São apresentadas as soluções aproximadas de alguns problemas de transporte de carga em dielétricos, inexpugnáveis ainda a um tratamento rigoroso, usando-se o Método de Galerkin. Com ele reduz-se o sistema de equações a derivadas parciais, que descrevem o transporte na presença de armadilhas, em um sistema de equações diferenciais ordinárias que são, então, integradas numericamente. Sempre que possível, a solução aproximada é comparada com alguma exata ou quase-exata, como a que se obtém da integração numérica direta do sistema de equações a derivadas parciais com o Método das Diferenças Finitas. Três diferentes condições de contorno são empregadas aqui: circuito aberto, curto circuito e circuito fechado com uma voltagem aplicada entre os eletrodos; em alguns casos considera-se temperatura variável. Este método requer que seja escollhida a priori, a forma da distribuição de carga livre; verifica-se que a corrente é mais sensível a esta distribuição do que o potencial de superfície, que sempre resulta muito próximo do exato, mesmo quando a aproximação parece grosseira. / Approximate solutions for some problems of charge transport in dielectrics, unsolved yet by exact methods, are presented using Galerkin\'s Method. This allows to transforming the system of partial differential equations, describing transport with trapping, into a system of ordinary differential equations which are, then, integrated numerically. Whenever possible, a comparison is made between this approximate solution with some exact or quasi-exact solution as, for example, that obtained from the direct numerical integrated of the system of partial differential equations using the Finite Difference Method. Three different boundary conditions are considered here: open circuit, short circuit and closed circuit with a voltage applied between the electrodes; in some cases the temperature was allowed to vary. Use of Galerkin\'s Method requires a priori choice of the free charge distribution; there results that the current is more sensitive to this distribution than the surface potential which leads to good results even when the approximation seems crude.

Este trabalho apresenta um modelo teórico que procura explicar a origem de um pico anômalo de corrente, observado na fase faraeletrica do copolímero P(VDF/TrFE), nos processos de descarga. Quando uma voltagem tipo V(t) = Vo S(t) é aplicado em materiais isolantes, correntes elétricas observadas em medidas, de carga-descarga decaem aproximadamente com T-n e obedecem ao principio da superposição linear (PSL). Esse comportamento e também observado em polímeros dielétricos. Entretanto, nos processos de descarga, acima da temperatura de Curie do copolímero p (VDF/TrFE), surge um pico anomalo de corrente que viola, aparentemente, o (PSL). Apresentamos um modelo de migração de cargas de espaço em presença de armadilhas, que pode ser responsável pelo pico de corrente e que estaria superposta a corrente de descarga da absorção dielétrica, conforme o modelo, durante os processos de carga, impurezas extrínsecas seriam varridas, pelo campo, do volume da amostra e ficariam, preferencialmente, presas em armadilhas de superfície. Nos processos de descarga, essas cargas se deslocariam em direção ao interior da amostra, devido a repulsão Coulombiana. Esse mecanismo de condução gera um pico de corrente. A aplicação do modelo, permitiu-nos inferir valores de parâmetros como a mobilidade dos portadores de carga e o tempo de transito. / A model based on space charge migration to explain an anomalous electric current peak on films of P (VDF/TrFE) copolymer in its paraelectric phase is presented. In general step-voltage current measurements of insulating polymeric materials obey the Principle of Linear Super-position. However, an unexpected anomalous peak was observed in discharge currents in measurements performed with P (VDF/TrFE) above the Curie temperature. We have assumed that space charges were dragged from the bulk of the sample by the external field during the charge measurement, and trapped close to the surface of the sample in a region where the concentration of traps would be very high. Two methods were developed to calculate the magnitude of the current peak: I) assuming a given space charge distribution during the discharge measurement, and II) considering the movement of thin discrete layers of charge under the influence of the internal field.

Estuda-se o movimento de carga espacial em dielétricos em uma dimensão na condição de circuito aberto e de curto-circuito. Soluções em simetria plana, cilíndrica e esférica são obtidas, incluindo a condutividade em alguns casos / One-dimensional space-charge motion in dielectrics is studied in open and in short-circuit condition. Solutions in plane, cylindrical and pheripherical symmetry are obtained . Conductivity is included in some cases

A finalidade principal deste trabalho é o da obtenção de soluções numéricas de movimento de carga espacial livre em que as grandezas pertinentes dependem de mais de uma coordenada espacial (casos não unidimensionais). Mais especificamente, os problemas estudados versam sobre casos em que um eletródio plano a um dado potencial emite carga não uniformemente ao longo de sua extensão e procura-se obter a deformação das linhas de campo daí resultantes, no espaço até um outro eletródio aterrado disposto paralelamente ao primeiro. Um método baseado em soluções tentativas bem como um outro global usando a \"transformação hodográfica\" recentemente formulada por Budd e Wheeler são apresentados. E também realizado um estudo breve sobre a validade da muito empregada \"suposição de Deutsch\" / The aim of the present work is to obtain numerical solutions of free space charge motion in cases in which the pertinent quantites depend on more than one space coordinate (non uni-dimensional cases). More specifically, the problems under concern here are those in which a plane electrode, at a constant potential, emit charge in a non-uniform way along them and the resultant deformation of the field lines, in the space between it and another plane grounded electrode parallelly disposed, is looked for. A method based on guessed solutions and also a global one using the hodographic transformation recently developed by Budd and Wheeler are presented. A brief study of the validity of the often employed \"Deutsch assumption\" is also carried out

O contínuo Tempo Randon Walk desenvolvido por Scher e Montroll é generalizado a fim de incluir as transições de taxas de espaço e o tempo. Tomando o limite contínuo, no passeio de equações aleatórias, uma equação geral de uma carga espacial transportada é obtida. A equivalência entre hopping e transporte através de estados expandidos com armadilhas é mostrado para realizar no espaço uma carga caso. Uma análise dos transportes um processo com dois processos simultâneos condução, por um armadilhagem e alargado a outros estados e por hopping através de armadilhas, é realizada. O Método Substituição Funcional (EFM) é introduzida a fim de obter resultados semi-markofian de Markofian soluções. Soluções específicas são obtidas em campo caso o alto e aproximadas queridos no espaço cobrar caso. Uma aproximação analítica de solução para o potencial superficial de decadência de uma sólida com armadilhas cobrado pela Corona é dada pela utilização do FSM. / The continuous Time Randon Walk developed by Scher and Montroll is generalized in order t o include space and time transitions rates. Taking the continuum limit in the random walk equations a general equation for space charge transport is obtained. The equivalence between hopping and transport via extended states with traps is shown to hold in the space charge case. An analysis of a transport process with two simultaneous conduction process, one by trapping and extended states and the other by hopping via traps, is carried out. The Functional Substitution Method (FSM) is introduced in order to get semi-markofian results from Markofian solutions. Specific exact solutions are obtained in the high field case and approximated ones in the space charge case. An approximated analytical solution for the superficial potential decay of a solid with traps charged by Corona is given by use of FSM.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1976. / Vita. / Bibliography: leaves 231-251. / Sc.D.

Portable mass spectrometers provide convenience for applications where conventional mass spectrometers are not suitable. However, a series of miniaturization issues show up in small mass spectrometers, specifically mass analyzers, that need to be thoroughly addressed before further miniaturization. The work in this dissertation focuses on miniaturization issues of ion trap mass analyzers. Space-charge is one of the major issues in small ion traps affecting their analytical performance. It limits ion trapping capacity when ion-ion repulsion causes spreading of a packet of ions. Simulation studies on the relationship between different trap dimensions and trapping capacity was done on a geometry-optimized cylindrical ion trap. A reasonable way of scaling the two important operating parameters (trapping voltage and trapping frequency as functions of the trap dimension) was discussed and applied in the simulation. The trapping capacity (N) decreased with the physical trap dimension (r0) as expected, and N is scaled exponentially as r0. Scaling laws for trapping parameters are proposed, confirmed by SIMION simulations that evaluate the space charge issue in small ion traps. This effect represents a practical limit in ion trap miniaturization.Geometry deviation is another issue that cannot be neglected in miniaturized ion traps, especially in small linear ion traps (LIT). The LIT our group is working on consists of an assembly of two plates, of which each was made by lithographically patterning a series of electrodes on an insulating plate. It is a promising way of expanding the trap capacity at a small trap dimension. However, misalignment of the two plates might seriously affect its performance, specifically resolution and signal intensity. Simulations were done on the misalignment of two-plate planar LIT in the six possible degrees of freedom (DOF) of misalignment between the two plates. Each DOF's influence on the mass resolution and the ion detection efficiency were discussed. Preliminary data from a previous ceramic plate design was collected while most of the misalignment experiments were done on an improved version. A platform was designed incorporating four motorized stages to precisely control the alignment of the ion trap in vacuum. The new plate design was demonstrated to achieve a better than unit resolution for toluene and deuterated toluene after the plates were aligned. The impact on the resolution and signal intensity from pitch, x-, y- and z-displacement were also experimentally studied.

The classical capacitance voltage characteristics based on the depletion approximation, is adequate at reverse bias, but introduces errors at high forward bias. Because of its inherent simplicity and compactness this classical depletion model is well studied and widely used in circuit simulators. In this work, a new model for the semiconductor space charge region (SCR) capacitance, based on physical justification, will be derived. This new model takes three input parameters, C0 , Vbi and m, thus eliminating the fitting parameter FC currently used in SPICE. This new model is applicable for any applied voltage and will be compared with the SCR capacitance extracted from the numerical device simulator PISCES, and with the SCR capacitance models proposed by Gummel and Poon and by DeGraaff and Klaassen.

The accomplishments presented in this thesis are the development of three models for simulation of space charge generation in the phosphor layer of alternating current thin-film electroluminescent (ACTFEL) devices and the results from simulation of these models. First, a single sheet charge model is developed and simulated. The single sheet charge model is a model that simplifies the problem of modeling an arbitrary distribution of space charge across the phosphor layer by lumping all of the space charge into a sheet of charge at a specified location in the phosphor layer. In this model and all subsequent models, space charge creation is assumed to occur by field emission from bulk traps or by impact ionization of deep-level traps. A fairly exhaustive parametric variation study of the single sheet charge model is performed and the results are presented and discussed. The results show space charge effects that are quite dependent on several parameters such as the number of bulk traps in the phosphor layer, the location of the sheet of charge, the capture efficiency for space charge annihilation, and the characteristic field for impact ionization of the deep-level traps. The second model considered is a logical extension of the single sheet charge model, the two sheet charge model, which models the space charge distribution as two sheets of charge rather than one. This model has potential application in the simulation of ACTFEL devices which exhibit large and/or symmetrical space charge effects. The final model developed is an equivalent circuit/SPICE model of the single sheet charge model. Actually, two models are developed, one for space charge creation by field emission and one for impact ionization of deep-levels. Two SPICE models are required because of functional differences in the dependencies of space charge creation. The results of a simulation showing overshoot generated by SPICE are given for the field emission equivalent circuit. / Graduation date: 1996

Organic semiconductors are very attractive for thin film Organic Photodetectors (OPDs) since they possess a number of desirable attributes for optical sensing including high absorption coefficients over visible and ultraviolet wavelengths and compatibility with large-area deposition processes such as ink-jet, screen printing, and solution processing. OPDs, in general, utilize a vertical device architecture where the photoactive organic semiconductor layers are sandwiched between top and bottom electrodes that provide electrical contact. More recently, an interest in utilizing a lateral device architecture instead of the vertical one, has emerged. In this architecture, the two contacts are positioned on the two sides of the photoactive material with respect to the direction of the incoming signal, separated by a small gap. However, the factors governing lateral OPDs’ photo-response are still not well understood. In this thesis, we fabricate top-contact lateral OPDs using a thermal evaporation only fabrication process. We study the factors governing both the dark and photo currents of lateral OPDs. The effect of the wide gap between the two electrodes on the current-voltage characteristics is discussed and the role of space charge limited conduction is investigated. The contributions in the photoresponse of light scattering through the active layers as well as the back reflection of light at the metallic contacts are emphasized. The reproducibility over repeated operation cycles of both dark and photo currents values is explored. Exposure to light of the lateral OPD is found to lead to a significant increase in the dark current. The role of the conductivity enhancement in the channel due to light-induced trap filling is investigated. External quantum efficiency and detectivity estimates are given for deep ultraviolet lateral (DUV) OPDs. A comparison with vertical DUV OPDs performances is provided. Finally, the use of a phosphorescent sensitizer doped in the absorbing bottom layer to improve top-contact lateral OPDs efficiency is discussed.