200 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 1, JANUARY 2003 Amorphous Silicon Active Pixel Sensor Readout Circuit for Digital Imaging Karim S. Karim, Member, IEEE, Arokia Nathan, Senior Member, IEEE, and John Alan Rowlands Abstract—The most widely used architecture in large-area amorphous silicon (a-Si) flat panel imagers is a passive pixel sensor (PPS), which consists of a detector and a readout switch. While the PPS has the advantage of being compact and amenable toward high-resolution imaging, reading small PPS output sig- nals requires external column charge amplifiers that produce additional noise and reduce the minimum readable sensor input signal. This work presents a current-mediated amorphous silicon active pixel readout circuit 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. Results indicate excellent small-signal linearity along with a high, and programmable, charge gain. In addition, the active pixel circuit shows immunity to shift in threshold voltage that is characteristic of a-Si devices. Preliminary circuit noise results and analysis appear promising for its use in noise-sensitive, large-area, medical diagnostic imaging applications such as digital fluoroscopy. Index Terms—Active pixel sensor (APS), amorphous silicon (a-Si), digital fluoroscopy, integrated pixel amplifier, medical imaging. I. INTRODUCTION A MORPHOUS silicon (a-Si) active matrix flat panel imagers (AMFPIs) have gained considerable significance in large-area flat panel digital imaging applications [1], in view of their large-area readout capability. The pixel, forming the fundamental unit of the imager, consists of a detector and readout circuit to efficiently transfer the collected electrons to external readout electronics for data acquisition. The pixel architecture most widely used is based on the passive pixel sensor (PPS) [1]. An example is the amorphous selenium (a-Se)-based photoconductor detection scheme where the readout circuit consists of a storage capacitor and a thin-film transistor (TFT) readout switch [2]. The storage capacitor accumulates signal charge during the integration period and transfers the collected charge to an external charge amplifier via the TFT switch during readout. While the PPS architecture has the advantage of being compact and thus amenable to high-resolution imaging, reading the small output signal of Manuscript received April 9, 2002; revised September 4, 2002. This work was supported by the DALSA/NSERC Industrial Research Chair Program, Commu- nications and Information Technology Ontario, and the Natural Sciences and Engineering Research Council of Canada. The review of this paper was arranged by Editor E. Fossum. K. S. Karim and A. Nathan are with the Department of Electrical and Com- puter Engineering, University of Waterloo, Waterloo, ON, N2L 3G1, Canada. J. A. Rowlands is with the Department of Medical Biophysics, University of Toronto Sunnybrook and Women’s College Health Sciences Centre, Toronto, ON, M4N 3M5, Canada. Digital Object Identifier 10.1109/TED.2002.806968 the PPS for low-input-signal, large-area applications (e.g., fluoroscopy [2]) is extremely challenging and requires costly, high-performance, and sometimes custom-made charge am- plifiers [3]. More importantly, if external noise sources (e.g., charge amplifier noise) are comparable to the input, there is a significant reduction in pixel dynamic range. This paper reports an integrated pixel amplifier circuit using a-Si TFTs based on the CMOS active pixel sensor (APS) technology [4]. The APS performs in situ signal amplification providing higher immu- nity to external noise, hence preserving the dynamic range. The current-mediated APS readout circuit was previously presented as a short note [5]. This paper constitutes a comprehensive version providing additional details on its operation, design, and performance pertinent to gain, noise, and a-Si metastability. II. OPERATION Unlike a conventional PPS, which has one TFT switch, there are three TFTs in the APS pixel architecture. This could under- mine the fill factor if conventional methods of placing the sensor and TFTs are used [3]. Therefore, in an effort to optimize the fill factor, the TFTs may be embedded underneath the sensor to pro- vide high-fill-factor imaging systems [1], [2]. Central to the APS illustrated in Fig. 1 is a source-follower circuit, which produces a current output (C-APS) to drive an external charge amplifier. Here, the APS array architecture is assumed to be column-parallel, i.e., one charge amplifier per column so that an entire row can be read out simultaneously. The C-APS operates in three modes. • Reset mode: the RESET TFT switch is pulsed ON and charges up to through the TFTs ON-resistance. is usually dominated by the detector (e.g., a-Se photoconductor [2] or a-Si photodiode [3] detection layer) capacitance. • Integration mode: after reset, the RESET and READ TFT switches are turned OFF. During the integration period , the input signal generates photocarriers discharging by and decreases the potential on by a small-signal voltage . • Readout mode: after integration, the READ TFT switch is turned ON for a sampling time , which connects the APS pixel to the charge amplifier and an output voltage is developed across proportional to . Operating the READ and RESET TFTs in the linear region re- duces the effect of inter-pixel threshold voltage nonuni- formities. Although the saturated AMP TFT causes the C-APS to suffer from FPN, using CMOS-like off-chip double sampling 0018-9383/03$17.00 © 2003 IEEE