Electrical conductivity in granular media and Branly's coherer: A simple experiment

accepted for publication in American Journal of Physics (to be published between February 2005 and June 2005)We show how a simple laboratory experiment can illustrate certain electrical transport properties of metallic granular media. At a low critical imposed voltage, a transition from an insulating to a conductive state is observed. This transition comes from an electro-thermal coupling in the vicinity of the microcontacts between grains where microwelding occurs. Our apparatus allows us to obtain an implicit determination of the microcontact temperature, which is analogous to the use of a resistive thermometer. The experiment also illustrates an old problem, the explanation of Branly's coherer effect - a radio wave detector used for the first wireless radio transmission, and based on the sensitivity of the metal fillings conductivity to an electromagnetic wave

An ultra-high switching frequency step-down DC-DC converter based on Gallium Arsenide devices

A 50MHz to 100MHz dc-dc power converter using Gallium Arsenide power switches is studied. GaAs Schottky rectifiers with high breakdown voltage and very small Ron x Con switching quality factor have been fabricated. A 10V to 5V (or 8V) prototype with an output power of 2.6 Watts and a power efficiency of 77% has been reported

0.3µm-N-HIGFET capabilities for microwave power apllications

0.3µm HIGFET transistors have been realized for microwave power applications. Power measurements at 3.5GHz using load-pull testbench have been carried out. Transistors exhibit an output saturated power of 16dBm and a power density of 370mW/mm for a 3V drain-to-source voltage

HETEROSTRUCTURE FIELD EFFECT TRANSISTOR, PHYSICAL ANALYSIS AND NEW STRUCTURES

Les principaux phénomènes physiques qui se produisent dans les MODFET conventionnels AlGaAs/GaAs sont décrits brièvement : structures quantiques, propriétés de transport - influence du champ électrique. La modélisation des composants permet d'obtenir des informations très utiles sur leur fonctionnement et les principaux effets physiques qui interviennent tout en permettant d'en prévoir les performances. Les problèmes posés par les réalisations technologiques ou le fonctionnement dans des conditions particulières (haute température, grande puissance) sont examinés. On décrit les nouvelles structures telles que les MODFET inversés, les SISFET et les MODFET à couches pseudomorphiques ainsi que leurs performances potentielles.Physical phenomena that occur in conventional AlGaAs-GaAs MODFETs are briefly described. This covers quantum effects, carrier transport properties, influence of electric field... Progress in device modeling allow to obtain valuable information on device behaviour as well as various intervening physical effects and to predict the performance. Limitations resulting from both technological imperfections and specific device operations, e.g. low temperature, large power, are investigated. New structures such as inverted MODFET, SISFET, and pseudomorphic MODFETs are described and their potential capabilities are discussed

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

Specific methodology for the design of new monolithic millimeter wave integrated circuits. Combination of several simulation and modeling tools.

The design of new millimeter wave monolithic integrated circuits needs the availability of accurate model tools such as physical simulations and experimental parameters extraction methods. Accurate electrical models have been built even in the difficult case of submicrometer dual gate pseudomorphic HEMTs. Their validity has been clearly demonstrated by the design of 60 GHz mixers

Structure MESFET AsGa bigrille distribuée. Performances potentielles en amplification

We present a study on the amplification capabilities of a GaAs distributed dual-gate MESFET structure, with the gate G1 as input and the gate G2 as output, the drain electrode being R.F. grounded. A model, based on the Spice program, allows us to investigate, successively, the influences of the termination impedances, the width, the losses and the bias conditions of the gate lines. A comparison with the single gate distributed structure is reported. Lastly, the device feasability is demonstrated through measurements carried out from a first realization.Nous présentons une étude des potentialités en amplification d'une structure MESFET AsGa bigrille distribuée, avec entrée sur la grille G1, sortie sur la grille G2, le drain étant découplé à la masse. Une modélisation, utilisant le logiciel Spice, permet d'examiner, successivement, les influences des impédances terminales, du développement, des pertes et de la polarisation des lignes de grilles. Une comparaison avec la structure monogrille distribuée est donnée. Enfin, la faisabilité du dispositif amplificateur est établie par des essais pratiqués sur une première réalisation