Active pixel imagers incorporating pixel‐level amplifiers based on polycrystalline‐silicon thin‐film transistors

Abstract

Active matrix, flat-panel imagers ͑AMFPIs͒ employing a 2D matrix of a-Si addressing TFTs have become ubiquitous in many x-ray imaging applications due to their numerous advantages. However, under conditions of low exposures and/or high spatial resolution, their signal-to-noise performance is constrained by the modest system gain relative to the electronic additive noise. In this article, a strategy for overcoming this limitation through the incorporation of in-pixel amplification circuits, referred to as active pixel ͑AP͒ architectures, using polycrystalline-silicon ͑poly-Si͒ TFTs is reported. Compared to a-Si, poly-Si offers substantially higher mobilities, enabling higher TFT currents and the possibility of sophisticated AP designs based on both n-and p-channel TFTs. Three prototype indirect detection arrays employing poly-Si TFTs and a continuous a-Si photodiode structure were characterized. The prototypes consist of an array ͑PSI-1͒ that employs a pixel architecture with a single TFT, as well as two arrays ͑PSI-2 and PSI-3͒ that employ AP architectures based on three and five TFTs, respectively. While PSI-1 serves as a reference with a design similar to that of conventional AMFPI arrays, PSI-2 and PSI-3 incorporate additional in-pixel amplification circuitry. Compared to PSI-1, results of x-ray sensitivity demonstrate signal gains of ϳ10.7 and 20.9 for PSI-2 and PSI-3, respectively. These values are in reasonable agreement with design expectations, demonstrating that poly-Si AP circuits can be tailored to provide a desired level of signal gain. PSI-2 exhibits the same high levels of charge trapping as those observed for PSI-1 and other conventional arrays employing a continuous photodiode structure. For PSI-3, charge trapping was found to be significantly lower and largely independent of the bias voltage applied across the photodiode. MTF results indicate that the use of a continuous photodiode structure in PSI-1, PSI-2, and PSI-3 results in optical fill factors that are close to unity. In addition, the greater complexity of PSI-2 and PSI-3 pixel circuits, compared to that of PSI-1, has no observable effect on spatial resolution. Both PSI-2 and PSI-3 exhibit high levels of additive noise, resulting in no net improvement in the signal-to-noise performance of these early prototypes compared to conventional AMFPIs. However, faster readout rates, coupled with implementation of multiple sampling protocols allowed by the nondestructive nature of pixel readout, resulted in a significantly lower noise level of ϳ560 e ͑rms͒ for PSI-3.

Theoretical investigation of the noise performance of active pixel imaging arrays based on polycrystalline silicon thin film transistors

“…Interestingly, while increasing the number of samples is generally expected to progressively decrease total noise, as is indeed observed for the two‐stage designs by virtue of averaging the signal of multiple samples, this is clearly not the case for the single‐stage designs. This is a direct consequence of the fact that increasing the number of samples necessitates decreasing the ADDRESS on‐time per sample (see Fig.…”

“…(Note that the reported values for signal gain approach, but do not reach, the first order approximation of 19.3 given by the ratio of C DATA to C PD – mainly as a result of the complexity of the source‐follower behavior under the broad range of operating conditions explored. This observation also applies to the other single‐stage designs, as well as to the two‐stage designs for which the first order approximation for signal gain is given by the ratio of C DATA to C FB …”

“…In addition, poly‐Si offers the advantage of large‐area processing as well as a relatively high level of radiation damage resistance . A number of prototype arrays based on poly‐Si have been previously developed, fabricated, and investigated under diagnostic imaging conditions . The prototypes, referred to as PSI‐1, PSI‐2, and PSI‐3, incorporate pixel designs comprising one, three, and five TFTs respectively.…”

Performance of in-pixel circuits for photon counting arrays (PCAs) based on polycrystalline silicon TFTs

“…In addition, poly-Si transistors have demonstrated good radiation tolerance (Li et al , 2006). Prototype active pixel imagers incorporating in-pixel amplification based on poly-Si TFTs have demonstrated over an order of magnitude increase in signal, as well as the ability to perform advanced readout techniques (El-Mohri et al , 2009). Such techniques allow significantly improved signal-to-noise ratio performance compared to that of conventional a-Si:H-based AMFPIs.…”

TFT backplane technologies for advanced array applications

“…Various designs have been tested, including source follower and operational amplifier types. [9] Single photon counting pixel electronics can further improve the low dose performance the x-ray detector by discriminating and rejecting the background noise, and this technology is of considerable interest for medical imaging. [10] An x-ray photon typically generates a detected charge that is proportional to the incident x-ray energy.…”