Correlation between skull thickness asymmetry and scalp potentialestimated by a numerical model of the head

The contribution of asymmetric skull thickness to the scalp potential amplitude was investigated numerically. The model consisted of four conductive layers representing the scalp, the skull, the cerebrospinal fluid, and the cortex with a current dipole in the occipital region. The potential created by the dipole was calculated assuming quasistatic formulation and linear media. The governing equation was discretized by the finite volume method to ensure the conservation of fluxes even in regions with abrupt changes of the conductivity. The large set of the algebraic equations for the electric potential was solved iteratively by the successive overrelaxation method. The model confirmed previous experimental studies suggesting that the potential amplitude is 60% smaller on the side with the thicker bone if the asymmetry of the skull thickness exceeds 40%. The model developed suggests that skull thickness asymmetry can create nonnegligible asymmetries in the potential measured on the scalp above homotopic points of the two hemispheres     

Effects of layer thickness variations on vertical-cavitysurface-emitting DBR semiconductor lasers

A theoretical analysis of the influence of layer thickness variation in vertical-cavity surface-emitting lasers with distributed Bragg reflectors (DBRs) on lasing wavelength is presented. It is shown that changing the active region length of one of the layers in the DBR mirror by only one unit cell (0.56 nm) is sufficient to produce shifts in the lasing wavelength up to 0.12 nm (for an AlGaAs laser). This could limit the precision with which a desired wavelength, its reproducibility, and its uniformity across a large wafer can be obtained. Possible influences on the linewidth of broad area devices are also discussed     

Correlation between structure and resistivity variations of the live human skull

A study on correlation between structure and resistivity variations was performed for live adult human skull. The resistivities of 388 skull samples, excised from 48 skull flaps of patients undergoing surgery, were measured at body temperature (36.5 degrees C) using the well-known four-electrode method in the frequency range of 1-4 MHz. According to different structures of the skull samples, all the 388 samples were classified into six categories and measured their resistivities: standard trilayer skull (7943 +/- 1752 ohm x cm, 58 samples), quasi-trilayer skull (14,471 +/- 3061 ohm x cm, 110 samples), standard compact skull (26,546 +/- 5374 ohm x cm, 62 samples), quasi-compact skull (19,824 +/- 3232 ohm x cm, 53 samples), dentate suture skull (5782 +/- 1778 ohm x cm, 41 samples), and squamous suture skull (12747 +/- 4120 ohm x cm, 64 samples). The results showed that the skull resistivities were not homogenous and were significantly influenced by local structural variations. The presence of sutures appeared to decrease the overall resistivity of particular regions largely and dentate suture decreased the resistivity more than squamous suture. The absence of diploe appeared to increase skull resistivity. The percentage on thickness of diploe would be the primary factor in determining the resistivity of the skull sample without suture. From resistivity spectra results, an inverse relationship between skull resistivity and signal frequency was found.     

An adaptive image sharpening scheme based on local intensity variations

Unsharp masking-based approaches are widely used in consumer electronics and printing technology for increasing the sharpness of the image. In the classical approaches, such improvements are achieved by adding the high-frequency details to the underlying image without considering any noise present in the image. As a result, such approaches yield visually poor results on noise-deteriorated images. In this paper, we propose an adaptive unsharp masking scheme which can tolerate the noise content, i.e., proposed algorithm will perform sharpening operation on the required regions thereby reducing the visual effects of the noise. Experimentally, it has been found out that the proposed approach yields better visual results than classical unsharp masking approach in the presence of noise.     

The influence of nitride thickness variations on the switching speed of MNOS memory transistors

The influence of nitride thickness variations on the switching speed of MNOS memory transistors is examined. The switching time constant is calculated as a function of the nitride thickness using a model of modified Fowler-Nordheim injection. The calculated characteristics compare well with measured characteristics and show a strong dependence on the nitride thickness.     

Effects of myocardial wall thickness on SPECT quantification

The effects of changing myocardial wall thickness in single photon emission computed tomography (SPECT) imaging are characterized, and a method which may be used to compensate for these effects is presented. The underlying principle is that the phenomena of attenuation, Compton scatter, and finite resolution can be separated and treated independently. Only finite resolution and its effects, along with a proposed method for correcting these effects, are addressed. A cardiac phantom with varying wall thickness (9-23 mm) was developed to characterize the dependence effects on (201)Tl myocardial SPECT images. Correction factors in the form of recovery coefficients have been developed with the use of a convolution simulation, and are shown to improve substantially the agreement of counts extracted from SPECT images of the phantom with the actual (201)Tl concentration. The degree of improvement, however, is markedly affected by external attenuation. Clinical application of this method will require corrections for attenuation and scatter or the development of regional recovery coefficients which include these effects.     

Intrinsic threshold voltage fluctuations in decanano MOSFETs due tolocal oxide thickness variations

Intrinsic threshold voltage fluctuations introduced by local oxide thickness variations (OTVs) in deep submicrometer (decanano) MOSFETs are studied using three-dimensional (3-D) numerical simulations on a statistical scale. Quantum mechanical effects are included in the simulations employing the density gradient (DG) formalism. The random Si/SiO₂ and gate/SiO₂ interfaces are generated from a power spectrum corresponding to the autocorrelation function of the interface roughness. The impact on the intrinsic threshold voltage fluctuations of both the parameters used to reconstruct the random interface and the MOSFET design parameters are studied using carefully designed simulation experiments. The simulations show that intrinsic threshold voltage fluctuations induced by local OTV become significant when the dimensions of the devices become comparable to the correlation length of the interface. In MOSFETs with characteristic dimensions below 30 nm and conventional architecture, they are comparable to the threshold voltage fluctuations introduced by random discrete dopants     

Quantifying local stiffness variations in radiofrequency ablations with dynamic indentation

Elastographic imaging can be used to monitor ablation procedures; however, confident and clear determination of the ablation boundary is essential to ensure complete treatment of the pathological target. To investigate the potential for ablation boundary representation on elastographic images, local variations in the viscoelastic properties in radiofrequency-ablated regions that were formed in vivo in porcine liver tissue were quantified using dynamic indentation. Spatial stiffness maps were then correlated to stained histology, the gold standard for the determination of the ablation periphery or boundary. Regions of interest in 11 radiofrequency ablation samples were indented at 18-24 locations each, including the central zone of complete necrosis and more peripheral transition zones including normal tissue. Storage modulus and the rate of stiffening were both greatest in the central ablation zone and decreased with radial distance away from the center. The storage modulus and modulus contrast at the ablation outer transition zone boundary were 3.1 ± 1.0 kPa and 1.6 ± 0.4, respectively, and 36.2 ± 9.1 kPa and 18.3 ± 5.5 at the condensation boundary within the ablation zone. Elastographic imaging modalities were then compared to gross pathology in ex vivo bovine liver tissue. Area estimated from strain, shear-wave velocity, and gross pathology images were 470, 560, and 574 mm(2), respectively, and ablation widths were 19.4, 20.7, and 23.0 mm. This study has provided insights into spatial stiffness distributions within radiofrequency ablations and suggests that low stiffness contrast on the ablation periphery leads to the observed underestimation of ablation extent on elastographic images.     

Effect of layer thickness variations on propagation delay andpenetration depth of a quarter-wave distributed Bragg reflector

The authors derive an analytical formula for the change of propagation delay and phase penetration depth of a quarter-wave distributed Bragg reflector (DBR) owing to thickness and/or composition variations. The analytical expression provides a very good estimate of change in penetration depth with arbitrary thickness perturbation in one or more layers of the DBR. For a Fabry-Perot cavity clad on both sides by two quarter-wave DBRs, the shift in the Fabry-Perot mode due to thickness variations in one or both of the DBRs can be simply obtained by calculation using the change in penetration depths of the corresponding DBR or both DBRs     

Effects of spacer thickness on noise performance of bipolar transistors

The effects of spacer thickness on noise performance of a bipolar junction transistor with different emitter widths and operation frequencies are examined. The minimum noise figure (NF/sub min/) derived from the Y-parameters as well as the base (r/sub B/) and emitter resistance (r/sub E/) obtained from the device simulation is used as a measure of noise characteristics. Furthermore, the noise resistance (R/sub n/), optimum source admittance (Y/sub sop/), and the associated gain (G/sub A,assc/) are also given in this brief. To achieve the minimum value of NF/sub min/, the spacer thickness should be targeted to an optimal value, and its value is frequency and geometry dependent.     

Diagnosis of a soft short and local variations of parameters occurring simultaneously in analog CMOS circuits

This paper is devoted to fault diagnosis of nonlinear analog CMOS circuits designed in nanometer technology. A method that allows diagnosing a single soft short and local parameter variations, occurring simultaneously, is developed. The method exploits DC measurements at limited number of points in the test phase. The diagnostic test leads to a system of nonlinear algebraic equations, not given in explicit analytical form, that may have multiple solutions. The solutions determine the sets comprising one soft short value and several values of the preliminary selected parameters. To find them an extended simplicial algorithm is developed. It allows tracing different space curves, leading to different solutions. Moreover, a procedure for selecting the actual solution from among the obtained ones is proposed. For illustration a representative numerical example is discussed in detail.     

Effects of plate thickness and stack distribution ofquasi-phase-matched materials on nonlinear frequency generation

Analytical results indicate that the plate-thickness tolerance of the GaAs plates in a quasi-phase-matched (QPM) stack with respect to the coherence length is not a critical parameter. Rather, proper placement order of the plates of different thicknesses that make up the stack is essential for generating efficient nonlinear conversion; conversion efficiency approaching that from a stack of plates with plate thickness equal exactly to the coherence length can be obtained. In effect, such ordered stacking optimizes the relative phase of the nonlinear process as it propagates down the stack and enhances the conversion efficiency. Furthermore, the analysis shows that random-order stacking of plates of different thicknesses produces a large variation in conversion efficiency, varying from one random stack to another. This may not be desirable in practice; one would normally prefer to have a process that produces QPM stacks with good and predictable performance. A single-pass three-wave coupled nonlinear frequency interaction model with temporal and spatial pulse profiles is used to analyze the behavior of the nonlinear conversion process in a quasi-phase-matched stack. Second-harmonic generation of CO₂ laser radiation in GaAs plates is used as an example     

Effects of circuit variations on the class E tuned power amplifier

The operation of the class E tuned power amplifier may be described by a set of equations based on Fourier component analysis. Previous publications have derived an optimum operating mode in which the collector efficiency of an idealized circuit is 100 percent. Since real amplifiers are made from nonideal components and are subject to nonideal loads, it is necessary to determine the effects of deviations from the ideal. The effects of variations in component values and duty cycle are determined from the basic equations. Numerical results of variations in load reactance, shunt capacitance, load resistance, frequency, and duty cycle are presented. The amplifier was found to be quite tolerant of reasonable circuit variations. With proper output filtering, the amplifier can be operated over nearly an octave bandwidth with less than a 5 percent reduction in efficiency.     

Effect of substrate thickness and metallization thickness on dispersion characteristics of CPW

Approximate conformal mapping techniques have been used for analysing the effect of finite substrate thickness on coplanar wave guide (CPW). Calculations for impedance and effective dielectric constant are presented for CPW's with finite substrate thicknesses. Analytical formulation are presented for calculations. Network analytical methods of electromagnetic fields are employed to evaluate the effect of thick metal coating on CPW. Dispersion characteristics of CPW have been plotted for various metallization thicknesses. Effect of thick metal coating on guide wavelength is also plotted. Increase in metallization thickness of CPW causes an increase in wavelength. Due to this fact characteristic impedance and effective dielectric constant decreases.     

Effects of border variations due to spatial quantisation on binary-image template matching

Quantised binary images are formed by projecting the binary intensity silhouette or line edge of a shape onto a spatial quantisation grid. The quantised shape depends on the orientation and translation of the object's image. Different complexities in the borders, due to the quantisation effects, are part of the error arising in a binary template-match process. The letter suggests a model to approximate the variation statistics as a result of uncertainty in translation only. To isolate the quantisation effects, noise-free images are considered.     

Effects of channel thickness variation on bias stress instability of InGaZnO thin-film transistors

Here, we report on the effects of channel (or active) layer thickness on the bias stress instability of InGaZnO (IGZO) thin-film transistors (TFTs). The investigation on variations of TFT characteristics under the electrical bias stress is very crucial for commercial applications. In this work, the initial electrical characteristics of the tested TFTs with different channel layer thicknesses (40, 50, and 60 nm) are performed. Various gate bias (V_GS) stresses (10, 20, and 30 V) are then applied to the tested TFTs. For all V_GS stresses with different channel layer thickness, the experimentally measured threshold voltage shift (ΔV_th) as a function of stress time is precisely modeled with stretched-exponential function. It is indicated that the ΔV_th is generated by carrier trapping but not defect creation. It is also observed that the ΔV_th shows incremental behavior as the channel layer thickness increases. Thus, it is verified that the increase of total trap states (N_T) and free carriers resulted in the increase of ΔV_th as the channel layer thickness increases.     

Effects of transverse doping variations on the transient responseof silicon avalanche shaper devices

Two-dimensional (2-D) drift-diffusion simulations were performed to study the transient response of silicon avalanche shaper (SAS) devices that are used in high-power switching and pulse sharpening applications. The role of transverse doping variations on the transient device response has been studied. Our results clearly reveal a potential for filamentary current conduction. The filamentation, however, is shown to be strongly dependent on the transverse doping characteristics, and hence in principle, could he tailored     

Effects of local photoexcitation of high-concentration charge carriers in silicon

The phenomena occurring at the local pulsed photoexcitation of intrinsic nonequilibrium highconcentration charge carriers in silicon are experimentally investigated. The effect of a substantial increase in the lifetime of photoexcited charge carriers is found. It is shown that the effect of a substantial increase in the lifetime of charge carriers is caused by a change in the degree of degeneracy and displacement of the impurityrecombination level towards the Fermi level due to local thermoelastic deformation of the crystal and the corresponding distribution of the concentration of nonequilibrium charge carriers.     

本征层厚度对非晶硅叠层电池电流匹配的影响

采用PECVD技术制备a-Si:H/a-Si:H叠层双结非晶硅电池,研究了本征层厚度对叠层电池功率及短路电流的影响。通过调节顶电池和底电池本征层的沉积时间,得到不同厚度比例的本征层(di1:di2),经过实验对比发现I层总体厚度为650 nm,di1:di2=1:5时得到的电池组件短路电流(Isc)和最大功率(Pmax)都是最大值。此时叠层电池的电流得到了较好的匹配,实现了工艺参数的优化。