Addressable Photosensing Elements for 2-Dimensional Image Sensors Using a-Si Alloy P-I-N Diodes
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ADDRESSABLE PHOTOSENSING ELEMENTS FOR 2-DIMENSIONAL IMAGE SENSORS USING a-Si ALLOY P-I-N DIODES
G. DE CESARE, P. DI ROSA, S. LA MONICA, R.SALOTTI, R. RITA, L. SCHIRONE Universith di Roma, Dip. Ing. Elettronica, Via Eudossiana 18, 00184 Roma, Italy
ABSTRACT We describe amorphous silicon alloy addressable photo-sensitive elements whose operation has been experimentally verified in 2-dimensional image sensors. The investigated structures consist of p-i-n photodiodes (PD) stacked and back-to-back connected to n-i-p blocking diodes (BD). A first developed device is based on a TCO-p+(Si)-i(Si)-n+(Si)i(SiC)-p+(SiC)-Metal structure. Due to an optimization of the thickness and of the energy gap of the layers, we obtained a rectification ratio between the current levels in forward and 4 reverse bias conditions If/Ir=10 , under AM1.5 illumination, filtered at 500 nm. A better rectification ratio If/Ir>10 6 , under white AM1.5 light, has been achieved by introducing a metallic film inside the n+ layer, for light shielding; this further technological step allows both the independent optimization of BD's and PD's structures, and the increase of the operation speed. INTRODUCTION Large area image sensors market is expanding in graphics and communications applications [1]. Hydrogenated amorphous silicon (a-Si:H) fits the requirements for the material to be used in the imaging surfaces [2], as it can be deposited over large areas by PECVD: this is a low temperature process, producing a rather uniform thin film, which can be patterned by well developed photolithography techniques. The mechanically scanning linear image sensors will be replaced by 2-D imaging surfaces obtaining very compact, reliable and low cost facsimile or image scanner equipment [3, 5]. Furthermore, the prospects of large area 2-dimensional detector devices appear attractive in the field of medical X-ray imaging [4]. The imaging surfaces are divided into individual pixels, which are addressed via analogical switches connecting the photo-sensors to output line: devices have been developed both using TFTs as integrated switching elements [3,5], and using blocking diodes [6]. In this work we investigate diode-based pixel structures, using p-i-n photodiodes (PD), stacked and back-to-back connected to n-i-p blocking diodes (BD). When a BD is forward-biased the signal photogenerated by the corresponding PD can flow across the data line; Structure #1 'M eae etal.a.. ....... .* ... . . p:Lty'pe a-Si .i-.ty~pe. a.-S8. . .
Structure #2 Structur ' ":" . . .. .Metal. a . . *.. . .............. M.etal.'.. . .2 .. p-type a-SiC .... p-type a-Si ii-type, a.-S'.i C. . . i-.type. a-Si . . . . . - - - - - - n-type aa-S- i . . . . . . . ...
n-type a-Si . .. . .. ..
n-type a-Si Metal . . . . . . . . . .n.-type a-Si i-type a-Si
i-type a-Si
. .
p-ty.pe*a-Si. ....
p-ty'pe a.-Si ...
Gl ass1i. . . . s. .. Fig.
Glass . . .. ati c. . . . e.
Glass . . . ... .ucu
i-type a-Si p-type a-Si
............
Fig. I Pixelschematic structure
Mat. Res. Soc. Symp. Proc. Vol. 297. c1993 Mate
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