CMOS image system wiht high responsivity and high dynamic range

Orientador: Jacobus Willibrordus SwartTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de ComputaçãoResumo: O trabalho apresentado nesta tese endereça dois importantes desafios impostos pela evolução da tecnologia CMOS, a diminuição da responsividade das junções e a redução da tensão de alimentação. Um fotodetector de alta responsividade e um sistema de imagem CMOS multiamostrado no domínio do tempo são propostos nesta tese. Como fototransistor de elevada responsividade propõem-se nesta tese o uso do Transistor Bipolar Lateral Controlado por Porta (GC-LBJT) operando como fototransistor de 4 terminais. Apresenta-se a análise do princípio de funcionamento e o desenvolvimento de um circuito equivalente CC. A fotoresposta do GC-LBJT é investigada em duas diferentes configurações, coletor-comum com tensão porta-base constante e emissor-comum com tensão porta-emissor constante. A característica da fotoresposta é associada às equações do dispositivo em ambas as configurações mostrando os principais parâmetros do dispositivo que determinam o ganho. Na configuração coletor-comum, a característica da fotoresposta varia de aproximadamente linear a sublinear por meio da tensão de controle VGB. Na configuração emissor-comum, o dispositivo apresenta fotoresposta sublinear e baixa excursão para toda faixa de tensão de controle (VGB) utilizada. Explorando a característica controlável do GC-LBJT em ambas as configurações, o fototransistor GC-LBJT pode apresentar ganho e responsividade maiores do que 10+6 e 10+4 A/W respectivamente. Propõe-se o método de múltipla-amostragem para sistemas de imagem CMOS no domínio do tempo. O pixel é composto por um comparador e um circuito de memória de um bit. O método de múltipla-amostragem no domínio do tempo permite reduzir o circuito de memória integrado ao pixel de 8 bits tipicamente para um único bit. O resultado da amostra armazenado na memória de um bit no pixel é lida externamente de forma síncrona e o valor do sinal do pixel é codificado de acordo com o instante da amostra no tempo. O número de bits e a velocidade de operação do circuito limitam a dimensão máxima da matriz. Além disso, este trabalho apresenta a influência da não-linearidade da capacitância do fotodiodo na característica da fotoresposta dos sistemas de imagem CMOS no domínio do tempo. Estudo do comportamento do ruído de padrão fixo e o temporal em sistema de imagem no domínio do tempo também são apresentadosAbstract: This thesis adresses two important challenges imposed by CMOS technology trends, the reduction of the junctions's responsivity and voltages levels. A new photodetector with high responsivity and a multi-sampling time domain image system are investigated. This thesis proposes to use the gate controlled lateral bipolar junction transistor (GCLBJT) as a four terminal phototransistor as photodetector with high responsivity. This work presents the photopolarization principle, gain current mechanism of the GC-LBJT in conjuction with DC equivalent circuit development. The GC-LBJT photo response is analysed in two different configurations, common colector with constant gate-base voltage and common emmiter with constant gate-emitter voltage. The photoresponse is related to device equations in both configurations. In the common colector with constant gate-base voltage configuration the photo response characteristic changes from linear to sublinear according to the VGB control voltage. In the common emmiter configuration, the device presents sublinear photo response and small changes for full range of the VGB control voltage used. Exploring the GC-LBJT controllable characteristic, the GC-LBJT phototransistor presents high and controllable gain all over the range of irradiation used, for both configurations. The multi-sampling method for time domain CMOS image systems is proposed. The pixel's architecture is composed by a comparator and a single bit memory circuit. The multisampling method in time-domain allows reducing memory circuits integrated per pixel with eight bit tipically to a single bit. The sample result stored in the single bit memory of the pixel is externally read in a synchronous way and the pixel signal value is coded according to the sampling instant. The number of the bits and the speed of circuit's operation define the upper limit of the matrix size. In addition, this work presents the influence of non-linearity on photoresponse characteristic for systems operating in time domain. The behavior of fixed and temporal pattern noise study in time domain image system is also presentedDoutoradoEletrônica, Microeletrônica e OptoeletrônicaDoutor em Engenharia Elétric

Circuits analogiques de lecture pour un capteur d'images couleur CMOS à photodétecteurs sans filtre

RÉSUMÉ La résolution spatiale des capteurs d’images couleur actuels, qu’ils soient de type CCD (Charge-Coupled Device) ou à base de circuits CMOS (Complementary Metal-Oxide-Semiconductor), est limitée par leur méthode de discrimination des couleurs. En fait, comme la plupart des photodétecteurs ne sont sensibles qu’à l’intensité de la lumière et non pas à sa longueur d’onde, il faut déposer un filtre chromatique à la surface du capteur pour ne transmettre que la longueur d’onde désirée vers chaque photodétecteur. On extrapole par la suite la couleur équivalente d’un regroupement de pixels à l’aide d’algorithmes mathématiques, ce qui restreint l’atteinte d’une résolution spatiale comparable à celle d’une pellicule chimique photosensible de bonne qualité. De surcroît, la déposition du filtre chromatique réduit la sensibilité du capteur en absorbant une partie de l’énergie lumineuse, ce qui impose une restriction supplémentaire au niveau de la gamme dynamique. Dans le but de corriger ces lacunes, l’équipe de recherche du professeur Yves Audet a développé et breveté un photodétecteur innovateur permettant la discrimination des couleurs sans recourir à un filtre chromatique. Réalisable en technologie CMOS standard, ce photodétecteur est un candidat idéal pour l’intégration des circuits périphériques de traitement des signaux et la réalisation d’un circuit intégré de type COC (Camera On Chip). Le présent travail de recherche s’inscrit dans le processus de développement et de caractérisation de ce photodétecteur. Plus spécifiquement, il consiste à concevoir les circuits analogiques de traitement de signal en vue d’obtenir un capteur d’images intégré complet comportant trois sorties analogiques qui correspondent aux couleurs détectées par le photodétecteur. Pour ce faire, une nouvelle topologie de pixel actif APS (Active Pixel Sensor) a été développée à partir des caractéristiques expérimentales du photodétecteur. Par la suite, les circuits analogiques de traitement du signal périphériques ont été conçus afin de compléter le système d’acquisition d’images. Le prototype a été fabriqué en technologie 0.25μm 3.3V et les résultats obtenus témoignent du fonctionnement adéquat des circuits conçus.----------ABSTRACT Many of today’s CCD (Charge-Coupled Device) and CMOS (Complementary Metal-Oxide-Semiconductor) color image sensors exhibit a lower than desirable spatial resolution on behalf of their color detection and extrapolation mechanisms. Indeed, most photodetector units respond to light energies whose wavelengths are within their sensitive range, without actually distinguishing the actual wavelength. Hence, a method for separating colors must be implemented. The most popular of these methods consists in placing a CFA (Color Filter Array) atop the pixel array in order to define the wavelength received by every pixel. As it requires many pixels to determine a single element of color information in the image, the spatial resolution is limited to numbers well below those of a high-quality photo film. Furthermore, optical filtering reduces sensor sensitivity by absorbing part of the energy contained in the incoming light, thus limiting the available dynamic range. In order to alleviate these shortcomings, Professor Yves Audet’s design team has developed and patented an innovative photodetector structure with integrated color detection that doesn’t require the addition of a CFA. Since this photodetector can be fabricated using a standard CMOS technology, signal processing circuits can be implemented on the same chip to produce a COC (Camera On Chip). This work is part of the ongoing development and characterization process of the filterless photodetector. More specifically, it consists of the design of the analog circuits required to achieve a complete integrated image sensor having three analog signal outputs corresponding to the detected colors. Based on previously collected experimental data about the photodetector, an innovative APS (Active Pixel Sensor) architecture was created. Then, the peripheral analog signal processing circuits were designed to complete the analog image capture system. The prototype has been fabricated using 0.25μm 3.3V CMOS technology and experimental results demonstrate the functionality of the circuits