Direct Capacitance Research Articles

Pressure Dependence of the Low-Frequency Dielectric Constant in III-VI Semiconductors

In this work we report on the pressure dependence of the low-frequency dielectric constant parallel to the c-axis (ε∥) in GaS, GaSe, and InSe as obtained from direct capacitance measurements. A large increase of ε∥ with pressure has been observed. The pressure change of the lattice polarizability along the c-axis is calculated in the framework of a rigid-ion model from the change of the angle of the anion–cation bond with respect to the layer plane, which results in a slight increase of the lattice contribution. Consequently, the pressure behaviour of ε∥ is proposed to arise from the large increase of the electronic polarizability along the c-axis. This is explained through a decrease of the Penn gap, whose energy and pressure coefficients are shown to scale with those of the indirect gap in these compounds. A supplementary and reversible step-increase of ε∥ has been observed at 1.6 GPa in GaS. This increase has been associated to a phase transition that was reported by other authors.

Gallium Phosphate Thin Films: Synthesis and Dielectric Properties

Deposits of dielectric gallium phosphate thin films on silicon and gallium arsenide semiconductors have been obtained by pyrolysis of an aerosol produced by ultrasonic spraying (“pyrosol” process) and containing the organometallic precursors gallium acetylacetonate and tributyl phosphate. The composition and microstructure of the layers are discussed with respect to the experimental conditions. For this paper, electrical properties were also studied. Using the metal–insulator–semiconductor structure, the behavior of conductivity under direct current, capacitance, and dielectric loss was studied in terms of frequency, temperature, and chemical composition. The sites involved in the conduction process are structural defects directly related to the oxygen excess present in deposits which leads to the formation ofM–O dangling bonds (M=Ga or P).

Influence of heat treatment on magnetostrictions of Finemet Fe 73.5CU 1Nb 3Si 3.5B 9

The investigation of magnetostriction of metallic glass of the Finemet type upon structural changes has been performed on samples with different thermal history. One set of samples has been annealed for 1 h at different temperatures ranging from 573 to 873 K; another set of samples has been annealed at 823 K for different times. Measurements of saturation magnetostriction s, volume magnetostriction ω and ∂ω/∂H have been performed using direct capacitance method. Spontaneous volume magnetostriction ΔVhas been determined from dilatometric measurements and has been found to be 15 × 10 −3. For the first set of annealed samples the temperature dependence of s exhibits local maxima at 573 and 733 K, above which a steep decrease of λ s by about one order of magnitude is observed. The dependence of the value of ∂ω/∂H on the annealing temperature exhibits local minima at 573 and 793 K and local maximum at 723 K. These extremes in λ s and ∂ω/∂H are related to the changes in CSRO and TSRO, to relaxation processes and to internal stresses. In the latter case the magnetostriction of the samples strongly decreases after annealing even for a few minutes from the value of 24 × 10 −6 by about one order of magnitude. The quantity ∂ω/∂H follows the crystallization process, exhibiting, however, maxima at 30 and 180 min annealing time. Amorphous Finemet samples crystallized after annealing at temperatures above 753 K into nanocrystalline phase containing supersaturated solid solution α-Fe(Si), Fe 3Si, (FeSi) 3B, Fe 2B and Fe 3B.

Molecular properties of alternative refrigerants derived from dielectric-constant measurements

A review of the current work in Lisbon on the measurement of the dielectric constant of the liquid phase of some environmentally acceptable refrigerants proposed as alternative replacements of the chlorofluorocarbons (CFCs), responsible for the destruction of the ozone layer, is presented. Measurements on HCFC 141b, HCFC 142b, HCFC 123, HFC 134a, HFC 152a, and HFC 32 samples of stated purities of 99.8 mass% or better were performed as a function of pressure and temperature, in the temperature range from 200 to 300 K and at pressures up to 20 MPa. The ratio of the capacitances of a cell filled with the sample and under vacuum was measured with a direct capacitance method. The dielectric-constant measurements have a repeatability of 0.003% and an accuracy of 0.1%. The theory developed by Vedam et al. based on the Eulerian Strain and the Kirkwood equation for the variation of the modified molar polarization with temperature and density were applied to obtain the dipole moments of the refrigegrants in the liquid state, to obtain a physical insight of the molecular behavior, and to understand the equilibrium configuration of these liquids.

The dielectric constant of liquid HFC 134a and HCFC 142b

This paper presents measurements of the dielectric constant of HFC 134a and HCFC 1426, as a function of pressure and temperature, in the temperature range from 200 to 300 K and pressures up to 20 M Pa, using a direct capacitance method, The samples used had a stated purity of 99.8 and 99.9%, respectively, The values of the dielectric constant have a precision of 0.01 % and an accuracy of 0.1%, The data obtained were correlated as a function of density and pressure, The theory developed by Vedam et al,, based on the Eulerian strain. and the Kirkwood equation for the variation of modified molar polarization with temperature and density were applied to analyze the data and to obtain the dipole moment of both refrigerants in the liquid state.

DX center in AlGaAsSb:Te and a mechanism of its ionization

The Te-related DX center in Al0.3Ga0.7As0.02Sb0.98 layers of p-n junctions and avalanche photodiodes has been studied by capacitance–voltage, deep level transient spectroscopy, and direct capacitance transient measurements. A process of DX center ionization under high reverse bias conditions has been observed at temperatures where the thermal emission is negligible. The kinetics and reverse bias dependence of this process has been studied and analyzed. A mechanism of photon generation in the high-electric-field region of the devices and a subsequent photoionization of the DX center has been proposed.

Effect of prenatal steroids on skin surface hydrophobicity in the premature rat.

The skin surface of the newborn rat at term is highly hydrophobic. This surface hydrophobicity plays a putative role in the transition from an aqueous to a gaseous environment at birth and is dependent on the presence of an intact periderm. Glucocorticoids given to pregnant dams, during late gestation, will accelerate formation of the stratum corneum and reduce transepidermal water loss in prematurely delivered pups. We tested the related hypotheses that surface hydrophobicity and maturation of the periderm are developmentally accelerated by prenatal exposure to steroids. Thirty pregnant Sprague-Dawley rats received either normal saline or 0.5 mg/kg betamethasone on d 17 of gestation. After cesarean delivery on d 18, 19, and 20, dorsal skin surface hydrophobicity was quantified by direct surface electrical capacitance (SEC) measurement. Initial skin surface hydration at birth was significantly lower in steroid-treated pups than in control pups at gestational ages 19 and 20 d (3060 +/- 1379 versus 4441 +/- 153 pF and 646 +/- 295 versus 1493 +/- 1019 pF, respectively, p < 0.001, mean +/- SD). Likewise, after desorption of amniotic fluid, baseline skin hydration was significantly lower in steroid-treated pups than in control pups at gestational ages 19 and 20 d (1862 +/- 1560 pF versus 4278 +/- 97 pF and 60 +/- 56 pF versus 128 +/- 264 pF, p < 0.001). Scanning and transmission electron microscopy showed morphologic maturation of the periderm after steroid treatment. These results demonstrate accelerated development of both functional and structural correlates of skin surface hydrophobicity in the premature rat after prenatal exposure to steroids.

Rapid electrical methods for the detection and enumeration of food spoilage yeasts

Methods for the electrical detection of food spoilage yeasts have been investigated; these include direct conductance, direct capacitance and indirect conductance measurement. Although these methods can be used to detect and enumerate yeasts, the two direct methods must be linked to the correct growth medium. The optimum medium for direct conductance monitoring was found to be Yeast Carbon Base and ammoniun tartrate (CBAT), whereas either CBAT or Yeast Carbon Base and ammonium sulphate (CBAS) could be used for direct capacitance monitoring. CBAT could also be used in indirect conductance systems; however, in some cases a medium was not required in indirect systems, as the food sample itself provided growth requirements. Generally, a direct capacitance system was better than a direct conductance system for yeast detection and enumeration, as it detected more yeast species. Indirect conductance was equivalent to direct capacitance.

Direct capacitance measurements of small geometry MOS transistors

Methods for measuring the intrinsic capacitances of small geometry MOS transistors are described. The influence of short- and narrow-channel effects on the capacitance characteristics of MOS transistors is evaluated. The results are compared with long-channel devices. It is shown that the presented capacitance methods can be used to study the physics of short-channel transistors.

A Capacitance Technique for Investigating Viscous Shear in a Hydrodynamic Contact

An experimental, investigation was conducted to examine the factional healing in a viscous lubricant film and also to investigate lubricant film thickness measurement. A hydrodynamic contact was achieved by loading a shoe against a rotating, lubricated disk. Shoe and disk temperatures were measured as functions of rotation speed and interface pressure. Film thicknesses were determined by a direct capacitance technique. The study revealed shoe temperatures in the range of 306–334°K and film thicknesses of 1.39–2.14 × 10−2 mm for sliding velocities of 3.28–14.05 m·s−1 and interface pressures 2.68–4.64 × 105 Pa. The trend of film, thickness is seen to be consistent with existing slider bearing theory. A relationship of T ∼U0.425 emerges as the dependence of shoe temperature on sliding velocity.

Deep Ti donor in GaAs

Ti-doped GaAs crystals grown by the liquid-encapsulated Czochralski method were investigated by using the observation of direct capacitance transients. The charge transfer transitions to the deep Ti donor level close to midgap are unambiguously identified in both n-type and p-type material. The energy position of Ti3+/Ti4+ is determined with high precision of Ec−(0.87±0.01) eV at 300 K and the respective cross sections are σn∞=(7±3)×10−15 cm2 for electron capture and σp∞=(10±5)×10−16 cm2 for hole capture. The position of the Ti2+/Ti3+ acceptor is confirmed to be at Ec−(0.19±0.01) eV. Its cross section for electron capture is σn∞=(3±1)×10−16 cm2.

A direct depletion capacitance measurement technique to determine the doping profile under the gate of a MOSFET

A technique to directly measure the depletion capacitance between the inversion layer and the substrate in the determination of the doping profile under the gate of a MOS device is described. The measurement technique is less sensitive to stray capacitance and can be carried out under steady-state conditions.The technique, therefore, lends itself to direct measurement on small-area devices thus avoiding the need for large-area devices as in other techniques. Doping profiles obtained using this method on MOS transistors as small as W × L = 18.8 × 3.1 µm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> are compared with profiles Obtained by the pulse method on large-area MOS capacitors. The comparison shows good agreement in the range where the technique is applicable.

Long-term gain changes in mercury-level tiltmeters of the Stacey type

A uniaxial mercury-level short baselength tilt transducer constructed at the University of Alberta is described. The design closely follows the differential capacitance tiltmeter ofStaceyet al. (1969), but differs in that aluminum is used for the beam, and how the mercury cups are supported beneath the beam. It also allows gain verification by comparison of the direct capacitances with a known capacitance. It has been found that the gain factor for the instrument changes with time, because of the relaxation of the mercury surfaces. This has been monitored for several years for four instruments at two sites. The gains change rapidly for several months after installation, but more slowly as time progresses. The magnitudes of such changes in instruments of this type may be as large as 3 percent.

Accuracy of effective channel-length extraction using the capacitance method

A comparison of two different methods for measuring the effective channel length of a MOSFET has been made. It has been determined the effective channel length obtained by the direct capacitance method depends on doping concentration and concentration gradient of the source-drain junctions. Due to the inner fringing capacitance and turn-on of the source-drain regions, the capacitance method tends to predict a longer effective channel length as compared to the resistance method.

A high voltage electrometer for recording and iontophoresis with fine-tipped, high resistance microelectrodes

A high voltage electrometer is described which incorporates a controlled current source, direct current monitoring, balance bridge, electrode resistance, and capacitance compensation test circuits. This device is suitable for making biophysical measurements and iontophoresing dyes or enzymes through extremely fine micropipettes with impedances an order of magnitude higher than conventional micropipettes. Such electrodes are useful for recording from the small neurons of the central nervous system in vivo.

An exact theory of direct capacitance frequency modulation

Recent mathematical results are used to give an exact treatment of the problem of frequency modulation as achieved by varying C or L (capacitance or inductance). Stability regions are found for arbitrary fractional deviation of the frequency. In various limits the exact results of the theory are supported by simple analytical expressions.

The Capacitances and Surface-Charge Distributions of a Shielded Balanced Pair

The capacitance matrix of a straight pair of uniform wires symmetrically placed in a shield is determined theoretically. Exact expressions for the elements of the capacitance matrix are determined as particular elements of the inverse of an infinite matrix which relates the Fourier coefficients of the surface-charge densities on the inner conductors and the shield to the applied voltage excitations on the cable conductors. If the wire diameter is small relative to the wire separation, and if the wire separation is small relative to the shield diameter, then accurate numerical approximations for the elements of the capacitance matrix are obtained to any degree of accuracy by suitably truncating the infinite matrix. Once the elements of the capacitance matrix are determined, then the distributions of the surface-charge densities on the peripheries of the inner conductors, and the shield are determined for any arbitrary excitation of the cable structure. In particular, the various capacitances associated with the cable structure, e.g., the direct, ground, and mutual capacitances, are determined from a comparison of the surface-charge densities resulting from a "balanced" excitation and a "longitudinal" excitation. The Fourier coefficients of the surface-charge densities are required to determine the propagation parameters and the associated propagation modes of the cable structure. The surface-charge distributions are evaluated numerically for a typical standard production cable using 22-gauge wires. The results of this paper will be extended by a perturbational method to include twisted wires in a shield; also, certain types of asymmetries in the cable geometry will be considered. Hence, the propagation constants and the associated propagation modes of unbalanced and/or twisted shielded pair cables can also be determined.

A direct reading capacitance plethysmograph.

An improved capacitance system for blood flow measurement by venous occlusion plethysmography is described. It is shown both theoretically and experimentally, that the change in the capacitance between the surface of the segment and an encircling flexible screen cuff at a fixed distance from the skin is related to the fractional change in volume of the part by a simple constant which can be determined from the initial capacitance. Thus, neither calibration nor measurement of segment volume is needed.

An Automatic Computer-Controlled System for the Measurement of Cable Capacitance

A description is given of the application of automation to a classical measurement problem-determining the capacitance parameters of multiconductor communication cables. These parameters are important in the manufacture of cables and determine such factors as transmission efficiency, crosstalk, and noise. Although the desirability of automating these measurements has long been recognized, the problem of controlling stray capacitance in the switching equipment and connecting leads has previously prevented such automation. This problem has now been overcome by the use of a three-terminal guarded automatic bridge to measure the individual direct capacitances between conductors and a digital computer to calculate the desired parameters from these measurements. The benefits that have been realized by the inclusion of the computer in the measurement system are emphasized-increased speed, elimination of errors, flexibility of operation, data reduction and presentation, and self-testing.

The guarded sphere as a gauging electrode in length metrology

The direct capacitance between a guarded sphere and a plane reference surface is shown to vary with spacing in accordance with an approximate theory, provided the spacing is reasonably small. A similar relationship is shown to hold for the capacitance between a guarded sphere and a circular cylinder. By means of these electrode systems displacements or dimensional differences may be measured in terms of capacitance with a precision to better than 0.01 μm. The accuracy is not affected by small errors of angle of the electrode in relation to the reference surface. The choice of the necessary capacitance bridge is discussed and some typical applications of spherical capacitance gauges are described.