Two terminal large area single and double p-i-n devices for image and color recognition
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Two terminal large area single and double p-i-n devices for image and color recognition 1
P. Louro, 1M. Vieira, 1A. Fantoni, 1M. Fernandes, 2C.. Nunes de Carvalho, 2G. Lavareda Electronics Telecommunications and Computer Dept., ISEL, Lisbon, Portugal. CFM, Complexo I, IST, Lisbon, Portugal ABSTRACT Large area hydrogenated amorphous silicon single and stacked p-i-n structures with low conductivity doped layers are proposed as monochrome and color image sensors. All have the same intrinsic layer and in the doped layers the resistivity and optical gap was controlled through the addition of methane to the doping gas. The current-voltage characteristics and the spectral sensitivity under different illumination conditions are analyzed. The sensor output characteristics are evaluated under different read-out voltages and scanner wavelengths. To extract information on image range, intensity and color, a modulated light beam scans the sensor active area at three appropriate bias voltages and the photoresponse in each scanning position (“sub-pixel”) is recorded. The investigation of the sensor output under different scanner wavelengths and varying electrical bias reveals that the response can be tuned, thus enabling color separation. The operation of the sensor is exemplified and supported by a numerical simulation. INTRODUCTION
Image sensing device is an apparatus for transforming light image into a sequential electronic signal. They are usually based on arrays of sensing elements. They capture light on a grid of small pixels on their surfaces and image sensing is performed using two basic techniques: line scanning and area scanning [1, 2]. Resolution is not the only factor governing the quality of the images. Equally important is the color. Silicon based devices are monochrome in nature. Color image processing is usually performed with the aid of three-color sequential filters or integral color filter arrays overlaying the devices. In this respect, voltage bias controlled color detection by two-terminal thin film devices is very interesting since one in-stead of three or four pixels will be sufficient for color discrimination, there is a remarkable gain in spatial resolution, and color filters as a major cost driver are no longer needed. Various structures and sequences have been suggested [3, 4, 5, 6]. In our group efforts have been devoted towards the development of a new kind of image sensor [7, 8]. This sensor consists on one large cell detector or pixel (picture element) where the optical 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. 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 intends to evaluate the possibility of color selectivity in single and stacked large area p-i-n cells by controlling parameters like scanner wavelength an
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