Browsing by Subject "Field-effect transistor"

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    Charge transport in single crystal organic semiconductors
    (2013-10) Xie, Wei
    Organic electronics have engendered substantial interest in printable, flexible and large-area applications thanks to their low fabrication cost per unit area, chemical versatility and solution processability. Nevertheless, fundamental understanding of device physics and charge transport in organic semiconductors lag somewhat behind, partially due to ubiquitous defects and impurities in technologically useful organic thin films, formed either by vacuum deposition or solution process. In this context, single-crystalline organic semiconductors, or organic single crystals, have therefore provided the ideal system for transport studies. Organic single crystals are characterized by their high chemical purity and outstanding structural perfection, leading to significantly improved electrical properties compared with their thin-film counterparts. Importantly, the surfaces of the crystals are molecularly flat, an ideal condition for building field-effect transistors (FETs). Progress in organic single crystal FETs (SC-FETs) is tremendous during the past decade. Large mobilities ~ 1 - 10 cm2V-1s-1 have been achieved in several crystals, allowing a wide range of electrical, optical, mechanical, structural, and theoretical studies. Several challenges still remain, however, which are the motivation of this thesis. The first challenge is to delineate the crystal structure/electrical property relationship for development of high-performance organic semiconductors. This thesis demonstrates a full spectrum of studies spanning from chemical synthesis, single crystal structure determination, quantum-chemical calculation, SC-OFET fabrication, electrical measurement, photoelectron spectroscopy characterization and extensive device optimization in a series of new rubrene derivatives, motivated by the fact that rubrene is a benchmark semiconductor with record hole mobility ~ 20 cm2V-1s-1. With successful preservation of beneficial π-stacking structures, these rubrene derivatives form high-quality single crystals and exhibit large ambipolar mobilities. Nevertheless, a gap remains between the theory-predicted properties and this preliminary result, which itself is another fundamental challenge. This is further addressed by appropriate device optimization, and in particular, contact engineering approach to improve the charge injection efficiencies. The outcome is not only the achievement of new record ambipolar mobilities in one of the derivatives, namely, 4.8 cm2V-1s-1 for holes and 4.2 cm2V-1s-1 for electrons, but also provides a comprehensive and rational pathway towards the realization of high-performance organic semiconductors. Efforts to achieve high mobility in other organic single crystals are also presented. The second challenge is tuning the transition of electronic ground states, i.e., semiconducting, metallic and superconducting, in organic single crystals. Despite an active research area since four decades ago, we aim to employ the electrostatic approach instead of chemical doping for reversible and systematic control of charge densities within the same crystal. The key material in this study is the high-capacitance electrolyte, such as ionic liquids (ILs), whose specific capacitance reaches ~ μF/cm2, thus allowing accumulation of charge carrier above 1013 cm-2 when novel transport phenomena, such as insulator-metal transition and superconductivity, are likely to occur. This thesis addresses the electrical characterization, device physics and transport physics in electrolyte-gated single crystals, in the device architecture known as the electrical double layer transistor (EDLT). A detailed characterization scheme is first demonstrated for accurate determination of several key parameters, e.g., carrier mobility and charge density, in organic EDLTs. Further studies, combining both experiments and theories, are devoted to understanding the unusual charge density dependent channel conductivity and gate-to-channel capacitance behaviors. In addition, Hall effect and temperature-dependent measurements are employed for more in-depth understandings of the transport mechanism in these unconventional devices at the extreme charge densities. Inspiringly, a truly metallic state is within reach of this type of device structure. Overall, this thesis demonstrates high mobility, high charge density and high performance organic single crystal transistors, with versatile fabrication techniques, comprehensive electrical and structural characterizations, well-developed theories and models and advanced transport measurements.
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    Device modeling of field-effect transistors with nanocrystalline channels.
    (2011-06) Steinke, Isaiah Peter
    Due to certain limitations of silicon for particular device applications, there has been increasing interest in the use of compound semiconductors. However, the growth of compound semiconductors in single crystal form is not always feasible on a large scale or even wanted for particular applications, such as solar cells or thin film transistors. The material for these applications is usually polycrystalline, and the presence of grain boundaries limits the performance of these devices. In our work, we present two models that take into account the effect of grain boundaries in nanocrystalline field-effect transistors. In our "macroscopic" model, we modify the field-effect mobility to include terms dependent upon the local carrier density and the longitudinal field along the channel. These terms are motivated by the expected carrier density and field dependences of transport across grain boundaries. In general, we find that the addition of each mobility term separately changes the carrier profile along the channel in opposite ways, and the inclusion of these terms increases the magnitude of the current. Furthermore, the addition of the longitudinal field dependent mobility term is only significant for large values of drain bias, i.e. near saturation. The limitation of the macroscopic model is that it inherently averages over the grains present in the channel. In order to further study the role of grains in the channel, we developed our "mesoscopic" model that incorporates ideas from percolation theory. Here, individual grains are represented as sites in our percolation problem, while the bonds represent the energy barriers between neighboring grains. The relative occupation of sites and bonds is connected to the carrier statistics of the device, whereby the carriers can be either free carriers in the grain or trapped carriers at the grain boundary. The relative occupations are controlled by the applied gate bias. Through the combination of a site-bond percolation problem and the carrier statistics, we describe the behavior of the transistor near threshold and illustrate a method to determine the threshold voltage.