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Title: Active pixel imagers incorporating pixel-level amplifiers based on polycrystalline-silicon thin-film transistors

Abstract

Active matrix, flat-panel imagers (AMFPIs) employing a 2D matrix of a-Si addressing TFTs have become ubiquitous in many x-ray imaging applications due to their numerous advantages. However, under conditions of low exposures and/or high spatial resolution, their signal-to-noise performance is constrained by the modest system gain relative to the electronic additive noise. In this article, a strategy for overcoming this limitation through the incorporation of in-pixel amplification circuits, referred to as active pixel (AP) architectures, using polycrystalline-silicon (poly-Si) TFTs is reported. Compared to a-Si, poly-Si offers substantially higher mobilities, enabling higher TFT currents and the possibility of sophisticated AP designs based on both n- and p-channel TFTs. Three prototype indirect detection arrays employing poly-Si TFTs and a continuous a-Si photodiode structure were characterized. The prototypes consist of an array (PSI-1) that employs a pixel architecture with a single TFT, as well as two arrays (PSI-2 and PSI-3) that employ AP architectures based on three and five TFTs, respectively. While PSI-1 serves as a reference with a design similar to that of conventional AMFPI arrays, PSI-2 and PSI-3 incorporate additional in-pixel amplification circuitry. Compared to PSI-1, results of x-ray sensitivity demonstrate signal gains of {approx}10.7 and 20.9 for PSI-2 and PSI-3,more » respectively. These values are in reasonable agreement with design expectations, demonstrating that poly-Si AP circuits can be tailored to provide a desired level of signal gain. PSI-2 exhibits the same high levels of charge trapping as those observed for PSI-1 and other conventional arrays employing a continuous photodiode structure. For PSI-3, charge trapping was found to be significantly lower and largely independent of the bias voltage applied across the photodiode. MTF results indicate that the use of a continuous photodiode structure in PSI-1, PSI-2, and PSI-3 results in optical fill factors that are close to unity. In addition, the greater complexity of PSI-2 and PSI-3 pixel circuits, compared to that of PSI-1, has no observable effect on spatial resolution. Both PSI-2 and PSI-3 exhibit high levels of additive noise, resulting in no net improvement in the signal-to-noise performance of these early prototypes compared to conventional AMFPIs. However, faster readout rates, coupled with implementation of multiple sampling protocols allowed by the nondestructive nature of pixel readout, resulted in a significantly lower noise level of {approx}560 e (rms) for PSI-3.« less

Authors:
; ; ; ; ; ;  [1]
  1. Department of Radiation Oncology, University of Michigan Medical Center, Ann Arbor, Michigan 48109 (United States)
Publication Date:
OSTI Identifier:
22100575
Resource Type:
Journal Article
Journal Name:
Medical Physics
Additional Journal Information:
Journal Volume: 36; Journal Issue: 7; Other Information: (c) 2009 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0094-2405
Country of Publication:
United States
Language:
English
Subject:
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; 61 RADIATION PROTECTION AND DOSIMETRY; AMPLIFIERS; DESIGN; PHOTODIODES; POLYCRYSTALS; QUANTUM EFFICIENCY; READOUT SYSTEMS; SEMICONDUCTOR MATERIALS; SIGNALS; SPATIAL RESOLUTION; THIN FILMS; X RADIATION

Citation Formats

El-Mohri, Youcef, Antonuk, Larry E., Koniczek, Martin, Qihua, Zhao, Yixin, Li, Street, Robert A., Jengping, Lu, and Palo Alto Research Center. Active pixel imagers incorporating pixel-level amplifiers based on polycrystalline-silicon thin-film transistors. United States: N. p., 2009. Web. doi:10.1118/1.3116364.
El-Mohri, Youcef, Antonuk, Larry E., Koniczek, Martin, Qihua, Zhao, Yixin, Li, Street, Robert A., Jengping, Lu, & Palo Alto Research Center. Active pixel imagers incorporating pixel-level amplifiers based on polycrystalline-silicon thin-film transistors. United States. doi:10.1118/1.3116364.
El-Mohri, Youcef, Antonuk, Larry E., Koniczek, Martin, Qihua, Zhao, Yixin, Li, Street, Robert A., Jengping, Lu, and Palo Alto Research Center. Wed . "Active pixel imagers incorporating pixel-level amplifiers based on polycrystalline-silicon thin-film transistors". United States. doi:10.1118/1.3116364.
@article{osti_22100575,
title = {Active pixel imagers incorporating pixel-level amplifiers based on polycrystalline-silicon thin-film transistors},
author = {El-Mohri, Youcef and Antonuk, Larry E. and Koniczek, Martin and Qihua, Zhao and Yixin, Li and Street, Robert A. and Jengping, Lu and Palo Alto Research Center},
abstractNote = {Active matrix, flat-panel imagers (AMFPIs) employing a 2D matrix of a-Si addressing TFTs have become ubiquitous in many x-ray imaging applications due to their numerous advantages. However, under conditions of low exposures and/or high spatial resolution, their signal-to-noise performance is constrained by the modest system gain relative to the electronic additive noise. In this article, a strategy for overcoming this limitation through the incorporation of in-pixel amplification circuits, referred to as active pixel (AP) architectures, using polycrystalline-silicon (poly-Si) TFTs is reported. Compared to a-Si, poly-Si offers substantially higher mobilities, enabling higher TFT currents and the possibility of sophisticated AP designs based on both n- and p-channel TFTs. Three prototype indirect detection arrays employing poly-Si TFTs and a continuous a-Si photodiode structure were characterized. The prototypes consist of an array (PSI-1) that employs a pixel architecture with a single TFT, as well as two arrays (PSI-2 and PSI-3) that employ AP architectures based on three and five TFTs, respectively. While PSI-1 serves as a reference with a design similar to that of conventional AMFPI arrays, PSI-2 and PSI-3 incorporate additional in-pixel amplification circuitry. Compared to PSI-1, results of x-ray sensitivity demonstrate signal gains of {approx}10.7 and 20.9 for PSI-2 and PSI-3, respectively. These values are in reasonable agreement with design expectations, demonstrating that poly-Si AP circuits can be tailored to provide a desired level of signal gain. PSI-2 exhibits the same high levels of charge trapping as those observed for PSI-1 and other conventional arrays employing a continuous photodiode structure. For PSI-3, charge trapping was found to be significantly lower and largely independent of the bias voltage applied across the photodiode. MTF results indicate that the use of a continuous photodiode structure in PSI-1, PSI-2, and PSI-3 results in optical fill factors that are close to unity. In addition, the greater complexity of PSI-2 and PSI-3 pixel circuits, compared to that of PSI-1, has no observable effect on spatial resolution. Both PSI-2 and PSI-3 exhibit high levels of additive noise, resulting in no net improvement in the signal-to-noise performance of these early prototypes compared to conventional AMFPIs. However, faster readout rates, coupled with implementation of multiple sampling protocols allowed by the nondestructive nature of pixel readout, resulted in a significantly lower noise level of {approx}560 e (rms) for PSI-3.},
doi = {10.1118/1.3116364},
journal = {Medical Physics},
issn = {0094-2405},
number = 7,
volume = 36,
place = {United States},
year = {2009},
month = {7}
}