Abstract:
An X-ray flat panel detector includes an X-ray photosensitive film which generates signal charges upon being exposed to incident X-rays, pixel electrodes which are arrayed in contact with the X-ray photosensitive film, a bias voltage application unit which applies a bias voltage to the X-ray photosensitive film so as to make the pixel electrodes collect holes or electrons, which serve as the signal charges generated by the X-ray photosensitive film and have a higher mobility, capacitors which are arranged in correspondence with the pixel electrodes and store the charges generated by the X-ray photosensitive film, switching thin-film transistors which are arranged in correspondence with the pixel electrodes and read the charges in the capacitors, scanning lines which supply a control signal to OPEN/CLOSE-control the switching thin-film transistors, and signal lines which are connected to the switching thin-film transistors to read the charges when the switching thin-film transistors are opened.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-307609, filed Oct. 3, 2001, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an X-ray flat panel detector used in a medical X-ray diagnosis apparatus or the like and, more particularly, to an X-ray flat panel detector which uses a thin-film transistor as a switching element in each of two-dimensionally arrayed X-ray detection pixels.  
           [0004]    2. Description of the Related Art  
           [0005]    In recent years, X-ray flat panel detectors for electrically detecting an X-ray image have been developed as X-ray diagnosis apparatuses. In such an X-ray flat panel detector, X-ray photosensitive pixels which generate signal charges when being irradiated with X-rays are two-dimensionally arrayed. Signal charges generated by these photosensitive pixels are stored in signal charge storing sections and read out by thin-film transistors serving as switching elements.  
           [0006]    [0006]FIG. 1 shows the circuit arrangement of one pixel of this X-ray flat panel detector. As shown in FIG. 1, the X-ray flat panel detector has a plurality of pixels  101  that are arranged in an array. Each pixel  101  is constituted by a thin-film transistor (to be referred to as a TFT hereinafter)  102  made of amorphous silicon (to be referred to as a-Si hereinafter) and used as a switching element, an X-ray charge conversion film  103  made of Se, a pixel capacitor (to be referred to as a Cst hereinafter)  104 , and a protective TFT  111 . The Cst  104  is connected to a Cst bias line  105 .  
           [0007]    The switching TFT  102  has a gate connected to a scanning line  107  and a source connected to a signal line  108 . The TFT  102  is ON/OFF-controlled by a scanning line driving circuit  109 . A terminal of the signal line  108  is connected to a signal detection amplifier  110 . The source and gate of the protective TFT  111  are connected to the drain of the switching TFT  102 . The drain of the protective TFT  111  is connected to a power supply  113  through a bias line  112 . Note that the protective TFT  111  is sometimes omitted.  
           [0008]    When X-rays become incident, they are converted into charges by the X-ray charge conversion film  103 . The charges are stored in the Cst  104 . When the scanning line  107  is driven by the scanning line driving circuit  109  to turn on the switching TFTs  102  of one row connected to one scanning line  107 , the stored signal charges are transferred to the amplifier  110  side through the signal line  108 . With change-over switches (not shown), the charges are input to the amplifier  110  for each pixel and converted into a dot sequential signal that is displayable on a CRT or the like. The charge amount and the output amplitude of the amplifier  110  change depending on the amount of x-ray light incident on the pixels  101 . To prevent extra charges from being stored in the Cst  104 , charges more than the bias voltage are removed from the bias line  112  by the protective TFT  111 .  
           [0009]    However, the presence of a protective circuit such as the protective TFT  111  poses the following problems. First, since the pixel circuit is complicated, the yield of non-defective TFT array boards decreases. Second, when the number of pixels is increased and the pixel size is reduced to obtain a higher resolution, it becomes difficult to form interconnections from the protective TFTs  111 , switching TFTs  102 , and the like at a sufficiently small pitch. Third, since the occupation area of the protective TFT  111  in each size-reduced pixel is large, no sufficient area can be ensured for the Cst  104 .  
           [0010]    In addition, when the number of pixels is increased and the pixel size is reduced to obtain a higher resolution, the following problems have occurred. To externally connect a TAB having a driving circuit for driving switching TFTs and an LSI for reading a signal outside the detection pixel area, these interconnections must be connected to the TFT array board on which the switching TFT  102 , x-ray charge conversion film  103 , Cst  104 , and protective TFT  111  are arranged. At this time, if the pixel size is reduced, it becomes difficult to connect the interconnections at a sufficiently small pitch. In addition, since the ratio of the TFT area in the small pixel is too high, the signal storage capacitor cannot have a sufficient area. Furthermore, when the number of pixels increases, the signal read time per line shortens. However, in the TFT formed from a-Si, a signal cannot be sufficiently read in the short address time. For this reason, in the conventional X-ray flat panel detector and, more particularly, an X-ray flat panel detector using a-Si TFTs, when the pixel size is reduced and the number of pixels is increased to obtain a higher resolution, no satisfactory detection image can be obtained because of shortage in TFT driving capability.  
           [0011]    For an X-ray detector for medical use, it is important to do diagnosis using an X-ray dose as low as possible to suppress the influence on the body. To do this, it is important to reduce the OFF current of a switching TFT to make it possible to detect a small amount of charges generated at a low X-ray dose. On the other hand, a satisfactory image is necessary for accurate diagnosis. In this case, it is necessary to sense an image with a high S/N ratio at a high X-ray dose. Hence, it is preferable to sense an image at an X-ray dose ranging from low level to high level. For this purpose, a switching TFT must normally operate even at a high pixel voltage corresponding to a large amount of charges. In addition, the OFF current of the switching TFT must be reduced. Many switching TFTs used in such an X-ray flat panel detector form defects by X-ray irradiation, and their characteristic degrades. The switching TFT must have a TFT characteristic that allows the TFT to operate even after degradation.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    The present invention has been made in consideration of the above situation, and has as its object to provide an X-ray flat panel detector which can prevent it from becoming complex for the protective function against a high voltage and capable of acquiring a satisfactory image even when the pixel size is reduced and the pixel number and bus line number are increased.  
           [0013]    The present invention may provide an X-ray flat panel detector comprises: an X-ray photosensitive film which generates signal charges upon being exposed to incident X-rays; a plurality of pixel electrodes which are two-dimensionally arrayed in contact with the X-ray photosensitive film; a bias voltage application unit which applies a bias voltage to the X-ray photosensitive film so as to make the plurality of pixel electrodes collect holes or electrons, which serve as the signal charges generated by the X-ray photosensitive film and have a higher mobility; a plurality of capacitors which are arranged in correspondence with the pixel electrodes and store the charges generated by the X-ray photosensitive film; a plurality of switching thin-film transistors which are arranged in correspondence with the pixel electrodes and read the charges stored in the capacitors; a plurality of scanning lines which supply a control signal to OPEN/CLOSE-control the plurality of switching thin-film transistors; and a plurality of signal lines which are connected to the plurality of switching thin-film transistors to read the charges when the switching thin-film transistors are opened.  
           [0014]    The present invention may provide an X-ray flat panel detector comprises: an X-ray photosensitive film which generates signal charges upon being exposed to incident X-rays; a plurality of pixel electrodes which are two-dimensionally arrayed in contact with the X-ray photosensitive film; a bias voltage application unit which applies a bias voltage to the X-ray photosensitive film so as to make the plurality of pixel electrodes collect holes or electrons, which serve as the signal charges generated by the X-ray photosensitive film and have a higher mobility; a plurality of capacitors which are arranged in correspondence with the pixel electrodes and store the charges generated by the X-ray photosensitive film; a plurality of switching thin-film transistors which are arranged in correspondence with the pixel electrodes and read the charges stored in the capacitors by using, as carriers, the holes or electrons, which serve as the signal charges generated by the X-ray photosensitive film and have a higher mobility; a plurality of scanning lines which supply a control signal to OPEN/CLOSE-control the plurality of switching thin-film transistors; and a plurality of signal lines which are connected to the plurality of switching thin-film transistors to read the charges when the switching thin-film transistors are opened.  
           [0015]    The present invention may provide an X-ray flat panel detector comprises: an X-ray photosensitive film which is substantially made of a material having a hole mobility higher than an electron mobility and generates signal charges upon being exposed to incident X-rays; a plurality of pixel electrodes which are two-dimensionally arrayed in contact with the X-ray photosensitive film; a bias voltage application unit which applies a bias voltage to the X-ray photosensitive film so as to make the plurality of pixel electrodes collect holes; a plurality of capacitors which are arranged in correspondence with the pixel electrodes and store the charges generated by the X-ray photosensitive film; a plurality of p-channel thin-film transistors which are arranged on an insulating substrate in correspondence with the pixel electrodes, have a polysilicon film formed into an island shape on the insulating substrate, and read the signal charges stored in the capacitors; a plurality of scanning lines which supply a control signal to OPEN/CLOSE-control the plurality of p-channel thin-film transistors; and a plurality of signal lines which are connected to the plurality of p-channel thin-film transistors to read the signal charges when the p-channel thin-film transistors are opened.  
           [0016]    The present invention may provide an X-ray flat panel detector comprises: an X-ray photosensitive film which is substantially made of a material having a higher electron mobility than a hole mobility and generates electrons and holes upon being exposed to incident X-rays; a plurality of pixel electrodes which are two-dimensionally arrayed in contact with the X-ray photosensitive film; a plurality of capacitors which are arranged in correspondence with the pixel electrodes and store the electrons generated by the X-ray photosensitive film; a plurality of n-channel thin-film transistors which are arranged in correspondence with the pixel electrodes and read the electrons stored in the capacitors; a plurality of scanning lines which supply a control signal to OPEN/CLOSE-control the plurality of n-channel thin-film transistors; and a plurality of signal lines which are connected to the plurality of n-channel thin-film transistors to read the electrons when the n-channel thin-film transistors are opened.  
           [0017]    The present invention may provide an X-ray flat panel detector comprises: an X-ray photosensitive film which generates signal charges upon being exposed to incident X-rays; a plurality of pixel electrodes which are two-dimensionally arrayed in contact with the X-ray photosensitive film; a bias voltage application unit which applies a bias voltage to the X-ray photosensitive film; a plurality of capacitors which are arranged in correspondence with the pixel electrodes and store the charges generated by the X-ray photosensitive film; a plurality of switching thin-film transistors which comprises a p-channel thin-film transistor made of polysilicon and are arranged in correspondence with the pixel electrodes and read the charges stored in the capacitors; a plurality of scanning lines which supply a control signal to OPEN/CLOSE-control the plurality of switching thin-film transistors; and a plurality of signal lines which are connected to the plurality of switching thin-film transistors to read the charges when the switching thin-film transistors are opened.  
           [0018]    The present invention may provide an X-ray flat panel detector comprises: an X-ray photosensitive film which generates signal charges upon being exposed to incident X-rays; a plurality of pixel electrodes which are two-dimensionally arrayed in contact with the X-ray photosensitive film; a bias voltage application unit which applies a bias voltage to the X-ray photosensitive film; a plurality of capacitors which are arranged in correspondence with the pixel electrodes and store the charges generated by the X-ray photosensitive film; a plurality of switching thin-film transistors which comprises a p-channel thin-film transistor made of polysilicon and with LDD structure and are arranged in correspondence with the pixel electrodes and read the charges stored in the capacitors; a plurality of scanning lines which supply a control signal to OPEN/CLOSE-control the plurality of switching thin-film transistors; and a plurality of signal lines which are connected to the plurality of switching thin-film transistors to read the charges when the switching thin-film transistors are opened. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0019]    [0019]FIG. 1 is a circuit diagram showing the circuit arrangement of one pixel of an X-ray flat panel detector;  
         [0020]    [0020]FIG. 2 is a circuit diagram showing the basic arrangement of an X-ray flat panel detector according to the first embodiment of the present invention;  
         [0021]    [0021]FIG. 3A is a plan view showing the structure of a pixel  1 ;  
         [0022]    [0022]FIG. 3B is a sectional view showing the structure of the pixel  1 ;  
         [0023]    [0023]FIG. 3C is a view showing a change in leading edge of the switching characteristic of an n-channel p-Si TFT used in a conventional X-ray flat panel detector before and after X-ray irradiation;  
         [0024]    [0024]FIG. 4 is a view for explaining the operation of the X ray flat panel detector according to the first embodiment;  
         [0025]    [0025]FIG. 5A is a graph showing the diode characteristic of a p-channel p-Si TFT when a voltage of 10 V is applied in a gate-off state together with the TFT characteristic of the p-channel TFT  
         [0026]    [0026]FIG. 5B is a graph showing the off-leakage current of a p-channel TFT Id that is formed by the process of the first embodiment using p-Si;  
         [0027]    [0027]FIG. 5C is a graph showing the characteristic of the p-channel TFT of the first embodiment before and after X-ray irradiation;  
         [0028]    [0028]FIG. 6A is a plan view showing the structure of one pixel of an X-ray flat panel detector according to the second embodiment of the present invention;  
         [0029]    [0029]FIG. 6B is a sectional view showing the structure of one pixel of the X-ray flat panel detector;  
         [0030]    [0030]FIG. 7 is a circuit diagram showing a driver circuit in an X-ray flat panel detector according to the third embodiment of the present invention; and  
         [0031]    [0031]FIG. 8 is a sectional view of the driver circuit shown in FIG. 7. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    The first to third embodiments of the present invention will be described below with reference to the accompanying drawing.  
         [0033]    First Embodiment  
         [0034]    [0034]FIG. 2 is a circuit diagram showing the basic arrangement of an X-ray flat panel detector according to the first embodiment of the present invention.  
         [0035]    X-ray detection pixels  1  for converting incident X-rays into an electrical signal are two-dimensionally arrayed to form an image sensing region  2 . Each pixel  1  comprises an X-ray charge conversion film  1   a  for converting incident X-rays into an electrical signal, a pixel electrode  1   b  connected to the X-ray charge conversion film  1   a,  a storage capacitor  1   c  connected to the pixel electrode  1   b,  and a switching thin-film transistor (TFT)  1   d  having a source connected to the pixel electrode  1   b.  FIG. 2 shows a simple 2×2 pixel arrangement. In fact, an m×n pixel arrangement including a number of rows and a number of columns is formed.  
         [0036]    An important point of the present invention is that the direction of a bias voltage to be applied to the X-ray charge conversion film  1   a  is selected in accordance with the type of X-ray charge conversion film  1   a.  More specifically, when the X-ray charge conversion film  1   a  having a hole mobility higher than the electron mobility is used, a bias voltage is applied to the X-ray charge conversion film  1   a  in a direction in which holes are collected by the pixel electrode. On the other hand, when an X-ray photosensitive film having an electron mobility higher than the hole mobility is used, a bias voltage is applied to the X-ray charge conversion film  1   a  in a direction in which electrons are collected by the pixel electrode. For a more detailed description, an X-ray flat panel detector which uses the X-ray charge conversion film  1   a  having a hole mobility higher than the electron mobility will be exemplified in the following embodiments.  
         [0037]    In the image sensing region  2 , a plurality of scanning lines  3  and a plurality of signal lines  4  are arranged to be perpendicular to each other. The scanning lines  3  are connected to the gates of the switching TFTs  1   d  in the image sensing region  2  and to a gate driver  5  for selectively driving the pixels outside the image sensing region  2 . The signal lines  4  are connected to the drains of the switching TFTs  1   d  in the image sensing region  2  and to a noise correction circuit  6 . A signal on signal lines  4  is read and output by a read circuit  7  outside the image sensing region  2 .  
         [0038]    [0038]FIG. 3A is a plan view showing the structure of the pixel  1 . FIG. 3B is a sectional view showing the structure of the pixel  1 . An SiN x  (50 nm)/SiO 2  (100 nm) film serving as an undercoat insulating film  11  is formed on a glass substrate  10 . A 50-nm thick amorphous Si (a-Si) film is formed on the undercoat insulating film  11 .  
         [0039]    The a-Si film is converted into polysilicon by ELA (Excimer Laser Anneal) to form a 50-nm thick polysilicon (p-Si) film  12 . The p-Si film  12  is etched to form a transistor region island  12 - 1  and capacitor region island  12 - 2 . A 150-nm thick gate SiO 2  film  13  is formed by PCVD or thermal CVD. A 300-nm thick gate electrode  14 - 1  and a 300-nm thick gate electrode  14 - 2  are formed on the transistor region island  12 - 1  and the capacitor region island  12 - 2 , respectively, as the MoW gates.  
         [0040]    Next, using the gate electrodes or a resist as a mask, B is doped by ion implantation at a high concentration of 1×10 14  cm −2  to 5×10 16  cm −2 , preferably, 1×10 15  cm −2  to 1×10 16  cm −2 , and in this embodiment, 3×10 15  cm −2  to form p + -regions  15 . That is, a drain  15 - 1  and source  15 - 2  formed from the p + -regions are formed in the transistor region, and a p + -region  15 - 3  is formed in the capacitor region. A gate width W and gate length L are defined as, e.g., W/L=10/5 μm.  
         [0041]    Next, a 500nm thick SiO 2  film  16  serving as a dielectric interlayer is formed by PCVD. Holes are formed in the source/drain contact portion of the SiO 2  film  16  to form a signal line  17 - 1  connected to the drain  15 - 1  of the TFT  1   d,  a Cs line  17 - 2  connected to the source  15 - 2  of the TFT id and the p + -region  15 - 3  of the storage capacitor  1   c,  and a capacitor line  17 - 3  connected to the storage capacitor by an Mo/Al/Mo film  17 . Next, a 400nm thick SiN x  film (not shown) for passivation is formed by PCVD. After that, a protective film  18  is formed by coating an acrylic resin to 2 to 5 μm, and preferably, 3 μm. A contact portion is formed using an acrylic photosensitive resin by exposing and developing it. A 100nm thick ITO film is formed and patterned into a desired shape, thereby forming an ITO pixel electrode  19 .  
         [0042]    Then, a p-Se layer  21 , i-type Se layer  22 , and n-Se layer  23  are formed in this order as an X-ray photosensitive film  20 . More specifically, a p-Se or p-As 2 Se 3  film  21  is formed to 5 to 100 μm, and preferably, 10 μm by vapor deposition. An undoped high-resistance Se film  22  having a thickness of 900 μm is formed on this film. An n-Se layer  23  is further formed to 5 to 100 μm, and preferably, 10 μm. Cr is deposited to 100 nm and Al is deposited to 300 nm on the uppermost portion, thereby forming an upper electrode  25 .  
         [0043]    Since the above structural elements are formed on a glass substrate, all manufacturing steps are executed at a temperature not higher than 500° C. so as to obtain an optimal TFT array. For this reason, the p-Si used in the X-ray charge conversion film  1   a  has characteristics different from those of the p-Si used in an LSI which is usually baricated at higher temperatures.  
         [0044]    The X-ray photosensitive film  1   a  can be made of any photosensitive material as far as it has a high hole mobility. More specifically, a photosensitive material such as GaP, AlSb, and a-Si can be used. On the other hand, for an X-ray photosensitive film whose electron mobility is higher than the hole mobility, CdZnTe, CdSe, HgI 2 , PbI 2 , ZnTe, CdTe, CdS, or the like can be used.  
         [0045]    In addition, a bias voltage is applied to the X-ray charge conversion film  1   a  through the upper electrode  25  such that holes or electrons, which have a higher mobility, are collected by the pixel electrode  1   b.  In this case, since the X-ray charge conversion film  1   a  is formed using a photosensitive material having a hole mobility higher than the electron mobility, a bias voltage is applied such that holes are collected by the pixel electrode  1   b  while using the upper electrode  25  as a positive pole.  
         [0046]    The switching thin-film transistor (TFT)  1   d  has a polarity corresponding to the type of X-ray charge conversion film  1   a.  More specifically, when the hole mobility in the X-ray charge conversion film  1   a  is higher than the electron mobility, a p-channel TFT is used as the switching thin-film transistor id. On the other hand, when the electron mobility in the X-ray charge conversion film  1   a  is higher than the hole mobility, an n-channel TFT is used as the switching thin-film transistor  1   d.  In this embodiment, since the X-ray charge conversion film  1   a  having a hole mobility higher than the electron mobility is used, the switching thin-film transistor (TFT)  1   d  is a p-channel TFT.  
         [0047]    The switching thin-film transistor  1   d  is a policrystallin-Si TFT (to be referred to as a “p-Si TFT” hereinafter). The p-Si TFT is used due to, e.g., the following reason.  
         [0048]    To drive a highly precise X-ray flat panel detector having a number of pixels, a TFT having a high switching speed is necessary. A conventional amorphous-Si TFT (to be referred to as an “a-Si TFT” hereinafter) has a low electron mobility and therefore cannot sufficiently drive the X-ray flat panel detector. Therefore, an p-Si TFT with a high mobility is used as a TFT having a high switch speed. The mobilities of electrons and holes are higher than those of a-Si by about two orders of magnitude. The mobilities of electrons and holes of p-Si are 100-400 and 50-200, respectively.  
         [0049]    Although it is possible to use monocrystalline silicon, a substrate formed of this material is small in size, and a wide-area TFT array required of an X-ray detector cannot be easily formed using such a small-sized substrate. In contrast, p-Si can be formed on a glass substrate and is therefore suitable for realizing a wide-area TFT array.  
         [0050]    However, the present inventors found that an n-channel p-Si TFT largely degrades the leading edge of the switching characteristic by X-ray irradiation, as shown in FIG. 3C. The degradation in leading edge of the switching characteristic is not observed in an a-Si TFT but is especially conspicuous in a p-Si TFT. Due to this degradation, the present inventors found that the n-channel p-Si TFT is not always suitable for the X-ray flat panel detector. In addition, to cause the TFT to act as a protective diode, it must be driven at a high drain voltage Vd.  
         [0051]    The present inventors found as a result of extensive studies that a p-channel p-Si TFT has a high drain breakdown voltage Vd and a high X-ray resistance and therefore is suitable as the switching thin-film transistor  1   d.  The conspicuous effect of usage of a p-channel p-Si TFT will the described later in detail.  
         [0052]    Conventionally, p-Si TFTs have been used in a liquid crystal display device (TFT-LCD). Since the LCD does not require resistance to X-rays, n-channel p-Si TFTs with improved mobility have been used. The inventors discovered for the first time that the use of p-channel TFTs was desirable where X-ray irradiation was executed. According to this arrangement, when a positive voltage is applied to the upper electrode  25 , such a bias voltage can be applied to the X-ray charge conversion film  1   a  that holes are collected by the pixel electrode  1   b.  With this bias voltage, holes generated in the X-ray charge conversion film  1   a  in accordance with the incident amount of X-rays can be stored in the storage capacitor  1   c  as signal charges. When the switching TFT  1   d  made of a p-channel p-Si TFT is turned on through the scanning line  3 , the stored signal charges can be read to the signal line  4  at a high speed and high S/N ratio. More specifically, the X-ray flat panel detector operates in the following manner.  
         [0053]    [0053]FIG. 4 is a view for explaining the operation of the X-ray flat panel detector according to this embodiment. Referring to FIG. 4, a positive voltage of 10 V is applied from the upper electrode  25 , thereby applying a bias voltage to the X-ray photosensitive film  1   a.  The switching TFT  1   d  is driven, as shown in FIG. 4. That is, in the gate-off state, a voltage of, e.g., 10 V is applied. The switching TFT is turned on by applying a voltage of, e.g., −25 V as a gate pulse. At the time of application of the gate pulse, X-ray irradiation is stopped. In imaging, X-ray irradiation is stopped at the time of gate pulse application. For sequential fluoroscopy, a gate pulse is appropriately applied during X-ray irradiation, and signal charges are read.  
         [0054]    With this operation, the pixel is irradiated with very strong X-rays, and the pixel potential greatly rises. When the potential becomes equal to or more than the sum of the gate pulse off voltage and the threshold value of the switching TFT, the switching TFT is turned on, and charges in the pixel (extra charges in the storage capacitor) flow to the signal line.  
         [0055]    Since the pixel potential never becomes equal to or more than the sum of the gate pulse off voltage and the threshold value of the switching TFT, breakdown of the gate insulating film of the switching TFT does not occur. For this reason, the switching TFT can be protected from a high voltage.  
         [0056]    [0056]FIG. 5A shows the diode characteristic of the p-channel TFT when a voltage of 10 V is applied in the gate-off state as well as the TFT characteristic of the p-channel TFT.  
         [0057]    That is, according to the X-ray flat panel detector of this embodiment, since the switching TFT can have the function of a protective circuit, no protective circuit for preventing extra charges from being stored in the storage capacitor  1   c  need be separately prepared. Unlike the conventional X-ray flat panel detector shown in FIG. 1, in a TFT array board on which a bias line for a protective TFT is arranged, no interconnections for externally connecting a driving circuit for driving the switching TFT or a TAB having an LSI for reading a signal need be connected. As a result, even when the pixel size is reduced, the interconnections can be connected at a sufficiently small pitch. In addition, since no protective circuit is necessary, a sufficient area can be ensured for the storage capacitor even in a smaller pixel.  
         [0058]    The X-ray flat panel detector according to this embodiment, a p-channel TFT formed from p-Si is used as the switching TFT  1   d.  With this arrangement, the following effects can be obtained.  
         [0059]    As the first effect, since holes having a high mobility are detected as charges in the Se film having a hole mobility higher than the electron mobility, the S/N ratio can be increased. More specifically, in the Se film serving as an X-ray photosensitive film, if electrons having a low mobility are used as charges to be stored in the storage capacitor, spatial charges are readily generated by slow mobility electrons trapped at defect sites. For this reason, the electrons are attracted by the Coulomb force and reach neighboring pixels. This degrades the resolution. In addition, possibility of appearance of image ghost due to the stored spatial charges occurs. In this embodiment, however, since holes which have a high mobility and rarely generate spatial charges are stored in the storage capacitor, the resolution rarely degrades, and image ghost rarely occurs.  
         [0060]    As the second effect, the breakdown voltage of the switching TFT  1   d  for the drain voltage can be made sufficiently high. According to experiments by the present inventors, a p-channel TFT manufactured by the technique described in this embodiment can operate at a voltage Vd of about 25 V even when the gate length L is 2 μm. To the contrary, in an n-channel TFT of a conventional X-ray flat panel detector, the drain breakdown voltage is about ½, i.e., 15 V. The degradation in TFT characteristic due to the drain voltage occurs because the high-energy carriers accelerated by the drain field enter the gate insulating film, generate a defect and degrade it.  
         [0061]    In the X-ray flat panel detector of this embodiment, a p-channel p-Si TFT is used as the switching TFT  1   d.  Hence, since the hole mobility is lower than the electron mobility, and the energy in the drain is low, the degradation in TFT characteristic by the drain voltage can be reduced. In addition, the defective charges that degrade the TFT characteristic have positive charges. Hence, holes serving as carriers in the switching TFT id run just separately from the surface of the gate instating film having the defect and therefore can reduce the influence from the defective charges.  
         [0062]    As the third effect, the X-ray flat panel detector according to this embodiment uses a p-channel TFT as the switching TFT  1   d  and therefore has a high X-ray resistance. More specifically, a switching TFT used in an X-ray flat panel detector generally forms a defect by X-ray irradiation and its characteristic degrades. The conventional X-ray flat panel detector cannot normally operate in some cases due to the degradation in characteristic. However, as will be described below, the inventors of the present invention discovered that in the case of an n-channel p-Si TFT, the degradation was so marked that the flat panel detector did not function normally. The inventors also discovered that in the case of a p-channel p-Si TFT, the X-ray irradiation did not much degrade the Vth and S-factor of the TFT than n-ch p-Si TFTs, and the flat panel detector functioned satisfactorily. As can be understood, for example, from the data shown in Table 1 of L. K. Wang “X-ray Lithography Induced Radiation Damage in CMOS and Bipolar Devices”, Journal of Electronic Materials, Vol. 21, No. 7, 1992, the phenomenon described above has a similar effect to that of the phenomenon wherein the adverse effects of the X-ray irradiation, namely, variations in “Vth” and variations in the inclination S-factor in the switching region, are less marked in a p-channel SiMOS transistor than in an n-channel SiMOS transistor. At any rate, the phenomenon described above is more marked in the case of polycrystalline Si.  
         [0063]    As the fourth effect, the off-leak current can be reduced, and even a small amount of charges produced with a small dose of X-rays can be detected with a high S/N ratio. FIG. SB illustrates an off-leak current that flows in a p-channel TFT  1   d  formed of P-Si and manufactured in such a method as provided in the present invention. As shown in FIG. 5B, the off-leak current can be reduced when the switching TFT  1   d  has an LDD structure. This phenomenon will be detailed later in relation to the second embodiment.  
         [0064]    [0064]FIG. 5C is a graph showing the characteristic of the p-channel TFT of this embodiment before and after X-ray irradiation. Referring to FIG. 5C, ⋄ indicates the characteristic before irradiation, and □ indicates the characteristic after irradiation. As compared to before X-ray irradiation, after X-ray irradiation, a threshold value Vth of the TFT changes, and the gradient of the sub-threshold value becomes moderate. The degradation due to X-ray irradiation is smaller in a p-channel p-Si TFT than in an n-channel p-Si TFT. Hence, to keep the current driving capability, a higher voltage Vd is necessary. As the voltage Vd increases, the TFT characteristic degrades. However, since Vd breakdown voltage and X-ray resistance are higher in the p-channel TFT than in an n-channel TFT, the TFT can be driven even at a higher voltage Vd. Accordingly, since the amount of signal charges that can be stored in the storage capacitor can be increased, a signal can be detected at a high S/N ratio without any saturation even for strong X-rays.  
         [0065]    Second Embodiment  
         [0066]    [0066]FIG. 6A is a plan view showing the structure of one pixel of an X-ray flat panel detector according to the second embodiment of the present invention. FIG. 6B is a sectional view showing the structure of one pixel of the X-ray flat panel detector. The same reference numerals as in FIG. 3A denote the same parts in FIGS. 6A and 6B, and a detailed description thereof will be omitted.  
         [0067]    The second embodiment is different from the above-described embodiment in that the switching TFT has an LDD (Lightly Doped Drain) structure. The process until an undercoat insulating film  11 , p-Si film  12 , gate SiO 2  film  13 , and gate electrodes  14  are formed on a glass substrate  10  is the same as in the first embodiment.  
         [0068]    Next, using the gate electrodes or a resist as a mask, B is doped by ion implantation at 1×10 11  cm −2  to 5×10 14  cm −2 , preferably, 3×10 12  cm −2  to 5×10 13  cm −2 , and in this embodiment, 2×10 13  cm −2  to form p − -regions  35 - 1  and  35 - 2  of LDDs. This almost corresponds to an impurity concentration of 1×10 17  cm −3  to 1×10 20  cm −3 . The LDD length is preferably 0.5 to 5 μm, and preferably, 1 to 4 μm. In this embodiment, the LDD length is 2 μm. In addition, W/L=10/5 μm.  
         [0069]    Next, using a resist as a mask, B is doped by ion implantation at a high concentration of 1×10 14  cm −2  to 5×10 16  cm −2 , preferably, 1×10 15  cm −2  to 1×10 16  cm −2 , and in this embodiment, 3×10 15  cm −2  to form p + -regions  15 - 1  and  15 - 2  of as source and drain electrodes.  
         [0070]    Subsequently, as in the first embodiment, holes are formed in the source/drain contact portion to form a signal line  17 - 1  and a Cs line  17 - 2  by an Mo/Al/Mo film. In addition, an SiN x  film for passivation and a protective film  18  made of an acrylic resin are formed. After that, a contact portion is formed, and a pixel electrode  19  made of an ITO film is formed. Then, an X-ray photosensitive film  20  is formed, and an electrode  25  is formed on the uppermost portion.  
         [0071]    [0071]FIG. 5B shows comparison between the off-leakage current of the p-channel TFT formed from p-Si in the above way and that of a TFT without any LDD. When the LDD is formed, the off current can be decreased as compared to the TFT without any LDD. The off current of a TFT for driving a pixel in an X-ray detector must be 1×10 −12  A or less. When the LDD is formed, the off current can sufficiently be made as small as 3×10 −14  A or less.  
         [0072]    Signals in liquid crystal TFTs are higher in level than those in X-ray detectors, and the off-current of the TFTs is on the order of 1×10 −10 A. For this reason, the liquid crystal TFTs are not p-channel p-Si TFTs of LDD structure and are therefore suitable for use in X-ray flat panel detectors.  
         [0073]    When Se is used as an X-ray photosensitive film, Se acts as a photodiode that has an especially high resistance and very small leakage current. For this reason, when the leakage current when the switch TFT is OFF is reduced, a small signal by weak X-rays can also be processed. Hence, a highly sensitive X-ray detector can be implemented. The present inventors found by studies that, in p-Si TFTs, a p-channel p-Si TFT having an LDD can make the current in the OFF state smallest. Hence, when Se which has very small dark current characteristics is used as the X-ray photosensitive film, and a p-channel p-Si TFT is used as a switching element, an X-ray flat panel detector whose sensitivity is much higher than the prior art can be implemented.  
         [0074]    Since the p-channel TFT has a high drain breakdown voltage as compared to an n-channel TFT, a larger X-ray signal can be processed. Accordingly, the dynamic range increases.  
         [0075]    As for the TFT characteristics before and after X-ray irradiation, after X-ray irradiation, a threshold value Vth changes, and the gradient of the sub-threshold value becomes moderate, as in FIG. 5C described in the first embodiment. However, since the X-ray flat panel detector can be driven at a higher voltage Vd, and the amount of signal charges that can be stored in the storage capacitor can be increased because of use of the p-channel TFT, as in the first embodiment, a signal can be detected without any saturation even for strong X-rays. Hence, the dynamic range can be increased.  
         [0076]    In addition, since the sub-threshold is little affected by the X-ray irradiation, the amount of signal charges that can be subjected to signal processing can be increased.  
         [0077]    Third Embodiment  
         [0078]    [0078]FIG. 7 is a circuit diagram showing a driver circuit in an X-ray flat panel detector according to the third embodiment of the present invention. FIG. 8 is a sectional view of the driver circuit. The same reference numerals as in FIG. 3B denote the same parts in FIG. 8, and a detailed description thereof will be omitted.  
         [0079]    In this embodiment, a driver circuit arranged in a peripheral circuit for driving a switching TFT is constituted using a p-channel TFT and n-channel TFT. Each TFT is manufactured simultaneously with TFTs in the image sensing region.  
         [0080]    As in the image sensing region, an SiN x  (50 nm)/SiO 2  (100 nm) film serving as an undercoat insulating film  11  is formed on a glass substrate  10 . A 50-nm thick a-Si film is formed on the undercoat insulating film  11 . The a-Si film is converted into polysilicon by ELA to form a 50-nm thick p-Si film  12 . The p-Si film  12  is etched to form peripheral circuit islands  12 - 3  and  12 - 4  together with a transistor region island  12 - 1  and capacitor region island  12 - 2 . A 150-nm thick gate SiO 2  film  13  is formed by PCVD or thermal CVD.  
         [0081]    Next, 300-nm thick MoW gates  14  are formed. In this case, gate electrodes  14 - 3  and  14 - 4  of CMOS transistors in peripheral circuits are formed together with a gate electrode  14 - 1  in the transistor region and a gate electrode  14 - 2  in the capacitor region in the image sensing region.  
         [0082]    Next, as in the image sensing region, using the gate electrodes or a resist as a mask, B is doped at 2×10 13  cm −2  to form p − -regions  35 - 4  and  35 - 5  of an LDD. This almost corresponds to an impurity concentration of 1×10 17  cm −3  to 1×10 20  cm −3 . The LDD length is, e.g., 2 μm. In addition, W/L=10/5 μm. Next, using a resist as a mask, B is doped at a high concentration of 3×10 15  cm −2  to form p + -regions  15 - 4  and  15 - 5  of source and drain electrodes.  
         [0083]    Next, independently of the image sensing region, using the gate electrodes or a resist as a mask, P is doped by ion implantation at 1×10 11  cm −2  to 5×10 14  cm −2 , preferably, 3×10 12  cm −2  to 5×10 14  cm −2 , and in this embodiment, 2×10 13  cm −2  to form n − -regions  55 - 4  and  55 - 5  of LDDs. This almost corresponds to an impurity concentration of 3×10 16  cm −3  to 2×10 21  cm −3 . The LDD length is preferably 0.5 to 5 μm, and preferably, 1 to 4 μm. In this embodiment, the LDD length is, e.g., 2 μm. In addition, W/L=10/5 μm. Next, using a resist as a mask, P is doped at a high concentration of 1×10 14  cm −2  to 5×10 16  cm −2 , preferably, 3×10 14  cm −2  to 5×10 15  cm −2 , and in this embodiment, 2×10 15  cm −2  to form n − -regions  45 - 4  and  45 - 5  of source and drain electrodes.  
         [0084]    Next, as in the image sensing region, a 500nm thick SiO 2  film  16  serving as a dielectric interlayer is formed by PCVD. Holes are formed in the source/drain contact portion to form a signal line  17 - 1  and Cs line  17 - 2  by an Mo/Al/Mo film. Simultaneously, interconnections  54 - 1  and  54 - 2  connected to the gate electrodes  14 - 4  and  14 - 5  are formed. After that, an SiN x  film for passivation is formed by PCVD. In addition, a protective film  18  is formed by coating an acrylic resin to 2 to 5 μm, and preferably, 3 μm. Since the protective film  18  is made of a photosensitive resin, contact holes can be formed only by exposure and development.  
         [0085]    With the above process, the pixel circuit is formed using a p-channel TFT while the peripheral driving circuit is formed using a CMOS made of a p-channel TFT and n-channel TFT.  
         [0086]    When the driving circuit having the CMOS structure of TFTs made of p-Si manufactured in this embodiment is used, signal charges can be sufficiently read even in a short address time. Hence, pixels at a small pitch can be driven. Accordingly, an X-ray flat panel detector for mammography inspection with a pixel pitch of 60 μm can be manufactured. In the prior art, since mounting at a 60-μm pitch is impossible, an X-ray flat panel detector with such small pixels can hardly be manufactured. In addition, according to the X-ray flat panel detector of this embodiment, the circuit characteristic of the peripheral circuit can be improved, and power consumption can be reduced.  
         [0087]    Modifications  
         [0088]    The present invention is not limited to the above-described embodiments. As a highly sensitive X-ray photosensitive film, not only the above-described Se film but also a polycrystalline or single-crystal efficient X-ray photosensitive material such as PbTe, HgTe, or ZnS, or a mixed crystal thereof can be used. The highly sensitive X-ray photosensitive film only needs to have a thickness capable of sufficiently absorbing X-rays. In addition, the thickness of a high-resistive semiconductor film is selected such that optical carriers (electrons or holes) can run through the high-resistive film in a time about {fraction (1/10)} the address time.  
         [0089]    The substrate is not limited to the glass substrate, and any other substrate can be used as long as a TFT can be formed on it. Since the X-ray photosensitive film used in the embodiments can be coated at a low temperature, a plastic substrate with a low heat resistance may be used as a substrate. In this case, the entire X-ray flat panel detector can have plasticity. For the TFT structure, the gates may be formed either on the upper side or on the lower side.  
         [0090]    As the protective film, inorganic SiN x  or SiO 2 , organic polyimides (ε=about 3.3; breakdown voltage=300 V/mm), benzocyclobutene (ε=about 2.7; breakdown voltage=400 V/mm), an acrylic photosensitive resin HRC available from JSR KK (ε=about 3.2), a black resist, or the like is used. These materials may be stacked as needed. As the protective film, a fluorine-based resin is also effective because of its low relative permittivity (ε=about 2.1). The protective film need not always be photosensitive. However, a photosensitive material is effective because it can easily be patterned.  
         [0091]    Various changes and modifications can be made without departing from the spirit and scope of the present invention. For example, in the above embodiments, an X-ray flat panel detector that employs the direct conversion scheme for converting incident X-rays into electron-hole pairs using a photoelectric conversion film has been exemplified. However, the technical idea of the present invention can also be applied to an X-ray flat panel detector that employs the indirect conversion scheme for temporarily converting incident X-rays into light using phosphor and then converting the light into electron-hole pairs using a photoelectric conversion film.