Noise Characterization of Polycrystalline Silicon Thin Film Transistors for X-ray Imagers Based on Active Pixel Architec
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Noise Characterization of Polycrystalline Silicon Thin Film Transistors for X-ray Imagers Based on Active Pixel Architectures L. E. Antonuk, M. Koniczek, J. McDonald, Y. El-Mohri, Q. Zhao, and M. Behravan Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, 48109 ABSTRACT An examination of the noise of polycrystalline silicon thin film transistors, in the context of flat panel x-ray imager development, is reported. The study was conducted in the spirit of exploring how the 1/f, shot and thermal noise components of poly-Si TFTs, determined from current noise power spectral density measurements, as well as through calculation, can be used to assist in the development of imagers incorporating pixel amplification circuits based on such transistors.
INTRODUCTION Active matrix, flat panel imagers (AMFPIs) are based on large area arrays (presently up to ~43×43 cm2) whose pixels typically consist of a single hydrogenated amorphous silicon (aSi:H) thin film transistor (TFT) coupled to a pixel storage capacitor. While such imagers are used in many medical applications, the relatively modest size of the signal generated per detected x-ray by the imager, relative to the electronic (i.e., dark) noise, results in significant loss of imaging performance under conditions of low exposure or very small pixel sizes. To overcome this limitation, a significant increase in signal relative to noise is required [1]. One approach, which preserves the advantages of conventional AMFPIs (e.g., large area, compact), involves the use of polycrystalline silicon (poly-Si) TFTs to create an amplification circuit in every pixel – analogous to the pixel architecture of CMOS active pixel sensors. The considerably higher electron and hole mobilities of poly-Si TFTs, compared to those of a-Si:H TFTs, enable faster switching times and considerably more complex pixel circuits. Initial prototype arrays with 1and 2-stage poly-Si pixel amplifiers have been developed and show encouraging results [2,3]. For such arrays, a critical issue is the degree to which noise properties of the individual poly-Si TFTs in the pixel circuit limit the performance of the entire imager. In order to fully explore this question, a detailed knowledge of the noise behavior of the individual transistors is required. While many studies of poly-Si TFT noise have been reported [4,5], it is highly desirable to acquire noise data from individual TFTs having the same design, and made in the same manner as those used in actual prototype arrays, since the results should be representative of their performance in pixel circuits. This paper describes a methodology for acquiring and utilizing noise data obtained from individual poly-Si TFTs for purposes of imager development. The methodology is illustrated through measurements of current noise power spectral density. This research was conducted in the spirit of a pilot study toward understanding the relationship between the range of conditions over which good quality data can be obtained, and how such informat
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