMTs normalmente apagado o enhancement mode han atraído la atención de la comunidad científica dedicada al desarrollo de circuitos amplificadores y conmutadores de potencia, ya que su utilización disminuiría significativamente el consumo de potencia; además de requerir solamente una tensión de alimentación negativa, y reducir la complejidad del circuito y su coste. Durante esta tesis se han evaluado varias técnicas utilizadas para la fabricación de estos dispositivos: el ataque húmedo para conseguir el gate-recess en heterostructuras de InAl(Ga)N/GaN; y tratamientos basados en flúor (plasma CF4 e implantación de F) de la zona debajo de la puerta. Se han llevado a cabo ataques húmedos en heteroestructuras de InAl(Ga)N crecidas sobre sustratos de Si, SiC y zafiro. El ataque completo de la barrera se consiguió únicamente en las muestras con sustrato de Si. Por lo tanto, se puede deducir que la velocidad de ataque depende de la densidad de dislocaciones presentes en la estructura, ya que el Si presenta un peor ajuste del parámetro de red con el GaN. En relación a los tratamientos basados en flúor, se ha comprobado que es necesario realizar un recocido térmico después de la fabricación de la puerta para recuperar la heteroestructura de los daños causados durante dichos tratamientos. Además, el estudio de la evolución de la tensión umbral con el tiempo de recocido ha demostrado que en los HEMTs tratados con plasma ésta tiende a valores más negativos al aumentar el tiempo de recocido. Por el contrario, la tensión umbral de los HEMTs implantados se desplaza hacia valores más positivos, lo cual se atribuye a la introducción de iones de flúor a niveles más profundos de la heterostructura. Los transistores fabricados con plasma presentaron mejor funcionamiento en DC a temperatura ambiente que los implantados. Su estudio a alta temperatura ha revelado una reducción del funcionamiento de todos los dispositivos con la temperatura. Los valores iniciales de corriente de drenador y de transconductancia medidos a temperatura ambiente se recuperaron después del ciclo térmico, por lo que se deduce que dichos efectos térmicos son reversibles. Se han estudiado varios aspectos relacionados con el funcionamiento de los HEMTs a diferentes temperaturas. En primer lugar, se han evaluado las prestaciones de dispositivos de AlGaN/GaN sobre sustrato de Si con diferentes caps: GaN, in situ SiN e in situ SiN/GaN, desde 25 K hasta 550 K. Los transistores con in situ SiN presentaron los valores más altos de corriente drenador, transconductancia, y los valores más bajos de resistencia-ON, así como las mejores características en corte. Además, se ha confirmado que dichos dispositivos presentan gran robustez frente al estrés térmico. En segundo lugar, se ha estudiado el funcionamiento de transistores de InAlN/GaN con diferentes diseños y geometrías. Dichos dispositivos presentaron una reducción casi lineal de los parámetros en DC en el rango de temperaturas de 25°C hasta 225°C. Esto se debe principalmente a la dependencia térmica de la movilidad electrónica, y también a la reducción de la drift velocity con la temperatura. Además, los transistores con mayores longitudes de puerta mostraron una mayor reducción de su funcionamiento, lo cual se atribuye a que la drift velocity disminuye más considerablemente con la temperatura cuando el campo eléctrico es pequeño. De manera similar, al aumentar la distancia entre la puerta y el drenador, el funcionamiento del HEMT presentó una mayor reducción con la temperatura. Por lo tanto, se puede deducir que la degradación del funcionamiento de los HEMTs causada por el aumento de la temperatura depende tanto de la longitud de la puerta como de la distancia entre la puerta y el drenador. Por otra parte, la alta densidad de potencia generada en la región activa de estos transistores conlleva el auto calentamiento de los mismos por efecto Joule, lo cual puede degradar su funcionamiento y Habilidad. Durante esta tesis se ha desarrollado un simple método para la determinación de la temperatura del canal basado en medidas eléctricas. La aplicación de dicha técnica junto con la realización de simulaciones electrotérmicas han posibilitado el estudio de varios aspectos relacionados con el autocalentamiento. Por ejemplo, se han evaluado sus efectos en dispositivos sobre Si, SiC, y zafiro. Los transistores sobre SiC han mostrado menores efectos gracias a la mayor conductividad térmica del SiC, lo cual confirma el papel clave que desempeña el sustrato en el autocalentamiento. Se ha observado que la geometría del dispositivo tiene cierta influencia en dichos efectos, destacando que la distribución del calor generado en la zona del canal depende de la distancia entre la puerta y el drenador. Además, se ha demostrado que la temperatura ambiente tiene un considerable impacto en el autocalentamiento, lo que se atribuye principalmente a la dependencia térmica de la conductividad térmica de las capas y sustrato que forman la heterostructura. Por último, se han realizado numerosas medidas en pulsado para estudiar el atrapamiento de carga en HEMTs sobre sustratos de SiC con barreras de AlGaN y de InAlN. Los resultados obtenidos en los transistores con barrera de AlGaN han presentado una disminución de la corriente de drenador y de la transconductancia sin mostrar un cambio en la tensión umbral. Por lo tanto, se puede deducir que la posible localización de las trampas es la región de acceso entre la puerta y el drenador. Por el contrario, la reducción de la corriente de drenador observada en los dispositivos con barrera de InAlN llevaba asociado un cambio significativo en la tensión umbral, lo que implica la existencia de trampas situadas en la zona debajo de la puerta. Además, el significativo aumento del valor de la resistencia-ON y la degradación de la transconductancia revelan la presencia de trampas en la zona de acceso entre la puerta y el drenador. La evaluación de los efectos del atrapamiento de carga en dispositivos con diferentes geometrías ha demostrado que dichos efectos son menos notables en aquellos transistores con mayor longitud de puerta o mayor distancia entre puerta y drenador. Esta dependencia con la geometría se puede explicar considerando que la longitud y densidad de trampas de la puerta virtual son independientes de las dimensiones del dispositivo. Finalmente se puede deducir que para conseguir el diseño óptimo durante la fase de diseño no sólo hay que tener en cuenta la aplicación final sino también la influencia que tiene la geometría en los diferentes aspectos estudiados (funcionamiento a alta temperatura, autocalentamiento, y atrapamiento de carga). ABSTRACT GaN-based high electron mobility transistors have been under extensive research due to the excellent electrical properties of GaN and its related alloys (high carrier concentration, high mobility, and high critical electric field). Although these devices have been recently included in commercial applications, some performance and reliability issues need to be addressed for their expansion in the market. Some of these relevant aspects have been studied during this thesis; for instance, the fabrication of enhancement mode HEMTs, the device performance at high temperature, the self-heating and the charge trapping. Enhancement mode HEMTs have become more attractive mainly because their use leads to a significant reduction of the power consumption during the stand-by state. Moreover, they enable the fabrication of simpler power amplifier circuits and high-power switches because they allow the elimination of negativepolarity voltage supply, reducing significantly the circuit complexity and system cost. In this thesis, different techniques for the fabrication of these devices have been assessed: wet-etching for achieving the gate-recess in InAl(Ga)N/GaN devices and two different fluorine-based treatments (CF4 plasma and F implantation). Regarding the wet-etching, experiments have been carried out in InAl(Ga)N/GaN grown on different substrates: Si, sapphire, and SiC. The total recess of the barrier was achieved after 3 min of etching in devices grown on Si substrate. This suggests that the etch rate can critically depend on the dislocations present in the structure, since the Si exhibits the highest mismatch to GaN. Concerning the fluorine-based treatments, a post-gate thermal annealing was required to recover the damages caused to the structure during the fluorine-treatments. The study of the threshold voltage as a function of this annealing time has revealed that in the case of the plasma-treated devices it become more negative with the time increase. On the contrary, the threshold voltage of implanted HEMTs showed a positive shift when the annealing time was increased, which is attributed to the deep F implantation profile. Plasma-treated HEMTs have exhibited better DC performance at room temperature than the implanted devices. Their study at high temperature has revealed that their performance decreases with temperature. The initial performance measured at room temperature was recovered after the thermal cycle regardless of the fluorine treatment; therefore, the thermal effects were reversible. Thermal issues related to the device performance at different temperature have been addressed. Firstly, AlGaN/GaN HEMTs grown on Si substrate with different cap layers: GaN, in situ SiN, or in situ SiN/GaN, have been assessed from 25 K to 550 K. In situ SiN cap layer has been demonstrated to improve the device performance since HEMTs with this cap layer have exhibited the highest drain current and transconductance values, the lowest on-resistance, as well as the best off-state characteristics. Moreover, the evaluation of thermal stress impact on the device performance has confirmed the robustness of devices with in situ cap. Secondly, the high temperature performance of InAlN/GaN HEMTs with different layouts and geometries have been assessed. The devices under study have exhibited an almost linear reduction of the main DC parameters operating in a temperature range from room temperature to 225°C. This was mainly due to the thermal dependence of the electron mobility, and secondly to the drift velocity decrease with temperature. Moreover, HEMTs with large gate length values have exhibited a great reduction of the device performance. This was attributed to the greater decrease of the drift velocity for low electric fields. Similarly, the increase of the gate-to-drain distance led to a greater reduction of drain current and transconductance values. Therefore, this thermal performance degradation has been found to be dependent on both the gate length and the gate-to-drain distance. It was observed that the very high power density in the active region of these transistors leads to Joule self-heating, resulting in an increase of the device temperature, which can degrade the device performance and reliability. A simple electrical method have been developed during this work to determine the channel temperature. Furthermore, the application of this technique together with the performance of electro-thermal simulations have enabled the evaluation of different aspects related to the self-heating. For instance, the influence of the substrate have been confirmed by the study of devices grown on Si, SiC, and Sapphire. HEMTs grown on SiC substrate have been confirmed to exhibit the lowest self-heating effects thanks to its highest thermal conductivity. In addition to this, the distribution of the generated heat in the channel has been demonstrated to be dependent on the gate-to-drain distance. Besides the substrate and the geometry of the device, the ambient temperature has also been found to be relevant for the self-heating effects, mainly due to the temperature-dependent thermal conductivity of the layers and the substrate. Trapping effects have been evaluated by means of pulsed measurements in AlGaN and InAIN barrier devices. AlGaN barrier HEMTs have exhibited a de crease in drain current and transconductance without measurable threshold voltage change, suggesting the location of the traps in the gate-to-drain access region. On the contrary, InAIN barrier devices have showed a drain current associated with a positive shift of threshold voltage, which indicated that the traps were possibly located under the gate region. Moreover, a significant increase of the ON-resistance as well as a transconductance reduction were observed, revealing the presence of traps on the gate-drain access region. On the other hand, the assessment of devices with different geometries have demonstrated that the trapping effects are more noticeable in devices with either short gate length or the gate-to-drain distance. This can be attributed to the fact that the length and the trap density of the virtual gate are independent on the device geometry. Finally, it can be deduced that besides the final application requirements, the influence of the device geometry on the performance at high temperature, on the self-heating, as well as on the trapping effects need to be taken into account during the device design stage to achieve the optimal layout.

AlGaAs/GaAs photovoltaic converters for high power narrowband radiation

AlGaAs/GaAs-based laser power PV converters intended for operation with high-power (up to 100 W/cm(2)) radiation were fabricated by LPE and MOCVD techniques. Monochromatic (lambda = 809 nm) conversion efficiency up to 60% was measured at cells with back surface field and low (x = 0.2) Al concentration 'window'. Modules with a voltage of 4 V and the efficiency of 56% were designed and fabricated.

Power Converter Topologies for a High Performance Transformer Rectifier Unit in Aircraft Applications

This paper presents some power converter architectures and circuit topologies, which can be used to achieve the requirements of the high performance transformer rectifier unit in aircraft applications, mainly as: high power factor with low THD, high efficiency and high power density. The voltage and the power levels demanded for this application are: three-phase line-to-neutral input voltage of 115 or 230V AC rms (360 – 800Hz), output voltage of 28V DC or 270V DC(new grid value) and the output power up to tens of kilowatts.