a photosensor for realizing an image sensor which can meet the requirements of high resolution , high operation speed and high signal - to - noise ratio is disclosed . the photosensor comprises a circuit substrate , a thin film transistor formed on the circuit substrate and an amorphous silicon photodiode formed on the substrate integral with the thin transistor between the drain and gate electrodes thereof . also formed on the circuit substrate adjacent to the thin film transistor and photodiode are a charging switch element for coupling the photodiode to a dc power source to charge an inter - electrode capacitance of the photodiode , a charge storage capacitor charged by a channel current of the thin film transistor controlled by an inter - electrode capacitance voltage of the photodiode which varies in response to incident light after the inter - electrode capacitance has been charged , and a detecting switch element for coupling the capacitor to an output amplifier . the charging and detecting switch elements are each formed of a thin film transistor .