A new CLSP Sensor for Image Recognition and Color Separation
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A new CLSP sensor for Image recognition and color separation M. Vieira, M. Fernandes, A. Fantoni, P. Louro, and R. Schwarz Electronic and Communications Dept., ISEL, Lisbon, Portugal. ABSTRACT Large area p-i-n a-SiC:H heterostructures are used as LSP color sensors. For reading out the color signals, three appropriated voltages have to be successively applied in order to combine afterwards the information to yield a color image. The highly resistive and wide band gap doped layers confine the photogenerated carriers at the illuminated regions and driven by the scanner extract information on the image shape and intensity. The bias voltage controls the potential profile across the main generation region leading to color sensitivity. As the positive applied voltage increases the reversed electrical field in the bulk shifts toward the main generation regions, and successively suppresses the ac component of the photocurrent at each primary color allowing color extraction. The device performance is analyzed and the scanning technique for color separation improved. The influence of the optical and electrical bias on image contrast, resolution and color extraction is discussed. A physical model for image and color recognition is presented and supported by a two dimensional simulation. INTRODUCTION Studies on the use of glass/ITO/p /i /n /metal a-SiC:H structures as Laser Scanned Photodiode (LSP) image sensors have recently shown its potential capability [1, 2] as monochrome image sensing devices. These devices are fundamentally different from the other electrically scanned image sensors [3, 4]. They consist on one single large cell detector or pixel (picture element) and the image is scanned by sequentially detecting scene information at discrete XY coordinates. The read-out of the injected carriers is achieved by measuring the ac component of the current, iac in short circuit mode. The LSP detects an optical image with a spatial resolution of 30 µm in a flux range of two orders of magnitude and present a sensitivity of 6.5 µWcm-2. Advantages to this approach are high resolution, uniformity of measurement along the sensor and the cost/simplicity of the detector. The design allows a continuous sensor without the need for pixel-level patterning, and so can take advantage of the amorphous silicon technology. It can also be integrated into a system where the signal processing can be performed by an ASIC chip underneath. Resolution is not the only factor governing the quality of the images. Equally important is the color. The LSP under short circuit is monochrome in nature. It is not able to determine the varying amounts of red, green and blue (RGB) information presented by the picture. For simultaneous image and color detection the wavelength filtering property of the silicon has to be combined with the sensor responsivity dependence on the applied voltage. This work aims to clarify possible improvements, physical limits and performance of the so-called CLSP (Color Laser Scanned Photodiode) when used as a full color image sensor.
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