Cette thèse a exploré comment, en mélangeant des matériaux avec des propriétés électriques différentes, il est possible de fabriquer des transistors avec des performances accrues. Des transistors organiques à effet de champ basés sur un oligothiophène (DH4T) ont été fabriqués et optimisés jusqu’à ce que les mobilités mesurées fussent supérieures à celles observées dans des films évaporés. Ces résultats ont été obtenus par le contrôle précis des interfaces et de la vitesse d’évaporation. Des polymères de type p ont été mélangés à des polymères de type n. Chaque solution obtenue a été utilisée pour la fabrication de transistors ambipolaires. Les transistors ont été caractérisés et il a été possible de fabriquer des transistors avec des mobilités équilibrées pour chaque paire de polymères. Des transistors à effet de champ basés sur un mélange de P3HT et d’une molécule photochromique (DAE-Me) ont été fabriqués. Le courant a été mesuré pendant et entre les irradiations et il a été démontré qu’une mémoire non-volatile à multiple niveaux peut être fabriquée / This thesis explored how, by blending of materials with different electrical characteristics, it is possible to fabricate transistors with new or improved performances. First, organic field-effect transistors based on a single oligothiophene, DH4T, were fabricated and optimized until the measured mobility was superior to that observed in vacuum deposited films. This was achieved through careful tuning of the interfaces using self-assembled monolayers and by strong control of the solvent- evaporation rate. P-type polymers were blended with an n-type polymer. Each resulting solution was used for the fabrication of ambipolar field-effect transistors. These devices were characterized and it was found that for each pair of p- and n-type polymers, a transistor with balanced mobilities and high Ion/Ioff could be fabricated. Finally field-effect transistors based on a blend of P3HT and a photoswitchable diarylethene (DAE-Me) were fabricated. The current was measured during and between irradiations and it was demonstrated that a non-volatile multilevel memory could be fabricated.

The growing applications for IoT devices have caused an increase in the study of low power consuming circuit design to meet the requirement of devices to operate for various months without external power supply. Scaling down the conventional CMOS causes various complications to design due to CMOS properties, therefore various non-conventional CMOS design techniques are being proposed that overcome the limitations. This thesis focuses on some of those emerging and novel low power design technique namely Adiabatic logic and low power devices like Magnetic Tunnel Junction (MTJ) and Carbon Nanotube Field Effect transistor (CNFET). Circuits that are used for large computations (multipliers, encryption engines) that amount to maximum part of power consumption in a whole chip are designed using these novel low power techniques.

La détection de faibles concentrations d’acides nucléiques est essentielle pour certaines applications comme la biologie médicale, où elle permet le diagnostic d’une multitude de pathologies par l’identification de biomarqueurs spécifiques. Par rapport aux techniques traditionnelles de détection par voie biochimique, la détection électrique par effet de champ présente l’avantage d’être une mesure directe, sans marquage, et à réponse rapide. Les transistors à nanofils semiconducteurs sont des dispositifs prometteurs qui permettent potentiellement d’atteindre des limites de détection très basses et une sensibilité élevée, grâce à leur grand rapport surface/volume et leurs propriétés électroniques uniques. Le carbure de silicium (SiC) est un matériau semiconducteur dont les qualités le rendent particulièrement adapté aux applications visées, telles que sa très grande stabilité physico-chimique et biocompatibilité.Dans cette thèse, des transistors à effet de champ à base de nanofils de Si et SiC ont été conçus dans une approche descendante pour être fabriqués par photolithographie. Un procédé de fabrication basé sur la filière silicium a été développé et optimisé afin de réaliser des dispositifs à nanofils et à nanorubans de Si de manière reproductible. Une étude détaillée a permis de démontrer la stabilité chimique supérieure des nanofils de SiC par rapport aux nanofils de Si en conditions physiologiques. Fort de ce résultat, nous avons exploré deux approches pour l’élaboration d’une couche mince de SiC autour de ces nanostructures de Si, pour leur conférer cette résistance chimique en milieu liquide. Ces dispositifs cœur-coquille Si/SiC reproductibles ont finalement été fonctionnalisés et intégrés dans un système microfluidique complet afin de réaliser des premières mesures novatrices de détection de pH et d’ADN en temps réel et en milieu liquide. / Sensing of low concentrations of nucleic acids is essential to a variety of applications such as bio-medical analysis, in which case it allows the diagnosis of pathologies by identifying specific biomarkers. Compared to traditional sensing techniques based on biochemistry, the advantage of electrical field-effect detection is that it relies on a direct, label-free, and fast-response measurement. Transistors based on semiconducting nanowires are promising devices that theoretically enable very low detection limits and a high sensitivity, thanks to their high surface-to-volume ratio and unique electronic properties. Silicon carbide (SiC) is a semiconductor material with qualities such as very high physical and chemical stability and high biocompatibility, which make it particularly suited for aforementioned applications.In this thesis, field-effect transistors based on Si and SiC nanowires were designed with a top-down approach to be fabricated using photolithography techniques. The Si-based process was developed and optimized in order to fabricate reproducible devices made of nanowires and nanoribbons. A detailed study was conducted to demonstrate the superior chemical stability of SiC nanowires over Si nanowires under physiological conditions. Based on these results, we investigated two ways of elaborating a thin SiC layer around these Si nanostructures to provide them with its chemical resistance in liquid medium. These reproducible core-shell Si/SiC devices were eventually functionalized and integrated into a microfluidic system in order to achieve novel measurements of DNA detection in real time and in liquid media.

Organic semiconductors (OSCs) have already been shown to have great potential to play an important role in the future of clean energy generation (organic solar cells) and provide energy efficient lighting (organic light-emitting diodes, OLED). Prior research has found that the light-emission efficiency of OLED is severely limited by the magnetic state (technically the spin-configuration) of the light-emission process. In this thesis, we work on the processes using external magnetic fields that can overcome these magnetic limitations. A major focus of this research is to enhance the performance of OLED, while at the same time to unravel the scientific mechanisms by which magnetic fields act on OSCs devices. Thermally activated delayed fluorescence (TADF) is a next-generation OLED emission technology which enables nearly 100% light-emission efficiency without using heavy precious metals. TADF characteristics depend on the probability of reverse intersystem crossing (RISC) from the triplet excited states (T1) to singlet excited states (S1). The conversion (T1 to S1) process depends strongly on spin dynamics, thus we predict a dramatic magnetic field effects (MFEs) in such TADF OLED devices. In subsequent experiments we observed that changes in TADF devices due to various forms of electrical stress can lead to enormous increases in magnetic field effects (MFEs) on the current (> 1400%) and electroluminescence (> 4000%). Our work provides a flexible and inexpensive pathway towards magnetic functionality and field sensitivity in current organic devices. Such OLED pave the way for novel magnetic sensitive OSCs devices with integrated optical, electronic and magnetic characteristics. Organic magnetoresistance (OMAR) has been observed to alter the current and efficiency of OLED without any ferromagnetic components. Here we utilizes slight alterations to the device properties, the addition of a radical-doped functional layer, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere, to address the assumption about the importance of the hyperfine interaction and to attempt to differentiate between the different models for OMAR. A feature where the magnitude of OMAR exhibits a plateau over a wide range of doping fraction was observed at all temperatures investigated. This phenomenon is well explained by a theory in which a single dopant spin strongly interacts, by exchange, with one of the bottleneck sites. A similar can be used to explain the efficiency increases observed in organic solar cells for certain doping fractions.

Le développement des transistors organiques, ces dernières années, a permis une nette amélioration de leurs performances et de leur stabilité. Ceci a été possible, notamment, grâce à une meilleure compréhension des mécanismes régissant le transport de charges dans ces dispositifs. Cependant, certains phénomènes restent encore à éclaircir, en particulier au niveau de l’interface entre le semi-conducteur et le diélectrique. Le piégeage des porteurs de charges qui est une des principales causes de perturbations du transport de charges dans les transistors organiques, en est un. Cette thèse se propose donc, d’étudier ce phénomène dans des transistors à base de pentacène.Les groupements polaires, et plus particulièrement les groupements hydroxyles, présents à l’interface entre l’isolant et le semi-conducteur, sont les principaux responsables du piégeage des porteurs de charges dans les transistors organiques. Afin de limiter leur présence, une technologie basée sur l’emploi d’une couche interfaciale diélectrique passivante, pauvre en groupements hydroxyles, à base de fluorure de calcium, a été mise en place. L’influence de cette couche sur le comportement de transistors à base de pentacène a été étudiée, de même que le vieillissement de ces dispositifs sous différentes conditions de stockage (sous vide et à l’air) et sous contrainte électrique.Ainsi, il a été mis en évidence qu’une couche de fluorure de calcium d’une épaisseur trop importante (de l’ordre de 5 nm) modifie la morphologie de la couche de pentacène, ce qui se traduit par une quasi-disparition du transport de charges dans le pentacène en configuration de transistor à effet de champ. Les études de vieillissement ont montré que sous l’effet de la couche interfaciale de CaF2, même d’une très fine épaisseur (de quelques nanomètres), une quantité plus importante d’humidité est présente dans la couche de pentacène, probablement à cause de la nature hygroscopique du fluorure de calcium. / These recent years, Organic Field-Effect Transistor (OFET) development has significantly improved it performances and it stability. This was made possible, through a better understanding of the mechanisms governing charge transport in these devices. However, some phenomena remain unclear, in particular, at the interface between the semiconductor and the dielectric. Charge carrier trapping which is one of the main causes of charge transport disturbance in organic transistors, is one of them. So, this work aims to investigate such phenomena in pentacene-based transistors.Polar groups and particularly, hydroxyl groups, located at the insulator-semiconductor interface, are the main sources of charge carriers trapping in OFET. To prevent their presence, an OFET fabrication technology based on a passivating dielectric, poor of hydroxyl groups, calcium fluoride-based interfacial layer has been developed. Effect of this layer on pentacene-based transistors operation has been studied, as well as these devices aging under different storage atmosphere (in vacuum and in air) and under electrical stress.Thus, it has been highlighted that an interfacial layer of calcium fluoride with a too high thickness (around 5 nm) changes pentacene layer morphology which results in a quasi-disappearance of charge transport in pentacene in OFET configuration. Aging studies showed that under the effect of CaF2 interfacial layer, even with a very thin thickness (a few nanometers), a greater quantity of moisture is induced in pentacene layer probably due to the hygroscopic nature of calcium fluoride.

This thesis consists of three main parts, starting with the use of improved nitridation processes to grow acceptable quality gate oxides on silicon carbide (SiC)[1]–[7], to the comprehensive investigation of basic electron-hole generation process in 4H SiC-based metal–oxide–semiconductor (MOS) capacitors [8], [9], and concluding with the experimental demonstration and analysis of nonvolatile characteristics of 4H SiC-based memory devices [10]–[15]. In the first part of the thesis, two improved versions of nitridation techniques have been introduced to alleviate oxide-growth rate and toxicity problems. Using a combination of nitridation and oxidation processes, a sandwich technique (nitridation–oxidation–nitridation) has been proposed and verified to solve the lengthy and expensive oxide-growing process in direct nitric oxide (NO) gas [1]. The nitrogen source from the toxic-NO gas has been replaced by using a nontoxic nitrous oxide (N2O) gas. The best combination of process parameters in this gas is oxide-growing temperature at 1300oC with 10% N2O [2], [3]. The quality of nitrided gate oxides obtained by this technique is lower than the sandwich technique [6], [13]. Using 4H SiC-based MOS with nitrided gate oxides grown by either of the abovementioned nitridation techniques, the fundamentals of electron-hole generation have been investigated using high-temperature capacitance–transient measurements. The contributions of carrier generation, occurring at room temperature, in the bulk and at the SiC–SiO2 interface are evaluated and compared using a newly developed method [8], [9]. The effective bulk-generation rates are approximately equal for both types of nitrided oxides, whereas the effective surface-generation rates have been shown to exhibit very strong dependencies on the methods of producing the nitrided gate oxide. Based on analysis, the prevailing generation component in a SiC-based MOS capacitor with nitrided gate oxide is at SiC–SiO2 interface located below the gate. Utilizing the understanding of electron-hole generation in SiC, the nonvolatile characteristics of memory device fabricated on SiC have been explored. The potential of developing a SiC-based one-transistor one-capacitor (1T/1C) nonvolatile-dynamic memory (NDM) has been analyzed using SiC-based MOS capacitors as storage elements or test structures. Three possible leakage mechanisms have been evaluated [10]–[16]: (1) leakage via MOS capacitor dielectric, (2) leakage due to electron-hole generation in a depleted MOS capacitor, and (3) junction leakage due to generation current occurred at a reverse-biased pn junction surrounding the drain region of a select metal–oxide– semiconductor field–effect–transistor (MOSFET). Among them, leakage through capacitor oxide remains an important factor that could affect the nonvolatile property in the proposed device, whereas others leakage mechanisms are insignificant. Based on the overall results, the potential of developing a SiC-based 1T/1C NDM is encouraging.

<p>While the car fleet becomes increasingly larger it is important to lower the amounts of pollutants from each individual diesel or gasoline engine to almost zero levels. The pollutants from these engines predominantly originate from high NO_x emissions and particulates, in the case when diesel is utilized, and emissions at cold start from gasoline engines. One way of treating the high NO_x levels is to introduce ammonia in the diesel exhausts and let it react with the NO_x to form nitrogen gas and water, which is called SCR (Selective Catalytic Reduction). However, in order to make this system reduce NO_x efficiently enough for meeting future legislations, closed loop control is required. To realize this type of system an NO_x or ammonia sensor is needed. The cold start emissions from gasoline vehicles are primarily due to a high light-off time for the catalytic converter. Another reason is the inability to quickly heat the sensor used for controlling the air-to-fuel ratio in the exhausts, also called the lambda value, which is required to be in a particular range for the catalytic converter to work properly. This problem may be solved utilizing another, more robust sensor for this purpose.</p><p>This thesis presents the efforts made to test the SiC-based field effect transistor (SiC-FET) sensor technology both as an ammonia sensor for SCR systems and as a cold start lambda sensor. The SiC-FET sensor has been shown to be highly sensitive to ammonia both in laboratory and engine measurements. As a lambda sensor it has proven to be both sensitive and selective, and its properties have been studied in lambda stairs both in engine exhausts and in the laboratory. The influence of metal gate restructuring on the linearity of the sensor has also been investigated. The speed of response for both sensor types has been found to be fast enough for closed loop control in each application.</p> / On the day of the public defence of the doctoral thesis, the status of article III was: in press. Report code: LiU-Tek-Lic-2003:50.

It has been found that non-magnetic organic semiconductors can show some magnetic responses in low magnetic field (<100 >mT). When applying magnetic field, the electroluminescence, electrical current, photocurrent, and photoluminescence could change with magnetic field, which are called magnetic field effects. Magnetic field effects are generated through spin-dependent process affected by the internal magnetic interaction. In nonmagnetic materials, hyperfine interaction has been supposed to dominantly affect the spin-dependent process recently. But the conclusion was made in weak spin-orbital coupling organic semiconductor. The hyperfine interaction might not be the main reason responsible for magnetic field effects in strong spin-orbital coupling materials. Therefore, the study of magnetic field effects in strong spin-orbital coupling organic semiconductor is important to get a whole view of the origin of the magnetic field effects in nonmagnetic organic semiconductors. This dissertation will clarify the generation mechanism of magnetic field effect in nonmagnetic organic semiconductors and further explore how the strong spin-orbital coupling affecting the magnetic field effect. It has been found the intermolecular excited states are important inter-median for magnetic field effects. The change of intersystem crossing at intermolecular excited states will change the singlet/triplet ratio and further generate magnetic field effects through different recombination and dissociation properties of singlet and triplet intermolecular excited states. Both the energy transfer effect coupled spin orbital coupling and energy transfer effect free spin orbital-coupling are discussed in the dissertation. The tuning of the magnetic field effect by adjusting the spin-orbital coupling is also established through distance effect and interface effect. It has been found that changing inter-molecular spin-orbital coupling is a critical factor to generate magnetic field effects in organic semiconductors. And the sensitivity of different magnetic field effects to strong spin-orbital coupling strength is depending on the final product. The internal magnetic interaction can be hyperfine interaction, spin orbital coupling and spin-spin interaction between electrons. The hyperfine interaction and spin orbital coupling are important in nonmagnetic organic semiconductors. But the electron spin-spin interaction is important in magnetic organic semiconductors. The magnetocurrent for magnetic and nonmagnetic organic semiconductors at different temperature has been compared.

The temperature dependent electrical characteristics of silicon carbide power transistors – 4H-SiC metal semiconductor field-effect transistors (MESFETs) and 4H-SiC bipolar junction transistors (BJTs) have been investigated through simulation and experimental approaches. Junction temperatures and temperature distributions in devices under large power densities have been estimated. The DC and RF performance of 4H-SiC RF Power MESFETs have been studied through two-dimensional electro-thermal simulations using commercial software MEDICI and ISE. The simulated characteristics of the transistors were compared with the measurement results. Performance degradation of transistors under self-heating and high operating temperatures have been analyzed in terms of gate and drain characteristics, power density, high frequency current gain and power gain. 3D thermal simulations have been performed for single and multi-finger MESFETs and the simulated junction temperatures and temperature profiles were compared with the results from electro-thermal simulations. The reduction in drain current caused by self-heating was found to be more prominent for transistors with more fingers and it imposes a limitation on both the output power and the power density (in W/mm) of multi-fingered large area devices. Thermal issues for design of high power multi-fingered SiC MESFETs were also investigated. A couple of useful ways to reduce the self-heating effects were discussed. Trap-induced performance instabilities of the devices were analyzed by carrying out DC, transient, and pulse measurements at room and elevated temperatures. Electrical characteristics of 4H-SiC BJTs have been measured. A reduction in current gain at elevated temperatures was observed. Based on the collector current-voltage diagram measured at three different ambient temperatures the junction temperature was extracted using the assumption that the current gain only depends on the temperature. Temperature measurements have been carried out for SiC BJTs. Thermal images of a device under operation were recorded using an infrared camera. 3D thermal simulations were conducted using FEMLAB. Both the simulations and the measurement showed a significant temperature increase in the vicinity of the device when operated at high power densities, thus causing the decrease of the DC current gain. The junction temperatures obtained from the thermal imaging, simulation and extraction agree well.

Label-free detection of DNA has been successfully demonstrated on field effect transistor (FET) based devices. Since conducting organic materials was discovered and have attracted more and more research efforts by their profound advantages, this work will focus on utilizing an organic field effect transistor (OFET) as DNA sensor. An OFET constructed with a transporting fluidic channel, WetOFET, forms a fluid-polymer (active layer) interface where the probe DNA can be introduced. DNA hybridization and non-hybridization after injecting target DNA and non-target DNA were monitored by transistor characteristics. The Hysteresis area of transfer curve increased after DNA hybridization which may be caused by the increasing electrostatic screening induced by the increasing negative charge from target DNA. The different morphology of coating surface could also influence the OFET response.