High electrical performance liquid-phase HBr oxidation gate insulator of InAlAs/InGaAs metamorphic MOS-mHEMT

The InAlAs/InGaAs metal-oxide-semiconductor metamorphic high electron mobility transistors (MOS-mHEMTs) were demonstrated by using liquid-phase HBr treatment technology to form a high-quality gate insulator layer. In this study, liquid-phase HBr treatment technology was used instead of traditional plasma-assisted chemical vapor deposition (PECVD) because the proposed technology can prevent the device from plasma-induced damage. The novel HBr + ultraviolet (UV) illumination treated InGaAs provided a lower surface states such that MOS structure can be efficiently obtained. Besides, based on the atomic force microscopy (AFM) measurement, the native oxides film formed by HBr + UV illumination treatment also provided a better surface roughness compared to traditional NH₄OH and only HBr treatment solutions. It is beneficial for reducing the surface traps and lowering the leakage current in MOS-mHEMTs. Based on the flicker noise and load-pull power measurement results, HBr + UV treatment mHEMT achieved a low flicker noise at high current level and the power-added efficiency can be enhanced up to 9%. Therefore, the novel liquid phase method of HBr + UV illumination treatment exhibited a highly potential for low noise microwave power device applications.     

Analysis of the diametral compression method for determining the tensile strength of transparent magnesium aluminate spinel

Attempts were made to determine the inherent tensile strength of a coarse-grained, hot-pressed magnesium aluminate spinel (MgAl₂O₄) using the diametral compression test. Thick (9.6 mm) disk specimens were machined from a large (356 mm square) plate of spinel. Two pairs of tungsten carbide (WC) platens, one with flat surfaces and the other with a 20° half-arc and radius matched to the disk diameter, were used to transfer the applied load. Specimens tested using the platens with the matched radius had strength values almost 50% higher than those tested using flat platens. Images of the fracture process captured using a high-speed camera showed that irrespective of the type of platens used, fracture consistently initiated at the loading interface, resulting in an invalid test. These results show that the diametral compression test method is not appropriate for determining the tensile strength of this spinel and it raises concerns about the applicability of the method for any advanced ceramic.     

What types of safety nets would be most efficient and effective for protecting small farmers and the poor against volatile food prices?

Smallholder farmers in dryland areas, including the Arab region, face many risks, including those relating to weather and markets. While risk taking is necessary for economic development, many risks are so severe that responses to them entail long-term losses. Social risk management is a family of tools that facilitates efficient risk management and can help eliminate risk-related poverty traps. Safety nets form an important part of the social risk management toolkit; it is therefore important to evaluate their effectiveness. This review has identified several considerations for the design and implementation of safety nets to improve food security among small-scale producers in dryland areas. The main conclusions are: safety nets must be part of an integrated risk management strategy; they should be designed to contribute to long-term agricultural growth; and more effort is needed to establish efficient risk management mechanisms prior to crises.     

Energy-Efficient Routing for Frequency-Hop Wireless Networks

The use of adaptive-transmission protocols in wireless, store-and-forward, packet communication networks may result in large differences in the energy requirements of the alternative paths that are available to the routing protocol. Routing metrics can provide quantitative measures of the quality and energy efficiency of the paths from the source to the destination. Such measures are required if the routing protocol is to take advantage of the potential energy savings that are made possible by an adaptive-transmission protocol. An energy-efficient protocol suite for routing and adaptive transmission in frequency-hop wireless networks is described and evaluated, several routing metrics are compared, and tradeoffs among energy efficiency, delay, and packet success probability are investigated.     

Fast predictions of variance images for fan-beam transmission tomography with quadratic regularization

Accurate predictions of image variances can be useful for reconstruction algorithm analysis and for the design of regularization methods. Computing the predicted variance at every pixel using matrix-based approximations [1] is impractical. Even most recently adopted methods that are based on local discrete Fourier approximations are impractical since they would require a forward and backprojection and two fast Fourier transform (FFT) calculations for every pixel, particularly for shift-variant systems like fan-beam tomography. This paper describes new "analytical" approaches to predicting the approximate variance maps of 2-D images that are reconstructed by penalized-likelihood estimation with quadratic regularization in fan-beam geometries. The simplest of the proposed analytical approaches requires computation equivalent to one backprojection and some summations, so it is computationally practical even for the data sizes in X-ray computed tomography (CT). Simulation results show that it gives accurate predictions of the variance maps. The parallel-beam geometry is a simple special case of the fan-beam analysis. The analysis is also applicable to 2-D positron emission tomography (PET).     

A steerable complex wavelet construction and its application to image denoising.

This work addresses the design of a novel complex steerable wavelet construction, the generation of transform-space feature measurements associated with corner and edge presence and orientation properties, and the application of these measurements directly to image denoising. The decomposition uses pairs of bandpass filters that display symmetry and antisymmetry about a steerable axis of orientation. While the angular characterization of the bandpass filters is similar to those previously described, the radial characteristic is new, as is the manner of constructing the interpolation functions for steering. The complex filters have been engineered into a multirate system, providing a synthesis and analysis subband filtering system with good reconstruction properties. Although the performance of our proposed denoising strategy is currently below that of recently reported state-of-the-art techniques in denoising, it does compare favorably with wavelet coring approaches employing global thresholds and with an "Oracle" shrinkage technique, and presents a very promising avenue for exploring structure-based denoising in the wavelet domain.     

Performance Analysis of the Spectral Correlation Discriminator Array

Mobile communications suffer from cochannel interference, adjacent channel interference and fading. The cell capacity or the number of users per cell is limited by the interference. In this paper we analyze a new blind adaptive array structure called the spectral correlation discriminator array (SCDA), designed to reject cochannel interference for advanced mobile phone service (AMPS) signals. The blind adaptive array exploits the spectral redundancy in the AMPS signals which arises due to the presence of the supervisory audio tone (SAT). SCDA is shown to provide an improvement of 25–30 dB in signal-to-interference plus noise ratio (SINR) in static multipath channels and 10–15 dB improvement of SINR in Rayleigh fading channels. The SCDA is compared to the least squares constant modulus array (LSCMA) and is shown to have better signal selectivity properties. The reduction in interference power can significantly reduce the frequency reuse factor.     

Structural Studies of Anomalous Behavior in the Silica-Alumina Gel System

This paper compares the structure ofAl₂O₃-SiO₂ xerogels prepared with Al₂O₃ contents ranging from 21 to 75 wt% (13 to 64 mol%). The 47 wt% Al₂O₃ xerogel (Al/Si ≅ 1) exhibits anomalously low surface area (≅1 m²/g) and skeletal density compared with other Al₂O₃-Si₂ compositions. Based on the results of nitrogen adsorption/condensation, helium displacement, high-resolution transmission electron microscopy (HRTEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS), we attribute the low surface area and skeletal density (density of the solid phase which is inaccessable to helium) to the formation of closed micropores, whereas the higher surface area materials exhibit a slightly coarser texture comprising open pores with radii of ≅1 nm. X-ray diffraction (XRD) and ²⁹SI and ²⁷Al magic-angle spinning nuclear magnetic resonance (MASNMR) indicate no anomalous behavior in the 47% sample on molecular length scales. HRTEM indicates the presence of a small fraction of crystallites which is supported by the SAXS results, but it is unknown if this crystallinity is related to low surface area. Low-field ¹H NMR spin-lattice relaxation measurements show that the physical structure of all of the 'wet' gels is similar implying that pore closure occurs during drying. Consistent with this idea, gel surface area and density increased significantly when the pore fluid (water) was replaced with a lower surface tension fluid (formamide, dioxane, ethanol. water/surfactant).     

Thermal stability investigations of AlGaN/GaN HEMTs with various high work function gate metal designs

The operation of high power RF transistor generates a huge amount of heat and thermal effect is a major consideration for improving the efficiency of power transistors. AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates have been studied extensively because of their high thermal conductivity. This study comprehensively investigates the DC, low frequency noise, microwave and RF power performance of Al_0.27Ga_0.73N/GaN HEMTs on silicon substrates at temperatures from room temperature to 100 °C using high work function metals such as palladium (Pd) and iridium (Ir) gate metals. Although the conventional Ni gate exhibited a good metal work function with AlGaN, which is beneficial for increasing the Schottky barrier height of HEMTs, the diffusion of Ni metal toward the AlGaN and GaN layers influences the DC and RF stability of the device at high temperatures or over long-term operation. Pd and Ir exhibited less diffusion at high temperature than Ni, resulting in less degradation of device characteristics after high temperature operation.     

Materialized view selection as constrained evolutionary optimization

One of the important issues in data warehouse development is the selection of a set of views to materialize in order to accelerate a large number of on-line analytical processing (OLAP) queries. The maintenance-cost view-selection problem is to select a set of materialized views under certain resource constraints for the purpose of minimizing the total query processing cost. However, the search space for possible materialized views may be exponentially large. A heuristic algorithm often has to be used to find a near optimal solution. In this paper, for the maintenance-cost view-selection problem, we propose a new constrained evolutionary algorithm. Constraints are incorporated into the algorithm through a stochastic ranking procedure. No penalty functions are used. Our experimental results show that the constraint handling technique, i.e., stochastic ranking, can deal with constraints effectively. Our algorithm is able to find a near-optimal feasible solution and scales with the problem size well.