1. Field of Invention
The invention relates to an image sensing panel and apparatus and, in particular, to an active matrix image sensing panel and apparatus.
2. Related Art
In the conventional X-ray imaging technology, images are produced by applying X-ray exposure to photographic film. In recent years, however, with the well development of semiconductor technology, X-ray imaging is improved to the so-called digital radiography (DR) technology that uses a flat and digital image sensing panel to produce images.
The principle of the digital radiography technology is briefly illustrated as below. When X-ray enters into an image sensing apparatus, a scintillator will convert X-ray into visible light that is then sensed by photo sensors and thus converted into electric signals read out by data lines through thin-film transistors (TFT), and the images will be produced after the electric signals are processed. As for the current DR technology, the photo sensor is improved to a silicon-based photodiode from a charge coupled device (CCD), and also, X-ray can be directly converted into electric signals without the scintillator.
In general, a commonly-used TFT device is such as a metal oxide TFT, and a commonly-used photo sensor is such as an NIP amorphous silicon (a-Si) photodiode. The gate electrode of the metal oxide TFT is generally operated by a voltage of negative polarity (e.g. −5V), and the bias polarity of the NIP a-Si photodiode is also negative polarity. Therefore, the electrons generated by the photo sensor due to the illumination will move towards the bottom electrode of the photo sensor, so that the levels of the bottom electrode and the source electrode of the TFT will drop off. However, if the levels of the bottom electrode and the source electrode of the TFT continuously descend because of the illumination of high-intensity light, the voltage difference between the gate and source electrodes will continuously ascend. Accordingly, when the voltage difference exceeds the threshold voltage of the TFT, the TFT will be turned on and thus the source electrode starts to leak electricity to the data line. Therefore, the image produced by the processing of the image processing module will be distorted.
Therefore, it is an important subject to provide an active matrix image sensing panel and apparatus that can prevent the leakage current problem to keep the image sensing undistorted.