Nader Safavian

Dynamic-effect compensating technique for stable a-Si:H AMOLED displays

A reliable driving scheme that can compensate for temporal instability of hydrogenated amorphous silicon (a-Si:H) backplane and OLED degradation is a prerequisite for the operation of a-Si:H AMOLED displays. In particular, for large-area and high-resolution displays, the technique should be accurate and fast. Although several driving schemes have been proposed to compensate for the static effect of the V/sub T/-shift, they are prone to the dynamic effects associated with charge injection and clock feed-through. The simulation results show that the dynamic effects can induce up to 10% error in the pixel current. A new driving scheme that enables control of the static and dynamic effects of the V/sub T/-shift is presented.

Stable a-Si:H circuits based on short-term stress stability of amorphous silicon thin film transistors

Hydrogenated amorphous silicon (a-Si:H) technology is interesting for large-area active matrix structure due to its good uniformity over large-area, low-temperature, and low-cost fabrication, and its industrial accessibility. However, the circuits implemented in this technology suffer from the instability of the material under prolonged bias stress. To improve the circuit stability, we present a circuit design technique based on the stability of a-Si:H thin film transistors (TFTs) under short-term bias stress. Here, an a-Si:H local current source (LCS) is used to adjust the circuit current bias. Since the LCS circuit is under stress for a small fraction of operation time, its current remains stable. The measurement and analysis of the LCS circuit indicate that the a-Si:H TFT is stable under short-term bias stress for over 50000h. Also, we present a pixel circuit based on this technique for active matrix organic light emitting diode displays.

A novel current scaling active pixel sensor with correlated double sampling readout circuit for real time medical x-ray imaging

This paper presents a new current-programmed, current-output active pixel sensor (APS) based on a novel current scaling scheme and suitable for real time X-ray imaging (fluoroscopy) and a current mode CMOS readout circuit. The circuit is designed using hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) technology. The simulation results based on the measured characteristics of a-Si:H TFTs show that the proposed pixel circuit can successfully scale up the current data during readout and therefore provide higher gain compared with the conventional current mode APS circuits. The design can also compensate for characteristic variations (e.g. mobility and threshold voltage shift) in a-Si:H TFTs under prolonged gate voltage stress. The readout circuit exploits correlated double sampling (CDS) technique to reduce the offset current, low frequency noise and fixed-pattern noise (FPN) on the array operation.

Noise performance of high fill factor pixel architectures for robust large-area image sensors using amorphous silicon technology

Large area digital imaging made possible by amorphous silicon thin-film transistor (a-Si TFT) technology, coupled with a-Si photo-sensors, provides an excellent readout platform to form an integrated medical image capture system. Major development challenges evolve around optimization of pixel architecture for detector fill factor, signal propagation performance, and manufacturability, while suppressing noise stemming from pixel array and external electronics. This work analyzes a novel vertically integrated pixel design based on signal readout and noise performance, and compares with conventional co-planar and continuous detector architectures. In addition, the analysis will consider various substrate options including glass and robust substrates such as polymer and metal foil. Our evaluation have demonstrated state-of-the-art radiographic detector system with electronic noise under 2000 electrons at 150 p.s frame time for an imaging arrays on robust substrate.

Photon Counting Pixel and Array in Amorphous Silicon Technology for Large Area Digital Medical Imaging Applications

A single photon counting Voltage Controlled Oscillator (VCO) based pixel architecture in amorphous silicon (a-Si) technology is reported for large area digital medical imaging. The VCO converts X-ray generated input charge into an output oscillating frequency signal. Experimental results for an in-house fabricated VCO circuit in a-Si technology are presented and external readout circuits to extract the image information from the VCOs frequency output are discussed. These readout circuits can be optimized to reduce the fixed pattern noise and fringing effects in an imaging array containing many such VCO pixels. Noise estimations, stability simulations and measurements for the fabricated VCO are presented. The reported architecture is particularly promising for large area photon counting applications (e.g. low dose fluoroscopy, dental computed tomography (CT)) due to its very low input referred electronic noise, high sensitivity and ease of fabrication in low cost a-Si technology.

Modeling and Characterization of the Hydrogenated Amorphous Silicon Metal Insulator Semiconductor Photosensors for Digital Radiography

Because of the inherent desired material and technological attributes such as low temperature deposition and high uniformity over large area, the amorphous silicon (a-Si:H) technology has been extended to digital X-ray diagnostic imaging applications. This paper reports on design, fabrication, and characterization of a MIS-type photosensor that is fully process-compatible with the active matrix a-Si:H TFT backplane. We discuss the device operating principles, along with measurement results of the transient dark current, linearity and spectral response.

Investigation of gain non-uniformities in the two TFT current programmed amorphous silicon active pixel sensor for fluoroscopy, chest radiography, and mammography tomosynthesis applications

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.

Characterization of bias induced metastability of amorphous silicon thin film transistor based passive pixel sensor switch and its impact on biomedical x-ray imaging application

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.

Current mode active pixel sensor architectures for large area digital imaging

The most widely used architecture in large area flat panel imagers is the amorphous silicon passive pixel sensor (PPS), which consists of a detector and a readout switch. While the PPS has the advantage of being compact, reading small PPS output signals requires external column charge amplifiers that produce additional noise and reduce the minimum readable sensor input signal. This work compares the SNR, metastability, area, and off-panel complexity of amorphous silicon active pixel sensor (APS) readout circuits based on three and two transistor current mode and current programmed designs that perform on-pixel amplification of noise-vulnerable sensor input signals to minimize the effect of external readout noise sources associated with ¿off-chip¿ charge amplifiers. The APS circuits presented are poised to replace PPS circuits for fast readout, noise-sensitive, large area, digital medical imaging applications such as dual mode radiography/fluoroscopy and mammography tomosynthesis.

TFT active image sensor with current-mode readout circuit for digital x-ray fluoroscopy

In this paper, an innovative current-programmed, current-output active TFT image sensor suitable for real time x-ray imaging (fluoroscopy) using hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) technology coupled with a transimpedance feedback column amplifier for pixel signal readout is presented. Simulation results show that this new TFT circuit can successfully compensate for variations in a-Si:H TFT characteristics under prolonged gate voltage stress. The readout is fast enough to fulfill the timing requirements of digital fluoroscopy. Dynamic effects such as charge injection, charge feed-through and drain-source voltage variation as well as additive noise of the pixel TFTs induce error on the output current of the pixel. To explore the dependence of this error on pixel parameters, concise analytical expressions are derived which can be used to reduce the amount of the output current error by proper pixel design.