Showing papers on "Capacitance published in 2007"

Carrier statistics and quantum capacitance of graphene sheets and ribbons

Abstract: In this work, fundamental results for carrier statistics in graphene two-dimensional sheets and nanoscale ribbons are derived. Though the behavior of intrinsic carrier densities in two-dimennsional graphene sheets is found to differ drastically from traditional semiconductors, very narrow (sub-10nm) ribbons are found to be similar to traditional narrow-gap semiconductors. The quantum capacitance, an important parameter in the electrostatic design of devices, is derived for both two-dimensional graphene sheets and nanoribbons.

Downscaling of self-aligned, all-printed polymer thin-film transistors

Abstract: Printing is an emerging approach for low-cost, large-area manufacturing of electronic circuits, but it has the disadvantages of poor resolution, large overlap capacitances, and film thickness limitations, resulting in slow circuit speeds and high operating voltages. Here, we demonstrate a self-aligned printing approach that allows downscaling of printed organic thin-film transistors to channel lengths of 100–400 nm. The use of a crosslinkable polymer gate dielectric with 30–50 nm thickness ensures that basic scaling requirements are fulfilled and that operating voltages are below 5 V. The device architecture minimizes contact resistance effects, enabling clean scaling of transistor current with channel length. A self-aligned gate configuration minimizes parasitic overlap capacitance to values as low as 0.2–0.6 pF mm−1, and allows transition frequencies of fT = 1.6 MHz to be reached. Our self-aligned process provides a way to improve the performance of printed organic transistor circuits by downscaling, while remaining compatible with the requirements of large-area, flexible electronics manufacturing.

Electrical measurements in the study of immersed paint coatings on metal. I. Comparison between capacitance and gravimetric methods of estimating water-uptake

Abstract: An electrical capacitance method of estimating the water-uptake of paint films on metal immersed in sea-water has given, for many paint formulations, values agreeing with those obtained by a gravimetric method. For a few paints, however, pronounced disagreement was shown between values obtained by the two methods. Theoretical considerations show that such differences indicate, in general, a type of water distribution within the film departing considerably from the random, uniform distribution assumed in calculating the ‘capacitance’ values. Several types of distribution are discussed. The capacitance method can be used for routine testing of the water-uptake of paints only on a type of paint known to give good agreement between the two methods; where applicable, it has decided advantages over gravimetric methods.

High performance, waveguide integrated Ge photodetectors

Abstract: Photonic systems based on complementary metal oxide semiconductor (CMOS) technology require the integration of passive and active photonic devices. The integration of waveguides and photodetector is one of the most important technologies. We report a Ge p-i-n photodetector that is monolithically integrated with silicon oxynitride and silicon nitride waveguides. All processes and materials are CMOS compatible and can be implemented in the current integrated circuit process technology. The small size of the devices results in low absolute dark current. The waveguide-coupled Ge devices show high efficiency (~90%) over a wide range of wavelengths well beyond the direct band gap of Ge, resulting in a responsivity of 1.08 A/W for 1550 nm light. The device speed of 7.2 GHz at 1V reverse bias is strongly affected by the capacitance of the probe pads. The high-performance of the devices at low voltage (≤ 1V) facilitates the integration with CMOS circuits.

A dielectric-modulated field-effect transistor for biosensing

Abstract: Interest in biosensors based on field-effect transistors (FETs), where an electrically operated gate controls the flow of charge through a semiconducting channel, is driven by the prospect of integrating biodetection capabilities into existing semiconductor technology. In a number of proposed FET biosensors, surface interactions with biomolecules in solution affect the operation of the gate or the channel. However, these devices often have limited sensitivity. We show here that a FET biosensor with a vertical gap is sensitive to the specific binding of streptavidin to biotin. The binding of the streptavidin changes the dielectric constant (and capacitance) of the gate, resulting in a large shift in the threshold voltage for operating the FET. The vertical gap is fabricated using simple thin-film deposition and wet-etching techniques. This may be an advantage over planar nanogap FETs, which require lithographic processing. We believe that the dielectric-modulated FET (DMFET) provides a useful approach towards biomolecular detection that could be extended to a number of other systems.

Electrochemical double-layer capacitance of MoS2 nanowall films

Abstract: Edge-oriented MoS 2 films synthesized by single source precursor chemical vapor deposition exhibit a high density of nanowalls, which can potentially exhibit excellent electrochemical charge storage properties. The electrochemical double-layer capacitance of layered, nanowalled MoS 2 film has been investigated in this work using electrochemical impedance spectroscopy. We show that edge-oriented MoS 2 thin films can behave as a supercapacitor at alternating current frequencies up to 100 Hz. The supercapacitor performance is comparable to that of carbon nanotube array electrodes. In addition to double-layer capacitance, diffusion of the ions into the films at slow scan rates gives rise to faradaic capacitance, which enhances the capacitance significantly. The real and imaginary part of the capacitance of the MoS 2 films was analyzed as a function of frequency, in order to obtain information on the relaxation time constant and frequency dependence of the supercapacitor properties.

High-Power Density Design of a Soft-Switching High-Power Bidirectional dc–dc Converter

Abstract: A bidirectional dc-dc converter typically consists of a buck and a boost converters. In order to have high-power density, the converter can be designed to operate in discontinuous conducting mode (DCM) such that the passive inductor can be minimized. The DCM operation associated current ripple can be alleviated by interleaving multiphase currents. However, DCM operation tends to increase turnoff loss because of a high peak current and its associated parasitic ringing due to the oscillation between the inductor and the device output capacitance. Thus, the efficiency is suffered with the conventional DCM operation. Although to reduce the turnoff loss a lossless capacitor snubber can be added across the switch, the energy stored in the capacitor needs to be discharged before device is turned on. This paper adopts a gate signal complimentary control scheme to turn on the nonactive switch and to divert the current into the antiparalleled diode of the active switch so that the main switch can be turned on under zero-voltage condition. This diverted current also eliminates the parasitic ringing in inductor current. For capacitor value selection, there is a tradeoff between turnon and turnoff losses. This paper suggests the optimization of capacitance selection through a series of hardware experiments to ensure the overall power loss minimization under complimentary DCM operating condition. According to the suggested design optimization, a 100-kW hardware prototype is constructed and tested. The experimental results are provided to verify the proposed design approach.

Noise analysis and reduction in solid-state nanopores

Abstract: The electrical noise characteristics of ionic current through organic and synthetic nanopores have been investigated. Comparison to proteinaceous alpha-Hemolysin pores reveals two dominant noise sources in silicon nitride nanometre-scale pores: a high-frequency noise associated with the capacitance of the silicon support chip (dielectric noise), and a low-frequency current fluctuation with 1/ f α characteristics (flicker noise). We present a technique for reducing the dielectric noise by curing polydimethylsiloxane (PDMS) on the nanopore support chip. This greatly improves the performance of solid-state nanopore devices, yielding an unprecedented signal-to-noise ratio when observing dsDNA translocation events and ssDNA probe capture for force spectroscopy applications. (Some figures in this article are in colour only in the electronic version)

Polymer electrolyte-gated organic field-effect transistors: low-voltage, high-current switches for organic electronics and testbeds for probing electrical transport at high charge carrier density.

Abstract: We report the fabrication and extensive characterization of solid polymer electrolyte-gated organic field-effect transistors (PEG-FETs) in which a polyethylene oxide (PEO) film containing a dissolved Li salt is used to modulate the hole conductivity of a polymer semiconductor. The large capacitance (approximately 10 microF/cm2) of the solution-processed polymer electrolyte gate dielectric facilitates polymer semiconductor conductivities on the order of 103 S/cm at low gate voltages ( 3 cm2/V.s. PEG-FETs fabricated with gate electrodes either aligned or intentionally nonaligned to the channel exhibited dramatically different electrical behavior when tested in vacuum or in air. Large differences in ionic diffusivity can explain the dominance of either electrostatic charging (in vacuum) or bulk electrochemical doping (in air) as the device operational mechanism. The use of a larger anion in the polymer electrolyte, bis(trifluoromethanesulfonyl)imide (TFSI-), yielded transistors that showed clear current saturation and square law behavior in the output characteristics, which also points to electrostatic (field-effect) charging. In addition, negative transconductances were observed using the PEO/LiTFSI electrolyte for all three polymer semiconductors at gate voltages larger than -3 V. Bias stress measurements performed with PEO/LiTFSI-gated bottom contact PEG-FETs showed that polymer semiconductors can sustain high ON currents for greater than 10 min without large losses in conductance. Collectively, the results indicate that PEG-FETs may serve as useful devices for high-current/low-voltage applications and as testbeds for probing electrical transport in polymer semiconductors at high charge density.

Self-Capacitance of High-Voltage Transformers

Abstract: The calculation of a transformer's parasitics, such as its self capacitance, is fundamental for predicting the frequency behavior of the device, reducing this capacitance value and moreover for more advanced aims of capacitance integration and cancellation. This paper presents a comprehensive procedure for calculating all contributions to the self-capacitance of high-voltage transformers and provides a detailed analysis of the problem, based on a physical approach. The advantages of the analytical formulation of the problem rather than a finite element method analysis are discussed. The approach and formulas presented in this paper can also be used for other wound components rather than just step-up transformers. Finally, analytical and experimental results are presented for three different high-voltage transformer architectures.

Electrical Capacitance Volume Tomography

Abstract: A dynamic volume imaging based on the principle of electrical capacitance tomography (ECT), namely, electrical capacitance volume tomography (ECVT), has been developed in this study. The technique generates, from the measured capacitance, a whole volumetric image of the region enclosed by the geometrically three-dimensional capacitance sensor. This development enables a real-time, 3-D imaging of a moving object or a real-time volume imaging (4-D) to be realized. Moreover, it allows total interrogation of the whole volume within the domain (vessel or conduit) of an arbitrary shape or geometry. The development of the ECVT imaging technique primarily encloses the 3-D capacitance sensor design and the volume image reconstruction technique. The electrical field variation in three-dimensional space forms a basis for volume imaging through different shapes and configurations of ECT sensor electrodes. The image reconstruction scheme is established by implementing the neural-network multicriterion optimization image reconstruction (NN-MOIRT), developed earlier by the authors for the 2-D ECT. The image reconstruction technique is modified by introducing into the algorithm a 3-D sensitivity matrix to replace the 2-D sensitivity matrix in conventional 2-D ECT, and providing additional network constraints including 3-to-2-D image matching function. The additional constraints further enhance the accuracy of the image reconstruction algorithm. The technique has been successfully verified over actual objects in the experimental conditions

Capacitance wire-mesh sensor for fast measurement of phase fraction distributions

Abstract: We introduce a new wire-mesh sensor based on capacitance (permittivity) measurements. The sensor can be used to measure transient phase fraction distributions in a flow cross-section, such as in a pipe or other vessel, and is able to discriminate fluids having different relative permittivity (dielectric constant) values in a multiphase flow. We designed and manufactured a prototype sensor which comprises two planes of 16 wires each. The wires are evenly distributed across the measuring cross-section, and measurement is performed at the wire crossings. Time resolution of the prototype sensor is 625 frames per second. Sensor and measuring electronics were evaluated showing good stability and accuracy in the capacitance measurement. The wire-mesh sensor was tested in a silicone oil/water two-phase bubbly flow.

Eliminating ground current in a transformerless photovoltaic application

Abstract: A single phase converter can be used for low-power grid connected applications. In photovoltaic applications it is possible to remove the transformer in the inverter in order to reduce losses, costs and size. Galvanic connection of the grid and the DC sources in transformerless systems can introduce additional leakage currents because of the earth parasitic capacitance. This currents increase conducted and radiated electromagnetic emissions, harmonics injected in the utility grid and system losses. Amplitude and spectrum of ground current depends on the converter topology, on the switching strategy and on the resonant circuit formed by the ground capacitance, the converter, the AC filter and the grid. In this paper, the leakage current in a 1.5 kW photovoltaic installation is measured. The installation includes a string of sixteen panels, a full bridge inverter and a LCL filter. Influence of inverter topology and modulation strategy on the magnitude of the leakage current is presented. Finally, the use of neutral point clamped inverters in transformerless photovoltaic applications is studied.

A Low-Dropout Regulator for SoC With $Q$ -Reduction

Abstract: A low-dropout regulator for SoC, with an advanced Q-reduction circuit to minimize both the on-chip capacitance and the minimum output-current requirement down to 100 muA, is introduced in this paper. The idea has been implemented in a standard 0.35-mum CMOS technology (VTHN ap 0.55 V and |VTHP| ap 0.75 V). The required on-chip capacitance is reduced to 6 pF, comparing to 25 pF for the case without Q-reduction circuit. From the experimental results, the proposed regulator-circuit implementation enables voltage regulation down to a 1.2-V supply voltage, and a dropout voltage of 200 mV at 100-mA maximum output current.

Electro-chemical and biological properties of carbon nanotube based multi-electrode arrays

Abstract: A novel class of micro-electrodes was fabricated by synthesizing high density carbon nanotube islands on lithographically defined, passivated titanium nitride conductors on a silicon dioxide substrate. Electrochemical characterization in phosphate buffered saline of these new electrodes reveals superb electrochemical properties marked by featureless rectangular cyclic voltammetry curves corresponding to a DC surface specific capacitance and a volume specific capacitance as high as 10 mF cm−2 and 10 F cm−3, respectively. These electrodes are also characterized by a slowly varying impedance magnitude over the range of 1 Hz to 20 kHz. High fidelity extracellular recordings from cultured neurons were performed and analysed to validate the effectiveness of the fabricated electrodes. The enhanced electrochemical properties of the electrodes, their flexible and simple micro-fabrication preparation procedure as well as their bio-compatibility and durability suggest that carbon nanotube electrodes are a promising platform for high resolution capacitive electrochemical applications.

Record RF performance of 45-nm SOI CMOS Technology

Abstract: We report record RF performance in 45-nm silicon-on- insulator (SOI) CMOS technology. RF performance scaling with channel length and layout optimization is demonstrated. Peak fT's of 485 GHz and 345 GHz are measured in floating- body NFET and PFET with nearby wiring parasitics (i.e., gate- to-contact capacitance) included after de-embedding, thus representing FET performance in a real design. The measured fT's are the highest ever reported in a CMOS technology. Body- contacted FETs are also analyzed that have layout optimized for high-frequency analog applications. Employing a notched body contact layout, we reduce parasitic capacitance and gate leakage current significantly, thus improving RF performance with low power. For longer than minimum channel length and a body-contacted NFET with notched layout, we measure a peak fT of 245 GHz with no degradation in critical analog figures of merit, such as self-gain.

Design of Class E Amplifier With Nonlinear and Linear Shunt Capacitances for Any Duty Cycle

Abstract: One of the main advantages of class E amplifiers for RF and microwave applications relies on the inclusion of a shunt capacitance in the tuned output network. At high frequencies, this capacitance is mainly provided by the output parasitic capacitance of the device with perhaps a linear external one for fine adjustments. The device's output capacitance is nonlinear and this influences the design parameters, frequency limit of operation, and performance of the class E amplifier. This paper presents a design method for the class E amplifier with shunt capacitance combining a nonlinear and linear one for any duty cycle, any capacitance's nonlinear dependence parameters, and any loaded quality factor of the tuned network. Nonlinear design with possibly different duty cycles is of relevance to maximize power or, alternatively, frequency utilization of a given device. Experimental, simulated, and compared results are presented to prove this design procedure

Gate capacitance coupling of singled-walled carbon nanotube thin-film transistors

Abstract: The electrostatic coupling between singled-walled carbon nanotube (SWCNT) networks/arrays and planar gate electrodes in thin-film transistors (TFTs) is analyzed both in the quantum limit with an analytical model and in the classical limit with finite-element modeling. The computed capacitance depends on both the thickness of the gate dielectric and the average spacing between the tubes, with some dependence on the distribution of these spacings. Experiments on transistors that use submonolayer, random networks of SWCNTs verify certain aspects of these calculations. The results are important for the development of networks or arrays of nanotubes as active layers in TFTs and other electronic devices.

A Varactor-Tunable High Impedance Surface With a Resistive-Lumped-Element Biasing Grid

Abstract: A high impedance surface consisting of metallic square patches electrically connected through vias to the ground plane beneath them is made tunable. Tunability is achieved by connecting adjacent patches with varactor diodes thus altering the capacitance between the patches and hence the surface's resonance frequency. The varactor diodes are biased with the aid of a resistive grid. The grid is made resistive using surface mount resistors. Using an approximate equivalent circuit the effect of the varactor diode resistance is investigated for normal plane wave incidence. It is shown that at resonance, a small varactor resistance may lead to a significant absorption. The potential use of a waveguide simulator to characterize approximately the performance of the proposed metamaterial structure is investigated.

Electrochemical Study of High Electrochemical Double Layer Capacitance of Ordered Porous Carbons with Both Meso/Macropores and Micropores

Abstract: Porous carbons with large meso/macropore surface areas were prepared by the colloidal-crystal-templating technique. The porous carbons exibited extremely high specific electrochemical double layer (EDL) capacitance of 200−350 F g-1 in an aqueous electrolyte (1 M H2SO4). The pore structure dependence of the capacitance was studied mainly by means of cyclic voltammetry and is discussed in detail. From the sweep rate dependence of the series resistance and capacitance, it was found that the ion-penetration depth at the porous electrode surface was finite and decreased with an increasing sweep rate. Peaks around the point of zero charge, which were observed in addition to typical rectangular voltammograms, were explained well by the potential drop in pores. The surface area dependence of the capacitance revealed that the contribution of the meso/macropore surface is as great as that of the plane electrodes and that only the part of the micropore surface adjacent to the opening mouths is effective.

Electric field gating with ionic liquids

Abstract: The authors show that ionic liquids are well suited to specialized electric field gating applications in which large surface charge densities can be induced on the surfaces of low-carrier density thin-film metals. Using either coplanar or overlay gate configurations, they demonstrate field-induced resistance changes on the order of a factor of 104 for thin conducting InOx films. The areal capacitances and field effect mobilities noticeably exceed those that can be achieved using AlOx dielectrics. In addition, the charge state can be frozen in by reducing the temperature, thus providing an opportunity for electric field tuning of metal-insulator transitions in a variety of thin-film systems.

Electrophoretic and dielectrophoretic field gradient technique for separating bioparticles.

Abstract: We describe a new device for separation of complex biological particles and structures exploiting many physical properties of the biolytes. The device adds a new longitudinal gradient feature to insulator dielectrophoresis, extending the technique to separation of complex mixtures in a single channel. The production of stronger local field gradients along a global gradient allows particles to enter, initially transported through the channel by electrophoresis and electroosmosis, and to be isolated according to their characteristic physical properties, including charge, polarizability, deformability, surface charge mobility, dielectric features, and local capacitance. In this work, the separation mechanism is described in terms of the relevant electromechanical principles, and proof-of-principle is demonstrated using various bacteria cells as model systems. The results demonstrate the selectivity of the technique and suggest that it may form the foundation for a versatile and useful tool for separating mix...