Caractérisation et résultats de fiabilité de transistors à haute mobilité électronique (HEMT)

The telecommunication systems require introduction of high performance devices especially for microwave applications. The emergence of molecular beam epitaxy as a growth technique allows the fabrication of heterostructure-based performing devices. Thus, this communication will focus on the reliability of technologies used for the development of field effect transistor using heterostructures and called HEMT (high electron mobility transistor).     

Diodes lasers en régime de forte modulation sinusoïdale de courant : modélisation et expériences â 1,3 µm

We present a theoretical and experimental study of laser diodes under strong sinusoidal current modulation. Situations where the diodes are biased above threshold are considered. The first part describes different behaviors occurring under strong modulation. The GaAs lasers are compared to the InGaAsP lasers. The major result of numerical calculations shows that the existence of anomalous behaviors (period doubling, chaos,...) can be predicted from the weak-signal response of the laser. In a second part, we report on the expérimentais results obtained with an InGaAsP laser at 1.3 µ.m and their comparison with the theoretical calculations from a rate equation model. This kind of laser does not exhibit any anomalous behaviour under strong modulation, but distorsion effects occur. The calculated spectra of the laser intensity are in excellent agreement with measurements, which validates both the use of the rate equation model and the choice of the laser-parameters. In contrast, the spectral analysis at the optical frequency shows that the optical phase-equation previously used by many authors is uncomplete and that longitudinal and/or transversal variations of the α-factor have to be taken into account.     

In Situ TEM Observation of the Behavior of an Individual Fullerene-Like MoS<Subscript>2</Subscript> Nanoparticle in a Dynamic Contact

Inorganic fullerene-(IF)-like nanoparticles made of metal dichalcogenides (IF-MoS₂, IF-WS₂) have been known to be effective as anti-wear and friction modifier additives under boundary lubrication. The lubrication mechanism of these nanoparticles has been widely investigated in the past and even if the exfoliation and third body transfer of molecular sheets onto the asperities constitute the prevalent mechanism for the improved tribological behavior of IF nanoparticles, it has also been suggested that a rolling friction process could also play a role for well crystallized and spherical particles. In this study, in situ Transmission Electron Microscopy (TEM) observations of the behavior of single IF-MoS₂ nanoparticles were conducted using a sample holder that combines TEM and Atomic Force Microscopy (AFM) which simultaneously can apply normal and shear loads. It was shown that depending on the test conditions, either a rolling process or a sliding of the fullerenes could be possible. These in situ TEM observations are the first carried out with IF nanoparticles.     

Kinetics of phase separation in polyurethane/polystyrene semi-1 interpenetrating polymer networks. 1. Light transmission studies

The phase separation of different in-situ semi-1 interpenetrating polymer networks (IPNs) based on polyurethane and polystyrene has been followed by light transmission. The effect of the presence ab initio of small amounts of homopolystyrene in the initial reaction mixture on the phase separation process has also been examined. If gelation of the polyurethane occurs before the onset of phase separation, the latter is impeded or strongly limited, and transparent semi-1 IPNs are obtained. In the opposite case, phase separation is macroscopic and the material is turbid.     

Estimation of parameters of a multifractal process

Multifractal functions are widely used to model irregular signals such as turbulence, data stream or road traffic. Here, we consider multifractal functions defined as lacunar wavelet series observed in a white noise model. These random functions are statistically characterized by two parameters. The first parameter governs the intensity of the wavelet coefficients while the second one governs its sparsity. We construct estimators of these two parameters and discuss statistical properties of this important model: the rate of the Fisher information and a testing procedure to check the multifractal feature of an observed noisy signal.      

Solving agreement problems with failure detectors: a survey

This paper surveys problems related to achieving agreement in distributed systems. Various agreement problems can be specified as a variant of the basic consensus problem. Unfortunately, this fundamental problem cannot be solved in asynchronous systems if crash failures can occur. In order to overcome this impossibility result, Chandra and Toueg have augmented the asynchronous system model with the notion of failure detectors. A failure detector is associated with each process of the distributed computation and is responsible for detecting external failures. Suspicions are essentially implemented using time-out mechanisms, which means that (1) the detection of a real failure is usually delayed and (2) a failure detector can make mistakes by incorrectly suspecting a process to have crashed. In this paper, various classes of failure detectors are presented. All are specified by a completeness property and an accuracy property. A completeness property puts a condition on the detection of crashed processes, while an accuracy property restricts the possible mistakes made by a failure detector. After reviewing the hierarchy of failure detector classes, two particular solutions to the consensus problem are examined. In each case, a different class of failure detectors is used. The proposed solutions have liveness properties and deliver their expected results provided that a minimal set of well-defined conditions is satisfied. After a detailed presentation of the consensus problem, various agreement problems are reviewed and their relationship with the consensus problem are underlined. Finally, for each agreement problem, we indicate under which minimum assumptions a solution can be found.     

Une note sur le cylindre des langages déterministes

La famille des langages déterministes constitue un cylindre non principal. Nous redémontrons ce résultat de S.A. Greibach en décrivant cette famille comme l'union d'une infinité de cylindres emboîtés qui constituent une hiérarchie stricte, chacun d'entre eux étant ici un cylindre principal.     

Le diagnostic des circuits intégrés par faisceau d’électrons

The purpose of this synthetic article is to show how the electron beam testing technique of prototype integrated circuits after a first run has now reached maturity. Firstly the physics of measurement has been the subject of many improvements which means that results, both more reliable and more precise (in voltage and in time) can be obtained more quickly. Moreover this improved performance has led to the development of fully comprehensive diagnosis stations so ergonomic that the user forgets his scanning electron microscope, stations which become the final link in the chain of computer aided design.     

Kinetics of phase separation in polyurethane/polystyrene semi-1 interpenetrating polymer networks. 2. Microscopy observations and theoretical approach

The phase separation process of in-situ polyurethane/polystyrene semi-1 interpenetrating polymer networks has been followed by optical microscopy completed by image analysis. The final morphology has been examined by scanning electron microscopy, and its relation to the phase separation process has been established. When phase separation occurs before gelation of the medium, polystyrene is dispersed both as large and small noduli in the polyurethane network; a polyurethane-rich interphase separates the noduli from the matrix. These observations explain the previous results on the turbidity of such systems measured by light transmission. In the opposite cases, no visible phase separation is observed on the micrographs. The thermodynamics of the phase separation process has also been examined.