Optimization of multiheterojunction AlGaAs/GaAs HEMT's for microwave power amplification

A theoretical and experimental study concerning a three channels power HEMT is presented. The structure has been, first, optimized using an adequate simulation. Then many technological realizations have been achieved at the laboratory. Measurements performed with these devices give results very encouraging and permit to foresee superior performances relatively to that of GaAs power MESFET's

DIFFUSION AND NOISE IN GAAS MATERIAL AND DEVICES

International audienceThe variation of the diffusion coefficent D(E) versus the electric field strength E, is determined at 300 K in n-type GaAs (N(D) = 3 x 10(-17) cm-3), using pulsed high-frequency noise measurements. D(E) is found to increase slightly at low field, then to decrease down to one tenth of its ohmic value near the threshold field. "Long" (greater-than-or-equal-to 4-mu-m) real n+-n-n+ Gunn diodes, with an arbitrary doping profile, can be modeled. Comparisons are made, and excellent agreement is found, between experimental and theoretical characteristics of two real diodes, with notch and with gradual doping profiles. The doping profile N(D)(x) is shown to have a considerable influence on the diode behavior, concerning the electric field profile as well as the noise characteristics. Using the impedance field method, the noise current is modeled and found to be very sensitive in the D(E) variation law, in particular in the range of 2.5-4 kV/cm. The agreement between the experimental noise and the computed noise of real diodes is found to be quite satisfactory when using D(E) determined in the present work, and important discrepancies occur when using other D(E) variations, both theoretical and experimental, found in the literature

Comment réaliser des TEC AsGa de transconductance supérieure à 700 mS/mm et à fréquence de coupure élevée

En utilisant un modèle prenant en compte les phénomènes de transport non stationnaire, nous montrons que les performances de MESFETs fonctionnant à tension grille positive dépendent de façon importante de la géométrie et des caractéristiques de la zone entre la source et la grille ; en particulier pour obtenir des performances maximales il faut que la valeur de la résistance carrée de la couche épitaxiée ainsi que la distance entre le début de la zone creusée (« recess ») du côté source et la grille soient les plus faibles possible. Sur ce principe, nous avons réalisé des MESFET ayant des transconductances hyperfréquences supérieures à 700 mS/mm avec des fréquences de coupure proches de 50 GHz. Nous décrivons ici l'ensemble des procédés permettant la modélisation, la réalisation technologique et la caractérisation hyperfréquences finale

HIGH-FREQUENCY NOISE AND DIFFUSION-COEFFICIENT OF HOT-ELECTRONS IN BULK AL0.25GA0.75AS

International audienceWe present experimental results of hot electron noise in Si doped Al0.25Ga0.75As test structures using pulsed high-frequency noise measurements. Noise temperature and longitudinal diffusion coefficient as functions of field strength are compared with those of Si doped GaAs. We find significant changes at high fields. Results are discussed in regard of electron scattering mechanisms

Challenges of tracking topical discussion networks online

Attempts to map online networks, representing relationships between people and sites, have covered sites including Facebook, Twitter, and blogs. However, the predominant approach of static network visualization, treating months of data as a single case rather than depicting changes over time or between topics, remains a flawed process. As different events and themes provoke varying interactions and conversations, it is proposed that case-by-case analysis would aid studies of online social networks by further examining the dynamics of links and information flows. This study uses hyperlink analysis of a population of French political blogs to compare connections between sites from January to August 2009. Themes discussed in this period were identified for subsequent analysis of topic-oriented networks. By comparing static blogrolls with topical citations within posts, this research addresses challenges and methods in mapping online networks, providing new information on temporal aspects of linking behaviors and information flows within these systems