Le principal objectif de cette thèse est de montrer le potentiel pour l’électronique organique de films moléculaires minces liés de façon covalente au substrat et déposés par greffage électro- chimique. Ces couches organiques de 5 à 100 nm d’épaisseur visent à proposer une alternative aux films minces organiques d’épaisseur supérieure à 100 nm et aux couches mono-moléculaires autoassemblées d’épaisseur comprise entre 1 et 5 nm.Ce travail a d’abord permis d’établir les conditions optimales de greffage de trois différents sels de diazonium : un dérivé de la tris-bipyridine fer (II), un sel de diazonium comportant une longue chaîne fluorée et un autre comportant une fonction thiol. En particulier, un contrôle fin de l’épaisseur des films est démontré sur des électrodes patternées micrométriques adaptées à la réalisation de dispositifs.L’électrogreffage de doubles couches est ensuite étudié. Il consiste à utiliser une électrode electrogreffée par des molécule électroactives, ici le dérivé de la tris-bipyridine fer (II), comme électrode de travail pour l’électrogreffage d’un second sel de diazonium. Cette technique permet de former des couches organiques d’épaisseur contrôlée par la première couche et présentant des fonctions terminales contrôlées par le choix du second composé (ici, fonctions thiols ou chaînes fluorées). L’intérêt de ces couches fonctionnelles est ensuite évalué dans des jonctions verticales métal-molécules-métal utilisant différents types d’électrodes supérieures : des électrodes imprimées à partir d’une solution de nanoparticules d’or, suivant un procédé élaboré dans cette thèse, et des électrodes fabriquées à partir de métaux évaporés sous vide. Enfin, des transistors à base de MoS2 utilisant 30 nm de ces couches greffées comme diélectrique de grille sont fabriqués et étudiés. Leurs performances (mobilité électronique de 46 cm2.(V.s)-1, rapport ION/IOFF de 9.107, etc.) confirment la qualité de ces isolants organiques électrogreffés. La méthode s’avère ainsi efficace et versatile pour la préparation de couches organiques robustes d’épaisseur contrôlée et aux propriétés de surface ajustables. / The main objective of this PhD thesis is to show the potential for organic electronics of molecular thin films covalently bounded and formed by electrochemical grafting. These 5 to 100 nm thick layers aim to propose an alternative to organic thin films of thickness above 100 nm and to self-assembled monolayers of thickness between 1 and 5 nm.This work first establishes the optimal electrografting conditions of three diazonium salts : a derivative from the tris-bipyridine iron (II), a diazonium salt with a long fluorinated chain and another with a thiol function). In particular, a fine tuning of the thickness of the resulting layers is demonstrated on micrometric patterned electrodes.Double layer electrografting is then studied. It consists in using an electrode electrografted with electroactive molecules, here the tris-bipyridine iron (II) derivative, as a working electrode for the electrografting of a second diazonium salt. This technique allows the formation of organic double-layers of thickness controlled by the first layer and presenting terminal functions controlled by the choice of the second compound (here, thiol functions or fluorinated chains).The potential of these layers is then evaluated in vertical metal-molecules-metal junctions using various top electrodes : electrodes printed from an aqueous gold nanoparticle ink through a method developed in this thesis, and electrodes made by metal evaporation in vacuum. To conclude, field-effect transistors based on MoS2 using these electrografted thin layers as gate-dielectric are fabricated and studied. Their performances (electronic mobility of 46 cm2.(V.s)-1, ION/IOFF ratio of 9.107,etc.) confirm the quality of these organic electrografted insulators. The method is thus efficient and versatile for the preparation of robust organic layers with adjustable surface properties and thickness.

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Les capteurs biologiques basés sur l’électronique nanométrique ont la propriété intéressante d’être à l’échelle des molécules étudiées. Plus spécifiquement, grâce à leurs propriétés électroniques exceptionnelles, les transistors à effet de champ à base de graphène (TECG) permettent des mesures électriques locales à grandes vitesses d’acquisition et sur de longues durées, offrant un cadre idéal pour la biodétection et l’étude de la cinétique moléculaire. Le présent mémoire traite de l’analyse, la mesure et la fonctionnalisation des TECG dans l’optique d’en faire des biocapteurs performants. En introduction, nous décrirons les propriétés électroniques du graphène ainsi que les principaux concepts reliés aux transistors de graphène et à la détection biologique. Puis, nous établirons les trois objectifs qui seront élaborés en autant de chapitres. Dans le premier chapitre, nous présenterons une revue de littérature critique qui cible l’analyse statistique et l’assemblage de l’interface de détection comme facteurs déterminants de la performance à l’aide d’analyses originales et d’une description approfondie de l’état du domaine. Dans le deuxième chapitre, nous présenterons des ajustements concrets aux sysèmes expérimentaux basés sur les recommandations émises dans la revue. D’abord, nous améliorons la productivité de la fabrication des transistors, puis développons une instrumentation permettant de mesurer plusieurs capteurs en parallèle. Dans le troisième chapitre, nous prendrons avantage de ces modifications pour présenter dans le deuxième article une méthode permettant une fonctionnalisation du graphène à la fois contrôlée et solide. En utilisant le voltage de grille, nous initions et suspendons la fonctionnalisation covalente du graphène aux sels de diazonium afin d’obtenir le taux de greffage désiré, tout en observant la réaction en temps-réel. Ainsi, par nos avancées méthodologiques et d’instrumentation, nous résolvons un enjeu critique du développement de la chimie de surface, centrale à la performance de biodétection. / Nanoscale electronics are a promising tool for biosensing as they fit their target’s size and allow for local, fast-paced measurements over long time scales. Because of their exceptional electronic properties, graphene field-effect transistors (GFETs) are excellent candidates for biosensing and studying molecular kinetics. This work discusses the analysis, measurement, and functionalization of GFETs as optimized biosensors. In the introduction, we describe the electronic properties of graphene and the main concepts related to GFETs and biodetection. We also establish the three aims of the project, elaborated in three chapters. The first chapter contains a critical literature review that uses original analyses and a thorough state-of-the-field to target statistical analysis and the biorecognition interface assembly as determining factors in sensing performance. In the second chapter, we present the practical adjustments to the experimental systems based on the review’s recommendations. First, we increase the productivity of device fabrication, then we develop a multiplexed electrical measurement setup. In the third chapter, we take advantage of these modifications to present in the second article a method for stable and controlled functionalization. Using the gate voltage, we start and stop the covalent functionalization of graphene with aryldiazonium salts to get the desired grafting level, while observing the reaction in real-time. Thus, with our advances in methodology and instrumentation, we solve a critical aspect of surface chemistry, central for biodetection performance

Organic field-effect transistors (OFETs) have attracted tremendous attention due to their flexibility, transparency, easy processiblity and low cost of fabrication. High-performance OFETs are required for their potential applications in the organic electronic devices such as flexible display, integrated circuit, and radiofrequency identification tags. One of the major limiting factors in fabricating high-performance OFET is the large interfacial barrier between metal electrodes and OSC which results in low charge injection from the metal electrodes to OSC. In order to overcome the challenge of low charge injection, carbon nanotubes (CNTs) have been suggested as a promising electrode material for organic electronic devices. In this dissertation, we study the effect of carbon nanotube (CNT) density in CNT electrodes on the performance of organic field effect transistor (OFETs). The devices were fabricated by thermal evaporation of pentacene on the Pd/single walled CNT (SWCNT) electrodes where SWCNTs of different density (0-30/um) were aligned on Pd using dielectrophoresis (DEP) and cut via oxygen plasma etching to keep the length of nanotube short compared to the channel length. From the electronic transport measurements of 40 devices, we show that the average saturation mobility of the devices increased from 0.02 for zero SWCNT to 0.06, 0.13 and 0.19 cm2/Vs for low (1-5 /µm), medium (10-15 /µm) and high (25-30 /µm) SWCNT density in the electrodes, respectively. The increase is three, six and nine times for low, medium and high density SWCNTs in the electrode compared to the devices that did not contain any SWCNT. In addition, the current on-off ratio and on-current of the devices are increased up v to 40 times and 20 times with increasing SWCNT density in the electrodes. Our study shows that although a few nanotubes in the electrode can improve the OFET device performance, significant improvement can be achieved by maximizing SWCNT/OSC interfacial area. The improved OFET performance can be explained due to a reduced barrier height of SWCNT/pentacene interface compared to metal/pentacene interface which provides more efficient charge injection pathways with increased SWCNT/pentacene interfacial area.

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Power device robustness refers to the device capability of withstanding abnormal events in power electronics applications, which is one of the key device capabilities that are desired in numerous applications. While the current robustness test methods and qualification standards are developed across the 70 years of Silicon (Si) device history, their applicability to the recent wide bandgap (WBG) power devices is questionable. While the market of WBG power devices has exceeded $1 billion and is fast growing, there are many knowledge gaps regarding their robustness, including the failure or degradation physics, testing methods, and lifetime extraction. This dissertation work studies the robustness of Gallium Nitride (GaN) power device. The structures of many GaN power devices are fundamentally different from Si or Silicon Carbide (SiC) power devices, leading to numerous open questions on GaN power device robustness. Based on the device structure, this dissertation is divided into two parts: The first half discusses the robustness of lateral GaN high electron mobility transistor (HEMT), which recently sees rapid adoption among wide range of applications such as the power adapter and chargers, data center, and photovoltaic panels. The absence of p-n junction between the source and drain of GaN HEMT results in the lack of avalanche mechanism. This raises a concern on the device capability of withstanding surge-energy or overvoltage stress, which hinders the penetration of GaN HEMTs in broader applications. To address this concern, the study begins with conducting the single-event unclamped inductive switching (UIS) test on two mainstream commercial p-gate GaN HEMTs with the Ohmic- and Schottky-type gate contacts, where the GaN HEMT is found to withstand surge energy through a resonant energy transfer between the device capacitance and the loop inductance. The failure mechanism is identified to be a pure electrical breakdown determined by device transient breakdown voltage (BV). The BV of GaN HEMT is further found to be "dynamic" from the switching tests with various pulse widths and frequencies, which is further explained by the time-dependent buffer trapping. This dynamic BV (BVDYN) phenomenon indicates that the static or single-pulse test may not reveal the true BV of GaN HEMT in high frequency switching applications. To address this gap, a novel testbed based on a zero-voltage-switching converter with an active clamping circuit is developed to enable the stable switching with kilovolt overvoltage and megahertz frequency. The overvoltage failure boundaries and failure mechanisms of four commercial p-gate GaN HEMTs from multiple vendors are explored. In addition to the frequency-dependent BVDYN, two new failure mechanisms are observed in some devices, which are attributable to the serious carrier trapping in GaN HEMTs under the high-frequency overvoltage switching. At last, based on the findings in the high frequency overvoltage test (HFOT), a physics-based lifetime model for commercial GaN HEMTs utilizing the device on resistance (RON) shift is established and validated by experimental results. Overall, the switching-based test methodology and experimental results provide critical references for the overvoltage protection and qualification of GaN power HEMTs. The second half of the dissertation discusses the robustness of the vertical GaN fin-channel junction field effect transistor (Fin-JFET), a promising pre-commercialized GaN power device with the p-n junction embedded between the gate and drain which enables the avalanche breakdown. The robustness study on GaN JFET follows similar test approaches as Si metal-oxide-semiconductor field-effect transistor (MOSFET) with two key interests: the avalanche and short circuit capabilities. The avalanche breakdown is first explored via the single-event and repetitive UIS tests and under various gate drivers, from which an interesting "avalanche-through-fin-channel" mechanism is discovered. By leveraging this avalanche path, the electro-thermal stress migrates from the main blocking p-n junction to the n-GaN fin channel, resulting in a very favorable failure-to-open-circuit signature. The single-pulse critical avalanche energy density (EAVA) of vertical GaN Fin-JFET is measured to be as high as 10 J/cm2, which is much higher than the Si MOSFET and comparable to the SiC MOSFET. The short circuit capability is explored utilizing the hard-switching fault on the 650-V rated GaN Fin-JFET, with a gate driving circuit identical to the switching application to best mimic device operation in converters. The short circuit withstanding time is measured to be 30.5 µs at an input voltage of 400 V, 17.0 µs at 600 V, and 11.6 µs at 800 V, all among the longest reported for 600-700 V normally-off transistors. In addition, the failure-to-open-circuit signature is also shown in the single-event and repetitive short circuit tests; all devices retain the avalanche breakdown after failure, which is highly desirable for system applications. These results suggest that, while GaN HEMT is already available in market, vertical GaN Fin-JFET shows superior avalanche and short-circuit robustness and thereby can unlock great potential of GaN devices for applications like automotive powertrains, motor drives, and grids. / Doctor of Philosophy / In recent years, many power electronics applications such as data centers and electric vehicles have witnessed a rapid increase in the adoption of wide bandgap (WBG) power devices. The Gallium Nitride (GaN) device is one of the most attractive candidates in WBG devices, owing to its good tradeoff between breakdown voltage and on resistance, as well as the small gate charge that enables high frequency switching. For power devices, their robustness against overvoltage and overcurrent stresses is as important as their performance under normal operations. However, the new material, new device structure, and new device physics in GaN power devices brought up many open knowledge gaps in their robustness study, particularly under the dynamic operation in switching circuits. This dissertation presents the work in exploring the robustness of GaN power devices. Based on the device structure, the discussion is divided in two parts: The first half of the dissertation focuses on the overvoltage robustness of the lateral GaN High Electron Mobility Transistor (HEMT), the commercially available device covering 30 to 900 V voltage classes. A key feature of this device is the lack of p-n junction between source and drain, leading to an absence of avalanche capability. The study is conducted on mainstream, commercial p-gate GaN HEMTs, with a combination of circuit testing, microscale failure analysis, and physics-based device simulation. The main contribution is on three aspects: identifying the single-event and high-frequency repetitive overvoltage boundaries of GaN HEMT, unveiling the failure and degradation mechanisms under transient overvoltage conditions, and providing guidelines to GaN HEMT device users with proper robustness test methodology for device qualification and screening. The second half of the dissertation focuses on the robustness of vertical GaN fin-channel junction field effect transistor (Fin-JFET), a promising pre-commercial GaN power device with the p-n junction implemented between the source and drain. The robustness tests follow the classic approaches deployed for Silicon power devices, where both the avalanche and short circuit capabilities are investigated. From the single-event and repetitive test results, the GaN JFET shows excellent avalanche robustness with a desirable failure-to-open-circuit behavior, as well as a critical avalanche energy (EAVA) of 10 J/cm2 that is higher than the Silicon metal-oxide-semiconductor field-effect transistor (MOSFET) and comparable to the Silicon Carbide MOSFET. For a 650-V rated GaN Fin-JFET, a record high 30.5 μs short circuit time is demonstrated under the hard-switching fault condition at 400 V input voltage. Overall, the results show great potential of GaN power devices for the power electronics applications that involve more stressful operation conditions for devices.

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