An innovative hydrogenated amorphous silicon (a-Si:H) p-i-n photodiode based X-ray detector for medical imaging applications was developed in this work, and the improvements of the device were also discussed. The detector consists of an a-Si:H p-i-n photodiode and a stacked dielectric layer, such as silicon nitride (SiNx), deposited on p-layer of this p-i-n diode (n-i-p-SiNx), as the major charge storage element. The detector operates as a capacitor, formed by this dielectric layer, in parallel with a reverse-biased p-i-n diode during the detection cycle. Consequently, the capacitance, for accumulating the photon-converted charges, of the p-i-n diode was enlarged by this stacked dielectric layer without decreasing the active area of the detector. As a result, the dynamic range, linearity and data retention capability of this novel detector are significantly improved. In particular, the photo sensitivity and charge storage capability of this novel detector can be separately optimized, and the drastically improved data retention, due to the high density and long release time of the trapped electrons in p-layer of the p-i-n diode, could facilitate this novel detector to be employed in the low dosage flux and long exposure applications. (C) 2003 Elsevier Science B.V. All rights reserved.