High Resistivity Regions Research Articles

A prototype system and reconstruction algorithms for electrical impedance technique in medical body imaging.

We have developed an impedance imaging system to reconstruct cross-sectional images of the body's electrical characteristics based on static tissue impedance. The hardware system consists of a data collection subsystem and the Intel 380 host microcomputer system with an Intel 80286 microprocessor, an Intel 80287 numeric data processor, and an Intel 80186 microprocessor-based display board. The system is capable of initiating a data collection from an array of current-sensing electrodes and reconstructing impedance images based on these data measurements. We have tested the data collection subsystem with physical phantom models, and we have found that the prototype system is capable of discriminating high resistivity regions in contrast with the low resistivity background. Our system is flexible in that each electrode's function (sensing currents, applying voltages, grounding body surfaces, and disconnected from the body) can be programmed individually so that a variety of electrode configurations for different projection techniques can be tested for optimal system performance. Various reconstruction algorithms have been developed and tested particularly for this imaging modality. Since a computer body model is needed for some impedance reconstruction algorithms, we have created two- and three-dimensional computer body models based on the finite element method approach, and verified our finite element modelling technique by building physical phantoms and comparing measured experimental results with simulation results predicted by the computer model. We have found that the sensitivity is a function of position, pixel size (image resolution) and background resistivity. We have also tried to compensate the low sensitivity of impedance changes in the central region.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal venous circulation of the gastroesophageal junction: A route to understanding varices

A study into the normal anatomy of the venous circulation of the gastroesophageal junction was undertaken using three complementary techniques (radiology, corrosion casting, and morphometry). Four distinct zones of venous drainage were defined as follows: (a) gastric zone, characterized by a longitudinal venous distribution; (b) palisade zone, composed of parallel vessels arranged in groups, lying mainly within the lamina propria; (c) perforating zone, characterized by “treble clef” shaped veins, which collect and channel blood into extrinsic veins; and (d) truncal zone, composed of four or five deep lying descending veins. This venous system appeared to be mainly distributed within the esophageal mucosal folds. The anatomic pattern suggests that venous flow is bidirectional at the palisade zone, which acts as a high-resistance watershed region between the portal and azygos systems. In patients with portal hypertension this normal vascular system has to accommodate greatly increased venous flow, and the anatomy as demonstrated here offers insight into variceal development.

Space-charge-limited current diode model for amorphous silicon solar cells and their degradation

A quantitative space-charge limited-current (SCLI) diode model for the rectifying current-voltage properties of diodes with a discrete trap distribution is extended to the case of diode solar cells with a continuous distribution of traps in the energy band gap. Analyzing hydrogenated amorphous silicon (a-Si:H) solar cells indicates that their light-exposed degradation causes two major changes in their high resistance, intrinsic regions. These are a sharp increase in the equilibrium and steady-state carrier concentrations and a change in the distribution of traps in the energy band gap. The measured trap concentration densities span the 1014–1018 cm−3eV−1 magnitude range at 0.8–0.2 eV below the conduction-band edge. The measured carrier concentrations cover the 105–1012 cm−3 range. The model indicates that higher device efficiencies are obtained when the quasi-Fermi level in the high resistance region lies just above a peak in the trap concentration density in the band gap. This peak steady-state density in a-Si:H is above 1017 cm−3 eV−1 and is located approximately 0.5–0.6 eV below the conduction-band edge. Fermi–Dirac statistics are employed in the model and quantitative explanations are given for the dark and light current-voltage curves of a-Si:H under forward and reverse bias. Conditions are defined that predict maximum a-Si:H fill factors in the 0.74–0.76 range and device efficiencies at the 11%–13% levels. The trap distributions that give higher device performance are defined, and a method for measuring these distributions is described and illustrated. The model indicates that the specific conductivity changes measured on light-exposed a-Si:H by Staebler and Wronski would tend to increase the fill factor and efficiency of a-Si:H solar cells.

Formation of p+ and High Resistivity Regions in GaAs-AIGaAs Heterojunctions

ABSTRACTThe formation of p+ and high resistivity layers in GaAs-AIGaAs heterostructures is useful in a number of applications, including the fabrication of heterojunction bipolar transistors (HBTs). Control of the implanted dopant profile during annealing is paramount for rapidly diffusing acceptor species such as Be and Mg, and we show SIMS data of the dopant profile in implanted heterojunctions as a function of annealing temperature in the range 700–900°C for 1–5 sec. Annealing of implanted Be at ≥800°C even at a dose of 2×10l5cm−2 results in complete activation whereas Mg shows only 30% activation under these conditions. There is no significant interface disordering visible by TEM or RBS for either case. Multiple energy O implants (up to and including 1 MeV) were used to render the entire heterostructure resistive; for doses of ∼1013cm−2, the O bombarded layers showed resistivities of 108 Ω/□ after 525°C annealing.

Photoelectrical fatigue effect in Bi12TiO20 crystals

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Application of protective devices to radial overhead line networks

The paper reviews the need for overhead line switching equipment and its use in developing countries for the control of long radial feeders into remote districts. The requirements for pole-mounted equipment and relaying characteristics are discussed showing the need for sensitive earth fault discrimination, particularly to detect broken conductors in regions of high earth resistance. Designs of pole-mounted, autoreclosers are discussed. Sectionalising equipment is also reviewed in the context of fault management on spur lines.

Frequency Spectrum of CdS Acoustic Domains for Acoustic Domain Injection Technique

The purpose of this paper is to study the frequency spectrum of the CdS acoustic domain generator to measure the physical parameters of II-VI or III-V compound semiconductors. It is concluded that the low frequency fluxes below 1 GHz are caused by the piezoelectric shock in the high resistivity region due to insufficient ohmic contact near the cathode electrode.

Self-aligned notched planar InP transferred-electron oscillators

A procedure is described for the fabrication of planar InP transferred-electron oscillators (TEO's). In this procedure, a localized high resistivity region is self-aligned to the cathode contact. The self-aligned structure, which also included a selectively implanted n+ anode, produced a pulsed power output of 220 mW with an efficiency of 9.5 percent at 9.1 GHz and a CW power output of 80 mW with an efficiency of 5.3 percent at 5.5 GHz. This is the highest reported power output and efficiency for a planar transferred-electron device in this frequency range.

Strong carrier freezeout above 77 K in tellurium-doped buried-channel MOS transistors

Semiconductor devices are almost universally based on an assumption of full ionization of dopant impurities, a natural condition at room temperature for conventional shallow-energy-level (activation energy ∼ 0.05 eV) dopant species. At temperature T < 30 K for conventional dopants, freezeout of the equilibrium carrier density becomes severe and uncompensated dopants are in the neutral charge state. The high resistivity of such frozen-out regions might have useful applications for device structures. However, the temperature (< 30 K) required makes such applications generally impractical. Freezeout-induced high-resistivity regions at 77 K would be considerably more interesting. We demonstrate strong freezeout of a buried-channel MOS transistor at 77 K using tellurium as the channel dopant.

High resistivity in InP by helium bombardment

Helium implants over a fluence range from 1011 to 1016 ions/cm2, reproducibly form high resistivity regions in both p- and n-type InP. Average resistivities of greater than 109 Ω cm for p-type InP and of 103 Ω cm for n-type InP are reported. Results are presented of a Monte Carlo simulation of helium bombardment into the compound target InP that yields the mean projected range and the range straggling.

Channeled substrate buried heterostructure InGaAsP/InP laser employing a buried Fe ion implant for current confinement

A channeled substrate buried heterostructure InGaAsP/InP laser is demonstrated using a hybrid technique of Fe ion implantation followed by liquid phase epitaxy. A high resistivity region is formed by the implantation and subsequent anneal of Fe into an n-type InP substrate, and this is used to provide a self-aligned current confinement barrier layer. The use of an in situ anneal prior to liquid phase epitaxy minimizes the number of processing steps. Pulsed threshold currents as low as 22 mA have been achieved on devices utilizing broad area metal contacts.

Xenopus neural crest cell migration in an applied electrical field.

Xenopus neural crest cells migrated toward the cathode in an applied electrical field of 10 mV/mm or greater. This behavior was observed in relatively isolated cells, as well as in groups of neural crest cells; however, the velocity of directed migration usually declined when a cell made close contact with other cells. Melanocytes with a full complement of evenly distributed melanosomes did not migrate of their own accord, but could be distorted and pulled by unpigmented neural crest cells. Incompletely differentiated melanocytes and melanocytes with aggregated melanosomes displayed the same behavior as undifferentiated neural crest cells, that is, migration toward the cathode. An electrical field of 10 mV/mm corresponded to a voltage drop of less than 1 mV across the diameter of each cell; the outer epithelium of Xenopus embryos drives an endogenous transembryonic current that may produce voltage gradients of nearly this magnitude within high-resistance regions of the embryo. We, therefore, propose that electrical current produced by the skin battery present in these embryos may act as a vector to guide neural crest migration.

Modelling of interdiffusion of hydrogen and alkali ions in glass surfaces: Electrical resistivity

The properties of the individual diffusion coefficients of the interdiffusing hydrogen and alkali ions in glasses, leached in water, are investigated by model calculations. The applied criterion is that the theoretical resistivity curve must be consistent with Wikby's observations that in the reacted glass a thin region of high resistivity exist whereas the resistivity in the outer surface layer is a fraction of the bulk resistivity. A concentration dependence of only the diffusion coefficient of the sodium ions, simulating a mixed-alkali effect passing to a gel-layer effect, suffices to fulfil this criterion as well as to give a concentration profile which is consistent with experimental concentration profiles.

Selected-area molecular beam epitaxy on ion-implanted GaAs substrates

We present a method of selective growth of GaAs which utilises ion implantation for defining conductive monocrystalline and high-resistivity nonmonocrystalline regions on a GaAs substrate. By molecular beam epitaxy, nonmonocrystalline GaAs is obtained on the implanted areas of the GaAs substrate. Selected area molecular beam epitaxy is performed directly on an unmasked substrate.

Time resolved DC electroluminescence studies in ZnS:Mn, Cu powder phosphors

Time resolved studies of DC electroluminescence (EL) from ZnS:Mn, Cu powder phosphors are reported. An emission band arising from the Mn2+, 4T1 to 6AI transition and a very broad band attributed to radiative interband transitions of hot electrons are detected. The hot electron band is used as a probe of the onset of avalanching in the devices. EL emission is found to occur simultaneously for the two bands. Detailed studies of the EL onsets reveal that luminescence emission occurs, to within 100 ns, when the voltage across the high resistivity region is sufficient for avalanching, without any additional delay being present. This is consistent with the Mn2+ being excited by direct impact by hot electrons. The rate of decay of the Mn luminescence is found to increase with increasing Mn concentration. Various concentration quenching mechanisms are discussed to account for this behaviour. High current densities are deduced from the rate of increase of luminescence rise transients. Only 1 to 5% of the Mn ions in the phosphor can be excited.

High resistivity interfacial layers due to compensation by Sulfur in (Al,Ga)As devices

In p-n junctions grown by LPE in the (Al,Ga)As system, high concentrations of Sulfur can accumulate on the p side of the junction. As a result, a closely compensated, high resistivity region can extend for 0.1-1.0 µm into the p layer. Since sulfur creates deep electron traps in Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</inf> As with a thermal activation energy in bulk p-type material of 0.2 eV, both electrical and optical properties of (Al,Ga)As diode devices can be affected by the interfacial sulfur concentrations.

Mössbauer study ofCo0.99557Fe0.005S2in external magnetic fields: Magnetic structure of CoS2and the quadrupole splitting of the FeS2-CoS2-NiS2system

The M\"ossbauer effect of $^{57}\mathrm{Fe}$-doped ferromagnetic Co${\mathrm{S}}_{2}$ was measured at 4.2 K in external magnetic fields up to 40 kOe. Ferromagnetic powder spectra were analyzed by taking into account the effect of the ferromagnetic moment. The hyperfine field induced by the ferromagnetic moment is -16 kOe when the moment lies along the principal axis of the electric field gradient and -7 kOe when the moment is perpendicular to it. The sign of the electric field gradient is negative at the $^{57}\mathrm{Fe}$ nucleus. Using these results, we conclude that the spectrum at zero external field is explicable from the collinear magnetic structure with a [111] easy direction of Co spins. The quadrupole splitting is negative in the Fe${\mathrm{S}}_{2}$-Co${\mathrm{S}}_{2}$-Ni${\mathrm{S}}_{2}$ system. The quadrupole splitting changes around the metal-semiconductor phase boundary because of a delocalization of ${e}_{g}$ electrons in the metallic region. The absolute value in the low-resistivity region is smaller than that in high-resistivity regions by about 0.1 mm/sec.

The asymmetrical field-controlled thyristor

A new device structure has been developed for field-controlled thyristors. In this structure, the uniformly doped n base of the conventional device has been replaced with a very lightly doped region near the gate and a more heavily doped region at the anode. This change in the base doping profile results in a significant improvement in the tradeoff between the forward-blocking voltage capability and the on-state forward-voltage drop. In addition, the high-resistivity region around the gate area allows the device to pinch off anode current flow at zero gate bias due to the built-in potential of the gate junction. The devices can, however, be triggered to the on-state by applying a small forward gate voltage and exhibit a forward voltage drop in the on-state which is much lower than that of conventional devices. The high resistivity of the channel area between gates also results in these devices having dc blocking gains in excess of 60, which is the highest value achieved in devices of this type. Further, because these devices have been fabricated using conventional planar processing techniques, this structure is suitable for high-volume production with high processing yields.

Nonlinear current-voltage relationships in cultured macrophages.

Intracellular recordings of cultured mouse thioglycolate-induced peritoneal exudate macrophages reveal that these cells can exhibit two different types of electrophysiological properties characterized by differences in their current-voltage relationships and their resting membrane potentials. The majority of cells had low resting membrane potentials (-20 to -40 mV) and displayed current-voltage relationships that were linear for inward-going current pulses and rectifying for outward-going pulses. Small depolarizing transients, occurring either spontaneously or induced by current pulses, were seen in some cells with low resting membrane potentials. A second smaller group of cells exhibited more hyperpolarized resting membrane potentials (-60 to -90 mV) and S-shaped current-voltage relationships associated with a high-resistance transitional region. Cells with S-shaped current-voltage relationships sometimes exhibited two stable states of membrane potential on either side of the high-resistance transitional region. These data indicate that macrophages exhibit complex electrophysiological properties often associated with excitable cells.

Semidistributed neuristor using plasma-coupled modulating transistors

Operation of an integrated active line using the plasma-coupled semiconductor device has been confirmed. The neuristor consists of modulating transistors (m.t.) which have a common high-resistivity base region. The construction, principle of operation and results of investigation of this device are described.