Amorphous Silicon Active Pixel Sensor Readout Circuit Architectures for Medical Imaging
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Amorphous Silicon Active Pixel Sensor Readout Circuit Architectures for Medical Imaging K.S. Karim1, A. Nathan1, and J.A. Rowlands Sunnybrook and Women’s College Health Science Center, Toronto, ON M4N 3M5 Canada. 1 Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON N2L 3G1 Canada. ABSTRACT Results from previous work indicate the feasibility of the amorphous silicon (a-Si) active pixel sensor (APS) readout circuit that performs on-pixel amplification for low-noise, real time imaging applications (e.g. fluoroscopy). In this paper, the noise contribution of the APS readout circuit is examined. In addition, due to the metastable nature of a-Si, the APS stability is discussed. Unlike conventional PPS systems with one thin film transistor (TFT) switch per pixel, the APS has three TFTs per pixel. The large number and sizes of a-Si TFTs can reduce the pixel fill factor if the TFTs are not embedded beneath the sensor as in continuous layer sensor architectures. In this paper, we present preliminary APS noise and stability measurements along with considerations of fully overlapped pixel architectures on APS performance. INTRODUCTION Large area amorphous silicon (a-Si) active matrix flat-panel imagers have gained considerable significance in digital diagnostic medical imaging applications. The most widely used imager architecture is the passive pixel sensor (PPS), which consists of a detector and a readout switch. While the PPS has the advantage of being compact and amenable towards high-resolution imaging, reading the small PPS output signals requires external circuitry such as column charge amplifiers that produce additional noise and reduce the minimum readable sensor input signal and hence dynamic range. This work presents a current mode amorphous silicon active pixel that performs on-pixel amplification of noise-vulnerable sensor input signals to minimize the effect of external readout noise sources associated with “off-chip” charge amplifiers. Preliminary results, reported in [1], were encouraging and indicate a programmable charge gain, which is promising for diagnostic medical imaging applications. This work examines the noise performance of the APS for continuous time and sampled operation. The latter is particularly useful for prediction of noise performance in medical imaging. In addition, the inherent metastability of a-Si TFTs, which manifests itself mainly as threshold voltage shifts and subthreshold slope degradation, causes the APS gain to decrease over time. Thus, an examination of the effects of metastability on the APS charge gain was performed. Lastly, the feasibility of a continuous layer architecture for APS pixels is examined. APS NOISE Current-noise-power spectrum (SID in A2/Hz) measurements were previously presented [2] for a-Si TFTs fabricated at the University of Waterloo using an in-house fully wet-etch 260oC process. The results of flicker noise current PSD measurements were consistent with previous findings [3][4]. Specifically, it appeared that the fabricated
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