This thesis deals with the development and application of Monte Carlo simulations to study electron transport in bulk GaN in the wurtzite crystal structure and the properties of field effect transistors made from the material. There is a particular emphasis on transport in the high electric field regime and transistors operating at high voltages. The simulation model includes five sets of non-parabolic conduction band valleys which can be occupied by electrons during high field transport. The effects on electron transport of impurities and the relevant phonon scattering mechanisms have been considered. Results for electron transport at both low and high electric field are presented and compared with the properties of GaN in the zincblende structure, of other group-III nitride semiconductors, and of GaAs. The dependence of the transport properties on the material parameters is discussed and also with regard to the temperature, donor concentration and electric field magnitude and direction. The transport properties of electrons in wurtzite GaN n+-i(n)-n+ diodes are also explored, including the effect of the upper valleys and the temperature on hot electron transport. Simulations have also been carried out to model the steady-state and transient properties of GaN MESFETs that have recently been the subject of experimental study. It has been suggested that traps have a substantial effect on the performance of GaN field effect transistors and we have developed a model of a device with traps to investigate this suggestion. The model includes the simulation of the capture and release of electrons by traps whose charge has a direct effect on the current flowing through the transistor terminals. The influence of temperature and light on the occupancy of the traps and the /- V characteristics are considered. It is concluded that traps are likely to play a substantial role in the behaviour of GaN field effect transistors. Further simulations were performed to model electron transport in AlGaN/GaN hetero-junction FETs. So called HFET structures with a 78 nm Alo.2Gao.8N pseudomorphically strained layer have been simulated, with the inclusion of spontaneous and piezoelectric polarization effects in the strained layer. The polarization effects are shown to not only increase the current density, but also improve the electron transport by inducing a higher electron density close to the positive charge sheet that occurs in the channel.

As Si-manufacturing compatible SiGe HBTs are making rapid in-roads into RF through mm-wave circuit applications, with performance levels steadily marching upward, the use of these devices under extreme environment conditions are being studied extensively. In this work, test structures of SiGe HBTs were designed and put into extremely low temperatures, and a new negative differential resistance effect and a novel collector current kink effect are investigated in the cryogenically-operated SiGe HBTs. Theory based on an enhanced positive feedback mechanism associated with heterojunction barrier effect at deep cryogenic temperatures is proposed. The accumulated charge induced by the barrier effect acts at low temperatures to enhance the total collector current, indirectly producing both phenomena. This theory is confirmed using calibrated 2-D DESSIS simulations over temperature. These unique cryogenic effects also have significant impact on the ac performance of SiGe HBTs operating at high-injection. Technology evolution plays an important role in determining the magnitude of the observed phenomena, and the scaling implications are addressed. Circuit implication is discussed.

This thesis presents an overview of the simulation, design, and measurement of state-of-the-art Silicon-Germanium Hetro-Junction Bipolar Transistor (SiGe HBT) low-noise amplifiers (LNAs). The LNA design trade-off space is presented and methods for achieving an optimized design are discussed. In Chapter 1, we review the importance of LNAs and the benefits of SiGe HBT technology in high frequency amplifier design. Chapter 2 introduces LNA design and basic noise theory. A graphical LNA design approach is presented to aid in understanding of the high-frequency LNA design process. Chapter 3 presents an LNA design optimization method for power constrained applications. Measured results using this design technique are highlighted and shown to have record performance. Lastly, in Chapter 4, we highlight cryogenic noise performance and present measured results from cryogenic operation of SiGe HBT LNAs. We demonstrate in this thesis that SiGe HBT LNAs have the capability to meet the demanding needs for next generation wireless systems. The aim of the analysis presented herein is to provide designers with the fundamentals of designing SiGe HBT LNAs through relevant design examples and measured results.

This work represents several years' research into the field of radiation hardening by design. The unique characteristics of a SiGe HBT, described in Chapter 1, make it ideally suitable for use in extreme environment applications. Chapter 2 describes the total ionizing dose effects experienced by a SiGe HBT, particularly those experienced on an Earth-orbital or lunar-surface mission. In addition, the effects of total dose are evaluated on passive devices. As opposed to the TID-hardness of SiGe transistors, a clear vulnerability to single-event effects does exist. This field is divided into three chapters. First, the very nature of single-event transients present in SiGe HBTs is explored in Chapter 3 using a heavy-ion microbeam with both bulk and SOI platforms [31]. Then, in Chapter 4, a new device-level SEU-hardening technique is presented along with circuit-design techniques necessarily for its implementation. In Chapter 5, the circuit-level radiation-hardening techniques necessarily to mitigate the effects shown in Chapter 3 are developed and tested [32]. Finally, in Chapter 6, the performance of the SiGe HBT in a cryogenic testing environment is characterized to understand how the widely-varying temperatures of outer space may affect device performance. Ultimately, the built-in performance, TID-tolerance, and now-developing SEU-hardness of the SiGe HBT make a compelling case for extreme environment electronics. The low-cost, high-yield, and maturity of Si manufacturing combine with modern bandgap engineering and modern CMOS to produce a high-quality, high-performance BiCMOS platform suitable for space-borne systems.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Recently, several SiGe HBT devices fabricated on CMOS-compatible silicon on insulator (SOI) substrates (SiGe HBTs-on-SOI) have been demonstrated, combining the well-known SiGe HBT performance with the advantages of SOI substrates. These new devices are especially interesting in the context of extreme environments - highly challenging surroundings that lie outside commercial and even military electronics specifications. However, fabricating HBTs on SOI substrates instead of traditional silicon bulk substrates requires extensive modifications to the structure of the transistors and results in significant trade-offs. The present work investigates, with measurements and TCAD simulations, the performance and reliability of SiGe heterojunction bipolar transistors fabricated on silicon on insulator substrates with respect to operation in extreme environments such as at extremely low or extremely high temperatures or in the presence of radiation (both in terms of total ionizing dose and single effect upset).

L’électronique organique reste un domaine de recherche prolifique grâce à la diversité de structures moléculaires accessible par la synthèse organique. Les matériaux moléculaires offrent des possibilités de mises en forme inédites comme les techniques de dépôt en solution, utilisables dans la conception de dispositifs organiques sur supports plastiques. Nos travaux de recherche mêlent électrochimie, conception et caractérisations électriques de dispositifs et mesures capteur. Ils abordent deux thématiques : l’élaboration de nouveaux transducteurs à base de polyanilines substituées et de phtalocyanines et l’étude de l’influence de la modification électrochimique de surface sur le comportement de dispositifs connus.Nous avons tout d’abord élaboré des hétérojonctions latérales polymère – phtalocyanine en utilisant les propriétés des polyanilines. De par leur géométrie, ces nouveaux dispositifs diffèrent de l’hétérojonction MSDI (Molecular Semiconductor – Doped Insulator), transducteur conductimétrique bicouche développé au laboratoire pour la détection de gaz comme l’ozone ou l’ammoniac. La caractérisation complète de la poly(2,3,5,6-tétrafluoroaniline) indique que ce polymère est peu conducteur, en comparaison de la polyaniline dont la conductivité peut être augmentée par dopage acido-basique. La présence des fluors empêche l’émergence du régime conducteur que l’on retrouve en milieu acide pour la polyaniline et la poly(2,5-diméthoxyaniline). Ces trois polymères, électrodéposés sur électrodes d’ITO interdigitées ont permis, après sublimation de la bisphtalocyanine de lutécium, de construire les hétérojonctions latérales. Le comportement électrique des différents dispositifs, étudié notamment par spectroscopie d’impédance, diffère en fonction de la nature des substituants de la polyaniline électrodéposée. Enfin, des mesures capteur ont montré la possibilité de détecter l’ammoniac en milieu humide, avec une limite de détection sub ppm.De précédents travaux sur l’élaboration de MSDI ont souligné le rôle primordial des interfaces, notamment dans le cas des n-MSDI qui présentent une hétérojonction p-n. Outre le changement de la nature chimique de la sous-couche employée, une autre manière de jouer sur les interfaces est de modifier électrochimiquement la surface des électrodes interdigitées, par réduction de sels de diazonium. Nous avons ainsi greffé différents benzènes substitués, dont certains ont conduit à la formation de multicouches comme l’ont révélée des mesures de microbalance à quartz électrochimique. Les différentes modifications de surface ont surtout joué le rôle de barrière isolante, amplifiant le comportement non linéaire des caractéristiques courant-tension des MSDI. Le greffage du 2,5-diméthoxybenzène a permis d’améliorer significativement la sensibilité à l’ammoniac de la MSDI à base d’hexadécafluorophtalocyanine de cuivre et de bisphtalocyanine de lutétium, avec une limite de détection de l’ordre de 200 ppb. / Organic electronics remains a fruitful research field thanks to the diversity of molecular structures reachable by organic synthesis. Molecular materials offer convenient shaping processes, such as solution processing techniques, which can be used for the fabrication of organic devices on plastic substrates.Our works can be summarized as the elaboration of conductometric devices thanks to electrochemistry and the study of their electrical and sensing properties. They deal with two topics: the development of new transducers based on substituted polyanilines and phthalocyanines and the study of the influence of electrochemical modifications on the behavior of known devices.We first developped polymer - phthalocyanine lateral heterojunctions using the properties of polyanilines. Because of their geometry, these new devices differed from the MSDI heterojunction (Molecular Semiconductor - Doped Insulator), a bilayer-based conductometric transducer developed in the laboratory for the detection of gases such as ozone or ammonia. The comprehensive study of poly (2,3,5,6-tetrafluoroaniline) indicated that this material was a poor conducting polymer, compared to polyaniline whose conductivity can be increased by acid-base doping. The presence of fluorine atoms prevented the emergence of the conductive regime found in acidic medium for polyaniline and poly (2,5-dimethoxyaniline). These three polymers, electrodeposited on interdigitated ITO electrodes, allowed us, after sublimation of the lutetium bisphthalocyanine, to build lateral heterojunctions. The electrical behavior of the different devices, studied in particular by impedance spectroscopy, differed according to the nature of the substituents of the electrodeposited polyaniline. Finally, sensing measurements revealed their efficiency to detect ammonia in humid atmosphere, with a sub-ppm limit of detection.Previous works on the development of MSDI emphasized the primary role of interfaces, particularly in the case of n-MSDI that contained a p-n heterojunction. In addition to the modification of the chemical nature of the underlayer, another way to play with the interfaces is to electrochemically modify the surface of the interdigitated electrodes by reducing diazonium salts. Thus, we grafted various substituted benzenes, some of which led to the formation of multilayers as revealed by electrochemical quartz microbalance measurements. The various surface modifications mainly acted as an insulating barrier that amplified the nonlinear behavior of the current-voltage characteristics of MSDI. The grafting of 2,5-dimethoxybenzene significantly improved the ammonia sensitivity of MSDI based on copper hexadecafluorophthalocyanine and lutetium bisphthalocyanine, with a limit of detection of around 200 ppb.

Ce travail s’inscrit dans le cadre du développement de nouvelles applications des matériaux III-Nitrure en général, et de l’alliage InGaN en particulier, pour la réalisation de cellules solaires à base de multi-jonction. Les nombreux avantages du matériau InGaN, à savoir son coefficient d’absorption élevé (105 cm−1), sa résistance thermique élevée, et sa tolérance aux radiations ainsi que sa bande interdite couvrant presque tout le spectre solaire (0.64 - 3.4eV), en font un sérieux candidat pour les dispositifs photovoltaïques. Ainsi une cellule solaire à quadruple jonctions permettrait l’obtention d’une efficacité au-delà de 50%. Cependant, les enjeux technologiques tels que la séparation de phase, le manque de substrat approprié donnant lieu à de forte densité de dislocations, et la difficulté de réalisation du dopage de type-p, sont considérés comme des obstacles pour atteindre les performances attendues. L’objectif de ce travail est d’étudier deux nouvelles approches qui peuvent résoudre les problèmes cités précédemment pour la réalisation de cellules solaires de haute efficacité à base d’InGaN. La première approche est dite approche "semibulk". Elle consiste à élaborer une structure multicouches InGaN/GaN épaisse avec une optimisation de l’épaisseur de chaque couche (InGaN et GaN), de façon que les couches de GaN soient suffisamment épaisses pour être efficaces, et assez mince pour permettre le transport des porteurs de charges par effet tunnel. Les couches InGaN quant à elles, doivent être assez épaisses et nombreuses afin d’absorber efficacement le rayonnement lumineux et suffisamment minces pour éviter la relaxation et l’apparition de dislocations. La deuxième approche consiste en la croissance de nanostructures InGaN qui autorise une incorporation d’indium élevée avec un matériau complètement relaxé et sans dislocation. La complète relaxation du matériau permet en outre de s’affranchir de l’effet piézoélectrique qui conduit à une chute du rendement. Nous avons pu démontrer que les cellules photovoltaïques à base d’In0.08Ga0.92N réalisées suivant l’approche "semibulk" présentent un pic de rendement quantique de 85%, ainsi qu’une efficacité de conversion en conditions AM 1.5G, presque trois fois plus élevée que l’état de l’art. Les premiers résultats obtenus sur les cellules photovoltaïques à base de nanostructures d’In0.08Ga0.62N sont très encourageants / The InGaN material system, with high absorption coefficient (105 cm−1) and a bandgap from 0.64 eV to 3.4 eV spanning the entire visual spectrum, make the development of all-InGaN multijunction solar cells with overall conversion efficiency larger than 50% theoretically possible. However, to reach this goal high-quality and thick InGaN layers with high indium concentration are required, which is not a trivial task. Studies of InGaN-based junctions with an indium mole fraction exceeding 0.3 are rare due to issues such as strong phase separation and relaxation of the layer due to lattice mismatch with the substrate which lead to InGaN layers with large dislocation density and indium-clustering. These material problems, significantly limit the performance of InGaN-based photovoltaic cells, and whatever the indium content, performance still remains far from the theoretical ones. The objective of this study is to investigate new approaches that may overcome the issues of phase separation and high dislocation density in InGaN materials with high indium concentration, for the realization of high efficiency InGaN based solar cells. Two novel approaches are proposed that may overcome the basic challenges involved in the InGaN hetero-junction solar cells. The first approach consists in the growth of a thick multi-layered InGaN/GaN absorber, called Semibulk. These GaN interlayers need to be thick enough to be effective and thin enough to allow carrier transport through tunneling. The InGaN layers need to be thick and numerous enough to absorb efficiently the incoming light beam, and thin enough to remain fully strained and without phase separation. The second approach consists in the growth of InGaN nano-structures to achieve high quality thick InGaN epitaxial layers with high indium concentration. It allows the elimination of the preexisting dislocations in the underlying template. It also allows strain relaxation of InGaN layers without any dislocations, leading to higher indium incorporation and reduced piezoelectric effect. The electro-optical characterization of semibulk In0.08Ga0.92N PV devices show a maximum external quantum efficiency (EQE) of 85%, which is the maximum EQE peak reported so far for an InGaN PIN heterojunction solar cell. The voltage dependence of the current density, under AM 1.5G solar spectrum for the semibulk In0.08Ga0.92N solar cells results in values of Jsc, Voc, fill factor (FF) and power conversion efficiency (PCE) as 0.57 mA/cm2, 1.04 V, 65% and 0.39% respectively. A comparison of the results to the literature show that the Jsc is four to five times of what has been reported for a bulk In0.08Ga0.92N PV structure. This value of Jsc lead to a PCE for the semibulk In0.08Ga0.92N-based PV cell which is at least three times higher than the PCE for the bulk In0.08Ga0.92N structure under AM 0 solar spectrum. For our second approach, high crystalline structural quality for InGaN nano-structures with 35% of indium concentration has been obtained. The electro-optical characterization for In0.09Ga0.91N nano-structure PV cells shows a significant enhancement in the performance of the devices. The PV devices result in a Jsc and Voc of 12 mA/cm2 and 1.89 V under concentrated light respectively

Heterojunctions were generated between tungsten trioxide and tetraphenyl porphyrin with reduced graphene oxide or exfoliated graphite support for mineralisation of acid blue 25 dye under visible light radiation. Moreover, degradation of pharmaceuticals was conducted using p-n heterojunctions between WO3 and Co3O4 and a direct Z-scheme heterojunction between BiOI and Co3O4 prepared using in-situ method and solvothermal self-assembly method respectively. The synthesized materials were characterised using Raman, FTIR, SEM/EDS, TEM, XRD, TGA, BET, UV-Vis and PL techniques. UV-Vis, TOC and HPLC-QTOF-MS were used to study the degradation efficiency and pathway. Scavenger trapping experiments were conducted to propose the charge transfer mechanisms. The highest degradation efficiency (99 %) was achieved for the dye and the pharmaceuticals using visible light. The mineralisation ability of the fabricated nanomaterials was pH dependent with acidic conditions favouring the removal of the dye (pH 5) while alkaline conditions favoured the mineralisation of pharmaceuticals (pH 10 – 11). / Civil and Chemical Engineering

Organische Solarzellen bieten die Aussicht auf eine ökologische und zugleich ökonomische Energiequelle. Nachteile des Konzepts liegen in der z.T. geringen Stabilität der für Absorption und Ladungstransport verwendeten Moleküle und einer unvollständigen Ausnutzung des Sonnenspektrums. Zur Verbesserung beider Merkmale werden in dieser Arbeit einzelne organische Bestandteile durch anorganische Materialien mit hoher Stabilität und breiten Absorptionsbanden ersetzt. Insbesondere werden als Absorber kolloidale Quantenpunkte (QP) verwendet, denen aufgrund nicht-linearer und durch Größeneffekte steuerbarer optischer Eigenschaften in der Photovoltaik der dritten Generation großes Interesse gilt. Dazu werden dünne anorganisch-organische Filme mit einem Verfahren hergestellt, das auf Wechselwirkungen zwischen Partikeln in Lösung und geladenen Oberflächen beruht (electrostatic layer-by-layer self-assembly). TiO2-Nanokristalle als Elektronenleiter, kolloidale CdTe- und CdSe-QP als Absorber und konjugierte Polymere als Lochleiter werden in die Filme integriert und diese als aktive Schichten in photovoltaischen Zellen verwendet. Die Struktur der Filme wird zunächst mittels AFM, SEM, XPS sowie durch eine Beladung mit organischen Farbstoffen untersucht. Sie weisen Porosität auf einer Skala von Nanometern sowie eine kontrollierbare Dicke und Mikrostruktur auf. Darauf aufbauend werden durch weitere lösungsbasierte Prozessschritte photovoltaische Zellen gefertigt und Zusammenhänge zwischen Struktur und Zellenleistung elektronisch und spektroskopisch untersucht. Einflussfaktoren der Zelleffizienz wie die Ladungsträgererzeugung und interne Widerstände können so bestimmt und die Effizienz von CdSe-QP als Sensibilisatoren nachgewiesen werden. Die Arbeit demonstriert die Eignung der gewählten Methoden und Zelldesigns zur Herstellung von photovoltaischen Zellen und eröffnet neue Ansätze für die Entwicklung und Fertigung insbesondere auf QP basierender Zellen. / Organic solar cells offer the prospect of a both ecological and economical energy source. Drawbacks of the concept are low stabilities of the molecules used for absorption and charge transport and an incomplete utilization of the solar spectrum. In order to improve both these characteristics, individual organic components are replaced by inorganic materials with a high stability and broad absorption bands in this work. In particular, colloidal quantum dots (QDs) are used as absorbers, the non-linear and size controllable optical properties of which are attracting great interest in third generation photovoltaics. For this application, inorganic/organic thin films are produced with a method based on interactions between particles in solution and charged surfaces (electrostatic layer-by-layer self-assembly). TiO2-nanocrystals as electron conductors, colloidal CdTe- and CdSe-QDs as absorbers and conjugated polymers as hole conductors are integrated into the films, which are used as active layers in photovoltaic cells. The structure of the films is investigated by AFM, SEM, XPS and by loading the films with organic dye molecules. The films show porosity on a nanometer scale as well as a controllable thickness and microstructure. Complemented by further solution based processing steps, photovoltaic cells are manufactured and correlations between the structure and performance of the cells are investigated both electronically and spectroscopically. Individual factors that determine the cell efficiency, such as carrier generation and internal resistances, are determined and the efficiency of CdSe-QDs as sensitizers is demonstrated. This work proves the suitability of the chosen methods and cell designs for manufacturing photovoltaic cells and opens up new approaches for the development and manufacture of in particular QD-based solar cells.