JP2012134827A - Radiation image detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation image detector for reducing the measurement frequency of an offset charge amount and facilitating radiography without enlarging a device or raising a price.SOLUTION: On a substrate, a plurality of pixels including a switching element, a photoelectric conversion element and an element with electric capacity as one set of elements are two-dimensionally arrayed. The radiation image detector includes, on the pixels, a radiation sensor 11 for which a fluorescence conversion film for converting X-rays made incident from the outside to light is laminated, and a gate driving circuit 13 for applying a drive voltage to one arbitrarily selected row selection line and applying a voltage in the direction opposite to the drive voltage to an adjacent row selection line.

Description

  Embodiments described herein relate generally to a radiographic image detector that can reduce the number of offset charge measurement times.

  As a new generation X-ray diagnostic detector, a radiological image detector using an active matrix has attracted much attention. By irradiating the radiation image detector with X-rays, an X-ray image or a real-time X-ray image is output as a digital signal. In addition, since it is a flat solid detector, it is highly expected in terms of image quality and stability. For this reason, many universities and manufacturers are working on research and development.

  Radiation image detectors are roughly classified into two methods, a direct method and an indirect method.

  The direct method is a method in which X-rays are directly converted into a charge signal by a photoconductive film such as a-Se and led to a charge storage capacitor.

  On the other hand, the indirect method is a method in which X-rays are received by a fluorescent conversion film and converted into visible light, and the visible light is converted into a charge signal by an a-Si photodiode or the like and led to a charge storage capacitor.

  Many of the radiation image detectors currently in practical use employ the indirect method.

  A conventional indirect radiation image detector forms an image detection unit by two-dimensionally arranging pixels including a photodiode that converts visible light into a charge signal and a TFT transistor as a switching element on a glass substrate, On top of that, a fluorescence conversion film for converting radiation into visible light is provided.

  In this indirect radiation image detector, X-rays incident from the outside are converted into visible light inside the fluorescence conversion film, and this visible light is incident on the photodiode in the pixel of the image detection unit and then converted into electric charge. The electric charge is accumulated in the photodiode or in the capacitor element connected in parallel.

  The X-ray information converted into electric charges passes through each switching element (TFT transistor) connected to the photodiode for each pixel group connected to the row selection line arranged in the row direction in each pixel and column direction. The signal is transmitted to the outside of the image detection unit through the connected signal line.

  The potential of the row selection line for driving the TFT transistor is normally changed to the potential of only one row selection line, thereby turning on the TFT transistors in all pixels corresponding to a specific row. By sequentially changing the row selection line for changing the potential, a signal from a pixel corresponding to a specific row is discharged to the outside. By referring to the position of the signal line from which the charge has been discharged and the position of the selection line whose potential has fluctuated at that time, it is possible to calculate the X-ray incident position and intensity.

  The charge signal discharged to the outside of the glass substrate is input to an integrating amplifier connected to each signal line. The charge information input to the integrating amplifier is amplified, converted into a potential signal, and output. The potential signal output from the integrating amplifier is converted into a digital value by an analog / digital converter, and finally edited as an image signal and output to the outside of the radiation image detector.

JP 2009-128023 A

  Since the radiation image detector is mainly intended to image X-rays transmitted through the human body, X-ray irradiation to the human body is suppressed to the minimum necessary from the viewpoint of preventing adverse health effects. Therefore, the intensity of X-rays incident on the radiation image detector is very weak, and the amount of charge output from the TFT transistor inside the radiation image detector is extremely small.

  The X-ray image information output from the radiation image detector includes sensitivity differences peculiar to individual pixels and offset charges as noise. In particular, the presence or absence of the offset charge amount is a weak image quality of the X-ray image. Greatly affects.

  The offset charge amount is mainly generated by a leakage current of a photodiode or a TFT constituting a pixel inside the TFT panel. It is known that the leakage current from these semiconductor elements changes greatly with respect to the element temperature, and is caused by changes in the ambient temperature of the radiation image detector and temperature changes due to heat generated from the electric circuit inside the X-ray detector. It will change.

  Therefore, when an X-ray image is taken using a radiographic image detector, the amount of offset charge is measured in response to the temperature change inside the radiographic image detector due to changes in the ambient temperature, the heat generation state of the detector, and the heat dissipation conditions. Thus, it is necessary to correct the image using the latest information on the offset charge amount with respect to the photographed X-ray image.

  For this reason, it is necessary to measure the offset charge amount every several minutes, particularly when performing a weak X-ray image diagnosis. However, during the measurement of the offset charge amount, it is necessary to temporarily stop X-ray imaging. If complicated measurement of the offset charge amount is made, there is an inconvenience that an emergency X-ray image diagnosis cannot be performed.

  In order to avoid such inconveniences, it is necessary to control the temperature inside the radiation image detector with high accuracy using a Peltier device, a large-capacity heat dissipation structure and a fan, and to suppress changes in the amount of offset charge from individual pixels. However, an increase in the size and cost of the radiation image detector is inevitable.

  The present invention has been made in view of the above problems, and provides a radiation image detector that facilitates X-ray imaging by reducing the frequency of measurement of the offset charge amount without increasing the size and cost of the apparatus. For the purpose.

  In order to achieve the above-described object, the radiation image detector includes a switching element having a gate, a source, and a drain to perform a switching function, a photoelectric conversion element that converts light into a charge signal, and an element having an electrical capacitance. A plurality of pixels included as one set of elements are two-dimensionally arranged on the substrate, and the switching element extends in the row direction on the substrate in the arbitrary pixel. And one of the plurality of signal lines extending in the column direction, the photoelectric conversion element is connected to the switching element, and the element having the electric capacity is connected to the switching element. And formed between the adjacent row selection line for driving the adjacent switching element adjacent in the column direction and the switching element. Applying a driving voltage for driving the switching element from a conductive state to an insulating state to a radiation sensor on which a fluorescent conversion film to be converted is laminated and one row selection line arbitrarily selected from the plurality of row selection lines And a gate drive circuit for applying a potential change in a direction opposite to a drive potential change for driving the switching element from a conductive state to an insulated state to an adjacent row select line adjacent to the row select line to which the drive voltage is applied. It is characterized by providing.

  The radiation image detector includes a switching element that has a gate, a source, and a drain to perform a switching function, a conversion element that directly converts radiation into a charge signal, and an element that has an electric capacitance as a set of elements. A plurality of pixels are two-dimensionally arranged on the substrate, and within the arbitrary pixel, the switching element is one of a plurality of row selection lines extending in the row direction on the substrate and Connected to one of a plurality of signal lines extending in the column direction, the conversion element is connected to the switching element, and the element having the electric capacity is adjacent to the switching element in the column direction. A radiation sensor formed between an adjacent row selection line for driving the switching element and the switching element, and one row arbitrarily selected from the plurality of row selection lines A gate for applying a driving voltage for driving the gate to the selection line and applying a voltage in a direction opposite to the driving voltage to the adjacent row selection line adjacent to the row selection line to which the driving voltage is applied with respect to the drain And a drive circuit.

The schematic diagram which shows the structure of the radiographic image detector which concerns on one embodiment of this invention. The perspective view which shows an example of the radiation sensor used for the radiographic image detector of FIG. The ideal equivalent circuit schematic of the image detection part of FIG. The ideal equivalent circuit diagram inside a pixel. FIG. 2 is a plan view illustrating an example of a gate drive circuit and a read circuit in FIG. 1. FIG. 6 is a plan view showing an example of a circuit pattern formed on the surface of the image detection unit 25. FIG. 3 is an equivalent circuit diagram inside a pixel in consideration of parasitic capacitance. FIG. 3 is an equivalent circuit diagram inside a pixel including a correction capacitor. FIG. 9 is an equivalent circuit diagram of an image detection unit having the pixel of FIG. 8. It is the schematic which shows an example of the circuit pattern of the pixel containing a correction | amendment capacitor | condenser, (a) is a top view, (b) is the BB sectional drawing of (a).

  Embodiments of the present invention will be described below with reference to the drawings.

(Overall configuration of radiation image detector)
FIG. 1 shows a configuration of a radiation image detector according to an embodiment of the present invention.

  The radiation image detector 10 is two-dimensionally arranged in a radiation sensor 11 having a phosphor layer that converts incident X-rays into light and an image detection unit that converts incident light into an electrical signal, and the image detection unit. A gate drive circuit 13 for sequentially applying a drive voltage for each scan line for each pixel, a drive control circuit 15 for generating a drive signal for determining a scan timing in the column direction for the gate drive circuit 13, and a pixel in a selected row A read circuit 17 that reads and amplifies the electric signal, a read control circuit 18 that generates a read signal that determines the read timing of the read circuit 17, and an offset charge that is subtracted from the image signal mixed with the offset charge as noise. A correction circuit 19 that performs image correction and a display device 20 that displays an image signal after performing noise correction are provided.

(Radiation sensor 11)
FIG. 2 shows an example of a specific configuration of the radiation sensor 11.

  The radiation sensor 11 includes a fluorescence conversion film 23 that converts incident X-rays 21 into fluorescence, and an image detection unit 25 that converts the fluorescence into image information using an electrical signal.

  The image detection unit 25 is formed by providing a circuit layer 29 on which a large number of pixels 28 including photodiodes and thin film transistors (TFTs) are arranged on a holding substrate 27 mainly composed of a glass substrate.

  FIG. 3 shows an ideal equivalent circuit of the image detection unit 25, and FIG. 4 shows an ideal equivalent circuit inside the pixel 28.

  The pixel 28 a includes a thin film transistor 31 and a photodiode 33, a row selection line (gate line) 35 is connected to the gate of the thin film transistor 31, a signal line 37 is connected to the source of the thin film transistor 31, and the drain of the thin film transistor 31 is connected to the drain. A photodiode 33 and a capacitor 36 are connected in parallel. The capacitor 36 is a capacitance between the electrodes of the photodiode 33.

  Further, an integrating amplifier 41 having a function of amplifying a charge signal transmitted through the signal line 37 and outputting the amplified signal to the outside is connected to the signal line 37 on a one-to-one basis.

  Furthermore, the row selection line (gate line) 35 is connected to a specific signal line of the gate driver 63 shown in FIG.

(Gate drive circuit 13 and reading circuit 17)
FIG. 5 shows an example of a specific configuration of the gate driving circuit 13 and the reading circuit 17 of FIG.

  The gate driving circuit 13 includes a gate driver 63 and a row selection circuit 65, and the reading circuit 17 includes an integrating amplifier 41, an A / D (analog / digital) converter 67, and a driver 69.

  The gate driver 63 has a function of sequentially changing the voltages of a large number of signal lines connected to the radiation sensor 11 when receiving a signal from the outside. A row selection circuit 65 is connected to the gate driver 63.

  The row selection circuit 65 has a function of sending a signal to the corresponding gate driver 63 according to the scanning direction of the X-ray image, and is connected to the drive control circuit 15 of FIG.

  Further, the integrating amplifier 41 is connected to a driver 69 via an A / D converter 67. The driver 69 is connected to the reading control circuit 18 shown in FIG. 1, and reads the signal digitized by the A / D converter 67 by the reading signal from the reading control circuit 18.

(Operation of Radiation Image Detector 10)
Below, operation | movement of said radiographic image detector 10 is demonstrated.

  In the initial state, electric charge is stored in the capacitor 36 in FIG. 4, and a reverse bias voltage is applied to the photodiodes 33 connected in parallel. The voltage at this time is the same as the voltage applied to the signal line 37. Since the photodiode 33 is a kind of diode, even if a reverse bias voltage is applied, almost no current flows. Therefore, the electric charge stored in the capacitor 36 is held without decreasing.

  In this state, when the incident X-ray 21 shown in FIG. 2 enters the fluorescence conversion film 23, the high-energy X-rays are converted into a large number of low-energy visible light inside the fluorescence conversion film 23. Part of the fluorescence generated inside the fluorescence conversion film 23 reaches the photodiode 33 disposed on the surface of the image detection unit 25.

  Fluorescence incident on the photodiode 33 shown in FIG. 4 is converted into charges consisting of electrons and holes inside the photodiode 33, and reaches both terminals of the photodiode 33 along the direction of the electric field applied to the capacitor 36. Thus, the current flowing through the photodiode 33 is observed.

  The current generated inside the photodiode 33 flows into the capacitor 36 connected in parallel, and acts to cancel the electric charge stored in the capacitor 36. As a result, the charge stored in the capacitor 36 decreases, and the potential difference generated between the terminals of the capacitor 36 also decreases compared to the initial state.

  In FIG. 5, the gate driver 63 has a function of sequentially changing the potentials of a large number of control lines, but there is only one control line whose potential changes at a specific time. The source-drain terminal of the thin film transistor 31 connected in parallel to the row selection line 35 connected to the control line changes from an insulating state to a conductive state.

  A specific voltage is applied to each signal line 37 in FIG. 4, and the voltage is applied to the capacitor 36 connected through the source and drain terminals of the thin film transistor 31 connected to the row selection line 35 whose potential has changed. Will be.

  Since the capacitor 36 is in the same potential state as the signal line 37 in the initial state, if the charge amount of the capacitor 36 is not changed from the initial state, the capacitor 36 does not move the charge from the signal line 37. . However, in the capacitor 36 connected in parallel with the photodiode 33 into which the fluorescence generated inside the fluorescence conversion film 23 from the incident X-ray 21 from the outside is incident, the charge stored inside is decreased, The potential of the state has changed. For this reason, a movement of electric charge is generated from the signal line 37 through the thin film transistor 31 in the conductive state, and the amount of electric charge stored in the capacitor 36 returns to the initial state. In addition, the amount of electric charge that has moved becomes a signal flowing through the signal line 37 and is transmitted to the outside.

  The current flowing through the signal line 37 in FIG. 4 is input to the corresponding integrating amplifier 41, and the integrating amplifier 41 integrates the current flowing within a predetermined time and outputs a voltage corresponding to the integrated value to the outside. By performing this operation, the amount of charge flowing through the signal line within a certain time can be converted into a voltage value. As a result, a charge signal corresponding to the intensity distribution of the fluorescence generated inside the fluorescence conversion film 23 by the incident X-ray 21 is generated inside the photodiode 33, and this charge signal is converted into potential information by the integrating amplifier 41. Is done.

  The potential generated by the integrating amplifier 41 is sequentially converted into a digital signal by the A / D converter 67 shown in FIG. The signal having the digital value is subjected to removal of offset charges by the correction circuit 19 shown in FIG. 1 via the driver 69, and then the rows and columns of the pixels arranged in the circuit layer 29 by an image synthesis circuit (not shown). Are sequentially arranged and output to the outside as an image signal.

  Image information based on electrical signals output to the outside can be easily imaged by a display device 20 such as a normal display device, and an X-ray image can be observed as an image by visible light.

(Circuit pattern formed on the surface of the image detection unit 25)
FIG. 6 shows an example of a circuit pattern formed on the surface of the image detection unit 25.

  In this circuit pattern, a plurality of row selection lines 35 arranged in the horizontal direction and a plurality of signal lines 37 arranged in the vertical direction are orthogonal to each other, and these wirings are electrically insulated by the insulating film 30. It has become.

  A photodiode 33 is formed in a region surrounded by the row selection line 35 and the signal line 37, and a bias line 26 is disposed above the photodiode 33. The upper part of the photodiode 33 and the bias line 26 are electrically connected, and a bias voltage is applied to each photodiode 33.

  The thin film transistor 31 uses a partial region of the row selection line 35 as the gate electrode 40, and similarly, a part of the signal line 37 serves as the source electrode 43 and a part of the lower electrode 24 of the photodiode 33 serves as the drain electrode 42. By using it, a switching element is formed.

  Since the image detection unit 25 has the circuit pattern shown in FIG. 6, the actual equivalent circuit of the pixel is different from the ideal equivalent circuit shown in FIG. In particular, since the thin film transistor 31 has a structure in which the semiconductor film and the insulating film 30 are interposed between the gate electrode 40 and the drain electrode 42 and between the gate electrode 40 and the source electrode 43, It is known that a capacitance component called parasitic capacitance is generated between the drain electrode 42 and between the gate electrode 40 and the source electrode 43.

  Particularly problematic here is the parasitic capacitance between the gate electrode 40 and the drain electrode 42 of the thin film transistor 31.

(Equivalent circuit inside the pixel 28 in consideration of parasitic capacitance)
FIG. 7 shows an equivalent circuit inside the pixel 28b in consideration of the parasitic capacitance.

  In this equivalent circuit, a pixel 28 b is formed by a thin film transistor 31, a photodiode 33, and a capacitor 36 in a region surrounded by the row selection line 35 and the signal line 37. In addition, the gate electrode and drain of the thin film transistor 31 are formed. In this structure, a parasitic capacitance 22 is added between the electrodes.

  Incidentally, parasitic capacitance is also generated between the gate electrode and the source electrode, but it is not shown because it is irrelevant to the present embodiment.

(Operation of the radiation image detector 10 having the pixel 28b having an equivalent circuit in consideration of the parasitic capacitance)
Also in the radiographic image detector 10 having pixels having the equivalent circuit shown in FIG. 7, the thin film transistor 31 driven by the row selection line 35 becomes conductive, as in the operation of the radiographic image detector shown in FIG. 4 described above. As a result, the capacitor 36 whose potential difference has been reduced by the charge generated in the photodiode 33 is supplemented by the signal line 37 through the thin film transistor 31.

  At this time, in order to bring the thin film transistor 31 into a conductive state, it is necessary to change the voltage of the row selection line 35. In the case of a general thin film transistor 31, the gate voltage for making the drain-source conductive state is about 20V, and the gate voltage for making the non-conductive state about -5V.

  Accordingly, a voltage of about 20 V is applied to the row selection line 35 in order to bring the thin film transistor 31 connected to the arbitrary row selection line 35 into a conductive state. At this time, charge flows from the signal line 37 through the thin film transistor 31 that is in a conductive state, and the capacitor 36 is charged. At the same time, the charge is charged to the parasitic capacitance 22.

  After the charge information is read by the above operation, the thin film transistor 31 connected to the row selection line 35 is changed to a non-conductive state by changing the voltage of the row selection line 35 to about −5V. As a result, the photodiode 33 and the capacitor 36 separated from the signal line 37 are isolated from other circuits.

  When there is no X-ray incident on the radiation image detector, no fluorescence is generated by the incident X-ray, so that no current is generated in the photodiode 33 and the potential difference of the capacitor 36 is maintained.

  Here, when there is no parasitic capacitance like the pixel 28a shown in FIG. 4, the potential of the drain electrode of the thin film transistor 31 connected to the capacitor 36 is equal to the potential of the source electrode. Even in the non-conducting thin film transistor 31, a resistance having a finite value exists between the drain and the source, but since no potential difference is generated between the drain and the source, the non-conducting thin film transistor 31 has a drain-source connection. There is no leakage current.

  However, in the case where the parasitic capacitance 22 exists as in the pixel 28b shown in FIG. 7, when a voltage of about 20 V is applied to the gate electrode of the thin film transistor 31 through the row selection line 35 to make the thin film transistor 31 conductive, normally Charges are accumulated in the parasitic capacitance 22 from the signal line 37 set to 0 V through the thin film transistor 31 in a conductive state.

The parasitic capacitance generated in a normal radiation image detector has a value of about 10 fF (fF: 10 ⁻¹⁵ F). Therefore, the parasitic capacitance 22 generated between the gate electrode of the thin film transistor 31 to which a voltage of about 20 V is applied and the drain electrode which is in a conductive state and is turned on at the same potential as the signal line 37 through the source electrode has 200 fC ( The charge of about charge = voltage × capacitance = 20 × 10 = 200) is accumulated in the parasitic capacitor 22.

  On the other hand, when a voltage of about −5 V is applied to the gate electrode of the thin film transistor 31 through the row selection line 35, the thin film transistor 31 changes to a non-conductive state. The thin film transistor 31 in the period for detecting X-rays incident on the radiation image detector 10 from the outside needs to be kept in a non-conductive state, and the thin film transistor 31 is in a non-conductive state for most of the period.

When the thin film transistor 31 is turned off, the photodiode 33 and the capacitor 36 constituting the pixel are electrically separated from the signal line 37. At this time, the electric charge accumulated in the parasitic capacitance 22 is distributed to the capacitor 36 and the parasitic capacitance 22. Since the capacitance of the capacitor 36 of a normal radiation image detector is about 1 pF (pF: 10 ⁻¹² F), most (99%) of the charge amount (about 200 fC) accumulated in the parasitic capacitance 22 of 10 fF is the capacitor 36. Will move to.

  Since most of the amount of charge accumulated in the parasitic capacitance 22 is distributed to the capacitor 36, the terminal voltage of the capacitor 36 changes. In this example, this corresponds to a voltage change of about 0.2 V (voltage = charge / capacity = 200/1000 = 0.2).

  Since the terminal voltage of the capacitor 36 changes, the drain voltage of the thin film transistor 31 connected to the capacitor 36 also changes. As a result, a potential difference of about 0.2 V is generated between the drain and source of the thin film transistor 31. Although the thin film transistor 31 is in a non-conductive state through the row selection line 35, it is inevitable that a finite resistance exists between the drain and the source. Therefore, a leakage current is generated due to the potential difference between the drain and the source, and the charge accumulated in the capacitor 36 flows into the signal line 37 through the thin film transistor 31.

  In order to read out the charge information generated by the photodiode 33 inside the pixel by the X-rays incident from the outside, the thin film transistor 31 is made conductive through the row selection line 35. In this case, the charge information generated in the photodiode 33 is set. In addition, the amount of charge reduced by the leakage current is also read out.

  This leakage current component is information unrelated to the X-ray image, and appears as noise in the X-ray image. The amount of leakage current is measured in advance, and the influence can be removed by taking the difference of the component from the image information.

  However, since the amount of leakage current varies greatly depending on the temperature of the thin film transistor 31 made of a semiconductor, it is necessary to measure the amount of leakage current as the temperature inside the radiation image detector 10 changes. Since an X-ray image cannot be taken during this measurement period, it becomes a great hindrance when actually performing a medical practice using X-rays. For this reason, it is considered to incorporate a correction capacitor in the pixel as described below.

(Equivalent circuit inside pixel 28c including correction capacitor)
FIG. 8 shows an equivalent circuit inside the pixel including the correction capacitor, and FIG. 9 shows an equivalent circuit of the image detection unit 25 having the pixel including the correction capacitor.

  The equivalent circuit shown in FIG. 8 further includes a correction capacitor 38 in addition to the elements of the equivalent circuit shown in FIG. The correction capacitor 38 is formed between the drain electrode of the thin film transistor 31 connected to the row selection line 35a and the row selection line 35b adjacent to the selection line 35a.

  In the equivalent circuit shown in FIG. 9, a large number of row selection lines 35 arranged in the horizontal direction and a large number of signal lines 37 arranged in the vertical direction are arranged orthogonally in a grid pattern and surrounded by the row selection lines 35 and the signal lines 37. The pixels 28c shown in FIG. 8 are arranged in a grid pattern in the region. The signal line 37 is connected to the integrating amplifier 41.

  In FIG. 9, the parasitic capacitance 22 shown in FIG. 8 is omitted from the drawing.

(Circuit pattern of pixel 28c including correction capacitor)
FIGS. 10A and 10B show examples of circuit patterns of the image detection unit 25 of the radiation image detector 10 having the pixel 28c including the correction capacitor.

  The circuit pattern shown in FIG. 10 is different from the circuit pattern shown in FIG. 6 in that the lower electrode 24 ′ formed in the lower layer of the photodiode 33 connected to the row selection line 35a is arranged in an adjacent row. That is, the insulating film 30 is superimposed on the row selection line 35b to which it belongs. Thereby, the correction capacitor 38 is formed.

  As described above, the lower electrode 24 ′ of the photodiode 33 is formed in the same layer as the drain electrode 42 of the thin film transistor 31. As described above, the row selection lines 35 a and 35 b are formed in the same layer as the gate electrode 40 of the thin film transistor 31.

  Therefore, the correction capacitor 38 can be formed in the same layer as the gate and drain of the thin film transistor 31. Therefore, the correction capacitor 38 can be formed efficiently and easily, and the change in the TFT manufacturing process can be minimized, and the increase in the product price can be minimized.

(Operation of Radiation Image Detector 10 Having Pixel 28c Including Correction Capacitor)
The operation of the radiation image detector 10 having the pixel 28c including the correction capacitor 38 shown in FIGS. 8 to 10 will be described.

  First, an X-ray image incident on the radiation image detector 10 from the outside is converted into visible light inside the fluorescence conversion film 23 and irradiated on the surface of the image detection unit 25 as a visible light image.

  A circuit having an equivalent circuit shown in FIG. 9 is formed inside the image detection unit 25, and the pixels 28c shown in FIG. 8 are formed in a grid pattern. Charge is stored in the capacitor 36 shown in FIG. 8, and a reverse bias voltage is applied to the photodiode 33.

  During the period for taking an X-ray image, the thin film transistor 31 is in a non-conductive state, and a gate voltage (about -5 V) for making the connected thin film transistor 31 non-conductive is supplied from the row selection line 35a. Yes.

  When the X-ray image capturing period ends, the thin film transistor 31 is turned on to read the X-ray image stored in the pixel 28c, and the drain-source resistance of the thin film transistor 31 is lowered.

  As a result, the signal line 37 connected to the source electrode 43 of the thin film transistor 31 and the capacitor 36 connected to the drain electrode 42 of the thin film transistor 31 are electrically connected, and the charge generated in the photodiode 33 due to the X-ray irradiation. As a result, the potential of the capacitor 36 whose potential difference is reduced returns to the initial state. At that time, the amount of electric charge flowing through the signal line 37 is amplified by an integrating amplifier 41 connected to the outside and output as an X-ray image.

  When the X-ray image is read, the voltage of the row selection line 35a is changed (about 20V), thereby bringing the thin film transistor 31 connected to the row selection line 35a into a conductive state. At this time, charges corresponding to the potential difference (about 20 V) from the signal line 37 are accumulated in the parasitic capacitance 22.

  Conventionally, during the readout period of pixels belonging to the row selection line 35a, the potential of the row selection line 35b that drives the adjacent row is not changed, but in this embodiment, the row selection adjacent to the row selection line 35a is selected. Line 35b is changed simultaneously.

  Specifically, when the pixel 28c is designed so that the capacitance of the parasitic capacitance 22 is 10 fF, the capacitance of the correction capacitor 38 is 100 fF, and the capacitance of the capacitor 36 is 1 pF, the thin film transistor 31 connected to the row selection line 35a. Is changed from a voltage (about -5V) to turn off to a voltage (20V) to turn on, the voltage of the row selection line 35b is set, for example, from -5V to -7.5V at the same timing. Note that a constant potential of −3 V is supplied to the bias line.

  When the adjacent row selection lines are changed at the same time, when the thin film transistor 31 is in the conductive state, a charge of 200 fC is accumulated in the parasitic capacitor 22 and a charge of −750 fC is accumulated in the correction capacitor 38. . Further, a charge of -3 pC is accumulated in the capacitor.

  When the readout of the charge information from the pixel is completed and the voltage (−5V) that makes the connected thin film transistor 31 non-conductive is supplied to the row selection line 35a, the thin film transistor 31 becomes non-conductive. At the same time, the voltage of the adjacent row selection line 35b supplies the normal voltage (−5V). In this state, the charge amount accumulated in the parasitic capacitor 22 and the correction capacitor 38 is redistributed. As a result of this redistribution, −50 fC is stored in the parasitic capacitance 22, −500 fC is stored in the correction capacitor 38, and −3 pC is stored in the capacitor 36, and before and after the state of the thin film transistor 31 is changed, The total amount of charge will not change.

  The charge accumulated in the capacitor 36 does not change before and after the thin film transistor 31 is changed from the conductive state to the non-conductive state by the above driving. When the thin film transistor 31 is conductive, the drain electrode 42 of the thin film transistor 31 connected to the capacitor 36 and the source electrode 43 connected to the signal line 37 have the same potential.

  Thereafter, immediately after the thin film transistor 31 is changed to the non-conductive state, the charge state of the capacitor 36 does not change, and the potential difference of the capacitor 36 does not change. Therefore, even if the thin film transistor 31 is changed to a non-conductive state, a potential difference between the drain and the source of the thin film transistor 31 does not occur.

  For this reason, as long as there is no X-ray incident from the outside, no charge is generated from the photodiode 33, and the charge amount of the capacitor 36 does not change. Therefore, the drain voltage of the thin film transistor 31 does not change, and the potential difference between the drain and the source of the thin film transistor 31 is kept at 0V. Therefore, when there is no X-ray incidence from the outside, no leakage current is generated between the drain and source of the thin film transistor 31 which is in a non-conductive state but has a finite resistance value.

  In the above operation, the voltage of the row selection line 35b for driving the pixels belonging to the row adjacent to the row selection line 35a is changed. At this time, the thin film transistors of the pixels belonging to the row selection line 35b need to be kept non-conductive. There is. This is necessary so that signals from adjacent pixels are not mixed. In the case of a general thin film transistor 31, in order to maintain a non-conductive state even if the gate voltage is changed by several volts in the negative direction, even if it is changed from -5V to -7.5V as in the present embodiment, It is easy to keep the thin film transistor in a non-conductive state.

(Effect of this embodiment)
In a radiographic image detector that requires detection of a weak X-ray image, the leakage current greatly impairs the quality of the X-ray image. In particular, in the case of detecting a very dark X-ray image, in the case of a conventional radiation image detector, the leakage current is larger than the signal amount due to incident X-rays.

  However, a significant reduction in leakage current can be realized by using the radiation image detector 10 having the pixel 28c including the correction capacitor of the present embodiment. Thereby, it becomes possible to reduce the offset charge amount of the pixel in which the leakage current occupies a large proportion.

  Therefore, it is possible to greatly reduce the number of measurement of the offset charge amount that makes it impossible to take an X-ray image without accurately controlling the temperature inside the detector using a Peltier element, a large-capacity heat dissipation structure and a fan. A radiation image detector that facilitates radiography can be provided.

(Other embodiments)
In the above embodiment, an example of the indirect method in which the photodiode 33 is provided as the radiation sensor 11 is shown. However, a direct method in which X-rays are directly converted into a charge signal by a photoconductive film and led to a charge storage capacitor. The same applies to. In this case, the same effect can be obtained by omitting the fluorescence conversion film 23 and using a photoconductive film such as a-Se instead of the photodiode 33 of the image detection unit 25.

10: Radiation image detector 11: Radiation sensor 13: Gate drive circuit 15: Drive control circuit 17: Reading circuit 18: Reading control circuit 19: Correction circuit 20: Display device 21: Incident X-ray 22: Parasitic capacitance 23: Fluorescence conversion Films 24 and 24 ': Lower electrode 25: Image detection unit 26: Bias line 27: Holding substrate 28: Pixel 28a: Pixel 28b: Pixel 28c: Pixel 28d: Pixel 29: Circuit layer 30: Insulating film 31: Thin film transistor 33: Photo Diode 34: Parasitic capacitance 35: Row selection line 35a: Row selection line 35b: Row selection line 36: Capacitor 37: Signal line 38: Correction capacitor 40: Gate electrode 41: Integration amplifier 42: Drain electrode 43: Source electrode 63: Gate Driver 65: Row selection circuit 67: A / D converter 69: Driver

Claims (6)

A pixel including a switching element having a gate, a source, and a drain for performing a switching function, a photoelectric conversion element for converting light into a charge signal, and an element having an electric capacitance as a set of elements is two-dimensionally formed on the substrate. In the arbitrary pixel, the switching element is one of a plurality of row selection lines extending in the row direction on the substrate and a plurality of signals extending in the column direction. Connected to one of the lines, the photoelectric conversion element is connected to the switching element, and the element having the electric capacity is an adjacent row selection that drives an adjacent switching element adjacent to the switching element in a column direction. A radiation sensor in which a fluorescence conversion film is formed between the X-ray and the switching element, and a fluorescent conversion film that converts X-rays incident from the outside into light is laminated on the pixel;
A drive voltage for driving the switching element from a conduction state to an insulation state is applied to one arbitrarily selected row selection line among the plurality of row selection lines, and the row selection line to which the drive voltage is applied A gate drive circuit for applying a potential change in a direction opposite to a drive potential change for driving the switching element from a conductive state to an insulated state to an adjacent row selection line;
A radiation image detector comprising:   A lower electrode is provided extending in a column direction below the photoelectric conversion element in the pixel connected to the arbitrary row selection line, and an insulating film is provided on the adjacent row selection line adjacent to the row selection line. The radiographic image detector according to claim 1, wherein the element having the electric capacitance is formed by overlapping with each other. Furthermore, image signal information is read from the photoelectric conversion element connected to the switching element connected to any one of the row selection lines to which the driving voltage is applied via the signal line, and by X-rays A reading circuit that reads the amount of offset charge outside of shooting,
A noise correction circuit for correcting the image signal information based on the offset charge amount;
The radiation image detector according to claim 1, further comprising: A pixel including a switching element that has a gate, a source, and a drain to perform a switching function, a conversion element that directly converts radiation into a charge signal, and an element having an electric capacitance as a set of elements is two-dimensionally formed on the substrate. In the arbitrary pixel, the switching element is one of a plurality of row selection lines extending in the row direction on the substrate and a plurality of signals extending in the column direction. Connected to one of the lines, the conversion element is connected to the switching element, and the element having the electric capacity drives an adjacent switching element adjacent to the switching element in a column direction. And a radiation sensor formed between the switching element and
A drive voltage for driving the switching element from a conduction state to an insulation state is applied to one arbitrarily selected row selection line among the plurality of row selection lines, and the row selection line to which the drive voltage is applied A gate drive circuit for applying a potential change in a direction opposite to a drive potential change for driving the switching element from a conductive state to an insulated state to an adjacent row selection line;
A radiation image detector comprising:   A lower electrode is provided extending in a column direction below the conversion element in the pixel connected to the arbitrary row selection line, and an insulating film is provided on an adjacent row selection line adjacent to the row selection line. 5. The radiation image detector according to claim 4, wherein the element having the electric capacitance is formed by overlapping the elements. Furthermore, image signal information is read through the signal line from the conversion element connected to the switching element connected to any one of the row selection lines to which the driving voltage is applied, and X-ray imaging is performed. A reading circuit that reads the amount of offset charge outside;
A noise correction circuit for correcting the image signal information based on the offset charge amount;
The radiation image detector according to claim 4, further comprising:

Images