Simplified wide dynamic range CMOS image sensor with 3t APS reset-drain actuation

Abstract

An image sensor is an array of small light-sensitive cells called pixel sensors. A pixel, or picture pix element el, is the smallest unit of an image. Therefore a pixel sensor is the smallest cell of an image sensor, which is able to detect a single image dot. Such image dot is then used to reconstruct a complete image frame. Image sensors built in CMOS technology are nowadays largely employed either in professional cameras or in personal mobile devices with embedded cameras. One of the most important features of a good image sensor is the ability to accommodate in the same image regions of high and low luminosity, a feature known as the sensor dynamic range. Many techniques, found in the literature, are able to extend the sensor dynamic range, each with their advantages and disadvantages, either by extending the saturation level or by reducing the noise floor. Non-idealities introduced during fabrication produce unwanted image artifacts known as fixed-pattern noise FPN and are among the main contributors to the noise floor in CMOS image sensors. Techniques applied to reduce FPN in image sensors with large dynamic range are still a challenge, because in applying these techniques other essential features of the sensor are jeopardized, as its processing speed, the reduction of photosensitive area, or the increase in fabrication costs. The main goal of this work is the improvement of CMOS image sensors, operating with wide dynamic range for general practical purposes, through the application of simple circuit control techniques, without interfering in the fabrication process of the imager. In order to achieve this goal, the present work proposes a simple and innovative approach to increase the dynamic range of the basic and most popular pixel sensor architecture currently employed in CMOS image sensors. The proposed idea differs from the current state-of-art in two essential points. First, because it is able to increase the sensor dynamic range without the need of boosting the level of any control signal above that of the supply voltage. Second, because, other than any different techniques found in literature, it is able to reduce FNP in this sort of image sensor by means of a simple control strategy. Boosting the level of some control signals is necessary, in previous techniques found in the literature, in order to avoid the use of more complex pixel-sensor architectures, which reduces the sensor photosensitive area. On the other hand, internal voltage boosting is harmful because it tends to reduce the life time of the imager. The simplified way to reduce FPN in the herein proposed technique is able to keep unaltered important features of the sensor, as those cited above, at no additional fabrication cost.