abstract: The drive towards device scaling and large output power in millimeter and sub-millimeter wave power amplifiers results in a highly non-linear, out-of-equilibrium charge transport regime. Particle-based Full Band Monte Carlo device simulators allow an accurate description of this carrier dynamics at the nanoscale. This work initially compares GaN high electron mobility transistors (HEMTs) based on the established Ga-face technology and the emerging N-face technology, through a modeling approach that allows a fair comparison, indicating that the N-face devices exhibit improved performance with respect to Ga-face ones due to the natural back-barrier confinement that mitigates short-channel-effects. An investigation is then carried out on the minimum aspect ratio (i.e. gate length to gate-to-channel-distance ratio) that limits short channel effects in ultra-scaled GaN and InP HEMTs, indicating that this value in GaN devices is 15 while in InP devices is 7.5. This difference is believed to be related to the different dielectric properties of the two materials, and the corresponding different electric field distributions. The dielectric effects of the passivation layer in millimeter-wave, high-power GaN HEMTs are also investigated, finding that the effective gate length is increased by fringing capacitances, enhanced by the dielectrics in regions adjacent to the gate for layers thicker than 5 nm, strongly affecting the frequency performance of deep sub-micron devices. Lastly, efficient Full Band Monte Carlo particle-based device simulations of the large-signal performance of mm-wave transistor power amplifiers with high-Q matching networks are reported for the first time. In particular, a CellularMonte Carlo (CMC) code is self-consistently coupled with a Harmonic Balance (HB) frequency domain circuit solver. Due to the iterative nature of the HB algorithm, this simulation approach is possible only due to the computational efficiency of the CMC, which uses pre-computed scattering tables. On the other hand, HB allows the direct simulation of the steady-state behavior of circuits with long transient time. This work provides an accurate and efficient tool for the device early-stage design, which allows a computerbased performance evaluation in lieu of the extremely time-consuming and expensive iterations of prototyping and experimental large-signal characterization. / Dissertation/Thesis / Ph.D. Electrical Engineering 2011

abstract: This work is an investigation into the information provided by the concurrent use of in situ reflection high energy electron diffraction (RHEED) and reflection electron energy loss spectroscopy (REELS). The two analytical methods were employed during growth of metal, semiconductor and superconductor thin films by molecular beam epitaxy (MBE). Surface sensitivity of the REELS spectrometer was found to be less than 1 nm for 20 KeV electrons incident at a 2 degree angle to an atomically flat film surface, agreeing with the standard electron escape depth data when adjusted incident angle. Film surface topography was found to strongly influence the REELS spectra and this was correlated with in situ RHEED patterns and ex situ analysis by comparison with atomic force microscopy (AFM). It was observed in all the experimental results that from very smooth films the plasmon peak maxima did not fall at the predicted surface plasmon values but at slightly higher energies, even for nearly atomically flat films. This suggested the REELS plasmon loss spectra are always a combination of surface and bulk plasmon losses. The resulting summation of these two types of losses shifted the peak to below the bulk plasmon value but held its minimum to a higher energy than the pure surface plasmon value. Curve fitting supported this conclusion. / Dissertation/Thesis / Ph.D. Engineering Science 2012

abstract: Group III-nitride semiconductors have been commercially used in the fabrication of light-emitting diodes and laser diodes, covering the ultraviolet-visible-infrared spectral range and exhibit unique properties suitable for modern optoelectronic applications. InGaN ternary alloys have energy band gaps ranging from 0.7 to 3.4 eV. It has a great potential in the application for high efficient solar cells. AlGaN ternary alloys have energy band gaps ranging from 3.4 to 6.2 eV. These alloys have a great potential in the application of deep ultra violet laser diodes. However, there are still many issues with these materials that remain to be solved. In this dissertation, several issues concerning structural, electronic, and optical properties of III-nitrides have been investigated using transmission electron microscopy. First, the microstructure of In_xGa_1-xN (<italic>x</italic> = 0.22, 0.46, 0.60, and 0.67) films grown by metal-modulated epitaxy on GaN buffer /sapphire substrates is studied. The effect of indium composition on the structure of InGaN films and strain relaxation is carefully analyzed. High luminescence intensity, low defect density, and uniform full misfit strain relaxation are observed for <italic>x</italic> = 0.67. Second, the properties of high-indium-content InGaN thin films using a new molecular beam epitaxy method have been studied for applications in solar cell technologies. This method uses a high quality AlN buffer with large lattice mismatch that results in a critical thickness below one lattice parameter. Finally, the effect of different substrates and number of gallium sources on the microstructure of AlGaN-based deep ultraviolet laser has been studied. It is found that defects in epitaxial layer are greatly reduced when the structure is deposited on a single crystal AlN substrate. Two gallium sources in the growth of multiple quantum wells active region are found to cause a significant improvement in the quality of quantum well structures. / Dissertation/Thesis / Doctoral Dissertation Physics 2014

abstract: In this work, the insight provided by our sophisticated Full Band Monte Carlo simulator is used to analyze the behavior of state-of-art devices like GaN High Electron Mobility Transistors and Hot Electron Transistors. Chapter 1 is dedicated to the description of the simulation tool used to obtain the results shown in this work. Moreover, a separate section is dedicated the set up of a procedure to validate to the tunneling algorithm recently implemented in the simulator. Chapter 2 introduces High Electron Mobility Transistors (HEMTs), state-of-art devices characterized by highly non linear transport phenomena that require the use of advanced simulation methods. The techniques for device modeling are described applied to a recent GaN-HEMT, and they are validated with experimental measurements. The main techniques characterization techniques are also described, including the original contribution provided by this work. Chapter 3 focuses on a popular technique to enhance HEMTs performance: the down-scaling of the device dimensions. In particular, this chapter is dedicated to lateral scaling and the calculation of a limiting cutoff frequency for a device of vanishing length. Finally, Chapter 4 and Chapter 5 describe the modeling of Hot Electron Transistors (HETs). The simulation approach is validated by matching the current characteristics with the experimental one before variations of the layouts are proposed to increase the current gain to values suitable for amplification. The frequency response of these layouts is calculated, and modeled by a small signal circuit. For this purpose, a method to directly calculate the capacitance is developed which provides a graphical picture of the capacitative phenomena that limit the frequency response in devices. In Chapter 5 the properties of the hot electrons are investigated for different injection energies, which are obtained by changing the layout of the emitter barrier. Moreover, the large signal characterization of the HET is shown for different layouts, where the collector barrier was scaled. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2016

abstract: Two major challenges in the transformer-less, single-phase PV string inverters are common mode leakage currents and double-line-frequency power decoupling. In the proposed doubly-grounded inverter topology with innovative active-power-decoupling approach, both of these issues are simultaneously addressed. The topology allows the PV negative terminal to be directly connected to the neutral, thereby eliminating the common-mode ground-currents. The decoupling capacitance requirement is minimized by a dynamically-variable dc-link with large voltage swing, allowing an all-film-capacitor implementation. Furthermore, the use of wide-bandgap devices enables the converter operation at higher switching frequency, resulting in smaller magnetic components. The operating principles, design and optimization, and control methods are explained in detail, and compared with other transformer-less, active-decoupling topologies. A 3 kVA, 100 kHz single-phase hardware prototype at 400 V dc nominal input and 240 V ac output has been developed using SiC MOSFETs with only 45 μF/1100 V dc-link capacitance. The proposed doubly-grounded topology is then extended for split-phase PV inverter application which results in significant reduction in both the peak and RMS values of the boost stage inductor current and allows for easy design of zero voltage transition. A topological enhancement involving T-type dc-ac stage is also developed which takes advantage of the three-level switching states with reduced voltage stress on the main switches, lower switching loss and almost halved inductor current ripple. In addition, this thesis also proposed two new schemes to improve the efficiency of conventional H-bridge inverter topology. The first scheme is to add an auxiliary zero-voltage-transition (ZVT) circuit to realize zero-voltage-switching (ZVS) for all the main switches and inherent zero-current-switching (ZCS) for the auxiliary switches. The advantages include the provision to implement zero state modulation schemes to decrease the inductor current THD, naturally adaptive auxiliary inductor current and elimination of need for large balancing capacitors. The second proposed scheme improves the system efficiency while still meeting a given THD requirement by implementing variable instantaneous switching frequency within a line frequency cycle. This scheme aims at minimizing the combined switching loss and inductor core loss by including different characteristics of the losses relative to the instantaneous switching frequency in the optimization process. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

abstract: Light Emitting Diodes even with their longer life, robust build and low power consumption, they are still plagued by some problems the most significant of which are the current droop and thermal droop. Current droop causes a lowering in the Internal Quantum Efficiency with increased current injection while thermal droop lowers the whole Internal Quantum Efficiency curve with increase in temperature. The focus here was understanding effects of thermal droop and develop a method to control it. Shockley Read Hall recombination plays a dominant role in the thermal droop effect when the current injection is low. Since the blue light emitting diode is based on Gallium Nitride, we need to take into consideration the effect of piezoelectric polarization in the quantum wells. The effects of the piezoelectric fields were studied based on the Gallium Nitride plane orientations. It was found in a Gallium Nitride light emitting diodes simulation study that more the number of quantum wells, lower would be the Radiative recombination rate. The problem of exacerbated spatial separation of electron hole wavefunctions in a thick single quantum well structure lead to the development of a dual well structure where one well assisted the other during high temperature operations. The Electron Blocking Layer was reduced in thickness and was made only 10 nm thick with a 5 nm Gallium Nitride buffer between it and the active region wells. The main reason for reducing the electron blocking layer thickness was to reduce the valance band offset and improve hole transport into the active region. Three different dual well designs were simulated of 3nm, 6nm and 9nm wide wells. The output parameters like the Power Spectral Density, Electron bound density, Light Output Power and Electron-Hole wavefunction overlaps were calculated. It was found that one of the wells acted as an assisting well where it had very little radiative recombination activity in it at room temperature. As the temperature increased, it was observed that the electrons in the main well started to overflow out of it and into the assisting well where the radiative recombination rate increased significantly. This lead to a boost in Internal Quantum Efficiency. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2017

abstract: In this work, an advanced simulation study of reliability in millimeter-wave (mm-wave) GaN Devices for power amplifier (PA) applications is performed by means of a particle-based full band Cellular Monte Carlo device simulator (CMC). The goal of the study is to obtain a systematic characterization of the performance of GaN devices operating in DC, small signal AC and large-signal radio-frequency (RF) conditions emphasizing on the microscopic properties that correlate to degradation of device performance such as generation of hot carriers, presence of material defects and self-heating effects. First, a review of concepts concerning GaN technology, devices, reliability mechanisms and PA design is presented in chapter 2. Then, in chapter 3 a study of non-idealities of AlGaN/GaN heterojunction diodes is performed, demonstrating that mole fraction variations and the presence of unintentional Schottky contacts are the main limiting factor for high current drive of the devices under study. Chapter 4 consists in a study of hot electron generation in GaN HEMTs, in terms of the accurate simulation of the electron energy distribution function (EDF) obtained under DC and RF operation, taking into account frequency and temperature variations. The calculated EDFs suggest that Class AB PAs operating at low frequency (10 GHz) are more robust to hot carrier effects than when operating under DC or high frequency RF (up to 40 GHz). Also, operation under Class A yields higher EDFs than Class AB indicating lower reliability. This study is followed in chapter 5 by the proposal of a novel π-Shaped gate contact for GaN HEMTs which effectively reduces the hot electron generation while preserving device performance. Finally, in chapter 6 the electro-thermal characterization of GaN-on-Si HEMTs is performed by means of an expanded CMC framework, where charge and heat transport are self-consistently coupled. After the electro-thermal model is validated to experimental data, the assessment of self-heating under lateral scaling is considered. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2018

Submitted by Cíntia de Jesus Soares Bisetto (cintiabiares@hotmail.com) on 2018-06-13T15:45:57Z No. of bitstreams: 1 Cíntia Bisetto.pdf: 4340025 bytes, checksum: 0627332eb0ebbde2a894c6b8cb95ec1d (MD5) / Rejected by Adriana Aparecida Puerta null (dripuerta@rc.unesp.br), reason: Prezada Cíntia, O documento enviado para a coleção Rio Claro foi recusado pelo(s) seguinte(s) motivo(s): - Falta capa, que é elemento obrigatório. Deve conter as informações: a) nome da instituição (opcional); b) nome do autor; c) título; d) subtítulo, se houver; e) local (cidade) da instituição onde foi apresentado; f) ano de depósito (da entrega). O documento enviado não foi excluído. Para revisá-lo e realizar uma nova tentativa de envio, acesse: https://repositorio.unesp.br/mydspace Em caso de dúvidas entre em contato pelo email repositoriounesp@reitoria.unesp.br. Agradecemos a compreensão e aguardamos o envio do novo arquivo. Atenciosamente, Biblioteca Campus Rio Claro Repositório Institucional UNESP https://repositorio.unesp.br on 2018-06-13T17:59:14Z (GMT) / Submitted by Cíntia de Jesus Soares Bisetto (cintiabiares@hotmail.com) on 2018-06-13T20:06:10Z No. of bitstreams: 1 merged.pdf: 2742880 bytes, checksum: 20163a6a5b61b75d5bc053947140c699 (MD5) / Approved for entry into archive by Adriana Aparecida Puerta null (dripuerta@rc.unesp.br) on 2018-06-14T17:06:38Z (GMT) No. of bitstreams: 1 bisetto_cjs_me_rcla.pdf: 2742880 bytes, checksum: 20163a6a5b61b75d5bc053947140c699 (MD5) / Made available in DSpace on 2018-06-14T17:06:38Z (GMT). No. of bitstreams: 1 bisetto_cjs_me_rcla.pdf: 2742880 bytes, checksum: 20163a6a5b61b75d5bc053947140c699 (MD5) Previous issue date: 2018-04-19 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Neste estudo, procuramos compreender as relações entre Brasil e Uruguai neste início de século. Iniciaremos discorrendo um pouco sobre a história do Uruguai e sobre os aspectos da sua aproximação com o Brasil. Abordaremos as relações comerciais entre os dois países e os projetos de integração regional, sobre o FOCEM-MERCOSUL e sobre a IIRSA-COSIPLAN-UNASUL, com foco nas relações Brasil-Uruguai. Após tecer considerações sobre a participação uruguaia (em menor dimensão, também sobre a participação brasileira) nessas iniciativas, interessa-nos analisar as relações entre os dois países no âmbito do GAN, instituído em 2012, o Grupo de Alto Nível Brasil-Uruguai (GAN) é expressivo da maior aproximação entre os dois países. A pesquisa, cujos resultados apresentamos a seguir, abarcou a temática da integração regional, centrando nas relações Brasil-Uruguai, com ênfase a iniciativas no campo energético e obras de infraestrutura como recuperação de ferroviárias, hidrovias, pontes e estradas. O Parque Eólico Artilleros no Uruguai recebeu mais atenção por se tratar de uma obra bancada em parte pelo Brasil, inserida na estratégia de disseminação de energias renováveis do governo uruguaio e no projeto de internacionalização da Eletrobras. A temática sobre o expansionismo de empresas do Brasil no Uruguai, considerado por alguns analistas da ótica do (sub)imperialismo, também se faz presente, embora não seja objeto central de reflexão na pesquisa realizada, abordamos o assunto defendendo que o processo é mais complexo, a depender das negociações entre os países envolvidos. / In this study, we intend to understand the relationship between Brazil and Uruguay at the beginning of this century. We will start talking a little about the history of Uruguay and the aspects of its approximation to Brazil. We will approach the commercial relations between these two neighboring countries and the regional integration projects, talking about the FOCEM-MERCOSUR and the IIRSA-COSIPLAN-UNASUR at the length, focusing on Brazil-Uruguay relationships. After talking about the Brazilian and Uruguayan participations in these initiatives, we take interest in analyzing the relationships between the two countries in the GAN fields. Established in 2012, the High-Level Group Brazil-Uruguay (GAN) is meaningful to the biggest recent approach between them. The present research will comprise the integration carried out on the energy areas and other infrastructure work, such as the recovery of the railroads, waterways, bridges and roads with the challenge of surpassing the transportation difficulty on the borders. What matters, above all, is to highlight initiatives in the energy area like the Artilleros Wind Park in Uruguay, whose structure is mostly sponsored by Brazil, inserted in the dissemination strategy for renewable energy. It is also intended to explain the charges against the Brazilian expansionism through the projects and work designed to improve the bilateral relationship with Uruguay, which would be considered imperialist, and we may ponder on the subject defending that the process is complex depending on the negotiations between the involved countries.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-06-28T12:40:48Z No. of bitstreams: 1 leandrodecastroguarieri.pdf: 1096642 bytes, checksum: e2c0ee03009dadfdf2e72f0f3138fdd6 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-08-07T21:08:56Z (GMT) No. of bitstreams: 1 leandrodecastroguarieri.pdf: 1096642 bytes, checksum: e2c0ee03009dadfdf2e72f0f3138fdd6 (MD5) / Made available in DSpace on 2017-08-07T21:08:56Z (GMT). No. of bitstreams: 1 leandrodecastroguarieri.pdf: 1096642 bytes, checksum: e2c0ee03009dadfdf2e72f0f3138fdd6 (MD5) Previous issue date: 2007-08-15 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho apresentamos cálculos para o espalhamento Raman eletrônico via flutuação de densidade de carga para sistemas a base de GaN dopados. As estruturas estudadas foram a super-rede δ-dopada e os sistemas de GaN uniformemente dopados. No caso da super-rede periodicamente dopada, a estrutura eletrônica foi determinada utilizando-se a Teoria do Funcional Densidade na aproximação de densidade local. Para os sistemas uniformemente dopados, a natureza das excitações elementares Raman foi investigada teoricamente e experimentalmente, tais sistemas consistem de estruturas cúbicas de GaN uniformemente dopadas (tipo-n) crescidas sobre substratos de GaAs. / In this work we present calculations of eletronic Raman cross-sections of doped GaN based system via charge - density mechanism. The studied structures were δ-doped superlattices and uniform doped GaN systems. In the case of periodically doped superlattice the eletronic structure was obtained using Density Funcional Theory in the local density approximation. Concerning the uniform doped system, the nature of their Raman elementary excitations were investigated theoretically and experimentally. Such structures consisted of cubic GaN uniformly doped (type-n) grown on GaAs substrates.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第14626号 / 工博第3094号 / 新制||工||1460(附属図書館) / 26978 / UT51-2009-D338 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 川上 養一, 教授 藤田 静雄, 教授 木本 恒暢 / 学位規則第4条第1項該当