Image and color sensitive detector based on double p-i-n/p-i-n a-SiC:H photodiode
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A13.4.1
Image and color sensitive detector based on double p-i-n/p-i-n a-SiC:H photodiode M. Vieira1, M. Fernandes1, P. Louro1, A. Fantoni1, Y. Vygranenko1,G. Lavareda2, C.N. Carvalho2. 1 Electronics Telecommunication and Computer Dept. ISEL, R.Conselheiro EmÃdio Navarro, 1949-014 Lisboa, Portugal Tel: +351 21 8317290, Fax: +351 21 8317114, [email protected] ; 2 CFM-UTL-Av. Rovisco Pais, Lisbon, Lisbon, Portugal, [email protected] .
ABSTRACT A two terminal image and color sensitive detector based on two stacked sensing/switching p-i-n a-SiC:H diodes is presented. The imaging is performed in a write-read simultaneous process: the write exposure, which converts the optical image into a localized packet of charges and the readout which performs the charge to current conversion by detecting the photocurrent generated by a light beam scanner. By sampling the absorption region at appropriated voltages it is possible to extract separately the RGB integrated information with a good rejection ratio. Readout of 1000 lines per second is achieved allowing continuous and fast color recognition and image detection. A trade-off between the sensing and the switching diodes thicknesses is established in order to enhance the green recognition. When it is used a thin a-SiC:H, optimized for red transmittance and blue collection, and a switching absorber thick enough to absorb most of the incoming green light ,the detector behaves itself as a filter giving information about the wavelength and the position of the optical image. INTRODUCTION Amorphous silicon technology is attractive for large area image detection since the materials can be deposited for large area substrates by glow discharge. Color sensitivity in crystalline silicon video cameras is obtained by using different detection channels with three CCD arrays, or by using a mosaic of filters deposited directly onto one single solid state sensor [1, 2]. This method can also be used for large area amorphous silicon sensor arrays and requires three sensors for each pixel. Several attempts to achieve structures capable of modifying their sensitivity spectrum by simply changing applied bias have been reported in the literature [3, 4, 5, 6]. Those approaches simplify the interconnections as only two terminals are necessary. In our group large area hydrogenated amorphous silicon single and stacked p-i-n structures with low conductivity doped layers were proposed as Laser Scanned Photodiode (LSP) image sensors [7, 8, 9]. These sensors are different from the other electrically scanned image sensors as they are based on only one sensing element with an opto-mechanical readout system. No pixel architecture is needed. Advantages to this approach are large area imaging, high resolution, and uniformity of measurement along the sensor. Through the tailoring of the sensor configuration (thickness; absorption coefficient of the absorbers layers) and readout parameters (applied bias) this work evaluates the possibility of using two stacked sensing/switching p-i-n photodiodes as a high-sensiti
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