Contact Capacitance Research Articles

Features of the charge transfer in structures based on thin layers of bismuth-modified arsenic triselenide

The influence of the amount of introduced bismuth impurity on the charge transport and accumulation in amorphous As2Se3 layers has been studied. A relation between the relaxation transport processes and the change in the internal structure of the material under study has been found. The physical parameters characterizing the electronic processes (contact capacitance, density of localized states responsible for the charge accumulation in the contact region, charge constant of the contact region, and charge-carrier mobility) have been determined in the arsenic triselenide layers. The mechanisms of the effects observed are discussed.

A 2D Lumped Parameter Model for Prediction of Temperature in C/C Composite Disk Pair in Dry Friction Contact

In applications involving substantial friction, surface failure is an inevitable phenomenon. Friction induced failure typically involves the generation of considerable heat. Existence of significant frictional force leads to relatively high interface temperature as a result of dynamic nature of flash temperatures at the contact areas. A first step in predicting friction induced failure is to develop an accurate thermomechanical model of the friction system. A thermomechanical model is developed in this paper based on a lumped parameter representation of a two-disk brake. A disk is viewed as consisting of three main regions: (1) the surface contact, (2) the friction interface, and (3) the bulk. The lumped parameter model is obtained by dividing a disk into a number of concentric rings and stacked layers. The friction layer contains both the interface and contact elements, each includes the equivalent thermal capacitance and conductive resistance. The contact capacitance and resistance are described in terms of the elastic contact interaction between the surfaces of the two disks. Therefore, they are obtained using the Greenwood and Williamson model for contact of rough surfaces. Each is described as a statistical summation of the micron-scale interaction of the surfaces.

Bias stress instability in organic transistors investigated by ac admittance measurements

In this paper, the bias stress effect (BSE) in organic field-effect transistors has been analyzed by an alternative experimental approach based on ac admittance (Y=G+jωC) measurements. conductance (C) and capacitance (G) curves have been recorded as a function of frequency at different times of the bias stress experiments and simultaneously fitted through a transmission line circuit, able to separately model the conducting properties of the channel and contact regions. The determination of the time behavior of the model fitting parameters is assumed as the starting point for a quantitative analysis of the BSE occurrence. This experimental procedure clarifies that both channel resistance (Rch) and contact resistance (Rc) are largely affected by the BSE, while the channel capacitance (Cch), related to the charge accumulation sheet, and the contact capacitance (Cc) result almost unchanged.

An RF Circuit Model of a Quantum Point Contact

We develop a realistic, physics based, practical RF circuit model for the AC impedance of a quantum point contact that includes the ohmic contacts, the on-chip ?lead? resistance and kinetic inductance, and the quantum point contact impedance itself. The kinetic inductance of the electrons in the ?leads? in series with the quantum point contact capacitance form a resonant tank circuit whose resonant frequency depends on the width of the quantum point contact channel. These measurements probe devices in the following qualitative regime: they are in the ballistic limit, and the measurement frequency is higher than the electron scattering frequency.

Device physics of highly sensitive thin film polyfluorene copolymer organic phototransistors

We report on solution processed, highly light sensitive thin film transistors (TFTs) based on poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). Transistors without heat treatment showed the highest saturation mobility, while devices annealed at 280°C showed the highest drain current. The latter annealed transistors were found to give highly stable and reproducible performance over many light cycles. Measurements were carried out using an inorganic light emitting diode (LED) light source with a peak wavelength of 465nm and 19nm bandwidth from 0to400μW∕cm2 light intensity on TFTs with an F8T2 film thickness of 30nm. The TFT OFF current was found to increase both with light intensity and gate bias. The bulk photogenerated carrier density was calculated to change from 5×1011to1×1013cm−3 over the measured light intensity range. The TFT saturation mobility did not change with light intensity, remaining constant at 1.2×10−4cm2∕Vs. The TFT ON current instead increased due to a shift in the turn-on voltage VT. This changed from −27to−20V over the measured light intensity range, initially changing rapidly but then saturating at higher intensity values. Contact resistance RC measurements showed large values in the dark. RC rapidly decreases with increasing light intensity, again saturating at higher values. From these results, we propose a phototransistor model in which illumination varies the device performance by effecting injection. By considering this shift in RC as photoassisted barrier lowering which additionally varies the width of the region depleted of carriers between the injecting interface and the channel, it is possible to explain the observed shift in VT as a change in the fraction of the gate bias dropped across the contact capacitance CC. By operating the phototransistor at a value of Vg=−5V (below VT), it was possible to achieve a highly linear response of the photocurrent with light intensity. Alternatively, by operating at a value of Vg=−40V (above VT), it was possible to maximize the photoresponsivity within the measured range. A photoresponsivity of 18.5A∕W at 5μW∕cm2 light intensity was achieved.

In situ characterization of microdroplet interfacial properties in digital microfluidic systems

Real-time characterization of digital microfluidic lab-on-a-chip devices is important for biological and chemical applications in which the properties of the microdroplet are time variant. In this paper, a method for in situ characterization of microdroplet interfacial properties is introduced. The proposed characterization method relies on two submodules, namely the contact angle and capacitance sampling submodules, in a digital microfluidic system. In the contact angle measurement submodule, the microdroplet profile is acquired and an accurate contact angle is determined. In the capacitance sampling submodule, the capacitance of the system is measured by means of an activation voltage signal. For verification purposes, the results obtained from the proposed method are compared to the Lippmann-Young equation. The results are in excellent agreement with previously reported values. Finally, the proposed submodules are used to characterize the interfacial properties of a microdroplet containing an aqueous solution of bovine serum albumin (BSA) in which adsorption is a predominant effect. The results show the temporal behaviour of both microdroplet interfacial properties and dielectric characteristics.

Investigation of the processes of charge transfer and charge accumulation in As2Se3 amorphous layers prepared by different methods

The influence of the method used for preparing As2Se3 amorphous layers on the processes of charge transfer and charge accumulation in these structures is studied. Different physical parameters, such as the contact capacitance, the width of the charge accumulation region, and the time of transit of charge carriers through the samples, which characterize the occurring electronic processes, are determined for the layers under investigation.

Kinetics of polarization current in a Pb3O4 wide-gap photoconductor

The results of the study of isothermal polarization of Pb3O4 high-resistivity semiconductor layers in a dc electric field of various intensities with measurements without and with illumination are presented. To interpret the data obtained, the relay mechanism of charge transport over local states in the band gap is introduced. The values of the parameters describing the relaxation phenomena such as contact capacitance, width of the charge accumulation region, and effective charge mobility are obtained.

A Semianalytical Method to Determine Parasitic Elements of Quantum-Well Laser

A parameter-extraction approach for the quantum- well laser, which combines the analytical approach and the empirical optimization procedure, is developed in this paper. The initial values of the parasitic pad capacitance and the feedline inductance are extracted by using a set of closed-form expressions derived from above-threshold mode input reflection coefficient on wafer measurement, and the extrinsic contact resistance and capacitance determined by using the analytical method are described as functions of the parasitic pad capacitance and the feedline inductance. Advanced Design System (Agilent commercial software) is then used to optimize only the parasitic pad capacitance and the feedline inductance with very small dispersions of initial values. An excellent fit between measured and simulated input reflection coefficients under cutoff and above-threshold biased conditions in the frequency range of 50 MHz-40 GHz is obtained.

Equivalent Circuit Analysis for Time-Coordinated Non-arcing Operation of Reed Switches

We proposed a method for suppressing arc ignition in mechanical contact devices using a transient current switch and a capacitor. We applied the method to conventional reed switches. For the electric circuit analysis, we clarified the momentary voltage-current characteristics at breaking operation of reed switches by FEM analysis. We could also estimate the capacitance of the contact electrodes at the metal bridge rupture by FEM analysis, and would derive the non-arcing condition using SPICE simulation. The suitable capacitor value in the transient current circuit for arc ignition suppression would be depend on the load impedance, the power supply, the time depending contact resistance R(t)s, the contact capacitance, and the minimum arc voltage and current.

The study of an electromagnetic levitating micromotor for application in a rotating gyroscope

An electromagnetic micromotor with alumina rotor, which is stably levitated, rotated, sensed and controlled by independent coils and capacitance structure, is presented. Levitated force with lateral stability is produced by the electromagnetic interaction between the rotor and four groups of levitation planar coils with inter and outer conductors carrying opposite phase high frequency AC current. The rotation is realized by a four-phase induction micromotor composed of the rotor and eight rotation planar coils carrying AC current. Eight pieces of sector sense electrode and the rotor constructed a no contact capacitance displacement sense system. The torque returning the rotor to its equilibrium position is produced by four groups of planar coils. The prototype device with the rotation speed of 3035 rpm, the levitation height of 200 μm and factor scale of 0.212 V/°, has been designed, fabricated, tested successfully. The study illustrates that the micromotor can be applied in a rotating gyroscope.

Body composition modeling in the calf using an equivalent circuit model of multi-frequency bioimpedance analysis

An equivalent electrical circuit model is used to describe the response of different tissue components in the calf to multi-frequency current. This model includes seven electrical components: skin resistance, contact capacitance, fat resistance, fat capacitance, extracellular resistance, intracellular resistance and cell membrane capacitance. Calf bioimpedance was measured on 30 pts using a multi-frequency bioimpedance device (Xitron 4200) with a range of frequency from 5 kHz to 1000 kHz. MRI was performed on each measured calf to provide body composition components: fat, muscle mass and bone. An equivalent circuit containing seven parameters (P1, P2, P3, P4, Q1, Q2, Q3) was constructed to represent the model. To identify the effect of different body compositions on their parameters, subjects were subgrouped according to (1) their range of fat mass: F1 > 0.4 kg, F2 > 0.4 & F2 < 0.25 kg and F3 < 0.25 kg; (2) their range of muscle mass: M1 > 1.2 kg, M2 < 1.2 & M2 > 1.0 kg and M3 < 0.25 kg. Curve fitting and simulation programs (Matlab Toolbox) were used to obtain the solution of the electrical equations. The results show a decrease in impedance with an increase in excitation frequency that differed among subjects with different fat contents. Simulation results show a high correlation (R2 > 0.98) between the bioimpedance measurements and the value calculated from the model. There are significant differences in parameters P1 (32.5 ± 5.9 versus 26 ± 4.4, p < 0.05), P3 (−15 330 ± 3352 versus −10 973 ± 3448, p < 0.05) and P4 (42 640 versus 24 191, p < 0.05) between groups F1 and F3. P2 is significantly different (1045 ± 442 versus 1407 ± 349, p < 0.05) between groups M1 and M2. The parameters that characterize the bioimpedance data depend upon many more tissue characteristics of electrical properties than those incorporated in current models and they are affected by aspects of body composition that are not considered in the fitting of bioimpedance data. This study shows a new model and methodology to analyze bioimpedance data and further work is likely to lead to much better understanding of electrical properties of body tissue.

Contact Charging Between Metals Revisited

The theory of contact charging between metals was established in 1951 by Harper and confirmed experimentally in 1975 by Lowell who measured the net charge after separation of pure metals in contact and related it to the contact potential difference and capacitance of the surfaces using a specialized measurement technique. It is generally assumed that these contact charges can be ignored for metals used in common industrial processes because of charge backflow on separation. A series of experiments carried out on commonly used industrial materials shows that net charge does exist on spherical metal balls rolling out of a metal tube into a Faraday pail. The experiments were carried out using three different tube materials (copper, brass, aluminum), five different ball materials (copper, brass, aluminum, steel, and stainless steel), for ball diameters ranging from 0.24 to 1.27 cm. The surfaces of the materials were only rinsed with water and dried. Analysis of the results shows general agreement with the Harper theory and confirms the presence of net charge on metals separated in this simulated industrial operation, i.e., moving metal parts off a conducting conveyor. This process is analyzed from the point of view of possible discharge hazards and the reasons why this phenomenon is not more widely observed in industrial processes are discussed.

Depletion layer and contact capacitance in non-uniformly doped semiconductors

We consider the dependence of the capacitance C of the semiconductor depletion layer versus the contact bias V. When the concentration of doping impurities varies in the plane parallel to the contact, we show that the standard Schottky dependence C−2(V) is no longer a straight line. We compute the effective concentration Neff from the slope of the dependence C−2(V) by using the perturbation and variation methods and compare Neff with previous theoretical and experimental data.

Nanoimpedance Microscopy and Spectroscopy

ABSTRACTOne of the key limiting factors in current-based scanning probe microscopies (SPM) is the quality of tip-sample contact and stray capacitance in the probe-surface junction. We conduct impedance spectroscopy over a broad frequency range (40Hz∼110MHz) through an AFM tip to quantify local electrical properties. Equivalent circuit for the tip-surface contact is constructed based on the impedance data and is used to study the mechanisms of relaxation in the near-tip region. Relative contributions of tip-surface contact and materials properties to the signal are discussed. This technique, referred to as Nanoimpedance Microscopy/Spectroscopy, is demonstrated in the imaging of an electronic ceramic: a ZnO varistor. Analysis of impedance spectra allows separation of tip-surface interactions and grain boundary behavior.

The effect of annealing on the junction properties of CdTe/CdS cells with carbon contacts

The effects of annealing on the properties of CdTe/CdS cells with carbon contacts are studied for CdTe sputtered cells. Both open circuit voltage and short circuit current decrease as the annealing time increases. This degradation is attributed to the diffusion of impurities that exist in the carbon paste. Results obtained from contact resistance measurement and capacitance–voltage measurement at different frequencies for annealed cells confirm this assumption.

Relationship between wetting and electrical contact properties of pure metals and alloys on semiconducting barium titanate ceramics

In this work, the relationship between the thermodynamic properties of metal–ceramic interfaces and their electrical contact properties was explored for semiconducting BaTiO3 ceramics. The role of chemical reactivity at the interface was quantified through sessile drop measurements of the work of adhesion (Wad). The interfacial properties were characterized with impedance spectroscopy to quantify contact resistance and capacitance. It was observed that interfaces with a strong chemical interaction between the metal and the oxygen anion in the ceramic showed large Wad and correspondingly low contact resistances. It is believed that an oxygen depleted region near the surface of the BaTiO3 was formed as a result of the strong metal-oxygen interactions and led to a large Wad and an Ohmic contact.

Quantitative two-dimensional carrier profiling of a 400 nm complementary metal–oxide–semiconductor device by Schottky scanning capacitance microscopy

Carrier profiling of a 400 nm complementary metal–oxide–semiconductor device has been accomplished by combining metal–semiconductor capacitance–voltage profiling techniques with two-dimensional scanning probe microscopy. When a metal probe is brought into contact with a semiconductor, a space-charged depletion region and therefore a capacitor is formed at the junction. By applying a small ac voltage, the voltage derivative of the contact capacitance can be measured with a lock-in amplifier. The amplitude of the derivative signal is a function of the carrier concentration, and the sign gives the type of carrier. The present work concentrates on the two dimensional (2D) carrier profiling of a 400 nm metal–oxide–semiconductor field effect transistor. The results demonstrate that this technique is capable of quantitative 2D characterization of semiconductor devices.

The effect of contact capacitance on current-voltage characteristics of stationary metal contacts

This paper investigates experimentally the significance of the effective contact capacitance, i.e., the interfacial capacitance during the current flow, for a wide range of stationary metal contacts operating under high charge injection rates. The effective capacitance of metallic interfaces depends on the ratio between the apparent contact area (which is optically determined) and the effective contact area (which injects the electronic charges). Silver contacts having series resistance values significantly less than the contact resistance were subjected to ac high current densities (up to 500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). The obtained i(t) and v(t) profiles were further analyzed to obtain I-V curves. Due to the phase shift between i(t) & v(t) profiles the I-V curve within a single period of the stimulating current will produce a closed loop. The area of the loop determines the interfacial electrical energy. According to the obtained results the electrical energy storage at a given metal contact, increases at: 1) higher ampacity values; 2) lower operating temperatures; 3) higher clamping forces between the joints (elastic deformation regime) each of the above parameters acting independently. The experimental results were obtained for AgSnO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and OFHC contacts operated in a wide temperature range, varying between −130 °C and +40 °C. The observed response of the electrical contacts is mainly characterized by the implications of the asperity contact model and dominating charge transport processes across the metallic interfaces. When standard simple equivalent circuits are used to determine contact impedance, the effective capacitance of current carrying metal contacts acquires exceptionally high values.

Two-dimensional imaging of charge carrier profiles using local metal–semiconductor capacitance–voltage measurement

We will report a variation of the scanning capacitance microscopy for two-dimensional delineation of semiconductor charge carrier profiles based on the measurement of the local Schottky contact capacitance. When a metal probe is brought into contact with a semiconductor, a space-charged depletion region and therefore a capacitor is formed at the junction. By applying a small ac voltage, the voltage derivative of the contact capacitance can be measured with a lock-in amplifier. The amplitude of the derivative signal is a function of the carrier concentration, and the sign gives the type of carrier. We have carried out the local contact capacitance–voltage measurements on standard doping concentration samples and two-dimensional (2D) carrier concentration profiling on device structures. The results demonstrate that the contact capacitance measurement is capable of quantitative 2D characterization of semiconductor devices. These preliminary results also demonstrate that this technique, when used as a method for carrier profiling, is relatively insensitive to the sample surface condition.