KEYWORDS: X-rays, Sensors, Imaging systems, Amorphous silicon, Signal to noise ratio, Prototyping, X-ray imaging, Amplifiers, Modulation transfer functions, Active sensors
Active pixel sensor (APS) circuits are an alternate to passive pixel sensor (PPS) technology which, when integrated with
a direct detection amorphous selenium (a-Se) photoconductor, can enable high performance, digital x-ray imaging
applications such as real-time fluoroscopy due to their better
signal-to-noise ratios at low dose. This paper presents
experimental imaging results from a prototype 64×64 APS pixel array fabricated in a-Si technology. The prototype APS
array is coated with a one millimeter thick layer of a-Se and the experimental results are evaluated using a standard
radiography x-ray beam quality RQA5. The APS experimental results are compared with a standard real-time detector
(FPD14) imaging array under the same x-ray beam conditions. In addition, we will theoretically examine the best
achievable performance for our APS array fabricated in
state-of-the-art a-Si technology and compare the results to
state-of-the-art PPS panels for real-time fluoroscopy.
A detailed experimental and theoretical investigation of noise in both current mode and voltage mode amorphous silicon
(a-Si) active pixel sensors (APS) has been performed. Both flicker (1/f) and thermal are considered in this study. The
experimental result in this paper emphasizes the computation of the output noise variance. The theoretical analysis shows
that the voltage mode APS has an advantage over the current mode APS in terms of the flicker noise due to the operation
of the readout process. The experimental data are compared to the theoretical analysis and are in good agreement.
A 2-TFT current-programmed, current-output active pixel sensor in amorphous silicon (a-Si:H) technology is introduced
for digital X-ray imaging, and in particular, for mammography tomosynthesis and fluoroscopy. Pixel structure, operation
and characteristics are presented. The proposed APS circuit was fabricated and assembled using an in-house bottom gate inverted staggered a-Si:H thin film transistor (TFT) process. Lifetime, transient performance as well as sensitivity to temperature measurements were carried out. An off-panel current amplifier with double sampling capability required for 1/f noise reduction is proposed and implemented in CMOS 0.18 micron technology. The results are promising and demonstrate that the proposed APS compensates for electrical and thermal stress causing shift in the threshold voltage of a-Si TFTs.
Active Matrix Flat Panel Imagers (AMFPIs) based on amorphous silicon (a-Si:H) thin film transistor (TFT) array is the
most promising technology for large area biomedical x-ray imaging.
a-Si:H TFT exhibits a metastable shift in its
characteristics when subject to prolonged gate bias that results in a change in its threshold voltage (VΤ) and a
corresponding change in ON resistance (RON). If not properly accounted for, the VΤ shift can be a major constraint in
imaging applications as it contributes to the fixed pattern noise in the imager. In this work, we investigated the timedependent
shift in VΤ (ΔVΤ) of a-Si:H TFTs stressed with the same bipolar pulsed bias used for static (chest radiography,
mammography, and static protein crystallography) and real time imaging (low dose fluoroscopy at 15, 30 and 60
frames/second, and dynamic protein crystallography). We used the well known power law model of time dependent ΔVT
to estimate the change in RON over time. Our calculation showed that RON can be decreased ~ 0.03 % per frame and ~ 5
% over 10,000 hours at 30 frames/second. We verified the theoretical results with measurement data. The implication of
TFT metastability on the performance (NPS, and DQE) of biomedical imagers is discussed.
KEYWORDS: Computer programming, X-rays, Sensors, Amorphous silicon, Imaging systems, Capacitors, Capacitance, Active sensors, Transistors, Signal to noise ratio
A dual mode current-programmed, current-output active pixel sensor (DCAPS) in amorphous silicon (a-Si:H) technology
is introduced for digital X-ray imaging, and in particular, for hybrid fluoroscopic and radiographic imagers. Here, each
pixel includes an extra capacitor that selectively is coupled to the pixel capacitance to realize the dual mode behavior.
Pixel structure, operation and characteristics are presented. The proposed DCAPS circuit was fabricated and assembled
using an in-house bottom gate inverted staggered a-Si:H thin film transistor (TFT) process. Gain, lifetime, transient
performance as well as noise analysis were carried out. The results are promising and demonstrate that the DCAPS
enables dual mode X-ray imaging while compensating for the long term electrical and thermal stress related a-Si TFT
threshold voltage (Vt) shift.
KEYWORDS: Amplifiers, Transistors, Capacitance, Sensors, Active sensors, X-rays, Fluoroscopy, Amorphous silicon, Signal to noise ratio, Imaging systems
In this paper, we derive the input referred noise in terms of the on-pixel transistor device dimensions of the main noise
sources of our array, namely, the flicker noise of the pixel thin-film transistors (TFTs), and the reset noise. Theoretical
calculations and simulation results show that it is desirable to minimize the amplifier TFT gate dimensions, L1 and W1,
and to maximize the read-out TFT gate width, W2. Noise curves are presented as a function of transistor dimensions,
allowing the designer to choose appropriate device dimensions when designing flat-panel imaging circuits. In addition, it
is demonstrated how the optimal amplifier TFT gate width, W1, for the lowest-noise design, changes as a function of the
extraneous sense node capacitance. The noise simulations indicate that with proper device dimension design, it is
possible to achieve sub-500 electron input referred noise performance.
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 element and a readout switch. While the PPS has the advantage of being compact and amenable toward high-resolution imaging, reading small PPS output signals requires external column charge amplifiers that produce additional noise and reduce the minimum readable sensor input signal. In contrast, on-pixel amplifiers in a-Si technology reduce readout noise by decoupling off-pixel noise sources, such as external charge amplifier and data line noise, from the sensor input. The off-pixel noise is reduced by the charge gain of the pixel amplifier, allowing for low-noise performance. Theoretical calculations and simulations of gain, linearity, metastability, pixel area requirements and noise indicate the applicability of the amplified a-Si pixel architectures for low-exposure, real-time fluoroscopy. In addition, the detailed noise results allow for the computation of noise performance as a function of transistor dimensions for both amorphous silicon and polysilicon technologies, allowing the designer to choose appropriate device dimensions when designing flat-panel imaging circuits.
KEYWORDS: Transistors, Signal to noise ratio, Amplifiers, Photodiodes, Sensors, Interference (communication), Active sensors, Imaging systems, Resistors, Standards development
As the sizes of imaging arrays become larger both in pixel count and area, the possibility of pixel defects increases during manufacturing and packaging, and over the lifetime of the sensor. To correct for these possible pixel defects, a Fault Tolerant Active Pixel Sensor (FTAPS) with redundancy at the pixel level has been designed and fabricated with only a small cost in area. The noise of the standard Active Pixel Sensor (APS) and FTAPS, under normal operating conditions as well as under the presence of optically stuck high and low faults, is analyzed and compared. The analysis shows that under typical illumination conditions the total noise of both the standard APS and FTAPS is dominated by the photocurrent shot noise. In the worst case (no illumination) the total mean squared noise of the FTAPS is only 15.5% larger than for the standard APS, while under typical illumination conditions the FTAPS noise increases by less than 0.1%. In the presence of half stuck faults the noise of the FTAPS compared to the standard APS stays the same as for the FTAPS without defects. However, simulation and experimental results have shown that the FTAPS sensitivity is greater than two times that of the standard APS leading to an increased SNR by more than twice for the FTAPS with no defects. Moreover, the SNR of a faulty standard APS is zero whereas the SNR of the FTAPS is reduced by less than half.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.