Direct-Conversion X-Ray Detector with 50-µm High-Gain Pixel Amplifiers for Low-X-Ray-Dose Digital Mammography

Abstract

This study proposes an active pixel sensor (APS) design for readout signal amplification to replace the traditional passive pixel sensor (PPS) design. The APS design adds only one extra thin-film transistor (TFT) amplifier, giving it the high signal sensitivity of the APS architecture and the high spatial resolution of the PPS architecture. The mobility of the amorphous silicon (a-Si) TFT is 0.56 cm2/Vs, the signal amplification of the APS circuit is more than 10×, the on/off current ratio is more than 107, and the leakage current is on the order of 1 fA. A 200-µm-thick layer of amorphous selenium (a-Se) as the X-ray photoconductor was deposited on the APS array with 160 × 160 pixels of various sizes, down to 50 µm. The 40-kVp X-ray spectra obtained with tube currents of 40 and 50 mA, respectively, were used for measuring the gray-level contrast induced by the X-rays. The experimental results show that the gray-level contrasts for the 50-µm pixels were 5.57 ± 0.89 and 13.19 ± 1.94 and those for the 70-µm pixels were 15.94 ± 2.98 and 24.19 ± 2.76 under 40-kVp X-ray exposure at 40 and 50 mA, respectively.