1,056 research outputs found

    Photo-effects on Current Transport in Back-gate Graphene Field-effect Transistor

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    Graphene, which has attracted wide attention because of its two-dimensional structure and high carrier mobility, is a promising candidate for potential application in optics and electronics. In this dissertation, the photonic effects on current transport in back-gate graphene field-effect transistor is investigated. Chemical vapor deposition (CVD) on metal provides a promising way for large area, controllability and high quality graphene film. The transfer and back-gate transistor fabrication processes are proposed in this dissertation. The theoretical analysis of photodetector based on back-gate graphene field-effect transistor has been done. It is shown that the photo-electronic current consists of current contributions from photovoltaic, photo-thermoelectric and photo-bolometric effects. A maximum external responsivity close to 0.0009A/W is achieved at 30ÎĽW laser power source and 633nm wavelength. The photodiode based on graphene/silicon Schottky barrier is also. A computed 238.8 W-1 photocurrent to dark current ratio normalized by the power source (633nm wavelength and 10mW laser) is obtained. An equivalent circuit model of the graphene/silicon Schottky barrier diode compatible with SPICE simulation is developed and simulated photo-response characteristics are presented using analog behavior modeling which are in close agreement with the theoretical analysis. Besides the optical applications, graphene based-transistors can also be used in applications related to space electronics. The irradiation effects including oxide trap charge and graphene layer traps charges are investigated. A semi-empirical model of graphene back-gate transistors before and after irradiation is predicted

    Study on Electrolyte-gated Graphene Nanoelectronic Biosensors for Biomarker Detection

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    Biosensors are called upon to provide valuable benefits for human society in vital fields such as disease diagnosis, food inspection, environment monitoring, etc. Among the various biosensor architectures, the field effect transistor (FET) biosensors are promising as the next generation nanoelectronic biosensors, particularly attractive for point-of-care biomedical applications. The FET biosensors typically operate by measuring the conductance change of the semiconducting channel induced by the adsorption of the target biomolecules on it. The superior properties of graphene, including the unique electronic characteristics, facile functionalization and good biocompatibility, etc., make it an ideal building block for the FET biosensors. In this dissertation, we present studies on the electrolyte-gated graphene field effect transistor (EGGFET) biosensor and its application for the label-free detection of biomarkers. Poly(methyl methacrylate) (PMMA) residues have long been a critical challenge for the transfer of the chemical vapor deposited (CVD) graphene, which is critical to obtain reliable devices. To address this issue, we first studied the degradation of the PMMA residues upon thermal annealing using Raman spectroscopy. An electrolytic cleaning method is shown to be effective to remove these post-annealing residues, resulting in a clean, residue-free graphene surface. The performance of the EGGFET biosensor is demonstrated by the successful detection of human immunoglobulin G (IgG) using IgG-aptamer as the bioreceptor. The gate voltage with the minimum conductivity (Dirac) in the transfer curve of the EGGFET biosensor is used for the quantitative measurement of IgG concentration. In EGGFET biosensors, the graphene channels are directly exposed to the electrolytes, of which the composition, concentration and pH may vary during the testing. The response of the EGGFET biosensors is found to be susceptible to these variations which might lead to high uncertainty or even false results. We present an EGGFET immunoassay which allows well regulation over the matrix effect. The performance is demonstrated by the detection of human IgG from serum. The detection range of the EGGFET immunoassay for IgG detection is estimated to be around 2-50 nM with a coefficient of variation (CV) of less than 20%. The limit of detection (LOD) is around 0.7 nM. Different from the metal-oxide-semiconductor field effect transistors (MOSFET), the gate voltage is applied on the electrolyte and the electrical double layer (EDL) at the electrolyte-graphene interface serves as the gate dielectric in EGGFET. We studied the capacitance behavior of the electrolyte-graphene interface; the results suggest that the electrolyte-graphene interface exhibits a complex constant phase element (CPE) behavior (1 = 0 () ) with both 0 and varying as functions of the gate voltage. The EDL capacitance and the quantum capacitance are determined which allows us to extract the carrier density and mobility in graphene. This study give insight into the device physics of the EGGFET biosensor and is instructive for the design of the EGGFET biosensors on the device level

    Field-Effect Transistors and Optoelectronic Devices Based on Emerging Atomically Thin Materials

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    Development of field-effect transistors and their applications is advancing at a relentless pace. Since the discovery of graphene, a single layer of carbon atoms, the ability to isolate and fabricate devices on atomically thin materials has marked a paradigm shift in the timeline of transistor technologies. In this thesis, electrical and optical properties of atomically thin structures of graphene and tungsten disulfide (WS2) are investigated. Transport in graphene side-gated transistors and contact resistance at the metal-WS2 interface are presented. Finally, the optoelectronic performance of the hybrid graphene-WS2 devices is examined. Presently, atomically thin semiconductors grown by chemical vapour deposition are of growing interest by a broad scientific community. For this work of thesis, an air stable material which requires non-toxic gases for the growth such as WS2 is selected. A considerable contact resistance at the metal/WS2 interface is found to hamper the electrical performance of WS2 transistors. The possible origin of this contact resistance is presented in this thesis. The graphene field-effect transistors with graphene side gates are fabricated by a single step of electron beam lithography and an O2 etching procedure. A comparative study of the electrical transport properties as a function of a bias applied to the side and back gate is conducted. The side gates allow for a much more efficient modulation of the charge density in the graphene channel owing to the larger maximum electric field which can experimentally be accomplished. Furthermore, the leakage between the side gate and the graphene channel is studied in a vacuum environment. It is found that the transport between graphene and the side gate is associated with Fowler-Nordheim tunnelling and Frenkel-Poole transport. More specifically, for voltages less than 60 V, the Frenkel-Poole transport dominates the transport, whereas the Fowler-Nordheim tunnelling governs the transport at higher bias. Finally, optoelectronic properties of graphene-WS2 heterostructure are explored. An ionic polymer is used as a top gate to enhance the screening of long-lived trap charges. Responsivities as large as 10^6 A/W under illumination with 600 nm wavelength of light are demonstrated at room temperature. The fall and rise time are in the order of milliseconds due to the screening of the traps by the ionic polymer. This study is the first presentation of the transition metal dichalcogenide (TMDC)-graphene hybrid heterostructure with such a high photoresponsivity and fast response times

    Graphene on Silicon Hybrid Field-Effect Transistors

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    The combination of graphene with silicon in hybrid devices has attracted attention extensively over the last decade. Most of such devices were proposed for photonics and radiofrequency applications. In this work, we present a unique technology of graphene-on-silicon heterostructures and their properties as solution-gated transistors. The graphene-on-Silicon field-effect transistors (GoSFETs) were fabricated exploiting various conformations of drain-source regions doping and channel material dimensions. The fabricated devices were electrically characterized demonstrating hybrid behavior with features specific to both graphene and silicon. Although GoSFET's transconductance and carrier's mobility were found to be lower than in conventional silicon and graphene field-effect transistors (SiFETs and GFETs), it was demonstrated that the combination of both materials within the hybrid channel contribute uniquely to the charge carrier transport. A comprehensive physics-based compact modeling was specifically developed, showing excellent agreement with the experimental data. The model is employed to rationalize the observed hybrid behavior as the theoretical results from the electrostatics and the carrier transport under a drift-diffusion approach show that graphene acts as a shield for the silicon channel, giving rise to a non-uniform potential distribution along it, especially at the subthreshold region. This graphene screening effect is shown to strongly affect the device subthreshold swing when compared against a conventional SiFET due to a non-negligible diffusion current in this operation regime

    Insulators for 2D nanoelectronics: the gap to bridge

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    Nanoelectronic devices based on 2D materials are far from delivering their full theoretical performance potential due to the lack of scalable insulators. Amorphous oxides that work well in silicon technology have ill-defined interfaces with 2D materials and numerous defects, while 2D hexagonal boron nitride does not meet required dielectric specifications. The list of suitable alternative insulators is currently very limited. Thus, a radically different mindset with respect to suitable insulators for 2D technologies may be required. We review possible solution scenarios like the creation of clean interfaces, production of native oxides from 2D semiconductors and more intensive studies on crystalline insulators

    Two-Dimensional Electronics and Optoelectronics

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    The discovery of monolayer graphene led to a Nobel Prize in Physics being awarded in 2010. This has stimulated further research on a wide variety of two-dimensional (2D) layered materials. The coupling of metallic graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) and black phosphorus have attracted a tremendous amount of interest in new electronic and optoelectronic applications. Together with other 2D materials, such as the wide band gap boron nitride nanosheets (BNNSs), all these 2D materials have led towards an emerging field of van der Waal 2D heterostructures. The papers in this book were originally published by Electronics (MDPI) in a Special Issue on “Two-Dimensional Electronics and Optoelectronics”. The book consists of eight papers, including two review articles, covering various pertinent and fascinating issues concerning 2D materials and devices. Further, the potential and the challenges of 2D materials are discussed, which provide up to date guidance for future research and development

    Towards RF graphene devices: A review

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    Graphene has been targeted for a wide variety of applications due to its characteristics. It is a zero-bandgap material, has high conductivity, and high carrier mobility, which makes it a promising material for radiofrequency applications. This review examines the applications of graphene in the design of radiofrequency building blocks, their performance, and current hurdles. Initially, graphene passive devices (inductors, capacitors, antennas, and waveguides) are analyzed, as well as their current modelling techniques. Then, radiofrequency transistors and their modelling are reported and discussed. An insight on the current state of radiofrequency devices is provided which more specifically targets graphene oscillators, multipliers, and mixers. Finally, the current fabrication issues and techniques are analyzed and discussed, providing a global overview on the application of graphene for radiofrequency electronics.Work supported by PTDC/EEI-TEL/29670/2017 - (POCI-01-0145-FEDER-029670), co-financed by the European Regional Development Fund (ERDF), through COMPETE 2020, grant SFRH/BD/141462/2018, grant SFRH/BD/137529/2018, grant UIDB/04436/2020, grant UIDP/04436/2020, and grant UIDB/04650/2020

    Short channel effects in graphene-based field effect transistors targeting radio-frequency applications

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    Channel length scaling in graphene field effect transistors (GFETs) is key in the pursuit of higher performance in radio frequency electronics for both rigid and flexible substrates. Although two-dimensional (2D) materials provide a superior immunity to Short Channel Effects (SCEs) than bulk materials, they could dominate in scaled GFETs. In this work, we have developed a model that calculates electron and hole transport along the graphene channel in a drift-diffusion basis, while considering the 2D electrostatics. Our model obtains the self-consistent solution of the 2D Poisson's equation coupled to the current continuity equation, the latter embedding an appropriate model for drift velocity saturation. We have studied the role played by the electrostatics and the velocity saturation in GFETs with short channel lengths L. Severe scaling results in a high degradation of GFET output conductance. The extrinsic cutoff frequency follows a 1/L^n scaling trend, where the index n fulfills n < 2. The case n = 2 corresponds to long-channel GFETs with low source/drain series resistance, that is, devices where the channel resistance is controlling the drain current. For high series resistance, n decreases down to n= 1, and it degrades to values of n < 1 because of the SCEs, especially at high drain bias. The model predicts high maximum oscillation frequencies above 1 THz for channel lengths below 100 nm, but, in order to obtain these frequencies, it is very important to minimize the gate series resistance. The model shows very good agreement with experimental current voltage curves obtained from short channel GFETs and also reproduces negative differential resistance, which is due to a reduction of diffusion current.Comment: 27-pages manuscript (10 figures) plus 6 pages of supplementary information. European Union Action H2020 (696656) / Department d'Universitats, Recerca i Societat de la Informaci\'o of the Generalitat de Catalunya (2014 SGR 384) / Ministerio de Econom\'ia y Competitividad of Spain (TEC2012-31330 and TEC2015-67462-C2-1-R) / MINECO FEDE
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