Empirical noise performance of prototype active pixel arrays employing polycrystalline silicon thin‐film transistors

Abstract

In the spirit of overcoming the signal-to-noise limitations of active matrix, flat-panel imagers (AMFPIs) which employ array circuits based on a-Si:H thin-film transistors (TFTs), an empirical investigation of the noise properties of prototype active pixel arrays based on polycrystalline silicon (poly-Si) TFTs is reported. Like a-Si:H, poly-Si supports fabrication of large area, monolithic x-ray imaging arrays and offers good radiation damage resistance, while providing electron and hole mobility orders of magnitude higher. Compared to pixel circuits typically consisting of a single addressing switch in an AMFPI array, the pixel circuit of an active pixel array includes an amplifier that magnifies the imaging signal prior to readout by external acquisition electronics. Also, while readout erases signal stored in the pixels for AMFPI arrays, active pixel arrays allow multiple nondestructive readout, which can be used to reduce noise. The prototype arrays investigated in this paper were developed to explore the effect of variation in amplifier design on noise. Methods: A pair of prototype arrays incorporating single-stage and two-stage poly-Si pixel amplifiers were examined. The arrays incorporate various amplifier designs in which dimensions of some of the three (or four) poly-Si TFTs per pixel circuit for the single-stage array, and some of the five poly-Si TFTs for the two-stage array, were varied. The arrays were operated using a recently developed electronic data acquisition system that allows variation of operational conditions such as voltages and timing of control signals. The arrays were operated in the absence of radiation in various correlated multiple sampling modes, with and without the injection of charge directly into the pixel circuits for measurements of in-pixel gain and pixel noise. Pixel noise, referred back to the input of the pixel amplifier, was compared to predictions generated by a sophisticated circuit simulation model. Results: Across the various pixel circuit designs, the median in-pixel gain for the single-stage and two-stage arrays was measured to be 99.3 and 925, respectively. These gain levels were sufficient to reduce the contribution of external noise, defined as the electronic additive noise in the absence of noise contributions from circuitry in the pixel and referred back to the input of the pixel amplifier, to less than 340 e. As a result, median pixel noise results as low as~695 e and 866 e, acquired using eight samples, were observed from the best-performing single-stage and two-stage designs, respectively. While the magnitude of pixel noise predicted by simulation was lower than the measured results, there was generally good agreement between simulation and measurement for the functional dependence of noise on operating voltages, timing, and sampling mode. Conclusions: The single-stage and two-stage arrays examined in this study demonstrated pixel noise well below that typically demonstrated by AMFPIs. Through proper design, it should be possible to maintain the noise levels...

Minimization of image lag in polycrystalline mercuric iodide converters through incorporation of Frisch grid structures for digital breast tomosynthesis

“…A different approach for improving signal-to-noise involves modification of the pixel circuit to include one or more amplification stages to magnify the pixel signal prior to readout-a strategy commonly referred to as the active pixel concept (Fossum 1993). The development of such active pixel arrays using TFTs fabricated with polycrystalline silicon, a semiconductor material offering electron mobilities two orders of magnitude higher than a-Si:H as well as similar radiation damage resistance and the same large-area fabrication capabilities, is being explored (Koniczek et al 2020). In addition, active pixel arrays based on crystalline silicon (c-Si) transistors and fabricated using standard CMOS fabrication techniques are under investigation for use in digital breast tomosynthesis (DBT) (Zhao et al 2015, Peters et al 2016, Choi et al 2019.…”

Photon counting-energy integrating hybrid flat panel detector systems for image-guided interventions: an experimental proof-of-concept

“…The imaging capability and performance of existing C-arm systems are strongly limited by the existing scintillator-based, energy integrating FPDs. The long transmission distances of analog signals in the a-Si(H) thin-film transistor (TFT) array and the passive pixel design results in pronounced electronic noise in the detector (Koniczek et al 2020 ); the down-weight of low-energy x-ray photons in energy integrating detectors is highly suboptimal for imaging iodine; the afterglow of scinitllator and the slow charge carrier mobilities in the a-Si impose a fundamental limit to the achievable temporal resolution and introduce undesirable lag effects; the light spatial spreading in the scintillator imposes a fundamental limit to the achievable spatial resolution. This limits the detector’s ability to resolve fine structures such as stent kinking and narrowing and corresponding vessel stenosis during IGIs; there is also a strong tradeoff between spatial resolution and scintillator thickness, which is currently only around 600 μ m (only 30% of the scinitllator thickness in MDCT detectors) and yields a low x-ray absorption efficiency.…”

Theoretical investigation of the signal performance of HgI2 x-ray converters incorporating a Frisch grid structure at mammographic energies

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