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This work is focused on the electrical characterization of polymer field effect transistors. Conventional method of characterizing organic polymeric semiconductors includes field-effect mobility measurement and optical time-of-flight measurement of drift mobility. In this dissertation we have introduced a new method that combines the advantages of both these methods. It involves the injection of carriers at the source of a transistor using a voltage pulse followed by their subsequent extraction at the drain. The delay between the two events is used to extract the velocity of carriers. The electronic time-of-flight method is a fast, simple and direct method to determine the charge transport properties of the semiconductor. In addition it also presents itself as a source of information for understanding injection into the semiconductor and determining the trap distribution. Theoretical modeling of transport was performed. Simulation was also done to include effect of non-idealities that are forbiddingly difficult to be solved analytically. Time of flight measurements of drift mobility were performed in organic transistors with varying semiconductors and dielectrics. It was observed that the electronic time-of-flight mobility lies in the range of the field-effect mobility. Variation in drift mobility was also observed with the applied pulse voltage. This was explained to be caused due to a combination of the increase in mobility with gate voltage and the increase in drift mobility at high lateral fields. Finally mobility measurements were done on transistors with varying channel length and it was concluded that the mobility increases proportional to the exponential square root of the electric field. Finally a derivation of the pulse voltage method is discussed that involves the use of a small signal electronic impulse instead of a large signal voltage pulse. It was shown that this method could not be used to calculate the drift velocity in a polymer transistor as it is valid only for low conductivity materials whose dielectric relaxation time is lower that the transit time of the carriers injected.

The scaling of metal-oxide-semiconductor (MOS) field-effect transistors (FETs) has continued for over four decades, providing device performance gains and considerable economic benefits. However, continuing this scaling trend is being impeded by the increase in dissipated power. Considering the exponential increase of the number of transistors per unit area in high speed processors, the power dissipation has now become the major challenge for device scaling, and has led to tremendous research activity to mitigate this issue, and thereby extend device scaling limits. In such efforts, non-planar device structures, high mobility channel materials, and devices operating under different physics have been extensively investigated. Non-planar device geometries reduce short-channel effects by enhancing the electrostatic control over the channel. The devices using high mobility channel materials such as germanium (Ge), SiGe, and III-V can outperform Si MOSFETs in terms of switching speed. Tunneling field-effect transistors use interband tunneling of carriers rather than thermal emission, and can potentially realize low power devices by achieving subthreshold swings below the thermal limit of 60 mV/dec at room temperature. In this work, we examine two device options which can potentially provide high switching speed combined with reduced power, namely germanium nanowire (NW) field-effect transistors (FETs) and tunneling field-effect transistors (TFETs). The devices use germanium (Ge) – silicon-germanium (Si[subscript x]Ge[subscript 1-x]) core-shell nanowires (NWs) as channel material for the realization of the devices, synthesized using a 'bottom-up' growth process. The device design and material choice are motivated by enhanced electrostatic control in the cylindrical geometry, high hole mobility, and lower bandgap by comparison to Si. We employ low energy ion implantation of boron and phosphorous to realize highly doped contact regions, which in turn provide efficient carrier injection. Our Ge-Si[subscript x]Ge[subscript 1-x]­ core-shell NW FETs and NW TFETs were fabricated using a conventional CMOS process and their electrical properties were systematically characterized. In addition, TCAD (Technology computer-aided design) simulation is also employed for the analysis of the devices. / text

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published_or_final_version / Mathematics / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Increasing oil prices and concerns about global warming have reinforced the interest in biofuels for domestic and district heating, most commonly through combustion of solid biomass like wood logs, hog fuel and pellets. Combustion at non-optimal conditions can, however, lead to substantial emissions of noxious compounds like unburned hydrocarbons, carbon monoxide, and nitrogen oxides as well as the generation of soot. Depending on the rate of combustion more or less air is needed per unit time to completely oxidize the fuel; deficiency of air leading to emissions of unburned matter and too much of excess air to slow combustion kinetics and emissions of mainly carbon monoxide. The rate of combustion is influenced by parameters like fuel quality – moisture and ash content etc. – and in what phase the combustion takes place (in the gas phase through combustion of evaporated substances or on the surface of char coal particles), none of which is constant over time. The key to boiler operation, both from an environmental as well as a power to fuel economy point of view, is thus the careful adjustment of the air supply throughout the combustion process. So far, no control schemes have been applied to small-scale combustors, though, mainly due to the lack of cheap and simple means to measure basic flue gas parameters like oxygen, total hydrocarbon, and carbon monoxide concentrations. This thesis reports about investigations on and characterization of silicon carbide (SiC) based Metal Insulator Semiconductor (MIS) field effect gas sensors regarding their utility in emissions monitoring and combustion control applications as well as the final development of a sensor based control system for wood fired domestic heating systems. From the main sensitivity profiles of such sensor devices, with platinum (Pt) and iridium (Ir) as the catalytic metal contacts (providing the gas sensing ability), towards some typical flue gas constituents as well as ammonia (NH3), a system comprising four individual sensors operated at different temperatures was developed, which through the application of Partial Least Squares (PLS) regression, showed good performance regarding simultaneous monitoring of propene (a model hydrocarbon) and ammonia concentrations in synthetic flue gases of varying content. The sensitivity to CO was, however, negligible. The sensor system also performed well regarding ammonia slip monitoring when tested in real flue gases in a 5.6 MW boiler running SNCR (Selective Non-Catalytic reduction of nitrogen oxides with ammonia). When applied to a 200 kW wood pellet fuelled boiler a similar sensor system was, however, not able to follow the flue gas hydrocarbon concentration in all encountered situations. A PCA (Principal Components Analysis) based scheme for the manipulation of sensor and flue gas temperature data, enabling monitoring of the state of combustion (deficiency or too much of excess air), was however possible to develop. The discrepancy between laboratory and field test results was suspected and later on shown to depend on the larger variation in CO and oxygen concentrations in the flue gases as compared to the laboratory tests. Detailed studies of the CO response characteristics for Pt gate MISiC sensors revealed a highly non-linear sensitivity towards CO, a large response only encountered at high CO/O2 ratios or low temperatures. The response exhibits a sharp switch between a small and a large value when crossing a certain CO/O2 ratio at constant operating temperature, correlated to the transition from an oxygen dominated to an almost fully CO covered Pt surface, originating from the difference in adsorption kinetics between CO and O2. Indications were also given pointing towards an increased sensitivity to background hydrogen as being the mediator of at least part of the CO response. Some general characteristics regarding the response mechanism of field effect sensors with differently structured metal contacts were also indicated. The CO response mechanism of Pt metal MISiC sensors could also be utilized in developing a combustion control system based on two sensors and a thermocouple, which when tested in a 40 kW wood fired boiler exhibited a good performance for fuels with extremely low to normal moisture content, substantially decreasing emissions of unburned matter.

La unión de las propiedades de los CNT con los principios de reconocimiento molecular se presenta como una base adecuada para el desarrollo de sensores altamente específicos. El objetivo de la presente tesis ha sido desarrollar sensores químicos, del tipo transistores de efecto campo (CNTFET), basados en interacciones receptor-analito, mediante el empleo de los nanotubos de pared sencilla (SWCNT), que actúan como transductores de la señal analítica.Las principales etapas de la parte experimental han sido: Crecimiento de SWCNT con la técnica de deposición química en fase vapor. Integración de los SWCNTs en sistemas CNTFET. Empleo del CNTFET como base del sensor en distintos campos utilizando modelos de reconocimiento molecular. Dependiendo del tipo de funcionalización de los SWCNTs se pueden obtener sensores para proteínas, iones, etc. Como resultado, se han desarrollado y estudiado sensores basados en CNTFETs para la detección distintos analitos de interés, como son la Inmunoglobulina G Humana, los iones potasio y el dióxido de azufre. / The general objective of this thesis is to develop chemical sensors whose sensing capacities are based on the principle of molecular recognition and where the transduction is carried out by single-walled carbon nanotubes (SWCNT).The sensing device used is the carbon nanotube field-effect transistor (CNTFET). The new structure of the CNTFET allows nanotubes to be integrated at the surface of the devices, thus exploiting SWCNTs' sensitivity to changes in their environment. The functionalization of SWCNTs with several types of molecular receptors such as antibodies, ion selective membranes, and synthetic receptors, achieve a high selectivity towards the analyte of interest. This thesis shows that CNTFETs can be used for the successful selective detection of different types of target analytes. These can be biomolecules such as antigens, small compounds such as cations or gas-phase compounds such as SO2.