CMOS camera employing a double junction active pixel

Solid-state image sensors usually employ a checker-board array of red, green and blue (RGB) filters to perform colour imaging. Due to this, only one third of the incident light gets converted into output signal. Additionally, in the most commonly used arrangement, twice as many pixels are allocated to sense green compared to red or blue, producing an uneven sampling of the three colour signals throughout the image. This results in more severe aliasing in the red and blue channels relative to green. This thesis investigates the colour imaging performance that can be obtained by combining the inherent spectral dependence of light absorption depth in silicon with a suitable colour filter array. This colour filter array can contain less colours of wider bandwidth, which makes more efficient use of the illumination. A novel active pixel is developed which contains two photo-junctions stacked vertically in the silicon. Device simulation of the spectral response, along with quantum efficiency measurement, is used to assess the raw spectral selectivity of the double junction structure in two standard CMOS technologies. The response with various colour filter combinations is also presented showing that colour imaging with only two filters should be possible. Active pixel circuitry suitable for such a photo-diode structure is discussed, and the calculated noise performance for the double junction pixel is compared with the standard 3-transistor active pixel. This analysis shows that comparable performance can be achieved using the new pixel type. The design of a simple analogue output 184 x 154 pixel image sensor using the double junction pixel is described. A full characterisation of the performance of the sensor is presented and results compared with a standard sensor. In order to complete a camera system, the processing needed to obtain colour images is implemented in software. Example images from the new camera are given and the colorimetric accuracy of the sensor with different filter combinations is discussed. Results show that the colour reproduction, though acceptable, is not yet as accurate as with a standard RGB sensor. Test patterns are used to examine the aliasing artifacts of the camera. Again, comparison with the standard approach is used to highlight the relative strengths and weaknesses. Finally, a critical discussion of the double junction approach is given. Suggestions for improvements to the techniques used in this work are made, along with ideas for future research that depend on predicted developments in image sensor technology. Declaration of originality I hereby declare that the research recorded in this thesis and the thesis itself was composed and originated entirely by myself in the Department of Electronics and Electrical Engineering at The University of Edinburgh, and at STMicroelectronics Imaging Division, Edinburgh. Other people who have contributed significantly to this work are acknowledged at the appropriate places in the thesis text.