Nanoscale conductive pattern of the homoepitaxial AlGaN/GaN transistor.

The gallium nitride (GaN)-based buffer/barrier mode of growth and morphology, the transistor electrical response (25-310 °C) and the nanoscale pattern of a homoepitaxial AlGaN/GaN high electron mobility transistor (HEMT) have been investigated at the micro and nanoscale. The low channel sheet resistance and the enhanced heat dissipation allow a highly conductive HEMT transistor (Ids > 1 A mm(-1)) to be defined (0.5 A mm(-1) at 300 °C). The vertical breakdown voltage has been determined to be ∼850 V with the vertical drain-bulk (or gate-bulk) current following the hopping mechanism, with an activation energy of 350 meV. The conductive atomic force microscopy nanoscale current pattern does not unequivocally follow the molecular beam epitaxy AlGaN/GaN morphology but it suggests that the FS-GaN substrate presents a series of preferential conductive spots (conductive patches). Both the estimated patches density and the apparent random distribution appear to correlate with the edge-pit dislocations observed via cathodoluminescence. The sub-surface edge-pit dislocations originating in the FS-GaN substrate result in barrier height inhomogeneity within the HEMT Schottky gate producing a subthreshold current.

Nanoscale investigation of AlGaN/GaN-on-Si high electron mobility transistors.

AlGaN/GaN HEMTs are devices which are strongly influenced by surface properties such as donor states, roughness or any kind of inhomogeneity. The electron gas is only a few nanometers away from the surface and the transistor forward and reverse currents are considerably affected by any variation of surface property within the atomic scale. Consequently, we have used the technique known as conductive AFM (CAFM) to perform electrical characterization at the nanoscale. The AlGaN/GaN HEMT ohmic (drain and source) and Schottky (gate) contacts were investigated by the CAFM technique. The estimated area of these highly conductive pillars (each of them of approximately 20-50 nm radius) represents around 5% of the total contact area. Analogously, the reverse leakage of the gate Schottky contact at the nanoscale seems to correlate somehow with the topography of the narrow AlGaN barrier regions producing larger currents.

Assessment of postinfarction ventricular septal ruptures by transesophageal Doppler echocardiography.

Transthoracic Doppler echocardiography has been shown to be a sensitive modality for the diagnosis of acute septal ruptures after myocardial infarctions. Transesophageal echocardiography has been shown to improve diagnostic accuracy and image quality in many clinical settings. We performed transesophageal Doppler echocardiography in 10 patients with acute septal ruptures. Transesophageal echocardiography provided improved visualization of the rupture morphology (6 of 10 by transthoracic versus 10 of 10 by transesophageal imaging), better detection of multiple rupture sites (2 by transthoracic, 5 by transesophageal study) and better detail of the direction of shunt flow. On the basis of the transesophageal echocardiographic appearance, we propose that septal ruptures after acute myocardial infarctions be classified as simple or complex, consistent with pathologic criteria for left ventricular septal and free wall ruptures. Transesophageal echocardiography proved a useful and safe adjunct to transthoracic imaging, overcoming the technical limitations in these critically ill patients.

Boron, molybdenum, and selenium in aquatic food chains from the lower San Joaquin River and its tributaries, California.

Boron (B), molybdenum (Mo), and selenium (Se) were measured in water, sediment, particulate organic detritus, and in various biota--filamentous algae, net plankton, macro-invertebrates, and fishes--to determine if concentrations were elevated from exposure to agricultural subsurface (tile) drainage during the spring and fall 1987, in the San Joaquin River, California. Concentrations of B and Se, but not Mo, were higher in most samples from reaches receiving tile drainage than in samples from reaches receiving no tile drainage. Maximum concentrations of Se in water (0.025 microgram/mL), sediment (3.0 micrograms/g), invertebrates (14 micrograms/g), and fishes (17 micrograms/g) measured during this study exceeded concentrations that are detrimental to sensitive warmwater fishes. Toxic threshold concentrations of B and Mo in fishes and their foods have not been identified. Boron and Mo were not biomagnified in the aquatic food chain, because concentrations of these two elements were usually higher in filamentous algae and detritus than in invertebrates and fishes. Concentrations of Se were lower in filamentous algae than in invertebrates and fishes; however, concentrations of Se in or on detritus were similar to or higher than in invertebrates and fishes. These observations suggest that high concentrations of Se accumulated in invertebrates and fishes through food-chain transfer from Se-enriched detritus rather than from filamentous algae.

Measles in rural Ohio county.

Between December 23, 1975, and March 31, 1976, 169 cases of measles were reported from Defiance County, Ohio, a small rural county in the northwest corner of the State. The outbreak spread from a single junior high school basketball player to eventually involve 19 of the 28 county schools. Among the affected schools, measles attack rates varied from 0.3-7.2% with a mean of 2.0%. A likely source of illness was determined for 160 of the 169 cases (95%). Intraschool transmission was most common, accounting for 97 of the 169 cases (57%) followed by sibling contact for 23 cases (14%). The pattern of measles spread was complex and would have been difficult to predict in advance even if surveillance systems reported each case the day it occurred. A control program held between February 2 and February 20, 1976, vaccinated 5145 of the 11,114 (46.3%) county schoolchildren. Forty-four cases of measles occurred 4 or more days following school clinics, 22 (50%) in children who requested measles vaccine at school clinics, 17 of whom were actually vaccinated. Most of the other cases occurred in students whose parents thought their children to be protected. Measles is a disease which spreads rapidly in a complex pattern over wide geographic areas. A control program vaccinating a large proportion of the children without definitive history of adequate vaccination or disease was apprently effective in curtailing the outbreak.