Influence of the operating temperature on the design and utilization of 94 GHz pulsed silicon IMPATT diodes

International audienceThe RF and thermal behavior of 94-GHz P/sup +/PNN/sup +/ double drift flat doping profile silicon impact ionization avalanche transit time (IMPATT) diodes for high-power pulsed operation is investigated by means of time domain electrical oscillator models. It is demonstrated that these diodes have a limited optimum temperature range of operation, associated to specific matching and bias conditions, to achieve a stable and high power operation. This restriction necessitates a thermal control when the oscillator must operate over a wide ambient temperature range. Highly doped, short active zone length diodes appear to have the best potential for high power performance

Simulation globale du fonctionnement à 94 GHz d'un oscillateur ATT pulsé, basée sur le couplage de modèles physique, électrique, thermique et électromagnétique

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Millimeter-wave pulsed oscillator global modeling by means of electromagnetic, thermal, electrical and carrier transport physical coupled models

International audienceThe time domain modeling of the operation of a 94-GHz pulsed silicon IMPATT oscillator, based on a physical approach, is described in this paper. It relies on the coupling of electrical, thermal, electromagnetic, and carrier transport physical models. The model has been used to study the high-power stable operation of a 94-GHz oscillator. The results of a comparison between simulations, using two different types of passive radio-frequency load circuits, including experimental measurements, are presented and discussed. They tend to demonstrate that it is now possible to develop accurate millimeterwave-circuit predictive models even for application based on a nonlinear thermal and electrical transient operation such as IMPATT oscillators

94 GHz pulsed silicon IMPATT oscillator modelling

A new type of 94-GHz pulsed silicon impact avalanche and transit time (IMPATT) oscillator numerical modeling is described. It consists of a set of three models of increasing complexity; namely, a pure sine model, time-domain isothermal model, and time-domain electro-thermal model, which basically rely on a diode one-dimensional bipolar drift-diffusion model embedded in a time-domain circuit modeling. In this paper, they are first used to investigate the 94-GHz diode intrinsic operation and performance. Secondly, the load-impedance level has been optimized. In each case, the thermal behavior is especially considered. Thirdly, pulse-operation-simulation results are compared with experiments performed at Thomson CSF, Radars et Contre-Mesures, Elancourt, France. Finally, some improvements of the present modeling are discussed in Section VI

Force nanoscopy as a versatile platform for quantifying the activity of antiadhesion compounds targeting bacterial pathogens

The development of bacterial strains that are resistant to multiple antibiotics has urged the need for new antibacterial therapies. An exciting approach to fight bacterial diseases is the use of antiadhesive agents capable to block the adhesion of the pathogens to host tissues, the first step of infection. We report the use of a novel atomic force microscopy (AFM) platform for quantifying the activity of antiadhesion compounds directly on living bacteria, thus without labeling or purification. Novel fullerene-based mannoconjugates bearing 10 carbohydrate ligands and a thiol bond were efficiently prepared. The thiol functionality could be exploited as a convenient handle to graft the multimeric species onto AFM tips. Using a combination of single-molecule and single-cell AFM assays, we demonstrate that, unlike mannosidic monomers, multivalent glycofullerenes strongly block the adhesion of uropathogenic Escherichia coli bacteria to their carbohydrate receptors. We expect that the nanoscopy technique developed here will help designing new antiadhesion drugs to treat microbial infections, including those caused by multidrug resistant organisms

Forces in yeast flocculation

In the baker’s yeast Saccharomyces cerevisiae, cell–cell adhesion (“flocculation”) is conferred by a family of lectin-like proteins known as the flocculin (Flo) proteins. Knowledge of the adhesive and mechanical properties of flocculins is important for understanding the mechanisms of yeast adhesion, and may help controlling yeast behaviour in biotechnology. We use single-molecule and single-cell atomic force microscopy (AFM) to explore the nanoscale forces engaged in yeast flocculation, focusing on the role of Flo1 as a prototype of flocculins. Using AFM tips labelled with mannose, we detect single flocculins on Flo1-expressing cells, showing they are widely exposed on the cell surface. When subjected to force, individual Flo1 proteins display two distinct force responses, i.e. weak lectin binding forces and strong unfolding forces reflecting the force-induced extension of hydrophobic tandem repeats. We demonstrate that cell–cell adhesion bonds also involve multiple weak lectin interactions together with strong unfolding forces, both associated with Flo1 molecules. Single-molecule and single-cell data correlate with microscale cell adhesion behaviour, suggesting strongly that Flo1 mechanics is critical for yeast flocculation. These results favour a model in which not only weak lectin-sugar interactions are involved in yeast flocculation but also strong hydrophobic interactions resulting from protein unfolding

Pleiotropic effects of rfa-gene mutations on Escherichia coli envelope properties

International audienceMutations in the rfa operon leading to severely truncated lipopolysaccharide (LPS) structures are associated with pleiotropic effects on bacterial cells, which in turn generates a complex phenotype termed deep-rough. Literature reports distinct behavior of these mutants in terms of susceptibility to bacteriophages and to several antibacterial substances. There is so far a critical lack of understanding of such peculiar structure-reactivity relationships mainly due to a paucity of thorough biophysical and biochemical characterizations of the surfaces of these mutants. In the current study, the biophysicochemical features of the envelopes of Escherichia coli deep-rough mutants are identified from the molecular to the single cell and population levels using a suite of complementary techniques, namely microelectrophoresis, Atomic Force Microscopy (AFM) and Isobaric Tag for Relative and Absolute Quantitation (iTRAQ) for quantitative proteomics. Electrokinetic, nanomechanical and proteomic analyses evidence enhanced mutant membrane destabilization/permeability, and differentiated abundances of outer membrane proteins involved in the susceptibility phenotypes of LPS-truncated mutants towards bacteriophages, antimicrobial peptides and hydrophobic antibiotics. In particular, inner-core LPS altered mutants exhibit the most pronounced heterogeneity in the spatial distribution of their Young modulus and stiffness, which is symptomatic of deep damages on cell envelope likely to mediate phage infection process and antibiotic action. Lipopolysaccharides (LPS) cover surface of the outer membrane of Gram-negative bacteria. They are tripar-tite molecules composed of lipid A, core oligosaccharides usually containing glucose, heptose, galactose, 2-keto-3-deoxyoctonate (KDO), and a highly variable O-antigen component (O-antigen is missing in Escherichia coli K-12). LPS act as a protective and permeable barrier against large molecules and hydrophobic compounds from the environment. They are positioned among phospholipids and proteins of the outer membrane, and contribute to the structural properties of the latter. In Escherichia coli, genes involved in the LPS synthesis are organized according to three operons in the rfa (also known as waa) locus (Fig. 1A). The first operon contains rfaD (or gmhD), rfaF, rfaC and rfaL genes. The three first genes encode proteins involved in the biosynthesis and transfer of the two first heptose residues in the inner core of LPS, whereas rfaL encodes a ligase required for the attachment of O-antigen. The second operon contains (i) rfaQ and rfaK (or waaU) that encodes the heptosyltransferases adding the third and fourth heptose residues, respectively, (ii) genes rfaG (or waaG), rfaI (or waaO) and rfaJ (or warR /waaJ) encoding the gluco-syltransferases that add the three glucose residues in the LPS outer core, (iii) rfaB that encodes the galactosyl-transferase adding the galactose residue to the first glucose, (iv) rfaY and rfaP that encodes kinases responsible for phosphorylation of heptoses, (v) rfaZ involved in the KDO attachment during LPS core biosynthesis, and finally (vi) rfaS that encodes a protein necessary for the attachment of rhamnose to the LPS core by linkage to the KDOII residue. The short kdtA operon contains kdtA (or waaA) that encodes the KDO transferase adding the two KDO residues to the lipid A and kdtB (or coaD) that is not involved in the LPS synthesis 1-3. A defining feature of E. coli LPS is the presence of phosphoryl substituents on the LPS core-heptose residues, essential fo