Patent Publication Number: US-9887232-B2

Title: Photodetector circuit and semiconductor device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photodetector circuit and a semiconductor device including the photodetector circuit. 
     2. Description of the Related Art 
     In a variety of fields, semiconductor devices including circuits (hereinafter, also referred to as “photodetector circuits”) which receive light from the outside and output signals corresponding to the amount of incident light are used. 
     Examples of photodetector circuits are a photodetector circuit including a CMOS circuit (hereinafter, also referred to as a CMOS sensor), and a CMOS sensor includes a photoelectric conversion element (e.g., a photodiode) which enables current corresponding to the amount of incident light to flow and a signal output circuit which holds a potential based on the amount of light entering the photoelectric conversion element and outputs a signal corresponding to the potential. 
     Note that a CMOS sensor detects the amount of light entering a photoelectric conversion element by performing, in a signal output circuit including a MOS transistor, an operation in which a potential (also referred to as charge) held in the signal output circuit is initialized (also referred to as a reset operation), an operation in which a potential corresponding to the amount of photocurrent flowing through the photoelectric conversion element is generated (also referred to as a potential generation operation), and an operation in which a signal corresponding to the potential is output (also referred to as an output operation). 
     As an example of semiconductor devices including photodetector circuits, an image display device in which a photodetector circuit is provided in each of a plurality of pixels arranged in a matrix can be given (e.g., see Patent Document 1). 
     In the image display device, in the case where an object to be detected (e.g., a pen or a finger) exists on a display screen, part of light emitted from the image display device is reflected by the object to be detected and the amount of reflected light is detected by the photodetector circuit, whereby a region on the display screen where the object to be detected exists can be detected. 
     Further, as an example of semiconductor devices including photodetector circuits, a medical diagnostic imaging device provided with a scintillator and a flat panel detector including a plurality of photodetector circuits can be given (e.g., see Patent Document 2). 
     In the medical diagnostic imaging device, a human body is irradiated with radiation (e.g., X-rays) emitted from a radiation source, radiation which passes through the human body is converted to light (e.g., visible light) by the scintillator, and imaging data is composed by detecting the light with a photodetector circuit included in the flat panel detector, whereby an image of the inside of the human body can be obtained as electronic data. 
     However, in a semiconductor device which obtains a variety of data with the use of a photodetector circuit provided therein as described above, a signal (also referred to as a detection signal) output from the photodetector circuit is a composite signal including not only a signal needed for obtaining data (also referred to as an essential signal) but also an unnecessary signal (also referred to as a noise signal) in some cases. 
     For example, in the above image display device, a signal corresponding to “light which is reflected by the object to be detected to enter the photodetector circuit”, which is output from the photodetector circuit, is an essential signal; on the other hand, a signal corresponding to “light (external light) which enters from the outside of the device, such as sunlight or fluorescent light”, which is output from the photodetector circuit, is a noise signal. 
     Further, in the above medical diagnostic imaging device, since in light emitted by the scintillator, there occurs a phenomenon (what is called afterglow) in which light emission continues even after radiation emission stops, light received by the flat panel detector might include both “light emitted due to radiation emission” and “light emitted by afterglow”. 
     In this case, a signal corresponding to “light emitted due to radiation emission” which is output from a photodetector circuit is an essential signal; on the other hand, a signal corresponding to “light emitted by afterglow” which is output from a photodetector circuit is a noise signal. 
     In order to solve the above-described problem in that a detection signal output from a photodetector circuit includes not only an essential signal but also a noise signal, it is effective to remove only a noise signal selectively from a composite signal. To achieve that, for example, as an image display device, a device including photodetector circuits (CMOS sensors) arranged in a matrix is proposed as in Non-Patent Document 1. 
     In an image display device in Non-Patent Document 1 (see FIG. 3 in Non-Patent Document 1), in each of photodetector circuits (referred to as photosensors in Non-Patent Document 1) arranged in a matrix, a transistor M 1 , a transistor M 2 , and a capacitor C INT  function as a signal output circuit and an element D 1  functions as a photoelectric conversion element. 
     In addition, after a reset operation and a potential generation operation are performed in the photodetector circuits in odd-numbered rows in a period during which an object to be detected is irradiated with light by turning on a backlight, a reset operation and a potential generation operation are performed in the photodetector circuits in even-numbered rows in a period during which the object to be detected is not irradiated with light by turning off the backlight. 
     Note that the time interval of blinking the backlight is short, and it can be considered that the object to be detected hardly moves between when the backlight is on and when the backlight is off. 
     After that, output operations are performed at the same time in the photodetector circuits in two adjacent rows, and a difference between detection signals thereof is obtained. Then, this operation is performed sequentially, so that output operations are performed in the photodetector circuits in all the rows. 
     A difference between detection signals thus obtained using photodetector circuits in two adjacent rows is an accurate signal including only an essential signal because a signal (noise signal) corresponding to the amount of light entering the photodetector circuit when the backlight is off is removed from a signal (composite signal) corresponding to the amount of light entering the photodetector circuit when the backlight is on. 
     In other words, a plurality of detection signals (at least two or more detection signals) are obtained using photodetector circuits, and an accurate detection signal is obtained using the plurality of detection signals. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2006-079589 
         [Patent Document 2] Japanese Published Patent Application No. 2003-250785 
       
    
     Non-Patent Document 
     
         
         [Non-Patent Document 1]K. Tanaka, et al., “A System LCD with Optical Input Function using Infra-Red Backlight Subtraction Scheme”, SID 2010 Digest, pp. 680-683 
       
    
     SUMMARY OF THE INVENTION 
     However, in a structure described in Non-Patent Document 1, at least two adjacent photodetector circuits are needed in order to obtain detection signals in different periods (when the backlight is on and when the backlight is off). 
     Therefore, in the case where there is a difference between characteristics (e.g., light-receiving sensitivity) of photoelectric conversion elements of two photodetector circuits, a detection signal output from the two photodetector circuits include the difference between characteristics of the photoelectric conversion elements. 
     In view of the above problem, an object of one embodiment of the disclosed invention is to provide a photodetector circuit capable of obtaining detection signals in different periods without being affected by characteristics of a photoelectric conversion element. 
     In addition, an object of one embodiment of the disclosed invention is to provide a semiconductor device including the above photodetector circuit. 
     In order to solve the above problem, in one embodiment of the disclosed invention, a photodetector circuit has a configuration in which n signal output circuits (n is a natural number of 2 or more) are connected to one photoelectric conversion element. Further, the n signal output circuits each include the following: a transistor whose gate potential varies in accordance with the amount of light entering the photoelectric conversion element and which outputs a signal corresponding to the gate potential; a first switching element which is connected between the photoelectric conversion element and the transistor and holds the gate potential of the transistor; and a second switching element which controls the signal output from the transistor. 
     In the case where the above configuration of a signal output circuit is employed, the gate potential of the transistor can be held by turning off the first switching element; thus, data based on the amount of light entering the photoelectric conversion element can be held in different signal output circuits in different periods. In the n signal output circuits, data in different periods (data based on the amount of light entering the photoelectric conversion element) is held, and then, the second switching elements are turned on; thus, signals in different periods can be obtained without being affected by the characteristics of the photoelectric conversion element. 
     In other words, according to one embodiment of the present invention, a photodetector circuit includes a photoelectric conversion element and n signal output circuits (n is a natural number of 2 or more) connected to the photoelectric conversion element. The n signal output circuits each include a transistor whose gate potential varies in accordance with the amount of light entering the photoelectric conversion element and which outputs a signal corresponding to the gate potential; a first switching element which is connected between the photoelectric conversion element and the transistor and holds the gate potential; and a second switching element which controls output of the signal. Gate potentials held in the n signal output circuits are based on the amount of light entering the photoelectric conversion element in different periods. After the gate potentials are held in the n signal output circuits, signals corresponding to the gate potentials are output from the n signal output circuits. 
     When the photodetector circuit has the above-described configuration, the photodetector circuit can obtain signals in different periods without being affected by characteristics of the photoelectric conversion element. 
     In the above-described photodetector circuit, by providing a wiring which is connected to the second switching elements in the n signal output circuits and transmits signals for controlling the operation of the second switching elements, the number of wirings necessary for performing on/off operations of the second switching elements in the signal output circuits can be reduced. In addition, since signals can be output from the n signal output circuits at the same time, the signals can be obtained in a short period. 
     Further, in the case where a transistor including an oxide semiconductor material in a channel formation region is used as the first switching element in the above photodetector circuit, the first switching element has extremely low off-state current, and thus can hold the gate potential of the transistor. Thus, a signal output from the signal output circuit is an extremely accurate signal including data corresponding to the amount of light entering the photoelectric conversion element. 
     Note that in the case where a transistor including an oxide semiconductor material in a channel formation region is used as each of the second switching element and the transistor in addition to the first switching element, elements included in the signal output circuits can be manufactured in the same steps; thus, time and cost for manufacturing photodetector circuits can be reduced. 
     Further, in the case where the above-described photodetector circuit is used for a semiconductor device, with a configuration in which the photodetector circuits are arranged in a matrix and gate potentials are held in the n signal output circuits in all the photodetector circuits arranged in a matrix, and then, n signals corresponding to the gate potentials are output from the photodetector circuits, signals in different periods can be obtained from all the photodetector circuits in a short period. 
     Specific examples of semiconductor devices include radiation imaging devices, for example. In the case where the above-described photodetector circuit is used for a radiation imaging device, the radiation imaging device includes a radiation source, a scintillator which outputs light by receiving radiation output from the radiation source, a photodetector mechanism including the photodetector circuits arranged in a matrix and a photodetector circuit control portion which controls operations of the photodetector circuits, and a detection signal comparison portion which compares signals output from the photodetector circuit control portion. The photodetector mechanism may have a structure in which gate potentials are held in the n signal output circuits in all the photodetector circuits arranged in a matrix, the n signals corresponding to the gate potentials are output from the photodetector circuits, and then, the detection signal comparison portion compares the n signals output from the photodetector circuits. 
     Examples of semiconductor devices other than radiation imaging devices are image display devices, for example. In the case where the above-described photodetector circuit is used for an image display device, the image display device includes a display portion in which pixels including a display element and a photodetector circuit are arranged in a matrix, a display element control portion which controls the operations of the display elements, a photodetector circuit control portion which controls the operations of the photodetector circuits, and an image signal generation portion which generates image signals by using signals output from the photodetector circuit control portion. Gate potentials are held in the n signal output circuits in all the photodetector circuits in the pixels arranged in a matrix and then, the n signals corresponding to the gate potentials are output from the photodetector circuits, and the image signal generation portion generates image signals from the n signals output from the photodetector circuits. 
     Further, one embodiment of the present invention is a method for operating a photodetector circuit which includes a photoelectric conversion element and n signal output circuits (n is a natural number of 2 or more) connected to the photoelectric conversion element. The n signal output circuits each include a transistor whose gate potential varies in accordance with the amount of light entering the photoelectric conversion element and which outputs a signal corresponding to the gate potential, a first switching element which is connected between the photoelectric conversion element and the transistor and which holds the gate potential, and a second switching element which controls the signal output from the transistor. The method includes the steps of: holding potentials based on the amount of light entering the photoelectric conversion element as gate potentials by turning off the first switching elements in the n signal output circuits in different periods independently of the signal output circuits; and outputting signals corresponding to the gate potentials by turning on the second switching elements. 
     By driving the photodetector circuit by the above-described operation method, in the photodetector circuit, signals corresponding to the amount of light entering the photoelectric conversion element in different periods can be obtained in a short period without being affected by characteristics of the photoelectric conversion element. 
     Note that in the above-described method for operating the photodetector circuit, by performing operations for initializing the gate potentials in the n signal output circuits at the same time, the gate potentials in the n signal output circuits can be reset at the same time; thus, signals can be obtained in a short period. 
     Further, in the above-described method for operating the photodetector circuit, by performing the operations of turning on the second switching elements and the operations of turning off the second switching elements in the n signal output circuits at the same time, signals can be output from the n signal output circuits at the same time; thus, the signals can be obtained in a short period. 
     According to one embodiment of the present invention, a photodetector circuit has a configuration in which n output circuits (n is a natural number of 2 or more) are connected to a photoelectric conversion element, and the output circuit includes a transistor whose output signal varies in accordance with the level of a potential generated, a first switching element which prevents leakage of the potential from the output circuit, and a second switching element which controls the output signal from the transistor. In the n output circuits, after the signals are held in different periods (at different timings) in the output circuits, the signals are output from the n output circuits. 
     Thus, a photodetector circuit capable of obtaining signals in different periods without being affected by characteristics of a photoelectric conversion element can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1A  illustrate a configuration of a photodetector circuit and  FIG. 1B  is an operation flow chart of the photodetector circuit; 
         FIG. 2  is an operation flow chart of a photodetector circuit; 
         FIG. 3A  illustrates a configuration of a photodetector circuit and  FIG. 3B  is an operation flow chart of the photodetector circuit; 
         FIG. 4  is an operation flow chart of a photodetector circuit; 
         FIG. 5  illustrates a configuration of a photodetector circuit; 
         FIG. 6  illustrates a configuration of a photodetector circuit; 
         FIGS. 7A to 7C  each illustrate an operational amplifier circuit; 
         FIGS. 8A and 8B  illustrate a layout of a photodetector circuit; 
         FIGS. 9A and 9B  illustrate a layout of a photodetector circuit; 
         FIGS. 10A and 10B  illustrate a structure of a radiation imaging device; 
         FIGS. 11A to 11D  illustrate operations of a radiation imaging device; 
         FIG. 12  illustrates a structure of an image display device; 
         FIG. 13  illustrates a configuration of an image display device; 
         FIGS. 14A and 14B  illustrate operations of an image display device; and 
         FIG. 15  illustrates a configuration of a photodetector circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described below in detail with reference to the accompanying drawings. Note that embodiments described below can be implemented in many different modes, and it is easily understood by those skilled in the art that modes and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the following description of the embodiments. In the drawings for explaining the embodiments, the same parts or parts having a similar function are denoted by the same reference numerals, and description of such parts is not repeated. 
     Note that in the embodiments described below, “one terminal” of a transistor refers to one of a source electrode and a drain electrode, and “the other terminal” of the transistor refers to the other of the source electrode and the drain electrode. That is, when one terminal of the transistor is the source electrode, the other terminal of the transistor refers to the drain electrode. 
     Note that “electrical connection” in this specification corresponds to the state where current, voltage, or potential can be supplied or transmitted. Therefore, a state of electrical connection means not only a state of direct connection but also a state of indirect connection through a circuit element such as a wiring, a resistor, a diode, or a transistor, in which current, voltage, or potential can be supplied or transmitted. 
     Unless otherwise specified, in the case of an n-channel transistor, the off-state current in this specification is a current that flows between a source electrode and a drain electrode when the potential of a gate electrode is less than or equal to 0 V with the potential of the source electrode as a reference potential while the potential of the drain electrode is higher than those of the source electrode and the gate electrode. Moreover, in the case of a p-channel transistor, the off-state current in this specification is a current that flows between a source electrode and a drain electrode when the potential of a gate electrode is greater than or equal to 0 V with the potential of the source electrode as a reference potential while the potential of the drain electrode is lower than those of the source electrode and the gate electrode. 
     Embodiment 1 
     In this embodiment, a configuration and an operation method of a photodetector circuit are described with reference to  FIGS. 1A and 1B  and  FIG. 2 . 
     &lt;Configuration of Photodetector Circuit&gt; 
       FIG. 1A  shows an example of a circuit diagram illustrating a configuration of a photodetector circuit. The photodetector circuit includes a photoelectric conversion element  100  and two signal output circuits (a first signal output circuit  101  and a second signal output circuit  102 ) connected to the photoelectric conversion element  100 . 
     &lt;Photoelectric Conversion Element&gt; 
     As the photoelectric conversion element  100 , a photodiode is illustrated in  FIGS. 1A and 1B . The photodiode generates current by irradiation with light from the outside, and the value of photocurrent varies in accordance with the intensity of incident light. Note that the photoelectric conversion element  100  is not limited to a photodiode. For example, the photoelectric conversion element  100  may be a variable resistor. The variable resistor can include a pair of electrodes and an amorphous silicon layer having i-type conductivity provided between the pair of electrodes. The i-type amorphous silicon layer can be used in a manner similar to that of a photodiode because the resistance of the i-type amorphous silicon layer varies by irradiation with light. 
     One of the electrodes of the photoelectric conversion element  100  is connected to a wiring  111  (also referred to as a wiring PR) and the other of the electrodes of the photoelectric conversion element  100  is connected to the first signal output circuit  101  and the second signal output circuit  102 . 
     Needless to say, one of the electrodes of the photoelectric conversion element  100  may be connected to the first signal output circuit  101  and the second signal output circuit  102  and the other of the electrodes of the photoelectric conversion element  100  may be connected to the wiring  111 . 
     The signal output circuits (the first signal output circuit  101  and the second signal output circuit  102 ) hold potentials including the amount of light entering the photoelectric conversion element  100  as data in the circuits and output detection signals corresponding to the potentials to the outside. 
     In the description of this embodiment, the two signal output circuits (the first signal output circuit  101  and the second signal output circuit  102 ) have the same structure; thus, components included in the signal output circuits are denoted by the same reference numerals. For example, both a transistor in the first signal output circuit  101  and a transistor in the second signal output circuit  102  are referred to as a “transistor  120 ”. 
     &lt;Detection Circuit&gt; 
     The first signal output circuit  101  includes the following: a transistor  120  whose gate potential varies in accordance with the amount of light entering the photoelectric conversion element  100  and which outputs a signal corresponding to the gate potential; a first switching element  121  which is connected between the photoelectric conversion element  100  and the transistor  120 , controls the connection state therebetween, and holds a potential applied to a gate of the transistor  120 ; and a second switching element  122  which controls the signal output from the transistor  120 . 
     The gate of the transistor  120  in the first signal output circuit  101  is connected to a wiring  112  (also referred to as a wiring FD 1 ), one of a source and a drain of the transistor  120  in the first signal output circuit  101  is connected to a wiring  113  (also referred to as a wiring VR), and the other of the source and the drain of the transistor  120  in the first signal output circuit  101  is connected to one of electrodes of the second switching element  122 . 
     Since the first switching element  121  in the first signal output circuit  101  holds the potential applied to the gate of the transistor  120 , it is preferable that the first switching element  121  have extremely low leakage current in an off state. 
     As an example of a switching element which has low leakage current in an off state, a transistor which includes an oxide semiconductor material in a channel formation region can be given. 
     The above oxide semiconductor material preferably contains at least indium (In) or zinc (Zn). In particular, In and Zn are preferably contained. The oxide semiconductor material preferably contains, in addition to In and Zn, gallium (Ga) serving as a stabilizer that reduces variations in electrical characteristics among transistors including the oxide semiconductor material. Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) is preferably contained as a stabilizer. 
     As another stabilizer, one or plural kinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) may be contained. 
     The bandgap of a film using an oxide semiconductor material is greater than or equal to 3.0 eV (electron volts), which is much wider than the bandgap of silicon (1.1 eV). 
     The off-resistance of a transistor (resistance between a source and a drain of the transistor in an off state) is inversely proportional to the concentration of carriers thermally excited in a channel formation region. Since the bandgap of silicon is 1.1 eV even in a state where there is no carrier caused by a donor or an acceptor (i.e., even in the case of an intrinsic semiconductor), the concentration of thermally excited carriers at room temperature (300 K) is approximately 1×10 −7  cm −3 . 
     The bandgap of a film using an oxide semiconductor material is generally as wide as 3.0 eV or more as described above, and the concentration of thermally excited carriers in a film with a bandgap of 3.2 eV, for example, is approximately 1×10 −7  cm −3 . When the electron mobility is the same, the resistivity is inversely proportional to the carrier concentration, and thus the resistivity of the semiconductor with a bandgap of 3.2 eV is 18 orders of magnitude higher than that of silicon. 
     Since a transistor that uses such a wide bandgap oxide semiconductor material in a channel formation region can achieve extremely low off-state current. 
     Further, the transistor is used as the first switching element  121 , and after the gate potential of the transistor  120  varies in accordance with the amount of light entering the photoelectric conversion element  100 , the first switching element  121  is turned off, whereby the gate potential of the transistor  120  can be held in the wiring  112  for a long time. 
     Although a transistor including an oxide semiconductor material in a channel formation region is described above as an example of the first switching element  121 , another switching element having low off-state current may be used. For example, a transistor using a magnetoresistance effect (also referred to as a spin transistor or the like), a transistor using a ferroelectric material for a gate insulating film (also referred to as a ferroelectric transistor or the like), or the like can be used. 
     A signal corresponding to the gate potential of the transistor  120  is output from the drain (or source) of the transistor  120 . Accordingly, the signal can be regarded as a “signal including the amount of light entering the photoelectric conversion element  100  as data”. 
     One of a source and a drain of the transistor including an oxide semiconductor material in a channel formation region, which is used as the first switching element  121 , is connected to the other of the electrodes of the photoelectric conversion element  100 , the other of the source and the drain of the transistor including an oxide semiconductor material in a channel formation region is connected to the gate of the transistor  120 , and a gate of the transistor including an oxide semiconductor material in a channel formation region is connected to a wiring  114  (also referred to as a wiring TX 1 ). 
     Although in this embodiment and the like, a transistor including an oxide semiconductor material in a channel formation region is used as the first switching element  121 , the first switching element  121  is not limited to a transistor as long as it is an element capable of switching on and off of a connection state (conduction state), and a variety of known techniques can be used. 
     One of a source and a drain of the second switching element  122  in the first signal output circuit  101  is connected to the other of the source and the drain of the transistor  120 , the other of the source and the drain of the second switching element  122  in the first signal output circuit  101  is connected to a wiring  115  (also referred to as a wiring OUT), and a gate of the second switching element  122  in the first signal output circuit  101  is connected to a wiring  116  (also referred to as a wiring SE 1 ). 
     In the case where a transistor is used as the second switching element  122  as illustrated in  FIG. 1A , by setting Vgs (a voltage difference between a gate and a source when the source is used as a reference) of the transistor to a voltage sufficiently higher than the threshold voltage, a signal output from the transistor  120  is output to the wiring  115  (OUT). 
     Note that an integrator circuit may be connected to the wiring  115  (OUT). Connecting the integrator circuit to the wiring  115  (OUT) increases S/N, enabling detection of weaker light. A specific configuration example of the integrator circuit will be described in Embodiment 2. 
     The second signal output circuit  102  includes the following: the transistor  120  whose gate potential varies in accordance with the amount of light entering the photoelectric conversion element  100  and which outputs a signal corresponding to the gate potential; the first switching element  121  which is connected between the photoelectric conversion element  100  and the transistor  120 , controls the connection state therebetween, and holds a potential applied to a gate of the transistor  120 ; and the second switching element  122  which controls the signal output from the transistor  120 . 
     The gate of the transistor  120  in the second signal output circuit  102  is connected to a wiring  132  (also referred to as a wiring FD 2 ), one of a source and a drain of the transistor  120  in the second signal output circuit  102  is connected to the wiring  113  (also referred to as the wiring VR), and the other of the source and the drain of the transistor  120  in the second signal output circuit  102  is connected to one of electrodes of the second switching element  122 . 
     The wiring connected to the one of the source and the drain of the transistor  120  in the second signal output circuit  102  is the same as the wiring  113  in the first signal output circuit  101 . 
     Since the first switching element  121  in the second signal output circuit  102  holds the potential applied to the gate of the transistor  120 , the first switching element  121  preferably has extremely low off-state current, and for example, a transistor including an oxide semiconductor material in a channel formation region can be used. For the description of the transistor including an oxide semiconductor material in a channel formation region, the above “description of the first signal output circuit  101 ” can be referred to. 
     Since the transistor including an oxide semiconductor material in a channel formation region has extremely low off-state current, the transistor is used as the first switching element  121 , and after the gate potential of the transistor  120  varies in accordance with the amount of light entering the photoelectric conversion element  100 , the first switching element  121  is turned off, whereby the gate potential of the transistor  120  can be held in the wiring  132  for a long time. 
     Further, a signal (hereinafter, the signal output from the second signal output circuit  102  is also referred to as a second signal) corresponding to the gate potential of the transistor  120  is output from the drain (or source) of the transistor  120 . 
     One of a source and a drain of the transistor including an oxide semiconductor material in a channel formation region, which is used as the first switching element  121 , is connected to the other of the electrodes of the photoelectric conversion element  100 , the other of the source and the drain of the transistor including an oxide semiconductor material in a channel formation region is connected to the gate of the transistor  120 , and a gate of the transistor including an oxide semiconductor material in a channel formation region is connected to a wiring  134  (also referred to as a wiring TX 2 ). 
     Although in this embodiment and the like, a transistor including an oxide semiconductor material in a channel formation region is used as the first switching element  121 , the first switching element  121  is not limited to a transistor as long as it is an element capable of switching on and off of a connection state (conduction state). 
     One of a source and a drain of the second switching element  122  in the second signal output circuit  102  is connected to the other of the source and the drain of the transistor  120 , the other of the source and the drain of the second switching element  122  in the second signal output circuit  102  is connected to the wiring  115  (also referred to as the wiring OUT), and the gate of the second switching element  122  in the second signal output circuit  102  is connected to a wiring  136  (also referred to as a wiring SE 2 ). 
     The wiring connected to the other of the source and the drain of the second switching element  122  in the second signal output circuit  102  is the same as the wiring  115  in the first signal output circuit  101 . 
     When the photodetector circuit has the above-described structure, by turning on the first switching elements  121  in the signal output circuits at different timings, the amount of light entering the photoelectric conversion element  100  at different timings can be detected. By turning off the first switching elements  121 , the data can be held as gate potentials; thus, for example, even when light entering the photoelectric conversion element  100  in a first period is light generating a composite signal, a potential including the light as data is held in the first signal output circuit  101 , light generating a noise signal is detected in a second period, and a potential including the light as data is held in the second signal output circuit  102 , whereby a signal necessary for generation of an essential signal can be obtained from the photodetector circuit. 
     In this embodiment, it is preferable that the transistor  120  have high mobility because the transistor  120  amplifies an electrical signal generated by the photoelectric conversion element  100 . 
     As an example of the transistor  120  having high mobility, a thin film transistor including amorphous silicon, microcrystalline silicon, polycrystalline silicon, single crystal silicon, or the like in a channel formation region can be given. 
     Further, the transistor  120  needs low off-state current characteristics in order to prevent output of an unnecessary potential to the wiring  113  (VR). For these reasons, it is also effective to use a transistor using an oxide semiconductor material, which achieves both high mobility and low off-state current in a channel formation region, as the transistor  120 . 
     In this embodiment, the second switching element  122  preferably has high mobility because of controlling output of a signal from the signal output circuit. 
     As an example of the second switching element  122  having high mobility, a thin film transistor including amorphous silicon, microcrystalline silicon, polycrystalline silicon, single crystal silicon, or the like in a channel formation region can be given. 
     Further, the second switching element  122  needs low off-state current characteristics in order to prevent output of an unnecessary potential to the wiring  115  (OUT). For these reasons, it is also effective to use a transistor using an oxide semiconductor material, which achieves both high mobility and low off-state current in a channel formation region, as the second switching element  122 . 
     The use of transistors including an oxide semiconductor material in a channel formation region as all the components (the transistor  120 , the first switching element  121 , and the second switching element  122 ) in each of the signal output circuits can simplify the manufacturing process of the signal output circuits. 
     When a semiconductor material capable of providing higher mobility than an oxide semiconductor material, such as polycrystalline or single crystal silicon, is used for the channel formation regions of the transistor  120  and the second switching element  122 , data can be read from the signal output circuit at high speed. 
     Connecting a capacitor to the wiring  115  (OUT) is effective in stabilizing the potential of the wiring  115  (OUT). 
     In  FIG. 1A , the transistor  120  and the second switching element  122  are connected in series in this order between the wiring  113  (VR) and the wiring  115  (OUT); alternatively, the transistor  120  and the second switching element  122  may be connected in reverse. 
     In  FIG. 1A , the transistor  120  has a gate only on one side of a semiconductor layer; however, the transistor  120  may have a pair of gates placed so that the semiconductor layer is sandwiched therebetween. When the transistor  120  has a pair of gates placed so that the semiconductor layer is sandwiched therebetween, one of the gates can function as a front gate to which the potential of the wiring  112  (or the wiring  132 ) is applied, and the other gate can function as a backgate that controls the threshold voltage or the like of the transistor  120 . In this case, the potential applied to the other gate preferably ranges from −20 V to +2 V with reference to the source potential. If a change in the threshold voltage of the transistor  120  does not adversely affect the operation of the signal output circuit when the potential applied to the other gate varies in the above range, the other gate may be electrically isolated (floating). 
     The above is the description of the configuration of the circuits in the photodetector circuit. A layout example of the configuration of the circuits illustrated in  FIG. 1A , which is described in this embodiment, will be described in Embodiment 4. 
     Although the photodetector circuit described in this embodiment includes one photoelectric conversion element and two signal output circuits connected to the photoelectric conversion element, it may include n signal output circuits (n is a natural number of 2 or more). For example, as illustrated in  FIG. 15 , a structure in which one photoelectric conversion element and four signal output circuits (the first signal output circuit  101 , the second signal output circuit  102 , a third signal output circuit  103 , and a fourth signal output circuit  104 ) are provided may be used. Since one photoelectric conversion element is shared by four signal output circuits, sharing wirings and a large-area photoelectric conversion element can be achieved. Alternatively, in the case where the area of the photoelectric conversion element does not need to be increased, the area of the photodetector circuit can be reduced. 
     Further, the configuration of the photodetector circuit described in this embodiment may be a configuration in which a transistor  501  is added to each of the first signal output circuit  101  and the second signal output circuit  102  as illustrated in  FIG. 5 . A gate of the transistor is electrically connected to the wiring  111  (PR), one of a source and a drain of the transistor is electrically connected to the wiring  112  (FD 1 ) (or the wiring  132  (FD 2 )), and the other of the source and the drain of the transistor is electrically connected to a wiring  502   a  (or a wiring  502   b ). The one of the electrodes of the photoelectric conversion element  100  is electrically connected to a wiring  503 . Here, the wiring  503  is a signal line (low potential line) for applying a reverse bias to the photoelectric conversion element  100 . Further, the wiring  502   a  and the wiring  502   b  are signal lines (high potential lines) for resetting the wiring  112  (FD 1 ) (or the wiring  132  (FD 2 )) to a high potential. 
     The transistor  501  functions as a reset transistor for resetting the wiring  112  (FD 1 ) (or the wiring  132  (FD 2 )). Accordingly, unlike in the detection circuit in  FIG. 1A , the reset operation using the photoelectric conversion element  100  is not performed, and a reverse bias is always applied to the photoelectric conversion element  100 . The wiring  112  (FD 1 ) and the wiring  132  (FD 2 ) can be reset by setting the potential of the wiring  111  (PR) high. 
     The transistor  501  can be formed using a silicon semiconductor such as amorphous silicon, microcrystalline silicon, polycrystalline silicon, or single crystal silicon; however, when leakage current is large, the charge accumulation portion cannot hold charge long enough. For this reason, like the transistor  120 , it is preferable to use a transistor including a semiconductor layer (at least a channel formation region) formed using an oxide semiconductor material, which achieves extremely low off-state current. 
     &lt;Operation Flow Chart of Photodetector Circuit&gt; 
     Next, an operation flow chart of the photodetector circuit illustrated in  FIG. 1A  will be described with reference to  FIG. 1B . 
     In  FIGS. 1B, 114S, 112S, and 116S  correspond to potentials of the wiring  114  (TX 1 ), the wiring  112  (FD 1 ), and the wiring  116  (SE 1 ) in the first signal output circuit  101 , and  134 S,  132 S, and  136 S correspond to potentials of the wiring  134  (TX 2 ), the wiring  132  (FD 2 ), and the wiring  136  (SE 2 ) in the second signal output circuit  102 . Further,  11 S and  115 S correspond to potentials of the wiring  111  (PR) and the wiring  115  (OUT) which are used in common in the first signal output circuit  101  and the second signal output circuit  102 . Note that the potential of the wiring  113  (VR) is fixed at a low level. 
     First, at a time T 1 , the potential (the signal  11 S) of the wiring  111  is set high and the potential (the signal  114 S) of the wiring  114  (TX 1 ) in the first signal output circuit  101  is set high (i.e., a reset operation starts). 
     Thus, a forward bias is applied to the photoelectric conversion element  100  and the potential (the signal  112 S) of the wiring  112  (FD 1 ) in the first signal output circuit  101  becomes high. Note that the potential (the signal  115 S) of the wiring  115  (OUT) is precharged to high. 
     Next, at a time T 2 , the potential (the signal  11 S) of the wiring  111  (PR) is set low and the potential (the signal  114 S) of the wiring  114  (TX 1 ) in the first signal output circuit  101  is set high (i.e., the reset operation finishes and a potential generation operation starts). 
     Thus, reverse current flows through the photoelectric conversion element  100  in accordance with the amount of light entering the photoelectric conversion element  100 , and the potential (the signal  112 S) of the wiring  112  (FD 1 ) in the first signal output circuit  101  starts to be lowered. 
     Since the amount of reverse current increases when the photoelectric conversion element  100  is irradiated with light, the speed of decrease in the potential (the signal  112 S) of the wiring  112  (FD) in the first signal output circuit  101  changes in accordance with the amount of incident light. In other words, the channel resistance between the source and the drain of the transistor  120  in the first signal output circuit  101  changes in accordance with the amount of light entering the photoelectric conversion element  100 . 
     Then, at a time T 3 , the potential (the signal  114 S) of the wiring  114  (TX 1 ) in the first signal output circuit  101  is set low (i.e., the potential generation operation finishes). 
     Since the first switching element  121  in this embodiment and the like is a transistor which includes an oxide semiconductor material in a channel formation region as described above and thus has extremely low off-state current, the potential applied to the gate of the transistor  120  in the first signal output circuit  101  can be held in the wiring  112  (FD 1 ) until an output operation is performed later. 
     Note that when the potential (the signal  114 S) of the wiring  114  (TX 1 ) in the first signal output circuit  101  is set low, the potential of the wiring  112  (FD) sometimes changes because of parasitic capacitance between the wiring  114  (TX 1 ) and the wiring  112  (FD 1 ) in the first signal output circuit  101 . A large amount of potential change makes it impossible to obtain an accurate amount of charge generated by the photoelectric conversion element  100  during the potential generation operation. 
     Examples of effective measures to reduce the amount of potential change include reducing the capacitance between the gate and the source (or between the gate and the drain) of the transistor used as the first switching element  121 , increasing the gate capacitance of the transistor  120 , and providing a storage capacitor to connect the wiring  112  (FD 1 ) in the first signal output circuit  101 . Note that in  FIG. 1B , the potential change can be ignored by the adoption of these measures. 
     Next, also in the second signal output circuit  102 , in order to hold a potential including the amount of light entering the photoelectric conversion element  100  as data in the second signal output circuit  102 , a “reset operation” and a “potential generation operation” are performed in a manner similar to those of the above operations in the first signal output circuit  101 . Thus, the potential including the amount of light entering the photoelectric conversion element  100  as data can be held in the wiring  132  until an output operation is performed later (the operations from the time T 4  to the time T 6  correspond to the reset operation and the potential generation operation). 
     Then, at a time  17 , when the potential (the signal  116 S) of the wiring  116  (SE 1 ) in the first signal output circuit  101  is set high (i.e., the output operation starts), current corresponding to the gate potential of the transistor  120  flows between the source and the drain of the second switching element  122 , so that the potential (the signal  115 S) of the wiring  115  (OUT) decreases. Note that precharge of the wiring  115  (OUT) is terminated before the time T 7 . 
     Here, the speed of decrease in the potential (the signal  115 S) of the wiring  115  (OUT) depends on the channel resistance between the source and the drain of the transistor  120  in the first signal output circuit  101 . That is, the speed of decrease in the potential (the signal  115 S) of the wiring  115  (OUT) changes in accordance with the amount of light entering the photoelectric conversion element  100  during the potential generation operation in the first signal output circuit  101 . 
     Then, at a time T 8 , when the potential (the signal  116 S) of the wiring  116  (SE 1 ) in the first signal output circuit  101  is set low (i.e., the output operation finishes), current flowing between the source and the drain of the second switching element  122  is stopped and the potential (the signal  115 S) of the wiring  115  (OUT) becomes a fixed value. 
     Here, the fixed value varies in accordance with the amount of light entering the photoelectric conversion element  100  during the potential generation operation in the first signal output circuit  101 . Thus, by obtaining the potential (the signal  115 S) of the wiring  115  (OUT), the amount of light entering the photoelectric conversion element  100  during the potential generation operation in the first signal output circuit  101  can be found out. That is, a signal output from the first signal output circuit  101  after the output operation is a detection signal in the first signal output circuit  101 . 
     More specifically, in the case where the amount of light entering the photoelectric conversion element  100  is large, in the first signal output circuit  101 , the potential (the signal  112 S) of the wiring  112  (FD 1 ) becomes lower and the gate potential of the transistor  120  becomes lower; thus, the speed of decrease in the potential (the signal  115 S) of the wiring  115  (OUT) becomes lower. As a result, the potential of the wiring  115  (OUT) becomes higher. 
     Alternatively, in the case where the amount of light entering the photoelectric conversion element  100  is small, in the first signal output circuit  101 , the potential (the signal  112 S) of the wiring  112  (FD 1 ) becomes higher and the gate potential of the transistor  120  becomes higher, thus, the speed of decrease in the potential (the signal  1155 S) of the wiring  115  (OUT) becomes higher. As a result, the potential of the wiring  115  (OUT) becomes lower. 
     Next, the wiring  115  (OUT) is precharged. 
     Also in the second signal output circuit  102 , an “output operation” is performed in a manner similar to that of the above operation in the first signal output circuit  101 . Thus, a detection signal in the second signal output circuit  102  is obtained (the operations between the time T 9  and the time T 10  correspond to the output operation). 
     As described above, potentials (data) based on the amount of light entering the photoelectric conversion element  100  in different periods (a potential generation operation period in the first signal output circuit  101  and a potential generation operation period in the second signal output circuit  102 ) can be held in the signal output circuits by using the transistor  120  and the first switching element  121 . Further, after the potentials are held in all the signal output circuits, a detection signal is obtained from each of the signal output circuits by using the second switching element in the signal output circuit; thus, detection signals in different periods can be obtained without being affected by characteristics of the photoelectric conversion element. 
     The above is the description of the operation flow chart of the photodetector circuit of this embodiment. 
     &lt;Different Operation Flow Chart of Photodetector Circuit&gt; 
     Note that the operation flow chart of the photodetector circuit described in FIG.  1 A may be an operation flow chart different from the above operation flow chart described using  FIG. 1B . The operation flow chart different from the above-described operation flow chart is described below using  FIG. 2 . 
     First, at a time T 1 , the potential (the signal  111 S) of the wiring  111  is set high and the potential (the signal  114 S) of the wiring  114  (TX 1 ) in the first signal output circuit  101  and the potential (the signal  134 S) of the wiring  134  (TX 2 ) in the second signal output circuit  102  are set high (i.e., a reset operation starts). 
     In the operation flow chart described in  FIG. 1B , reset operations are performed in the first signal output circuit  101  and the second signal output circuit  102  in separate steps. By performing reset operations in the first signal output circuit  101  and the second signal output circuit  102  at the same time as illustrated in  FIG. 2 , a period from the reset operation start to the output operation end (period from the time T 1  to the time T 10 ) can be shortened, so that detection signals in different periods can be obtained in a short period. 
     Note that since the following operation flow chart is the same as that in the above operation flow chart described using  FIG. 1B  except for the operation flow chart between the time T 4  and the time T 5 , the operation flow chart described using  FIG. 1B  can be referred to for the following operation flow chart. 
     The above is the description of the different operation flow chart of the photodetector circuit. 
     In the case of using the above-described operation, it is preferable that capacitance in the wiring  112  (FD 1 ) and the wiring  132  (FD 2 ) be larger than wiring capacitances between the photoelectric conversion element  100  and the first switching element  121  in the first signal output circuit  101  and between the photoelectric conversion element  100  and the first switching element  121  in the second signal output circuit  102 . 
     Embodiment 2 
     In this embodiment, a photodetector circuit whose structure and operation method are different from those in Embodiment 1 will be described with reference to  FIGS. 3A and 3B  and  FIG. 4 . 
     &lt;Configuration of Photodetector Circuit&gt; 
       FIG. 3A  shows an example of a circuit diagram illustrating a configuration of a photodetector circuit. The photodetector circuit includes, as in Embodiment 1, the photoelectric conversion element  100  and two signal output circuits (a first signal output circuit  301  and a second signal output circuit  302 ) connected to the photoelectric conversion element  100 . 
     &lt;Photoelectric Conversion Element&gt; 
     Although a photodiode is used as the photoelectric conversion element  100  as in Embodiment 1, the photoelectric conversion element  100  is not limited to a photodiode. 
     One of the electrodes of the photoelectric conversion element  100  is connected to the wiring  111  (PR) and the other of the electrodes of the photoelectric conversion element  100  is connected to the first signal output circuit  301  and the second signal output circuit  302 . 
     The signal output circuits (the first signal output circuit  301  and the second signal output circuit  302 ) hold potentials including the amount of light entering the photoelectric conversion element  100  as data in the circuits and output detection signals corresponding to the potentials (data) to the outside. 
     &lt;Detection Circuit&gt; 
     Although this embodiment is similar to Embodiment 1 in that the first signal output circuit  301  and the second signal output circuit  302  illustrated in  FIG. 3A  each include the transistor  120 , the first switching element  121 , and the second switching element  122  as components, different points are as follows: a wiring which controls the operation state of the second switching element  122  is shared by the first signal output circuit  301  and the second signal output circuit  302 ; and different wirings are used in the first signal output circuit  301  and the second signal output circuit  302  to output a detection signal. 
     Specifically, in the photodetector circuit illustrated in  FIG. 1A , the second switching element  122  in the first signal output circuit  101  is connected to the wiring  116  (SE 1 ), and the second switching element  122  in the second signal output circuit  102  is connected to the wiring  136  (SE 2 ). 
     In contrast, in the photodetector circuit illustrated in  FIG. 3A , the second switching element  122  in the first signal output circuit  301  and the second switching element  122  in the second signal output circuit  302  are both connected to a wiring  316  (SE). 
     Further, in the photodetector circuit illustrated in  FIG. 1A , the second switching element  122  in the first signal output circuit  101  and the second switching element  122  in the second signal output circuit  102  are both connected to the wiring  115  (OUT). 
     In contrast, in the photodetector circuit illustrated in  FIG. 3A , the second switching element  122  in the first signal output circuit  301  is connected to a wiring  315  (OUT 1 ) and the second switching element  122  in the second signal output circuit  302  is connected to a wiring  335  (OUT 2 ). 
     When the photodetector circuit has the above-described structure, detection signals can be output from the first signal output circuit  301  and the second signal output circuit  302  at the same time; thus, detection signals can be obtained in a short period. 
     Note that the configuration of the photodetector circuit described in this embodiment may be a configuration in which a transistor  601  is added to each of the first signal output circuit  301  and the second signal output circuit  302  as illustrated in  FIG. 6 . A gate of the transistor is electrically connected to the wiring  111  (PR), one of a source and a drain of the transistor is electrically connected to the wiring  112  (FD 1 ) (or the wiring  132  (FD 2 )), the other of the source and the drain of the transistor is electrically connected to a wiring  602   a  (or a wiring  602   b ), and the one of the electrodes of the photoelectric conversion element  100  is electrically connected to a wiring  603 . Here, the wiring  603  is a signal line (low potential line) for always applying a reverse bias to the photoelectric conversion element  100 . Further, the wiring  602   a  and the wiring  602   b  are signal lines (high potential lines) for resetting the wiring  112  (FD 1 ) (or the wiring  132  (FD 2 )) to a high potential. 
     The transistor  601  functions as a reset transistor for resetting the wiring  112  (FD 1 ) (or the wiring  132  (FD 2 )). Accordingly, unlike in the detection circuit in  FIG. 3A , the reset operation using the photoelectric conversion element  100  is not performed, and a reverse bias is always applied to the photoelectric conversion element  100 . The wiring  112  (FD 1 ) and the wiring  132  (FD 2 ) can be reset by setting the potential of the wiring  111  (PR) high. 
     The transistor  601  can be formed using a silicon semiconductor such as amorphous silicon, microcrystalline silicon, polycrystalline silicon, or single crystal silicon; however, when leakage current is large, the charge accumulation portion cannot hold charge long enough. For this reason, like the transistor  120 , it is preferable to use a transistor including a semiconductor layer (at least a channel formation region) formed using an oxide semiconductor material, which achieves extremely low off-state current. 
     &lt;Operation Flow Chart of Photodetector Circuit&gt; 
     Next, an operation flow chart of the photodetector circuit illustrated in  FIG. 3A  will be described with reference to  FIG. 3B . 
     First, as in the operation flow chart of the photodetector circuit described in Embodiment 1, reset operations and potential generation operations are performed in the first signal output circuit  301  and the second signal output circuit  302  from the time T 1  to the time T 6 . 
     Next, at the time T 7 , output operations are performed in the first signal output circuit  301  and the second signal output circuit  302 . Although the output operations are sequentially performed in the first signal output circuit  101  and the second signal output circuit  102  in Embodiment 1, in the operation flow chart of the photodetector circuit in this embodiment, the output operations are performed in the first signal output circuit  301  and the second signal output circuit  302  at a time (the potential (the signal  316 S) of the wiring  316  (SE) is set high) as illustrated in  FIG. 3B . 
     Thus, current corresponding to the gate potential of the transistor  120  flows between the source and the drain of the second switching element  122  in each of the first signal output circuit  301  and the second signal output circuit  302 , whereby the potential (the signal  315 S) of the wiring  315  (OUT 1 ) and the potential (the signal  335 S) of the wiring  335  (OUT 2 ) are decreased. 
     Then, at the time T 8 , when the potential (the signal  316 S) of the wiring  316  (SE) in the first signal output circuit  301  is set low (i.e., the output operation finishes), current flowing between the source and the drain of the second switching element  122  in each of the first signal output circuit  301  and the second signal output circuit  302  is stopped, so that the potential (the signal  315 S) of the wiring  315  (OUT 1 ) which serves as a transmission path of a detection signal output from the first signal output circuit  301  and the potential (the signal  335 S) of the wiring  335  (OUT 2 ) which serves as a transmission path of a detection signal output from the second signal output circuit each have a fixed value. 
     As illustrated in  FIG. 3A , the wiring (wiring  316  (SE)) which controls the operation state of the second switching element  122  is shared by the first signal output circuit  301  and the second signal output circuit  302  and different wirings (the wiring  315  (OUT 1 ) and the wiring  335  (OUT 2 )) are used in the first signal output circuit  301  and the second signal output circuit  302  as wirings for outputting detection signals, so that output of a detection signal from the first signal output circuit  301  and output of a detection signal from the second signal output circuit  302  can be performed at a time; thus, detection signals in different periods can be obtained in a short period. 
     The above is the description of the operation flow chart of the photodetector circuit in this embodiment. 
     &lt;Different Operation Flow Chart of Photodetector Circuit&gt; 
     Note that the operation flow chart of the photodetector circuit described in  FIG. 3A  may be an operation flow chart different from the above operation flow chart described using  FIG. 3B . The operation flow chart different from the above-described operation flow chart is described below using  FIG. 4 . 
     First, at a time T 1 , the potential (the signal  111 S) of the wiring  111  is set high and the potential (the signal  114 S) of the wiring  114  (TX 1 ) in the first signal output circuit  301  and the potential (the signal  134 S) of the wiring  134  (TX 2 ) in the second signal output circuit  302  are set high (i.e., a reset operation starts). 
     In the operation flow chart described in  FIG. 3B , reset operations are performed in the first signal output circuit  301  and the second signal output circuit  302  in separate steps. By performing reset operations in the first signal output circuit  301  and the second signal output circuit  302  at the same time as illustrated in  FIG. 4 , a period from the reset operation start to the output operation end (period from the time T 1  to the time T 8 ) can be shortened, so that detection signals in different periods can be obtained in a short period. 
     Note that since the following operation flow chart is the same as that in the above operation flow chart described using  FIG. 3B  except for the operation flow chart between the time T 4  and the time T 5 , the operation flow chart described using  FIG. 3B  can be referred to for the following operation flow chart. 
     The above is the description of the different operation flow chart of the photodetector circuit. 
     Embodiment 3 
     In this embodiment, examples of the configuration of an integrator circuit used to be connected to the wiring  115  (OUT) in Embodiment 1, and the wiring  315  (OUT 1 ) and the wiring  335  (OUT 2 ) in Embodiment 2. 
       FIG. 7A  illustrates an integrator circuit including an operational amplifier circuit (also referred to as an op-amp). An inverting input terminal of the operational amplifier circuit is connected to the wiring  115  (OUT), the wiring  315  (OUT 1 ), and the wiring  335  (OUT 2 ) through a resistor R. A non-inverting input terminal of the operational amplifier circuit is grounded. An output terminal of the operational amplifier circuit is connected to the inverting input terminal of the operational amplifier circuit through a capacitor C. 
     Here, the operational amplifier circuit is assumed to be an ideal operational amplifier circuit. In other words, it is assumed that input impedance is infinite (the input terminals draw no current). Since the potential of the non-inverting input terminal and the potential of the inverting input terminal are equal in a steady state, the potential of the inverting input terminal can be considered as a ground potential. 
     Relations (1), (2), and (3) are satisfied, where Vi is the potential of each of the wiring  115  (OUT), the wiring  315  (OUT 1 ), and the wiring  335  (OUT 2 ), Vo is the potential of the output terminal of the operational amplifier circuit, i 1  is a current flowing through the resistor R, and i 2  is a current flowing through the capacitor C.
 
 Vi=i 1· R   (1)
 
 i 2= C·dVo/dt   (2)
 
 i 1+ i 2=0  (3)
 
     Here, when charge in the capacitor C is discharged at the time t=0, the potential Vo of the output terminal of the operational amplifier circuit at the time t=t is expressed by (4).
 
 Vo =−(1/ CR )∫ Vidt   (4)
 
In other words, with a longer time t (integral time), the potential (Vi) to be read can be raised and output as the detection signal Vo. Moreover, lengthening of the time t corresponds to averaging of thermal noise or the like and can increase S/N of the detection signal Vo.
 
     In a real operational amplifier circuit, a bias current flows even when a signal is not input to the input terminals, so that an output voltage is generated at the output terminal and charge is accumulated in the capacitor C. It is therefore effective to connect a resistor in parallel with the capacitor C so that the capacitor C can be discharged. 
       FIG. 7B  illustrates an integrator circuit including an operational amplifier circuit having a structure different from that in  FIG. 7A . An inverting input terminal of the operational amplifier circuit is connected to the wiring  115  (OUT), the wiring  315  (OUT 1 ), and the wiring  335  (OUT 2 ), through a resistor R and a capacitor C 1 . A non-inverting input terminal of the operational amplifier circuit is grounded. An output terminal of the operational amplifier circuit is connected to the inverting input terminal of the operational amplifier circuit through a capacitor C 2 . 
     Here, the operational amplifier circuit is assumed to be an ideal operational amplifier circuit. In other words, it is assumed that input impedance is infinite (the input terminals draw no current). Since the potential of the non-inverting input terminal and the potential of the inverting input terminal are equal in a steady state, the potential of the inverting input terminal can be considered as a ground potential. 
     Relations (5), (6), and (7) are satisfied, where Vi is the potential of each of the wiring  115  (OUT), the wiring  315  (OUT 1 ), and the wiring  335  (OUT 2 ), Vo is the potential of the output terminal of the operational amplifier circuit, i 1  is a current flowing through the resistor R and the capacitor C 1 , and i 2  is a current flowing through the capacitor C 2 .
 
 Vi =(1/ C 1)∫ i 1 dt+i 1· R   (5)
 
 i 2= C 2· dVo/dt   (6)
 
 i 1+ i 2=0  (7)
 
     Here, assuming that charge in the capacitor C 2  is discharged at the time t=0, the potential Vo of the output terminal of the operational amplifier circuit at the time t=t is expressed by (9) when (8) is met, which corresponds to a high-frequency component, and (11) when (10) is met, which corresponds to a low-frequency component.
 
 Vo&lt;&lt;dVo/dt   (8)
 
 Vo =−(1/ C 2 R )∫ Vidt   (9)
 
 Vo&gt;&gt;dVo/dt   (10)
 
 Vo=−C 1/ C 2· Vi   (11)
 
     In other words, by appropriately setting the capacitance ratio of the capacitor C 1  to the capacitor C 2 , the potential (Vi) to be read can be raised and output as the detection signal Vo. Further, a high-frequency noise component of the input signal can be averaged by time integration, and S/N of the detection signal Vo can be increased. 
     In a real operational amplifier circuit, a bias current flows even when a signal is not input to the input terminals, so that an output voltage is generated at the output terminal and charge is accumulated in the capacitor C 2 . It is thus effective to connect a resistor in parallel with the capacitor C 2  so that the capacitor C 2  can be discharged. 
       FIG. 7C  illustrates an integrator circuit including an operational amplifier circuit having a structure different from those in  FIGS. 7A and 7B . A non-inverting input terminal of the operational amplifier circuit is connected to the wiring  115  (OUT), the wiring  315  (OUT 1 ), and the wiring  335  (OUT 2 ) through a resistor R and is grounded through a capacitor C. An output terminal of the operational amplifier circuit is connected to an inverting input terminal of the operational amplifier circuit. The resistor R and the capacitor C constitute a CR integrator circuit. The operational amplifier circuit is a unity gain buffer. 
     A relation (12) holds, where Vi is the potential of each of the wiring  115  (OUT), the wiring  315  (OUT 1 ), and the wiring  335  (OUT 2 ) and Vo is the potential of the output terminal of the operational amplifier circuit. Although Vo is saturated at the value of Vi, a noise component included in the input signal Vi can be averaged by the CR integrator circuit, and as a result, S/N of the detection signal Vo can be increased.
 
 Vo =(1/ CR )∫ Vidt   (12)
 
     The above are the examples of the configuration of the integrator circuit used to be connected to each of the wiring  115  (OUT), the wiring  315  (OUT 1 ), and the wiring  335  (OUT 2 ). Connecting the above-described integrator circuit to the wiring  115  (OUT), the wiring  315  (OUT 1 ), and the wiring  335  (OUT 2 ) increases S/N of the detection signal and enables weaker light to be detected; thus, a more accurate image signal can be generated in the semiconductor device. 
     Embodiment 4 
     In this embodiment, an example of the layout of the photodetector circuit in  FIG. 1A  and  FIG. 3A  described in Embodiment 1 will be described with reference to  FIGS. 8A and 8B  and  FIGS. 9A and 9B . 
     &lt;Example of Layout of Photodetector Circuit in  FIG. 1A &gt; 
       FIG. 8A  is a top view of the photodetector circuit illustrated in  FIG. 1A , and  FIG. 8B  is a cross-sectional view along the dashed-dotted line A 1 -A 2  in  FIG. 8A . 
     The photodetector circuit includes, over a substrate  860  on which an insulating film  861  is formed, a conductive film  811  serving as the wiring  111  (PR), a conductive film  812  serving as the wiring  112  (FD 1 ) in the first signal output circuit  101 , a conductive film  832  serving as the wiring  132  (FD 2 ) in the second signal output circuit  102 , a conductive film  813  serving as the wiring  113  (VR), a conductive film  814  serving as the wiring  114  (TX 1 ) in the first signal output circuit  101 , a conductive film  834  serving as the wiring  134  (TX 2 ) in the second signal output circuit  102 , a conductive film  815  serving as the wiring  115  (OUT), a conductive film  816  serving as the wiring  116  (SE 1 ) in the first signal output circuit  101 , and a conductive film  836  serving as the wiring  136  (SE 2 ) in the second signal output circuit  102 . 
     The photoelectric conversion element  100  includes a p-type semiconductor film  801 , an i-type semiconductor film  802 , and an n-type semiconductor film  803  that are stacked in this order. 
     The conductive film  811 , which serves as the wiring  111  (PR), is electrically connected to the p-type semiconductor film  801  that functions as one of the electrodes (the anode) of the photoelectric conversion element  100 . 
     A conductive film  841  functions as a wiring for connecting one of the source and the drain of the transistor  120  to the conductive film  813 . 
     A conductive film  842  functions as one of the source and the drain of the first switching element  121 . 
     A conductive film  843  functions as a wiring for connecting one of the source and the drain of the first switching element  121  in the first signal output circuit  101  to one of the source and the drain of the first switching element  121  in the second signal output circuit  102 . 
     A conductive film  844  functions as one of the source and the drain of the transistor  120 . 
     A conductive film  845  functions as the other of the source and the drain of the first switching element  121 . 
     A conductive film  846  functions as a wiring for connecting the other of the source and the drain of the transistor  120  to one of the source and the drain of the second switching element  122 . 
     A conductive film  847  functions as the gate of the second switching element  122  in the first signal output circuit  101 . 
     A conductive film  848  functions as the gate of the second switching element  122  in the second signal output circuit  102 . 
     A conductive film  849  functions as a wiring for connecting the gate of the second switching element  122  in the first signal output circuit  101  to the conductive film  816 . 
     A conductive film  850  functions as a wiring for connecting the gate of the second switching element  122  in the second signal output circuit to the conductive film  836 . 
     The conductive films  812 ,  814 ,  816 ,  832 ,  834 ,  836 ,  841 ,  843 ,  847 , and  848  can be formed by processing one conductive film formed over an insulating surface into a desired shape. Over these conductive films, a gate insulating film  862  is formed. Further, the conductive films  811 ,  813 ,  815 ,  842 ,  844 ,  845 ,  846 ,  849 , and  850  can be formed by processing one conductive film formed over the gate insulating film  862  into a desired shape. 
     Over the conductive films  811 ,  813 ,  815 ,  842 ,  844 ,  845 ,  846 ,  849 , and  850 , an insulating film  863  and an insulating film  864  are formed, and over the insulating films  863  and  864 , a conductive film  870  is formed. 
     It is preferable to use an oxide semiconductor for a semiconductor layer  880  of the first switching element  121 . In order to hold charge generated by light entering the photoelectric conversion element  100  in the conductive film  812  (FD 1 ) (or the conductive film  832  (FD 2 )) for a long time, a transistor having extremely low off-state current is preferably used as the first switching element  121  electrically connected to the conductive film. For that reason, the use of an oxide semiconductor material for the semiconductor layer  880  can increase the performance of the photodetector circuit. 
     In the photodetector circuit in  FIGS. 8A and 8B , the elements such as the transistors and the photoelectric conversion element  100  may overlap each other. This configuration can increase the pixel density and thus can increase the resolution of an imaging device. In addition, the area of the photoelectric conversion element  100  can be increased, and the sensitivity of the imaging device can be increased as a result. 
     &lt;Example of Layout of Photodetector Circuit in  FIG. 3A &gt; 
       FIG. 9A  is a top view of the photodetector circuit illustrated in  FIG. 3A , and  FIG. 9B  is a cross-sectional view along the dashed-dotted line B 1 -B 2  in  FIG. 9A . 
     The photodetector circuit includes, over a substrate  960  on which an insulating film  961  is formed, a conductive film  911  serving as the wiring  111  (PR), a conductive film  912  serving as the wiring  112  (FD 1 ) in the first signal output circuit  301 , a conductive film  932  serving as the wiring  132  (FD 2 ) in the second signal output circuit  302 , a conductive film  913  serving as the wiring  113  (VR), a conductive film  914  serving as the wiring  114  (TX 1 ) in the first signal output circuit  301 , a conductive film  934  serving as the wiring  134  (TX 2 ) in the second signal output circuit  302 , a conductive film  915  serving as the wiring  315  (OUT 1 ) in the first signal output circuit  301 , a conductive film  935  serving as the wiring  335  (OUT 2 ) in the second signal output circuit  302 , and a conductive film  916  serving as the wiring  316  (SE). 
     The photoelectric conversion element  100  includes a p-type semiconductor film  901 , an i-type semiconductor film  902 , and an n-type semiconductor film  903  that are stacked in this order. 
     The conductive film  911 , which serves as the wiring  111  (PR), is electrically connected to the p-type semiconductor film  901  that functions as one of the electrodes (the anode) of the photoelectric conversion element  100 . 
     A conductive film  941  is connected to the conductive film  913  serving as the wiring  113  (VR) and functions as part of the wiring  113  (VR). 
     A conductive film  942  is connected to the conductive film  914  serving as the wiring  114  (TX 1 ) or the conductive film  934  serving as the wiring  134  (TX 2 ), and functions as the gate of the first switching element  121 . 
     A conductive film  943  functions as one of the source and the drain of the first switching element  121 . 
     A conductive film  944  functions as the other of the source and the drain of the first switching element  121 . 
     A conductive film  945  functions as the other of the source and the drain of the transistor  120  and one of the source and the drain of the second switching element  122 . 
     A conductive film  946  functions as a wiring for connecting the conductive film  911  to the p-type semiconductor film  901 . 
     The conductive films  911 ,  912 ,  916 ,  932 ,  941 , and  942  can be formed by processing one conductive film formed over an insulating surface into a desired shape. Over these conductive films, a gate insulating film  962  is formed. Further, the conductive films  913 ,  914 ,  915 ,  934 ,  935 ,  943 ,  944 ,  945 , and  946  can be formed by processing one conductive film formed over the gate insulating film  962  into a desired shape. 
     Further, over the conductive films  911 ,  912 ,  916 ,  932 ,  941 , and  942 , an insulating film  963  and an insulating film  964  are formed, and over the insulating films  963  and  964 , a conductive film  970  is formed. 
     It is preferable to use an oxide semiconductor for a semiconductor layer  980  of the first switching element  121 . In order to hold charge generated by light entering the photoelectric conversion element  100  in the conductive film  912  (FD 1 ) (or the conductive film  932  (FD 2 )) for a long time, a transistor having extremely low off-state current is preferably used as the first switching element  121  electrically connected to the conductive film. For that reason, the use of an oxide semiconductor material for the semiconductor layer  980  can increase the performance of the photodetector circuit. 
     In the photodetector circuit in  FIGS. 8A and 8B , the elements such as the transistors and the photoelectric conversion element  100  may overlap each other. This configuration can increase the pixel density and thus can increase the resolution of an imaging device. In addition, the area of the photoelectric conversion element  100  can be increased, and the sensitivity of the imaging device can be increased as a result. 
     This embodiment can be combined with any of the other embodiments disclosed in this specification as appropriate. 
     Embodiment 5 
     The photodetector circuit described in any of the above embodiments can be provided in a variety of semiconductor devices. In this embodiment, as an example of a semiconductor device including a photodetector circuit, a radiation imaging device in which adverse effect of afterglow is reduced by including the photodetector circuit described in any of the above embodiments will be described with reference to  FIGS. 10A and 10B  and  FIGS. 11A to 11D . 
     Further, an image display device having a touch panel function obtained by including the photodetector circuit described in any of the above embodiments will be described with reference to  FIG. 12 ,  FIG. 13 , and  FIGS. 14A and 14B . 
     &lt;Structure Example of Radiation Imaging Device&gt; 
     A structure of a radiation imaging device including the photodetector circuit described in any of the above embodiments will be described with reference to  FIGS. 10A and 10B  and  FIGS. 1A to 11D . 
     As illustrated in  FIG. 10A , a radiation imaging device  1000  includes a radiation emission portion  1001 , a scintillator  1004  which receives radiation  1002  output from the radiation emission portion  1001  and outputs light  1003 , a photodetector mechanism  1005  which outputs a detection signal corresponding to the amount of incident light  1003 , and an image signal generation portion  1006  which generates an image signal by using a detection signal output from the photodetector mechanism  1005 . Further, the radiation imaging device  1000  is connected to an image display device  1007  and the image display device  1007  receives an image signal output from the image signal generation portion  1006 , so that internal data and the like of an object  1008  is displayed on the image display device  1007 . 
     A structure example of the photodetector mechanism  1005  will be described below with reference to  FIG. 10B . 
     &lt;Structure Example of Photodetector Mechanism&gt; 
     The photodetector mechanism  1005  described in this embodiment includes a photodetector portion  1010  in which photodetector circuits  1012  are arranged in a matrix of m rows and n columns, and a photodetector circuit control portion  1020  including a first photodetector circuit driver  1021  and a second photodetector circuit driver  1022  for controlling the photodetector circuits  1012 . 
     As the photodetector circuit  1012 , the photodetector circuit illustrated in  FIG. 1A  is used (needless to say, the photodetector circuit  1012  is not limited thereto). 
     The first photodetector circuit driver  1021  has a function of generating a signal output to the wiring  113  (VR) and the wiring  111  (PR) and a function of extracting detection signals, which are output from the first signal output circuit  101  and the second signal output circuit  102 , from the wiring  115  (OUT) in a selected row. Note that the first photodetector circuit driver  1021  is connected to the image signal generation portion  1006  which generates an image signal that is less affected by afterglow. 
     In addition, the first photodetector circuit driver  1021  includes a precharge circuit, and has a function of setting the potential of the wiring  115  (OUT) to a predetermined potential. Note that the first photodetector circuit driver  1021  can have a structure in which output of an analog signal from a photodetector circuit is extracted as it is to the outside of the radiation imaging device  1000  with the use of an operational amplifier or the like or a structure in which an analog signal is converted into a digital signal with the use of an A/D converter circuit and extracted to the outside of the radiation imaging device  1000 . 
     The second photodetector circuit driver  1022  has a function of generating a signal output to the wiring  114  (TX 1 ), the wiring  134  (TX 2 ), the wiring  116  (SE 1 ), and the wiring  136  (SE 2 ). 
     The above is the description of the structure example of the photodetector mechanism  1005 . 
     &lt;Operation Example of Radiation Imaging Device&gt; 
     Next, an example of the operation of the radiation imaging device  1000  having the above-described structure will be described with reference to  FIGS. 11A to 11D . 
     In the case where moving images, for example, for monitoring blood flow in vessels or temporally continuous still images are taken with a radiation imaging device, it is necessary to increase the time resolution of the radiation imaging device to obtain high-definition images; thus, it is desired that a period after stop of radiation emission before start of the next radiation emission be as short as possible. 
     However, in the case where the period after stop of radiation emission before start of the next radiation emission is short, light due to afterglow is output from a scintillator at the start of the next radiation emission in some cases. 
     When radiation emission starts in such a state, light output from the scintillator is a combination of light obtained by radiation emission and light due to afterglows of previous and earlier radiation emission; thus, a difference may arise between the amount of radiation through the object  1008 , which is received by the scintillator  1004 , and data corresponding to the amount of radiation received by the photodetector mechanism  1005 , which is output from the photodetector mechanism  1005 . 
     In view of the above, first, as illustrated in  FIGS. 11A and 11B , light  1101  output from the scintillator  1004  in a period  1111  just before the start of the next radiation emission is received by the photoelectric conversion element  100  in each of the photodetector circuits  1012  and potentials (data) (hereinafter, also referred to as potentials A) based on the amount of incident light are held in the first signal output circuits  101 . 
     The light  1101  output from the scintillator  1004  in the period  1111  can be regarded as light due to afterglows of the previous and earlier radiation emission. 
     Next, as illustrated in  FIGS. 11C and 11D , light  1102  output from the scintillator  1004  in a period  1112  during which the next radiation emission is performed is received by the photoelectric conversion element  100  in each of the photodetector circuits  1012  and potentials (data) (hereinafter, also referred to as potentials B) based on the amount of incident light are held in the second signal output circuits  102 . 
     The light  1102  output from the scintillator  1004  in the period  1112  can be regarded as light due to afterglows of the previous and earlier radiation emission. 
     Next, after the potentials A and the potentials B are held in all the photodetector circuits  1012 , a detection signal including the potential A as data and a detection signal including the potential B as data are output from each of the photodetector circuits  1012  to the image signal generation portion  1006 . 
     Then, in the image signal generation portion  1006 , an image signal (for one pixel) is generated using a difference between the two detection signals input from each of the photodetector circuits  1012  and imaging data is displayed on the image display device  1007  with the use of the image signal. 
     Here, the photodetector circuit  1012  has a structure in which one photoelectric conversion element and one signal output circuit are provided. 
     In the structure, when data held in the signal output circuit remains in the period  1111 , a potential (data) in the period  1112  cannot be obtained accurately. In other words, since a potential (data) in the period  1112  is added to a potential (data) in the period  1111 , output of a detection signal corresponding to the potential (data) (an output operation) and reset of the potential (data) held in the signal output circuit (an reset operation) are necessarily performed before the start of the period  1112 . 
     Since the amount of light emitted by afterglow is decreased as time passes, as an interval from the end of the period  1111  to the start of the period  1112  increases, an accurate image signal is less likely to be obtained when an image signal is generated using a difference between two detection signals as described above. Particularly in the case where the amount of temporal change of afterglow is large, the above problem becomes significant. 
     In contrast, in the case where in the photodetector circuit  1012 , the two signal output circuits (the first signal output circuit  101  and the second signal output circuit  102 ) are connected to the photoelectric conversion element  100  as illustrated in  FIG. 1A , turning off the first switching element  121  in the first signal output circuit  101  enables a potential (data) in the period  1111  to be held in the first signal output circuit  101  and further, only performing a reset operation on the second signal output circuit  102  after the period  1111  enables a potential (data) in the period  1112  to start to be obtained using the photoelectric conversion element  100  and the second signal output circuit  102 . Note that in the case where the photodetector circuit illustrated in  FIG. 3A  is used, a reset operation performed after the period  1111  is not necessary in some cases. 
     Thus, an image signal generated using a difference between two detection signals input from each of the photodetector circuits  1012  is an accurate image signal that is less affected by afterglow. 
     The above is the description of the radiation imaging device including the photodetector circuit described in any of the above embodiments. 
     Although the image signal generation portion  1006  is provided in the radiation imaging device  1000  so as to be connected to the photodetector mechanism  1005  in  FIGS. 10A and 10B , the image signal generation portion  1006  may be provided in the photodetector mechanism  1005 . Alternatively, the image signal generation portion  1006  may be provided outside the radiation imaging device  1000 . 
     Further, although the image display device  1007  is provided outside the radiation imaging device  1000  in  FIGS. 10A and 10B , the image display device  1007  may be provided in the radiation imaging device  1000 . 
     &lt;Structure Example of Image Display Device&gt; 
       FIG. 12  illustrates an example of a structure of an image display device including a plurality of pixels and a driver for driving the plurality of pixels. 
     An image display device  1200  includes a display portion  1240 , a display element control portion  1220 , and a photodetector circuit control portion  1230 . The display portion  1240  includes a plurality of pixels  1210  arranged in a matrix. 
       FIG. 12  illustrates an example where the pixel  1210  includes one display element  1201 R emitting red light, one display element  1201 G emitting green light, one display element  1201 B emitting blue light, and one photodetector circuit  1202 . The structure of the photodetector circuit  1202  can be similar to that described in any of the above embodiments can be used. 
     An example of a configuration of the pixel  1210  is described below with reference to  FIG. 13 . 
     &lt;Configuration Example of Pixel&gt; 
     The pixel  1210  described in this embodiment includes three display elements (the display element  1201 R, the display element  1201 G, and the display element  1201 B) and one photodetector circuit  1202 . Using the pixel  1210  as a basic configuration, a plurality of pixels  1210  are arranged in a matrix of m rows and n columns and form a display screen that also serves as a data input region.  FIG. 13  illustrates an example of the case where the photodetector circuit having the configuration in  FIG. 1A  is used as the photodetector circuit  1202  in the pixel  1210 . 
     Note that the number of display elements and the number of photodetector circuits included in each pixel is not limited to those illustrated in  FIG. 13 . The density of the photodetector circuits and that of the display elements may be the same or different. That is, one photodetector circuit may be provided for one display element; one photodetector circuit may be provided for two or more display elements; or one display element may be provided for two or more photodetector circuits. 
       FIG. 13  illustrates a configuration where the display element  1201 R, the display element  1201 G, and the display element  1201 B each include a liquid crystal element  1250  is illustrated as an example. The display element  1201 R, the display element  1201 G, and the display element  1201 B each include the liquid crystal element  1250 , a transistor  1252  serving as a switching element for controlling the operation of the liquid crystal element  1250 , and a capacitor  1254 . The liquid crystal element  1250  includes a pixel electrode, a counter electrode, and a liquid crystal layer to which a voltage is applied by the pixel electrode and the counter electrode. 
     Although not illustrated, a red color filter, a green color filter, and a blue color filter are provided on the light extraction side of the liquid crystal element  1250  in the display element  1201 R, the liquid crystal element  1250  in the display element  1201 G, and the liquid crystal element  1250  in the display element  1201 B, respectively. 
     A gate of the transistor  1252  is connected to a scan line GL (GL 1  or GL 2 ). One of a source and a drain of the transistor  1252  is connected to a signal line SL (SL 1  or SL 2 ), and the other of the source and the drain of the transistor  1252  is connected to a pixel electrode of the liquid crystal element  1250 . One of a pair of electrodes of the capacitor  1254  is connected to the pixel electrode of the liquid crystal element  1250 , and the other of the pair of electrodes of the capacitor  1254  is connected to a wiring COM supplied with a fixed potential. The signal line SL is supplied with a potential corresponding to an image to be displayed. When the transistor  1252  is turned on with a signal of the scan line GL, the potential of the signal line SL is supplied to one of the pair of the electrodes of the capacitor  1254  and the pixel electrode of the liquid crystal element  1250 . The capacitor  1254  holds charge corresponding to voltage applied to the liquid crystal layer. Contrast (gray scale) of light passing through the liquid crystal layer is made by utilizing the change in the polarization direction of the liquid crystal layer with voltage application, and images are displayed. As light passing through the liquid crystal layer, light emitted from the backlight is used. 
     In the configuration in  FIG. 13 , the operation of the display elements arranged in a matrix can be similar to that in a known display device. 
     Note that as the transistor  1252 , the transistor including an oxide semiconductor material in a channel formation region, which is described in any of the above embodiments, can be used. In the case of using the transistor, since its off-state current is extremely low, the capacitor  1254  is not necessarily provided. 
     Note that each of the display elements  1201 R,  1201 G, and  1201 B may further include another circuit element such as a transistor, a diode, a resistor, a capacitor, or an inductor as needed. 
     Although  FIG. 13  illustrates the case where the display element  1201 R, the display element  1201 G, and the display element  1201 B each include the liquid crystal element  1250 , another element such as a light-emitting element may be included. The light-emitting element is an element whose luminance is controlled with current or voltage, and specific examples thereof are a light emitting diode and an organic light-emitting diode (OLED). 
     The above is the description of the configuration example of the pixel  1210 . 
     The display element control portion  1220  includes a first display element driver  1221  which has a function of controlling the display elements  1201  and inputs a signal to the display element  1201  through a signal line through which an image signal is transmitted (also referred to as a “source signal line”) and a second display element driver  1222  which inputs a signal to the display element  1201  through a scan line (also referred to as a “gate signal line”). For example, the first display element driver  1221  has a function of giving a predetermined potential to the display elements  1201  in the pixels  1210  placed in the selected line. Further, the second display element driver  1222  has a function of selecting the display elements  1201  included in the pixels placed in a particular row. 
     The photodetector circuit control portion  1230  includes drivers for controlling the photodetector circuits  1202 , and specifically a first photodetector circuit driver  1231  which faces the first display element driver  1221  with the display portion  1240  provided therebetween, and a second photodetector circuit driver  1232  which faces the second display element driver  1222  with the display portion  1240  provided therebetween. 
     The first photodetector circuit driver  1231  has a function of generating a signal output to the wiring  111  (PR) and the wiring  113  (VR) and a function of extracting an output signal of a photodetector circuit in the pixel  1210  in a selected row from the wiring  115  (OUT). Note that the first photodetector circuit driver  1231  is connected to a detection signal comparison portion  1260  which determines whether an object to be detected exists over each pixel  1210  or not with the use of a plurality of detection signals output from each pixel  1210 . 
     In addition, the first photodetector circuit driver  1231  includes a precharge circuit, and has a function of setting the potential of the wiring  115  (OUT) to a predetermined potential. Note that the first photodetector circuit driver  1231  can have a structure in which output of an analog signal from a photodetector circuit is extracted as it is to the outside of the image display device  1200  with the use of an operational amplifier or the like or a structure in which an analog signal is converted into a digital signal with the use of an A/D converter circuit and extracted to the outside of the image display device  1200 . 
     The second photodetector circuit driver  1232  has a function of generating a signal output to the wiring  114  (TX 1 ), the wiring  134  (TX 2 ), the wiring  116  (SE 1 ), and the wiring  136  (SE 2 ). 
     The above is the description of the configuration example of the image display device  1200 . 
     &lt;Operation Example of Image Display Device&gt; 
     Next, an example of the operation of the image display device having the above-described configuration will be described with reference to  FIGS. 14A and 14B . 
     The photodetector circuit  1202  provided in the image display device  1200  can hold potentials (data) including the amount of light entering the photoelectric conversion element  100  in given periods as data in accordance with the number of signal output circuits included in the photodetector circuit  1202 , as described in the above embodiments. 
     For example, in the photodetector circuit in  FIG. 13 , potentials (data) in two periods can be held by using the first signal output circuit  101  and the second signal output circuit  102 . Note that in this embodiment, the two periods are denoted by a period A and a period B, and the period B comes after the period A. 
     The period A is a period during which an object to be detected, such as a finger, does not exist over the display element  1201  as illustrated in  FIG. 14A . Light (image) output from the display element  1201  with the use of the first display element driver  1221  and the second display element driver  1222  is output to the outside through a liquid crystal layer  1401 , a pair of alignment films  1402  between which the liquid crystal layer  1401  is sandwiched, a pair of electrodes  1403  between which the pair of alignment films  1402  are sandwiched, a color filter  1404 , a substrate  1405 , and the like. 
     Thus, slightly reflected light which is reflected by the substrate  1405  or the like, external light, or the like is input to the photodetector circuit  1202 , and a potential (data) (hereinafter, also referred to as a potential C) including the amount of incident light in the period A as data is held in the first signal output circuit  101 . 
     The period B is a period during which an object  1410  to be detected exists over the display element  1201  as illustrated in  FIG. 14B , and light (image) output from the display element  1201  with the use of the first display element driver  1221  and the second display element driver  1222  is partly absorbed by the object  1410 , the other light enters the photodetector circuit  1202 , and a potential (data) (hereinafter, also referred to as a potential D) including the amount of incident light in the period B as data is held in the second signal output circuit  102 . 
     Note that the amount of light entering the photodetector circuit  1202  in the period B is much larger than the amount of incident light in the period A. 
     After that, the potential C and the potential D are held in all the pixels  1210  in the display portion  1240 , and then, a detection signal including the potential C as data and a detection signal including the potential D as data are output from each of the pixels  1210  to the detection signal comparison portion  1260 . 
     Then, in the detection signal comparison portion  1260 , two detection signals input from each of the pixels  1210  are compared. In the case where a difference of a predetermined value (which may be determined by a practitioner as appropriate) or more is found, it is judged that the object  1410  exists over the pixel  1210 . 
     Here, the case where the photodetector circuit  1202  includes one photoelectric conversion element and one signal output circuit is described. 
     In the structure, when data held in the signal output circuit remains in the period A, a potential (data) in the period B cannot be obtained accurately. In other words, since a potential (data) in the period B is added to a potential (data) in the period A, output of a detection signal corresponding to a potential (data) (output operation) and reset of a potential (data) held in the signal output circuit (reset operation) are necessarily performed by the start of the period B. 
     Therefore, after the end of the period A and before the start of the period B, for example, in the case where an output operation is performed on data obtained in the period A and the object  1410  passes over the display element  1201  in a period during which a reset operation is performed on the data obtained in the period A, whether the object  1410  exists over each pixel  1210  or not cannot sometimes be judged accurately by the detection signal comparison portion  1260 , even using data obtained in the period A and data obtained in the period B. 
     In contrast, in the image display device  1200  described in this embodiment, two signal output circuits (the first signal output circuit  101  and the second signal output circuit  102 ) are connected to the photoelectric conversion element  100  as illustrated in  FIG. 13 ; thus, turning off the first switching element  121  in the first signal output circuit  101  enables a potential (data) in the period A to be held in the first signal output circuit. Further, only performing a reset operation on the second signal output circuit  102  after the period A enables a potential (data) in the period B to start to be obtained using the photoelectric conversion element  100  and the second signal output circuit  102 . 
     Thus, even in the case where the object  1410  moves extremely quickly, whether the object  1410  exists over each pixel  1210  or not can be judged accurately. 
     The above is the description of the image display device including the photodetector circuit described in any of the above embodiments. 
     Although the detection signal comparison portion  1260  is provided in the image display device  1200  so as to be connected to the first photodetector circuit driver  1231  in  FIG. 12 , the detection signal comparison portion  1260  may be provided in the first photodetector circuit driver  1231 . Alternatively, the detection signal comparison portion  1260  may be provided outside the image display device  1200 . 
     This application is based on Japanese Patent Application serial no. 2012-200495 filed with Japan Patent Office on Sep. 12, 2012, the entire contents of which are hereby incorporated by reference.