In recent years, it has become technically and economically feasible to use solid-state detector technology to display, store and transfer X-ray images. In this paper, we report the performance of a 33 cm x 41 cut amorphous silicon flat panel detector (FPD) based on an ion shower doped P-I-N photodiode/thin-film transistor (TFT) array. The p-layer of a diode is formed by an ion shower doping method instead of the conventional plasma-enhanced chemical vapor deposition method. Measurements of X-ray imaging performances are reported for general imaging metrics such as modulation transfer function, noise power spectrum and detective quantum efficiency. The presampling MTF was found to be 0.51 and 0.26 at 1 lp/mm and 2 lp/mm. The measured DQE at I lp/mm and 2 lp/mm was 0.35 and 0.17.