Digital radiology using active matrix readout of amorphous selenium: geometrical and effective fill factors

Flat panel self-scanned x-ray detectors using amorphous selenium (a-Se) as the photoconductor are being developed to replace both film/screen cassette systems for radiography and image intensifier (XRII)/video systems for fluoroscopy. These use a two-dimensional array of pixel electrodes to collect and readout the latent image charges formed on the photoconductor surface. The percentage of the area covered by the pixel electrodes (i.e., the geometrical fill factor fg) is always less than unity. In this paper, a novel approach is introduced to make the charge collection by pixel electrodes almost complete (i.e., a close to unity effective fill factor). The idea is to bend the electric field lines in the a-Se layer in such a way that image charges cannot land in the gap region between electrodes. This is achieved by depositing holes in the gap region, which is possible because there are charge traps available at the a-Se/insulator interface. The distribution of holes required in the gap region is calculated. Various factors associated with the feasibility of this approach as well as a method to deposit these holes are discussed. Application of the approach to the case of mammography is also included.