Browsing by Subject "Thin film transistors"

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Approaches and evaluation of architectures for chemical and biological sensing based on organic thin-film field-effect transistors and immobilized ion channels integrated with silicon solid-state devices
(2007) Fine, Daniel Hayes, 1978-; Dodabalapur, Ananth, 1963-
There is significant need to improve the sensitivity and selectivity for detecting chemical and biological agents. This need exists in a myriad of human endeavors, from the monitoring of production of consumer products to the detection of infectious agents and cancers. Although many well established methodologies for chemical and biological sensing exist, such as mass spectrometry, gas or liquid phase chromatography, enzymelinked immunosorbent (ELISA) assays, etc., it is the goal of the work described herein to outline aspects of two specific platforms which can add two very important features, low cost and portability. The platforms discussed in this dissertation are organic semiconductor field-effect transistors (OFETS), in various architectural forms and chemical modifications, and ion channels immobilized in tethered lipid bilayers integrated with solid state devices. They take advantage of several factors to make these added features possible, low cost manufacturing techniques for producing silicon and organic circuits, low physical size requirements for the sensing elements, the capability to run such circuits on low power, and the ability of these systems to directly transduce a sensing event into an electrical signal, thus making it easier to process, interpret and record a signal. In the most basic OFET functionality, many types of organic semiconductors can be used to produce transistors, each with a slightly different range of sensitivities. When used in concert, they can produce a reversible chemical "fingerprint". These OFETS can also be integrated with silicon transistors - in a hybrid device architecture - to enhance their sensitivity while maintaining their reversibility. The organic semiconductors themselves can be chemically altered with the use of small molecule receptors designed for specific chemicals or chemical functional groups to greatly enhance the interaction of these molecules with the transistor. This increases both sensitivity and selectivity for discrete devices. Specially designed nanoscale OFET configurations with individually addressable gates can enhance the sensitivity of OFETS as well. Finally, ion channels can be selected for immobilization in tethered lipid bilayer sensors which are already inherently sensitive to the analyte of choice or can be genetically modified to include receptors for many kinds of chemical or biological agents.
Charge transport and device engineering for improvement of thin film transistor
(2022-07-01) Wang, Xiao, Ph. D.; Dodabalapur, Ananth, 1963-; Register, Leonard F.; Kulkarni, Jaydeep P.; Akinwande, Deji; Ganesan, Venkat
Semiconductors used in thin film transistors (TFTs) include a wide range of materials, such as semiconducting polymers, organic molecules, and amorphous metal oxides, etc. Although TFTs have already found applications in fields such as display technology and flexible electronics, there are still several technical and scientific challenges that remain in TFTs areas including understanding of charge transport and device physics in high mobility TFTs, and in developing new applications with better-performing short channel devices. In this dissertation, we start from describing charge transport in TFTs with the assistance of a proposed physical model, then build a device model based on the fundamentals of the charge transport to investigate the performances of TFTs, and finally, develop experimental techniques to overcome performance bottle necks in short channel length TFTs. An extended multiple trap and release (MTR) model is proposed as the basis to understand the physics of charge transport. The extended MTR model uses Boltzmann transport theory with multiple scattering mechanisms, combined with a phenomenological transport reduction factor, which originates from the statistical nature of the transport, and multiple trap and release process to describe the charge transport in high mobility TFTs. The extended MTR model can be applied to various types of TFTs and provides a deeper understanding of the charge transport in such TFTs. Modeling thin film device based on the framework of the extended MTR model is accomplished by implementing a self-consistent Poisson and current continuity solver. Physical quantities such as carrier velocity, lateral electric fields and carrier distributions in TFTs are studied. The effect of contact resistance is investigated and analyzed in short channel TFTs. It is clear from the results of device modeling together with experimental data that the contact resistance, which is mainly due to the formation of Schottky barrier in metal-semiconductor contact region, is the major bottle neck that prevents the TFTs from further scaling down channel lengths. Two techniques are proposed to solve this bottle neck. One is to use doped graphene as contacts for TFTs to reduce the Schottky contact barrier. Another is to enhance the field injection of the carriers by patterning the graphene contacts into arrays of nanospikes. Both techniques are demonstrated to substantially reduce the contact resistance and facilitate scaling down channel lengths in organic TFTs well below a micrometer.
Circuit design and device modeling of zinc-tin oxide TFTs
(2011-05) Divakar, Kiran; Viswanathan, T. R., doctor of electrical engineering; Dodabalapur, Ananth, 1963-
Amorphous Oxide Semiconductors (AOS) are widely being explored in the field of flexible and transparent electronics. In this thesis, solution processed zinc-tin oxide (ZTO) n-channel TFT based circuits are studied. Inverters, single stage amplifiers and ring oscillators are designed, fabricated and tested. 7-stage ring oscillators with output frequencies up to 106kHz and 5-stage ring oscillators with frequencies up to 75kHz are reported. A stable three stage op-amp with a buffered output is designed for a gain of 39.9dB with a unity gain frequency of 27.7kHz. A 7-stage ring oscillator with output frequency close to 1MHz is simulated and designed. The op-amp and the ring oscillator are ready to be fabricated and tested. An RPI model for a-Si, adapted to fit the ZTO device characteristics, is used for simulation. Development of a new model based on the physics behind charge transport in ZTO devices is explored. An expression for gate bias dependent mobility in ZTO devices is derived.
Developing non-invasive processing methodologies and understanding the materials properties of solution-processable organic semiconductors for organic electronics
(2007-05) Dickey, Kimberly Christine, 1977-; Loo, Yueh-Lin, 1974-
Essential to the success of organic electronics, and in particular organic thin-film transistors, is the realization of stable, high-mobility, electrically-active organic materials that can enable low-cost solution-based processing methods. The development of viable solution-processable organic semiconductors helps make this possible. Consequently, understanding the materials properties of solution-processable organic semiconductors and how the processing conditions associated with device fabrication affect device performance are key to realizing low-cost organic electronics. In this work, we focused on understanding the processing-structure-property relationships of a solutionprocessable organic semiconductor, triethylsilylethylnyl anthradithiophene (TES ADT). Specifically, we demonstrated how a solvent-vapor annealing process can induce the crystallization of TES ADT post device processing. Bottom-contact thin-film transistors with annealed TES ADT routinely exhibit an average charge-carrier mobility of 0.1 cm 2 /V-s, which is sufficient to drive backplane circuitry in flexible display applications. Additionally, we demonstrated that the manner in which source and drain electrodes are defined significantly affects the performance of the resulting TES ADT thin-film transistors. Specifically, the yield of functioning top-contact TES ADT thinfilm transistors with electrodes defined by evaporation through a shadow mask directly on the organic semiconductor is low, and of the functioning devices, the charge-carrier mobility varies significantly (0.01 – 0.1 cm2 /V-s). In comparison, top-contact TES ADT thin-film transistors with electrodes defined separately and then laminated against the organic semiconductors have high yield and high charge-carrier mobility (0.2 ± 0.06 cm 2 /V-s). This result emphasizes the importance of adapting existing or developing new thin-film transistor fabrication techniques to overcome the materials limitations of organic semiconductors. Along the same vein, we also demonstrated an elastomeric stamp-based, solventless printing process, nanotransfer printing (nTP), for the additive patterning of copper electrodes and interconnects of feature sizes 1 – 500 μm. These printed copper patterns differ from similarly printed gold patterns in that they are not electrically conductive. Leaching the elastomeric stamps in hot toluene prior to printing, however, allowed us to routinely print conductive copper features with an average resistivity of 31 μΩ-cm. Another aspect of thin-film transistor fabrication that is crucial for optimal device performance (i.e., low off currents and low leakage currents) is the patterning and isolation of the organic semiconductor between neighboring devices. We demonstrated two novel techniques for patterning TES ADT. The first technique utilizes UV light in the presence of dichloroethane vapors to simultaneously pattern and crystallize TES ADT. TES ADT thin-film transistors patterned with this technique exhibit high chargecarrier mobility (0.1 cm2 /V-s) and low off currents (10-11 A). The second patterning technique uses a PDMS stamp to selectively remove TES ADT from the non-channel regions of the thin-film transistor. This technique can be used to pattern both as-spun and crystalline TES ADT thin films. Crystalline TES ADT thin-film transistors patterned with this technique exhibit an average charge-carrier mobility of 0.2 cm2 /V-s and low off currents on the order of 10-11 A, while amorphous TES ADT thin films that are first patterned and then crystallized exhibit an average charge-carrier mobility of 0.1 cm2 /V-s and off currents on the order of 10-10 A.
Device modeling and circuit design for ZTO based amorphous metal oxide TFTs
(2011-05) Joshi, Tanvi Dhananjay; Viswanathan, T. R., doctor of electrical engineering; Dodabalapur, Ananth, 1963-
Amorphous Oxide semiconductors have gained large interest in the display industry owing to their high carrier mobilities and low fabrication costs. In this thesis, n-channel solution based zinc-tin oxide (ZTO) thin-film transistors (TFTs) are studied from a circuit design perspective. The study includes an iterative process of circuit design, layout and test procedure of the fabricated devices in the lab. The device models used in circuit simulations are refined following the data fed back from each of these iterations which has enabled more accurate design of complex circuits using ZTO devices. The requirement and development of a physical compact model for performing accurate and predictive circuit simulations has been presented. The use of ZTO devices in low cost, transparent and flexible electronic applications has been investigated through the study of basic circuit blocks such as amplifiers, ring oscillators, inverters and a four stage Operational Amplifier.
Nanoscale organic and polymeric field-effect transistors and their applications as chemical sensors
(2005) Wang, Liang; Dodabalapur, Ananth, 1963-
This work mainly focused on fabricating of nanoscale polycrystalline organic and conjugated polymeric thin-film field-effect transistors and investigating their scaling behaviors of electrical transport and chemical sensing properties. Devices with channel lengths systematically ranging from a few hundred microns down to sub 10 nm were successfully fabricated with the techniques such as stencil mask, photolithography, electron beam lithography, and break junction. The use of a novel four-terminal geometry ensures that the active area for charge transport and vapor sensing is truly nanoscale, and eliminates undesirable spreading currents traveling over the large area outside the defined channel to reduce the background signal level. It was discovered that upon scaling channel lengths from micron scale down to nanoscale, the dominating factors for charge transport and vapor sensing in organic thin-film transistors become different. At small dimensions, injection limited transport and field-dependent mobility are the dominant mechanisms for transport through the gate-modulated channel at low and high longitudinal fields respectively. Furthermore for sub 10 nm channels, tunneling effect plays an important role. In micron scale devices, the drain current usually decreases as a sensing response upon exposure of the polycrystalline organic/polymeric semiconductor layer to the analyte, mainly because of the transistor threshold shift caused by the immobile charges at grain boundaries trapped by the dipolar analyte molecules. The vapor sensing behavior of nanoscale organic transistors is markedly different (in an opposite direction of response) from that of large-scale devices for the same analyte-semiconductor combination, due to the fact that the electrical transport in a nanoscale OTFT depends on its morphological structure and interface properties (such as the injection barrier at the metal-organic semiconductor contacts) which could be modulated by the delivery of analyte.
Spin and charge transport through carbon based systems
(2007) Jung, Suyong, 1976-; Yao, Zhen, Ph. D.
In this thesis, we investigate spin-dependent transport through ferromagnet-contacted single-walled carbon nanotubes (SWCNTs), in which charge transport shows the Fabry-Perot (FP) interference effect, the Kondo effect and the Coulomb blockade effect at low temperatures. Hysteric magnetoresistance (MR) is observed in all three transport regimes, which can be controlled by both the external magnetic field and the gate voltage. The MR in the FP interference regime can be well understood by a model considering the intrinsic electronic structure of SWCNTs and the quantum interference effect. In the strongly interacting Kondo regime, the Kondo effect is not suppressed by the presence of nearby ferromagnetism. Several observed MR features including the non-splitted zero-bias Kondo peak and positive MR switching can be explained by the strong Kondo effect and weak ferromagnetism in the leads. In the Coulomb blockade regime, several effects that can be associated with the magneto-Coulomb effect have been observed, and isolated spin accumulation and transport through the SWCNT quantum dot have been realized by a four-probe non-local measurements. We also studied charge transport behavior through organic semiconductor pentacene thin film transistors (OTFTs) in the limit of single- or a few molecular layers of pentacene films. The charge transport in these devices can be well explained by the multiple trapping and release model. The structural disorders induced by the physical and chemical causes, such as grain boundaries, interactions with gate insulator, metal contacts and ambient conditions can be responsible for the localized trap states in the ultrathin layer OTFTs, which are further confirmed by the electric force microscopy (EFM) measurements.