No description available.

No description available.

This thesis work contains electrical transport and low frequency (1/f) noise measurements in ultrathin graphene and Molybdenum disulphide (MoS2) field effect transistors (FET). From the measurements, We mainly focus on the origin of disorder in both the materials. To address the orgin of disorder in graphene, we study single and bilayer graphene-FET devices on SiO2 substrate. We observe that both conductivity and mobility are mainly determined by substrate induced long range, short range, and polar phonon scattering. For further confirmation, we fabricate suspended graphene devices which show extremely high mobility. We find that, in contrast to substrate-supported graphene, conductivity and mobility in suspended graphene are governed by the longitudinal acoustic phonon scattering at high temperature and the devices reach a ballistic limit at low temperature. We also conduct low frequency 1/f noise measurements, known to be sensitive to disorder dynamics, to extract more information on the nature of disorder. The measurements are carried out both in substrate-supported and suspended graphene devices. We find that 1/f noise in substarted graphene is mainly determined by the trap charges in the SiO2 substrate. On the other hand, noise behaviour in suspended graphene devices can not be explained with trap charge dominated noise model. More-over, suspended devices exhibit one order of magnitude less noise compared to graphene on SiO2 substrate. We believe noise in suspended graphene devices probably originate from metal-graphene contact regions. In the second part of our work, We present low temperature electrical transport in ultrathin MoS2 fields effect devices, mechanically exfoliated onto Si/SiO2 substrate. Our experiments reveal that the electronic states in MoS2 are localized well up to the room temperature over the experimentally accessible range of gate voltage. This manifests in two dimensional (2D) variable range hopping (VRH) at high temperatures, while below ~ 30 K the conductivity displays oscillatory structures in gate voltage arising from resonant tunneling at the localized sites. From the correlation energy (T0) of VRH and gate voltage dependence of conductivity, we suggest that the charged impurities are the dominant source of disorder in MoS2. To explore the origin of the disorder, we perform temperature dependent I - V measurements at high source-drain bias. These measurements indicate presence of an exponentially distributed trap states in MoS2 which originate from the structural inhomogeneity. For more detailed investigation, we employ 1/f noise which further confirms possible presence of structural disorder in the system. The origin of the localized states is also investigated by spectroscopic studies, which indicate a possible presence of metallic 1T-patches inside semiconducting 2H phase. From all these evidences, we suggest that the disorder is internal, and achieving high mobility in MoS2 FET requires a greater level of crystalline homogeneity.

Low-frequency noise (LFN) is characterized using in-house measurement systems in a variety of SiGe HBT generations. As technology scales to improve the performance and integration level, a large low-frequency noise variation in small geometry SiGe HBTs is first observed in 90 GHz peak fT devices. The fundamental mechanism of this geometry dependent noise variation is thought to be the superposition of individual Lorentzian spectra due to the presence of G/R centers in the device. The observed noise variation is the result of a trap quantization effect, and is thus best described by number fluctuation theory rather than mobility fluctuation theory. This noise variation continues to be observed in 120 GHz and 210 GHz peak fT SiGe HBT BiCMOS technology. Interestingly, the noise variation in the 210 GHz technology generation shows anomalous scaling behavior below about 0.2-0.3um2 emitter geometry, where the noise variation rapidly decreases. Data shows that the collector current noise is no longer masked by the base current noise as it is in other technology generations, and becomes the dominant noise source in these tiny 210 GHz fT SiGe HBTs. The proton response of LFN in SiGe HBTs is also investigated in this thesis. The results show that the relative increase of LFN is minor in transistors with small emitter areas, but significant in transistors with large emitter areas after radiation. A noise degradation model is proposed to explain this observed geometry dependent LFN degradation. A 2-D LFN simulation is applied to SiGe HBTs for the first time in order to shed light on the physical mechanisms responsible for LFN. A spatial distribution of base current noise and collector current noise reveals the relevant importance of the physical locations of noise sources. The impact of LFN in SiGe HBTs on circuits is also examined. The impact of LFN variation on phase noise is demonstrated, showing VCOs with small geometry devices have relatively large phase noise variation across samples.

The rapid progress of complementary-metal-oxide-semiconductor (CMOS) integrated circuit technology became feasible through continuous device scaling. The implementation of high-k/metal gates had a significantcontribution to this progress during the last decade. However, there are still challenges regarding the reliability of these devices. One of the main issues is the escalating 1/fnoise level, which leads to degradation of signal-to-noise ratio (SNR) in electronic circuits. The focus of this thesis is on low-frequency noise characterization and modeling of various novel CMOS devices. The devices include PtSi Schottky-barriers  for source/drain contactsand different high-kgatestacksusingHfO2, LaLuO3 and Tm2O3 with different interlayers. These devices vary in the high-k material, high-k thickness, high-k deposition method and interlayermaterial. Comprehensive electrical characterization and low-frequency noise characterization were performed on various devices at different operating conditions. The noise results were analyzed and models were suggested in order to investigate the origin of 1/f noise in these devices. Moreover, the results were compared to state-of-the-art devices. High constant dielectrics limit the leakage current by offering a higher physical dielectric thickness while keeping the Equivalent Oxide Thickness (EOT) low. Yet, the 1/f noise increases due to higher number of traps in the dielectric and also deterioration of the interface with silicon compared to SiO2. Therefore, in order to improve the interface quality, applying an interfacial layer (IL) between the high-k layer and silicon is inevitable. Very thin, uniform insitu fabricated SiO2 interlayers with HfO2 high-k dielectric have been characterized. The required thickness of SiO2 as IL for further scaling has now reached below 0.5 nm. Thus, one of the main challenges at the current technology node is engineering the interfacial layer in order to achieve both high quality interface and low EOT. High-k ILs are therefore proposed to substitute SiOx dielectrics to fulfill this need. In this work, we have made the first experiments on low-frequency noise studies on TmSiO as a high-k interlayer with Tm2O3 or HfO2 on top as high-k dielectric. The TmSiO/Tm2O3 shows a lower level of noise which is suggested to be related to smoother interface between the TmSiO and Tm2O3. We have achieved excellentnoise performancefor TmSiO/Tm2O3 and TmSiO/HfO2 gate stacks which are comparableto state-of-the-art SiO2/HfO2 gate stacks. / <p>QC 20151130</p>

Disertacijoje nagrinėjama triukšmo sklaidos nuo vėjo jėgainių problema. Pagrindiniai tyrimo objektai yra vėjo jėgainių parkų skleidžiamas triukšmas ir aplinkai nekenksmingos, cilindrinę struktūrą turinčios, žemo dažnio triukšmą slopinančios medžiagos. Darbe sprendžiami keli pagrindiniai uždaviniai: nustatomas ir įvertinamas žemo dažnio triukšmo ir infragarso susidarymas ir sklaida pramoninių vėjo jėgainių parkuose ir jų išorėje, taip pat nustatomos efektyvios gyvenviečių apsaugos priemonės nuo vėjo jėgainių keliamo žemo dažnio triukšmo. Disertaciją sudaro įvadas, trys skyriai, bendrosios išvados, rekomendacijos, naudotos literatūros ir autoriaus publikacijų disertacijos tema sąrašai. Įvadiniame skyriuje aptariama tiriamoji problema, darbo aktualumas, aprašomas tyrimų objektas, formuluojamas darbo tikslas bei uždaviniai, aprašoma tyrimų metodika, darbo mokslinis naujumas, darbo rezultatų praktinė reikšmė, ginamieji teiginiai. Įvado pabaigoje pristatomos disertacijos tema autoriaus paskelbtos publikacijos ir pranešimai konferencijose bei disertacijos struktūra. Pirmasis skyrius skirtas literatūros analizei. Jame pateikta vėjo jėgainių ir jų keliamo triukšmo apžvalga. Apžvelgtos vėjo energetikos būklė Lietuvoje, vėjo jėgainių konstrukcijos, vėjo jėgainių triukšmo formavimosi principas, triukšmo matavimo metodikos. Analizuojamos skleidžiamo aerodinaminio triukšmo mažinimo priemonės bei žemo dažnio triukšmo slopinimo būdai ir izoliuojančių medžiagų panaudojimas. Skyriaus... [toliau žr. visą tekstą] / The dissertation examines the issue of noise emission from the wind farms. The objects of investigation are wind farm noise and environmentally friendly low-frequency noise-deadening materials with cylindrical structure. The paper addresses a number of key objectives: to determine and assess low frequency noise and infrasound emission and dispersion both inside and outside the industrial wind farms, as well as to establish effective measures for protection of settlements against low-frequency noise generated by wind turbines. The dissertation consists of an introduction, three chapters, conclusions, recommendations, list of references and the author's publications on the topics. The introductory chapter discusses the research problem, the relevance of the investigation, describes the object of the research, states the aims and objectives, presents the research methodology, scientific novelty, practical significance of the results, and the statements for defending. It also highlights practical significance of the dissertation presents the author's publications and presentations at conferences, and explains the structure of the thesis. The first chapter is devoted to literature review. It provides an overview of general problems related to wind and noise produced by wind turbines, an overview of the outlook on wind energy in Lithuania, and a discussion of wind turbine constructions. The chapter presents the analysis of aerodynamic noise emitted by means of low-frequency noise... [to full text]

The dissertation examines the issue of noise emission from the wind farms. The objects of investigation are wind farm noise and environmentally friendly low-frequency noise-deadening materials with cylindrical structure. The paper addresses a number of key objectives: to determine and assess low frequency noise and infrasound emission and dispersion both inside and outside the industrial wind farms, as well as to establish effective measures for protection of settlements against low-frequency noise generated by wind turbines. The dissertation consists of an introduction, three chapters, conclusions, recommendations, list of references and the author's publications on the topics. The introductory chapter discusses the research problem, the relevance of the investigation, describes the object of the research, states the aims and objectives, presents the research methodology, scientific novelty, practical significance of the results, and the statements for defending. It also highlights practical significance of the dissertation presents the author's publications and presentations at conferences, and explains the structure of the thesis. The first chapter is devoted to literature review. It provides an overview of general problems related to wind and noise produced by wind turbines, an overview of the outlook on wind energy in Lithuania, and a discussion of wind turbine constructions. The chapter presents the analysis of aerodynamic noise emitted by means of low-frequency noise... [to full text] / Disertacijoje nagrinėjama triukšmo sklaidos nuo vėjo jėgainių problema. Pagrindiniai tyrimo objektai yra vėjo jėgainių parkų skleidžiamas triukšmas ir aplinkai nekenksmingos, cilindrinę struktūrą turinčios, žemo dažnio triukšmą slopinančios medžiagos. Darbe sprendžiami keli pagrindiniai uždaviniai: nustatomas ir įvertinamas žemo dažnio triukšmo ir infragarso susidarymas ir sklaida pramoninių vėjo jėgainių parkuose ir jų išorėje, taip pat nustatomos efektyvios gyvenviečių apsaugos priemonės nuo vėjo jėgainių keliamo žemo dažnio triukšmo. Disertaciją sudaro įvadas, trys skyriai, bendrosios išvados, rekomendacijos, naudotos literatūros ir autoriaus publikacijų disertacijos tema sąrašai. Įvadiniame skyriuje aptariama tiriamoji problema, darbo aktualumas, aprašomas tyrimų objektas, formuluojamas darbo tikslas bei uždaviniai, aprašoma tyrimų metodika, darbo mokslinis naujumas, darbo rezultatų praktinė reikšmė, ginamieji teiginiai. Įvado pabaigoje pristatomos disertacijos tema autoriaus paskelbtos publikacijos ir pranešimai konferencijose bei disertacijos struktūra. Pirmasis skyrius skirtas literatūros analizei. Jame pateikta vėjo jėgainių ir jų keliamo triukšmo apžvalga. Apžvelgtos vėjo energetikos būklė Lietuvoje, vėjo jėgainių konstrukcijos, vėjo jėgainių triukšmo formavimosi principas, triukšmo matavimo metodikos. Analizuojamos skleidžiamo aerodinaminio triukšmo mažinimo priemonės bei žemo dažnio triukšmo slopinimo būdai ir izoliuojančių medžiagų panaudojimas. Skyriaus... [toliau žr. visą tekstą]

A redução das dimensões dos dispositivos semicondutores para escalas submicrométricas impõe diversos desafios no projeto de circuitos integrados. O impacto das variações intrínsecas afetando parâmetros elétricos cresce em importância à medida que a área dos dispositivos adentra a faixa nanométrica. Dentre essas variações estão flutuações nas tensões e correntes de terminal causadas pelas diferentes formas de ruído intrínseco dos dispositivos MOS. Este trabalho apresenta um estudo sobre o impacto do ruído elétrico no desempenho de circuitos MOS. Um novo modelo para simulação do Random Telegraph Signal (RTS) no domínio do tempo é utilizado. Uma metodologia de simulação para contabilizar o ruído térmico em simulações transientes também é proposta. A partir desses modelos de simulação de dispositivos, o trabalho de pesquisa analisa o impacto da variabilidade de parâmetros elétricos em nível de circuito. As simulações focam na caracterização da pureza espectral em osciladores em anel de sinal diferencial. Diversas topologias são apresentadas e posteriormente comparadas em termos do jitter no período de oscilação. / The shrinking of semiconductors devices dimensions to submicron scales introduces many challenges in integrated circuit design. The impact of intrinsic variability affecting electrical parameters increases in importance as transistors enter the nanometric range. Among these variations are fluctuations in terminal voltages and currents caused by different forms of intrinsic noise of MOS devices A new model for Random Telegraph Signal (RTS) simulation in time-domain is utilized. A simulation methodology to account for thermal noise effects in transient simulations is also proposed. Using these simulation models, this research work analyses the impact of electrical noise at circuit level. The simulations focus on the characterization of spectral purity in differential ring oscillators. Different topologies are presented and compared in terms of jitter in the period of oscillation.

Les transistors organiques attirent actuellement beaucoup d'attention en raison des avantages uniques par rapport à leur homologue inorganique. En revanche, la compréhension physique du fonctionnement et du transport des porteurs de charge est très limitée. L'objectif de cette thèse est de contribuer à apporter une meilleure compréhension des transistors organiques. Le Chapitre 1 présente les semi-conducteurs organiques : le mécanisme de conduction, les paramètres essentiels, les matériaux typiques etc. Le Chapitre 2 discute des transistors organi-ques en termes de structures, de mécanismes de fonctionnement, de paramètres principaux et des procédés de fabrication. Le Chapitre 3 étudie la caractérisation statique. Après les méthodes classiques, la méthode de la fonc-tion Y est introduite. Subséquemment, des techniques pour extraire les paramètres principaux sont présentées sé-parément. Enfin, les résultats expérimentaux sur nos échantillons sont exposés. Sur la base des données mesurées, les travaux de modélisation sont présentés dans le chapitre 4. Premièrement, une solution de l'équation Poisson est introduite qui donne la distribution de potentiel et donc la distribution de porteurs dans le film organique. Avec la prise en compte des pièges, les résultats obtenus par simulation sont en bon accord avec les données expérimen-tales. A partir de mesures des caractéristiques de courant –tension effectuées à basse température, on propose une procédure d'analyse de la mobilité en utilisant l'intégrale de Kubo-Greenwood. Ensuite, prenant en compte la dis-tribution de porteurs dans le film organique, une solution de l'équation de Poisson est donnée et la mobilité effec-tive est calculée en fonction de la tension de grille et de la température. Le Chapitre 5 est consacré à l'analyse du bruit à basse fréquence. On étudie d'abord le bruit du canal où une domination du bruit provenant des contacts est observée. En conséquence, une méthode TLM pour l'extraction du bruit des contacts est présentée. Ensuite, un procédure d'analyse des sources de bruit dû au contact est aussi proposée. Les résultats de bruit obtenus sur des transistors organiques de différentes origines sont également discutés à la fin. / Organic transistors recently attract much attention because of their unique advantages over the conventional inorganic counterparts. However, the understanding of their operating mechanism and the carrier transport process are still very limited, this thesis is devoted to such a subject. Chapter 1 presents the organic semiconductors regarding carrier transport, parameters, typically applied materials. Chapter 2 describes the issues related to organic transistors: structure, operating mechanism, principal parameters and fabrication technologies. Chapter 3 deals with the static properties characterization. The commonly used methods are firstly presented and then the Y function method is introduced. Afterwards, the characterization methods for principles parameters are separately discussed. The experimental results on our organic transistors are finally described. Chapter 4 focuses on the mod-eling on the basis of the experimental data, regarding DC characteristics modeling with a solution for Poisson's equation, carrier mobility modeling with using Kubo-Greenwood integral as well as a theoretical analysis of OFETs' carrier mobility involving a solution of Poisson's equation. Chapter 5 analyzes the low-frequency noise in organic transistors. One firstly addresses the channel noise sources and then concentrates on the contact noise extraction and contact noise sources diagnosis. The noise measurements on other samples are also presented.

A redução das dimensões dos dispositivos semicondutores para escalas submicrométricas impõe diversos desafios no projeto de circuitos integrados. O impacto das variações intrínsecas afetando parâmetros elétricos cresce em importância à medida que a área dos dispositivos adentra a faixa nanométrica. Dentre essas variações estão flutuações nas tensões e correntes de terminal causadas pelas diferentes formas de ruído intrínseco dos dispositivos MOS. Este trabalho apresenta um estudo sobre o impacto do ruído elétrico no desempenho de circuitos MOS. Um novo modelo para simulação do Random Telegraph Signal (RTS) no domínio do tempo é utilizado. Uma metodologia de simulação para contabilizar o ruído térmico em simulações transientes também é proposta. A partir desses modelos de simulação de dispositivos, o trabalho de pesquisa analisa o impacto da variabilidade de parâmetros elétricos em nível de circuito. As simulações focam na caracterização da pureza espectral em osciladores em anel de sinal diferencial. Diversas topologias são apresentadas e posteriormente comparadas em termos do jitter no período de oscilação. / The shrinking of semiconductors devices dimensions to submicron scales introduces many challenges in integrated circuit design. The impact of intrinsic variability affecting electrical parameters increases in importance as transistors enter the nanometric range. Among these variations are fluctuations in terminal voltages and currents caused by different forms of intrinsic noise of MOS devices A new model for Random Telegraph Signal (RTS) simulation in time-domain is utilized. A simulation methodology to account for thermal noise effects in transient simulations is also proposed. Using these simulation models, this research work analyses the impact of electrical noise at circuit level. The simulations focus on the characterization of spectral purity in differential ring oscillators. Different topologies are presented and compared in terms of jitter in the period of oscillation.