Active matrix flat-panel imager (AMFPI) technology is being employed for an increasing variety of imaging applications. An important element in the adoption of this technology has been significant ongoing improvements in optical signal collection achieved through innovations in indirect detection array pixel design. Such improvements have a particularly beneficial effect on performance in applications involving low exposures and/or high spatial frequencies, where detective quantum efficiency is strongly reduced due to the relatively high level of additive electronic noise compared to signal levels of AMFPI devices. In this article, an examination of various signal properties, as determined through measurements and calculations related to novel array designs, is reported in the context of the evolution of AMFPI pixel design. For these studies, dark, optical, and radiation signal measurements were performed on prototype imagers incorporating a variety of increasingly sophisticated array designs, with pixel pitches ranging from 75 to 127 microm. For each design, detailed measurements of fundamental pixel-level properties conducted under radiographic and fluoroscopic operating conditions are reported and the results are compared. A series of 127 microm pitch arrays employing discrete photodiodes culminated in a novel design providing an optical fill factor of approximately 80% (thereby assuring improved x-ray sensitivity), and demonstrating low dark current, very low charge trapping and charge release, and a large range of linear signal response. In two of the designs having 75 and 90 microm pitches, a novel continuous photodiode structure was found to provide fill factors that approach the theoretical maximum of 100%. Both sets of novel designs achieved large fill factors by employing architectures in which some, or all of the photodiode structure was elevated above the plane of the pixel addressing transistor. Generally, enhancement of the fill factor in either discrete or continuous photodiode arrays was observed to result in no degradation in MTF due to charge sharing between pixels. While the continuous designs exhibited relatively high levels of charge trapping and release, as well as shorter ranges of linearity, it is possible that these behaviors can be addressed through further refinements to pixel design. Both the continuous and the most recent discrete photodiode designs accommodate more sophisticated pixel circuitry than is present on conventional AMFPIs--such as a pixel clamp circuit, which is demonstrated to limit signal saturation under conditions corresponding to high exposures. It is anticipated that photodiode structures such as the ones reported in this study will enable the development of even more complex pixel circuitry, such as pixel-level amplifiers, that will lead to further significant improvements in imager performance.
The development of the highest resolution, large-area, active-matrix, flat-panel imager (AMFPI) thus far reported is described. This imager is based on a 97 jtm pixel pitch array with each pixel comprising a single a-Si:H TFT coupled to a discrete a-Si:H n-i-p photodiode. While the initial configuration chosen for fabrication is a 2048x2048 pixel array, a larger monolithic array format of 3072x4096 pixels is also permitted by the design. When coupled to an overlying scintillator such as a phosphor screen or CsI:Tl, the array allows indirect detection of incident radiation. The array is operated in conjunction with a recently completed electronic acquisition system featuring asynchronous operation, a large addressing range, fast analog signal extraction and digitization, and 16-bit digitization. This imager, whose empirical characterization will be reported in a subsequent paper, was developed as an engineering prototype to allow investigation of the performance limits of the most aggressive array designs permitted by present active-matrix technology. The development of this new imager builds upon knowledge acquired from the iterative design, fabrication, and quantitative evaluation of earlier engineering prototypes based on a series of 127 im pitch arrays. This paper summarizes the general program of research leading to this new device and puts this in the context of world-wide developments in indirect and direct detection AMFPI technology. Some limitations of present AMFPI technology are described, and possible solutions are discussed.Specifically, the incorporation of multiplexers based on poly-crystalline silicon circuitry into the array design, to facilitate very high resolution imagers, are proposed. In addition, strategies to significantly improve AMFPI performance at very low exposures, such as those commonly encountered in fluoroscopy, involving the reduction of additive noise (such as through lower preamplifier noise) and the enhancement of system gain (such as through the use of lead iodide) are discussed and initial calculations illustrating potential levels ofperformance are presented.
To obtain detailed information about recombination processes near room temperature in a-Si:H we have measured the steady-state and transient response of luminescence, light-induced ESR, and photoconductivity in lightly doped and undoped samples. T¹ low-energy luminescence is at a different energy in nand p-type a-Si:H, and has an intensity-dependent decay. The results lead us to propose a new modelthat the radiative transition is the capture of a majority carrier into a neutral dangling bond, having a low radiative efficiency and only a small Stokes shift. Measurements of the quantum efficiency for generating light-induced ESR (or LESR) confirm that a transition through a danghng bond is the dominant recombination mechanism in all samples, and is predominately nonradiative. %'e discuss the trapping mechanism and conclude that a single multiphonon process does not seem possible. Instead wc suggest that thc ITlcchanlsm may bc a cascade through as yct unidentified excited states. Transient luminescence, LESR, and photoconductivity each show that the response time is much longer in doped samples than in undoped samples. I. INTRGDUCTIONA great deal of information has been obtained about the recombination of excess carriers in a-Si:H. The principal experiments used in these studies are luminescence, ' hght-1Ilduced ESR (LESR), ' photoconductlv1ty, and induced absorption.There are several recombination processes, both radiative and nonradiative, with ihe relative importance of each depending on the experimental conditions. Previous papers in this series have mostly explored the low-temperature mechanisms which are often dominated by a luminescence transition. This paper further investigates the high-temperature regime in which the recombination is predominately nonradiative. In particular, we compare undoped and doped samples and discuss the details of the recombination.At low temperature the dominant recombination processes are a radiative tunneling transition between bandtail states and nonradiative tunneling to dangling-bond defects. ' Surface and Auger recombination are present, but tend to be less important. ' Some recent measurements also suggest another process of, as yet, unknown origin. The low-temperature recombination is characterized by tunneling rather than extensive diffusion of carriers because the presence of 'a large density of band-tail states causes rapid trapping in states much deeper than kT. ' Near room temperature, carriers are more mobile so that carrier transport is an important factor in the recombination.The band-edge luminescence is strongly quenched, this effect being attributed to trapping at dangling bonds. ' There is a second weak luminescence transition at lower energy that tends to dominate near 300 K and which has also been associated with recombination at the defects. ' This band is seen particularly in doped samples in which the defect density is known to be high.The association of dangling bonds with recombination has been confirmed recently by time-of-fhght photoconductivity. " The...
This paper introduces new high-resolution amorphous Silicon (a-Si) image sensors specifically configured for demonstrating film-quality medical x-ray imaging capabilities. The device utilizes an x-ray phosphor screen coupled to an array of a-Si photodiodes for detecting visible light, and a-Si thin-film transistors (TFTs) for connecting the photodiodes to external readout electronics. We have developed imagers based on a pixel size of 127 tm x 127 xm with an approximately page-size imaging area of244nirn x 195mm, and array size of 1,536 data lines by 1,920 gate lines, for a total of2.95 million pixels.' More recently, we have developed a much larger imager based on the same pixel pattern, which covers an area of approximately 406mm x 293mm, with 2,304 data lines by 3,200 gate lines, for a total ofnearly 7.4 million pixels.2 This is very likely to be the largest image sensor array and highest pixel count detector fabricated on a single substrate. Both imagers connect to a standard PC and are capable oftaking an image in a few seconds.Through design rule optimization we have achieved a light sensitive area of 57% and optimized quantum efficiency for x-ray phosphor output in the green part ofthe spectrum, yielding an average quantum efficiency between 500 and 600 nm of -7O%. At the same time, we have managed to reduce extraneous leakage currents on these devices to a few fA per pixel, which allows for very high dynamic range to be achieved. We have characterized leakage currents as a function of photodiode bias, time and temperature to demonstrate high stability over these large sized arrays.At the electronics level, we have adopted a new generation of low noise, charge-sensitive amplifiers coupled to 12-bit AID converters. Considerable attention was given to reducing electronic noise in order to demonstrate a large dynamic range (over 4,000: 1) for medical imaging applications. Through a combination oflow data lines capacitance, readout amplifier design, optimized timing, and noise cancellation techniques, we achieve l,000e to 2,000e ofnoise for the page size and large size arrays, respectively. This allows for true 1 2-bit performance and quantum limited images over a wide range of x-ray exposures. Various approaches to reducing line correlated noise have been implemented and will be discussed. Images documenting the improved performance will be presented. Avenues for improvement are under development, including higher resolution 97 jim pixel imagers,3 further improvements in detective quantum efficiency, and characterization of dynamic behavior.
A new algorithm is described for deriving the density of states N(E) from the Fermi energy EF upwards toward the conduction band edge. This refinement in the analysis of space-charge-limited currents (SCLC) enables the accurate determination of N(E) by implicitly accounting for the spatial variations of physical quantities across the thickness of the diode. SCLC is measured in NiCr/n+/a-Si1−xGex:H/Pt diode structures. For a-Si:H samples, SCLC values for N(EF) are compared to those derived from admittance measurements on the same diodes. The two determinations agree in samples where 1016<N(EF) <1018 eV−1 cm−3. Arguments are presented that densities of states between 3×1014 and 1016 eV−1 cm−3 found by SCLC methods are more accurate than higher densities found from admittance measurements. Structure in N(E) inferred from a number of investigations is discussed. SCLC in sputtered a-Si0.7Ge0.3:H is also investigated, as a function of hydrogen content cH, optical gap, and photoluminescence intensity IPL. In this alloy increasing cH causes N(EF) to decrease, to a minimum of 3×1016 for cH=14 at. %. IPL increases inversely with N(EF), confirming the sensitivity of SCLC to bulk nonradiative recombination centers. It is concluded that the SCLC measurement and analysis constitute a relatively simple, straightforward, and generally applicable method of obtaining the density of states in the gap of amorphous semiconductors.
Following our previous report' concerning the development ofa 127 im resolution, 7.4 million pixel, 30 x 40 cm2 active area, flat panel amorphous Silicon (a-Si) x-ray image sensor, this paper describes enhancements in image sensor performance in the areas of image lag, linearity, sensitivity, and electronic noise. New process improvements in fabricating a-Si thin film transistor (TFT)/photodiode arrays have reduced first-frame image lag to less than 2%, and uniformity in photoresponse to < 5% over the entire 30 x 40 cm2 active area. Detailed analysis of image lag vs. time and x-ray dose will be discussed. An improved charge amplifier has been introduced to suppress image cross-talk artifacts caused by charge amplifier saturation, and system linearity has been optimized to eliminate banding effects among charge amplifiers. Preliminary sensitivity improvements through the deposition of CsI(Tl) directly on the arrays are reported, as well as overall imaging characteristics of this improved image sensor.
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