Amorphous silicon p-i-n photodetector with Frisch grid for high-speed medical imaging

Abstract

In indirect digital x-ray detectors, photodetectors such as hydrogenated amorphous silicon (a-Si:H) p-i-n photodetectors are used to convert the optical photons generated by the scintillating material to collectible electron-hole pairs. A problem that arises during the collection of the charges is that the mobility and lifetime of both types of carriers (electrons and holes) differ. In a-Si:H, the mobility of holes is much lower than that of electrons which leads to depth-dependent signal variations and causes the charge collection time to be extensive. It has been shown that the use of a Frisch grid can reduce the effect of the slower carriers in direct x-ray detectors. The Frisch grid is essentially a conducting grid that shields carriers from the collecting electrode until they are in close proximity. When the pixel electrodes are properly biased, the grid prevents the slow moving carriers (traveling away from the collecting electrode) from being collected and puts more weight on the fast moving carriers, thus allowing the total charge to be collected in less time. In this paper we investigate the use of a Frisch grid in a-Si:H p-i-n photodetectors for indirect x-ray detectors. Through simulations and theoretical analysis we determine the grid line sizes and positioning that will be most effective for practical p-i-n photodetector designs. In addition we compare the results of photodetectors with and without the grid to characterize the improvement achievable.