Circuit-level modelling and simulation of carbon nanotube devices

The growing academic interest in carbon nanotubes (CNTs) as a promising novel class of electronic materials has led to significant progress in the understanding of CNT physics including ballistic and non-ballistic electron transport characteristics. Together with the increasing amount of theoretical analysis and experimental studies into the properties of CNT transistors, the need for corresponding modelling techniques has also grown rapidly. This research is focused on the electron transport characteristics of CNT transistors, with the aim to develop efficient techniquesto model and simulate CNT devices for logic circuit analysis.The contributions of this research can be summarised as follows. Firstly, to accelerate the evaluation of the equations that model a CNT transistor, while maintaining high modelling accuracy, three efficient numerical techniques based on piece-wise linear, quadratic polynomial and cubic spline approximation have been developed. The numerical approximation simplifies the solution of the CNT transistor’s self-consistent voltage such that the calculation of the drain-source current is accelerated by at least two orders of magnitude. The numerical approach eliminates complicated calculations in the modelling process and facilitates the development of fast and efficient CNT transistor models for circuit simulation.Secondly, non-ballistic CNT transistors have been considered, and extended circuit-level models which can capture both ballistic and non-ballistic electron transport phenomena, including elastic scattering, phonon scattering, strain and tunnelling effects, have been developed. A salient feature of the developed models is their ability to incorporate both ballistic and non-ballistic transport mechanisms without a significant computational cost. The developed models have been extensively validated against reported transport theories of CNT transistors and experimental results.Thirdly, the proposed carbon nanotube transistor models have been implemented on several platforms. The underlying algorithms have been developed and tested in MATLAB, behaviourallevel models in VHDL-AMS, and improved circuit-level models have been implemented in two versions of the SPICE simulator. As the final contribution of this work, parameter variation analysis has been carried out in SPICE3 to study the performance of the proposed circuit-level CNT transistor models in logic circuit analysis. Typical circuits, including inverters and adders, have been analysed to determine the dependence of the circuit’s correct operation on CNT parameter variation

Effective network grid synthesis and optimization for high performance very large scale integration system design

制度:新 ; 文部省報告番号:甲2642号 ; 学位の種類:博士(工学) ; 授与年月日:2008/3/15 ; 早大学位記番号:新480

Evolutionary synthesis of analog networks

The significant increase in the available computational power that took place in recent decades has been accompanied by a growing interest in the application of the evolutionary approach to the synthesis of many kinds of systems and, in particular, to the synthesis of systems like analog electronic circuits, neural networks, and, more generally, autonomous systems, for which no satisfying systematic and general design methodology has been found to date. Despite some interesting results in the evolutionary synthesis of these kinds of systems, the endowment of an artificial evolutionary process with the potential for an appreciable increase of complexity of the systems thus generated appears still an open issue. In this thesis the problem of the evolutionary growth of complexity is addressed taking as starting point the insights contained in the published material reporting the unfinished work done in the late 1940s and early 1950s by John von Neumann on the theory of self-reproducing automata. The evolutionary complexity-growth conditions suggested in that work are complemented here with a series of auxiliary conditions inspired by what has been discovered since then relatively to the structure of biological systems, with a particular emphasis on the workings of genetic regulatory networks seen as the most elementary, full-fledged level of organization of existing living organisms. In this perspective, the first chapter is devoted to the formulation of the problem of the evolutionary growth of complexity, going from the description of von Neumann's complexity-growth conditions to the specification of a set of auxiliary complexity-growth conditions derived from the analysis of the operation of genetic regulatory networks. This leads to the definition of a particular structure for the kind of systems that will be evolved and to the specification of the genetic representation for them. A system with the required structure — for which the name analog network is suggested — corresponds to a collection of devices whose terminals are connected by links characterized by a scalar value of interaction strength. One of the specificities of the evolutionary system defined in this thesis is the way these values of interaction strength are determined. This is done by associating with each device terminal of the evolving analog network a sequence of characters extracted from the sequences that constitute the genome representing the network, and by defining a map from pairs of sequences of characters to values of interaction strength. Whereas the first chapter gives general prescriptions for the definition of an evolutionary system endowed with the desired complexity-growth potential, the second chapter is devoted to the specification of all the details of an actual implementation of those prescriptions. In this chapter the structure of the genome and of the corresponding genetic operators are defined. A technique for the genetic encoding of the devices constituting the analog network is described, along with a way to implement the map that specifies the interaction between the devices of the evolved system, and between them and the devices constituting the external environment of the evolved system. The proposed implementation of the interaction map is based on the local alignment of sequences of characters. It is shown how the parameters defining the local alignment can be chosen, and what strategies can be adopted to prevent the proliferation of unwanted interactions. The third chapter is devoted to the application of the evolutionary system defined in the second chapter to problems aimed at assessing the suitability in an evolutionary context of the local alignment technique and to problems aimed at assessing the evolutionary potential of the complete evolutionary system when applied to the synthesis of analog networks. Finally, the fourth chapter briefly considers some further questions that are relevant to the proposed approach but could not be addressed in the context of this thesis. A series of appendixes is devoted to some complementary issues: the definition of a measure of diversity for an evolutionary population employing the genetic description introduced in this thesis; the choice of the quantizer for the values of interaction strength between the devices constituting the evolved analog network; the modifications required to use the analog electronic circuit simulator SPICE as a simulation engine for an evolutionary or an optimization process

The CASCADE 10B thermal neutron detector and soil moisture sensing by cosmic-ray neutrons

This work connects the three domains of experimental nuclear physics, computational physics and environmental physics centered around the neutron. The CASCADE thermal neutron detector is based on a combination of solid 10B coatings in several layers, GEMs as gas amplification stages, a microstructured readout, multichannel ASICs and FPGA hardware triggered data acquisition. The detailed analysis to improve the system in terms of time-of-flight resolution for Neutron Resonance Spin Echo Spectroscopy required for a simulation model of the detector. The limitations of existing codes led to the development of the Monte Carlo transport code URANOS, which fully integrates the detector components and features a voxel-based geometry definition. The simulation could then successfully be applied to precisely understand neutron transport within the frame of Cosmic-Ray Neutron Sensing. This novel and interdisciplinary method offers the possibility to non-invasively measure soil moisture on the hectare scale using neutrons of the environmental radiation. The endeavor of this work led to the development of the footprint weighting function, which describes the neutron density change by different hydrogen pools in the air-ground interface. Significant influences of the near-field topology around the sensor were predicted by this work, experimentally verified and correction methods were successfully tested

Annual Review of Progress in Applied Computational Electromagnetics

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Etude de l’impact de micro-cavités (voids) dans les attaches de puces des modules électroniques de puissance

Power converters nowadays are required to function under harsh conditions in meeting energy efficiency and reliability requirement. Whereas, industrial specifications tend toward a higher level of power integration in respect to the cost constraint. As a result, the die attach is one of the key elements in power module packaging because of high current densities and high heat flow which are transported through. Void formation in the die attach may lead to performance degradation and premature aging of the component. This study introduces a methodology based on the comparison of numerical simulations and experimental campaigns. The obtained results help to improve our understanding on the electro-thermal behaviour of MOSFETs with solder voids. In this thesis, we depict a finite element model in which electro-thermal coupling of a MOSFET active layer is taken in to account. Simulation results will be correlated to the experimental responses. Later on, a parametric numerical study based on the response surface method (RSM) which minimizes the number of simulations and future tests will be exploited to quantify the impact of void position and size on several selective performance criteria. A future serial experimental study in respect to the same RSM design is expected in prospect, in order to fulfil the complementarity for this approach.Les convertisseurs électroniques de puissance sont voués à fonctionner sous des conditions applicatives de plus en plus sévères tout en respectant les impératifs d’efficacité énergétique et de fiabilité. Or, les besoins industriels tendent vers un plus haut niveau d’intégration fonctionnelle tout en améliorant le rapport qualité-prix. Dès lors, la solution utilisée pour le report des puces semi-conductrices est le siège de densités de courant importantes et d’un flux thermique élevé. La présence de défauts dans cette couche d’interconnexion peut conduire à la dégradation de ses performances et au vieillissement prématuré du composant. L’objectif de nos recherches est d’évaluer la pertinence d’une méthodologie basée sur la confrontation de simulations numériques et de campagnes expérimentales. L’objectif est d’améliorer la compréhension du comportement électrothermique en régime de conduction d’un transistor MOSFET en présence d’un void dans sa brasure. Dans cette manuscrite, nous présenterons la construction d’un modèle intégrant le couplage électrothermique de la partie active qui sera confronté à la réponse de résultats expérimentaux. Puis, une étude numérique basée sur la théorie des plans fractionnaires, qui minimise le nombre de simulations, sera exploitée afin de quantifier l’impact de la taille et de la position du défaut sur la réponse électrothermique du composant et de ses liaisons électriques. Les détails de la mise en place d’une étude expérimentale analogue permettront de mettre en perspective la complémentarité de cette approche