The photocurrent, device area, and parasitic capacitance of gapgate type thin-film transistor (TFT) and photodiode have been compared. With the higher photocurrent, less occupied area, and smaller parasitic capacitance, the gap-type TFT is suitable as photo device in active pixel sensor (APS) to sense low intensity light.
The large area TFT image sensors using PIN photodiode as the light sensing device attract attention owing to their application in optical fingerprint sensors under displays. Different structures of the pixel sensing circuits are compared in the aspect of output signal, settling time, photo current collection, as well as variation of TFT. Even though the active pixel sensing circuits (APS) are expected to provide better performance intuitively, the passive pixel sensing circuits (PPS) can surpass APS for practical purpose.
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