Patent Publication Number: US-2023138802-A1

Title: Semiconductor device and method for driving the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/878,757, filed May 20, 2020, now allowed, which is a divisional of U.S. application Ser. No. 15/462,956, filed Mar. 20, 2017, now U.S. Pat. No. 10,726,913, which is a continuation of U.S. application Ser. No. 15/284,662, filed Oct. 4, 2016, now U.S. Pat. No. 9,633,722, which is a continuation of U.S. application Ser. No. 14/148,885, filed Jan. 7, 2014, now U.S. Pat. No. 9,583,576, which is a continuation of U.S. application Ser. No. 13/166,029, filed Jun. 22, 2011, now U.S. Pat. No. 8,630,127, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2010-145339 on Jun. 25, 2010, all of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention disclosed herein relates to a semiconductor device using a semiconductor element, a method for manufacturing the semiconductor device, and a method for driving the semiconductor device. 
     In this specification, a semiconductor device refers to any device that can function by utilizing semiconductor properties. A semiconductor circuit, a storage device, an imaging device, a display device, an electro-optical device, an electronic device, and the like are all semiconductor devices. 
     BACKGROUND ART 
     Storage devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even when power is not supplied. 
     A typical example of a volatile storage device is a DRAM (dynamic random access memory). A DRAM stores data (information) in such a manner that a transistor included in a storage element is selected and electric charge is stored in a capacitor. 
     When data is read from a DRAM, electric charge in a capacitor is lost on the above principle; thus, another write operation is necessary every time data is read. Moreover, a transistor included in a storage element has a leakage current and electric charge flows into or out of a capacitor even when the transistor is not selected, so that the data retention time is short. For that reason, another write operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption of the storage element. Furthermore, since stored data is lost when power supply stops, an additional storage device using a magnetic material or an optical material is needed in order to hold the data for a long time. 
     Another example of a volatile storage device is an SRAM (static random access memory). An SRAM retains data by using a circuit such as a flip-flop and thus does not need a refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacity is increased because of the use of a flip-flop or the like. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops. 
     A typical example of a non-volatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding electric charge in the floating gate. Therefore, a flash memory has advantages in that the data retention time is extremely long (almost permanent) and a refresh operation which is necessary in a volatile storage device is not needed (e.g., see Patent Document 1). 
     However, a gate insulating layer included in a storage element deteriorates by tunneling current generated in writing, so that the storage element stops its function after a limited number of write operations. In order to reduce adverse effects of this problem, a method of equalizing the number of write operations for storage elements is employed, for example, in which case a complicated peripheral circuit is needed. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for applications in which data is frequently rewritten. 
     In addition, high voltage is necessary to inject electric charge in the floating gate or to remove the electric charge, and a circuit for generating high voltage is also necessary. Further, it takes a relatively long time to perform an operation for injecting or removing electric charge, so that it is not easy to increase the speed of write and erase operations. 
     REFERENCE 
     
         
         Patent Document 1: Japanese Published Patent Application No. S57-105889 
       
    
     DISCLOSURE OF INVENTION 
     In view of the foregoing problems, an object of one embodiment of the invention disclosed herein is to provide a semiconductor device with a novel structure in which stored data can be retained even when power is not supplied, and there is no limitation on the number of write cycles. 
     Another object is to provide a semiconductor device with high integration and large storage capacity. 
     Another object is to provide a highly reliable semiconductor device with high stability in operation. 
     Another object is to provide a semiconductor device capable of high-speed operation. 
     Another object is to provide a semiconductor device with low power consumption. 
     Each embodiment of the invention disclosed in this specification achieves at least one of the above objects. 
     In one embodiment of the invention disclosed herein, a storage gate including an oxide semiconductor is provided in a transistor. The storage gate is configured to be made to be a conductor, and a specific potential is supplied to the storage gate. After that, the storage gate is made to be an insulator to hold the potential (electric charge). 
     One embodiment of the present invention is a semiconductor device that includes: a memory cell including a transistor including a control gate and a storage gate; a word line; a data line; a read signal line; and a bit line. The control gate is electrically connected to the word line. The storage gate is electrically connected to the data line. One of a source and a drain of the transistor is electrically connected to the read signal line. The other of the source and the drain of the transistor is electrically connected to the bit line. 
     One embodiment of the present invention is a semiconductor device that includes a memory cell including: a transistor having a first gate and a second gate which includes an oxide semiconductor; a first wiring; a second wiring; a third wiring; and a fourth wiring. The first gate is electrically connected to the first wiring. The second gate is electrically connected to the second wiring. One of a source and a drain of the transistor is electrically connected to the third wiring. The other of the source and the drain of the transistor is electrically connected to the fourth wiring. 
     As the oxide semiconductor used for the storage gate or the second gate, an intrinsic (i-type) or substantially intrinsic oxide semiconductor is preferably used. An intrinsic oxide semiconductor (including an oxide semiconductor that becomes intrinsic by application of an electric field) has a sufficiently low carrier density (e.g., lower than 1×10 12 /cm 3 , or lower than 1.45×10 10 /cm 3 ), and thus serves as an insulator. 
     Data can be stored in the following manner: a potential of data to be stored is applied to the oxide semiconductor used for the storage gate or the second gate when the oxide semiconductor is made to serve as a conductor by application of an electric field, and the potential (electric charge) is held by making the oxide semiconductor intrinsic. 
     The storage gate is placed to overlap with a channel formation region in a semiconductor layer included in the transistor. 
     As a semiconductor for forming a channel of the transistor, a single crystal semiconductor, a polycrystalline semiconductor, a microcrystalline semiconductor, an amorphous semiconductor, or the like can be used. Examples of a semiconductor material are silicon, germanium, silicon germanium, silicon carbide, and gallium arsenide. Alternatively, an organic semiconductor material or the like may be used. 
     When an oxide semiconductor is used as a semiconductor for forming the channel of the transistor, a transistor with an extremely low off-state current can be realized. Thus, a semiconductor device with low power consumption can be provided. 
     In the above semiconductor device, data is written in the following manner: the potential of the control gate is set at a potential that makes the storage gate a conductor, a potential to be stored in the memory cell is supplied to the storage gate, and the potential of the control gate is set at a potential that makes the storage gate an insulator. 
     In the above semiconductor device, data is written in the following manner: the potential of the first wiring is set at a potential that makes the second gate a conductor, a potential to be stored in the memory cell is supplied to the second gate through the second wiring, and the potential of the first wiring is set at a potential that makes the second gate an insulator. 
     In the above semiconductor device, data is read in the following manner: the drain is supplied with (precharged to) electric charge for making the drain of the transistor have a first potential, and a change in the potential of the drain at the time when the potential of the source of the transistor is set at a second potential is detected. 
     In the above semiconductor device, data is read in the following manner: the fourth wiring is supplied with (precharged to) electric charge for making the fourth wiring have a first potential, and a change in the potential of the fourth wiring at the time when the potential of the third wiring is set at a second potential for reading is detected. 
     Note that in this specification and the like, a non-volatile semiconductor device indicates a semiconductor device that can retain data for a given period of time (at least 1×10 4  seconds or longer, preferably 1×10 6  seconds or longer) even when no power is supplied. 
     According to one embodiment of the present invention, the area of a semiconductor device can be reduced; thus, a semiconductor device with higher integration and larger storage capacity can be provided. 
     In addition, deterioration of a gate insulating layer hardly occurs because data writing does not need high voltage; thus, the number of write cycles and the reliability are greatly increased. 
     Further, an operation for erasing data is not needed, so that high-speed operation can be easily realized. 
     Data is stored by making an oxide semiconductor an insulator, so that the stored data can be retained for an extremely long time. In other words, a refresh operation becomes unnecessary or the frequency of refresh operations can be extremely low. Consequently, power consumption of the semiconductor device can be reduced. Moreover, stored data can be retained for a long time even when power is not supplied. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIGS.  1 A and  1 B  are circuit diagrams of a semiconductor device; 
         FIGS.  2 A and  2 B  are diagrams explaining transient characteristics of a capacitor including an oxide semiconductor; 
         FIGS.  3 A and  3 B  are timing charts illustrating operations of a semiconductor device; 
         FIG.  4    is a circuit diagram of a semiconductor device; 
         FIG.  5 A  is a plan view and  FIGS.  5 B and  5 C  are cross-sectional views of a semiconductor device; 
         FIGS.  6 A to  6 D  are cross-sectional views illustrating steps for manufacturing a semiconductor device; 
         FIGS.  7 A and  7 B  are cross-sectional views of semiconductor devices; and 
         FIGS.  8 A to  8 F  each illustrate an electronic device including a semiconductor device. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Examples of embodiments of the present invention will be described below with reference to the accompanying drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details can be changed in various ways 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. 
     A transistor is one of a variety of semiconductor elements, and can amplify current or voltage and perform a switching operation for controlling conduction and non-conduction, for example. A transistor in this specification includes an insulated-gate field effect transistor (IGFET) and a thin film transistor (TFT) in its category. 
     Note that the position, size, range, and the like of each component illustrated in drawings and the like are not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, and the like disclosed in the drawings and the like. 
     In this specification, ordinal numbers such as first, second, and third are used in order to avoid confusion among components, and the terms do not limit the number of components. 
     In addition, in this specification, the terms “electrode” and “wiring” do not have functional limitations. For example, an “electrode” is sometimes used as part of a “wiring”, and vice versa. Furthermore, the term “electrode” or “wiring” sometimes refers to a plurality of “electrodes” or “wirings” formed in an integrated manner. 
     Functions of a “source” and a “drain” are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be replaced with each other in this specification. 
     Note that in this specification and the like, the term “electrically connected” includes the case where components are connected through an object having any electric function. There is no particular limitation on an object having any electric function as long as electric signals can be transmitted and received between components that are connected through the object. 
     Examples of the object having any electric function are a switching element such as a transistor, a resistor, an inductor, a capacitor, and an element with a variety of functions as well as an electrode and a wiring. 
     Note that a voltage refers to a difference between potentials of two points, and a potential refers to electrostatic energy (electric potential energy) of a unit charge at a given point in an electrostatic field. Note that in general, a difference between a potential of one point and a reference potential (e.g., a ground potential) is merely called a potential or a voltage, and “potential” and “voltage” are used as synonymous words in many cases. Therefore, in this specification, “potential” can be replaced with “voltage” and vice versa, unless otherwise specified. 
     Embodiment 1 
     In this embodiment, an example of a circuit configuration and an operation of a semiconductor device according to one embodiment of the disclosed invention will be described with reference to  FIGS.  1 A and  1 B ,  FIGS.  2 A and  2 B ,  FIGS.  3 A and  3 B , and  FIG.  4   . In this embodiment, the case where an n-channel transistor (n-type transistor) is used will be described. 
       FIG.  1 A  illustrates a circuit configuration of a non-volatile memory cell  200  including a transistor  210 . The transistor  210  has a control gate  211  and a storage gate  212 . In  FIG.  1 A , the control gate  211  of the transistor  210  is electrically connected to a first wiring  201  (also referred to as a word line WL). The storage gate  212  of the transistor  210  is electrically connected to a second wiring  202  (also referred to as a data line DL). One of a source and a drain of the transistor  210  is electrically connected to a third wiring  203  (also referred to as a read signal line RL). The other of the source and the drain of the transistor  210  is electrically connected to a fourth wiring  204  (also referred to as a bit line BL). 
     A single crystal semiconductor, a polycrystalline semiconductor, a microcrystalline semiconductor, an amorphous semiconductor, or the like can be used as a semiconductor for forming a channel of the transistor  210 . Examples of a semiconductor material are silicon, germanium, silicon germanium, silicon carbide, and gallium arsenide. The transistor  210  including such a semiconductor material can operate at sufficiently high speed, so that stored data can be read at high speed, for example. In other words, high-speed operation of the semiconductor device can be realized. 
     Alternatively, an oxide semiconductor can be used as a semiconductor for forming the channel of the transistor  210 . An oxide semiconductor has a wide energy gap of 3.0 eV or more. In a transistor obtained by processing an oxide semiconductor under appropriate conditions, the leakage current between a source and a drain in the off state, that is, the off-state current per 1 μm of channel width at an operating temperature (e.g., at 25° C.) can be 100 zA (1×10 −19  A) or lower or 10 zA (1×10 −20  A) or lower, and further can be 1 zA (1×10 −21  A) or lower. Thus, a semiconductor device with lower power consumption can be provided. 
     The storage gate  212  of the transistor  210  is formed using an oxide semiconductor. It is known that there are an oxide semiconductor that becomes n-type when an electric field is applied and an oxide semiconductor that becomes p-type when an electric field is applied. In this specification, the case of using an oxide semiconductor that becomes n-type when an electric field is applied will be described as an example. As an oxide semiconductor used for the storage gate  212 , an intrinsic (i-type) or substantially intrinsic oxide semiconductor is preferably used. 
     An oxide semiconductor used for the storage gate  212  is not necessarily an intrinsic (i-type) or substantially intrinsic oxide semiconductor, and any oxide semiconductor that can become intrinsic by application of an electric field can be used. Note that the intrinsic (i-type) or substantially intrinsic oxide semiconductor is intrinsic without application of an electric field; therefore, in the case of using such an oxide semiconductor, circuit design can be easy and a semiconductor device with lower power consumption can be manufactured. 
     Here, measurement results of transient characteristics of a capacitor  300  that includes a stack of an insulator and an oxide semiconductor as a dielectric will be described with reference to  FIGS.  2 A and  2 B .  FIG.  2 A  is a schematic diagram illustrating a stacked structure of the measured capacitor  300 . 
     The capacitor  300  includes an oxide semiconductor  302  and an insulator  303  between an electrode  301  and an electrode  304 . The electrode  301  is in contact with the oxide semiconductor  302 . The electrode  304  is in contact with the insulator  303 . As the oxide semiconductor  302 , an In—Ga—Zn—O-based oxide semiconductor with a thickness of 30 nm was used. As the insulator  303 , 100-nm-thick silicon oxide was used. The overlap area of the electrode  301  and the electrode  304  was 1 mm 2 . 
     The transient current was measured with Agilent  4156 C Precision Semiconductor Parameter Analyzer (manufactured by Agilent Technologies, Inc.). The measurement was performed as follows. First, the value of a current flowing between the electrode  301  and the electrode  304  was measured for 60 seconds while the potential of the electrode  301  is set at 0 V and the potential of the electrode  304  is set at 2 V. Then, the value of a current flowing between the electrode  301  and the electrode  304  was measured for 60 seconds while the potential of the electrode  301  is set at 0 V and the potential of the electrode  304  is set at −2 V. 
       FIG.  2 B  shows the measurement results of the transient current. In  FIG.  2 B , the horizontal axis represents voltage application time (measurement time), and the vertical axis represents the absolute value of the current flowing between the electrode  301  and the electrode  304 . A curve  311  in  FIG.  2 B  represents a change in the value of the current flowing between the electrode  301  and the electrode  304  when a voltage of 2 V is applied to the electrode  304 . A curve  312  represents a change in the value of the current flowing between the electrode  301  and the electrode  304  when a voltage of −2 V is applied to the electrode  304 . 
     It is understood from the curves  311  and  312  that almost no current flows between the electrode  301  and the electrode  304  when a voltage of 2 V is applied to the electrode  304 , and then, when a voltage of −2 V is applied to the electrode  304 , a current which is apparently larger in amount than that in the case where a voltage of 2 V is applied to the electrode  304  flows between the electrode  301  and the electrode  304  for approximately 30 seconds after starting application of −2 V. Although not clear in  FIG.  2 B , it is confirmed that the amount of current represented by the curve  312  is larger than that represented by the curve  311  even after 30 seconds have passed. 
     The measurement results of the transient current can be explained as follows. When a voltage of 2 V is applied to the electrode  304 , a positive electric field is applied to the oxide semiconductor  302  through the insulator  303 , and the oxide semiconductor  302  becomes n-type. The n-type oxide semiconductor  302  is supplied with electric charge from the electrode  301  and functions as a conductor; thus, only the insulator  303  serves as a dielectric layer in the capacitor  300 , and almost no current flows between the electrode  301  and the electrode  304 . 
     Then, when a voltage of −2 V is applied to the electrode  304 , the oxide semiconductor  302  becomes i-type. At this time, electric charge in the oxide semiconductor  302  and near the interface between the electrode  301  and the oxide semiconductor  302  moves toward the electrode  301  immediately. On the other hand, electric charge in the oxide semiconductor  302  and away from the electrode  301  cannot move immediately but moves slowly because of effects of electric field drift caused by application of −2 V to the electrode  304 . 
     The oxide semiconductor has a wide energy gap of 3.0 eV or more. Moreover, the intrinsic (i-type) or substantially intrinsic oxide semiconductor has a sufficiently low carrier density (e.g., lower than 1×10 12 /cm 3 , or lower than 1.45×10 10 /cm 3 ) as compared to the carrier density of a general silicon wafer (approximately 1×10 14 /cm 3 ). 
     A semiconductor such as general silicon becomes n-type when a positive electric field is applied and becomes p-type when a negative electric field is applied. That is, such a semiconductor always serves as a conductor when an electric field is applied. On the other hand, an oxide semiconductor becomes n-type when a positive electric field is applied but remains i-type when a negative electric field is applied. 
     It is therefore likely that an oxide semiconductor serves as one of a conductor and an insulator in accordance with an electric field applied thereto and electric charge applied when the oxide semiconductor serves as a conductor can be retained even after the oxide semiconductor becomes an insulator. 
     Using the above properties of the oxide semiconductor, it is possible to realize a storage element in which writing and reading of data can be performed at high speed with lower voltage than a conventional element. 
     Next, an example of a write operation (write mode) and a read operation (read mode) for storing two-value data (e.g., data of 0 or 1) in the semiconductor device disclosed in this embodiment will be described with reference to timing charts of  FIGS.  3 A and  3 B . The timing charts of  FIGS.  3 A and  3 B  show changes over time in potentials or states of the components illustrated in  FIG.  1 A . In this embodiment, an operation of storing a high-level potential V DLH  or a low-level potential V DLL  in the memory cell  200  as two-value data and an operation of reading the stored data will be described. 
     In  FIGS.  3 A and  3 B , the first wiring  201  (the word line WL) is supplied with a high-level potential V WLH  or a low-level potential V WLL . The second wiring  202  (the data line DL) is supplied with a high-level potential V DLH  or a low-level potential V DLL . The third wiring  203  (the read signal line RL) is supplied with a high-level potential V RLH  or a low-level potential V RLL . The fourth wiring  204  (the bit line BL) is supplied with a high-level potential V BLH  or a low-level potential V BLL . 
     The potential of the storage gate  212  after the storage gate  212  becomes an insulator by V DLH  written thereto is denoted by V MLH . The potential of the storage gate  212  after the storage gate  212  becomes an insulator by V DLL  written thereto is denoted by V MLL . The potential of the storage gate  212  at the time of a read operation in the case where V DLH  has been written into the storage gate  212  in a write operation is denoted by V MLHR . The potential of the storage gate  212  at the time of a read operation in the case where V DLL  has been written into the storage gate  212  in a write operation is denoted by V MLLR . 
     In  FIGS.  3 A and  3 B , a voltage (hereinafter referred to as V thos ) that makes the storage gate  212  (indicated by ML in  FIGS.  3 A and  3 B ) formed using an oxide semiconductor a conductor (n-type) is assumed to be 1 V, and V WLH  is set at 4 V and V MLL  at −1 V. V DLH  is set at 2 V; V DLL , 0 V; V RLH , 0 V; V RLL , −5 V; V BLH , 0 V; and V BLL , −5 V. 
     Note that in this embodiment, V MLH  is 0 V and V MLL  is −1 V. Furthermore, V MLHR  is −2.5 V and V MLLR  is −3.5 V. The threshold voltage of the transistor  210  (indicated by TR in  FIGS.  3 A and  3 B ) is 2 V. 
     The insulated storage gate  212  is electrically floating (in a floating state). For that reason, when the potential of the first wiring  201  (the word line WL), the third wiring  203  (the read signal line RL), or the fourth wiring  204  (the bit line BL) is varied, the potential of the insulated storage gate  212  is varied accordingly. 
     The amount of the change in the potential of the insulted storage gate  212  is determined by the ratio of a capacitance (hereinafter “C CM ”) generated between the control gate  211  to which the first wiring  201  (the word line WL) is electrically connected and the storage gate  212  to a capacitance (hereinafter “C MS ”) generated between the storage gate  212  and a channel formation region of the transistor  210 . 
     For example, in the case where the value of C CM  is sufficiently larger than that of C MS , when the potential of the channel formation region is fixed and the potential of the control gate  211  is changed, the potential of the insulated storage gate  212  is changed by approximately the same amount as the change in the potential of the control gate  211 . 
     In the case where the value of C CM  is sufficiently smaller than that of C MS , when the potential of the channel formation region is fixed and the potential of the control gate  211  is changed, the potential of the insulated storage gate  212  is hardly changed. 
     In the case where the value of C CM  is the same as that of C MS , when the potential of the channel formation region is fixed and the potential of the control gate  211  is changed, the potential of the insulated storage gate  212  is changed by only half the amount of the change in the potential of the control gate  211 . 
     The potential of the channel formation region of the transistor  210  can be fixed by fixing the potentials of the third wiring  203  (the read signal line RL) and the fourth wiring  204  (the bit line BL) which are connected to the transistor  210 . 
     For example, in order to prevent the potential of the insulated storage gate  212  in a floating state from being affected by the change in the potentials of the third wiring  203  (the read signal line RL) and the fourth wiring  204  (the bit line BL) as much as possible, the value of C CM  is preferably 2 times or more, further preferably 5 times or more, still preferably 10 times or more that of C MS . 
     Further, in order to minimize effects of the change in potentials of all the wirings connected to the transistor  210  on the potential of the insulated storage gate  212  in a floating state, the value of C CM  is preferably 0.5 times or more and less than 2 times, further preferably 0.7 times or more and less than 1.5 times that of C MS . 
     In this embodiment, the case where the value of C CM  is the same as that of C MS  is described. 
     First, a write (rewrite) operation of data into the memory cell  200  will be described. Here, an operation for holding the high-level potential V MLH  in the storage gate  212  is described. First, as a first operation, the high-level potential V WLH  (4 V) is applied to the first wiring  201  (the word line WL) connected to the memory cell  200  that is selected to be subjected to data writing, and the high-level potential V DLH  (2 V) is applied to the second wiring  202  (the data line DL). 
     When the high-level potential V WLH  is applied to the first wiring  201  (the word line WL), the potential of the control gate  211  becomes the high-level potential V WLH . Accordingly, the potential difference between the second wiring  202  (the data line DL) and the control gate  211 , with the second wiring  202  (the data line DL) as a reference, becomes 2 V. In other words, the potential difference is more than V thos  (1 V); thus, the storage gate  212  becomes a conductor and the potential of the second wiring  202  (the data line DL) is supplied to the storage gate  212 . That is, the potential of the storage gate  212  becomes the high-level potential V DLH  (2 V). 
     Then, as a second operation, the low-level potential V WLL  (−1 V) is applied to the first wiring  201  (the word line WL). At this time, the potential of the second wiring  202  (the data line DL) is set to remain at the high-level potential V DLH . When the low-level potential V WLL  is applied to the first wiring  201  (the word line WL), the potential of the control gate  211  becomes the low-level potential V WLL . Accordingly, the potential difference between the second wiring  202  (the data line DL) and the control gate  211 , with the second wiring  202  (the data line DL) as a reference, becomes −3 V; thus, the storage gate  212  becomes an insulator. 
     Since V thos  is 1 V, the storage gate  212  serves as a conductor and is supplied with the high-level potential V DLH  when the difference between the potential of the first wiring  201  (the potential of the control gate  211 ) and the potential of the second wiring  202  (the data line DL) is more than 1 V (i.e., during the first operation). When the difference between the potential of the first wiring  201  (the potential of the control gate  211 ) and the potential of the second wiring  202  (the data line DL) becomes less than 1 V by the second operation, the storage gate  212  becomes an insulator to exist in a floating state. Consequently, the potential of the storage gate  212  is changed by the influence of the change in the potential of the first wiring  201  (the word line WL) corresponding to the ratio of C CM  to C MS . 
     Formula 1 represents the potential V MLH  of the storage gate  212  after the high-level potential V DLH  is supplied to the storage gate  212  and the potential of the control gate  211  is changed from the high-level potential V WLH  to the low-level potential V WLL . 
     
       
         
           
             
               
                 
                   
                     V 
                     MLH 
                   
                   = 
                   
                     
                       V 
                       DLH 
                     
                     - 
                     
                       
                         
                           C 
                           CM 
                         
                         
                           
                             C 
                             CM 
                           
                           + 
                           
                             C 
                             MS 
                           
                         
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             V 
                             DLH 
                           
                           + 
                           
                             V 
                             thos 
                           
                           - 
                           
                             V 
                             WLL 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                         
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In order to hold the low-level potential V MLL  in the storage gate  212 , the low-level potential V DLL  is supplied to the second wiring  202  (the data line DL) and the same write operation as above is performed. Formula 2 represents the potential V MLL  of the storage gate  212  after the low-level potential V DLL  is supplied to the storage gate  212  and the potential of the control gate  211  is changed from the high-level potential V WLH  to the low-level potential V WLL . 
     
       
         
           
             
               
                 
                   
                     V 
                     MLL 
                   
                   = 
                   
                     
                       V 
                       DLL 
                     
                     - 
                     
                       
                         
                           C 
                           CM 
                         
                         
                           
                             C 
                             CM 
                           
                           + 
                           
                             C 
                             MS 
                           
                         
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             V 
                             DLL 
                           
                           + 
                           
                             V 
                             thos 
                           
                           - 
                           
                             V 
                             WLL 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                         
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     That is, the high-level potential V MLH  is 0 V and the low-level potential V MLL  is −1 V in this embodiment. 
     When the storage gate  212  serves as an insulator, electric charge in the storage gate  212  cannot move. For that reason, the potential of the storage gate  212  can be kept at the high-level potential V MLH  or the low-level potential V MLL  even when the potential of the second wiring  202  connected to the storage gate  212  is changed. In such a manner, the high-level potential V DLH  written into the storage gate  212  is stored as the high-level potential V MLH , and the low-level potential V DLL  is stored as the low-level potential V MLL . 
     Note that in the write operation, the potential of the third wiring  203  (the read signal line RL) is set at V RLH  (0 V) and the potential the fourth wiring  204  (the bit line BL) at V BLH  (0 V). 
     It is preferable that both the high-level potential V MLH  and the low-level potential V MLL  which are stored in the storage gate  212  be a potential at which the transistor  210  is off. In particular, in a read operation in the case where a plurality of memory cells  200  are connected, a malfunction of a memory cell that is not subjected to data reading is prevented and data can be accurately read, resulting in improving the reliability of the semiconductor device. 
     Next, an operation of reading data stored in the memory cell  200  will be described.  FIG.  3 B  is a timing chart of the operation in the read mode. Here, an operation in the case where the high-level potential V MLH  is held in the storage gate  212  is described. 
     First, as a first operation, electric charge is supplied to the fourth wiring  204  (the bit line BL) so that the potential of the fourth wiring  204  (the bit line BL) is set at the high-level potential V BLH  (this operation is called precharge). At this time, the potential of the first wiring  201  (the word line WL) is set to remain at the low-level potential V WLL . There is no particular limitation on the potential of the second wiring  202  (the data line DL); here, the second wiring  202  has the high-level potential V DLH . Note that the high-level potential V BLH  and the low-level potential V RLL  are different from each other. 
     Then, as a second operation, the potential of the third wiring  203  (the read signal line RL) is set at the low-level potential V RLL . At this time, since the storage gate  212  is in a floating state, the potential of the storage gate  212  is affected by the change in the potential of the third wiring  203  (the read signal line RL) corresponding to the ratio of C CM  to C MS . 
     In the case where the storage gate  212  stores the high-level potential V MLH , when the potential of the third wiring  203  (the read signal line RL) is changed from the high-level potential V RLH  to the low-level potential V RLL , the potential V MLHR  of the storage gate  212  can be represented by Formula 3. 
     
       
         
           
             
               
                 
                   
                     V 
                     MLHR 
                   
                   = 
                   
                     
                       V 
                       MLH 
                     
                     - 
                     
                       
                         
                           C 
                           CM 
                         
                         
                           
                             C 
                             CM 
                           
                           + 
                           
                             C 
                             MS 
                           
                         
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             V 
                             RLH 
                           
                           - 
                           
                             V 
                             RLL 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                         
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     In other words, the potential V MLHR  is −2.5 V in this embodiment. Since the low-level potential V RLL  is −5 V at this time, the gate-source voltage of the transistor  210  is obtained in the following manner: V MLHR −V RLL =−2.5 V−(−5 V)=2.5 V. That is, the gate-source voltage is larger than the threshold voltage (2 V) of the transistor  210 ; thus, the transistor  210  is turned on. 
     When the transistor  210  is turned on, the potential of the third wiring  203  (the read signal line RL) is supplied to the fourth wiring  204  (the bit line BL) through the transistor  210 , so that the potential of the fourth wiring  204  (the bit line BL) is changed. 
     In the case where the storage gate  212  stores the low-level potential V MLL , when the potential of the third wiring  203  (the read signal line RL) is changed from the high-level potential V RLH  to the low-level potential V RLL , the potential V MLLR  of the storage gate  212  can be represented by Formula 4. 
     
       
         
           
             
               
                 
                   
                     V 
                     MLLR 
                   
                   = 
                   
                     
                       V 
                       MLL 
                     
                     - 
                     
                       
                         
                           C 
                           CM 
                         
                         
                           
                             C 
                             CM 
                           
                           + 
                           
                             C 
                             MS 
                           
                         
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             V 
                             RLH 
                           
                           - 
                           
                             V 
                             RLL 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                         
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     In other words, the potential V MLLR  is −3.5 V in this embodiment. Since the low-level potential V RLL  is −5 V at this time, the gate-source voltage of the transistor  210  is obtained in the following manner: V MLLR  V RLL =−3.5 V−(−5 V)=1.5 V. That is, the gate-source voltage cannot exceed the threshold voltage (2 V) of the transistor  210 . In that case, the transistor  210  remains off, and the potential of the fourth wiring  204  (the bit line BL) is not changed. 
     In such a manner, data stored in the storage gate  212  can be read by detecting a change in the potential of the fourth wiring  204  (the bit line BL) caused by changing the potential of the third wiring  203  (the read signal line RL) to the low-level potential V RLL . 
     Data (electric charge) held in the storage gate  212  is retained until the data is rewritten with new data in a write mode. The insulated oxide semiconductor has high resistivity and electric charge hardly moves therein; thus, the potential of the storage gate  212  can be retained for an extremely long time. 
     Note that in a flash memory, it is necessary to keep a proper distance between cells in order to prevent the potential of a control gate from adversely affecting a floating gate of an adjacent cell. This is one of factors inhibiting higher integration of the semiconductor device. The factor is attributed to the following basic principle of a flash memory: tunneling current is generated by application of a high electric field. 
     Further, because of the above principle of a flash memory, deterioration of an insulating film proceeds, which causes a limitation on the write cycles (approximately 10000 cycles). 
     The semiconductor device according to the invention disclosed herein does not use the above principle of charge injection by tunneling current. That is, unlike a flash memory, a high electric field for charge injection is not necessary. Consequently, it is not necessary to consider an influence of a high electric field from a control gate on an adjacent cell, which facilitates high integration. 
     Further, since charge injection by tunneling current is not used, there is no cause for deterioration of a memory cell. In other words, the semiconductor device according to the disclosed invention has higher durability and reliability than a flash memory. 
     In addition, the semiconductor device according to the disclosed invention has advantages over a flash memory in that a high electric field is not necessary and a large peripheral circuit (such as a booster circuit) is not necessary. 
     Note that an n-channel transistor in which electrons are majority carriers is used in the above description; it is needless to say that a p-channel transistor in which holes are majority carriers can be used instead of the n-channel transistor. In the case of using p-channel transistors, potentials supplied to the wirings are set on the basis of the above-described operation principle. 
       FIG.  1 B  illustrates an example of a circuit diagram of a semiconductor device with a memory capacity of m×n bits, including the semiconductor devices (the memory cells) illustrated in  FIG.  1 A .  FIG.  1 B  is a circuit diagram of a NOR semiconductor device in which memory cells  1200  are connected in parallel. 
     The semiconductor device illustrated in  FIG.  1 B  includes a memory cell array and peripheral circuits such as a first driver circuit  1221 , a second driver circuit  1222 , a third driver circuit  1223 , and a fourth driver circuit  1224 . The memory cell array includes m word lines WL, m read signal lines RL, n bit lines BL, n data lines DL, and a plurality of memory cells  1200  that are arranged in a matrix of m rows (in the vertical direction) and n columns (in the horizontal direction) (m and n are natural numbers). Here, the structure illustrated in  FIG.  1 A  is applied to the memory cell  1200 . 
     Each of the memory cells  1200  includes a transistor  1210 . The transistor  1210  has a control gate  1211  and a storage gate  1212 . The control gate  1211  is electrically connected to the word line WL. The storage gate  1212  is electrically connected to the data line DL. One of a source and a drain of the transistor  1210  is electrically connected to the read signal line RL. The other of the source and the drain of the transistor  1210  is electrically connected to the bit line BL. 
     A memory cell  1200  ( i,j ) of an i-th row and a j-th column (i is an integer of 1 to m and j is an integer of 1 to n) is electrically connected to a read signal line RL_i, a bit line BL_j, and a word signal line WL_i. 
     The word lines WL are electrically connected to the first driver circuit  1221 . The data lines DL are electrically connected to the second driver circuit  1222 . The read signal lines RL are electrically connected to the third driver circuit  1223 . The bit lines BL are electrically connected to the fourth driver circuit  1224 . Note that the first driver circuit  1221 , the second driver circuit  1222 , the third driver circuit  1223 , and the fourth driver circuit  1224  are separately provided here; alternatively, a decoder having one or a plurality of their functions may be used. 
     Although data can be written into the memory cell  1200  with the above-described write operation, when V WLH  is applied to a given word line WL, the storage gates  1212  of the all the transistors  1210  connected to the word line WL become conductors. Thus, when data is sequentially written into the memory cells  1200  one by one, data in the memory cell in which the write operation has finished might be changed. For that reason, in the write operation, it is preferable that data be concurrently written into all the memory cells connected to a selected word line WL. 
     Data can be read from the memory cell  1200  with the above-described read operation. Since data stored in the memory cell  1200  is a potential at which the transistor  1210  is off, data can be read from given memory cells  1200  one by one. Moreover, it is possible to concurrently read data in all the memory cells  1200  connected to the read signal line RL. 
     Note that an n-channel transistor (n-type transistor) in which electrons are majority carriers is used in the above description; it is needless to say that a p-channel transistor in which holes are majority carriers can be used instead of the n-channel transistor. In the case of using p-channel transistors, potentials supplied to the wirings are set on the basis of the above operation principle. 
     The semiconductor device disclosed in this embodiment does not include a capacitor that is needed for a DRAM in terms of the operation principle; therefore, the area per unit memory cell can be reduced and integration of the memory cells can be increased. For example, given that the minimum feature size is F, the area of a memory cell can be 15 F 2  to 25 F 2 . 
     In addition, in the semiconductor device disclosed in this embodiment, stored electric charge hardly moves because data is stored by making an oxide semiconductor an insulator. Thus, a refresh operation needed for a conventional DRAM can be unnecessary or performed much less often (e.g., about once a month or a year), so that power consumption of the semiconductor device can be sufficiently reduced. 
     Further, in the semiconductor device disclosed in this embodiment, data can be directly rewritten by rewriting of new data to a memory cell. For that reason, an erase operation which is necessary for a flash memory or the like is not needed, so that a reduction in operation speed due to the erase operation can be prevented. That is, high-speed operation of the semiconductor device can be realized. Moreover, a high voltage necessary for a conventional floating gate transistor to write and erase data is unnecessary; thus, power consumption of the semiconductor device can be further reduced. 
       FIG.  4    schematically illustrates a reading circuit for reading data stored in a memory cell. The reading circuit includes a transistor and a sense amplifier circuit. 
     At the time of reading data, a terminal A is connected to a bit line BL to which a memory cell subjected to data reading is connected. Furthermore, a bias potential Vbias is applied to a gate electrode of the transistor so that the potential of the terminal A is controlled. 
     The sense amplifier circuit outputs high data when the potential of the terminal A is higher than a reference potential Vref (e.g., 0 V) and outputs low data when the potential of the terminal A is lower than the reference potential Vref. Specifically, first, the transistor is turned on, and the bit line BL connected to the terminal A is precharged to the potential V BLH . Next, the memory cell subjected to data reading is set to a read mode, and the potential of the bit line BL connected to the terminal A is compared with the reference potential Vref. Accordingly, high data or low data is output as output data in accordance with the data stored in the memory cell. 
     By using the reading circuit in the above manner, data stored in the memory cell can be read. Note that the reading circuit of this embodiment is an example; another known circuit may be used. 
     The methods and structures described in this embodiment can be combined as appropriate with any of the methods and structures described in the other embodiments. 
     Embodiment 2 
     In this embodiment, an example of a structure and a manufacturing method of the semiconductor device described in Embodiment 1 will be described with reference to  FIGS.  5 A to  5 C ,  FIGS.  6 A to  6 D , and  FIGS.  7 A and  7 B . 
     &lt;Cross-Sectional Structure and Planar Structure of Semiconductor Device&gt; 
       FIGS.  5 A to  5 C  illustrate an example of a transistor  150  that can be used as a storage element.  FIG.  5 A  illustrates a planar structure of the transistor  150 .  FIG.  5 B  illustrates a cross section along X 1 -X 2  in  FIG.  5 A .  FIG.  5 C  is an enlarged view of a portion  180  in  FIG.  5 B . 
     In the transistor  150 , a base insulating layer  110  is provided over a substrate  100 . A control gate  101  is provided over the base insulating layer  110 . A first gate insulating layer  102  is provided over the control gate  101 . An electrode  103  is provided over the first gate insulating layer  102 . A storage gate  104  is provided in contact with the first gate insulating layer  102  and the electrode  103 . A second gate insulating layer  105  is provided over the storage gate  104 . A semiconductor layer  106  is provided over the second gate insulating layer  105 . A source electrode  107   a  and a drain electrode  107   b  are provided over the semiconductor layer  106 . An insulating layer  108  is provided over the semiconductor layer  106 , the source electrode  107   a , and the drain electrode  107   b . A protective insulating layer  109  is provided over the insulating layer  108 . The transistor  150  has a bottom-gate structure and an inverted staggered structure. 
     The electrode  103  is provided in contact with part of the storage gate  104  and supplies data to be stored to the storage gate  104 . The storage gate  104  is sandwiched between the first gate insulating layer  102  and the second gate insulating layer  105 . Moreover, the storage gate  104  is in contact with the first gate insulating layer  102  and the second gate insulating layer  105  at least in a region overlapping with a channel formation region of the semiconductor layer  106  (i.e., a region in the semiconductor layer  106 , placed between the source electrode  107   a  and the drain electrode  107   b ). The electrode  103  and the storage gate  104  are connected to each other at a position which does not overlap with the channel formation region of the semiconductor layer  106 . 
     The control gate  101  is positioned to overlap with the storage gate  104  and the channel formation region of the semiconductor layer  106 . The storage gate  104  is placed between the control gate  101  and the semiconductor layer  106 . 
     An edge  181  illustrated in  FIG.  5 C  is an edge of the electrode  103  in a region where the electrode  103  is in contact with the storage gate  104 . The control gate  101  is provided to overlap with the storage gate  104  and further overlaps with at least a region from the channel formation region of the semiconductor layer  106  to the edge  181 . With the above structure, when the storage gate  104  is made to be a conductor by applying a voltage to the control gate  101 , the region which overlaps with the semiconductor layer  106  and the region from the channel formation region to the portion in contact with the electrode  103  can be made to be a conductor, and data can be accurately written into the storage gate  104 . The control gate  101  is preferably provided to overlap with the electrode  103  beyond the edge  181 . 
     &lt;Method for Manufacturing Semiconductor Device&gt; 
     Next, an example of a method for manufacturing the transistor  150  will be described with reference to  FIGS.  6 A to  6 D . Note that unless otherwise specified, a photolithography process in this specification includes a step of forming a resist mask, a step of etching a conductive layer or an insulating layer, and a step of separating the resist mask. 
     First, the base insulating layer  110  and a conductive layer are formed over the substrate  100 . Then, through a first photolithography process, the conductive layer (as well as a wiring formed from the conductive layer) is partly removed by etching, and the control gate  101  is formed. Note that a resist mask may be formed by an inkjet method. Formation of a resist mask by an inkjet method needs no photomask; thus, manufacturing costs can be reduced. 
     There is no particular limitation on a substrate that can be used as the substrate  100 , and a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, a crystallized glass substrate, or the like can be used. 
     Alternatively, a flexible substrate may be used as the substrate  100 . In the case of using a flexible substrate, a transistor may be directly formed over a flexible substrate; alternatively, a transistor may be formed over another substrate and then separated from the substrate and transferred to a flexible substrate. Note that in order to separate the transistor from the substrate and transfer it to the flexible substrate, a separation layer is preferably provided between the substrate and the transistor. 
     The base insulating layer  110  can be formed with a single-layer structure or a stacked structure using at least one of the following insulating layers: an aluminum nitride layer, an aluminum oxide layer, an aluminum oxynitride layer, an aluminum nitride oxide layer, a silicon nitride layer, a silicon oxide layer, a silicon nitride oxide layer, and a silicon oxynitride layer. The base insulating layer  110  has a function of preventing diffusion of an impurity element from the substrate  100 . 
     When a halogen element such as chlorine or fluorine is contained in the base insulating layer  110 , a function of preventing diffusion of an impurity element from the substrate  100  can be further improved. The peak of the concentration of a halogen element contained in the base insulating layer  110 , measured by secondary ion mass spectrometry (SIMS), is preferably higher than or equal to 1×10 15 /cm 3  and lower than or equal to 1×10 20 /cm 3 . 
     The base insulating layer  110  may be formed using gallium oxide. Alternatively, the base insulating layer  110  may have a stacked structure of a gallium oxide layer and any of the above insulating layers. Gallium oxide is a material that is hardly charged, and thus can suppress variation in threshold voltage due to charge build-up of the insulating layer. Note that the base insulating layer  110  is not necessarily provided. 
     The control gate  101  can be formed with a single-layer structure or a stacked structure using a metal material such as molybdenum (Mo), titanium (Ti), tantalum (Ta), tungsten (W), aluminum (Al), copper (Cu), chromium (Cr), neodymium (Nd), scandium (Sc), or magnesium (Mg) or an alloy material containing any of these materials as a main component. 
     Then, the first gate insulating layer  102  is formed over the control gate  101  (see  FIG.  6 A ). The first gate insulating layer  102  can be formed using silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, tantalum oxide, gallium oxide, yttrium oxide, hafnium oxide, hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)), hafnium silicate to which nitrogen is added (HfSi x O y N z  (x&gt;0, y&gt;0, z&gt;0)), hafnium aluminate to which nitrogen is added (HfAl x O y N z  (x&gt;0, y&gt;0, z&gt;0)), or the like by plasma CVD, sputtering, or the like. The first gate insulating layer  102  is not limited to a single layer and may be a stack of different layers. For example, the first gate insulating layer  102  may be formed in the following manner: a silicon nitride layer (SiN y  (y&gt;0)) is formed by plasma CVD as a gate insulating layer A and a silicon oxide layer (SiO x  (x&gt;0)) is stacked over the gate insulating layer A as a gate insulating layer B. 
     Other than sputtering and plasma CVD, the first gate insulating layer  102  can be formed by, for example, high-density plasma CVD using microwaves (e.g., a frequency of 2.45 GHz). 
     In particular, the first gate insulating layer  102  is preferably formed using an insulating material containing the same kind of component as the oxide semiconductor that is formed later. Such a material is compatible with the oxide semiconductor, and the use of such a material for the first gate insulating layer  102  can keep the interface state between the oxide semiconductor and the first gate insulating layer  102  favorable. Here, “the same kind of component as the oxide semiconductor” means one or more of elements selected from constituent elements of the oxide semiconductor. For example, in the case where the oxide semiconductor is formed using an In—Ga—Zn-based oxide semiconductor material, gallium oxide is given as an insulating material containing the same kind of component as the oxide semiconductor. 
     In the case of employing a stacked structure, the first gate insulating layer  102  may have a stacked structure of a film formed using an insulating material containing the same kind of component as the oxide semiconductor and a film formed using a material different from that of the film. 
     Next, a conductive layer is formed over the first gate insulating layer  102 . Then, through a second photolithography process, the conductive layer is partly removed by etching, and the electrode  103  (as well as a wiring formed from the same layer as the electrode  103 ) is formed. Note that a resist mask may be formed by an inkjet method. Formation of a resist mask by an inkjet method needs no photomask; thus, manufacturing costs can be reduced. 
     For the conductive layer used for the electrode  103 , a metal containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, W, Nd, Sc, and Mg, a metal nitride containing any of the above elements as a component (e.g., titanium nitride, molybdenum nitride, or tungsten nitride), or the like can be used, for example. The electrode  103  may have a structure in which a refractory metal layer of Ti, Mo, W, or the like or a metal nitride layer of any of these elements (e.g., a titanium nitride layer, a molybdenum nitride layer, or a tungsten nitride layer) is stacked on one or both of a bottom surface and a top surface of a metal layer of Al, Cu, or the like. 
     Next, an oxide semiconductor layer with a thickness of 2 nm to 200 nm, preferably 5 nm to 30 nm is formed over the electrode  103  and the first gate insulating layer  102 . 
     In order that hydrogen, a hydroxyl group, and moisture may be contained in the oxide semiconductor layer as little as possible, it is preferable that the substrate  100  where the electrode  103  and the first gate insulating layer  102  are formed be subjected to preheating in a preheating chamber of a sputtering apparatus as pretreatment for forming the oxide semiconductor layer so that impurities such as hydrogen and moisture adsorbed on the substrate  100  are removed and exhausted. As an exhaustion unit provided in the preheating chamber, a cryopump is preferably employed. Note that this preheating treatment can be omitted. Further, before the insulating layer  108  is formed, this preheating may be similarly performed on the substrate  100  over which components up to and including the source electrode  107   a  and the drain electrode  107   b  are formed. 
     As an oxide semiconductor used for the oxide semiconductor layer, any of the following oxide semiconductors can be used, for example: an oxide of four metal elements, such as an In—Sn—Ga—Zn—O-based oxide semiconductor; an oxide of three metal elements, such as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, a Sn—Al—Zn—O-based oxide semiconductor, an In—Hf—Zn—O-based oxide semiconductor, an In—La—Zn—O-based oxide semiconductor, an In—Ce—Zn—O-based oxide semiconductor, an In—Pr—Zn—O-based oxide semiconductor, an In—Nb—Zn—O-based oxide semiconductor, an In—Pm—Zn—O-based oxide semiconductor, an In—Sm—Zn—O-based oxide semiconductor, an In—Eu—Zn—O-based oxide semiconductor, an In—Gd—Zn—O-based oxide semiconductor, an In—Tb—Zn—O-based oxide semiconductor, an In—Dy—Zn—O-based oxide semiconductor, an In—Ho—Zn—O-based oxide semiconductor, an In—Er—Zn—O-based oxide semiconductor, an In—Tm—Zn—O-based oxide semiconductor, an In—Yb—Zn—O-based oxide semiconductor, and an In—Lu—Zn—O-based oxide semiconductor; an oxide of two metal elements, such as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxide semiconductor, and an In—Ga—O-based oxide semiconductor; an In—O-based oxide semiconductor; a Sn—O-based oxide semiconductor; and a Zn—O-based oxide semiconductor. Further, SiO 2  may be contained in the above oxide semiconductor. 
     The oxide semiconductor layer preferably includes In, more preferably In and Ga. Dehydration or dehydrogenation is effective in order to obtain an i-type (intrinsic) oxide semiconductor layer. 
     Here, for example, an In—Ga—Zn—O-based oxide semiconductor refers to an oxide containing indium (In), gallium (Ga), and zinc (Zn), and there is no limitation on the composition ratio thereof. The In—Ga—Zn—O-based oxide semiconductor may contain an element other than In, Ga, and Zn. 
     For the oxide semiconductor layer, a thin film expressed by a chemical formula of InMO 3 (ZnO) m  (m&gt;0) can be used. Here, M represents one or more of metal elements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, or Ga and Co. 
     In this embodiment, the oxide semiconductor layer is formed by sputtering using an In—Ga—Zn—O-based oxide target. The oxide semiconductor layer can be formed by sputtering under a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas and oxygen. 
     As a target for forming the oxide semiconductor layer by sputtering, for example, an oxide target having a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [molar ratio] is used to form an In—Ga—Zn—O layer. Without limitation to the material and the composition of the target, an oxide target having a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:2 [molar ratio] may be used, for example. 
     In the case where an In—Zn—O-based material is used for the oxide semiconductor layer, a target used has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In 2 O 3 :ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In 2 O 3 :ZnO=10:1 to 1:2 in a molar ratio), further preferably In:Zn=15:1 to 1.5:1 (In 2 O 3 :ZnO=15:2 to 3:4 in a molar ratio). For example, when a target used for forming an In—Zn—O-based oxide semiconductor layer has an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z&gt;1.5X+Y is satisfied. 
     The filling rate of the oxide target is 90% to 100%, preferably 95% to 99.9%. With the use of a metal oxide target with a high filling rate, a dense oxide semiconductor layer can be deposited. 
     It is preferable that a high-purity gas from which impurities such as hydrogen, water, a compound having a hydroxyl group, and hydride are removed be used as a sputtering gas for forming the oxide semiconductor layer. 
     When the oxide semiconductor layer is formed, the substrate is held in a deposition chamber kept under a reduced pressure and the substrate temperature is set at 100° C. to 600° C., preferably 300° C. to 500° C. By heating the substrate during deposition, the impurity concentration in the oxide semiconductor layer can be reduced. In addition, damage by sputtering is reduced. Then, a sputtering gas from which hydrogen and moisture are removed is introduced into the deposition chamber while moisture remaining therein is removed, and the oxide semiconductor layer is formed with the use of the above target. 
     In order to remove moisture remaining in the deposition chamber, an entrapment vacuum pump such as a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump provided with a cold trap may be used. In the deposition chamber evacuated with a cryopump, a hydrogen atom, a compound containing a hydrogen atom, such as water (H 2 O) (preferably, also a compound containing a carbon atom), and the like are evacuated, so that the concentration of impurities in the oxide semiconductor layer formed in the deposition chamber can be reduced. 
     An example of the deposition conditions is as follows: the distance between the substrate and the target is 100 mm, the pressure is 0.6 Pa, the power of the DC power source is 0.5 kW, and the atmosphere is an oxygen atmosphere (the flow rate of oxygen is 100%). Note that it is preferable to use a pulsed direct-current power source because powder substances (also referred to as particles or dust) generated during deposition can be reduced and the film thickness can be uniform. 
     Next, first heat treatment is performed. With the first heat treatment, excessive hydrogen (including water and a hydroxyl group) in the oxide semiconductor layer is removed (dehydration or dehydrogenation) and the structure of the oxide semiconductor layer is ordered, so that defect levels in the energy gap can be reduced. Moreover, defects caused at the interface between the oxide semiconductor layer and the insulating layer in contact with the oxide semiconductor layer can be reduced. 
     The first heat treatment is performed at a temperature in the range of 250° C. to 750° C. or at a temperature higher than or equal to 400° C. and lower than the strain point of the substrate in a reduced pressure atmosphere, an inert gas atmosphere such as a nitrogen atmosphere or a rare gas atmosphere, an oxygen gas atmosphere, or an ultra-dry air (with a moisture content of 20 ppm (equivalent to a dew point of −55° C.) or lower, preferably 1 ppm or lower, further preferably 10 ppb or lower when measured with a dew-point meter using cavity ring-down laser spectroscopy (CRDS)). For example, the substrate is put in an electric furnace which is a kind of heat treatment apparatus, and the oxide semiconductor layer is subjected to heat treatment at 450° C. for one hour in a nitrogen atmosphere. 
     Note that a heat treatment apparatus is not limited to an electrical furnace, and may include a device for heating an object to be processed by heat conduction or heat radiation from a heating element such as a resistance heating element. For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus is an apparatus for heating an object by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressure sodium lamp, or a high-pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the high-temperature gas, an inert gas which does not react with an object by heat treatment, such as nitrogen or a rare gas like argon, is used. 
     For example, as the first heat treatment, GRTA may be performed in the following manner: the substrate is moved into an inert gas heated to a temperature as high as 650° C. to 700° C., heated for several minutes, and moved out of the inert gas heated to the high temperature. 
     When the heat treatment is performed in an atmosphere of an inert gas such as nitrogen or a rare gas, oxygen, or ultra-dry air, it is preferable that the atmosphere do not contain water, hydrogen, and the like. It is also preferable that the purity of nitrogen, oxygen, or the rare gas which is introduced into a heat treatment apparatus be set to 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (i.e., the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower). 
     Next, through a third photolithography process, the oxide semiconductor layer is partly removed by etching, and the storage gate  104  is formed (see  FIG.  6 B ). Note that a resist mask for forming the storage gate  104  may be formed by an inkjet method, in which case manufacturing costs can be reduced because a photomask is not used. 
     In the case where a contact hole is formed in the first gate insulating layer  102 , the contact hole can be formed at the same time as processing of the oxide semiconductor layer. 
     Note that the etching of the oxide semiconductor layer may be dry etching, wet etching, or both dry etching and wet etching. As an etchant used for wet etching of the oxide semiconductor layer, a mixed solution of phosphoric acid, acetic acid, and nitric acid can be used, for example. Alternatively, ITO- 07 N (produced by Kanto Chemical Co., Inc.) may be used. 
     After the resist mask is removed, oxygen may be introduced into the storage gate  104 . Oxygen can be introduced by oxygen plasma doping. Specifically, oxygen is turned into plasma with the use of radio-frequency (RF) power, and oxygen radicals and/or oxygen ions are introduced into the oxide semiconductor layer over the substrate. At this time, it is preferable to apply a bias to the substrate where the storage gate  104  is formed. By increasing the bias applied to the substrate, oxygen can be introduced more deeply. Oxygen may be introduced by ion implantation. 
     Oxygen for the doping (an oxygen radical, an oxygen atom, and/or an oxygen ion) may be supplied from a plasma generation apparatus with the use of a gas containing oxygen or from an ozone generation apparatus. Specifically, for example, oxygen can be generated with an apparatus for etching treatment on a semiconductor device, an apparatus for ashing on a mask, or the like to process the storage gate  104 . 
     By introduction of oxygen into the storage gate  104 , the oxide semiconductor layer contains excessive oxygen. When oxygen is introduced to the oxide semiconductor layer, a hydrogen atom is abstracted from an M-H bond to form an M-OH group. 
     That is, by the introduction of oxygen, a bond between a metal and hydrogen included in the oxide semiconductor or a bond between the oxygen and hydrogen of a hydroxyl group on the metal is cleaved to produce water. In particular, oxygen having a unpaired electron easily reacts with hydrogen remaining in the oxide semiconductor to produce water. Consequently, hydrogen or a hydroxyl group which is an impurity can be easily eliminated as water by heat treatment performed later. 
     After the introduction of oxygen to the storage gate  104 , second heat treatment is performed (preferably at 200° C. to 600° C., for example, at 250° C. to 550° C.). For example, the second heat treatment is performed at 450° C. for one hour in a nitrogen atmosphere. It is preferable that the above atmosphere do not contain water, hydrogen, or the like. 
     Through the above steps, dehydration or dehydrogenation of the oxide semiconductor layer can be performed with the introduction of oxygen and the heat treatment, whereby remaining impurities including a hydrogen atom (e.g., hydrogen, water, a hydroxyl group, or a hydride (also referred to as a hydrogen compound)), which cannot be removed completely in the first heat treatment, can be removed from the storage gate  104 . In addition, defects generated at the interface between the storage gate  104  and the insulating layer in contact with the storage gate  104  can be reduced. In such a manner, the oxide semiconductor used for the storage gate  104  can be purified to be an electrically intrinsic oxide semiconductor. 
     Then, the second gate insulating layer  105  is formed to cover the storage gate  104  and the electrode  103  (see  FIG.  6 C ). The second gate insulating layer  105  can be formed using a material and a method which are similar to those of the first gate insulating layer  102 . 
     Next, a semiconductor layer is formed over the second gate insulating layer  105 . Then, through a fourth photolithography process, the semiconductor layer is partly removed by etching, thereby forming the island-shaped semiconductor layer  106  in which a channel of the transistor  150  is formed. For the semiconductor layer  106 , a single crystal semiconductor, a polycrystalline semiconductor, a microcrystalline semiconductor, an amorphous semiconductor, or the like can be used. Examples of a semiconductor material are silicon, germanium, silicon germanium, silicon carbide, and gallium arsenide. The transistor  150  including such a semiconductor material can operate at sufficiently high speed, so that stored data can be read at high speed, for example. In other words, high-speed operation of the semiconductor device can be realized. Alternatively, an organic semiconductor material or the like may be used. 
     Alternatively, an oxide semiconductor can be used for the semiconductor layer  106 . The oxide semiconductor can be formed using a material and a method which are similar to those of the storage gate  104 . The oxide semiconductor is preferably purified by sufficient removal of impurities such as hydrogen or sufficient supply of oxygen. Specifically, the concentration of hydrogen in the oxide semiconductor layer is 5×10 19  atoms/cm 3  or lower, preferably 5×10 18  atoms/cm 3  or lower, further preferably 5×10 17  atoms/cm 3  or lower, for example. Note that the above hydrogen concentration of the oxide semiconductor layer was measured by SIMS (secondary ion mass spectrometry). 
     The oxide semiconductor purified by a sufficient reduction in hydrogen concentration, in which defect levels in the energy gap due to oxygen deficiency are reduced as a result of sufficient supply of oxygen, has a carrier concentration of less than 1×10 12 /cm 3 , preferably less than 1×10 11 /cm 3 , further preferably less than 1.45×10 10 /cm 3 . For example, the off-state current (per unit channel width (1 μm) here) at room temperature (25° C.) is 100 zA/μm (1 zA (zeptoampere) is 1×10 −21  A) or lower, preferably 10 zA/μm or lower. The off-state current at 85° C. is 100 zA/μm (1×10 −19  A/μm) or lower, preferably 10 zA/μm (1×10 −20  A/μm) or lower. The transistor  150  with extremely low off-state current characteristics can be obtained with the use of such an oxide semiconductor that is made to be intrinsic (i-type) or substantially intrinsic. 
     Next, a conductive layer serving as the source electrode and the drain electrode (as well as a wiring formed by the conductive layer) is formed over the second gate insulating layer  105  and the semiconductor layer  106 . The conductive layer used for the source and drain electrodes can be formed using a material and a method which are similar to those of the electrode  103 . Further, the conductive layer used for the source and drain electrodes may be formed using a conductive metal oxide. Examples of the conductive metal oxide are indium oxide (In 2 O 3 ), tin oxide (SnO 2 ), zinc oxide (ZnO), a mixed oxide of indium oxide and tin oxide (In 2 O 3 —SnO 2 , referred to as ITO), a mixed oxide of indium oxide and zinc oxide (In 2 O 3 —ZnO), and any of these metal oxide materials containing silicon oxide. 
     Through a fifth photolithography process, a resist mask is formed over the conductive layer, selective etching is performed to form the source electrode  107   a  and the drain electrode  107   b , and the resist mask is removed. Note that a resist mask may be formed by an inkjet method, in which case manufacturing costs can be reduced because a photomask is not used. 
     Next, the insulating layer  108  is formed over the semiconductor layer  106 , the source electrode  107   a , and the drain electrode  107   b  (see  FIG.  6 D ). The insulating layer  108  can be formed using a material and a method which are similar to those of the first gate insulating layer  102 . Sputtering is preferably employed for forming the insulating layer  108  in terms of low possibility of entry of hydrogen, water, and the like. In the case where the semiconductor layer  106  is formed using an oxide semiconductor, if hydrogen is contained in the insulating layer  108 , hydrogen might enter the oxide semiconductor or extract oxygen in the oxide semiconductor, which might cause a reduction in resistance of the oxide semiconductor (make the oxide semiconductor have n-type conductivity). Therefore, it is important to form the insulating layer  108  by a method through which hydrogen and an impurity containing hydrogen are not contained therein. 
     For the insulating layer  108 , an inorganic insulating material such as silicon oxide, silicon oxynitride, hafnium oxide, aluminum oxide, or gallium oxide can be typically used. Gallium oxide is a material that is hardly charged, and thus can suppress variation in threshold voltage due to charge build-up of the insulating layer. Note that in the case where an oxide semiconductor is used for the semiconductor layer  106 , a metal oxide layer containing the same kind of component as the oxide semiconductor may be formed as the insulating layer  108  or stacked over the insulating layer  108 . 
     In this embodiment, a 200-nm-thick silicon oxide layer is formed as the insulating layer  108  by sputtering. The substrate temperature in film formation is higher than or equal to room temperature and lower than or equal to 300° C. and is 100° C. in this embodiment. The silicon oxide layer can be formed by sputtering in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas and oxygen. As a target, a silicon oxide target or a silicon target can be used. For example, a silicon oxide layer can be formed by sputtering under an atmosphere containing oxygen with the use of silicon for the target. 
     In order to remove remaining moisture from the deposition chamber at the time of formation of the insulating layer  108 , an entrapment vacuum pump (e.g., a cryopump) is preferably used. When the insulating layer  108  is formed in the deposition chamber evacuated using a cryopump, the impurity concentration in the insulating layer  108  can be reduced. In addition, as an exhaustion unit for removing moisture remaining in the chamber used for depositing the insulating layer  108 , a turbo molecular pump provided with a cold trap may be used. 
     It is preferable that a high-purity gas from which impurities such as hydrogen, water, a compound containing a hydroxyl group, and a hydride are removed be used as a sputtering gas for forming the insulating layer  108 . 
     Then, third heat treatment may be performed in a reduced pressure atmosphere, an inert gas atmosphere, an oxygen gas atmosphere, or an ultra-dry air (preferably at 200° C. to 600° C., for example, 250° C. to 550° C.). For example, the third heat treatment may be performed at 450° C. for one hour in a nitrogen atmosphere. In the third heat treatment, part of the semiconductor layer (the channel formation region) is heated while being in contact with the insulating layer  108 . It is preferable that the above atmosphere do not contain water, hydrogen, or the like. 
     In the case where the semiconductor layer  106  is formed using an oxide semiconductor, when heat treatment is performed in a state where the semiconductor layer  106  is in contact with the insulating layer  108  containing oxygen, oxygen can be further supplied to the semiconductor layer  106  from the insulating layer  108  containing oxygen. 
     Through the above process, the transistor  150  is formed. The protective insulating layer  109  may be further formed over the insulating layer  108 . For the protective insulating layer  109 , an inorganic insulating material that hardly contains impurities such as moisture, a hydrogen ion, or OH −  and can prevent entry of such impurities from the outside, such as silicon nitride, aluminum nitride, silicon nitride oxide, or aluminum oxynitride is preferably used. In this embodiment, the protective insulating layer  109  is formed using silicon nitride (see  FIG.  6 D ). 
     A silicon nitride layer used as the protective insulating layer  109  is formed in the following manner: the substrate  100  over which layers up to and including the insulating layer  108  are formed is heated to a temperature ranging from 100° C. to 400° C., a sputtering gas containing high-purity nitrogen from which hydrogen and moisture are removed is introduced, and a silicon target is used. Like the insulating layer  108 , the protective insulating layer  109  is preferably deposited while moisture remaining in a treatment chamber is removed. 
     The protective insulating layer  109  is preferably formed in contact with the first gate insulating layer  102  or the base insulating layer  110  placed below the protective insulating layer  109 , and prevents entry of impurities such as moisture, a hydrogen ion, or OH −  from the vicinity of an edge portion of the substrate. 
     After the transistor  150  is formed, heat treatment may be further performed at 100° C. to 200° C. for 1 to 30 hours in the air. This heat treatment may be performed at a fixed heating temperature. Alternatively, it is possible that the following change in temperature is set as one cycle and repeated plural times: the temperature is increased from room temperature to a heating temperature and then decreased to room temperature. 
     Alternatively, without performing the first heat treatment, the second heat treatment may be performed under the conditions of the first heat treatment. 
     Note that in the transistor  150 , edges of the control gate  101 , the electrode  103 , the source electrode  107   a , and the drain electrode  107   b  are preferably tapered. Here, the taper angle is 30° to 60°, for example. Note that the taper angle refers to an inclination angle formed by a side surface and a bottom surface of a layer when the layer is observed from the direction perpendicular to the cross section of the layer (i.e., the plane perpendicular to the surface of the substrate). When the edges of the control gate  101 , the electrode  103 , the source electrode  107   a , and the drain electrode  107   b  are tapered, coverage thereof with a layer formed in a later step can be improved and disconnection can be prevented. 
       FIG.  7 A  illustrates a transistor  160  as a structural example of the case where a back gate  111  is formed in the transistor  150 . The back gate  111  is positioned so that the channel formation region of the semiconductor layer  106  is placed between the back gate  111  and the control gate  101  or the storage gate  104 . The back gate  111  can be formed using a material and a method similar to those of the control gate, the source electrode, the drain electrode, and the like. 
     In  FIG.  7 A , the back gate  111  is formed over the channel formation region of the semiconductor layer  106  with the insulating layer  108  and the protective insulating layer  109  placed therebetween.  FIG.  7 A  illustrates the example in which the back gate  111  is formed over the protective insulating layer  109 ; alternatively, the back gate  111  may be formed between the insulating layer  108  and the protective insulating layer  109 . 
     The back gate  111  may be connected to one of the source electrode  107   a  and the drain electrode  107   b , or may be connected nowhere and electrically floating (in a floating state). The placement of the back gate  111  can reduce variations in characteristics which are caused when a plurality of transistors are formed in the semiconductor device, and realize high stability in operation of the semiconductor device. 
     The oxide semiconductor used for the semiconductor layer in this embodiment is an intrinsic (i-type) oxide semiconductor or a substantially intrinsic oxide semiconductor. The intrinsic (i-type) oxide semiconductor or substantially intrinsic oxide semiconductor is obtained in such a manner that hydrogen having the property as a donor is removed from an oxide semiconductor as much as possible, and the oxide semiconductor is purified so as to contain impurities as little as possible. In other words, a feature of such an oxide semiconductor lies in that a purified i-type semiconductor or a substantially intrinsic semiconductor is obtained not by adding an impurity but by removing an impurity such as hydrogen or water as much as possible. That is, the oxide semiconductor layer used in the above transistor is an oxide semiconductor layer that is purified and made to be electrically intrinsic. 
     The number of carries in the purified oxide semiconductor is very small (close to zero), and the carrier concentration can be lower than 1×10 14 /cm 3 , preferably lower than 1×10 12 /cm 3 , further preferably lower than 1×10 11 /cm 3 . 
     Since the number of carriers in the oxide semiconductor is extremely small, the off-state current of a transistor can be reduced. The smaller amount of off-state current is more preferable. 
     Specifically, in a transistor in which the above oxide semiconductor is used for a channel formation region, the off-state current per 1 μm of channel width at room temperature can be 10 aA (1×10 −17  A/μm) or lower, further reduced to 1 aA (1×10 −18  A/μm) or lower, still further reduced to 1 zA (1×10 −21  A/μm) or lower, and still further reduced to 1 yA (1×10 −24  A/μm). 
     In addition, the on-state current of the transistor including the above oxide semiconductor in the channel formation region has almost no temperature dependence, and variation in off-state current is extremely small. 
     Furthermore, the transistor including the above oxide semiconductor in the channel formation region can be a highly reliable transistor in which the amount of change in the threshold voltage of the transistor by external stimuli (e.g., those in a bias-temperature stress (BT) test) can be reduced. 
     The transistor including the above oxide semiconductor can have relatively high field-effect mobility and thus can operate at high speed. 
     As described above, a semiconductor device that includes an oxide semiconductor and has stable electric characteristics can be provided. Therefore, a semiconductor device with high reliability can be provided. 
     Note that the method for manufacturing the bottom-gate transistor is described as an example in this embodiment; however, the transistor in this embodiment is not limited to having a bottom-gate structure. A transistor  170  illustrated in  FIG.  7 B  is an example of a top-gate transistor. Although the positions where each of the control gate  101  and the semiconductor layer  106  is stacked are different from those in the transistor  150 , the transistor  170  can be formed using a material and a method similar to those of the transistor  150 . 
     In the case where the back gate  111  is provided in the transistor  170 , the back gate  111  can be provided, for example, between the substrate  100  and the base insulating layer  110  so as to overlap with the semiconductor layer  106 . 
     Since the semiconductor layer  106  can be provided as the bottom layer in the top-gate transistor, when a single crystal substrate such as a silicon wafer is used as the substrate  100 , part of the single crystal substrate can be easily used as the semiconductor layer  106 . 
     This embodiment can be implemented in combination with other embodiments as appropriate. 
     Embodiment 3 
     In this embodiment, examples of an electronic device to which the semiconductor device described in the above embodiment is applied will be described with reference to  FIGS.  8 A to  8 F . Specifically, this embodiment explain applications of the semiconductor device described in the above embodiment to electronic devices such as a computer, a mobile phone handset (also referred to as a mobile phone or a mobile phone device), a personal digital assistant (including a portable game machine and an audio reproducing device), a camera such as a digital camera and a digital video camera, electronic paper, and a television device (also referred to as a television or a television receiver). 
       FIG.  8 A  illustrates a laptop personal computer including a housing  701 , a housing  702 , a display portion  703 , a keyboard  704 , and the like. The semiconductor device described in the above embodiment is provided in each of the housings  701  and  702 . Thus, it is possible to realize a laptop personal computer in which data is written and read at high speed, data can be retained for a long time, and power consumption is sufficiently low. 
       FIG.  8 B  illustrates a personal digital assistant (PDA). A main body  711  is provided with a display portion  713 , an external interface  715 , operation buttons  714 , and the like. Further, a stylus  712  or the like for operating the personal digital assistant is provided. The semiconductor device described in the above embodiment is provided in the main body  711 . Thus, it is possible to realize a personal digital assistant in which data is written and read at high speed, data can be retained for a long time, and power consumption is sufficiently low. 
       FIG.  8 C  illustrates an e-book reader  720  including electronic paper, and the e-book reader  720  includes two housings of a housing  721  and a housing  723 . The housing  721  and the housing  723  are provided with a display portion  725  and a display portion  727 , respectively. The housings  721  and  723  are connected by a hinge portion  737  and can be opened or closed with the hinge portion  737 . The housing  721  is provided with a power switch  731 , an operation key  733 , a speaker  735 , and the like. At least one of the housings  721  and  723  is provided with the semiconductor device described in the above embodiment. Thus, it is possible to realize an e-book reader in which data is written and read at high speed, data can be retained for a long time, and power consumption is sufficiently low. 
       FIG.  8 D  illustrates a mobile phone including two housings of a housing  740  and a housing  741 . The housing  740  and the housing  741  developed as illustrated in  FIG.  8 D  can shift by sliding so that one is lapped over the other; therefore, the size of the mobile phone can be reduced, which makes the mobile phone suitable for being carried. The housing  741  is provided with a display panel  742 , a speaker  743 , a microphone  744 , a pointing device  746 , a camera lens  747 , an external connection terminal  748 , and the like. The housing  740  is provided with a solar cell  749  for charging the mobile phone, an external memory slot  750 , and the like. An antenna is incorporated in the housing  741 . 
     The display panel  742  is provided with a touch panel. A plurality of operation keys  745  displayed as images are indicated by dashed lines in  FIG.  8 D . Note that the mobile phone includes a booster circuit for raising a voltage output from the solar cell  749  to a voltage needed for each circuit. 
     At least one of the housings  740  and  741  is provided with the semiconductor device described in the above embodiment. Thus, it is possible to realize a mobile phone in which data is written and read at high speed, data can be retained for a long time, and power consumption is sufficiently low. 
       FIG.  8 E  illustrates a digital camera including a main body  761 , a display portion  767 , an eyepiece  763 , an operation switch  764 , a display portion  765 , a battery  766 , and the like. The semiconductor device described in the above embodiment is provided in the main body  761 . Thus, it is possible to realize a digital camera in which data is written and read at high speed, data can be retained for a long time, and power consumption is sufficiently low. 
       FIG.  8 F  illustrates a television device  770  including a housing  771 , a display portion  773 , a stand  775 , and the like. The television device  770  can be operated with an operation switch of the housing  771  or a remote controller  780 . The housing  771  and the remote controller  780  are each provided with the semiconductor device described in the above embodiment. Thus, it is possible to realize a television device in which data is written and read at high speed, data can be retained for a long time, and power consumption is sufficiently low. 
     As described above, the electronic devices described in this embodiment each include the semiconductor device described in the above embodiment; thus, electronic devices with low power consumption can be realized. 
     EXPLANATION OF REFERENCE 
     
         
           100 : substrate,  101 : control gate,  102 : first gate insulating layer,  103 : electrode,  104 : storage gate,  105 : second gate insulating layer,  106 : semiconductor layer,  107   a : source electrode,  107   b : drain electrode,  108 : insulating layer,  109 : protective insulating layer,  110 : base insulating layer,  111 : back gate,  150 : transistor,  160 : transistor,  170 : transistor,  180 : portion,  181 : edge,  200 : memory cell,  201 : first wiring,  202 : second wiring,  203 : third wiring,  204 : fourth wiring,  210 : transistor,  211 : control gate,  212 : storage gate,  300 : capacitor,  301 : electrode,  302 : oxide semiconductor,  303 : insulator,  304 : electrode,  311 : curve,  312 : curve,  701 : housing,  702 : housing,  703 : display portion,  704 : keyboard,  711 : main body,  712 : stylus,  713 : display portion,  714 : operation button,  715 : external interface,  720 : e-book reader,  721 : housing,  723 : housing,  725 : display portion,  727 : display portion,  731 : power switch,  733 : operation key  735 : speaker,  737 : hinge portion,  740 : housing,  741 : housing,  742 : display panel,  743 : speaker,  744 : microphone,  745 : operation key,  746 : pointing device,  747 : camera lens,  748 : external connection terminal,  749 : solar cell,  750 : external memory slot,  761 : main body,  763 : eyepiece,  764 : operation switch,  765 : display portion,  766 : battery,  767 : display portion,  770 : television device,  771 : housing,  773 : display portion,  775 : stand,  780 : remote controller,  1200 : memory cell,  1210 : transistor,  1211 : control gate,  1212 : storage gate,  1221 : first driver circuit,  1222 : second driver circuit,  1223 : third driver circuit,  1224 : fourth driver circuit 
       
    
     This application is based on Japanese Patent Application serial No. 2010-145339 filed with Japan Patent Office on Jun. 25, 2010, the entire contents of which are hereby incorporated by reference.