fig1 shows a photosensor of this invention , formed in a staggered configuration on a substrate . the substrate 1 is an insulating plate having a smooth ( i . e ., lustrous ) surface . more specifically , it is an alkali metal - free glass plate having an optically polished major surface . a photodiode 2 of a sandwich structure and a thin film transistor ( tft ) 3 are formed on the major surface of substrate 1 , integrally and adjacent to each other . a charging transistor ( tft ) 4 and a detection transistor ( tft ) 5 are formed adjacent to tft 3 on substrate 1 . a capacitor 6 is formed between tfts 4 and 5 . more specifically , tft 3 has drain and source electrodes 11 and 12 formed on the major surface of substrate 1 . between drain and source electrodes 11 and 12 are sequentially formed an active layer 13 which consists of a - si : h film , an insulating layer 14 of sinx , and a gate electrode layer 15 consisting of , for instance , chromium . drain and source electrodes 11 and 12 may be formed by vacuum deposition of aluminum and by a subsequent photoetching process ( pep ). a - si : h film 13 and sinx film 14 may be formed by plasma cvd process , from 100 % sih 4 gas and sih 4 / n 2 mixture gas , respectively . gate insulating film 14 may be double - layer film made of a sinx layer and a siox layer , with the sinx layer provided on the side of active layer 13 . gate electrode 15 may be formed by pep . photodiode 2 has a cathode electrode 16 , which is formed of the same material and by the same process as gate electrode 15 , on a portion of drain electrode layer 11 that is not covered with insulating film 14 and on a portion of insulating film 14 . a - si : h film 17 is formed on a portion of cathode electrode 16 over insulating layer 14 . an anode electrode 18 consisting of transparent conductive film is formed on a - si : h film 17 and insulating layer 14 so as to partly contact gate electrode 15 . as a result , photodiode 2 is formed which has a sandwich structure with the a - si : h film sandwiched between the anode and cathode electrodes . evidently , gate electrode 15 and drain electrode 11 of tft 3 are electrically connected to respective anode and cathode electrodes 18 and 16 of photodiode 2 . ito may be used for a material of the transparent conductive film . light 20 is incident on a - si : h film 17 through transparent anode electrode 18 of photodiode 2 . since the tft and photodiode are formed adjacent to each other on the substrate , a wiring capacitance can be greatly reduced , improving the signal - to - noise ratio . charging tft 4 , detection tft 5 and capacitor 6 are formed integrally with photodiode 2 and tft 3 on substrate 1 . capacitor 6 is formed of source electrode layer 12 , insulating film 14 formed thereon , and electrode layer 21 formed on insulating film 14 . electrode 21 is spaced apart from gate electrode 15 of tft 3 , and a - si : h film 22 is formed between electrode layer 21 and gate electrode 15 . an insulating layer 23 is formed on electrode layer 21 , a - si : h film 22 and gate electrode 15 of tft 3 . a gate electrode 24 is formed on insulating film 23 . thus , charging tft 4 for charging the interelectrode capacitance of photodiode 2 is formed . gate electrode 15 of transistor 3 and electrode 21 of capacitor 6 are used as the drain and source electrodes of charging tft 4 , respectively . an electrode 24 is formed on the major surface of substrate 1 , and an active layer 25 , or a - si : h layer , is formed between electrodes 12 and 24 . insulating layer 14 is formed to extend on electrodes 11 and 12 and active layers 13 and 25 . a gate electrode 26 is formed over active layer 25 with insulating layer 14 interposed therebetween . in this way , detection tft 5 is formed . electrodes 12 and 24 are used as the drain and source electrodes of detection transistor 5 , respectively . a charging ( i . e ., reset ) pulse and a readout pulse are applied to the gate electrodes of charging tft 4 and detection tft 5 , respectively . the positive terminal of a power supply 2 with 5 to 6 v is connected to cathode electrode 16 of photodiode 2 , while the negative terminal of the power supply is connected to a common potential ( i . e ., ground ) terminal . capacitor electrode 21 is connected to a non - inverting input of an output amplifier 28 , and source electrode 24 of detection transistor 5 is connected to an inverting input of amplifier 28 . amplifier 28 is provided separately from the photosensor circuit formed on substrate 1 . the operation of the photosensor shown in fig1 will now be described with reference to fig2 i . e ., a circuit diagram thereof . charging tft ( switch ) 4 and detection tft ( switch ) 5 are normally off . they are turned on by a charging pulse and a readout pulse from a control circuit ( not shown ), respectively . when a charging pulse turns charging tft 4 on , the inter - electrode capacitance c of photodiode 2 is charged by power supply 27 . at this time , tft 3 is off since its gate electrode 15 is coupled to electrode 21 placed at ground potential through the channel of conducting tft 4 . the drain current of transistor 3 is a small off - current i d off . after charging tft 3 has been turned off , the inter - electrode capacitance c of photodiode 2 gradually discharges in accordance with photocurrent corresponding to the incident image information light 20 . the discharging of inter - electrode capacitance c raises the potential on gate electrode 15 of tft 3 . as a result , a drain current based on the gate potential flows in tft 3 . capacitor 6 is charged by this drain current . the extent of charging capacitor 6 depends on the intensity of light 20 incident on photodiode 2 . when the read - out pulse is applied to the gate electrode of detection tft 5 , detection tft 5 is turned on . as a result , the voltage across capacitor 6 is applied to output amplifier 28 , so that an output image information signal is obtained . fig3 shows a circuit diagram of an image sensor in which a plurality of photosensors as described above are arranged in a line . more specifically , in this image sensor , n photosensors , each of which corresponds to one picture element ( i . e ., one dot ) and is constituted by a photodiode 2 , a tft 3 , a charging tft 4 , a detection tft 5 and a capacitor 6 , are arranged in line on a substrate . each photosensor is wired in the manner as shown in fig2 . the n photosensors are successively enabled by control signals having charging pulses and read - out pulses . thus , the image information signal is taken from an output amplifier 28 . where the resolution is 16 dots / mm , n is approximately 3 , 500 . fig4 shows a relation between the signal - to - noise ratio of drain current i d of tft 3 and gate - source voltage v gs thereof . as is shown , the threshold voltage of tft 3 is 2 to 3 v , and the signal - to - noise ratio is not less than 100 db with respect to the drain current when the v gs changes by several volts . the signal voltage taken out from the photodiode having the sandwich structure ( i . e ., voltage across the inter - electrode capacitor ) is 0 to 7 v when the intensity of the incident light is 0 . 5 lx / sec . thus , an output signal having a dynamic range of approximately 100 db can be obtained by controlling the tft gate voltage with the signal voltage from the photodiode and by detecting the drain current . thus , an image information signal of a great signal - to - noise ratio can be obtained . besides , the noise voltage , such as switching noize and dark current noise , is on the order of the threshold voltage of tft . this indicates that the noise voltage tends to be compressed , to further improve the signal - to - noise ratio . fig5 shows the temperature - dependency 41 of an output signal of a sandwich type photodiode consisting of a tft and a photodiode , and also the temperature - dependency 42 of an output signal of the photosensor according to the invention . as is evident from fig5 a signal - to - noise ratio of 80 db or above can be realized in a temperature range of 0 to 80 ° c . this can meet the requirements of a high performance image sensor , i . e ., the resolution of 16 dots / mm and the readout speed of 1 msec . the substrate 1 may be made of glazed ceramic or organic film of a polyimide resin which can withstand a high temperature of 400 ° c . the active layer may be made of a - si : h film or polycrystalline silicon film formed by a vacuum deposition process or spattering process . the photodiode may have a pin structure or any other structure . further , photodiode 2 may be connected between the gate electrode and source electrode of tft 3 as shown in fig6 . it suffices to connect the photodiode between the drain or source electrode a channel , and the gate electrode . ( a channel is formed between the drain and source electrodes .) the structure of the photodiode and tft is not limited to the staggered structure shown in fig1 . further , the positional relationship among drain and source electrodes 11 and 12 , active layer 13 , insulating layer 14 and gate electrode 15 of the tft in the direction normal to the major surface of the substrate in fig1 may be reversed as shown in fig7 . that is , an inverse staggered structure may be adopted . further , the tft may take a coplanar structure in which electrodes 11 , 12 and 15 are formed on the same side of active layer 13 as shown in fig8 . further , the tft may have the structures shown in fig9 and 10 . in these figures , parts like those shown in fig1 are designated by the same numerals . fig9 shows a u mos or v mos transistor structure , in which an active layer 13 is sandwiched between drain and source electrodes 11 and 12 , and an insulating layer 14 has a u - or v - shape extending from the drain electrode to the source electrode . fig1 shows a sit ( static induction transistor ) structure , in which an active layer 10 is sandwiched between drain and source electrodes 11 and 12 , and a plurality of gate sections are provided . the above embodiment is designed such that capacitor 6 is charged by the drain current of the tft and subsequently the image information signal is read out through switch 5 . however , it may be arranged to convert the drain current of the tft directly into a voltage signal .