Gallium Oxide (Ga2O3) has emerged over the last decade as a new up-and-coming alternative to traditional wide bandgap semiconductors. It exists as five polymorphs (α-, β-, γ-, δ-, and ε-Ga2O3), of which β-Ga2O3 is the thermodynamically stable form, and the most extensively studied phase. β-Ga2O3 has a wide bandgap of ~4.8 eV and exhibits a superior figure-of-merit for power devices compared to other wide bandgap materials, such as SiC and GaN. These make β-Ga2O3 a promising candidate in a host of electronic and optoelectronic applications. Recent advances in β-Ga2O3 single crystals growth have also made inexpensive β-Ga2O3 single crystal grown from the melt a possibility in the near future. Despite the plethora of literature on β-Ga2O3-based devices, understanding of contacts to this material --- a device component that fundamentally determines device characteristics — remained lacking. For this research, ohmic and Schottky metal contacts to Sn-doped β-Ga2O3 (-201) single crystal substrates, unintentionally doped (UID) homoepitaxial β-Ga2O3 (010) on Sn-doped β-Ga2O3 grown by molecular beam epitaxy (MBE), and UID heteroepitaxial β-Ga2O3 (-201) epitaxial layers on c-plane sapphire by metal-organic chemical vapor deposition (MOCVD) were investigated. Each of the substrates was characterized for their structural, morphological, electrical, and optical properties, the results will be presented in the following document. Nine metals (Ti, In, Ag, Sn, W, Mo, Sc, Zn, and Zr) with low to moderate work functions were studied as possible ohmic contacts to β-Ga2O3. It was found that select metals displayed either ohmic (Ti and In) or pseudo-ohmic (Ag, Sn and Zr) behavior under certain conditions. However, the morphology was often a problem as many thin film metal contacts dewetted the substrate surface. Ti with a Au capping layer with post-metallization annealing treatment was the only consistently reliable ohmic contact to β-Ga2O3. It was concluded that metal work function is not a dominant factor in forming an ohmic contact to β-Ga2O3 and that limited interfacial reactions appear to play an important role. Prior to a systematic study of Schottky contacts to β-Ga2O3, a comparison of the effects of five different wet chemical surface treatments on the β-Ga2O3 Schottky diodes was made. It was established that a treatment with an organic solvent clean followed by HCl, H2O2 and a deionized water rinse following each step yielded the best results. Schottky diodes based on (-201) β-Ga2O3 substrates and (010) β-Ga2O3 homoepitaxial layers were formed using five different Schottky metals with moderate to high work functions: W, Cu, Ni, Ir, and Pt. Schottky barrier heights (SBHs) calculated from current-voltage (I-V) and capacitance-voltage (C-V) measurements of the five selected metals were typically in the range of 1.0 – 1.3 eV and 1.6 – 2.0 eV, respectively, and showed little dependence on the metal work function. Several diodes also displayed inhomogeneous Schottky barrier behavior at room temperature. The results indicate that bulk or near-surface defects and/or unpassivated surface states may have a more dominant effect on the electrical behavior of these diodes compared to the choice of Schottky metal and its work function. Lastly, working with collaborators at Structured Materials Industries (SMI) Inc., heteroepitaxial films of Ga2O3 were grown on c-plane sapphire (001) using a variety of vapor phase epitaxy methods, including MOVPE, and halide vapor phase epitaxy (HVPE). The stable phase β-Ga2O3 was observed when grown using MOVPE technique, regardless of precursor flow rates, at temperatures ranging between 500 – 850 °C. With HVPE growth techniques, instead of the stable β-phase, we observed the growth of the metastable α- and ε-phases, often a combination of the two. Cross-sectional transmission electron microscopy (TEM) shows the better lattice matched α-phase first growing semi-coherently on the c-plane sapphire substrate, followed by domain matched epitaxy of ε-Ga2O3 on top. Secondary ion mass spectrometry (SIMS) revealed that epilayers forming the ε-phase contain higher concentrations of chlorine, which suggests that compressive stress due to Cl- impurities may play a role in the growth of ε-Ga2O3 despite it being less than thermodynamically favorable.

En frekvensselektiv yta har en förmåga att välja och separera ut olika frekvenser. Inom bland annat kommunikationsteknik finns det ett intresse för dessa ytor. Detta examensarbete har fört ihop textil med denna tekniska applikation. Stela konstruktioner har översatts till textila konstruktioner med typiska textila egenskaper såsom böjlighet och elasticitet. Tidigare forskning har visat att det är möjligt att skapa textila ytor med frekvensselektiva egenskaper. Denna studie har spunnit vidare på detta och inbegriper ett flertal metoder där de textila materialens egenskaper och konstruktion nyttjas. Syftet har varit att undersöka hur textil kan användas i frekvensselektiva ytor och ifall textiltypiska egenskaper kan ge dessa ytor funktioner som inte annars är möjliga. Den teoretiska referensramen beskriver det elektromagnetiska fältet och de frekvenserna inom mikrovågsområdet vilket mätning utförs på samt mer ingående om frekvensselektiva ytors funktion. Både vävda och stickade prover har skapats och dessa har undersökts utifrån hur töjning, metalltäthet samt en multilagereffekt påverkar de frekvensselektiva egenskaperna. Detta har gjorts med elastiska vävkonstruktioner och uppbyggnad av ledande material med hjälp av en i tjockare och tätare bindning men också stapling av flera ytor ovanpå varandra. Resultatet presenteras i grafer av de mätningar som utförts och styrker slutsatsen som säger att det är möjligt att modifiera frekvensselektiviteten med hjälp av textila egenskaper. / Program: Textilingenjörsutbildningen

Neste trabalho, utilizamos o desenvolvimento recente do método DFT/LDA-1/2 para cálculos de estados excitados em materiais. Começamos com um resumo da teoria do funcional da densidade (DFT) e incluímos uma introdução ao método LDA-1/2 para cálculos de excitações em sólidos. Na compilação dos resultados esperamos ter demonstrado a utilidade do LDA-1/2 para cálculos de alinhamentos de bandas em junções semicondutor/semicondutor e semicondutor/isolante. A aplicação do método envolve o conhecimento da química básica dos sistemas. Para tanto, escolhemos sistemas importantes para diversas aplicações, e cujos modelos de simulação estão o limite ou fora do alcance de metodologias que envolvem alto custo computacional, mas que foram bem caracterizados experimentalmente. Concentramos nossas ações no estudo da capacidade preditiva do LDA-1/2 para alinhamentos de bandas, os chamados band offsets, particularmente importantes para a micro e optoeletrônica. Quando não foi possível compararmos nossos resultados com o experimento, procuramos a comparação com métodos estado-da-arte como GW. Bons resultados foram obtidos para band gaps e band offsets de interfaces A1As/GaAs, Si/SiO2, A1N/GaN e CdSe/CdTe, que representam os diferentes tipos de jun_c~oes poss__veis, com (e.g. A1As/GaAs, A1N/GaN) e sem (e.g. Si/SiO2, CdSe/CdTe) ^anions omuns, com (e.g. A1As/GaAs) e sem (e.g. CdSe/CdTe, Si/SiO2) casamento de parâmetros de rede e diferentes tipos de alinhamentos (\"straddling\", e.g. A1As/GaAs ou \"staggered\"e.g. CdSe/CdTe). Analisamos de maneira sistemática o comportamento do entorno do bandgap ao longo da interface, verificando plano a plano atômico o comportamento das bordas de valência e condução com LDA-1/2 em comparação com o LDA, ou comparando diferentes modelos dentro do LDA-1/2, como o caso do CdSe/CdTe e do Si/SiO2. Para o caso A1As/GaAs, aproveitamos o casamento de parâmetros de rede dos semicondutores constituintes e tentamos um modelo de interface de ligas A1xGa1-x As/GaAs para estudar a variação de valência, condução e bandgap em função da composição x. No AlN/GaN, estudamos também os offsets com as contribuições dos orbitais separadamente. Em todos os casos o LDA-1/2 levou-nos a resultados interessantes com modelos simples. A exploração de novas fronteiras de aplicação do método fez-se necessária com a diminuição da dimensionalidade dos sistemas, de 3D (bulk ) para 2D (interfaces) e depois para 1D, ou seja, _os quânticos (\"nanofios\"). Nosso material de estudo para os foi o ZnO que, além da motivação oriunda de conhecidas aplicações em optoeletrônica, apresenta desafios para simulações bulk com qualquer método, e que foi abordado com certo sucesso usando o LDA-1/2 anteriormente, sendo que para fios quânticos encontramos resultados interessantes em geometrias triangulares que facilitaram os modelos. Calculamos o bandgap ZnO bulk e de nanofios passivados e não passivados com hidrogênios usando LDA e LDA-1/2 sem polarização de spin. As estruturas de bandas e o bandgap como função do diâmetro do ano_o foram calculados e ajustes com funções de decaimento foram feitos para comparação, por extrapolação, dos bandgaps com valores experimentais. Foi possível comparar nossos resultados de fios com o bulk, e predizer uma faixa de variaação de bandgaps que os experimentais podem encontrar para nanofios triangulares de ZnO. Também foi feita análise de energias de confinamento em fios quânticos de ZnO, comparando o LDA com LDA-1/2. Finalmente, mostramos os resultados de uma oportunidade de aplicação do método a um material com defeitos, recentemente descoberto e promissor, e com enorme mercado potencial em fotocatálise, o Ti1-O4N. Nosso trabalho envolveu a aplicação do LDA-1/2 a um problema muito desafiador, e.g. a geração de energia limpa, especificamente a separação da molécula de água para produção de hidrogênio. O desafio maior vem da dificuldade de predição de bandgaps teoricamente, em particular para sistemas grandes como é o caso de modelos atomísticos com defeitos, devido aos altos custos computacionais envolvidos. Tais dificuldades forçam os pesquisadores a usarem parâmetros ajustáveis ou métodos semi-empíricos, ou modelos simplificados demais para descrever precisamente resultados experimentais. Isto dificulta o estudo dos sistemas fotocatalíticos potencialmente eficientes e que não foram ainda caracterizados ou otimizados. O LDA-1/2 é aqui validado para esta classe de materiais, abrindo assim a oportunidade para estudar sistemas mais realísticos e complexos para cálculos ainda mais precisos, particularmente para geração de energia limpa. Em particular, modelamos o TiO2 na estrutura rutile com nitrogênio substitucional, cuja estrutura eletrônica é ainda debatida. Foi a primeira aplicação do LDA-1/2 a sistemas com algum tipo de defeito, com ótimos resultados para o novo sistema Ti1- _O4N com vacâncias de Ti. / In this work, we used the recent development of DFT/LDA-1/2 method for calculations of excited states in materials. We begin with a summary of the density functional theory (DFT) and included an introduction to the method LDA-1/2 for calculations of excitations in solids. In compiling the results we hope to have demonstrated the usefulness of the LDA-1/2 for calculating alignments of bands at junctions semiconductor / semiconductor and semiconductor / insulator. The method involves the knowledge of basic chemical systems. To do this we chose systems important for several applications, and simulation models which are the limit or beyond the reach of methodologies involving high computational cost, but have been well characterized experimentally. We focus our actions in the study of the predictive capability of the LDA-1/2 for alignments of bands, the band called offsets, particularly important for micro and optoelectronics. When it was not possible to compare our results with experiment, we compared the methods with state of the art as GW. Good results were obtained for band gaps and band offsets of interfaces A1As/GaAs, Si/SiO2, A1N/GaN and CdSe / CdTe, which represent the different types of jun_c poss__veis-tions, with (eg A1As/GaAs, A1N/GaN) and without (eg Si/SiO2, CdSe / CdTe) ^ omuns anions with (eg A1As/GaAs) and without (eg CdSe / CdTe, Si/SiO2) matching network parameters and different types of alignments (\"straddling\" eg A1As/GaAs or \"staggered\" eg CdSe / CdTe). Systematically analyze the behavior of the environment along the interface bandgap, plane by plane scanning behavior of the edges atomic valence and conduction with LDA-half in comparison with LDA, or comparing templates within the LDA-1 / 2, as the case of CdSe / CdTe and Si/SiO2. For the case A1As/GaAs, we take the marriage of network parameters of semiconductor components and try an interface model alloys A1xGa1-x As / GaAs to study the variation of valence, conduction and bandgap as a function of composition x. In the AlN / GaN, we also studied the offsets with the contributions of the orbitals separately. In all cases the LDA-half led us to interesting results from simple models. The exploration of new frontiers of the method was necessary to decrease the dimensionality of the systems, the 3D (bulk) for 2D (interfaces) and then to 1D, ie, quantum _os (\"nanowires\"). Our study material for the ZnO was that, apart from the motivation coming from known applications in optoelectronics, presents challenges for bulk simulations with any method, and that was addressed with some success using the LDA-half earlier, and for wireless find interesting results in quantum triangular geometries that facilitated models. We calculate the bandgap and bulk ZnO nanowires passivated and not passivated with hydrogen using LDA and LDA-1/2 without spin polarization. The bandgap structures and strips as a function of the diameter of ano_o adjustments are calculated and decay functions for comparison were made by extrapolation of the bandgaps with experimental values. It was possible to compare our results with the bulk of wires, and predict a range of bandgaps that variaação can find experimental triangular ZnO nanowires. It was also made analysis of energy confinement in ZnO quantum wires, comparing LDA with LDA-1/2. Finally, we show the results of an opportunity to apply the method to a material with defects, newly discovered and promising, and with huge market potential in photocatalysis, the Ti1-O4N. Our work involved the application of LDA-1/2 to a very challenging problem, eg the generation of clean energy, specifically the separation of the water molecule for hydrogen production. The main challenge has been the difficulty of predicting bandgaps theoretically, in particular for large systems such as the model atomistic defects because of the high computational costs involved. These difficulties force the researchers to use adjustable parameters or semi-empirical methods, or other simplified models to accurately describe experimental results. This complicates the study of potentially efficient photocatalytic systems which have not yet been characterized or optimized. The LDA-1/2 is here validated for this class of materials, thus opening the opportunity to study more realistic and complex systems for more accurate calculations, particularly for clean energy generation. In particular, we modeled the structure of TiO2 in the rutile with substitutional nitrogen, whose electronic structure is still debated. It was the first application of the LDA-1/2 systems with some kind of defect, with excellent results for the new system Ti1-_O4N with Ti vacancies.

L'évolution des composants d'électronique de puissance se heurte aujourd'hui aux limites physiques du silicium. L'utilisation des semi-conducteurs à large bande interdite permettraient de dépasser ces limites. Parmi ces nouveaux matériaux, le diamant possède les propriétés les plus intéressantes pour l'électronique de puissance : champ de rupture et conductivité thermique les plus élevés parmi les solides, grandes mobilités des porteurs électriques, possibilité de fonctionnement à haute température. Les substrats de diamant synthétisés actuellement par des méthodes de dépôt en phase vapeur ont des caractéristiques cristallographiques compatibles avec l'exploitation de ces propriétés en électronique de puissance. L'utilisation technologique du diamant reste toutefois difficile ; ses propriétés de dureté et d'inertie chimique rendent son utilisation délicate. L'objet de ces travaux est dans un premier temps d'évaluer les bénéfices que pourrait apporter le diamant en électronique de puissance. Ensuite, différentes étapes technologiques nécessaires à la fabrication de composants sur diamant sont étudiées : dépôts de contacts électriques, dopage et gravure ionique. Enfin, une étude sur la fabrication de diodes Schottky est présentée. Les résultats obtenus permettent d'établir les perspectives à ces travaux et les challenges scientifiques et technologiques qu'il reste à relever. / The evolution of power electronic devices is getting more and more limited by the silicon intrinsic properties. This limitation could be overcome by using wide bandgap semiconductors. Among these materials, diamond properties are the more fitted for power electronics: the highest critical electric field and thermal conductivity amongst the solids, high carriers mobility, high temperature operation possibility. At this time, diamond samples grown by chemical vapour deposition methods exhibit crystallographic properties that are suitable for a use in power electronics. Though, the realization of diamond power devices remains difficult due to its hardness and chemical inertness, among others. First, this work aims at determining the profit that could represent diamond for power electronics. Second, different technologic steps that are necessary to the realisation of electronic devices are studied: ohmic contacts deposition, doping and ion etching. Finally, the first devices we realised, Schottky diodes, are presented. Their characterisation allows establishing new objectives for the future developments of our studies.

abstract: Photovoltaic (PV) energy has shown tremendous improvements in the past few decades showing great promises for future sustainable energy sources. Among all PV energy sources, III-V-based solar cells have demonstrated the highest efficiencies. This dissertation investigates the two different III-V solar cells with low (III-antimonide) and high (III-nitride) bandgaps. III-antimonide semiconductors, particularly aluminum (indium) gallium antimonide alloys, with relatively low bandgaps, are promising candidates for the absorption of long wavelength photons and thermophotovoltaic applications. GaSb and its alloys can be grown metamorphically on non-native substrates such as GaAs allowing for the understanding of different multijunction solar cell designs. The work in this dissertation presents the molecular beam epitaxy growth, crystal quality, and device performance of AlxGa1−xSb solar cells grown on GaAs substrates. The motivation is on the optimization of the growth of AlxGa1−xSb on GaAs (001) substrates to decrease the threading dislocation density resulting from the significant lattice mismatch between GaSb and GaAs. GaSb, Al0.15Ga0.85Sb, and Al0.5Ga0.5Sb cells grown on GaAs substrates demonstrate open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. In addition, a detailed study is presented to demonstrate the temperature dependence of (Al)GaSb PV cells. III-nitride semiconductors are promising candidates for high-efficiency solar cells due to their inherent properties and pre-existing infrastructures that can be used as a leverage to improve future nitride-based solar cells. However, to unleash the full potential of III-nitride alloys for PV and PV-thermal (PVT) applications, significant progress in growth, design, and device fabrication are required. In this dissertation, first, the performance of ii InGaN solar cells designed for high temperature application (such as PVT) are presented showing robust cell performance up to 600 ⁰C with no significant degradation. In the final section, extremely low-resistance GaN-based tunnel junctions with different structures are demonstrated showing highly efficient tunneling characteristics with negative differential resistance (NDR). To improve the efficiency of optoelectronic devices such as UV emitters the first AlGaN tunnel diode with Zener characteristic is presented. Finally, enabled by GaN tunnel junction, the first tunnel contacted InGaN solar cell with a high VOC value of 2.22 V is demonstrated. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

High efficiency solar cells can be attained by the development of two junctions one stacked on top of each other into tandem structures. So that, if a photon is not able to excite an electron-hole pair in the top cell can create a pair in the bottom cell, which has a smaller bandgap. For a two junction tandem device structure, the bandgap of the top cell should be 1.6-1.8eV and for the bottom cell should be 1eV to attain efficiencies in the range of 25%. Cadmium Zinc Telluride which has a tunable bandgap of 1.45- 2.2eV is a candidate for the top cell of the tandem structure. Cadmium Zinc Telluride (Cd1-xZnxTe) films were deposited by co-sputtering of CdTe and ZnTe. Deposition of Cd1-xZnxTe was studied in Ar and Ar/N2 ambient. Characterization of the films was done using transmission response, X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Secondary Electron Microscopy (SEM), current-voltage (I-V) and spectral response measurements. CZT deposited on CdS/SnO2 substrates showed improved performance compared to other heterojunction partners. Doped graphite and copper were utilized as back contacts for CZT devices. Post deposition annealing treatments with ZnCl2 on CZT films were done and their effect on the devices was also studied. The best combination of Voc and Jsc were 530mV and 3.66mA/cm² respectively.

The present thesis is a study of the evolution of defect states in devices based on wide bandgap semiconductors. The attention has been focused on light-emitting diodes based on GaN and Schottky diodes based on SiC, these latter a basic structure for the fabrication of high-power rectifiers and ionising particle detectors. In both cases, we studied the defects and their electronic properties by means of the following experimental techniques: current-voltage (I-V) measurements, in order to investigate the effect of imperfections on the transport properties of the material/device; capacitance-voltage (C-V) measurements, yielding the profile of concentration of charge carriers, and giving information on the influence of defects on this concentration; deep level transient spectroscopy (DLTS), a technique allowing for the identification and characterization of defect-originated electron levels in the gap. I also employed techniques, such as photocurrent spectroscopy (PC), allowing for the characterization of light absorption by the material and/or device versus varying photon energy. In both cases of SiC and GaN, the defect characterization was always interpreted in the framework of its influence on device operation. In the analysed LEDs the defect evolution was connected to the evolution of quantum efficiency, and in the SiC diodes we studied the effects of defect introduction on the charge collection efficiency (CCE) and on the leakage current of the device. Furthermore, for the interpretation of photocurrent spectra, I developed a model describing the generation of photocurrent considering the dispersion relations for the absorption coefficient and refractive index in the various device layers, as well as the internal reflection, transmission and interference phenomena involving the optical field within the device. The research yielded various interesting results: I detected many deep levels introduced by proton- and electron-irradiation in SiC. From the study of their annealing behaviour I concluded that one of these levels is related to a particular lattice defect, the carbon interstitial. By means of the analysis of the introduction rates of the levels and comparisons between proton and electron irradiation, I was able to distinguish between deep levels related to simple intrinsic defects and to defect complexes. In the case of the GaN LED, I found that the evolution of several independent properties are strongly correlated, meaning that a single degradation mechanism is responsible for the observed changes. In particular, I concluded that the degradation of the light emission intensity is due to the generation of defects in the active region of the device.

<p>Many analog systems need a stable power supply voltage that does not vary with temperature and time in order to operate properly. In a battery operated system the battery voltage is not stable, e.g. it decreases with decreasing temperature and with ageing. In that case a voltage regulator must be used, that regulates the battery voltage and generates a stable supply voltage to power other circuitry. </p><p>In this thesis a voltage regulator to be used in a battery operated system has been designed which meets the given specification of stability and power capabilities. A voltage reference, which is a commonly used devise in analog circuits, was also designed. The role of a reference voltage in an electrical system is the same as for a tuning fork in a musical ensemble; to set a standard to which other voltages are compared. </p><p>A functionality to detect when the lifetime of the battery is about to run out was also developed.</p>

SiC MESFETs and GaN HEMTs have an enormous potential in high-power amplifiers at microwave frequencies due to their wide bandgap features of high electric breakdown field strength, high electron saturation velocity and high operating temperature. The high power density combined with the comparably high impedance attainable by these devices also offers new possibilities for wideband power microwave systems. In this thesis, Class C switching response of SiC MESFET in TCAD and two different generations of broadband power amplifiers have been designed, fabricated and characterized. Input and output matching networks and shunt feedback topology based on microstrip and lumped components have been designed, to increase the bandwidth and to improve the stability. The first amplifier is a single stage 26-watt using a SiC MESFET covering the frequency from 200-500 MHz is designed and fabricated. Typical results at 50 V drain bias for the whole band are, 22 dB power gain, 43 dBm output power, minimum power added efficiency at P 1dB is 47 % at 200 MHz and maximum 60 % at 500 MHz and the IMD3 level at 10 dB back-off from P 1dB is below ‑45 dBc. The results at 60 V drain bias at 500 MHz are, 24.9 dB power gain, 44.15 dBm output power (26 W) and 66 % PAE. In the second phase, two power amplifiers at 0.7-1.8 GHz without feed back for SiC MESFET and with feedback for GaN HEMT are designed and fabricated (both these transistors were of 10 W). The measured maximum output power for the SiC amplifier at Vd = 48 V was 41.3 dBm (~13.7 W), with a PAE of 32 % and a power gain above 10 dB. At a drain bias of Vd= 66 V at 700 MHz the Pmax was 42.2 dBm (~16.6 W) with a PAE of 34.4 %. The measured results for GaN amplifier are; maximum output power at Vd = 48 V is 40 dBm (~10 W), with a PAE of 34 % and a power gain above 10 dB. The SiC amplifier gives better results than for GaN amplifier for the same 10 W transistor. A comparison between the physical simulations and measured device characteristics has also been carried out. A novel and efficient way to extend the physical simulations to large signal high frequency domain was developed in our group, is further extended to study the class-C switching response of the devices. By the extended technique the switching losses, power density and PAE in the dynamics of the SiC MESFET transistor at four different frequencies of 500 MHz, 1, 2 and 3 GHz during large signal operation and the source of switching losses in the device structure was investigated. The results obtained at 500 MHz are, PAE of 78.3%, a power density of 2.5 W/mm with a switching loss of 0.69 W/mm. Typical results at 3 GHz are, PAE of 53.4 %, a power density of 1.7 W/mm with a switching loss of 1.52 W/mm. / Report code: LIU-TEK-LIC-2008:32

Current trends in technology are moving to increased use of wireless communication with rapidly increasing data transmission rates and higher frequencies. Miniaturization is essential to allow electronics of increasing complexity to fit into smaller devices. Adaptive technologies allow a single system to operate across multiple wireless protocols, adjusting to changing conditions to minimize interference and enhance performance. Flexibility is essential as the use of wireless technology increases and spreads to new industries. The objective of this research is twofold: to develop novel reconfigurable electromagnetic structures and a novel process to fabricate microelectromechanical systems (MEMS) devices on flexible substrates. The novel electromagnetic structures are passive frequency-switchable parasitic antennas, conformal MEMS-tunable frequency selective surfaces (FSS) and MEMS-tunable electromagnetic bandgap (EBG) structures. Fabricating the reconfigurable conformal FSS and EBG structures requires the development of a new fabrication process to produce MEMS devices monolithically integrated onto a flexible substrate. Novel frequency-switchable parasitic antenna arrays are developed, fabricated and measured. The structure radiates efficiently when placed over metal and absorbing material, improving the range of conventional RFID systems, as well as minimizing blind spots to provide continuous coverage in a hemisphere. A novel analysis method is developed to characterize frequency-switchable parasitic patch arrays. The purpose of the analysis is to provide an approximation of the input impedance and variation of the radiation pattern with frequency. The analysis combines models based on electromagnetic theory and circuit theory to provide a fast and yet reasonable approximation of the parasitic array characteristics. The analysis also provides a good deal of physical insight into the operation of multi-mode parasitic patch arrays. The end result is an initial array design which provides a good starting point for full EM simulation and optimization. The new analysis method is validated alongside measured and simulated results, with good correlation for both impedance characteristics and far-field radiation patterns. A MEMS-based switched parasitic antenna array is designed, fabricated and measured with good correlation between simulated and measured results. The structure is a direct-coupled parasitic patch array which is capable of frequency steering and has additional MEMS-enabled beam-steering capabilities at each frequency. An EBG-based multi-mode radiating structure design is presented, which is capable of frequency-switchable beam steering. The antenna area is significantly reduced compared to the parasitic patch array structure, but at a considerable cost in terms of gain and efficiency. A novel MEMS process is developed to fabricate large numbers of high-performance MEMS devices monolithically integrated onto a rigid-flex organic substrate using low-temperature processes. The rigid-flex substrate is all dielectric, which is amenable to low-loss electromagnetic structures. The substrate provides mechanical support to the MEMS devices while maintaining overall flexibility. The adaptation of each fabrication process step to handle flexible substrates is analyzed and documented in detail. The newly-developed MEMS process is used to fabricate a MEMS reconfigurable frequency-selective surface. A practical bias network is incorporated into the structure design to ensure that all devices are actuated simultaneously. FSS structures operating in the Ku and Ka bands are fabricated and tested, with good correlation between simulated and measured results for individual devices as well as the entire FSS structures. The newly-developed MEMS process is also used to fabricate a MEMS reconfigurable electromagnetic bandgap structure. An EBG structure operating in the Ka band is fabricated and tested to verify the validity of the proposed concept.