Afrin Sultana

Threshold voltage instability of amorphous silicon thin-film transistors under constant current stress

We investigate the time-dependent shift in the threshold voltage of amorphous silicon thin-film transistor stressed with constant drain current. We observe a nonsaturating power-law time dependence, which is in contrast to the conventional stretched exponential that saturates at prolonged stress time. The result is consistent with the carrier-induced defect creation model and corroborates the nonlinear dependence of the rate of defect creation on the band-tail carrier density.

Design and feasibility of active matrix flat panel detector using avalanche amorphous selenium for protein crystallography

Protein crystallography is the most important technique for resolving the three-dimensional atomic structure of protein by measuring the intensity of its x-ray diffraction pattern. This work proposes a large area flat panel detector for protein crystallography based on direct conversion x-ray detection technique using avalanche amorphous selenium (a-Se) as the high gain photoconductor, and active matrix readout using amorphous silicon (a-Si:H) thin film transistors. The detector employs avalanche multiplication phenomenon of a-Se to make the detector sensitive to each incident x ray. The advantages of the proposed detector over the existing imaging plate and charge coupled device detectors are large area, high dynamic range coupled to single x-ray detection capability, fast readout, high spatial resolution, and inexpensive manufacturing process. The optimal detector design parameters (such as detector size, pixel size, and thickness of a-Se layer), and operating parameters (such as electric field across the a-Se layer) are determined based on the requirements for protein crystallography application. The performance of the detector is evaluated in terms of readout time (<1 s), dynamic range (approximately 10(5)), and sensitivity (approximately 1 x-ray photon), thus validating the detectors efficacy for protein crystallography.

Digital X-Ray Imaging Using Avalanche a-Se Photoconductor

This work presents the development of a novel detector comprised of an avalanche amorphous selenium (a-Se) photoconductor and an amorphous silicon (a-Si:H) passive pixel sensor for digital X-ray imaging, in particular, for low exposure imaging applications. Electrical compatibility of a high voltage (~1000 V) avalanche a-Se photoconductor with a low voltage (~25 V) a-Si:H pixel sensor is demonstrated. Single pixel readout is done as a proof of concept.

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.

Large area direct X-ray conversion detector for protein crystallography

This work proposes a large area detector for protein crystallography based on an amorphous silicon (a-Si:H) thin film transistor (TFT) pixel array backplane and an overlying amorphous selenium (a-Se) photoconductor for direct conversion of incident X-ray into image charge. To achieve high sensitivity, avalanche multiplication in a-Se is adopted to make the detector sensitive to each incident X-ray. The use of a-Si:H technology promises large area imaging of protein diffraction patterns at less expense compared to existing CCD and image plate detectors. In addition, a theoretical analysis shows that the detector exhibits fast readout speed (readout time < 1 s), high dynamic range (~106), and high sensitivity (~1 X-ray photon), thus validating the detectorpsilas use in protein crystallography.

Current stress metastability in a-Si:H thin film transistors

In this paper, we investigate the threshold voltage (VT) instability in a-Si:H TFTs subject to constant current stress. The gate voltage under such conditions continuously adjusts to keep the drain current constant. As such, existing voltage stress models fail to predict the resulting VT-shift. We propose a physically based model to predict VT-shift under current stress. The model follows a power law assuming that the VT-shift under moderate current stress is due to defect state creation in a-Si:H bulk and interfaces. Good agreement between simulation results and experimental data is obtained for various levels (2μA-15μA) of stress current at both room and elevated (75°C) temperatures.

Characterization of Current Programmed Amorphous Silicon Active Pixel Sensor Readout Circuit for Dual Mode Diagnostic Digital X-ray Imaging

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.

Effect of anomalous scattering and K-fluroscence reabsorption on the performance of selenium

Selenium (Se) has been successfully employed as a photoconductor to convert incident X-rays to electronic charges in direct conversion flat panel detectors for digital imaging systems. We have studied the effect of K-fluorescence reabsorption coupled with anomalous scattering on the performance of Se in protein crystallography detectors. Anomalous scattering is used as a method of phase calculation in crystallography. For occurrence of anomalous scattering, incident X-ray energy is made equal to the K-edge energy of Se. Our study indicates that significant K--fluorescence escape and reabsorption occurs at K-edge energy which degrades spatial resolution, sensitivity, and signal-to-noise ratio of the detector.

Selenium-based amorphous silicon flat-panel digital X-ray imager for protein crystallography

This work proposes a large-area detector for protein crystallography based on an amorphous silicon (a-Si:H) thin film transistor (TFT) pixel-array backplane and an overlying amorphous selenium (a-Se) photoconductor for direct conversion of incident X-rays into an image charge. To achieve high sensitivity, avalanche multiplication in a-Se is adopted to make the detector sensitive to each incident X-ray. The use of a-Si:H technology enables large-area imaging of protein diffraction patterns at less expense compared to existing charge coupled device (CCD) and imaging plate (IP) detectors. In addition, a theoretical analysis shows that the detector exhibits fast readout speed (readout time <1 s), high dynamic range (~106), high sensitivity (~1 X-ray photon), and high detective quantum efficiency (~0.7), thus validating its suitability for protein crystallography.

Amorphous silicon based direct x-ray conversion detector for protein structure analysis

Detailed knowledge of protein structure is vital to the understanding of numerous biological processes and to expedite advancements in pharmaceutical research. The atomic structure of protein can be determined by measuring the intensity of its X-ray diffraction pattern. The functional wavelength of X-ray for this purpose lies in the range of 0.01-1 nm. The diffraction pattern produced by X-ray of such wavelength can be read by a large area (~ 20 cm x 20 cm) detector. The detector should have good spatial resolution (FWHM ~ 2 pixels) to detect every Bragg peak in the pattern. In addition, a wide dynamic range (~106) is desirable to accurately measure the intensity of Bragg peaks. Amorphous silicon (a-Si:H) based detector is a potential candidate to meet these requirements; in particular, it is attractive by virtue of its inexpensive manufacturing process and large area compatibility compared to the existing CCD and image plate based detection techniques. In this work, we investigate by modeling the feasibility of an a-Si:H based detector for the study of protein structure. The proposed detector employs amorphous selenium (a-Se) photoconductor layer to directly convert the incident X-ray to a charge image, which is then electronically read by an array of a-Si:H thin film transistors. The modeling results show that the detector reasonably satisfies the requirements for determination of protein.