Two-Dimensional Amorphous Silicon Color Sensor Array
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TWO-DIMENSIONAL AMORPHOUS SILICON COLOR SENSOR ARRAY F. Lemmi*, M. Mulato, J. Ho, R. Lau, J. P. Lu and R. A. Street Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA, 94304 *present address: University of Rome “La Sapienza”, Roma - Italy
ABSTRACT This paper reports on the first full realization and characterization of a two-dimensional array of amorphous silicon (a-Si:H) color sensors, addressed by integrated amorphous siliconbased thin-film transistors (TFTs). The array includes 512 x 512 pixels with 75-µm pitch, or about 340 dpi. Each pixel features a color sensor realized by a p-i-n-i-p stack of doped and undoped a-Si:H layers, and the TFT. The color sensors are made of two back-to-back p-i-n diodes, which selectively sense the illumination according to the polarity of the applied bias voltage. The sensor layers are grown on top of the TFTs to improve the array fill factor. The p-in-i-p sensor stack is mesa-isolated into single sensors to reduce cross-talk. Images are acquired using two bias voltages and yield the red and blue/green components of the original with a good color separation. A color image is reconstructed using the information from the two images acquired. Aside from a color bias, which is expected for a two-color reconstruction, the imaging system works well. In particular, the array shows very low leakage currents, which enable a very large dynamic range and sensitivity. In the response of the array to a light pulse, the bottom thick diode ensures a fast drop in the signal after the flash, while the top thin diode exhibits some residual image lag. INTRODUCTION Amorphous silicon (a-Si:H) is studied for a variety of applications, including x-ray medical imaging systems and flat-panel document scanning [1, 2], solar cells [3], etc. This paper reports about the first full nitride or oxynitride p- or n- doped a-Si:H realization and characterization of a twometal intrinsic a-Si:H dimensional, 512 x 512 element array of aITO BCB Si:H color sensors. The sensors chosen for the array pixels are two-color detectors based on a p-i-n-i-p layer sequence [4]. The p i choice of a two-band sensitive device does n not allow a full color image, but enables the i p study of the active-matrix approach with multi-junction sensors, and ultimately the analysis of most issues regarding the use of a three-color detector, keeping the complexity to a lower level. The pixel pitch of the imager is 75 µm, and each pixel includes both a thin-film transistor (TFT) as the switching element, and a p-i-n-i-p sensor. A cross-section of the pixel is shown in Fig. 1, and more detail about the fabrication is available elsewhere. [5]. A passivation layer of oxynitride separates the sensor layers from the inverted-staggered TFT and a 2.3 µm Fig. 1. Schematic cross-section of a pixel of the color-sensor array.
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thick BCB [6] coating isolates the mesa-structured sensors and holds a common transparent indium tin oxide (ITO) bias plane for the whole array. EXPERIMENTAL RESULTS AND DISCUSSION Steady State Respons
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