Charles R. Westgate

Planar Strip Array (PSA) for MRI

Parallel, spatial‐encoded MRI requires a large number of independent detectors that simultaneously acquire signals. The loop structure and mutual coupling in conventional phased arrays limit the number of coils and therefore the potential reduction in minimum scan time achievable by parallel MRI tchniques. A new near‐field MRI detector array, the planar strip array (PSA), is presented that eliminates the coupling problems and can be extended to a very large number of detectors and high MRI frequencies. Its basic structure is an array of parallel microstrips with a high permittivity substrate and overlay. The electromagnetic (EM) wavelength can be adjusted with the permittivity, and the strip lengths tuned to a preselected fraction of the wavelength of the MRI frequency. EM wave analysis and measurements on a prototype four‐element PSA reveal that the coupling between the strips vanishes when the strip length is either an integer times a quarter wavelength for a standing‐wave PSA, or a half wavelength for a travelling‐wave PSA, independent of the spacing between the strips. The analysis, as well as phantom and human MRI experiments performed by conventional and parallel‐encoded MRI with the PSA at 1.5 T, show that the decoupled strips produce a relatively high‐quality factor and signal‐to‐noise ratio, provided that the strips are properly terminated, tuned, and matched or coupled to the preamplifiers. Magn Reson Med 45:673–683, 2001.

Characterization of subthreshold MOS mismatch in transistors for VLSI systems

MOS transistor mismatch is revisited in the context of subthreshold operation and VLSI systems. We report experimental measurements from large transistor arrays with device sizes typical for digital and analog VLSI systems (areas between 9 and 400µm2). These are fabricated at different production qualified facilities in 40-nm gate oxide,n-well andp-well, mask lithography processes. Within the small area of our test-strips (3 mm2), transistor mismatch can be classified into four categories: random variations, “edge,” “striation,” and “gradient” effects. The edge effect manifests itself as a dependence of the transistor current on its position with reference to the surrounding structures. Contrary to what was previously believed, edge effects extend beyond the outer most devices in the array. The striation effect exhibits itself as a position-dependent variation in transistor current following a sinusoidal oscillation in space of slowly varying frequency. The gradient effect is also a position-dependent spatial variation but of much lower frequency. When systematic effects are removed from the data, the random variations follow an inverse linear dependence on the square root of transistor area.

The role of bulk traps in metal-insulator contact charging

Metal-insulator contact charging models are presented for insulators having (a) traps uniformly distributed in the energy gap and (b) discrete traps in the energy gap. It is shown that in the former case the relationship between the magnitude of the charge transferred and the workfunction difference is linear, while in the latter case it ranges from an exponential to a quadratic depending on the band bending and the energetic position of the traps. Screening lengths are calculated for the two cases, and it is shown that the thickness of the insulator is usually not the limiting factor in contact charging, even for thin-film insulators. Finally, it is shown that the times required for the charge transfer can be short.

Comparison of the dielectric behavior of mixtures of methanol with carbon dioxide and ethane in the mixture-critical and liquid regions

Roskar, V., Dombro, R.A., Prentice, G.A., Westgate, C.R. and McHugh, M.A., 1992. Comparison of the dielectric behavior of mixtures of methanol with carbon dioxide and ethane in the mixture-critical and liquid region. Fluid Phase Equilibria, 77: 241-259. Dielectric data are reported for the CO2-methanol system at 35, 50 and 65°C and the ethane-methanol system at 35, 65 and 95°C at a number of composition and over a pressure range from the saturation pressure of the mixture to 300 bar. Experimental pressure-composition isotherms and molar volume data are also reported for the ethane-methanol system. The dielectric constant for both types of methanol mixtures can be increased by decreasing the temperature or by increasing the permanent dipole concentration of the mixture through increasing the mixture density or the methanol mole fraction. The dielectric constants for CO2-methanol mixtures are consistently greater than those for ethanemethanol mixtures over the entire pressure-temperature-composition space investigated in this study. The higher dielectric constants for CO2-methanol mixtures are conjectured to be a consequence of the quadrupole interactions between CO2 and methanol and to the contribution from the CO2 -methanol complex which is present in these mixtures.

An analytical SMASH procedure (ASP) for sensitivity-encoded MRI

The simultaneous acquisition of spatial harmonics (SMASH) method of imaging with detector arrays can reduce the number of phase‐encoding steps, and MRI scan time several‐fold. The original approach utilized numerical gradient‐descent fitting with the coil sensitivity profiles to create a set of composite spatial harmonics to replace the phase‐encoding steps. Here, an analytical approach for generating the harmonics is presented. A transform is derived to project the harmonics onto a set of sensitivity profiles. A sequence of Fourier, Hilbert, and inverse Fourier transform is then applied to analytically eliminate spatially dependent phase errors from the different coils while fully preserving the spatial‐encoding. By combining the transform and phase correction, the original numerical image reconstruction method can be replaced by an analytical SMASH procedure (ASP). The approach also allows simulation of SMASH imaging, revealing a criterion for the ratio of the detector sensitivity profile width to the detector spacing that produces optimal harmonic generation. When detector geometry is suboptimal, a group of quasi‐harmonics arises, which can be corrected and restored to pure harmonics. The simulation also reveals high‐order harmonic modulation effects, and a demodulation procedure is presented that enables application of ASP to a large numbers of detectors. The method is demonstrated on a phantom and humans using a standard 4‐channel phased‐array MRI system. Magn Reson Med 43:716–725, 2000.

Electron transport in rectifying semiconductor alloy ramp heterostructures

The electron transport properties of AlGaAs ramp diodes are investigated using a physical model which combines current transport through the heterostructure bulk with current across the abrupt heterointerface in a fully self-consistent numerical approach. Transport at the abrupt material discontinuity is described by thermionic and thermionic-field emission processes, whereas transport in regions of smoothly varying alloy composition is modeled by diffusion-drift mechanisms. Several devices whose bandgaps are graded over several thousand angstroms have been fabricated by molecular beam epitaxy (MBE) and tested at room and liquid-nitrogen temperatures. The experimentally observed rectification properties are compared with the simulated results over a wide range of DC bias. Through appropriate choice of alloy composition and doping profiles, majority carrier devices based on internal (bulk) barriers may be realized. >

Microwave Hall measurment techniques on low mobility semiconductors and insulators. I. Analysis

The relationships of microwave Hall mobilities and microwave conductivities in a semiconductor, placed at the center of a dual mode TE111 cavity, are expressed in terms of the loaded and unloaded voltage reflection coefficients, the sample and the cavity dimensions, the ratio of the output power to the input power of the cavity, and the magnetic field. Scattering

Analytical calculation of the quantum-mechanical transmission coefficient for a triangular, planar-doped potential barrier

Abstract The thermionic emission limit for current transport in planar-doped triangular barrier devices is determined by the quantum-mechanical reflection of the charge carriers at the barrier. In this paper we present an analytical calculation for the quantum-mechanical transmission and reflection coefficients for such a barrier.

Microwave Hall measurement techniques on low mobility semiconductors and insulators. II. Experimental procedures

An experimental technique for the measurement of Hall mobilities at microwave frequencies is described. The design of a dual mode TE111 cavity is described, and a comparison among receivers for the detection system is given. A procedure for tuning the cavity for orthogonality, degeneracy, and equal quality factors is presented. With a superheterodyne receiver and a well designed gold plated, probe‐coupled cavity, the minimum detectable mobility is 0.07 cm2/V⋅sec for a high conductivity sample.

Dielectric constant behavior of carbon dioxide-methanol mixtures in the mixture-critical and liquid-phase regions

Abstract The dielectric constant, measured with a frequency-domain technique, is reported for saturated mixtures of carbon dioxide and methanol from 90 to 115° C and at 0.28, 0.325, 0.40 and 0.65 mole fraction methanol. Isothermal dielectric information is obtained for the same compositions at 90 °C and at pressures from the saturation pressure to 240 bar. The data are interpreted using the Kirkwood-Frohlich equation, which incorporates the structure correlation factor, g, that depends on density and temperature. The behavior of g provides information on the type of fluid environment encountered in the mixture-critical and liquid-phase regions.