Active Pixel TFT Arrays for Digital Fluoroscopy in a-Si:H Technology
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Active Pixel TFT Arrays for Digital Fluoroscopy in a-Si:H Technology Jackson Lai, Nader Safavian, Arokia Nathan, and John A. Rowlands* Electrical and Computer Engineering Department, University of Waterloo, Waterloo, Ontario, Canada. * Imaging Research Program, Sunnybrook Health Science Centre, Toronto, Ontario, Canada. ABSTRACT Major development challenges in application of hydrogenated amorphous silicon (a-Si:H) technology to large area digital X-ray imaging and technological attributes such as low temperature deposition and high uniformity over large area evolve around dynamic imaging modalities such as fluoroscopy, which demands both high speed readout and signal amplification capabilities in addition to long term device stability. This work reports on initial results of a variety of TFT active pixel sensor (APS) structures in a-Si:H technology, each demonstrating unique capabilities such as enhancements in signal gain, TFT threshold voltage immunity, and real-time high speed readout. INTRODUCTION The use of a-Si:H thin film transistors (TFTs) in pixelated arrays has been extended from its role as switching devices, commonly used in liquid crystal displays, to on-pixel amplifiers that forms the basis of active pixel sensors (APS) [1] [2]. While the benefits of such APS circuit are evident, digital fluoroscopic imaging demands higher on-pixel gain, to ensure the continual improvement in X-ray safety standards, with real-time (30 frames per second) readout. This paper presents a 4-TFT pixel architecture that features on-pixel voltage amplification. This proposed design is capable of achieving a six-fold increase in signal voltage gain via the subthreshold operation of a global shutter TFT prior to any on-pixel amplifier. From a device reliability standpoint, the a-Si:H material is prone to device parameter degradation, in particular, threshold voltage shift (∆VT) upon prolonged gate voltage stress [3] [4]. Previously reported APS designs alleviate this instability issue by minimizing the duty cycle of gate pulses. However, such a method cannot mitigate long term pixel output degradation due to the fixed sampling time at the external sample-and-hold circuit [1]. Immunity to TFT VT shift enables higher device reliability and is accomplished through a two-step reset operation of a 5TFT APS circuit design. Together with the on-pixel current amplification, this APS circuit can deliver performance in terms of both signal gain and readout rate entailed by fluoroscopic imaging application. 4-TFT APS with global shutter Figure 1 shows the schematic of the APS circuit with global shutter TFT that decouples the storage and sensing capacitances [5]. The AMP TFT forms a source follower amplifier circuit with the READ TFT serving as the load when it is turned ON. Unlike previously reported APS designs, in which both charge storage and signal sensing are performed through a single nodal capacitance, this design performs each operation with a dedicated capacitor and the analogue biasing of the SHUTTER TFT. The storage ca
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