An innovative hydrogenated amorphous silicon (a-Si:H) p-i-n photodiode based x-ray detector for medical imaging applications has been developed in this work. Basically, the detector is a p-i-n photodiode, with a very simple modification by depositing a stacked silicon nitride (SiNx) layer on the p-layer (n-i-p-SiNx) or n-layer (p-i-n-SiNx) of this diode. The dielectric layer functioned as the major charge storage element of the pixel, and p-i-n as the photon-charge converter, separately. The charge storage capacity is larger as the nitride layer is thinner. Consequently, dynamic range, linearity, and data retention of the image array were significantly improved. The novel detector also offers a scheme to independently optimize the photo sensitivity and charge storage capacity of a p-i-n photodiode based pixel. Instead of the conventional p-i-n photodiodes, the novel detectors are proposed to employ in the active matrix, flat-panel imager, with the favor that the signal readout electronics and the TFT driving circuitry are unchanged. The changes include only the bias voltage, whch as a bi-level waveform, as well as the timing for turning on/off the switching thin film transistors (TFTs). The fundamentals of the n-i-p-SiNx and p-i-n-SiNx detectors are addressed, and the performances of these two novel detectors and the conventional p-i-n photodiode are compared. Additionally, the different performances, such as the speed, between n-i-p-SiNx and p-i-n-SiNx will be particularly discussed.