10,581 research outputs found

    Modeling a Schottky-barrier carbon nanotube field-effect transistor with ferromagnetic contacts

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    In this study, a model of a Schottky-barrier carbon nanotube field- effect transistor (CNT-FET), with ferromagnetic contacts, has been developed. The emphasis is put on analysis of current-voltage characteristics as well as shot (and thermal) noise. The method is based on the tight-binding model and the non- equilibrium Green's function technique. The calculations show that, at room temperature, the shot noise of the CNT FET is Poissonian in the sub-threshold region, whereas in elevated gate and drain/source voltage regions the Fano factor gets strongly reduced. Moreover, transport properties strongly depend on relative magnetization orientations in the source and drain contacts. In particular, one observes quite a large tunnel magnetoresistance, whose absolute value may exceed 50%.Comment: 8 pages, 4 figure

    Graphene Field Effect Transistors: Diffusion-Drift Theory

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    Based on explicit solution of current continuity equation in the graphene FET's channel the semi-classical diffusion-drift description of the carrier transport and I-V characteristics model has been developed. Role of rechargeable defects (interface traps) near or at the interface between graphene and insulated layers has also described.Comment: 24 pages, 13 figures, a chapter in "Graphene, Theory, Research and Applications", INTEC

    Design of a single-chip pH sensor using a conventional 0.6-μm CMOS process

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    A pH sensor fabricated on a single chip by an unmodified, commercial 0.6-/spl μm CMOS process is presented. The sensor comprises a circuit for making differential measurements between an ion-sensitive field-effect transistor (ISFET) and a reference FET (REFET). The ISFET has a floating-gate structure and uses the silicon nitride passivation layer as a pH-sensitive insulator. As fabricated, it has a large threshold voltage that is postulated to be caused by a trapped charge on the floating gate. Ultraviolet radiation and bulk-substrate biasing is used to permanently modify the threshold voltage so that the ISFET can be used in a battery-operated circuit. A novel post-processing method using a single layer of photoresist is used to define the sensing areas and to provide robust encapsulation for the chip. The complete circuit, operating from a single 3-V supply, provides an output voltage proportional to pH and can be powered down when not required

    High operating temperature in V-based superconducting quantum interference proximity transistors

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    Here we report the fabrication and characterization of fully superconducting quantum interference proximity transistors (SQUIPTs) based on the implementation of vanadium (V) in the superconducting loop. At low temperature, the devices show high flux-to-voltage (up to 0.52 mV/Ί0\ \textrm{mV}/\Phi_0) and flux-to-current (above 12 nA/Ί0\ \textrm{nA}/\Phi_0) transfer functions, with the best estimated flux sensitivity ∌\sim2.6 ΌΊ0/Hz\ \mu\Phi_0/\sqrt{\textrm{Hz}} reached under fixed voltage bias, where Ί0\Phi_0 is the flux quantum. The interferometers operate up to Tbath≃T_\textrm{bath}\simeq 2 K \textrm{K}, with an improvement of 70%\% of the maximal operating temperature with respect to early SQUIPTs design. The main features of the V-based SQUIPT are described within a simplified theoretical model. Our results open the way to the realization of SQUIPTs that take advantage of the use of higher-gap superconductors for ultra-sensitive nanoscale applications that operate at temperatures well above 1 K.Comment: Published version with Supplementary Informatio

    Phase-driven collapse of the Cooper condensate in a nanosized superconductor

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    Superconductivity can be understood in terms of a phase transition from an uncorrelated electron gas to a condensate of Cooper pairs in which the relative phases of the constituent electrons are coherent over macroscopic length scales. The degree of correlation is quantified by a complex-valued order parameter, whose amplitude is proportional to the strength of the pairing potential in the condensate. Supercurrent-carrying states are associated with non-zero values of the spatial gradient of the phase. The pairing potential and several physical observables of the Cooper condensate can be manipulated by means of temperature, current bias, dishomogeneities in the chemical composition or application of a magnetic field. Here we show evidence of complete suppression of the energy gap in the local density of quasiparticle states (DOS) of a superconducting nanowire upon establishing a phase difference equal to pi over a length scale comparable to the superconducting coherence length. These observations are consistent with a complete collapse of the pairing potential in the center of the wire, in accordance with theoretical modeling based on the quasiclassical theory of superconductivity in diffusive systems. Our spectroscopic data, fully exploring the phase-biased states of the condensate, highlight the profound effect that extreme phase gradients exert on the amplitude of the pairing potential. Moreover, the sharp magnetic response observed near the onset of the superconducting gap collapse regime can be exploited to realize ultra-low noise magnetic flux detectors.Comment: 7 pages, 5 color figures plus supporting inf
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