JP2005117281A - Image reading apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem of a conventional image reading apparatus employing a thin film transistor that although an area sensor incorporating a source follower circuit of a thin film transistor has been developed, a significant read error occurs because the source follower circuit is susceptible to a variation in threshold of the thin film transistor. <P>SOLUTION: A buffer circuit having a differential circuit is constituted of a thin film transistor and the output voltage of a light receiving element is taken out through the buffer circuit thus eliminating conventional problems such as deficient driving power, variation in threshold, and the like. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image reading apparatus, and more particularly to an image reading apparatus configured with a thin film semiconductor element. The present invention also relates to an electronic device using the image reading device.

  In recent years, with the progress of communication technology, portable electronic devices have become widespread. In the future, transmission of moving images and transmission of more information are expected. In addition, personal computers are also produced with mobile-friendly products due to their light weight. A large number of information terminals called PDAs that have begun in electronic notebooks are also being produced and spread. Some of them have an image reading function, and the printed information can be digitized.

  Conventional image reading apparatuses having such an image reading function, particularly image reading apparatuses using thin film transistors, include the following. FIG. 2 shows an equivalent circuit of a conventional image reading apparatus. The image reading apparatus shown here is a one-dimensional line sensor, and includes a shift register 201, switches 202 to 204, and photodiodes 205 to 207. The shift register sequentially drives the switches 202 to 204. When the switch 202 is turned on, the photodiode 205 is connected to the signal output line. The photodiode has a function of outputting a current corresponding to the intensity of light when irradiated with light. When the light hits the photodiode 205, a current flows from the photodiode 205 to the signal output line via the switch 202. Next, when the switch 202 is turned off and the switch 203 is turned on, the current of the photodiode 206 flows through the switch 203 to the signal output line. Do this sequentially. The output current of the photodiode for one line is sequentially output to the signal output line (see, for example, Patent Document 1).

  The image reading apparatus includes not only a one-dimensional line sensor but also a two-dimensional area sensor, an example of which is shown in FIG. In the area sensor, the pixels 300 are arranged in a matrix, and driving circuits for driving the pixels are arranged around the pixels. In FIG. 3, 2 × 2 pixels are shown for the sake of simplification, but are not limited to 2 × 2. The pixel 300 includes a photodiode 308, a switch 314, a source follower transistor 312, and a switch 310. A signal line reading circuit 337 that reads signals from the signal lines 335 and 336 includes a shift register 301, switches 302 and 303, source followers 304 and 305, and capacitors 306 and 307. The switches 302 and 303 are connected to the signal output line 324, and the power supply lines 325 and 326 apply a direct current potential to the source follower TFTs 304 and 305 and the capacitors 306 and 307, respectively.

  Next, the operation will be described. First, when the scanning line 330 is activated by the scanning line driving circuit 334, the switches 314 and 315 are turned on, and the potentials of the power supply lines 328 and 339 are applied to both ends of the photodiodes 308 and 309. At this time, a potential is applied to the photodiode so that a reverse bias is applied. Next, when the scanning line 330 becomes inactive, the switches 314 and 315 are turned off. Here, when the photodiode receives light, a current flows from the cathode of the photodiode to the anode, and the cathode potential of the photodiodes 314 and 315 decreases.

  A potential lower than the cathode potential of the photodiodes 314 and 315 by VGS of the source follower TFTs 312 and 313 is applied to the switches 310 and 311. Next, the scanning line drive circuit 334 activates the scanning line 327 and turns on the switches 310 and 311. Accordingly, the capacitors 306 and 307 are charged by the source follower TFTs 312 and 313. A potential lower than the potentials of the capacitors 306 and 307 by VGS of the source follower TFTs 304 and 305 is applied to the switches 302 and 303. The shift register 301 sequentially turns on the switches 302 and 303. First, the output of the source follower TFT 304 is output to the signal output line 324. In this way, sensing on the first line is completed.

  Next, when the scanning line 333 is activated by the scanning line driving circuit 334, the switches 322 and 323 are turned on, and the potentials of the power supply lines 331 and 332 are applied to both ends of the photodiodes 316 and 317. At this time, a potential is applied to the photodiode so that a reverse bias is applied. Next, when the scanning line 333 becomes inactive, the switches 322 and 323 are turned off. Here, when the photodiode receives light, a current flows from the cathode of the photodiode to the anode, and the cathode potential of the photodiodes 316 and 317 decreases.

A potential lower than the cathode potential of the photodiodes 316 and 317 by VGS of the source follower TFTs 320 and 321 is applied to the switches 318 and 319. Next, the scanning line driving circuit 334 activates the scanning line 331 and the switches 318 and 319 are turned on. Accordingly, the capacitors 306 and 307 are charged by the source follower TFTs 320 and 321. A potential lower than the potentials of the capacitors 306 and 307 by VGS of the source follower TFTs 304 and 305 is applied to the switches 302 and 303. The shift register 301 sequentially turns on the switches 302 and 303. First, the output of the source follower TFT 304 is output to the signal output line 324. In this way, sensing on the second line is completed. Thereafter, another row is sensed to enable two-dimensional sensing (see, for example, Patent Document 2).
JP-A-7-115223 JP 2001-308306 A

  In the conventional line sensor type image reading apparatus as described above, since the output current of the photodiode is taken out via a switch, there is a problem that the signal is weak and easily affected by noise. In addition, since the current output is small, when the current value is small, the current is used for charging / discharging the parasitic capacitance on the substrate, and there is a problem that an accurate current cannot be detected.

  Further, in the conventional area sensor type image reading apparatus, since the output of the photodiode is taken out by the voltage via the source follower transistor, there are few problems with the current output method as in the line sensor type image reading apparatus. However, the output voltage of the source follower circuit greatly depends on the threshold voltage of the transistors constituting the source follower. When a source follower circuit is configured with a thin film transistor, there is a problem that in-plane variation of the threshold voltage of the thin film transistor is large, so that the output voltage of the source follower varies from place to place and reading becomes inaccurate.

In order to solve the above problems, when the present inventor configures an image reading circuit using a thin film transistor, the image reading circuit is configured using a buffer circuit using a differential amplifier without using a source follower. Thought to do.
When using a differential amplifier, the output variation is not the absolute value variation of the threshold voltage, but depends on the relative variation of the transistor pair constituting the differential amplifier, so even if a thin film transistor with large in-plane variation is used, Good characteristics can be obtained.

  The present invention provides an image reading apparatus having a thin film transistor and a light receiving element on an insulating substrate, wherein the image reading apparatus includes a buffer circuit having a differential circuit composed of the thin film transistor, and means for selecting an output of the buffer circuit. And the light receiving element is electrically connected to the buffer circuit.

  The present invention is characterized in that, in the above, the means for sequentially selecting the buffer circuits is a shift register.

  The present invention provides an image reading apparatus having a thin film transistor and a light receiving element on an insulating substrate, wherein the light receiving element includes a first and a second buffer circuit having a differential circuit composed of the thin film transistor, and a first and a second buffer circuit. A switch, a signal output line, and a means for sequentially selecting the second switch, the light receiving element is electrically connected to the first buffer circuit, and one end of the first switch is connected to the first switch The first switch is electrically connected to the second buffer circuit, and one end of the second switch is electrically connected to the second buffer circuit. The second end of the second switch is electrically connected to the signal output line.

  The present invention is characterized in that, in the above, the means for sequentially selecting the second switch is a shift register.

  The present invention is characterized in that, in the above, the light receiving element is constituted by an amorphous silicon photodiode.

  In the above, the present invention is characterized in that the light receiving element is constituted by a polysilicon thin film element.

  The present invention is characterized in that a module having a display device has an image reading device composed of a light receiving element and a thin film transistor on an insulating substrate, and the image reading device is mounted on the display device.

  The present invention relates to a module having a display device, the module having an image reading device having a buffer circuit using a differential circuit composed of a light receiving element and a thin film transistor on an insulating substrate, the image reading device being a display device. It is characterized by being implemented above.

  In the above, the present invention is characterized in that the substrate constituting the display device and the substrate constituting the image reading device have the same composition.

  The present invention is characterized in that the display device is a liquid crystal display device.

  The present invention is characterized in that, in the above, the display device is an EL display device.

  The present invention is an electronic device including the above-described image reading device.

  The present invention is an electronic device including the above module.

  As described above, the image reading apparatus of the present invention includes a light receiving element and a buffer circuit having a differential circuit configured using thin film transistors, and noise superimposition of an output signal, which has been a problem of the prior art, It is possible to eliminate signal deterioration due to parasitic capacitance, signal fluctuation due to variations in threshold voltage of thin film transistors, and the like, and an image reading apparatus with a small reading error can be configured.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows an application of the present invention to a one-dimensional line sensor type image reading apparatus. The line sensor shown in FIG. 1 includes a shift register 101 and circuit arrays 102 and 103. In FIG. 1, only the circuit rows 102 and 103 are shown for simplification of explanation, but the number of circuit rows is not limited to two, and a larger number of circuit rows can be connected.

  The circuit array 102 includes a light receiving element 122, a first switch 120, a first buffer circuit 118 having a differential circuit, a second switch 116, a holding capacitor 114, a second buffer circuit 112 having a differential circuit, and a first buffer circuit 118. Switch 110. Similarly, the circuit array 103 includes a light receiving element 123, a fourth switch 121, a third buffer circuit 119 having a differential circuit, a fifth switch 117, a holding capacitor 115, a fourth buffer circuit 113 having a differential circuit, The sixth switch 111 is used. The signal output line 104 is connected to the switches 110 and 111, the switch control line 106 controls the switches 116 and 117, and the switch control line 109 controls the switches 120 and 121. The power line 105 applies a DC potential to the storage capacitors 114 and 115, and the power lines 108 and 107 apply a potential to the light receiving elements 122 and 123.

  Next, the operation of this embodiment will be described. Initially, the switch control line 108 becomes active, and the first switch 120 and the fourth switch 121 are turned on. As a result, the potentials of the power supply line 107 and the power supply line 108 are applied to both ends of the light receiving elements 122 and 123. This is called a reset state. Next, the switch control line 108 becomes inactive, and the first switch 120 and the fourth switch 121 are turned off. When light is irradiated, a light receiving current flows through the light receiving elements 122 and 123, and the current of the light receiving element decreases. Since the ends of the light receiving elements 122 and 123 that are not connected to the power supply line 107 are respectively connected to buffer circuits 118 and 119 having differential circuits, the potentials of the light receiving elements 122 and 123 pass through the buffer circuits 118 and 119. Are input to the switches 116 and 117.

  After a certain time after the first switch 120 and the fourth switch 121 are turned off, the switch control line 106 becomes active, and the second switch 116 and the fifth switch 117 are turned on. Accordingly, the capacitors 114 and 115 are charged by the buffer circuits 118 and 119. When charging is completed, the switch control line 106 becomes inactive, and the switches 116 and 117 are turned off.

  Since the capacitors 114 and 115 are respectively connected to the second buffer circuit 112 having a differential circuit and the fourth buffer circuit 113 having a differential circuit, the potentials of the capacitors 114 and 115 are passed through the buffer circuits 112 and 113. Are output to the third switch 110 and the sixth switch.

  The shift register 101 sequentially selects the switches 110 and 111. When the switch 110 is turned on, the output of the buffer circuit 112 is connected to the signal output line 104. When the switch 111 is turned on, the output of the buffer circuit 113 is connected to the signal output line. In this way, the outputs of the buffer circuits 112 and 113 are sequentially output to the signal output line. In the above description, the circuit arrays 102 and 103 have been described. However, it is possible to configure a one-dimensional line sensor type image reading apparatus by connecting three or more circuit arrays.

  In the buffer circuit using the differential circuit as shown in FIG. 1 and applying feedback, the input and output potentials of the buffer circuit can be made substantially the same. In a buffer circuit using a source follower as shown in the conventional example, a potential difference always occurs between the input and output, and the potential difference is affected by variations in threshold values of transistors, particularly variations in the substrate. It is not suitable for such a signal reading device. In particular, it is not suitable for a signal reading device using a thin film transistor with large variations in the substrate.

  As described above, according to the present invention, in the signal reading device, it is possible to configure a signal reading device with little variation by taking out the output of the light receiving element through a buffer circuit having a differential circuit. . In particular, the effect is remarkable in a signal reading device using a thin film transistor.

  The light receiving elements used in the present embodiment can be various light receiving elements such as photodiodes and phototransistors, and the types are not particularly limited.

  FIG. 4 is an equivalent circuit diagram in the case where a buffer circuit, particularly an operational amplifier type buffer circuit, is formed by using a thin film semiconductor element, particularly a TFT. This operational amplifier type buffer circuit includes a differential circuit composed of TFT 9501 and TFT 9502, a current mirror circuit composed of TFT 9503 and TFT 9504, a constant current source composed of TFT 9505 and TFT 9509, a common source circuit composed of TFT 9506, and TFT 9507. , An idle circuit composed of TFT 9508, a source follower circuit composed of TFT 9510 and TFT 9511, and a phase compensation capacitor 9512. The current mirror TFT and the source ground circuit TFT are connected to the high potential side power supply terminal, and the constant current source TFT is connected to the low potential power supply terminal.

  The operation of the operational amplifier type buffer circuit of FIG. 4 will be described below. When a + signal is input to the non-inverting terminal, the constant current source configured by the TFT 9505 is connected to the source of the TFT configuring the differential circuit, so that the drain current of the TFT 9501 becomes larger than the drain current of the TFT 9502. The drain current of the TFT 9503 is the same as the drain current of the TFT 9502 because the TFT 9504 and the TFT 9503 constitute a current mirror circuit. To change. Since the TFT 9506 is a P-type TFT, when the gate potential of the TFT 9506 decreases, the TFT 9506 operates in a more ON direction and the drain current increases. Accordingly, the gate potential of the TFT 9510 increases, and accordingly, the source potential of the TFT 9510, that is, the output terminal also increases.

  When a negative signal is input to the non-inverting input terminal, the drain current of the TFT 9501 becomes smaller than the drain current of the TFT 9502, and the drain current of the TFT 9503 is the same as the drain current of the TFT 9502; Due to the difference current, the gate potential of the TFT 9506 changes in a rising direction. Since the TFT 9506 is a P-type TFT, when the gate potential of the TFT 9506 increases, the TFT 9506 operates in a direction to turn off and the drain current decreases. Therefore, the gate potential of the TFT 9510 is lowered, and accordingly, the source potential of the TFT 9510, that is, the output terminal is also lowered. In this way, a signal in phase with the signal at the non-inverting input terminal is output from the output terminal.

  When a + signal is input to the inverting input terminal, the drain current of the TFT 9501 becomes smaller than the drain current of the TFT 9502, and the drain current of the TFT 9503 is the same as the drain current of the TFT 9502. Therefore, the difference current between the drain current of the TFT 9503 and the TFT 9501 As a result, the gate potential of the TFT 9506 changes in an increasing direction. Since the TFT 9506 is a P-type TFT, when the gate potential of the TFT 9506 increases, the TFT 9506 operates in a direction to turn off and the drain current decreases. Therefore, the gate potential of the TFT 9510 is lowered, and accordingly, the source potential of the TFT 9510, that is, the output terminal is also lowered.

  When a negative signal is input to the inverting input terminal, the drain current of the TFT 9501 becomes larger than the drain current of the TFT 9502, and the drain current of the TFT 9503 is the same as the drain current of the TFT 9502; The gate potential of the TFT 9506 changes in a decreasing direction due to the difference current of the drain current. Since the TFT 9506 is a P-type TFT, when the gate potential of the TFT 9506 decreases, the TFT 9506 operates in a more ON direction and the drain current increases. Accordingly, the gate potential of the TFT 9510 increases, and accordingly, the source potential of the TFT 9510, that is, the output terminal also increases. In this way, a signal having a phase opposite to that of the inverting input terminal is output from the output terminal.

  By electrically connecting the output terminal and the inverting input terminal in the above circuit, feedback can be applied and a buffer circuit with little variation can be configured.

In this embodiment, the differential circuit is made of an Nch TFT and the current mirror circuit is made of a Pch TFT. However, the present invention is not limited to this and may be reversed. In this case, the constant current source TFT is connected to the high potential side power supply terminal, and the current mirror circuit TFT and the source ground circuit TFT are connected to the low potential side power supply terminal.
In addition, the operational amplifier type buffer circuit used in the present invention is not limited to such a circuit form, and any circuit that satisfies a necessary function can be used.

  FIG. 5 is an equivalent circuit diagram in the case where a buffer circuit is formed using a thin film semiconductor element, particularly a TFT. This buffer amplifier is composed of a differential circuit composed of TFT 9601 and TFT 9602, a current mirror circuit composed of TFT 9603 and TFT 9604, and a constant current source composed of TFT 9605. The current mirror circuit TFT is connected to the high potential side power supply terminal, and the constant current source TFT is connected to the low potential side power supply terminal.

  The operation of the buffer circuit of FIG. 5 will be described below. When a + signal is input to the input terminal, the constant current source configured by the TFT 9605 is connected to the source of the TFT forming the differential circuit, and thus the drain current of the TFT 9601 becomes larger than the drain current of the TFT 9602. The drain current of the TFT 9604 is the same as the drain current of the TFT 9601 because the TFT 9604 and the TFT 9603 constitute a current mirror circuit, and the potential of the output terminal increases in the direction of the difference between the drain current of the TFT 9604 and the drain current of the TFT 9602. Change.

  When a negative signal is input to the input terminal, the drain current of the TFT 9601 becomes smaller than the drain current of the TFT 9602, and the drain current of the TFT 9604 is the same as the drain current of the TFT 9601. Therefore, the difference between the drain current of the TFT 9604 and the TFT 9602 Depending on the current, the potential of the output terminal changes in a decreasing direction.

In this embodiment, the differential circuit is made of NchTFT and the current mirror circuit is made of PchTFT. However, the present invention is not limited to this and may be reversed. In such a case, the constant current source TFT is connected to the high potential side power supply terminal, and the current mirror circuit TFT is connected to the low potential power supply terminal.
The buffer circuit used in the present invention is not limited to the circuit of this embodiment, and other circuits may be used as long as necessary functions are satisfied.

  FIG. 7 shows an example of a sectional structure of the image reading apparatus of the present invention. 7 includes a substrate 701, a base film 702, a thin film transistor active layer 703, a gate insulating film 704, a gate electrode 705, an interlayer insulating film 706, a wiring 707, a source electrode 708, a drain electrode 709, a transparent electrode 710, and a P-type electrode. 711, an I-type electrode 712, an N-type electrode 713, a second interlayer film 714, and extraction electrodes 715 and 716.

  Hereinafter, the manufacturing method will be described. A base film 702 is formed over an insulating substrate 701 such as a glass substrate, a quartz substrate, or a plastic. This base film is formed of an oxide film, a nitride film, a composite thereof, or a laminated film. Next, an amorphous silicon film is formed and then crystallized. Crystallization methods include laser crystallization and thermal crystallization, which may be combined. Further, thermal crystallization may be performed after adding the catalyst element. The laser is preferably continuous oscillation, but may be pulse oscillation. Lasers can use either gas oscillation or solid-state oscillation. Gas lasers include excimer laser, Ar laser, and Kr laser. Solid lasers include YAG laser, YVO4 laser, YLF laser, YAIO3 laser, glass laser, ruby laser, and alexander dry. There are laser diodes and sapphire lasers. Next, the crystallized silicon film is patterned to form island regions 703.

  Next, after forming a gate insulating film, a metal for a gate electrode is formed. Then, it is patterned to form a gate electrode. Further, P-type impurities and N-type impurities are doped and activated by heat or laser. As a result, the location where the impurity is doped becomes the source or drain region. Next, an interlayer film 706 is formed. As the interlayer film, an oxide film, a nitride film, a resin film, or the like can be used.

Next, a contact hole is opened and a metal to be a source / drain electrode is formed. Then, the metal for the source / drain electrode is patterned to form a wiring 707, a source electrode 708, and a drain electrode 709. Next, a transparent electrode 710 is formed and patterned. Then, an N-type amorphous silicon 711, an I-type amorphous silicon 712, and a P-type amorphous silicon 713 to be a light receiving element are formed and patterned. As a result, an amorphous silicon photodiode is formed. Next, a second interlayer film 714 is formed, a contact hole is opened, a metal for an extraction electrode is formed, and patterning is performed. Thus, an image reading apparatus having a thin film transistor and an amorphous silicon photodiode is formed.
As described above, in the present invention, an image reading device can be configured by forming a thin film transistor and a light receiving element over an insulating substrate, and thus a stick-shaped image reading device larger than a single crystal silicon chip can be configured. Become.

  FIG. 8 shows an embodiment of a cross-sectional structure of an image reading apparatus different from FIG. 8 includes a substrate 801, a base film 802, thin film transistor active layers 803 and 804, a gate insulating film 805, gate electrodes 806 and 807, an interlayer insulating film 808, a wiring 809, a source electrode 810, a drain electrode and a cathode electrode 811, An anode electrode 812, a second interlayer film 813, and extraction electrodes 814 and 815 are included.

  Hereinafter, the manufacturing method will be described. A base film 802 is formed over an insulating substrate 801 such as a glass substrate, a quartz substrate, or a plastic. This base film is formed of an oxide film, a nitride film, a composite thereof, or a laminated film. Next, an amorphous silicon film is formed and then crystallized. Crystallization methods include laser crystallization and thermal crystallization, which may be combined. Further, thermal crystallization may be performed after adding the catalyst element. The laser is preferably continuous oscillation, but may be pulse oscillation. Lasers can use either gas oscillation or solid-state oscillation. Gas lasers include excimer laser, Ar laser, and Kr laser. Solid lasers include YAG laser, YVO4 laser, YLF laser, YAIO3 laser, glass laser, ruby laser, and alexander dry. There are laser diodes and sapphire lasers. Next, the crystallized silicon film is patterned to form island regions 803 and 804.

Next, after forming a gate insulating film, a metal for a gate electrode is formed. Then, it is patterned to form gate electrodes 806 and 807. Further, P-type impurities and N-type impurities are doped and activated by heat or laser. As a result, the location where the impurity is doped becomes the source or drain region.
Here, a thin film transistor can be formed by doping impurities of the same polarity on both sides of the gate electrode, and a thin film PIN diode is formed by doping impurities of different polarities on both sides of the gate electrode. Can do. Next, an interlayer film 808 is formed. As the interlayer film, an oxide film, a nitride film, a resin film, or the like can be used.

Next, a contact hole is opened and a metal to be a source / drain electrode is formed. Then, the metal for the source / drain electrode is patterned to form a wiring 809, a source electrode 810, a drain / cathode electrode 811, and an anode electrode 812. Next, a second interlayer film 813 is formed, a contact hole is opened, a metal for an extraction electrode is formed, and patterning is performed to form extraction electrodes 814 and 815. As described above, an image reading device having a light receiving element of a thin film transistor and a thin film PIN diode is formed.
As described above, in the present invention, an image reading device can be configured by forming a thin film transistor and a light receiving element over an insulating substrate, and thus a stick-shaped image reading device larger than a single crystal silicon chip can be configured. Become.

  FIG. 6 shows an example of a module in which the image reading device of the present invention is mounted on a liquid crystal display device. In the liquid crystal display device of FIG. 6, a pixel portion 603, a source signal line driver circuit 605, and a gate signal line driver circuit 604 are formed on a TFT substrate 601 using thin film transistors, and a counter substrate 602 is attached. A liquid crystal material is inserted between the TFT substrate 601 and the counter substrate 602. The liquid crystal material is sealed with a sealing material (not shown). An FPC 606 (flexible printed circuit) is mounted on the TFT substrate, and necessary signals are supplied from the FPC 606 to the gate signal line driving circuit 604, the source signal line driving circuit 605, and the image reading device 607.

The image reading device is mounted on the TFT substrate 601. In this mounting, bumps may be placed on the TFT substrate, or electrical connection may be made using a conductive paste or the like. In addition, input / output of electrical signals of the image reading device may be separated from electrical signal input of the liquid crystal display device, and another FPC may be provided.
Further, the description has been given by taking the liquid crystal display device as an example of the display device, but this embodiment is not limited to the liquid crystal display device, and various displays other than the liquid crystal display device such as an EL display device and an electrophoretic display device. It can be used for modules using the device.

In addition, it is desirable to use the same type of substrate material for the substrate forming the image reading device of the present invention and the substrate forming the display device.
Since the image reading apparatus of the present invention can be formed on an insulating substrate unlike a single crystal silicon chip, the upper limit of the length is not 2 to 3 cm as in the case of a single crystal silicon chip. Since it is possible to form an image reading device and a stick-shaped image reading device having a length of 1, it is possible to mount on a large display device without having a complicated optical system. Such a module using the present invention can be used for various electronic devices.

  In the display device illustrated in FIG. 6, the pixel portion and the signal line driver circuit are integrally formed; however, the present invention is not limited to this, and the signal line driver circuit may be mounted with an IC chip by COG or using TAB. May be implemented.

  FIG. 9 shows an embodiment in which a module in which the image reading device of the present invention is mounted on the display device shown in Embodiment 5 is applied to a personal digital assistant (PDA). 9A shows a top view of a portable information terminal, FIG. 9B shows a long side view, and FIG. 9C shows a short side view. The portable information terminal of this embodiment includes a housing 901, a display unit 902, a roller 903, and a sensing slit 904. The display device described in Embodiment 5 is incorporated in the housing 901, and the display screen of the display device can be viewed from the display unit 902.

  On the short side, a slit 903 for capturing an image into the image reading device is opened, from which an optical image of the image can be captured. Since the image reading apparatus used here is a one-dimensional line sensor, a roller 903 is disposed as a distance meter for converting the image into two dimensions. The sensing distance can be calculated based on the number of rotations of the roller 903. A two-dimensional image can be constructed from this calculated value and the read image data.

FIG. 10 shows an internal structure diagram of the portable information terminal with a built-in image reading apparatus shown in FIG. 10 includes a housing 1001, a TFT substrate 1002, a counter substrate 1003, an image reading device 1004, a mirror 1005, a roller 1006, a light source 1007, and a lens 1008.
The material 1009 is irradiated with light from the light source 1007 through the slit. The light reflected by the material 1009 passes through the lens 1008, is reflected by the mirror 1005, and is input to the image reading device 1004. Optical information is converted into electrical information. In this way, the contents of the material 1009 can be read in this embodiment.

  The application range of the image reading apparatus of the present invention is extremely wide, and is not limited to the above-described portable information terminal, and can be applied to electronic devices in many fields such as a scanner and a personal computer.

The figure which shows the equivalent circuit of the image reading circuit of this invention. The figure which shows the equivalent circuit of the conventional image reading circuit. The figure which shows the equivalent circuit of the conventional image reading circuit. The figure which shows the equivalent circuit of the buffer circuit of this invention. The figure which shows the equivalent circuit of the buffer circuit of this invention. 1 is a diagram showing a display device on which an image reading device of the present invention is mounted. 1 is a cross-sectional view of an image reading apparatus according to the present invention. 1 is a cross-sectional view of an image reading apparatus according to the present invention. The figure which shows the Example which applied the image reading apparatus of this invention to PDA. The figure which shows the Example which applied the image reading apparatus of this invention to PDA.

Claims (13)

  In an image reading device having a thin film transistor and a light receiving element on an insulating substrate, the image reading device has a buffer circuit having a differential circuit composed of the thin film transistor, and means for selecting an output of the buffer circuit, An image reading apparatus, wherein the light receiving element is electrically connected to the buffer circuit.   2. The image reading apparatus according to claim 1, wherein the means for sequentially selecting the buffer circuits is a shift register.   In an image reading apparatus having a thin film transistor and a light receiving element on an insulating substrate, the light receiving element includes first and second buffer circuits having a differential circuit composed of the thin film transistor, first and second switches, and a signal. An output line and means for sequentially selecting the second switch, the light receiving element is electrically connected to the first buffer circuit, and one end of the first switch is connected to the first buffer circuit A first end of the first switch is electrically connected to the second buffer circuit; one end of the second switch is electrically connected to the second buffer circuit; An image reading apparatus characterized in that a multi-end of the switch of 2 is electrically connected to the signal output line.   4. The image reading apparatus according to claim 3, wherein the means for sequentially selecting the second switch is a shift register.   5. The image reading apparatus according to claim 1, wherein the light receiving element is formed of an amorphous silicon photodiode.   5. The image reading apparatus according to claim 1, wherein the light receiving element is formed of a polysilicon thin film element.   A module having a display device, comprising: an image reading device including a light receiving element and a thin film transistor on an insulating substrate; and the image reading device is mounted on the display device.   In a module having a display device, the module has an image reading device having a buffer circuit using a differential circuit constituted by a light receiving element and a thin film transistor on an insulating substrate, and the image reading device is mounted on the display device. A module characterized by that.   9. The module according to claim 7, wherein the substrate constituting the display device and the substrate constituting the image reading device have the same composition.   10. The module according to claim 7, wherein the display device is a liquid crystal display device.   The module according to claim 7, wherein the display device is an EL display device.   An electronic apparatus comprising the image reading device according to any one of claims 1 to 6. An electronic device comprising the module according to any one of claims 7 to 11.

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