Abstract:
A radiation reader including a conversion device, and a method of driving the radiation reader. The conversion device includes (a) a pixel including a conversion device for converting an incident radiation into an electric charge, and (b) a reading circuit including at least one field effect transistor for reading the electric charge from the pixel in an amplifying manner as a signal containing a noise. The reading circuit includes first storage means for storing the signal containing the noise, second storage means for storing the noise, and processing means for deriving an output signal not containing the noise based on a first output from the first storage means and a second output from the second storage means.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a photoelectric converter and a radiation reader, more particularly to a photoelectric converter capable of performing the read operation at higher sensitivity and higher speed and a radiation reader capable of reading the information for radiation represented by (α-rays, β-rays, γ-rays or x-rays at a high sensitivity.  
         [0003]     2. Related Background Art  
         [0004]     In the case of a photoelectric converter and a radiation reader for reading the information based on radiation by wavelength-converting the radiation into the sensitivity region of the photoelectric converter with a wavelength converter such as a photoluminescent body (for example, scintillator), electric charges based on the input information photoelectric-converted by a photoelectric-converting section is transferred to a capacitance to amplify a signal voltage.  
         [0005]     However, to amplify a signal voltage by transferring electric charges from the capacitance of a photoelectric-conversion element itself to an external capacitance like the case of reading signal charges with a conventional circuit, a relatively-large S/N ratio can be obtained but a parasitic capacitance may be formed on a signal line when arranging a plurality of sensors. For example, when arranging 2,000×2,000 area sensors equivalent to an X-ray film at a size of 200×200 μm per cell and manufacturing an area sensor with a size of 40×40 cm, a capacitance is formed because the gate and source of a transistor for transferring electric charges are overlapped each other. Because the above overlap corresponds to the number of pixels, one signal line has an overlap C gs  capacitance of 0.05 pF×2,000 area sensors=100 pF even if one area sensor has a capacitance of approx. 0.05 pF. Because a sensor capacitance C s  is equal to approx. 1 pF, when assuming a signal voltage generated in a sensor as V 1 , the output voltage V 0  of the signal line is obtained from the following expression. 
 
 V   0 =( C   s /( C   s   /C   gs +×1000))× V   1  
 
 Thus, the output voltage lowers to 1/100. 
 
         [0006]     That is, to constitute an area sensor having a large area, an output voltage is greatly lowered.  
         [0007]     Moreover, to read a dynamic image under the above condition, a sensitivity and a high-speed operation capable of reading  30  images per sec. or more are requested. Particularly, in the case of a non-destructive inspection including X-ray diagnosis, higher sensitivity allowing the number of signal charges to increase up to 100 to 400 times is requested because there is a request for minimizing the dose of X-rays to be irradiated.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is made to solve the above problems and its object is to provide a photoelectric converter capable of preventing an output voltage from lowering due to increase of a parasitic capacitance and resultantly having higher sensitivity and more-advanced performance and a radiation reader having the photoelectric converter.  
         [0009]     It is another object of the present invention to provide a photoelectric converter having a large opening ratio; that is, a large rate of the light-receiving-section region in an area necessary for one pixel and resultantly capable of achieving a high sensitivity and an advanced performance and a radiation reader having the photoelectric converter.  
         [0010]     It is still another object of the present invention to provide a photoelectric converter capable of reading a dynamic image.  
         [0011]     It is still another object of the present invention to provide a radiation reader capable of further reducing the dose of radiation such as X-rays.  
         [0012]     It is still another object of the present invention to provide a photoelectric converter having a semiconductor layer comprising in one pixel: a photoelectric conversion element, a reading field-effect transistor having a gate for receiving signal charges generated in the photoelectric conversion element and a source and a drain for reading a signal corresponding to the signal charges accumulated in the gate, selection-switch means set between the reading field-effect transistor and a power supply, and reset means for resetting the gate; wherein 
        the photoelectric conversion element, the reading field-effect transistor, the selection-switch means, and the reset means are formed on a common insulating support body.        
 
         [0014]     It is still another object of the present invention to provide a radiation reader comprising a photoelectric converter having a semiconductor layer comprising in one pixel: a photoelectric conversion element, a reading field-effect transistor having a gate for receiving signal charges generated in the photoelectric conversion element and a source and a drain for reading a signal corresponding to the signal charges accumulated in the gate, selection-switch means set between the reading field-effect transistor and a power supply, and reset means for resetting the gate; wherein the photoelectric conversion element, the reading field-effect transistor, the selection-switch means, and the reset means are formed on a common insulating support body; and a photoluminescent body set on the photoelectric conversion element to absorb radiation and emit light in a wavelength band which can be detected by the photoelectric conversion element.  
         [0015]     It is still another object of the present invention to provide a radiation reader having the following in one pixel: a radiation reading element which includes an electric-charge discharging layer for absorbing radiation and discharging electric charges and in which at least the electric-charge discharging layer is constituted so as to be held by two conductive layers, a reading field-effect transistor having a gate for receiving signal charges generated in the radiation reading element and a source and a drain for reading a signal corresponding to the signal charges accumulated in the gate, selection-switch means set between the reading field-effect transistor and a power supply, and reset means for resetting the gate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIGS. 1, 10  and  13  are schematic circuit diagrams for explaining one-pixel portion of the photoelectric section of a photoelectric converter;  
         [0017]      FIG. 2  is a schematic plan view for explaining a structure corresponding to the schematic circuit diagram of  FIG. 1 ;  
         [0018]      FIG. 3  is a schematic sectional view of the structure of  FIG. 2 , taken along the line  3 - 3  of  FIG. 2 ;  
         [0019]      FIG. 4  is a schematic circuit diagram showing a case of arranging two or more pixels of  FIG. 1  like a matrix;  
         [0020]      FIGS. 5, 6 ,  11 , and  14  are timing charts for explaining a case of driving when applying a radiation reader and a photoelectric converter;  
         [0021]      FIGS. 7, 8  and  9  are schematic sectional views for explaining a radiation reader;  
         [0022]      FIG. 12  is a schematic circuit diagram showing a circuit structure when arranging the pixels of  FIG. 10  like a matrix;  
         [0023]      FIG. 15  is a schematic circuit diagram showing a circuit structure when the pixels of  FIG. 13  like a matrix;  
         [0024]      FIG. 16  is a graph showing the relation between ON-resistance, channel width (W), and channel length (L) of a transistor;  
         [0025]      FIGS. 17, 18 ,  19  and  20  are schematic sectional views showing one-pixel portions of a radiation reader; and  
         [0026]      FIGS. 21 and 22  are illustrations showing X-ray absorbing characteristics of a shielding material. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     Embodiments of the present invention are described below by referring to the accompanying drawings. A radiation reader of the present invention for reading the information based on radiation is not restricted to an X-ray reader described below but it can be also applied to a reader for reading the information based on (α-rays, β-rays, and γ-rays.  
       First Embodiment  
       [0028]      FIG. 1  is a schematic circuit diagram showing one-pixel portion of a photoelectric converter of the present invention.  FIG. 2  is a schematic plan view of the radiation reader shown in  FIG. 1  and  FIG. 3  shows a case of combining the radiation reader of  FIG. 2  with a wavelength converting body to use the combination as a radiation reader, which is a schematic sectional view between A and B shown by the schematic plan view of  FIG. 2 .  
         [0029]     In each drawing, symbol  21  denotes a photoelectric conversion element,  300  denotes a capacitor for accumulating signal charges sent from the photoelectric conversion element  21 ,  22  denotes a MOS transistor to which the photoelectric conversion element  21  and a gate are connected,  23  denotes a MOS transistor serving as selection-switch means for selecting each pixel,  24  denotes a MOS transistor serving as reset means for resetting the gate of the MOS transistor  22  and the capacitor  300 ,  25  denotes a current source,  26  denotes a MOS transistor for outputting a signal,  27  denotes a terminal to be connected with a voltage source,  28  denotes a reset gate line,  29  denotes a vertical gate line, and  41  denotes a voltage source. Moreover, symbol V 2  and V 3  denote signals to be applied to the gates of the MOS transistors  24  and  23 ,  42  and  43  denote signal sources for outputting the signals V 2  and V 3 . When the photoelectric conversion element  21  has a sufficient capacitance, it is not always necessary to use the capacitor  300 .  
         [0030]     As shown in  FIG. 1 , the photoelectric conversion element  21  is connected to the gate of the MOS transistor  22  so as to be able to amplify and output a signal sent from a photoelectric conversion element. Therefore, it is possible to prevent an output voltage from lowering due to increase of a parasitic capacitance when arranging a plurality of photoelectric conversion elements. Moreover, a photoelectric conversion element of the present invention is not restricted to a photoelectric conversion element having the same layer structure as a thin-film transistor (TFT) described below.  
         [0031]     Methods for forming elements are described below in order by referring to  FIGS. 2 and 3 . In this case, an X-ray-signal reader is described. A MOS transistor is constituted with a TFT.  
         [0032]     First, Cr is deposited up to a thickness of approx. 500 Å as a lower metal layer  2  on a glass substrate  1  serving as an insulating material through sputtering and then, patterned through photolithography to etch unnecessary areas. Thereby, the lower electrode of the photoelectric conversion element  21 , the gate electrodes of the MOS transistors (TFT)  22  to  24 , the lower electrode of the capacitor  300 , and lower wirings  402  and  412  are formed.  
         [0033]     Then, an SiN (silicon nitride) layer  70 , an i layer  4 , and an n layer  5  are deposited in order up to thicknesses of approx. 2,000 Å, 5,000 Å, and 500 Å by the CVD method in the same vacuum state. These layers serve as the insulating layer, photoelectric-conversion semiconductor layer, and hole-injection preventive layer of the photoelectric conversion element  21 , the gate insulating layers, semiconductor layers, and ohmic contact layers of the TFTs  22  to  24 , and the intermediate layer of the capacitor  300 . Moreover, they are used as crossing-portion insulating layers of upper and lower wirings. Thicknesses of the layers are not restricted to the above values. It is possible to most preferably design each of the thicknesses in accordance with the voltage, current, electric charge, or incoming luminous energy used for a photoelectric converter. However, a thickness of 500 Å or more not allowing electrons and a hole to pass and capable of functioning as the gate insulating film of a TFT is preferable.  
         [0034]     After each layer is deposited, an area serving as a contact hole  408  is etched and then, Al is deposited up to a thickness of approx. 10,000 Å as the upper metal layer  6  through sputtering or the like. Moreover, by patterning the Al film through photolithography and etching unnecessary areas, the upper electrode of the photoelectric conversion element  21 , the source and drain electrodes serving as main electrodes of the TFTs  22  to  24 , the upper electrode of the capacitor  300 , and the upper wirings  406  and  416  are formed. At the same time, the lower wiring  402  and the upper wiring  406  are connected each other in the contact hole  408 .  
         [0035]     Moreover, by etching the n layers of only the channel sections of the TFTs  22  to  24  with a RIE and then, etching the unnecessary SiN layer  70 , i layer  4 , and n layer  5 , each element is separated. Thus, the photoelectric conversion element  21 , TFTs  22  to  24 , lower wirings  402  and  412 , and upper wirings  406  and  416 , and contact hole  408  are completed.  
         [0036]     Furthermore, to improve the durability, the upper side of each element is normally covered with a passivation film  50  of SiN or the like to form a photoluminescent body layer  51  on the film  50  as a wavelength conversion body. Incoming X-rays is converted by the photoluminescent body layer  51  into the light (typically, visible light) having a wavelength band which can be converted by the photoelectric conversion element  21 .  
         [0037]     As described above, the photoelectric converter is able to form the common lower metal layer  2 , SiN layer  70 , i layer  4 , n layer  5 , and upper metal layer  6  on which the photoelectric conversion element  21 , TFTs  22  to  24 , and capacitor  300 , and a wiring section  400  are simultaneously deposited by only etching each layer. Moreover, only one injection preventive layer is present in the photoelectric conversion element  21  and the layers can be formed in the same vacuum state. Furthermore, it is possible to form a gate insulating film and an i layer which are important for characteristics of a TFT in the same vacuum state. Furthermore, because the intermediate layer of the capacitor  300  includes an insulating layer not easily causing a leak due to heat, the capacitor  300  is formed by having preferable characteristics. Thus, the photoelectric converter can be manufactured at a low cost with an advanced performance.  
         [0038]     The above forming method can use amorphous silicon. When it is requested to improve the operation speed of a TFT, however, it is possible to improve the operation speed by forming an amorphous silicon layer and thereafter, changing the layer to a polysilicon layer through laser annealing.  
       Second Embodiment  
       [0039]      FIG. 4  is a schematic circuit diagram showing a photoelectric converter of the present invention. This embodiment makes it possible to output signals through successive scanning by arranging one-pixel portions of the photoelectric converter in  FIG. 1  like a matrix. Also, this embodiment is described below in accordance with a case of applying the embodiment as a radiation reader by using a wavelength conversion body.  
         [0040]     In  FIG. 4 , the photoelectric converter (photodiode or the like)  21  accumulates electric charges corresponding to the incoming luminous energy of the light in a photosensitive wavelength region obtained through photosensitive-conversion of X-rays. The photoelectric conversion element  21  is two-dimensionally arranged. Also in this case, when the photoelectric conversion element  21  has a sufficient capacitance, the capacitor  300  added to each pixel is unnecessary. One end of the photoelectric conversion element  21  is connected to the gate of a source-follower input MOS (Metal Oxide Silicon Transistor)  22 , the drain of the source-follower input MOS  22  is connected to the source of a vertical selection-switch MOS  23  and the source is connected to the load current source  25  through a vertical output line  35 , and the drain of the vertical selection-switch MOS (transistor)  23  is connected to the power-supply terminal  27  through a power-supply line  34  and these components constitute a source follower circuit.  
         [0041]     Moreover, symbol  24  denotes a reset switch, and its source is connected to the gate of the source-follower input MOS  22  and its drain is connected to the power-supply terminal  27  through the power-supply line  34 .  
         [0042]     When a signal voltage is generated in the gate of the source-follower input MOS  22  correspondingly to the electric charges accumulated in the photoelectric conversion element of each pixel, the above source follower circuit reads the signal voltage with the current amplified.  
         [0043]     The gate of the vertical selection-switch MOS  23  is connected to a vertical scanning circuit  32  by a vertical gate line  29 . The gate of the reset switch  24  is connected to the vertical scanning circuit  32  by a reset gate line  28 . Moreover, the output signal of the source follower circuit is output to an external unit through the vertical output line  35 , the horizontal transfer MOS switch  26 , a horizontal output line  30 , and an output amplifier  31 . The gate of the horizontal transfer MOS switch  26  is connected to a horizontal scanning circuit  33 .  
         [0044]     Operations of this circuit are described below. First, the circuit resets the photoelectric conversion element  21  by the reset switch  24 . Then, the circuit starts accumulation. A signal voltage is generated in the gate of the source-follower input MOS  22  correspondingly to the accumulated signal charges. After the accumulation time passes, signals of pixels selected by the vertical scanning circuit  32  and horizontal scanning circuit  33  are amplified by the source follower circuit and then, successively output through the output amplifier  31 .  
         [0045]     The structure of this circuit has advantages that a compact layout is realized because the power-supply line and reset power-supply line of the source follower are used in common and a source-follower output having a high linearity is obtained because the selection switch  23  is set to the power-supply side and thereby, the resistance of the selection switch  23  is absent between the source end and the constant-current source of the source-follower input MOS  22 .  
         [0046]     Then, an example of driving of the circuit of  FIG. 4  is described below by referring to  FIG. 5 .  
         [0047]      FIG. 4  is a timing chart showing an example of driving timing. In the case of this example, X-rays are continuously irradiated. Symbols D 1 , D 2 , D 3 , . . . , and DH denote driving of rows. For example, D 1  shows each timing related to the first row. In D 1 , φ 11  output from the vertical scanning circuit  32  denotes a reset pulse, φ 21  denotes a drive pulse for all lines of one row, and φ 31  denotes a read pulse output from the horizontal scanning circuit  33 . Thereby, the read pulse is sent to an analog-to-digital conversion circuit (A/D)  40  through the output amplifier  31  and stored in an X memory  41 .  
         [0048]     The potential of the photoelectric conversion element  21  on one-row line is reset by a pulse φ RESET1  in accordance with the reset pulse φ 11 , the light due to X-ray irradiation is detected by the photoelectric conversion element  21  for the time of approx. (T 1 -T 2 ) to accumulate electric charges in a capacitor C 1    300 . A transistor  23  is turned on by a pulse φ Drive1  to transfer a potential to each column C 2 . Then, potentials are successively output to the A/D from each column in accordance with a pulse φ Read1  through the output amplifier  31 .  
         [0049]     Thereafter, rows are read from D 2  to D N .  
         [0050]     The time of T 1  is equal to 33 msec (T 1 ≅1/30 sec) in the case of, for example, 30 frames per sec. When photoelectric conversion elements are arranged by 500×500 in rows and columns, it is necessary to read from D to D500, T 2  becomes almost equal to T 1 /500 which is equal to approx. 66 μsec, T 3  becomes equal to T 2 /500 which is equal to approx. 130 μsec.  
         [0051]     These times are determined by the number of frames and the number of pixels for 1 sec.  
         [0052]     For the above driving example, a case is described in which X-rays are continuously irradiated.  FIG. 6  shows a case of intermittently irradiating X-rays like a pulse as the second driving method.  
         [0053]     In the case of this example, since lines of all rows of a photoelectric conversion element are simultaneously reset, they are reset by reset pulses φ 11 , φ 31 , . . . , φ 1n . Then, X-rays are irradiated like a pulse for a time Tx.  
         [0054]     Thereafter, electric charges are successively read in accordance with the pulses φ 21 +φ 31 , . . . , φ 2n , and φ 3n  similarly to the case of the first driving described for  FIG. 5 .  
         [0055]     In the case of this example, an X-ray-pulse irradiation time Tx is required. Therefore, how to determine T 2  and T 3  is slightly different from the case of the above example. For example, when assuming that the number of pixels and the number of frames same as the case of the above example are used, T 2  is almost equal to (T 1 -T x )/500, T 1  is almost equal to 1/30 s, and T 3  is almost equal to T 2 /500.  
         [0056]     To completely derive the performance of a sensor, a problem may occur that the influence of a dark current during the accumulation time or the fluctuation of the offset potential of the source follower (SF) of each cell appears as fixed-pattern noises.  
         [0057]     In the first and second driving methods, the first method for eliminating fixed-pattern noises previously stores the output data of each element when X-rays are not irradiated in a memory and subtracts the dark current and the noises due to the offset potential of the SF from the output under X-ray irradiation. Thereby, it is possible to improve the sensor characteristic.  
         [0058]     In the second driving method, the second method executes a series of sequences not performing X-ray irradiation and uses the output when no X-ray irradiation is performed as noise data (N). Moreover, the photoelectric conversion output when X-ray irradiation is uniformly performed is used as (signal+noise) data values (S+N) and the difference ((S+N)−N) between the data values is calculated to obtain a signal output. Thereby, it is possible to correct noises.  
       Third Embodiment  
       [0059]      FIG. 7  is a schematic sectional view showing the structure of another embodiment in which a photoelectric converter of the present invention is applied to a radiation reader. It is possible to use the circuit of  FIG. 4  as an electric circuit. In  FIG. 7 , a PIN-type photodiode sensor has an upper electrode constituted with an ITO  710  and a first Al layer  712  serves as a wiring for supplying a negative potential to the ITO electrode  710 . A lower electrode is constituted with a second Al layer  708  and the second Al layer  708  is also formed on the upper side of thin-film transistors (for amplification, selection-switch, and reset transistor) through an insulating layer (SiN layer)  707 . Symbol  711  denotes an insulating film made of SiN or the like and  709  denotes a PIN junction layer.  
         [0060]     The second Al layer  708  serving as the lower electrode of the PIN-type photodiode sensor is connected with a Cr layer  715  and the Cr layer  715  is connected with a gate electrode  702  of the thin-film transistor for amplification (in  FIG. 7 , connection between the Cr layer  715  and the gate electrode  702  is schematically shown).  
         [0061]     The thin-film transistors (for amplification, selection-switch, and reset transistor) are constituted by being superimposed with the Cr gate electrode  702 , an insulating layer  703 , a semiconductor layer  704 , an ohmic layer  705 , and a source-drain electrode  706  on a glass substrate  701 .  
         [0062]     Moreover, it is possible to fabricate a capacitor by setting a Cr electrode below the second Al layer  708  through an insulating layer made of SiN or the like similarly to the structure of  FIG. 8 .  
         [0063]     Because a PIN-type photodiode sensor has a sensitivity in visible light, when using the sensor as a reader for radiation such as X-rays, a photoluminescent body  714  serving as a wavelength conversion body for converting X-rays into visible light is set on the upper side of the sensor through an insulting layer  713  made of PI (polyimide, SiO 2 , or SiN 4 ) as shown in  FIG. 7 .  
         [0064]     In the case of the structure of this embodiment, the PIN-type photodiode sensor and respective thin-film transistors (for amplification, selection-switch, and resetting) can use amorphous silicon or polysilicon.  
         [0065]     This embodiment above described makes it possible to obtain the following advantages.  
         [0066]     (1) Because a PIN-type photodiode sensor is also set on each thin-film transistor (though only an amplification transistor is illustrated in the form of a sectional view, selection-switch and reset transistors are actually included), it is possible to set a sensor opening ratio to a value closed to approx. 100% and further increase an S/N ratio.  
         [0067]     (2) When constituting the area sensor equivalent to an X-ray film shown in the conventional example by amplifying the signal charges accumulated by a PIN-type photodiode sensor with the amplifier (source follower amplifier) used for the first embodiment, it is possible to improve S/N correspondingly to the amplification factor of the amplifier.  
         [0068]     According to the above Items (1) and (2), it is possible to remarkably improve an S/N ratio compared to the case of a conventional area sensor (for example, an opening ratio is improved two times by the structure in Item (1) and charge amplification is improved 100 times by the structure in Item (2), and resultantly the S/N ratio is remarkably improved).  
       Fourth Embodiment  
       [0069]      FIG. 8  is a schematic sectional view showing the structure of an embodiment in which a photoelectric converter of the present invention is applied to a radiation reader. It is possible to use an embodied circuit by applying  FIG. 4 . In  FIG. 8 , an X-ray-direct-conversion-type sensor mainly made of PbI 2  constitutes the upper electrode with a first Al layer  813  and the lower electrode with a second Al layer  810 . By setting an insulating layer  812  made of PI (polyimide) or the like between the first Al layer  813  and PbI 2    811  of the upper electrode, it is prevented that electric charges are injected into the PbI 2    811  from the first Al layer  813  of the upper electrode. Moreover, in the case of this embodiment, it is possible to directly take out the electric charges due to incoming of X-rays by holding a material such as a-Se1, PbI 2 , HgI 2 , or Pb0 (in this case, PbI 2  is used) for producing a lot of electron-hole pairs in accordance with incoming of X-rays with upper and lower electrodes and applying an electric field between the upper and lower electrodes. The insulating layer  812  shown in  FIG. 8  is not always necessary. As shown in  FIG. 9 , it is possible to sufficiently take out output charges from a sensor not having the insulating layer  812  as outputs.  
         [0070]     A signal-charge accumulating capacitor is constituted with the second Al layer  810  of the lower electrode and the lowest Cr layer  808  to accumulate signal charges generated due to incoming of X-rays in the capacitor. Symbol  807  denotes an insulating film made of SiN or the like.  
         [0071]     In this case, the second Al layer  810  of the lower electrode is also formed on thin-film transistors (for amplification, selection switch, and resetting) through the insulating layer (SiN layer)  807  as shown in  FIG. 7  of the third embodiment.  
         [0072]     The second Al layer  810  serving as the upper electrode of the accumulating capacitor is connected with the gate electrode  802  of the amplifying thin-film transistor, serving as a lowest Cr layer (in  FIG. 8 , connection between the second Al layer  810  and the gate electrode  802  is schematically shown).  
         [0073]     The thin-film transistors (for amplification, selection-switch, and reset transistors) are constituted by being superimposed with the Cr gate electrode  802 , an insulting layer  803 , a semiconductor layer  804 , an ohmic layer  805 , and a source-drain electrode  806  on a glass substrate  801 .  
         [0074]     In the case of the structure of this embodiment, the thin-film transistor (for amplification, selection-switch, and resetting) can also use amorphous silicon or polysilicon similarly to the case of the third embodiment.  
         [0075]     According to the above embodiments, the following advantages can be obtained.  
         [0076]     (1) Because an X-ray-direct-conversion-type sensor is also set on each thin-film transistor (though only an amplification transistor is illustrated in the form of a sectional view, selection-switch and reset transistors are actually included), it is possible to set a sensor opening ratio to a value closed to approx. 100% and further increase the opening ratio approx. two times.  
         [0077]     (2) Because an X-ray-direct-conversion-type sensor mainly made of PbI 2  does not require that a photoluminescent body converts X-rays into visible light, it has a high collection efficiency after converting incoming X-rays into electric signal charges and resultantly, is superior in the efficiency of conversion from incoming X-rays into electric signal charges.  
         [0078]     Moreover, because a direct-conversion material is able to effectively collect electric charges in accordance with an electric field, it is possible to make the thickness relatively thick, increase the X-ray collection value, and raise the quantum efficiency.  
         [0079]     (3) By amplifying the signal charges accumulated by an X-ray-direct-conversion-type sensor mainly made of PbI 2  or the like with the amplifier (sour-follower amplifier) used for the first embodiment, S/N is improved compared to the case of the conventional type (transistor type).  
       Fifth Embodiment  
       [0080]      FIG. 10  shows another example of circuit applicable to the first to fourth embodiments. The circuit shown in  FIG. 10  is able to subtract an SF offset or the like from a signal.  FIG. 11  shows an example of timing of each of signals φ x , φ R , φ N , and φ S . In  FIG. 10 , symbol &lt;1&gt; denotes a reset random noise generated when resetting a sensor accumulation terminal section with a reset transistor and &lt;2&gt; denotes an offset-fixing pattern noise generated in a source follower section. X-rays show X-ray irradiation timing. However, it is also possible to continuously irradiate X-rays.  
         [0081]     To eliminate the above two types of noises, capacitors C T1  and C T2  are arranged similarly to the case of the circuit of  FIG. 10  to transfer signals accumulated in a dark state to the capacitor C T1  in accordance with the pulse φ N  and signals accumulated in a bright state to the capacitor C T2  in accordance with the pulse φ s , and input the signals in the capacitors C T2  and C T1  to a differential amplifier in accordance with the pulse φ H  and perform subtraction processing. Thereby, it is possible to obtain a sensor signal excluding the above noises &lt;1&gt; and &lt;2&gt;.  
         [0082]      FIG. 12  is a circuit diagram showing a photoelectric converter for outputting a signal through sequential scanning, constituted by arranging the photoelectric converters in  FIG. 10  like a matrix.  
         [0083]     The one-bit section in  FIG. 10  (shown by symbol S in  FIG. 12 ), that is, sensors and amplifiers (amplification transistor, selection transistor, and reset transistor) are arranged up to m sections in X direction and n sections in Y direction and the capacitors C T2  and C T1  are arranged on each signal line as described for  FIG. 10 .  
         [0084]     Therefore, by alternately reading m×n signal outputs in a dark state and a bright state by shift registers in X and Y directions and applying subtraction processing to the outputs by a differential amplifier, it is possible to obtain signal outputs having less noises and high S/N. In any case, it is possible to drive pixels arranged like a matrix through continuous or intermittent irradiation with a light source of X-rays or the like as described above.  
       Sixth Embodiment  
       [0085]      FIG. 13  shows still another example of circuit to be applied to the first to fourth embodiments.  FIG. 14  shows timings of signals φ x , φ R1 , φ R2 , φ N  and φ s . In this case, symbol &lt;1&gt; denotes a random noise generated when applying reset with a reset transistor according to the reset pulse φ R1 , &lt;2&gt; denotes a 1/f random noise generated while electric charges are accumulated in a source follower section, and &lt;3&gt; denotes an offset-fixing pattern noise generated in a source follower section. A sensor cell is reset by turning on the reset pulses φ R1  and φ R2  and then, accumulation by the sensor is started. In the case of an X-ray pulse, X-ray irradiation is started after the reset pulse φ R1  (pulse &lt;1&gt;) is turned off. It is also possible to continuously irradiate X-rays.  
         [0086]     To remove the above three types of noises, capacitors C T1  and C T2  are arranged similarly to the case of the circuit of  FIG. 13  to transfer signals accumulated in a dark state to the capacitor C T1  in accordance with the pulse φ N  and signals accumulated in a bright state to the capacitor C T2  in accordance with the pulse φ H , input the signals in the capacitors C T2  and C T1  to a differential amplifier in accordance with the pulse φ H  and perform subtraction processing. Thereby, it is possible to obtain a sensor signal excluding the above noises &lt;1&gt;, &lt;2&gt;, and &lt;3&gt;.  
         [0087]      FIG. 15  is a schematic circuit diagram showing a photoelectric converter for outputting a signal through sequential scanning by arranging the photoelectric converters in  FIG. 13  like a matrix. In  FIG. 15 , reset circuits G 1  and G 2  are built in. Also in the case of  FIG. 12 , it is possible to incorporate the reset circuits G 1  and G 2 .  
         [0088]     The one-bit section in  FIG. 13  (shown by symbol S in  FIG. 15 ), that is, sensors and amplifiers (amplification transistor, selection transistor, and reset transistor) are arranged up to m sections in X direction and n sections in Y direction and the capacitors C T2  and C T1  are arranged on each signal line as described for  FIG. 23 .  
         [0089]     Therefore, by alternately reading m×n signal outputs in a dark state and a bright state by shift registers in X and Y directions and applying subtraction processing to the outputs by a differential amplifier, it is possible to obtain signal outputs having less noises and high S/N. Each photoelectric conversion element can be driven by continuous X-ray irradiation and pulses similarly to the case of  FIG. 5  or  6 .  
         [0090]     Because n one-bit circuits are connected to each signal line, n overlapped capacitances C gs  of the source and gate of an amplification transistor are connected in parallel. In the case of an X-ray area sensor, n is equal to 500 to 2,000. Therefore, signal-line wiring capacitance C 2  becomes equal to C gs ((500 to 2,000). Thus, when C gs  increases, C 2  becomes a very large value.  
         [0091]     To read only a static image, increase of C 2  is not a large problem. However, to perform dynamic-image-like read, increase of C 2  greatly influences the read rate.  
         [0092]     In the case of  FIGS. 2, 3 ,  7 ,  8 , and  9 , a photoelectric conversion section having a light-receiving section (capacitance to be formed according to necessity), reset MOS, and source follower are formed on an insulating substrate and others are normally constituted with external circuits. Therefore, the time constant obtained as the product between the ON-resistance Ron and the capacitance C 2  of a transistor in a sensor cell is the largest problem.  
         [0093]      FIG. 16  shows an example of the data for the ratio between the ON-resistance, width (W), and channel length (L) of a transistor when using amorphous silicon as the material of the transistor. In  FIG. 16 , the broken line shows the calculated value when the thickness of amorphous silicon is 3,000 Å and the continuous line shows the calculated value when the thickness of the amorphous silicon is 1,000 Å.  
         [0094]     Symbols ▴, Δ, and denote measured data values.  
         [0095]     Normally, W/L ranges between 2 and 10 and the ON-resistance Ron ranges between 1 and 10 MΩ. Because C 2  normally has 16 to 50 pF though depending on design, RonC 2  ranges between 10 and 50 (sec. Normally, a pulse length (e.g. T 2  in  FIG. 5 ) requires a value three times larger than RonC 2  for read. Therefore, the above pulse length reaches 30 to 1,500 μsec. As described for  FIG. 5 , T 2  is equal to, for example, 66 μsec. Though this value can correspond to a switching speed in the minimum region, it may not be able to correspond to the switching speed in the normal range. Therefore, by decreasing the thickness of the i layer and increasing the W/L to 10 or more, it is possible to correspond to high-speed switching.  
         [0096]     In the case of the embodiment shown in  FIGS. 2 and 3 , the W/L cannot be set to a large-enough value for design of a transistor. However, in the case of the laminated-type photoelectric converter shown in  FIGS. 7, 8 , and  9 , it is possible to fabricate a transistor by using the substantially entire surface of a pixel or photoelectric-element forming region. Therefore, it is possible to completely reduce the Ron.  
         [0097]     In the case of the embodiment in  FIGS. 7, 8 , and  22 , a sufficient switching speed is obtained and therefore, a structure suitable for a dynamic image can be formed.  
       Seventh Embodiment  
       [0098]     In the case of the present invention, it is also possible to form a photoelectric conversion element and a switching element on a single-crystal substrate.  
         [0099]      FIG. 17  shows a schematic sectional view of a photoelectric converter having a photoelectric conversion section  1701  and a MOS transistor  1702  on a Si single-crystal substrate  510 . It is possible to form a photoelectric converter similarly to the case of forming a photoelectric conversion element and a TFT on the previously-described insulating substrate. However, when X-rays passing through a photoluminescent body ( 506 ) without being completely absorbed is absorbed in single crystal, extra carriers are produced in the Si single crystal and becomes noises against a signal caused by X-rays produced duel to the light emitted from the photoluminescent body.  
         [0100]     Therefore, as shown in  FIG. 17 , it is necessary to electrically separate the substrate  510  from a sensor or switch region (in  FIG. 17 , they are electrically separated from each other in accordance with the P-type and N-type). It is preferable to set the thickness of the P-type region (p-well)  501  to a value two or three times or less the absorption coefficient of the luminescent wavelength of the photoluminescent body  506  so that the light wavelength-converted by the photoluminescent body  506  can be completely detected. The p-well  501  is electrically separated from the substrate  510  by applying a reverse bias. Then, X-rays are absorbed by only a value equivalent to the thickness (2 to 3 μm or less) of the p-well  501 , noises due to direct X-ray absorption are decreased, and performances are improved.  
         [0101]     By decreasing the thickness of the p-well ( 501 ), X-rays are less absorbed and the noise characteristic is improved. Though depending on a photoluminescent body, when using a Gd-based material as the photoluminescent body  506 , 30 to 50% of X-rays are absorbed by the photoluminescent  506  and residual X-rays pass through the photoluminescent  506  without being wavelength-converted by the photoluminescent  506 . However, when absorbing all the X-rays with an Si substrate, it is impossible to decide the information obtained though the absorption as a signal or noise. Approximately 1/10,000 X-rays of approx. 50 KeV are absorbed by Si single crystal having a thickness of 2 nm. Therefore, it is possible to reduce noise components by electrically separating from the thickness of the above region.  
         [0102]     In the case of the photoelectric converter shown in  FIG. 17 , it is preferable to change the threshold voltage of the vertical selection switch  23  and that of the reset switch  24 . The reason is described below.  
         [0103]     First, the input MOS transistor  22  of the source follower of  FIG. 1  must meet the following conditional expression (1). 
 
V ds &gt;V gs −V th2   (1) 
 
         [0104]     In the above expression, ds denotes a potential difference between drain and source, V gs  denotes a potential difference between gate and source, and V th2  denotes a threshold voltage.  
         [0105]     In this case, it is assumed that the gate voltage when the reset switch  24  is turned on is V 2 , the gate voltage when the vertical selection switch  23  is turned on is V 3 , the drain voltage of the input MOS transistor  22  of the source follower is V 1 , the threshold voltage of the reset switch  24  is V th0 , the threshold voltage of the vertical selection switch  23  is V th1 , and the threshold voltage of the input MOS transistor  22  of the source follower is V th2 .  
         [0106]     When assuming a case in which both the reset switch  24  and the vertical selection switch  23  operate in a pentode region (region in which the bias (VDS) between source and drain is equal to or lower than a pinch-off voltage), the reset voltage V sig0  is expressed by the following expression (2). 
 
 V   sigo   =V   2   −V   th0   (2) 
 
         [0107]     Then, when assuming that the current flowing through the vertical selection switch  23  is equal to the current flowing through the source follower circuit, the following expression (3) is effectuated. 
 
 Ia=K ( V   3   −V   1   −V   th1 ) 2   (3) 
 
 K =½ ×μ×C   ox   ×W/L  
        μ: Mobility     C ox : Gate oxide film capacity for unit area     W: Gate width     L: Gate length        
 
         [0112]     In this case, a gradual channel approximation is used to simplify the explanation.  
         [0113]     By transforming the above expression, the following expression (4) is obtained. 
 
 V   1   =V   3   −V   th1 −{square root}{square root over (( Ia/K ))}  (4) 
 
         [0114]     By substituting the expressions (2) and (4) for the expression (1), the conditional expression for the source follower to operate in a linear operating region is shown by the following expression (5). 
 
 V   3   −V   th1 ( Ia/K )&gt; V   2   −V   th0   −V   th2   (5) 
 
         [0115]     As an example in which the reset switch  24  and vertical selection switch  23  operate in the pentode region, the gate voltages V 2  and V 3  have used a voltage equal to a power-supply voltage and the threshold voltages of the switches  23  and  24  have used the same value. In this case, the expression (5) is transformed into the following expression (6). 
 
IaY&lt;Vt  (6) 
 
 Thus, it is found that the current which can be flown through the source follower circuit is controlled by the threshold voltage of each switch. 
 
         [0116]     Therefore, when change to multiple pixel is accelerated and the load which must be driven by a source follower circuit increases, it is more preferable to change threshold voltages of the vertical selection switch  23  and reset switch  24  so as to meet the above expressions. For example, it is desired to change the threshold value (Vth) of respective transistors by on the order of 0.5 V to 1.0 V.  
         [0117]     Examples of raising the threshold voltage of the reset switch  24  shown in FIGS.  1  to  3  by 1 V compared to that of the vertical selection switch  23  are shown below.  
         [0118]     (a) By constituting the gate metal of the reset switch  23  with chromium and the gate metal of the vertical selection switch  23  with aluminum, the threshold of the reset switch  24  becomes approx. 2.5 V and that of the vertical selection switch  23  becomes approx. 1.5 V.  
         [0119]     (b) When constituting gate metals of the vertical selection switch  23  and reset switch  24  with aluminum, by applying +20 V to the potential V 2  of the gate metal of the reset switch  24  in common to all bits and moreover, grounding the potential V 3  of the gate metal of the vertical selection switch  23  and driving the switches  23  and  24  for approx. 3 hr at ordinary temperature, the threshold of the reset switch  24  becomes approx. 2.5 V and that of the vertical selection switch  23  is kept at approx. 1.5 V.  
         [0120]     Then, to form a photoelectric conversion element, a field-effect transistor (MOS transistor) whose gate is connected with the output side of the photoelectric conversion element on the single-crystal substrate, vertical selection switch, and reset switch, a method for changing threshold voltages of the vertical selection switch and reset switch is described below.  
         [0121]      FIG. 18  is a sectional view showing a method for changing threshold voltages. In  FIG. 18 , symbol  501  denotes a semiconductor substrate, in which a P-type semiconductor is shown. Symbol  502  denotes a gate electrode formed on the semiconductor substrate  501  through a gate oxide film, which is formed with, for example, polysilicon or polycide. Symbol  503  denotes a source region and a drain region formed in the semiconductor substrate  501  through ion implantation or the like and having a conducting type opposite to that of the semiconductor substrate  501 . A field-effect transistor is constituted with the above components. Moreover, to improve the durability, the upper side of each element is normally covered with a passivation film  505  made of SiN or the like to form a photoluminescent layer  506  on the film  505 . Incoming X-rays are converted into the light in a wavelength band which can be photoelectric-converted (typically, visible light) by a photoelectric conversion element on the photoluminescent layer  506 .  
         [0122]     In this case, by forming a channel doped layer  504  in the channel region of only a desired transistor, it is possible to easily make the threshold voltage of the transistor different from that of other transistors. For example, in the case of the example of  FIG. 18 , by doping the layer  504  with an N-type ion species it is possible to lower the threshold voltage of a transistor compared to that of a transistor not doped with the N-type ion species. However, by doping the layer  504  with a P-type ion species, it is possible to raise the threshold voltage. The change value can be accurately determined by controlling the concentration of the channel doped layer  504 .  
         [0123]     An N-type field-effect transistor is described above as an example. However, it is needless to say that the same advantage can be also obtained from a P-type field-effect transistor. Moreover, for this embodiment, a case is described in which the channel doped layer of one transistor is controlled. However, the present invention is not restricted to the above case. It is also possible to use a plurality of channel doped layers by setting each of them to the optimum condition.  
         [0124]     In the above description, the expression (3) of gradual channel approximation is used as an expression of the current flowing through a field-effect transistor. In addition to the case of the above ideal transistor, even if a slight shift occurs from the above expression because of progress of fining of a transistor, the advantage is not changed. It is essential to control the on-resistance of a field-effect transistor so as to meet the expression (1). For this reason, it is very effective means to change threshold voltages of a vertical selection switch and a reset switch.  
         [0125]     The structure shown in  FIG. 19  is used for another method for changing threshold voltages. In  FIG. 19 , symbol  601  denotes a well region provided only for a desired transistor region. Other structures are the same as those shown in  FIG. 18 . Also by the structure of  FIG. 19 , it is possible to easily control the threshold voltage of a desired transistor. Moreover, for  FIG. 19 , a case is described in which a P-type well region is formed in a P-type substrate. However, it is also possible to determine a desired threshold voltage by forming a plurality of P-type wells having different concentrations in an N-type substrate and controlling each concentration. Furthermore, though an N-type field-effect transistor is described as an example, it is needless to say that the same advantage can be obtained from a P-type field-effect transistor.  
         [0126]     It is also effective to form the direct-type photoelectric conversion element shown in  FIGS. 8 and 9  on a single-crystal substrate. In this case, it is preferable that a substrate can be electrically separated from a transistor similarly to the case of  FIG. 17 . Moreover, it is a matter of course that the circuit and operation previous described can be applied to a direct-type X-ray sensor.  
         [0127]     It is already described that, when using a single-crystal substrate as a sensor substrate, transmitted X-rays to be absorbed in the substrate are decreased. Moreover, it is possible not only to make the region of a p-well or the like thin but also to form a shielding layer at the upper side.  
         [0128]     For example, it is preferable to form the gate of a MOS transistor with a heavy metal instead of polysilicon. Specifically, it is preferable to form the electrode  502  of  FIG. 17  with a heavy metal (Pt, W, Mo, or Pd). In the case of a MOS transistor, it is preferable to form a metal into a double or triple structure and moreover, form the lower side with polysilicon and the upper side with a heavy-metal silicide or a heavy metal.  
         [0129]     Furthermore, as shown in  FIG. 20 , it is possible to form an X-ray shielding layer  600  between the photoluminescent body and the substrate at a portion other than the light detecting section.  
         [0130]      FIGS. 21 and 22  show X-ray absorption characteristics of platinum Pt and tungsten W which can be used as shielding materials. For example, by using a shielding layer made of Pt and having a thickness of 10 μm, it is possible to obtain X-ray shielding effects of 13% and 91% against X-rays of 50 KeV and 10 KeV. Particularly, the shielding layer is every effective for a low energy.  
         [0131]     When applying the direct-type structure shown in  FIGS. 8 and 9  to a single-crystal substrate, it is possible to use a shielding layer made of a heavy metal (e.g. Pt, W, Mo, or Pd), which is the second Al layer  810  instead of Al.  
         [0132]     As described above, the present invention makes it possible to prevent an output voltage from lowering due to increase of a parasitic capacitance caused when arranging a plurality of photoelectric conversion elements by sending signal charges generated in a photoelectric conversion element to the gate of a reading field-effect transistor and amplifying the signal charges with the reading field-effect transistor to output a signal.  
         [0133]     Besides, an photoelectric conversion element may be arranged on at least any one of the reading field-effect transistor, selection switch means, and reset means, so as to allow the opening ratio to be improved.  
         [0134]     Moreover, the present invention makes it possible to provide a photoelectric converter having higher sensitivity and more-advanced performance and radiation reader having the photoelectric converter.