L'objectif de ce travail de thèse est l'exploitation des oscillations de plasma bidimensionnelles dans les transistors à haute mobilité électronique à base d'InGaAs, matériaux de grand intérêt pour les applications Terahertz grâce à sa haute mobilité électronique. Ce travail s'insère dans le contexte d'études récentes dans lesquelles l'utilisation de dis- positifs basés sur l'excitation d'ondes de plasma bidimensionnelles a été proposée pour des applications Terahertz. Cette étude est menée au travers du développement d'un outil numérique de simulation basé sur le modèle hydrodynamique couplé avec l'équation de Poisson pseudo 2D. La réponse continue du courant à une excitation électrique de fréquence THz a été étudiée et l'influence sur les résonances de plasma des différents paramètres de transistor est mise en évidence. Une étude de la densité spectrale de la fluctuation du courant est alors présentée en vue d'établir une coordination entre le bruit dans les HEMT et la détection directe d'un signal électrique THz. La réponse de HEMT à différentes perturbation au niveau de drain et de la grille est enfin évaluée par le biais de la description du régime petit-signal, ce qui permettrait éventuellement une étude plus approfondie des ondes de plasma dans les transistors HEMT. / The objective of this work is the use of plasma oscillations mechanism in the electron mobility transistors channel that based of InGaAs, this materials characterize by it great interest for Terahertz thanks to its high electron mobility applications. This work registered in the context of recent studies in which the use of devices based on wave excitation of two-dimensional plasma has been proposed for Terahertz applications.This study is conducted through the development of a simulation tool based on the hydrodynamic model coupled with the Poisson equation 2D. the continued current response to THz electrical excitation has been studied and the influence of the different parameters of transistor on plasma resonances is demonstrated. A study of the spectral density of the current fluctuation is then presented, we demonstrate that the main resonances in the drain current noise spectrum are the same as those observed in the current response to an external THz excitation. The current response at different perturbation applied upon the drain and the gate of the HEMT is finally evaluated by means of the description of the small-signal equivalent circuit, which would possibly further studies of plasma oscillation in HEMT transistors.

Extracellular neuroelectronic interfacing has important applications in the fields of neural prosthetics, biological computation and whole-cell biosensing for drug screening and toxin detection. While the field of neuroelectronic interfacing holds great promise, the recording of high-fidelity signals from extracellular devices has long suffered from the problem of low signal-to-noise ratios and changes in signal shapes due to the presence of highly dispersive dielectric medium in the neuron-microelectrode cleft. This has made it difficult to correlate the extracellularly recorded signals with the intracellular signals recorded using conventional patch-clamp electrophysiology. For bringing about an improvement in the signalto-noise ratio of the signals recorded on the extracellular microelectrodes and to explore strategies for engineering the neuron-electrode interface there exists a need to model, simulate and characterize the cell-sensor interface to better understand the mechanism of signal transduction across the interface. Efforts to date for modeling the neuron-electrode interface have primarily focused on the use of point or area contact linear equivalent circuit models for a description of the interface with an assumption of passive linearity for the dynamics of the interfacial medium in the cell-electrode cleft. In this dissertation, results are presented from a nonlinear dynamic characterization of the neuroelectronic junction based on Volterra-Wiener modeling which showed that the process of signal transduction at the interface may have nonlinear contributions from the interfacial medium. An optimization based study of linear equivalent circuit models for representing signals recorded at the neuron-electrode interface subsequently iv proved conclusively that the process of signal transduction across the interface is indeed nonlinear. Following this a theoretical framework for the extraction of the complex nonlinear material parameters of the interfacial medium like the dielectric permittivity, conductivity and diffusivity tensors based on dynamic nonlinear Volterra-Wiener modeling was developed. Within this framework, the use of Gaussian bandlimited white noise for nonlinear impedance spectroscopy was shown to offer considerable advantages over the use of sinusoidal inputs for nonlinear harmonic analysis currently employed in impedance characterization of nonlinear electrochemical systems. Signal transduction at the neuron-microelectrode interface is mediated by the interfacial medium confined to a thin cleft with thickness on the scale of 20-110 nm giving rise to Knudsen numbers (ratio of mean free path to characteristic system length) in the range of 0.015 and 0.003 for ionic electrodiffusion. At these Knudsen numbers, the continuum assumptions made in the use of Poisson-Nernst-Planck system of equations for modeling ionic electrodiffusion are not valid. Therefore, a lattice Boltzmann method (LBM) based multiphysics solver suitable for modeling ionic electrodiffusion at the mesoscale neuron-microelectrode interface was developed. Additionally, a molecular speed dependent relaxation time was proposed for use in the lattice Boltzmann equation. Such a relaxation time holds promise for enhancing the numerical stability of lattice Boltzmann algorithms as it helped recover a physically correct description of microscopic phenomena related to particle collisions governed by their local density on the lattice. Next, using this multiphysics solver simulations were carried out for the charge relaxation dynamics of an electrolytic nanocapacitor with the intention of ultimately employing it for a simulation of the capacitive coupling between the neuron and the v planar microelectrode on a microelectrode array (MEA). Simulations of the charge relaxation dynamics for a step potential applied at t = 0 to the capacitor electrodes were carried out for varying conditions of electric double layer (EDL) overlap, solvent viscosity, electrode spacing and ratio of cation to anion diffusivity. For a large EDL overlap, an anomalous plasma-like collective behavior of oscillating ions at a frequency much lower than the plasma frequency of the electrolyte was observed and as such it appears to be purely an effect of nanoscale confinement. Results from these simulations are then discussed in the context of the dynamics of the interfacial medium in the neuron-microelectrode cleft. In conclusion, a synergistic approach to engineering the neuron-microelectrode interface is outlined through a use of the nonlinear dynamic modeling, simulation and characterization tools developed as part of this dissertation research.