The storage capacitor of the DEL stores which type of charge?

• A. Positive Charge
• B. Negative Charge
• C. Photons
• D. Electrons

Answer: (A)

When a DEL is exposed, the photodiode generates charge that is stored on the storage capacitor. The biasing and the way the thin-film transistor is connected cause the positive carriers (holes) to remain on the storage node while electrons are drawn away into the readout path. This leaves a net positive charge on the storage capacitor, and the amount of this positive charge (and its resulting voltage) is proportional to the x-ray exposure. When the TFT selects that DEL, the stored positive charge is read out as the image signal. Photons aren’t stored as a charge, and while electrons move during operation, the designed polarity of the storage is positive, not negative.