Systematic characterization of Cl2 reactive ion etchingfor improved ohmics in AlGaN/GaN HEMTs

Pre-metal-deposition reactive ion etching (RIE) was performed on an Al_0.3Ga_0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFM). The contact resistance decreased from about 0.45 Ωmm for unetched ohmics to a minimum of 0.27 Ωmm for 70 Å etched ohmics. The initial thickness of the AlGaN layer was 250 Å. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs     

A series active power filter based on a sinusoidalcurrent-controlled voltage-source inverter

A series active power filter working as a sinusoidal current source, in-phase with the mains voltage, has been developed and tested. The amplitude of the fundamental current in the series filter is controlled through the error signal generated between the load voltage and a pre-established reference. The control allows an effective correction of power factor, harmonic distortion and load voltage regulation. Compared with previous methods of control developed for series active filters, this method is simpler to implement because it is only required to generate a sinusoidal current, in-phase with the mains voltage, the amplitude of which is controlled through the error in the load voltage. The proposed system has been studied analytically and tested using computer simulations and experiments. In the experiments, it has been verified that the filter keeps the line current almost sinusoidal and in-phase with the line voltage supply. It also responds very quickly under sudden changes in load conditions, reaching its steady-state in about two cycles of the fundamental     

Film cracking and debonding in a coated fiber

A fracture mechanics based methodology for the determination of interface fracture toughness from crack spacing in a thin coated fiber is presented. The coating (film) may be regarded as the matrix material in typical experiments employing this configuration. Matrix crack spacing is considered to be the result of a competitive process between matrix segmentation and interface debonding which are assumed to be governed by critical energy release rate criteria. Matrix cracks are assumed to form by the process of channeling in the circumferential direction and steady state conditions are assumed at the matrix crack front in the channeling direction. Energy release rates are determined using domain integral procedures in conjunction with the finite element method. The minimum crack spacing is obtained as a function of applied stress for different values of interface fracture toughness. A methodology to relate the saturated crack spacing to interface fracture toughness is developed. Interfaces are classified into three categories: weak, intermediate and strong. It is shown that in experiments of this type, quantitative information about the interface fracture toughness can be obtained for intermediate interfaces while qualitative information may be obtained for weak and strong interfaces.     

The mechanics of compressive kinking in unidirectional fiber reinforced ductile matrix composites

Three distinct stages of kink band formation and propagation exist in ductile matrix composites subjected to compressive loading. These stages are called incipient kinking, transient kinking and kink band broadening. Each stage involves a different deformation mode. The mechanics governing each stage are discussed. Incipient kinking, where the peak load is attained, and kink band broadening, where the load attains a steady-state, are important in structural design. Two design philosophies are presented. References to pertinent literature are made throughout.     

Reactive Power Compensation Technologies: State-of-the-Art Review

This paper presents an overview of the state of the art in reactive power compensation technologies. The principles of operation, design characteristics and application examples of Var compensators implemented with thyristors and self-commutated converters are presented. Static Var generators are used to improve voltage regulation, stability, and power factor in ac transmission and distribution systems. Examples obtained from relevant applications describing the use of reactive power compensators implemented with new static Var technologies are also described.     

Influence of Manufacturing and Ripening Conditions on the Survival of Enterobacteriaceae in Manchego Cheese

The effects of cooking temperature, time and temperature of brine-salting, and temperature of ripening on the behavior of Enterobacteriaceae in Manchego cheese were studied. Fifty lots of cheese from raw ewe's milk manufactured and ripened under different conditions were investigated throughout a 60-day ripening period. Differences in pH values due to temperature-dependent whey retention accounted for the effect of cooking temperature on coliform and fecal coliform counts. Temperature of brine-salting had no effect on Enterobacteriaceae counts, but a significant effect of salting time on Enterobacteriaceae and fecal coliform counts was detected. Ripening temperature was the manufacturing variable with greatest influence (P<.01) on Enterobacteriaceae and coliform counts during the whole curing period.     

Spray forming technology for military applications

Over the past five years, the Annapolis Detachment, Carderock Division, Naval Surface Warfare Center (formerly David Taylor Research Center) has established a state- of- the- art spray forming facility for the study and exploitation of this near net shape manufacturing process. Spray forming is a single step gas atomization/deposition process which yields ferrous and nonferrous, near final shape, near fully dense preforms that has proven to be a viable and cost effective alternative to conventional metalworking technology for the production of material preforms with properties surpassing those of their cast and wrought counterparts. Current programs at Carderock Divison Naval Surface Warfare Center are aimed at optimization of the process, certification of the spray formed products and industrialization of the technology.     

Influence of cooking and drying processes at different temperatures on the nutritive value of the protein of mussels (Mytilus edulis)

The effect of steam cooking (96 degrees C for 15 minutes) and drying at two temperatures, 70 degrees C and 110 degrees C, on nutritive value of mussel protein was studied. The measurements were carried out by nitrogen balance techniques in growing rats, and the nutritional parameters studied were: CD, BV and NPU. The crude digestibility (CD) values were: 87 +/- 1 and 82 +/- 1, and the biological values (BV), 80 +/- 1 and 74 +/- 1 for raw mussels, dried at 70 degrees C and at 110 degrees C respectively. This implies a significant decrease in the protein nutritive values of the mussel dried at a higher temperature. Cooking prior to drying significantly improved the digestibility and the biological value of the mussel's protein. In effect, improvement was so great, that the different drying temperatures did not affect the previously cooked product in a different way; therefore, the CD (94 +/- 1 and 94 +/- 1) and the BV (90 +/- 1 and 90 +/- 2) were the same for the mussel's protein, cooked and dried at 70 degrees C or at 110 degrees C.