Surface leakage currents in SiN/sub x/ passivated AlGaN/GaN HFETs

A novel guarded surface leakage test structure is used to isolate the surface and bulk leakage contributions to gate current in AlGaN/GaN HFETs. Passivation with various recipes of SiN/sub x/ always resulted in the commonly observed increase in gate leakage, which was found to be dominated by bulk leakage through the AlGaN. However, high temperature deposited SiN/sub x/ recipes gave a 1-2 orders reduction in surface leakage, whereas low temperature deposition gave an increase. Gate lag measurements were found to correlate closely with the surface leakage component, giving direct evidence that the key device problem of current slump is associated with current flow at the AlGaN surface.     

Retinoblastoma: clinical and molecular diagnostic aspects

Retinoblastoma, a tumor of the immature retina concerns babies and young infants in particular. They make up for 14% of malignomas in the first years of life. There are two types of retinoblastoma: In the first two alleles of the gene Rb1 must be inactivated sequentially in the same retinoblast cell until this may escape control. In this case the retinoblastoma is always unilateral and unifocal. This is explained by the lower frequency of two mutations in one retinoblast. The other type, however, is inherited: One allele Rb1 is inactivated in all cells of the organism by mutation. The probability that a second mutation arrives in different retinoblasts is thus high. In this case bilateral multifocal tumors develop. Characterization of the Rb1 gene has permitted identification or at least determination of a haplotype in persons at risk. This knowledge is decisive for early recognition of babies at risk and for genetic counselling.     

Crystal Chemistry and Phase Transitions in Substituted Pollucites along the CsAlSi2O6-CsTiSi2O6.5 Join: A Powder Synchrotron X-ray Diffractometry Study

The crystal structures for a suite of substituted pollucites with the compositions CsTi _x Al_{1– x} Si₂O_{6+0.5 x} , 0 ≤ x ≤ 1, have been determined from Rietveld analysis of powder synchrotron XRD data. Our results indicate that the pollucite end member (CsAlSi₂O₆) has a tetragonal structure (space group I 4₁/ a ), whereas all other compositions are cubic (space group Ia 3 d ). The increased symmetry for the titanium-substituted structures is presumably due to the incorporation of additional O²⁻ anions (needed for compensating the charge imbalance between Ti⁴⁺ and Al³⁺), which effectively holds open the expanded cubic framework. In situ cooling experiments of the substituted phase CsTi_0.1Al_0.9Si₂O_6.05 reveal a displacive transformation to the tetragonal structure at ∼230 K. This transformation is tricritical in nature and is analogous to the tetragonal-to-cubic transition in pollucite on heating.     

Metastable Phase Selection and Partitioning for Zr(1−x)AlxO(2−x/2) Materials Synthesized with Liquid Precursors

Aqueous solutions of zirconium acetate and aluminum nitrate were spray pyrolyzed at 250°C and upquenched to different temperatures to yield metastable solid solutions of composition Zr_{(1− x )}Al_xO_{(2− x /2)}. An amorphous oxide forms first during pyrolysis which subsequently crystallizes as a single phase for x ≤ 0.57 (≤40 mol% Al₂O₃). The crystallization temperature increased with Al₂O₃ content. Electron diffraction, supported by Raman spectroscopy, indicates that the initial phase is tetragonal. At higher temperatures, the initial solid solation partitions to other metastable phases, viz., t -ZrO₂+γ-Al₂O₃, prior to achieving their equilibrium phase assemblage, m -ZrO₂+α-Al₂O₃. Partitioning yields a nanocomposite microstructure with grain sizes of 20–100 nm, compared to the 3 to 5 nm in the initial, single phase. Compositions containing 45 to 50 mol% Al₂O₃ concurrently crystallize and partition. The structure selected during crystallization and the partitioning phenomena are discussed in terms of diffusional constraints during crystallization, which are conceptually similar to those operating during rapid solidification.     

Environmental behavior of the chiral acetamide pesticide metalaxyl: enantioselective degradation and chiral stability in soil

Racemic metalaxyl is currently being replaced in many countries by metalaxyl-M, the fungicide enantiomerically enriched with the biologically active R-enantiomer. This "chiral switch" is expected to reduce the amount of pesticide released into the environment as well as potential side-effects on nontarget organisms. Detailed knowledge of the environmental behavior of such chiral compounds should include information on the chiral stability (interconversion of enantiomers). In the present study, the degradation/ dissipation of metalaxyl and its primary carboxylic acid metabolite (MX-acid) in soil was investigated under laboratory conditions using enantioselective gas chromatography mass spectrometry (GC-MS). Racemic and the enantiopure R- and S-compounds were incubated in separate experiments. The degradation of metalaxyl was shown to be enantioselective with the fungicidally active R-enantiomer being faster degraded than the inactive S-enantiomer, resulting in residues enriched with S-metalaxyl when the racemic compound was incubated. The relatively high enantioselectivity suggests that degradation/dissipation was largely biological. The data indicated a conversion of 40-50% of metalaxyl to MX-acid, and the remaining metalaxyl being degraded via other pathways. The degradation of MX-acid was also enantioselective. Metalaxyl and MX-acid were both configurationally stable in soil, showing no interconversion of R- to S-enantiomers, and vice-versa. Furthermore, the conversion of metalaxyl to MX-acid proceeded with retention of configuration. Degradation followed approximate first-order kinetics but showed significant lag phases.     

Comparison of Rat Hepatic Cholesterol Biosynthesis During Skim Milk Versus Whey Permeate Ingestion

Whey permeate is an ultrafiltrate of whey that is devoid of protein but contains lactose, salts, and other soluble low molecular weight compounds. These experiments compared cholesterol concentrations of blood plasma, hepatic lipids, and hepatic cholesterol biosynthesis of rats ingesting skim milk powder versus whey permeate powder. Groups of young male rats weighing 90 to 92 g were fed a casein-based diet into which skim milk powder or whey permeate powder was incorporated isocalorically. No effects of skim milk or whey permeate on plasma cholesterol concentrations were observed at any time during 5-wk of feeding. However, 3-hydroxy-3-methylglutaryl coenzyme A reductase activity was increased by either skim milk or whey permeate feeding. Hepatic cholesterol, triglyceride, and phospholipid concentrations at wk 5 were unchanged. Plasma and hepatic cholesterol responses of rats to whey permeate ingestion are similar to those that occur with skim milk consumption, and plasma and hepatic cholesterol concentrations do not reflect necessarily an increase in hepatic cholesterol biosynthesis.