Showing papers on "Capacitance published in 2003"

Electrochemical Characteristics and Impedance Spectroscopy Studies of Carbon-Carbon Supercapacitors

Abstract: This paper presents the results obtained on the electrochemical behavior of electrochemical capacitors assembled in nonaqueous electrolyte. The first part is devoted to the electrochemical characterization of carbon-carbon 4 cm2 cells systems in terms of capacitance, resistance, and cyclability. The second part is focused on the electrochemical impedance spectroscopy study of the cells. Nyquist plots are presented and the impedance of the supercapacitors is discussed in terms of complex capacitance and complex power. This allows the determination of a relaxation time constant of the systems, and the real and the imaginary part of the complex power vs. the frequency plots give information on the supercapacitor cells frequency behavior. The complex impedance plots for both a supercapacitor and a tantalum dielectric capacitor cells are compared. © 2003 The Electrochemical Society. All rights reserved.

Probing Biomolecular Interactions at Conductive and Semiconductive Surfaces by Impedance Spectroscopy: Routes to Impedimetric Immunosensors, DNA‐Sensors, and Enzyme Biosensors

Abstract: Impedance spectroscopy is a rapidly developing electrochemical technique for the characterization of biomaterialfunctionalized electrodes and biocatalytic transformations at electrode surfaces, and specifically for the transduction of biosensing events at electrodes or field-effect transistor devices. The immobilization of biomaterials, e.g., enzymes, antigens/antibodies or DNA on electrodes or semiconductor surfaces alters the capacitance and interfacial electron transfer resistance of the conductive or semiconductive electrodes. Impedance spectroscopy allows analysis of interfacial changes originating from biorecognition events at electrode surfaces. Kinetics and mechanisms of electron transfer processes corresponding to biocatalytic reactions occurring at modified electrodes can be also derived from Faradaic impedance spectroscopy. Different immunosensors that use impedance measurements for the transduction of antigen-antibody complex formation on electronic transducers were developed. Similarly, DNA biosensors using impedance measurements as readout signals were developed. Amplified detection of the analyte DNA using Faradaic impedance spectroscopy was accomplished by the coupling of functionalized liposomes or by the association of biocatalytic conjugates to the sensing interface providing biocatalyzed precipitation of an insoluble product on the electrodes. The amplified detections of viral DNA and single-base mismatches in DNA were accomplished by similar methods. The changes of interfacial features of gate surfaces of field-effect transistors (FET) upon the formation of antigen-antibody complexes or assembly of protein arrays were probed by impedance measurements and specifically by transconductance measurements. Impedance spectroscopy was also applied to characterize enzymebased biosensors. The reconstitution of apo-enzymes on cofactor-functionalized electrodes and the formation of cofactor-enzyme affinity complexes on electrodes were probed by Faradaic impedance spectroscopy. Also biocatalyzed reactions occurring on electrode surfaces were analyzed by impedance spectroscopy. The theoretical background of the different methods and their practical applications in analytical procedures were outlined in this article.

Theory of ballistic nanotransistors

Abstract: Numerical simulations are used to guide the development of a simple analytical theory for ballistic field-effect transistors. When two-dimensional (2-D) electrostatic effects are small (and when the insulator capacitance is much less than the semiconductor (quantum) capacitance), the model reduces to Natori's theory of the ballistic MOSFET. The model also treats 2-D electrostatics and the quantum capacitance limit where the semiconductor quantum capacitance is much less than the insulator capacitance. This new model provides insights into the performance of MOSFETs near the scaling limit and a unified framework for assessing and comparing a variety of novel transistors.

Two-dimensional beam steering using an electrically tunable impedance surface

Abstract: By covering a metal ground plane with a periodic surface texture, we can alter its electromagnetic properties. The impedance of this metasurface can be modeled as a parallel resonant circuit, with sheet inductance L, and sheet capacitance C. The reflection phase varies with frequency from +/spl pi/ to -/spl pi/, and crosses through 0 at the LC resonance frequency, where the surface behaves as an artificial magnetic conductor. By incorporating varactor diodes into the texture, we have built a tunable impedance surface, in which an applied bias voltage controls the resonance frequency, and the reflection phase. We can program the surface to create a tunable phase gradient, which can electronically steer a reflected beam over +/- 40/spl deg/ in two dimensions, for both polarizations. We have also found that this type of resonant surface texture can provide greater bandwidth than conventional reflectarray structures. This new electronically steerable reflector offers a low-cost alternative to a conventional phased array.

Chemical capacitance of nanostructured semiconductors: its origin and significance for nanocomposite solar cells

Abstract: The capacitance measured in dye-sensitised nanocrystalline TiO2 solar cells (DSSC) is interpreted in terms of a chemical capacitance, which is found to be a crucial feature for describing the dynamic operation of solar cells based on nanoscaled materials.

Double-layer and pseudocapacitance types of electrochemical capacitors and their applications to the development of hybrid devices

Abstract: The basis of the complementary use of electrochemical capacitors (so-called supercapacitors) in hybrid electric power generation by rechargeable batteries and fuel cells is explored. Electrochemical capacitors are of two types: one where the interfacial double-layer capacitance of high specific area carbon materials is the basis of electric charge storage (as ions and electrons); and the other where pseudocapacitance, associated with electrosorption and surface redox processes at high-area electrode materials, e.g. RuO2, or at conducting polymers, provides the basis of charge storage. The former, double-layer, type of capacitance stores charge non-faradaically while the latter type, pseudocapacitance, stores charge indirectly through faradaic chemical processes but its electrical behaviour is like that of a capacitor. Two types of hybrid battery/capacitor system are recognized: one based on combination of an electrochemical capacitor cell with a rechargeable battery or a fuel cell in a load-leveling function, e.g. in an electric vehicle power train; and the other based on combination of a faradaic battery-type electrode coupled internally with a capacitative electrode in a two-electrode hybrid module (termed an asymmetric capacitor). Optimization of operation of such systems in terms of balancing of active masses, of power and charge densities, and choice of maximum but limited states-of-discharge, is treated.

An RF circuit model for carbon nanotubes

Abstract: We develop an RF circuit model for single walled carbon nanotubes for both dc and capacitively contacted geometries. By modeling the nanotube as a nanotransmission line with distributed kinetic and magnetic inductance as well as distributed quantum and electrostatic capacitance, we calculate the complex, frequency dependent impedance for a variety of measurement geometries. Exciting voltage waves on the nanotransmission line is equivalent to directly exciting the yet-to-be observed one dimensional plasmons, the low energy excitation of a Luttinger liquid.

Cyclic Voltammetry Studies of Nanoporous Semiconductors. Capacitive and Reactive Properties of Nanocrystalline TiO2 Electrodes in Aqueous Electrolyte

Abstract: Extended networks of nanosized semiconductor particles permeated with an electrolyte display unique electrochemical behaviors. We report on a general investigation of the electrochemical properties of nanoporous electrodes by means of cyclic voltammetry. Models have been developed accounting for the fundamental characteristics of these electrodes: charge accumulation, charge transport, and interfacial charge transfer. These characteristics can be translated into simple electrical equivalents, which allow us to identify and classify the major features of voltammetry response according to the competition of the different processes during a voltammetric scan. A key point for describing the experimental observations is the potential dependence of the intrinsic film capacitance. The physical meaning of this capacitance is discussed in terms of the distribution of electronic states. We describe in detail the numerical simulation methods, and despite the simplicity of our approach, we show that these methods al...

Electronic device having sound output module

Abstract: the output amplifiers requiring a bias voltage can be activated or deactivated individually, and a bias acceleration circuit for rapidly increasing the magnitude of a bias voltage in time can be provided in or with the bias circuit, whereby even in case of the capacitance of the capacitor included in the bias circuit being increased for improving the power supply rejection ratio (PSRR), the rise in the bias voltage can be increased so that the pop sound which arises when the bias circuit is activated can be still diminished.

10-Gb/s limiting amplifier and laser/modulator driver in 0.18-/spl mu/m CMOS technology

Abstract: A limiting amplifier incorporates active feedback, inductive peaking, and negative Miller capacitance to achieve a voltage gain of 50 dB, a bandwidth of 9.4 GHz, and a sensitivity of 4.6 mV/sub pp/ for a bit-error rate of 10/sup -12/ while consuming 150 mW. A driver employs T-coil peaking and negative impedance conversion to achieve operation at 10 Gb/s while delivering a current of 100 mA to 25-/spl Omega/ lasers or a voltage swing of 2 V/sub pp/ to 50-/spl Omega/ modulators with a power dissipation of 675 mW. Fabricated in 0.18-/spl mu/m CMOS technology, both prototypes operate with a 1.8-V supply.

Application of Low-Viscosity Ionic Liquid to the Electrolyte of Double-Layer Capacitors

Abstract: The performance of a double-layer capacitor (DLC) composed of activated carbon electrodes and 1--ethyl-3-methylimidazolium fluoride (EMIF).2.3HF, which has extremely high conductivity with low viscosity, was examined and compared with those using the popular ionic liquid EMIBF 4 , conventional aqueous electrolyte 35 wt % H 2 SO 4 , and nonaqueous electrolyte 1 M Et 3 MeNBF 4 /propylene carbonate. The DLC using EMIF.2.3HF showed an intermediate capacitance and internal resistance between the aqueous and nonaqueous electrolyte systems due to its intermediate double-layer capacitance and electrolytic conductivity. EMIF.2.3HF afforded much higher capacitance than EMIBF 4 even at low temperatures, however, it had a lower decomposition voltage (∼2 V), resulting in lower energy density. The capacitance of EMIF.2.3HF was extremely dependent on the applied voltage.

Langmuir−Blodgett Film Deposition of Metallic Nanoparticles and Their Application to Electronic Memory Structures

Abstract: The Langmuir-Blodgett deposition of organically passivated gold nanoparticles is reported. A monolayer of these particles has been incorporated into a metal-insulator-semiconductor (MIS) structure. The MIS device exhibits a hysteresis in its capacitance versus voltage characteristic, the magnitude of which is dependent on the voltage sweep conditions. Charge storage in the layer of nanoparticles is thought to be responsible for this effect.

Charge control of parallel-plate, electrostatic actuators and the tip-in instability

Abstract: Controlling the charge, rather than the voltage, on a parallel-plate, electrostatic actuator theoretically permits stable operation for all deflections. Practically, we show that, using charge control, the maximum stable deflection is limited by 1) charge pull-in, in which the actuator snaps due to the presence of parasitic capacitance and 2) tip-in, in which the rotation mode becomes unstable. This work presents a circuit that controls the amount of charge on a parallel-plate, electrostatic actuator. This circuit reduces the sensitivity to parasitic capacitance, so that tip-in is the limiting instability. A small-signal model of the actuator is developed and used to determine the circuit bandwidth and gain requirements for stable deflections. Four different parallel-plate actuators have been designed and tested to verify the charge control technique as well as to verify charge pull-in, tip-in, and the bandwidth requirements. One design travels 83% of the gap before tip-in. Another design can only travel 20% of the gap before tip-in, regardless of whether voltage control or charge control is used.

MIM capacitor with metal nitride electrode materials and method of formation

Abstract: An MIM capacitor with low leakage and high capacitance is disclosed. A layer of titanium nitride (TiN) or boron-doped titanium nitride (TiBN) material is formed as a lower electrode over an optional capacitance layer of hemispherical grained polysilicon (HSG). Prior to the dielectric formation, the first layer may be optionally subjected to a nitridization or oxidation process. A dielectric layer of, for example, aluminum oxide (Al2O3) formed by atomic layer deposition (ALD) is fabricated over the first layer and after the optional nitridization or oxidation process. An upper electrode of titanium nitride (TiN) or boron-doped titanium nitride (TiBN) is formed over the dielectric layer.

Calculation and measurement of electromechanical coupling coefficient of capacitive micromachined ultrasonic transducers

Abstract: The electromechanical coupling coefficient is an important figure of merit of ultrasonic transducers. The transducer bandwidth is determined by the electromechanical coupling efficiency. The coupling coefficient is, by definition, the ratio of delivered mechanical energy to the stored total energy in the transducer. In this paper, we present the calculation and measurement of coupling coefficient for capacitive micromachined ultrasonic transducers (CMUTs). The finite element method (FEM) is used for our calculations, and the FEM results are compared with the analytical results obtained with parallel plate approximation. The effect of series and parallel capacitances in the CMUT also is investigated. The FEM calculations of the CMUT indicate that the electromechanical coupling coefficient is independent of any series capacitance that may exist in the structure. The series capacitance, however, alters the collapse voltage of the membrane. The parallel parasitic capacitance that may exist in a CMUT or is external to the transducer reduces the coupling coefficient at a given bias voltage. At the collapse, regardless of the parasitics, the coupling coefficient reaches unity. Our experimental measurements confirm a coupling coefficient of 0.85 before collapse, and measurements are in agreement with theory.

Varactor characteristics, oscillator tuning curves, and AM-FM conversion

Abstract: A simple analysis relates the small-signal specification of a varactor's capacitance to an oscillator's tuning curve. The notion of an effective capacitance across the amplitude of oscillation is introduced. The analysis also explains how the varactor converts AM noise on the oscillation into FM, which is phase noise. The analysis is experimentally validated.

Mott-Schottky Analysis of Nanoporous Semiconductor Electrodes in Dielectric State Deposited on SnO2 ( F ) Conducting Substrates

Abstract: This paper analyzes the dark capacitance of nanostructured electrodes in the dielectric state, with particular emphasis on TiO 2 electrodes deposited over a transparent conducting substrate of SnO 2(F). It is shown that at those potentials where the TiO 2 nanostructure is in the dielectric state, the capacitance is controlled by the contact SnO 2(F)/~electrolyte, TiO2). The partial or total covering of the substrate by a dielectric medium causes a modification of the Mott-Schottky plot of the bare substrate. We provide a mapping of the various Mott-Schottky curves that will appear depending on the film characteristics. If the dielectric nanoparticles completely block part of the substrate surface, the slope of the Mott-Schottky plot increases ~with the same apparent flatband potential! as an effect of area reduction. The covering of a significant fraction of the surface by a thin dielectric layer shifts the apparent flatband negatively. Measurements on several TiO 2 nanostructured electrodes show that the capacitance contribution of the semiconductor network in the dielectric state is very low, indicating that the field lines penetrate little into the TiO2 network, not much further than the first particle. The different surface covering observed for rutile-anatase and pure anatase colloids is explained by lattice matching rules with the substrate. By comparing different electrodes, the Helmholtz capacitance at the SnO2(F)/solution interface was calculated and the apparent flatband potential was corrected for the effect of band unpinning.

The Limitations of Energy Density of Battery/Double-Layer Capacitor Asymmetric Cells

Abstract: The formula describing the energy density of asymmetric cells, which consists of a battery-type electrode (such as lithium intercalated compound) and an electrochemical capacitor-type electrode (such as activated carbon), was derived. From the formula, the optimal mass (or volume) ratio of battery electrode to capacitor electrodes and electrolyte can be obtained for achieving the maximum theoretical gravimetric (or volumetric) energy density. The voltage swing of the cell during charge and discharge cycles was also described. Relationships between the energy density, ion concentration of the electrolyte, specific capacity of battery electrode, specific capacitance of capacitor electrode, and maximum operational voltage were also given. Three specific asymmetric systems, including carbon/LiPF 6 ethylene carbonate:dimethyl carbonate (EC:DMC)/Li x Ti 5 O 12 , carhon/LiPF 6 EC:DMC/WO 2 , and Ni(OH) 2 /KOH H 2 O/carbon were evaluated for their maximum theoretical energy density and swing voltage. It was found that for asymmetric cells using nonaqueous electrolyte, the maximum energy density (about 30 Wh/kg) was limited mainly by the electrolyte due to the low ion concentration; however, for asymmetric cells using aqueous electrolytes, the maximum energy density (about 40 Wh/kg) was limited mainly by the capacitor electrode. The maximum operational voltage always plays an important role in the maximum energy density.

Molecular quantum cellular automata cells. Electric field driven switching of a silicon surface bound array of vertically oriented two-dot molecular quantum cellular automata.

Abstract: The amine functionality of the linker on the dinuclear complex [trans-Ru(dppm)(2)(Ctbd1;CFc)(NCCH(2)CH(2)NH(2))][PF(6)] reacts with Si-Cl bonds of a chlorinated, highly B doped Si (111) surface to yield Si-N surface-complex bonds. The surface bound complex is constrained to a near vertical orientation by the chain length of the linker as confirmed by variable angle XPS. Oxidation of the dinuclear complex with ferrocenium ion or electrochemically generates a stable, biased Fe(III)-Ru(II) mixed-valence complex on the surface. Characterization of the array of surface bound complexes with spectroscopic as well as electrochemical techniques confirms the presence of strongly bound, chemically robust, mixed-valence complexes. Capping the flat array of complexes with a minimally perturbing mercury electrode permits the equalization of the Fe and Ru energy wells by an applied electric field. The differential capacitance of oxidized and unoxidized bound complexes is compared as a function of voltage applied between the Hg gate and the Si. The results show that electron exchange between the Fe and Ru sites of the array of dinuclear mixed-valence complexes at energy equalization generates a fluctuating dipole that produces a maximum in the capacitance versus voltage curve for each complex-counterion combination present. Passage through the capacitance maximum corresponds to switching of the molecular quantum cellular automata (QCA) cell array by the electric field from the Fe(III)-Ru(II) configuration to the Fe(II)-Ru(III) configuration, thereby confirming that molecules possess an essential property necessary for their use as elements of a QCA device.

A Hybrid Fe3 O 4 ­ MnO2 Capacitor in Mild Aqueous Electrolyte

Abstract: A hybrid electrochemical capacitor using MnO 2 and Fe 3 O 4 as active material for the positive and the negative electrode, respectively, has been designed. The electrodes have been individually tested in a mild aqueous electrolyte (0.1 M K 2 SO 4 ) to define the adequate balance of active material in the capacitor as well as the working voltage of a capacitor based on these two electrodes. The specific capacitances of MnO 2 and Fe 3 O 4 were 150 ′ 10 and 75 ′ 8 F/g, respectively whereas the specific capacitance of the Fe 3 O 4 /MnO 2 capacitor was equal to about 20 F/g of active material. The hybrid electrochemical capacitor has been cycled between 0 and 1.8 V for over 5000 constant current charge/discharge cycles. A real energy density of 7 Wh/kg was reproducibly measured with a real power density up to 820 W/kg.

Broadband ESD protection circuits in CMOS technology

Abstract: A broadband technique using monolithic T-coils is applied to electrostatic discharge (ESD) structures for both input and output pads. Fabricated in 0.18-/spl mu/m CMOS technology, the prototypes achieve operation at 10 Gb/s while providing a return loss of -20 dB at 10 GHz. The human-body model tolerance is 1000 V for the input structure and 800-900 V for the output structure.

A Strategy for Analysis and Modelling of Impedance Spectroscopy Data of Electroceramics: Doped Lanthanum Gallate

Abstract: Modelling of impedance spectroscopy (IS) data of electroceramics depends critically on the correct choice of equivalent electrical circuit so that the extracted parameters have physical significance. The strategy proposed to choose the correct circuit involves analysis of IS data in several of the four complex formalisms: impedance, electric modulus, admittance and permittivity together with consideration of the frequency and temperature dependence of data and the magnitude and temperature dependence of extracted resistance and capacitance values. This is demonstrated using IS data from oxide-ion conducting La0.80Sr0.20Ga0.83Mg0.17O2.82 ceramics over the range 182 to 280°C. Low temperature data are fitted first, to allow a full characterisation of the bulk response; some of the bulk parameters may then be fixed to enable fitting of the higher temperature data which increasingly feature lower frequency phenomena such as grain boundary impedances. The most appropriate circuit in this case is found to consist of a parallel combination of a resistor, capacitor and constant phase element (CPE) for the bulk response in series with a resistor and capacitor in parallel for a constriction resistance. The origin of the constriction resistance may be associated with the presence of plate-like LaSrGaO4 secondary phase within the grains and/or with the presence of pores at the grain boundaries. The importance of choosing (a) the correct equivalent circuit and (b) approximately correct input values for the various circuit parameters to be fitted and refined are demonstrated.

Current source design for electrical impedance tomography.

Abstract: Questions regarding the feasibility of using electrical impedance tomography (EIT) to detect breast cancer may be answered by building a sufficiently precise multiple frequency EIT instrument. Current sources are desirable for this application, yet no current source designs have been reported that have the required precision at the multiple frequencies needed. We have designed an EIT current source using an enhanced Howland topology in parallel with a generalized impedance converter (GIC). This combination allows for nearly independent adjustment of output resistance and output capacitance, resulting in simulated output impedances in excess of 2 GΩ between 100 Hz and 1 MHz. In this paper, the theoretical operation of this current source is explained, and experimental results demonstrate the feasibility of creating a high precision, multiple frequency, capacitance compensated current source for EIT applications.

Electrostatic charge and field sensors based on micromechanical resonators

Abstract: We have developed highly sensitive electrometers and electrostatic fieldmeters (EFMs) that make use of micromechanical variable capacitors. Modulation of the input capacitance, a technique used in macroscale instruments such as the vibrating-reed electrometer and the field-mill electrostatic voltmeter (ESV), moves the detection bandwidth away from the 1/f-noise-limited regime, thus improving the signal-to-noise ratio (SNR). The variable capacitors are implemented by electrostatically driven resonators with differential actuation and sensing to reduce drive-signal feedthrough. The resonators in the electrometer utilize a balanced comb structure to implement harmonic sensing. Two fabrication methods were employed - a hybrid technology utilizing fluidically self-assembled JFETs and SOI microstructures, and an integrated process from Analog Devices combining 0.8-/spl mu/m CMOS and 6-/spl mu/m-thick polysilicon microstructures. All devices operate in ambient air at room temperature. Measured data from one electrometer with an input capacitance of 0.7 pF indicates a charge resolution of 4.5 aC rms (28 electrons) in a 0.3 Hz bandwidth. The resolution of this electrometer is unequaled by any known ambient-air-operated instrument over a wide range of source capacitances. The EFM has a resolution of 630 V/m, the best reported figure for a MEMS device.