Investigation of self-heating and macroscopic built-in polarization effects on the performance of III-V nitride devices

The effect of hot phonons and the influence of macroscopic polarization-induced built-in fields on the performance of III-V nitride devices are investigated. Self-heating due to hot phonons is analyzed in AlGaN/GaN high electron mobility transistors (HEMTs). Thermal transport by acoustic phonons in the diffusive limit is modeled using a two-dimensional lattice heat equation. The effect of macroscopic polarization charges on the operation of blue and green InGaN-based quantum well structures is presented. To characterize these structures, the electronic part of the two-dimensional quantum well laser simulator MINILASE is extended to include nitride bandstructure and material models. A six-band k.p theory for strained wurtzite materials is used to compute the valence subbands. Spontaneous and piezoelectric polarization charges at the interfaces are included in the calculations, and their effects on the device performance are described. Additionally, k.p Hamiltonian for crystal growth directions that minimize the polarization-induced built-in fields are modeled, and valence band dispersion for the non-polar and semi-polar planes are also calculated. Finally, a design parameter subspace is explored to suggest epitaxial layer structures which maximize gain spectral density at a target wavelength for green InxGa1-xN-based single quantum well active regions. The dependence of the fundamental optical transition energy on the thickness and composition of barriers and wells is discussed, and the sensitivity of gain spectral density to design parameters, including the choice of buffer layer material, is investigated.Ph.D.Committee Chair: Yoder, Douglas; Committee Member: Graham, Samuel; Committee Member: Allen, Janet; Committee Member: Klein, Benjamin; Committee Member: Voss, Pau

Phylogenetic comparison of exonic US4, US7 and UL44 regions of clinical herpes simplex virus type 1 isolates showed lack of association between their anatomic sites of infection and genotypic/sub genotypic classification

Abstract Background HSV-1 genome is a mosaic of recombinants. Clinical Herpes simplex virus -1 (HSV1) isolates were already genotyped as A, B and C types based on nucleotide variations at Unique Short (US) 4 (gG) and US 7 (gI) regions through phylogeny. Analysis of Glycoprotein C (gC) exon present on the Unique Long (UL) region had also revealed the existence of different genotypes. Glycoprotein C is mainly involved in initial viral attachment to heparan sulphate on host cell surface facilitating the virus's binding and penetration into cell. As the amount of heparan sulphate on the host cell surface varies according to the cell type, it is plausible that different genotypes bind differentially to cell types. Hence, this study was framed to determine the existence of novel genotypes/sub genotypes in the US or UL regions which could associate with clinical entities. Results All the twenty five isolates analyzed in this study were of genotype A as per their gG gene sequences. In case of gI gene, 16 out of 25 were found to be type A and the remaining nine were type B putative intergenic recombinants. Intragenic recombinations were also encountered in both the US genes, with gG possessing novel subgenotypes, arbitrarily designated A1 and A2. The 9 type B isolates of gI genes also branched out into 2 clades due to genetic variations. Glycoprotein C of UL region had two distinct genotypic clades α and β, whose topological distribution was significantly different from that of the US region. Neither the US nor UL regions, however, showed any preference among the genotypes to a specific anatomic site of infection. Even the non synonymous variations identified in the functional domain of gC, were not confined to a particular genotype/clinical entity. Conclusion The analyses of the US and UL regions of the HSV-1 genome showed the existence of variegated genotypes in these two regions. In contrary to the documented literature, in which Asian strains were concluded as more conserved than European ones, our study showed the existence of a higher degree of variability among Indian strains. However, the identified novel genotypes and subgenotypes were not found associated with clinical entities.</p

Contrasting responses within a single neuron class enable sex-specific attraction in Caenorhabditis elegans

Animals find mates and food, and avoid predators, by navigating to regions within a favorable range of available sensory cues. How are these ranges set and recognized? Here we show that male Caenorhabditis elegans exhibit strong concentration preferences for sex-specific small molecule cues secreted by hermaphrodites, and that these preferences emerge from the collective dynamics of a single male-specific class of neurons, the cephalic sensory neurons (CEMs). Within a single worm, CEM responses are dissimilar, not determined by anatomical classification and can be excitatory or inhibitory. Response kinetics vary by concentration, suggesting a mechanism for establishing preferences. CEM responses are enhanced in the absence of synaptic transmission, and worms with only one intact CEM show nonpreferential attraction to all concentrations of ascaroside for which CEM is the primary sensor, suggesting that synaptic modulation of CEM responses is necessary for establishing preferences. A heterogeneous concentration-dependent sensory representation thus appears to allow a single neural class to set behavioral preferences and recognize ranges of sensory cues.Physic