Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an electronic device including a circuit using a photosensor and a transistor. For example, the present invention relates to an electronic device using, as its component, an electro-optical device typified by a liquid crystal display panel utilizing a touch panel. 
         [0003]    2. Description of the Related Art 
         [0004]    A CMOS sensor is used mostly in digital cameras or cellular phones and has an important imaging function. A CMOS sensor is a photosensor utilizing an amplifying function of a MOS transistor, and can be manufactured by a general CMOS process. In addition, a CMOS sensor requires lower driving voltage than a CCD sensor and thus leads to low power consumption of a solid-state imaging device. 
         [0005]    A display device using a touch sensor is attracting attention. A display device using a touch sensor is called a touch panel, a touch screen, or the like (hereinafter also referred to simply as a touch panel). A touch panel is used mostly in personal computers or cellular phones and allows image display and operation to be done on the same panel. Patent Document 1 discloses a display device using an optical touch sensor. 
         [0006]    A CMOS sensor performs three operations: storage of charge generated by a photodiode, read-out of the charge, and reset of the charge. In performing these three operations, a CMOS sensor uses a node that stores charge resulting from photocurrent generated by a photodiode (such a node is hereinafter referred to as an FD), and wire for supplying a control signal. Patent Document 2 discloses a structure of a solid-state imaging device with a layout that allows the potential of such an FD storing charge to be insusceptible to fluctuations in the potential of a signal line. 
       REFERENCE 
     [Patent Documents] 
       [0000]    
       
         [Patent Document 1] Japanese Published Patent Application No. 2001-292276 
         [Patent Document 2] Japanese Published Patent Application No. 2006-148513 
       
     
       SUMMARY OF THE INVENTION 
       [0009]    In case of presence of parasitic capacitance during these three operations which occurs between an FD that stores charge resulting from photocurrent generated by a photodiode and either wire that supplies a control signal or another signal line, changes in the signal causes changes in the potential of the FD. As a result, the output of the photosensor changes, so that imaging quality decreases. 
         [0010]    In a display device using a CMOS sensor, each pixel includes an FD. Therefore, as in the case of wire in an imaging element, in case of present of parasitic capacitance between a signal line in a display element and an FD, changes in the signal causes changes in the potential of the FD. As a result, the output of the photosensor changes, so that imaging quality decreases. 
         [0011]    An object of one embodiment of the present invention is to provide a pixel structure that prevents a decrease in imaging quality caused by changes in the output of the photosensor. 
         [0012]    One embodiment of the present invention provides a pixel layout structure in which a shielding wire is disposed between an FD and an imaging signal line (a PR line, a TX line, or an SE line) or between the FD and an image-display signal line in order to reduce or eliminate parasitic capacitance between the FD and a signal line for the purpose of suppressing changes in the potential of the FD. An imaging power supply line, an image-display power supply line, a GND line, a common line, or the like whose potential is fixed, such as a common potential line, is used as a shielding wire. 
         [0013]    One embodiment of the present invention disclosed in this specification suppresses changes in the potential of an FD (node) by using two capacity lines for the adjacent pixels as a first shielding wire and a second shielding wire and disposing these lines between the FD and a display signal line. Specifically, this is an electronic device including a pixel structure including: a first transistor electrically connected to a first pixel electrode and a first signal line; a second transistor electrically connected to a photodiode; and a third transistor electrically connected to a second pixel electrode and a second signal line. The second transistor is electrically connected to a node storing charge. A first shielding wire is provided between the node and the first signal line electrically connected to the first transistor. A second shielding wire is provided between the node and the second signal line electrically connected to the third transistor. Note that the phrase “a shielding wire is provided between the node and a signal line electrically connected to the first transistor” indicates a pixel layout in which a shielding wire is provided between the node and a signal line when seen from above. 
         [0014]    The above-stated structure resolves at least one of the above-mentioned problems. 
         [0015]    Another embodiment of the present invention suppresses changes in the potential of an FD (node) by using a capacity line for a pixel as a shielding wire, disposing this line between the FD and a display signal line, and disposing the same shielding wire between the FD and a TX line. Specifically, this is an electronic device including: a first transistor electrically connected to a pixel electrode and a first signal line; and a second transistor electrically connected to a photodiode. The second transistor is electrically connected to a node storing charge. A shielding wire is provided between the node and the first signal line electrically connected to the first transistor. The shielding wire is provided between the node and a second signal line electrically connected to a gate of the second transistor. 
         [0016]    The above-stated structure resolves at least one of the above-mentioned problems. 
         [0017]    Another embodiment of the present invention suppresses changes in the potential of an FD (node) by using a capacity line for a pixel as a shielding wire, disposing this line between the FD and a display signal line, and disposing the same shielding wire between the FD and a PR line. Specifically, this is an electronic device including: a first transistor electrically connected to a pixel electrode and a first signal line; and a second transistor electrically connected to a photodiode. The second transistor is electrically connected to a node storing charge. A shielding wire is provided between the node and the first signal line electrically connected to the first transistor. The shielding wire is provided between the node and a second signal line electrically connected to the photodiode. 
         [0018]    The above-stated structure resolves at least one of the above-mentioned problems. 
         [0019]    Another embodiment of the present invention suppresses changes in the potential of an FD (node) by using a capacity line for a pixel as a shielding wire, disposing this line between the FD and a display signal line, and disposing the same shielding wire between the FD and an SE line. Specifically, this is an electronic device including: a first transistor electrically connected to a pixel electrode and a first signal line; a second transistor electrically connected to a photodiode; a third transistor a gate of which is a node storing charge; and a fourth transistor electrically connected to the third transistor. The second transistor is electrically connected to the node. A shielding wire is provided between the node and the first signal line electrically connected to the first transistor. The shielding wire is provided between the node and a second signal line electrically connected to a gate of the fourth transistor. 
         [0020]    The above-stated structure resolves at least one of the above-mentioned problems. 
         [0021]    Reducing parasitic capacitance between a node and a signal line which may adversely affect the potential of the node can suppress fluctuations in the output of a photosensor, thereby preventing imaging quality from decreasing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIGS. 1A and 1B  are external views illustrating one embodiment of the present invention. 
           [0023]      FIG. 2  is a block diagram illustrating one embodiment of the present invention. 
           [0024]      FIG. 3  is an equivalent circuit diagram of a pixel, illustrating one embodiment of the present invention. 
           [0025]      FIG. 4  is a schematic view of a driver circuit for photosensors, illustrating one embodiment of the present invention. 
           [0026]      FIG. 5  is an equivalent circuit diagram of a pixel, illustrating one embodiment of the present invention. 
           [0027]      FIG. 6  is a timing diagram illustrating one embodiment of the present invention. 
           [0028]      FIG. 7  is a layout pattern illustrating a pixel, illustrating one embodiment of the present invention. 
           [0029]      FIGS. 8A and 8B  are a plan view and a cross-sectional view of a transistor connected to a photosensor and its periphery, illustrating one embodiment of the present invention. 
           [0030]      FIGS. 9A and 9B  are a diagram and a block diagram of an electronic device, illustrating one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following description and it is easily understood by those skilled in the art that the mode and details can be variously changed. Moreover, the present invention should not be construed as being limited to the description of the embodiments below. 
       Embodiment 1 
       [0032]    In this embodiment, an example of an electronic device  1030  including a display area  1032  in which an image is displayed using external light will be described with reference to  FIGS. 1A and 1B . 
         [0033]    The display area  1032  in the electronic device  1030  has a touch-input function for which photo sensors are used. A plurality of keyboard buttons  1031  is displayed on a display area region  1033  as illustrated in  FIG. 1A . The display area  1032  indicates the entire display region and includes the display area region  1033 . A user inputs information by touching desired keyboard buttons, so that the input information is displayed on the display area  1032 . 
         [0034]    An example of the usage of the electronic device  1030  will be described. For example, characters are input by either the user&#39;s fingers successively touching keyboard buttons displayed on the display area region  1033  or noncontact, and the resulting text is displayed on a region other than the display area region  1033 . After a set period of time during which no output signal of the photosensor is detected has passed from when the user removes his finger from the keyboard on the screen, the keyboard displayed on the display area region  1033  is erased automatically and the input text is displayed also on the display area region  1033 , so that the user can see the input text with the whole screen. In the case where input is performed again, the keyboard buttons can be displayed on the display area region  1033  again and character input can also be performed by forcing the device to detect an output signal of a photosensor by either the user&#39;s fingers successively touching the display area  1032  or noncontact. 
         [0035]    Alternatively, an image without the keyboard can be displayed on the display area  1032  not automatically but by the user pushing a switch  1034 , as illustrated in  FIG. 1B . The keyboard can be displayed and also made ready for touch input by pushing a keyboard display switch  1036 . 
         [0036]    In addition, the switch  1034 , a power supply switch  1035 , and the keyboard display switch  1036  may be displayed on the display area  1032  as switch buttons. Each operation may be performed by a touch on the displayed switch button. 
         [0037]    The electronic device  1030  includes at least a battery, and preferably includes a memory for storing data (e.g., a flash memory circuit, an SRAM circuit, or a DRAM circuit), a central processing unit (CPU), or a logic circuit. With a CPU or a memory, the device can install various kinds of software and thus can realize part or all of the functions of a personal computer. 
         [0038]    In addition, when a gradient detector such as a gyroscope or a triaxial acceleration sensor is provided in the electronic device  1030 , a function used in the electronic device  1030 , particularly a function relating to display and input performed on the display area can be switched by an arithmetic circuit in response to a signal from the gradient detector. Therefore, unlike an electronic device with an input key whose type, size, or location is predetermined, such as a built-in operation key, the electronic device  1030  can improve the user&#39;s convenience. 
         [0039]    Next, an example of a display panel included in the display area  1032  will be described with reference to  FIG. 2 . A display panel  100  includes a pixel circuit  101 , a display element control circuit, and a photosensor control circuit. The pixel circuit  101  includes a plurality of pixels  103 ,  104 ,  113 , and  114  and a plurality of photosensors  106  which are arranged in a matrix of rows and columns. Each of the pixels  103 ,  104 ,  113 , and  114  includes one display element. In this embodiment, one photosensor  106  is placed between the pixel  103  and the pixel  104  and between the pixel  113  and the pixel  114 . In other words, this embodiment employs a pixel layout structure in which one photosensor is used for four pixels. 
         [0040]    The pixels  103 ,  104 ,  113 , and  114  each include a liquid crystal element including a transistor, a storage capacitor, and a liquid crystal layer. The transistors are electrically connected to pixel electrodes  105 ,  115 ,  125 , and  135 . The transistor has a function of controlling injection or ejection of charge to/from the storage capacitor. The storage capacitor has a function of retaining charge corresponding to a voltage applied to the liquid crystal layer. Taking advantage of changes in polarization direction due to a voltage application to the liquid crystal layer, contrast of light passing through the liquid crystal layer (gray scale) is made, so that image display is realized. External light (sunlight or illumination light) which enters from the outside of a liquid crystal display device is used as the light passing through the liquid crystal layer. There is no particular limitation on the liquid crystal layer, and a known liquid crystal material (typically, a nematic liquid crystal material or a cholesteric liquid crystal material) may be used. For example, polymer dispersed liquid crystal (PDLC) or polymer network liquid crystal (PNLC) may be used for the liquid crystal layer so that a white image (a bright image) is displayed using scattered light by liquid crystal. 
         [0041]    Further, the display element control circuit is a circuit configured to control the pixels  103 ,  104 ,  113 , and  114  and includes a display element driver circuit  107  which inputs a signal to the pixel electrodes  105 ,  115 ,  125 , and  135  through the transistor via a signal line (also referred to as a source signal line) such as a video data signal line, and a display element driver circuit  108  which inputs a signal to the gate electrode of the transistor placed in each pixel via a scan line (also referred to as a gate signal line). 
         [0042]    For example, the display element driver circuit  108  connected to the scan lines has a function of selecting the display elements included in the pixels placed in a particular row. The display element driver circuit  107  connected to the signal lines has a function of applying a predetermined potential to the display elements included in the pixels placed in the selected row. Note that in the display element to which the display element driver circuit  108  connected to the scan lines applies high potential, the transistor is in a conduction state, so that the display element is supplied with charge from the display element driver circuit  107  connected to the scan lines. 
         [0043]    The photosensor  106  includes a transistor and a light-receiving element which has a function of generating an electrical signal when receiving light, such as a photodiode. 
         [0044]    The photosensor control circuit is a circuit configured to control the photosensors  106  and includes a photosensor reading circuit  109  connected to signal lines such as photosensor output signal lines and photosensor reference signal lines, and a photosensor driver circuit  110  connected to the scan lines. The photosensor driver circuit  110  connected to the scan lines has a function of performing reset operation and selecting operation, which will be described later, on the photosensors  106  included in the pixels placed in a particular row. Further, the photosensor reading circuit  109  connected to the signal lines has a function of taking out output signals of the photosensors  106  included in the pixels in the selected row. 
         [0045]    A circuit diagram of the pixel  103  and the photosensor  106  will be described in this embodiment with reference to  FIG. 3 . 
         [0046]    A pixel  301  includes a transistor  330 , a storage capacitor  331 , and a liquid crystal element  332 . A photosensor  302  includes a photodiode  351 , a transistor  352 , a transistor  353 , and a transistor  354 . 
         [0047]    A gate of the transistor  330  is electrically connected to a gate signal line  310 , one of a source and a drain of the transistor  330  is electrically connected to a video data signal line  311 , and the other of the source and the drain of the transistor  330  is electrically connected to one electrode of the storage capacitor  331  and one of the electrodes of the liquid crystal element  332 . The other electrode of the storage capacitor  331  is electrically connected to a capacitor line  312  and held at a fixed potential. The other electrode of the liquid crystal element  332  is held at a fixed potential. The liquid crystal element  332  is an element including a pair of electrodes and a liquid crystal layer provided between the pair of electrodes. 
         [0048]    When “H” (high-level potential) is applied to the gate signal line  310 , the transistor  330  applies the potential of the video data signal line  311  to the storage capacitor  331  and the liquid crystal element  332 . The storage capacitor  331  holds the applied potential. The light transmittance of the liquid crystal element  332  is changed in accordance with the applied potential. 
         [0049]    One electrode of the photodiode  351  is electrically connected to a photodiode reset signal line  341  (also referred to as a PR line), and the other electrode is electrically connected to one of a source and a drain of the transistor  352 . The other of the source and the drain of the transistor  352  is an FD (node). 
         [0050]    The transistor  352  has a function of controlling and holding the voltage of the FD (node). The gate of the transistor  352  is connected to a photosensor charge transmit signal line  342  (also referred to as a TX line). The signal line  342  has a function of controlling the switching of the transistor  352 . 
         [0051]    A gate of the transistor  354  is the FD (node). One of a source and a drain of the transistor  354  is electrically connected to a power source line  344 . Further, one of the source and the drain of the transistor  354  is electrically connected to one of a source and a drain of the transistor  353 . 
         [0052]    A gate of the transistor  353  is electrically connected to a photosensor reference signal line  345  (an SE line). The other of the source and the drain of the transistor  353  is electrically connected to a photosensor output signal line  343  (also referred to as an OUT line). 
         [0053]    Next, an example of a structure of the photosensor reading circuit  109  will be described with reference to  FIG. 4 . As an example, the display area includes pixels provided in 1024 rows and 768 columns. One display element is provided in each pixel in the rows and columns and one photosensor is provided to pixels in two rows and two columns. In other words, the display elements are provided in 1024 rows and 768 columns, and the photosensors are provided in 512 rows and 384 columns. In addition, this embodiment shows the case where a signal is output to the outside of the display device under the condition that photosensor output signal lines in two columns are regarded as one pair. In other words, one output is obtained from two photosensors provided between four pixels in two rows and two columns. 
         [0054]      FIG. 3  illustrates a circuit configuration of pixels showing four pixels and one photosensor provided in two rows and two columns. One display element is provided for each pixel and one photosensor is provided for four pixels.  FIG. 4  illustrates a circuit configuration of the photosensor reading circuit  109 , in which some photosensors are illustrated for explanation. Transistors each having a channel formation region including an oxide semiconductor can be used as transistors shown in  FIG. 3 . 
         [0055]    As an example, the case where a driving method in which, as illustrated in  FIG. 4 , a scan line driver circuit for photosensors drives pixels for four rows (that is, photosensors for two rows) simultaneously and shifts selected rows by one row including photosensors corresponding to pixels for two rows will be given. Here, photosensors in each row are continually selected in a period in which the scan line driver circuit shifts selected rows twice. Such a driving method facilitates improvement in frame frequency at the time of imaging by a photosensor. In particular, it is advantageous in the case of a large-sized display device. Note that outputs of photosensors in two rows are superimposed on the photosensor output signal line  343  at one time. All of the photosensors can be driven by repeating shift of selected rows 512 times. 
         [0056]    As illustrated in  FIG. 4 , in the photosensor reading circuit  109 , one selector is provided per pixels for 24 rows. The selector selects 1 pair from 12 pairs of photosensor output signal lines  343  (1 pair corresponds to photosensor output signal lines  211  for two columns) in the display area and obtains an output. In other words, the photosensor reading circuit  109  includes 32 selectors in total and obtains  32  outputs at one time. Selection is performed on all of the 12 pairs in each selector, whereby  384  outputs which correspond to one row of photosensors can be obtained in total. The selector selects 1 pair from the 12 pairs every time selected rows are shifted by the scan line driver circuit of photosensors, whereby outputs from all of the photosensors can be obtained. 
         [0057]    In this embodiment, the structure in which, as illustrated in  FIG. 4 , the photosensor reading circuit  109  connected to the signal lines takes out outputs of photosensors, which are analog signals, to the outside of the display device and the outputs are amplified with the use of an amplifier provided outside the display device and converted to digital signals with the use of an AD converter will be given. Needless to say, the following structure may also be employed: the AD converter is mounted on a substrate over which the display device is provided, and the outputs of photosensors are converted to digital signals and then the digital signals are taken out to the outside of the display device. 
         [0058]    A method for driving the photosensor circuit shown in  FIG. 3  will be described with a timing diagram of  FIG. 6 . 
         [0059]    In the case shown in  FIG. 6 , a binary signal is applied to a TX 910 , a PR 911 , and an SE 912  for convenience. Hereinafter, a high potential of the TX 910  is referred to as “High-TX”; a low potential of the TX 910 , “Low-TX”; a high potential of the PR 911 , “High-PR”; a low potential of the PR 911 , “Low-PR”; a high potential of the SE 912 , “High-SE”; and a low potential of the SE 912 , “Low-SE”. Note that each potential is actually an analog signal, so that each potential does not necessarily have two values and may have any number of values depending on conditions. 
         [0060]    First, at a time  901 , the TX 910  is brought “High-TX”. Next, at a time  902 , the PR 911  is brought “High-PR”. Then, an FD potential  913  becomes “High-PR” which is the same as the potential of the PR 911 . This is called a reset operation. 
         [0061]    When the PR 911  is brought “Low-PR” at a time  903 , the FD potential  913  remains “High-PR”, which makes a PIN photodiode reverse-biased (the start of a storage operation). At the same time, light enters an i-type layer in the PIN photodiode and a flow of reverse current is produced, so that the amount of charge stored on the FD changes according to the amount of light. 
         [0062]    When the TX 910  is brought “Low-TX” at a time  904 , movement of charge from the FD to the PIN photodiode stops, and the amount of charge stored on the FD is determined (the end of the storage operation). 
         [0063]    When the SE 912  is brought “High-SE” at a time  905 , charge is supplied from the power source line to the OUT line in accordance with the FD potential  913  (the start of a reading operation). 
         [0064]    When the SE 912  is brought “Low-SE” at a time  906 , supply of charge from the power source line to the OUT line is stopped, so that the OUT 914  is determined (the end of the reading operation). The use of the OUT 914  can reproduce a captured image. 
         [0065]    After that, the operation at the time  901  is performed and the same operations as those described above are repeated. 
         [0066]    In this embodiment, when the reset operation, the storage operation, and the reading operation are performed on the photosensors, a partial shadow of external light can be detected. In addition, when image processing or the like is performed on the detected shadow appropriately, a position where a finger, a pen, or the like touches the display device can be recognized. Operation corresponding to the position where the display device is touched, for example, as for input of characters, kinds of characters are regulated in advance, so that desired characters can be input. 
         [0067]    Note that in the display device in this embodiment, the partial shadow of external light is detected by the photosensors. Therefore, even if a finger, a pen, or the like does not touch the display device physically, when the finger, the pen, or the like gets close to the display device without contact and a shadow is formed, detection of the shadow is possible. Hereinafter, “a finger, a pen, or the like touches the display device” includes the case where the finger, the pen, or the like is close to the display device without contact. 
         [0068]    With the above structure, the display area  1032  can have a touch-input function. 
       Embodiment 2 
       [0069]    In this embodiment, the configuration of a circuit which is partly different from the circuit shown in  FIG. 3  is shown in  FIG. 5 , and an example of a pixel layout is shown in  FIG. 7 . 
         [0070]    The pixel circuit includes a display circuit  3501  and a sensor circuit  3502 . 
         [0071]    The display circuit includes a transistor  3530 , a liquid crystal element  3532 , and a capacitor  3531 . A gate of the transistor  3530  is connected to a signal line  3510 . One of a source and a drain of the transistor  3530  is connected to a signal line  3511 , and the other is connected to one electrode of the capacitor  3531  and one electrode of the liquid crystal element  3532 . The other electrode of the capacitor  3531  is connected to a signal line  3512 . The other electrode of the liquid crystal element  3532  corresponds to a common electrode. 
         [0072]    The sensor circuit includes a PIN photodiode  3551 , a FET-T  3552 , a FET-AMP  3554 , and a FET-S  3553 . A gate of the FET-T  3552  is connected to TX lines  3542  and  3547 . One of a source and a drain of the FET-T  3552  is connected to a cathode of the PIN photodiode  3551 , and the other is connected to a gate of the FET-AMP  3554 . 
         [0073]    A source of the FET-AMP  3554  is connected to a power source line  3544 , and a drain of the FET-AMP  3554  is connected to a source of the FET-S  3553 . A gate of the FET-S  3553  is connected to a SE line  3545 , and a source of the FET-S  3553  is connected to an OUT line  3543 . An anode of the PIN photodiode  3551  is connected to PR lines  3541  and  3546 . Transistors each having a channel formation region including an oxide semiconductor can be used as transistors shown in  FIG. 5 . 
         [0074]      FIG. 7  illustrates an example of a plan view of a pixel layout corresponding to the circuit diagram of  FIG. 5 . 
         [0075]    The pixel layout includes layouts of four display circuits  3610   a ,  3610   b ,  3610   c , and  3610   d , and a layout of a sensor circuit  3620 . The layouts of the display circuits  3610   a ,  3610   b ,  3610   c , and  3610   d  each include a selection FET  3603 , a Cs line  3601 , a video data line  3602 , a selection line  3600 , and a COM line  3604 . 
         [0076]    The layout of the sensor circuit  3620  includes a PIN photodiode  3630 , a FET-T including a semiconductor layer  3637 , a FET-AMP including a semiconductor layer  3638 , a FET-T including a semiconductor layer  3636 , an FD  3641 , a vertical TX line  3632 , a horizontal TX line  3640 , a vertical PR line  3631 , a horizontal PR line  3639 , an SE line  3635 , an OUT line  3633 , and a VDD line  3634 . There is no particular limitation on a material for the semiconductor layers  3636 ,  3637 , and  3638 . For example, a polycrystalline semiconductor film (e.g., a polysilicon film), a microcrystalline semiconductor film, or an oxide semiconductor film represented by the chemical formula InMO 3 (ZnO) m  (m&gt;0) can be used. Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like. In addition, the above oxide semiconductor film may contain SiO 2 . 
         [0077]    The pixel layout includes the following layers: semiconductor layers  3636 ,  3637 , and  3638 , a gate line layer  3651 , an SD line layer  3652 , a Si layer  3653 , and an ITO electrode layer  3654 . The vertical TX line  3632  and the vertical PR line  3631  are formed utilizing the SD line layer  3652 . The horizontal TX line  3640  and the horizontal PR line  3639  are formed utilizing the gate line layer  3651 . Horizontal lines and vertical lines have contacts with each other, thereby forming a mesh layout. 
         [0078]    In the pixel layout, the Cs line  3601  serving as a shielding wire is present between the FD  3641  and either the video data line  3602  or a signal line for the adjacent pixel. Therefore, parasitic capacitance between the FD  3641  and the plurality of signal lines is reduced, or changes in the potentials of neighbor elements due to the changes in the voltages of the signal lines are prevented, thereby avoiding changes in the potential of the FD. 
         [0079]    A feature of the pixel layout is the fact that the FD  3641  has a sufficient distance from the PR line, the SE line, and the OUT line for the pixel in which the FD  3641  is provided and for the adjacent pixel. Thus, changes in the potential of the FD  3641  can be suppressed and a display device with an imaging function that causes less noise in an output signal of a photosensor can be provided. 
         [0080]      FIG. 8A  is an enlarged view of a part of  FIG. 7 , and  FIG. 8B  is an example of a cross-sectional structure thereof. 
         [0081]    Note that portions shown in  FIG. 8A  and  FIG. 8B  that are common to those in  FIG. 7  use the same reference numerals as those in  FIG. 7 . 
         [0082]    In  FIG. 8B , the gate line layer  3651  included in the transistor electrically connected to the photodiode, and the horizontal PR line  3639  which are formed using the same material and in the same process are formed over a substrate  500 . Further, a gate insulating layer  502  covering the gate line layer  3651  and the horizontal PR line  3639  is formed, and the semiconductor layer  3637  is formed over the gate insulating layer  502  so as to overlap with the gate line layer  3651 . 
         [0083]    Moreover, the SD line  3652  that partly overlaps with the semiconductor layer  3637  is formed, and the first interlayer insulating layer  505  covering the SD line  3652  is formed. The PIN photodiode  3630  is a photoelectric conversion layer which is a stack of a silicon layer  3653   p  containing a p-type impurity element, an i-type amorphous silicon layer  3653   i , and a silicon layer  3653   n  containing an n-type impurity element. 
         [0084]    Further, a second interlayer insulating layer  509  covering the PIN photodiode  3630  and the first interlayer insulating layer  505  is formed, and the ITO electrode  3654  using a transparent conductive film is formed over the second interlayer insulating layer  509 . The ITO electrode  3654  serves as a contact electrode for connecting the SD line  3652  for the transistor including the semiconductor layer  3637  to the PIN photodiode  3630 . Note that the pixel electrode which is formed using the same material and in the same process as the ITO electrode  3654  is formed in the display region. 
         [0085]    The case where the transistor electrically connected to the PIN photodiode  3630  is a bottom-gate transistor has been shown in  FIG. 8B ; however, there is no particular limitation on the structure of the transistor. The transistor may have another bottom-gate structure or a top-gate structure instead. 
       Embodiment 3 
       [0086]    In this embodiment, an example of an electronic device including a display area (a touch panel) having the touch-input function that has been described in the above embodiments will be described. 
         [0087]      FIG. 9A  illustrates an information terminal that can include housings  9630 , a display area  9631 , operation keys  9632 , a solar battery  9633 , and a charge and discharge control circuit  9634 . The information terminal is provided with the solar battery  9633  and a touch panel so that the solar battery  9633  and the touch panel can be opened and closed freely. Note that in  FIG. 9A , a structure of the information terminal including a battery  9635  and a DCDC converter (hereinafter abbreviated as a converter  9636 ) is illustrated as an example of the charge and discharge control circuit  9634 . 
         [0088]    Note that a structure in which the solar battery  9633  is provided on each of a surface and a rear surface of the housing  9630  is preferable in order to charge the battery  9635  efficiently. The use of a lithium ion battery as the battery  9635  produces an advantage such as downsizing. 
         [0089]    The structure and the operation of the charge and discharge control circuit  9634  illustrated in  FIG. 9A  are described with reference to a block diagram in  FIG. 9B . The solar battery  9633 , the battery  9635 , the converter  9636 , a converter  9637 , switches SW 1  to SW 3 , and the display area  9631  are illustrated in  FIG. 9B , and the battery  9635 , the converter  9636 , the converter  9637 , and the switches SW 1  to SW 3  correspond to the charge and discharge control circuit  9634 . 
         [0090]    First, an example of the operation in the case where power is generated by the solar battery  9633  using external light will be described. The voltage of power generated by the solar battery is raised or lowered by the converter  9636  so that the power has a voltage for charging the battery  9635 . Then, when the power from the solar battery  9633  is used for the operation of the display area  9631 , the switch SW 1  is turned on and the voltage of the power is raised or lowered by the converter  9637  so as to be a voltage needed for the display area  9631 . In addition, when an image is not displayed on the display area  9631 , the switch SW 1  is turned off and the switch SW 2  is turned on so that charge of the battery  9635  may be performed. 
         [0091]    Note that although the solar battery  9633  is described as an example of a charging method, the battery  9635  may be charged with another method. In addition, a combination of the solar battery  9633  and another charging method may be used. 
         [0092]    Needless to say, one embodiment of the present invention is not necessarily the electronic device shown in  FIGS. 9A and 9B  as long as it includes a display area (a touch panel) having the touch-input function that has been described in the above embodiments. 
         [0093]    This embodiment can be implemented in appropriate combination with any structure described in the other embodiments. 
         [0094]    This application is based on Japanese Patent Application serial no. 2010-198928 filed with Japan Patent Office on Sep. 6, 2010, the entire contents of which are hereby incorporated by reference.

Technology Category: 5