Experimental Demonstration of the Small Pixel Effect in an Amorphous Photoconductor using a Monolithic Spectral Single Photon Counting Capable CMOS-Integrated Amorphous-Selenium Sensor

We directly demonstrate, for the first time, the small pixel effect in an amorphous photoconductor by measuring the intrinsic transient response of an amorphous selenium (a-Se) photoconductor pixel of various sizes monolithically combined with a CMOS sensor. Our front-end circuitry leverages the lowest electronic noise reported for aSe photon detection, $\sim 300 \mu \text{V}_{rms}$ measured, to demonstrate the first-ever fully CMOS integrated a-Se pulse heigh spectroscopy results. Fabricated using $0.18 \mu \text{m}$ CMOS mixedsignal technology, the chip contains four pixel arrays with 30, 60, 90, and $120 \mu \text{m}$-pitch, each pixel having its own dedicated charge-sensitive amplifier. Our measured results from a monoenergetic Gamma source demonstrate the capability of a-Se/CMOS pixel arrays to achieve a high count rate when the small pixel effect is leveraged in the high spatial resolution pixel arrays. The results demonstrated can expedite the development of energy discriminating single photon counting imaging detectors for large area mammography tomosynthesis and dedicated breast computed tomography, something that has not as yet been achieved at the commercial scale.