Abstract:
In order to obtain a good image without degradation of image quality by permitting accurate detection of a defective pixel and further compensation for the defective pixel even with occurrence of the defect originating in TFT during operation, it is made possible to detect the defective pixel by self-diagnosis. The detection is carried out in such a manner that in a dark state the voltage applied to the photoelectric conversion elements is changed from a first voltage in normal reading to a second voltage and outputs read out of the charged photoelectric conversion elements are compared with a predetermined threshold.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photoelectric conversion apparatus, a method for driving the photoelectric conversion apparatus, and an information processing apparatus having the photoelectric conversion apparatus and, more particularly, to a photoelectric conversion apparatus suitably used in X-ray image pickup apparatus, facsimile devices, scanners, and so on, a driving method of the photoelectric conversion apparatus, and an information processing apparatus provided therewith. 
     2. Related Background Art 
     FIG. 1 is a schematic circuit diagram to show the schematic structure of an example of the photoelectric conversion apparatus. In the figure, each pixel is composed of a photoelectric conversion element (a photodiode P 1  to P 4  in this example) and a thin film transistor (hereinafter abbreviated as TFT) T 1  to T 4 . Numeral  1  denotes a power source connected to the photoelectric conversion elements, for applying the bias voltage thereto. 
     Charges generated in the respective photoelectric conversion elements P 1  to P 4  by incident light are transferred to a reading unit  2  by the thin film transistors (hereinafter called TFTs). The reading unit  2  is normally composed of amplifiers, an analog multiplexer, an A-D converter, a memory, etc. not illustrated. Further, numeral  3  designates a gate drive unit for applying a gate pulse Vg 1  or Vg 2  for control of on/off of the TFTs to the gate electrodes of the TFTs T 1  to T 4 . The gate drive unit  3  is normally comprised of a shift register (not illustrated) or the like. 
     The photoelectric conversion elements P 1  to P 4  and the TFTs T 1  to T 4  are normally made of amorphous silicon materials or the like. 
     FIG. 2 is a timing chart to explain an example of reading operation of the photoelectric conversion apparatus. In the figure “Light” represents the timing of irradiation of light. After photocharges are accumulated in the respective photoelectric conversion elements P 1  to P 4  by the light irradiation, the gate drive unit  3  applies the gate pulse, as indicated by Vg 1  and Vg 2 , to switch the TFTs T 1 , T 3  on and then switch the TFTs T 2 , T 4  on, whereby the charges generated by the light are transferred to the reading unit  2 . The transferred charges are amplified, undergo A-D conversion, and are stored as image signals in the memory in the reading unit  2 , and the signals are outputted to a monitor or the like as occasion may demand. 
     It is, however, commonly known that the performance of TFTs is degraded, that is, the threshold voltage Vth varies during the operation, in cases of TFTs made of the amorphous silicon materials. Particularly, where the photoelectric conversion apparatus is composed of an array of many pixels, variations etc. in production can cause variations in degrees of degradation of the TFTs. There are cases wherein some heavily degraded TFTs fail to transfer the charge successfully, so as to lower the output of pixels, compose defective pixels, and degrade the image quality. 
     In order to correct the variations of output, a potential method employed was to detect the defective pixels caused by the operation, based on a white image obtained under irradiation of light or X-rays or the like. It is, however, difficult to irradiate a large area with uniform light in general, and there were some cases wherein normal pixels were detected as defective pixels because of dust or the like on an illumination system or on the apparatus. 
     As described above, the photoelectric conversion apparatus had the problem of degradation of image quality, where the defective pixels appeared due to the degradation or the like of the TFTs during the operation. Further, the apparatus had another problem that it was considerably hard to accurately detect the defective pixels appearing during the operation per se. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished in view of the above problems and an object of the present invention is to provide a photoelectric conversion apparatus, a driving method thereof, and an information processing apparatus provided therewith which permit accurate detection of the defective pixel or the like appearing during the operation of the photoelectric conversion apparatus or due to secular change of TFTs and which permit compensation for the defective pixels, so as to obtain a good image without substantial degradation of image quality. 
     Another object of the present invention is to provide a photoelectric conversion apparatus for reading information by arraying a plurality of pixels, each comprising a photoelectric conversion element and a thin film transistor connected to the element, and applying a voltage to gate electrodes of the thin film transistors to turn the thin film transistors on, the photoelectric conversion apparatus comprising a controllable power source for electrically charging the photoelectric conversion elements by changing a voltage applied to electrodes of the photoelectric conversion elements to which the thin film transistors are not connected, from a first voltage applied during normal reading to a second voltage and applying the second voltage to the electrodes in a dark state, and comparing means for comparing outputs read out of the charged photoelectric conversion elements with a predetermined threshold value to detect a defective pixel, and also to provide an information processing apparatus having the photoelectric conversion apparatus. 
     A further object of the present invention is to provide a method for driving a photoelectric conversion apparatus for reading information by arraying a plurality of pixels, each comprising a photoelectric conversion element and a thin film transistor connected to an output of the element, and applying a voltage to gate electrodes of the thin film transistors to turn the thin film transistors on, the apparatus having a reading mode and a self-diagnosis mode, the driving method comprising steps of electrically charging the photoelectric conversion elements by changing a voltage applied to electrodes of the photoelectric conversion elements to which the thin film transistors are not connected, from a first voltage applied in the reading mode to a second voltage and applying the second voltage to the electrodes in a dark state in the self-diagnosis mode, and comparing outputs read out of the charged photoelectric conversion elements with a predetermined threshold value to detect a defective pixel. 
     The present invention described above achieves the following operation; in the self-diagnosis mode the controllable power source changes and applies the voltage applied to the photoelectric conversion elements in the dark state, thereby charging the photoelectric conversion elements, not optically, but electrically, the charges are read out by the reading means, and the read outputs are compared with the predetermined threshold by the comparing means, so as to permit detection of the defective pixel. 
     Since the means for detecting the defective pixel by self-diagnosis has the function of switching two activity states of the reading mode and the self-diagnosis mode, the self-diagnosis can be performed even after activation of the apparatus by switching the mode into the self-diagnosis mode to find a defect due to a degraded TFT during the normal reading operation. Namely, the self-diagnosis can be performed at will of user or serviceman upon on of the main power supply, or by switching a changing switch. 
     By storing positional information of each defective pixel detected in the memory, the position of the defective pixel can be identified accurately and compensation by compensation means becomes easier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows schematically a circuit diagram of a substantial structure of an example of a photoelectric conversion apparatus; 
     FIG. 2 shows a timing chart for explaining an example of a reading operation by the photoelectric conversion apparatus; 
     FIG. 3 shows schematically a circuit diagram of an example of a desirable photoelectric conversion apparatus according to the present invention; 
     FIG. 4 shows a timing chart for explaining an example of an operation of the photoelectric conversion apparatus in FIG. 3; 
     FIG. 5A shows schematically a sectional view of an example of a photoelectric conversion element for use in the photoelectric conversion apparatus according to the present invention; 
     FIG. 5B shows schematically an equivalent circuit of one in FIG. 5A; 
     FIG. 6A shows schematically a sectional view of a photoelectric conversion element desirably for use in the photoelectric conversion apparatus according to the present invention; 
     FIG. 6B shows schematically an equivalent circuit of one in FIG. 6A; 
     FIG. 7 shows schematically a circuit diagram of the photoelectric conversion apparatus according to a second embodiment of the present invention; 
     FIG. 8A shows schematically a sectional view of another example of the photoelectric conversion element desirably for use in the photoelectric conversion apparatus of the present invention; 
     FIG. 8B shows schematically an equivalent circuit of one in FIG. 8A; 
     FIG. 9 shows a circuit diagram of photoelectric conversion apparatus according to a third embodiment of the present invention; 
     FIG. 10A shows schematically a structural diagram of an X-ray detecting photoelectric converter to which the present invention is adopted; 
     FIG. 10B shows schematically a sectional view of the X-ray detecting photoelectric converter in FIG. 10A; and 
     FIG. 11 shows an example in which the photoelectric conversion apparatus of the present invention is applied to an X-ray diagnosis system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail by reference to the drawings. 
     &lt;First Embodiment&gt; 
     FIG. 3 is a schematic circuit diagram to show an example of the preferred photoelectric conversion apparatus of the present invention. FIG. 4 is a timing chart to explain an example of the operation. Further, FIG. 5A shows a schematic, sectional view of an example of the photoelectric conversion elements used in the photoelectric conversion apparatus of the present invention, and FIG. 5B a schematic equivalent circuit thereof. The elements having the same functions as those in FIG. 1 are denoted by the same reference symbols. 
     In the present embodiment the photoelectric conversion elements are comprised of pin type photodiodes made of amorphous silicon materials as illustrated in FIG.  5 A. The pin type photodiodes are usually constructed in the structure of a stack of first electrode layer  11 , p-type amorphous silicon layer  12 , amorphous silicon semiconductor layer  13 , n-type amorphous silicon layer  14 , and second electrode layer  15  on glass substrate  10 . The pin type photodiode can be represented by a diode and a capacitor as illustrated in FIG.  5 B. 
     As illustrated in FIG. 3, the second electrode layers  15  of the pin photodiodes illustrated in FIG. 5A are connected in common to a bias line Vs and a controllable power supply  4  applies the bias thereto. The controllable power supply can apply at least two types of voltages Vs 1 , Vs 2 , as described hereinafter. 
     The TFTs (thin film transistors) T 1  to T 4  are TFTs connected to first electrodes of the respective photodiodes P 1  to P 4  and adapted for transferring charges generated in the photodiodes P 1  to P 4  and stored in the capacitors C 1  to C 4 , to the reading unit  2 . 
     The reading unit  2  is composed of amplifiers, an analog multiplexer, an A-D converter, a memory, etc. not illustrated. This reading unit  2  is normally composed of external IC or the like. Further, connected to the gate electrodes of the TFTs T 1  to T 4  is the gate drive unit  2  for applying the gate pulse Vg 1  or Vg 2  for control of on/off of the TFTs. A comparator  5  is also connected to the reading unit  2  to compare the output of the reading unit  2  with a threshold value (a threshold voltage Va in the example of FIG. 3) and write the comparison result in a memory for storage of defective position. The photodiodes and TFTs are normally deposited and formed by the amorphous silicon process or the like. 
     The photoelectric conversion apparatus of the present invention has a reading mode and a self-diagnosis mode in the operation. This can be implemented as follows; the user or the serviceman for carrying out maintenance of the apparatus switches the modes at will by a mode changing switch not illustrated; or the apparatus may be designed to carry out the self-diagnosis mode automatically, for example, with on of the unrepresented main power supply of the apparatus by a logical circuit configuration often used normally, a control program of a microcomputer, or the like, and thereafter turn the mode into the reading mode. 
     As illustrated in FIG. 3, the photoelectric conversion apparatus of the present invention has the reading mode and the self-diagnosis mode in the operation. 
     The reading mode will be described first. The controllable power source  4  is put in the state of the voltage Vs 2 . In this example of the pin photodiodes of the present embodiment, the relation of the voltages Vs 1  and Vs 2  is Vs 1 &gt;Vs 2  and, specifically, Vs 1 =15 V and Vs 2 =10 V, for example. The MIS type or selenium photodiodes in the subsequent embodiments are also charged in the negative even in the relation of Vs 1 &lt;Vs 2 . 
     In this state, the photodiodes are exposed to the light at the timing of on of Light in the figure and charges corresponding to quantities of light are stored in C 1  to C 4 . After that, the gate drive circuit successively applies the gate pulses Vg 1  and Vg 2  to the gate electrodes of the TFTs, whereupon the charges of the respective pixels are transferred to the reading unit  2 . Then the charges are amplified by the amplifiers not illustrated, are multiplexed, are converted into digital signals by the A-D converter, and are stored in the frame memory not illustrated. The digital image signals stored in the frame memory are subjected to offset correction and gain correction as occasion may demand, and are outputted to the monitor or the like. 
     The operation in the self-diagnosis mode will be described next. In this mode, the light (or X-rays) is not radiated (the dark state). 
     First, the controllable power source  4  is put in the state of the voltage Vs 2 . In this state the gate drive circuit applies an optional number of gate pulses to the gate electrodes to perform empty reading to read charges of the photoelectric conversion elements stored because of dark current or the like. In this description the empty reading operation turns the potential on the first electrode side of the photoelectric conversion elements to zero or the ground. The empty reading is effective, particularly, where the dark current is large and where the self-diagnosis of defect is carried out accurately. 
     Then the controllable power source  4  is switched into the state of the voltage Vs 1  while the TFTs are kept off. This turns the potential of the first electrodes of the photoelectric conversion elements or the pin type photodiodes into the equal potential of (Vs 1 -Vs 2 ) for all the photoelectric conversion elements. Namely, the photoelectric conversion elements can be charged electrically. In this state the gate drive circuit applies the gate pulses, whereby the charges electrically charged in the photoelectric conversion elements can be read out. 
     Signals read out here are used for the self-diagnosis of defective pixel. As long as the TFTs are free of degradation, the signal charges transferred to the reading unit are basically constant. However, if the TFTs undergo degradation because of secular change in use or the like, the transferred charges will decrease. Namely, the output becomes small. Therefore, a defect due to degradation of TFT can be detected by comparing the output in the self-diagnosis mode with the threshold by means of the comparator  5 . FIG. 3 is the illustration of the apparatus with an analog comparator, but like function can also be realized with a digital comparator using a memory. 
     Positional information of a pixel determined as a defect because of the output below the threshold is stored in the memory for storage of defect position. The defect position storing memory of FIG. 3 indicates normal pixels by 0 and a defective pixel by 1, and shows a state in which the pixel of P 2  is detected as a defect, as an example. The positional information of the defective pixel can be specified with correspondence between an address of the memory and the position of the pixel, for example, by storing the information of the pixels in the memory in order. 
     Further, the defective pixel is compensated for by interpolation using an average of adjacent pixel outputs by means of a compensation means not illustrated. Such interpolation means can be comprised of a DSP (digital signal processor) for carrying out an arithmetic operation by mutually referencing the data from the frame memory storing the image information and the data from the defect position storing memory. 
     A better image can be obtained by detecting the defect by the self-diagnosis and compensating for the defect as described above. 
     &lt;Second Embodiment&gt; 
     FIG. 6A is a schematic, sectional view of a photoelectric conversion element suitably applicable to the photoelectric conversion apparatus of the present invention and FIG. 6B a schematic equivalent circuit thereof. FIG. 7 is a schematic circuit diagram of the photoelectric conversion apparatus of the second embodiment. 
     In the present embodiment the photoelectric conversion elements are comprised of the MIS type sensors. As illustrated in FIG. 6A, the MIS type sensors of the present embodiment are constructed in the structure of a stack of first electrode layer  11 , amorphous silicon nitride film layer  16  as an insulating layer, amorphous silicon semiconductor layer  13 , n-type amorphous silicon layer  14 , and second electrode layer  15  on glass substrate  10 . As illustrated in the equivalent circuit diagram of FIG. 6B, the photoelectric conversion elements have the capacitance Csin, which is the capacitance of the amorphous silicon nitride film. The circuit diagram illustrated in FIG. 7 is different only in this point from the configuration of the circuit diagram of FIG. 3 described above, and the other structure is the same. 
     The operations of the present embodiment in the reading mode and in the self-diagnosis mode both can be carried out in similar fashion as in the first embodiment illustrated in FIG.  4 . 
     &lt;Third Embodiment&gt; 
     FIG. 8A is a schematic, sectional view of an example of another photoelectric conversion element suitably applicable to the photoelectric conversion apparatus of the present invention and FIG. 8B a diagram to show a schematic equivalent circuit thereof. FIG. 9 is a schematic circuit diagram of the photoelectric conversion apparatus of the third embodiment. 
     In the present embodiment the photoelectric conversion elements are constructed using amorphous selenium as a principal material. As illustrated in FIG. 8A, the photoelectric conversion elements of the present embodiment are constructed in the structure of a stack of third electrode layer  21 , first insulating layer  20 , first electrode layer  11 , charge injection inhibiting layer  19 , amorphous selenium semiconductor layer  18 , second insulating layer  17 , and second electrode layer  15  on glass substrate  10 . Since the amorphous selenium semiconductor layer  18  is sensitive to X-rays, an X-ray image can be obtained directly. 
     As illustrated in the equivalent circuit diagram of FIG. 8B, the present embodiment is different in possession of Cins 1 , Cse, R, and Cins 2  from Embodiment 1, wherein Cins 1  is the capacitance of the first insulating layer, Cse the capacitance of the amorphous selenium semiconductor layer, R the resistance of the amorphous selenium semiconductor, and Cins 2  the capacitance of the second insulating layer. As illustrated in the circuit diagram shown in FIG. 9, the present embodiment is different only in this point from Embodiment 1, and the other structure is the same as in Embodiment 1. 
     The operations of the present embodiment in the reading mode and in the self-diagnosis mode can be carried out in similar fashion as in the first embodiment illustrated in FIG.  4 . 
     The photoelectric conversion apparatus of the present invention described above can replace the conventional photoelectric conversion apparatus to construct the X-ray image pickup apparatus, the facsimile machines, the scanners, or the like and can also detect and correct the defective pixels in the self-diagnosis mode described above in such apparatus. 
     An example of the information processing apparatus will be described briefly using a preferred example of application of the photoelectric conversion apparatus of the present invention to the X-ray image pickup apparatus. 
     FIG.  10 A and FIG. 10B show an X-ray detecting photoelectric converter  6000  which adapts the present invention; FIG. 10A is a schematically structural diagram and FIG. 10B is a schematically sectional view. 
     The photoelectric converting element and the TFT are constituted in plural numbers inside an a-Si sensor substrate  6011  and connected with flexible circuit substrates  6010  on which shift registers SR 1  and integrated circuits IC for detection are mounted. The opposite side of the flexible circuit substrates  6010  are connected with a PCB 1  or a PCB 2 . A plurality of the a-Si sensor substrates  6011  are adhered onto a base  6012  so as to constitute a large-sized photoelectric converter. A lead plate  6013  is mounted under the base  6012  so as to protect memories  6014  in a processing circuit  6018  from X rays. A phosphor  6030 , which is a wavelength conversion element, such as CsI or the like is coated on or adhered to the a-Si sensor substrate  6011 . Further, numeral  6019  denotes a connector. In this embodiment, as shown in FIG. 10B, the whole is packed in a case  6020  made of carbon fiber. 
     FIG. 11 shows an applied example in which the photoelectric converter of the present invention is applied to an X-ray diagnosis system. 
     X rays  6060  emitted from an X-ray tube  6050  are transmitted through the chest  6062  of a patient or an examinee  6061  to be incident to a photoelectric converter  6040  on which a phosphor as a wavelength conversion element has been mounted. The incident X rays include the internal information of the patient. Here, the phosphor emits light in response to the incident X rays and the emitted light is photoelectrically converted to obtain the electric information. The electric information is then converted to be digitalized and an image on the electric information is processed by an image processor  6070  to be able to observe on a display  6080  in a control room. This information can be transferred to a remote place, such as a doctor room located in other place or the like, by way of a transmission means such as a telephone line  6090  and displayed on a display  6081  or stored in a storage means such as an optical disk by recorder  6085 , and this makes it possible to be diagnosed by a doctor in a remote place. Also, this information can be recorded on a film or recording medium as paper  6110  by a film processor or printer  6100 . 
     EFFECT OF THE INVENTION 
     As described above, the present invention permits the user or the serviceman to detect the defective pixel during the operation or with a lapse of time in use at an arbitrary time or on a periodical basis in the simple structure and with high accuracy, even after the apparatus has been mounted on equipment. 
     In addition, the present invention permits the defective pixel with a malfunction to be specified accurately and permits the specifying operation of the defective pixel to be carried out readily by the extremely simple operation and self-diagnosis mode. 
     Further, the present invention permits prevention of the degradation of image quality by properly compensating for the output of the defective pixel.