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Proceedings Paper

Performance analysis of a 127-micron pixel large-area TFT/photodiode array with boosted fill factor
Author(s): Richard L. Weisfield; William Yao; Tycho Speaker; Kungang Zhou; Richard E. Colbeth; Cesar Proano
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Paper Abstract

Sensor fill factor is one of the key pixel design requirements for high performance imaging arrays. In our conventional imaging pixel architecture with a TFT and a photodiode deposited in the same plane, the maximum area that the photodiode can occupy is limited by the size of the TFT and the surrounding metal lines. A full fill factor array design was previously proposed using a continuous sensor layer1. Despite the benefits of 100% fill factor, when applied to large-area applications, this array design suffers from high parasitic line capacitances and, thus, high line noise. We have designed and fabricated an alternative pixel structure in which the photodiode is deposited and patterned over the TFT, but does not overlap with the lines underneath. Separating the diode from the TFT plane allows extra space for an additional TFT which can be used for pixel reset and clipping excessive charge in the photodiode developed under high illumination. This reduces memory effect by 250%. The yield and the reliability are expected to improve as well since the TFTs and lines are buried underneath the diode. With the increased fill factor, we collect 56% more electrons per pixel, thereby improving the signal to noise ratio. The maximum signal to noise ratio is achieved when the increased signal and the undesirable parasitic capacitance on the data line are best optimized. Linearity, sensitivity, leakage, and MTF characteristics of a prototype X-ray imager based on this architecture are presented.

Paper Details

Date Published: 6 May 2004
PDF: 11 pages
Proc. SPIE 5368, Medical Imaging 2004: Physics of Medical Imaging, (6 May 2004); doi: 10.1117/12.535896
Show Author Affiliations
Richard L. Weisfield, dpiX, LLC (United States)
William Yao, dpiX, LLC (United States)
Tycho Speaker, dpiX, LLC (United States)
Kungang Zhou, dpiX, LLC (United States)
Richard E. Colbeth, Varian Medical Systems, Inc. (United States)
Cesar Proano, Varian Medical Systems, Inc. (United States)


Published in SPIE Proceedings Vol. 5368:
Medical Imaging 2004: Physics of Medical Imaging
Martin J. Yaffe; Michael J. Flynn, Editor(s)

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