Patent Publication Number: US-8542004-B2

Title: Semiconductor device and driving method of the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/022,407, filed Feb. 7, 2011, now allowed, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2010-028820 on Feb. 12, 2010, both of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The disclosed invention relates to a semiconductor device using a semiconductor element and a driving method thereof. 
     BACKGROUND ART 
     Memory devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when not powered, and a non-volatile device that holds stored data even when not powered. 
     A typical example of a volatile memory device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a memory element is selected and charge is accumulated in a capacitor. 
     Owing to the above principle, charge in the capacitor is lost when data in a DRAM is read out; thus, another writing operation is necessary every time data is read out. Moreover, since leakage current (off-state current) flows between a source and a drain of a transistor included in a memory element when the transistor is in an off state, charge flows into or out even if the transistor is not selected, which makes a data holding period short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to adequately reduce power consumption. Furthermore, since stored data is lost when power supply stops, another memory device utilizing a magnetic material or an optical material is needed in order to hold the data for a long time. 
     Another example of a volatile memory device is a static random access memory (SRAM). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need refresh operation, which is an advantage over a DRAM. However, cost per storage capacity is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops. 
     A typical example of a non-volatile memory 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 charge in the floating gate. Therefore, a flash memory has advantages in that the data holding period is extremely long (semi-permanent) and refresh operation which is necessary in a volatile memory device is not needed (e.g., see Patent Document 1). 
     However, in a flash memory, there is a problem in that a memory element does not function after a predetermined number of writing operations because a gate insulating layer included in the memory element deteriorates due to tunneling current generated in writing operations. In order to reduce effects of this problem, a method in which the number of writing operations is equalized among the memory elements can be employed, for example, but a complicated peripheral circuit is needed to employ this method. Moreover, even when such a method is employed, the fundamental problem about the lifetime is not resolved. In other words, a flash memory is not suitable for applications in which data is frequently rewritten. 
     In addition, high voltage is necessary for holding charge in the floating gate or removing the charge, and a circuit therefor is required. Further, it takes a relatively long time to inject or remove charge, and it is not easy to increase a speed of writing and erasing data. 
     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 disclosed invention is to provide a semiconductor device with a novel structure in which stored data can be held even when power is not supplied, and the number of times of writing is not limited. 
     In the disclosed invention, a semiconductor device is formed using a material which allows a sufficient reduction in off-state current of a transistor; for example, an oxide semiconductor material, which is a wide gap semiconductor, is used. When a semiconductor material which allows a sufficient reduction in off-state current of a transistor is used, the semiconductor device can hold data for a long period. 
     Further, one embodiment of the disclosed invention is a semiconductor device which is formed using a wide gap semiconductor and includes a potential change circuit which selectively applies a potential either equal to or different from a potential of a bit line to a source line. Thus, power consumption of the semiconductor device can be sufficiently reduced. 
     Specifically, examples of structures given below can be employed. 
     One embodiment of the present invention is a semiconductor device including a bit line, a source line, a potential change circuit, and a memory cell. The memory cell includes a first transistor including a first gate electrode, a first source electrode, a first drain electrode, and a first channel formation region; a second transistor including a second gate electrode, a second source electrode, a second drain electrode, and a second channel formation region; and a capacitor. The first channel formation region includes a semiconductor material different from a semiconductor material in the second channel formation region. The first gate electrode, the second drain electrode (or the second source electrode), and one of electrodes of the capacitor are electrically connected to each other to form a node which holds charge. The source line, one of terminals of the potential change circuit, and the first source electrode (or the first drain electrode) are electrically connected to each other. The bit line, the second source electrode (or the second drain electrode), and the first drain electrode (or the first source electrode) are electrically connected to each other. The potential change circuit selectively applies a potential to the source line, the potential being equal to or different from a potential of the bit line. 
     One embodiment of the present invention is a semiconductor device including a write word line, a read word line, a bit line, a source line, a potential change circuit, and a memory cell. The memory cell includes a first transistor including a first gate electrode, a first source electrode, a first drain electrode, and a first channel formation region; a second transistor including a second gate electrode, a second source electrode, a second drain electrode, and a second channel formation region; and a capacitor. The first channel formation region includes a semiconductor material different from a semiconductor material in the second channel formation region. The first gate electrode, the second drain electrode (or the second source electrode), and one of electrodes of the capacitor are electrically connected to each other to form a node which holds charge. The source line, one of terminals of the potential change circuit, and the first source electrode (or the first drain electrode) are electrically connected to each other. The bit line, the second source electrode (or the second drain electrode), and the first drain electrode (or the first source electrode) are electrically connected to each other. The read word line and the other of the electrodes of the capacitor are electrically connected to each other. The write word line and the second gate electrode are electrically connected to each other. The potential change circuit selectively applies a potential to the source line, the potential being equal to or different from a potential of the bit line. 
     In the above structure, the second channel formation region of the second transistor may include an oxide semiconductor. Further, the potential change circuit may have a function to apply a potential equal to a potential of the bit line to the source line, when a potential which turns the first transistor on is applied to the node. 
     Another embodiment of the present invention is a method for driving a semiconductor device where a gate electrode of a first transistor, a drain electrode (or a source electrode) of a second transistor, and one of electrodes of a capacitor are electrically connected to form a node which holds charge. In the method, a potential of a source electrode of the first transistor and a potential of a drain electrode of the first transistor are made equal to each other, when a potential which turns the first transistor on is applied to the node. 
     In the above structure, a channel formation region of the first transistor may include a semiconductor material different from a semiconductor material in a channel formation region of the second transistor. Further, the channel formation region of the second transistor may include an oxide semiconductor. Further, the potential of the source electrode (the drain electrode) of the first transistor and the potential of the drain electrode of the first transistor are made equal to each other by a potential change circuit controlling a potential of a source line connected to the source electrode of the first transistor. 
     Note that the term “equal” in describing potentials includes “substantially equal.” The technical idea of the disclosed invention lies in that the potential difference between the bit line and the source line is sufficiently reduced and thus current flowing in the bit line and the source line is suppressed. Consequently, “substantially equal” potentials, e.g., potentials which enable power consumption to be sufficiently reduced (to one hundredth or less) compared to the case where the potential of the source line is fixed to GND or the like, are included. In addition, potential deviation due to wire resistance or the like are reasonably acceptable. 
     Note that although the transistor may be formed using an oxide semiconductor in the above description, the disclosed invention is not limited to this. A material which can realize the off-state current characteristics equivalent to those of the oxide semiconductor, such as a wide gap material like silicon carbide (specifically, a semiconductor material whose energy gap Eg is larger than 3 eV) may be used. 
     Note that in this specification and the like, the term such as “over” or “below” does not necessarily mean that a component is placed “directly over” or “directly below” another component. For example, the expression “a gate electrode over a gate insulating layer” does not exclude the case where another component is provided between the gate insulating layer and the gate electrode. 
     In addition, in this specification and the like, the term such as “electrode” or “wiring” does not limit a function of a component. For example, an “electrode” is sometimes used as part of a “wiring”, and vice versa. Further, the term “electrode” or “wiring” can include the case where a plurality of “electrodes” or “wirings” are formed in an integrated manner. 
     Further, functions of a “source” and a “drain” are sometimes interchanged with each other when a transistor of opposite polarity is used or when the direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be interchanged 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 the object having any electric function as long as electric signals can be transmitted and received between the components connected through the object. 
     Examples of the object having any electric function include 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. 
     Since the off-state current of a transistor including an oxide semiconductor is extremely small, stored data can be held for an extremely long period by using the transistor. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed). 
     Further, a semiconductor device according to the disclosed invention does not need high voltage for writing data, and deterioration of the element does not become a problem. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate, and thus a problem such as deterioration of a gate insulating layer does not occur at all. That is, the semiconductor device according to the disclosed invention does not have a limit on the number of times of rewriting, which has been a problem of a conventional non-volatile memory, and thus has drastically improved reliability. Furthermore, since data is written by turning on or off the transistor, high-speed operation can be easily realized. Additionally, there is an advantage that operation for erasing data is not needed. 
     Since a transistor including a material which is not an oxide semiconductor can operate at sufficiently high speed, when it is combined with a transistor including an oxide semiconductor, a semiconductor device can perform operation (e.g., reading data) at sufficiently high speed. Further, a transistor including a material which is not an oxide semiconductor can suitably realize a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed. 
     Thus, a semiconductor device having a novel feature can be realized by provision of both the transistor including a material which is not an oxide semiconductor (a transistor capable of operation at sufficiently high speed, in general) and the transistor including an oxide semiconductor (a transistor whose off-state current is sufficiently small, in general). 
     Further, a potential of a source line and a potential of a bit line are made equal to each other when data is written, whereby a semiconductor device with sufficiently reduced power consumption can be realized. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A-1 ,  1 A- 2 , and  1 B are circuit diagrams of a semiconductor device. 
         FIG. 2  is a circuit diagram of a semiconductor device. 
         FIG. 3  is a circuit diagram of a semiconductor device. 
         FIG. 4  is a timing diagram. 
         FIGS. 5A and 5B  are a cross-sectional view and a plan view of a semiconductor device. 
         FIGS. 6A to 6D  are cross-sectional views of manufacturing steps of a semiconductor device. 
         FIGS. 7A to 7D  are cross-sectional views of manufacturing steps of a semiconductor device. 
         FIGS. 8A to 8D  are cross-sectional views of manufacturing steps of a semiconductor device. 
         FIGS. 9A to 9C  are cross-sectional views of manufacturing steps of a semiconductor device. 
         FIGS. 10A to 10F  illustrate electronic devices including a semiconductor device. 
         FIG. 11  is a graph showing characteristics of a transistor including an oxide semiconductor. 
         FIG. 12  is a diagram of a circuit for evaluating characteristics of a transistor including an oxide semiconductor. 
         FIG. 13  is a timing diagram of the circuit for evaluating characteristics of the transistor including an oxide semiconductor. 
         FIG. 14  is a graph showing the characteristics of the transistor including the oxide semiconductor. 
         FIG. 15  is a graph showing the characteristics of the transistor including the oxide semiconductor. 
         FIG. 16  is a graph showing the characteristics of the transistor including the oxide semiconductor. 
         FIG. 17  is a graph showing examination results of the memory window width. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments and examples of the present invention are described with reference to the drawings. Note that the present invention is not limited to the following description and it will be readily appreciated by those skilled in the art that modes and details can be modified in various ways without departing from the spirit and the scope of the present invention. Accordingly, the present invention should not be construed as being limited to the description of the embodiments and examples to be given below. 
     Note that the position, the size, the range, or the like of each structure illustrated in drawings and the like is not accurately represented in some cases for easy understanding. The disclosed invention is thus not necessarily limited by the position, size, range, or the like illustrated in the drawings and the like. 
     In this specification and the like, ordinal numbers such as first, second, and third are used in order to avoid confusion among components, and the terms do not limit the components numerically. 
     Embodiment 1 
     In this embodiment, a basic circuit structure and operation of a semiconductor device according to one embodiment of the disclosed invention will be described with reference to  FIGS. 1A-1 ,  1 A- 2 , and  1 B and  FIG. 2 . Note that in each of the circuit diagrams, “OS” may be written beside a transistor in order to indicate that the transistor includes an oxide semiconductor. 
     &lt;Basic Circuit  1 &gt; 
     First, the most basic circuit structure and its operation will be described with reference to  FIGS. 1A-1 ,  1 A- 2 , and  1 B. In the semiconductor device in  FIG. 1A-1 , a first wiring (a 1st line) and a source electrode (or a drain electrode) of a transistor  160  are electrically connected to each other, and a second wiring (a 2nd line) and the drain electrode (or the source electrode) of the transistor  160  are electrically connected to each other. In addition, a third wiring (a 3rd line) and a source electrode (or a drain electrode) of a transistor  162  are electrically connected to each other, and a fourth wiring (a 4th line) and a gate electrode of the transistor  162  are electrically connected to each other. In addition, a gate electrode of the transistor  160  and the drain electrode (or the source electrode) of the transistor  162  are electrically connected to one electrode of a capacitor  164 , and a fifth wiring (a 5th line) and the other electrode of the capacitor  164  are electrically connected to each other. 
     Here, a transistor including an oxide semiconductor is used as the transistor  162 , for example. Off-state current of a transistor including an oxide semiconductor is extremely small. Therefore, when the transistor  162  is in an off state, a potential in the gate electrode of the transistor  160  can be held for a very long time. The capacitor  164  facilitates holding of charge applied to the gate electrode of the transistor  160  and reading of the held data. 
     Note that there is no particular limitation on the transistor  160 . In order to increase the speed of reading data, it is preferable to use, for example, a transistor with high switching speed, such as a transistor formed using single crystal silicon. 
     Alternatively, the capacitor  164  may be omitted as in  FIG. 1B . 
     The semiconductor device in  FIG. 1A-1  can write, hold, and read data as described below, utilizing a characteristic in which the potential of the gate electrode of the transistor  160  can be held. 
     Firstly, writing and holding of data will be described. The potential of the fourth wiring is set to a potential at which the transistor  162  is turned on, whereby the transistor  162  is turned on. Thus, the potential of the third wiring is applied to the gate electrode of the transistor  160  and the capacitor  164 . That is, predetermined charge is applied to the gate electrode of the transistor  160  (writing of data). Here, charge for applying either two different levels of potential (hereinafter charge for applying a low potential is referred to as charge Q L  and charge for applying a high potential is referred to as charge Q H ) is applied. Note that charge for applying three or more different levels of potential may be employed to improve storage capacity. After that, the potential of the fourth wiring is set to a potential at which the transistor  162  is turned off, whereby the transistor  162  is turned off. Thus, the charge applied to the gate electrode of the transistor  160  is held (holding of data). 
     Since the off-state current of the transistor  162  is extremely small, the charge in the gate electrode of the transistor  160  is held for a long period. 
     Secondly, reading of data will be described. While a predetermined potential (a fixed potential) is applied to the first wiring, an appropriate potential (a read-out potential) is applied to the fifth wiring, whereby the potential of the second wiring changes depending on the amount of charge held in the gate electrode of the transistor  160 . This is because in general, when the transistor  160  is an n-channel transistor, an apparent threshold value V th     —     H  in the case where Q H  is held in the gate electrode of the transistor  160  is lower than an apparent threshold value V th     —     L  in the case where Q L  is held in the gate electrode of the transistor  160 . Here, an apparent threshold value refers to the potential of the fifth wiring which is needed to turn on the transistor  160 . Thus, by setting the potential of the fifth wiring to a potential V 0  which is between V th     —     H  and V th     —     L , charge held in the gate electrode of the transistor  160  can be determined. For example, in the case where Q H  is applied in writing, when the potential of the fifth wiring is set to V 0  (&gt;V th     —     H ), the transistor  160  is turned on. In the case where Q L  is applied in writing, even when the potential of the fifth wiring is set to V 0  (&lt;V th     —     L ), the transistor  160  remains off. Thus, the data held can be read by measuring the potential of the second wiring. 
     Note that in the case where memory cells are arrayed, it is necessary to read out data only from an intended memory cell. In the case where data of predetermined memory cell is read out and data of the other memory cells is not read out, a potential at which the transistor  160  is in an off state regardless of the state of the gate electrode, that is, a potential lower than V th     —     H , may be applied to the fifth wirings of the memory cells whose data is not to be read. Alternatively, a potential at which the transistor  160  is in an on state regardless of the state of the gate electrode, that is, a potential higher than V th     —     L , may be applied to the fifth wirings of the memory cells whose data is not to be read. 
     Thirdly, rewriting of data will be described. Rewriting of data is performed in a manner similar to that of the writing and holding of data. In other words, the potential of the fourth wiring is set to a potential at which the transistor  162  is turned on, whereby the transistor  162  is turned on. Accordingly, the potential of the third wiring (a potential for new data) is applied to the gate electrode of the transistor  160  and the capacitor  164 . After that, the potential of the fourth wiring is set to a potential at which the transistor  162  is turned off, whereby the transistor  162  is turned off. Thus, charge for the new data is applied to the gate electrode of the transistor  160 . 
     In the semiconductor device according to the disclosed invention, data can be directly rewritten by another writing of data as described above. Therefore, extracting of charge from a floating gate with the use of high voltage, which is needed in a flash memory or the like, is not needed and thus a reduction in operation speed caused by erasing operation, can be suppressed. In other words, high-speed operation of the semiconductor device can be realized. 
     Note that the drain electrode (or the source electrode) of the transistor  162  is electrically connected to the gate electrode of the transistor  160 , and thereby has an effect similar to that of a floating gate of a floating-gate transistor which is used as a non-volatile memory element. Hereinafter, the portion where the drain electrode (or the source electrode) of the transistor  162  and the gate electrode of the transistor  160  are electrically connected to each other is called a node FG in some cases. When the transistor  162  is off, the node FG can be regarded as being embedded in an insulator and charge is held in the node FG. The off-state current of the transistor  162  including an oxide semiconductor is smaller than or equal to one hundred thousandth of the off-state current of a transistor including a silicon semiconductor or the like; thus, loss of the charge accumulated in the node FG due to leakage current of the transistor  162  is negligible. That is, with the transistor  162  including an oxide semiconductor, a non-volatile memory device which can hold data without power supply can be realized. 
     For example, when the off-state current of the transistor  162  is smaller than or equal to 10 zA (1 zA (zeptoampere) is 1×10 −21  A) at room temperature (25° C.) and the capacitance value of the capacitor  164  is approximately 10 fF, data can be held for 10 4  seconds or longer. It should be appreciated that the holding time changes depending on the transistor characteristics and the capacitance value. 
     In the semiconductor device of the disclosed invention, a problem of deterioration of a gate insulating film (a tunnel insulating film), which occurs in a conventional floating-gate transistor, does not exist. That is, deterioration of a gate insulating film due to injection of electrons into a floating gate, which has been regarded as a problem, can be solved. This means that there is no limit on the number of times of writing in principle. In addition, high voltage which is needed for writing or erasing data in a conventional floating-gate transistor is not necessary. 
     The components such as transistors in the semiconductor device in  FIG. 1A-1  can be regarded as including a resistor and a capacitor as shown in  FIG. 1A-2 . That is, in  FIG. 1A-2 , the transistor  160  and the capacitor  164  are each regarded as including a resistor and a capacitor. R 1  and C 1  denote the resistance value and the capacitance value of the capacitor  164 , respectively. The resistance value R 1  corresponds to the resistance value of an insulating layer included in the capacitor  164 . R 2  and C 2  denote the resistance value and the capacitance value of the transistor  160 , respectively. The resistance value R 2  corresponds to the resistance value of a gate insulating layer at the time when the transistor  160  is on. The capacitance value C 2  corresponds to the capacitance value of so-called gate capacitance (capacitance between the gate electrode and the source electrode or drain electrode and capacitance between the gate electrode and the channel formation region). 
     A charge holding period (also referred to as a data holding period) is determined mainly by off-state current of the transistor  162  under the condition where gate leakage current of the transistor  162  is sufficiently small, R 1  is greater than or equal to ROS (R 1 ≧ROS), and R 2  is greater than or equal to ROS (R 2 ≧ROS), where ROS is the resistance value (also referred to as effective resistance) between the source electrode and the drain electrode at the time when the transistor  162  is off. 
     On the other hand, when the condition is not satisfied, it would be difficult to ensure a sufficient holding period even if the off-state current of the transistor  162  is small enough. This is because leakage current other than the off-state current of the transistor  162  (e.g., leakage current generated between the source electrode and the gate electrode) would be large. Thus, it can be said that the semiconductor device disclosed in this embodiment desirably satisfies the relation where R 1  is greater than or equal to ROS (R 1 ≧ROS), and R 2  is greater than or equal to ROS (R 2 ≧ROS). 
     It is desirable that C 1  be greater than or equal to C 2  (C 1 ≧C 2 ). This is because by increasing C 1 , the potential of the fifth wiring can be effectively applied to the node FG when the potential in the node FG is controlled by the fifth wiring, and thus the difference between the potentials applied to the fifth wiring (e.g., a potential of reading and a potential of not reading) can be reduced. 
     As described above, when the above relation is satisfied, a more favorable semiconductor device can be realized. Note that R 1  and R 2  are determined by the gate insulating layer included in the transistor  160  and an insulating layer included in the capacitor  164 , respectively. Similarly, C 1  and C 2  are determined by the gate insulating layer of the transistor  160  and an insulating layer included in the capacitor  164 , respectively. Therefore, the material, the thickness, and the like of the gate insulating layer are desirably set as appropriate so that the above relation may be satisfied. 
     In the semiconductor device described in this embodiment, the node FG has an effect similar to a floating gate of a floating-gate transistor in a flash memory or the like, but the node FG of this embodiment has a feature which is essentially different from that of the floating gate in the flash memory or the like. 
     In a flash memory, since a potential applied to a control gate is high, it is necessary to keep a proper distance between cells in order to prevent the potential from affecting a floating gate of the adjacent cell. This is one of the factors inhibiting high integration of the semiconductor device. The factor is due to a basic principle of a flash memory that tunneling current is generated by application of a high electric field. 
     In contrast, the semiconductor device according to this embodiment is operated by switching of a transistor including an oxide semiconductor and 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. Accordingly, an effect of a high electric field for a control gate on an adjacent cell does not need to be taken into account, and thus high integration can be facilitated. 
     In addition, it is also advantage over a flash memory that a high electric field is unnecessary and a large peripheral circuit (such as a booster circuit) is unnecessary. For example, the maximum value of voltage applied to the memory cell according to this embodiment (the difference between the highest potential and the lowest potential applied to terminals of the memory cell at the same time) can be 5 V or lower, preferably 3 V or lower, in each memory cell in the case where two-level (one-bit) data is written. 
     In the case where the dielectric constant ∈r 1  of the insulating layer included in the capacitor  164  is different from the dielectric constant ∈r 2  of the insulating layer included in the transistor  160 , C 1  can easily be made greater than or equal to C 2  (C 1 ≧C 2 ) while S 1  which is the area of the insulating layer included in the capacitor  164  and S 2  which is the area of an insulating layer forming gate capacitance of the transistor  160  satisfy the relation where 2×S 2  is greater than or equal to S 1  (2×S 2 ≧S 1 ), desirably S 2  is greater than or equal to S 1  (S 2 ≧S 1 ). In other words, C 1  can easily be made greater than or equal to C 2  while the area of the insulating layer included in the capacitor  164  is small. Specifically, for example, when a film formed of a high-k material such as hafnium oxide or a stack of a film formed of a high-k material such as hafnium oxide and a film formed of an oxide semiconductor is used for the insulating layer included in the capacitor  164 , ∈r 1  can be set to 10 or more, preferably 15 or more, and when a film formed of silicon oxide is used for the insulating layer forming the gate capacitance, ∈r 2  can be set to 3 to 4 inclusive. 
     Combination of such structures enables higher integration of the semiconductor device according to the disclosed invention. 
     Note that in addition to the increase in the degree of integration, a multilevel technique can be employed in order to increase the storage capacity of the semiconductor device. For example, three or more levels of data are written to one memory cell, whereby the storage capacity can be increased as compared to the case where two-level (one-bit) data is written. The multilevel technique can be achieved by, for example, supplying charge Q to the gate electrode of the first transistor, in addition to charge Q L  for supplying a low potential and charge Q H  for supplying a high potential. In this case, enough storage capacity can be ensured even in a circuit structure with a relatively large scale (e.g., 15 F 2  to 50 F 2  inclusive; F is the minimum feature size). 
     Note that an n-channel transistor (an n-type transistor) in which electrons are carriers is used in the above description, but it will be appreciated that a p-channel transistor in which holes are majority carriers can be used instead of the n-channel transistor. 
     &lt;Basic Circuit  2 &gt; 
     Next, a specific circuit structure and its operation will be described with reference to  FIG. 2 . The structure of a memory cell  170  in  FIG. 2  is similar to that in  FIG. 1A-1 . The first wiring in  FIG. 1A-1  corresponds to a source line SL in  FIG. 2 . The fourth wiring in  FIG. 1A-1  corresponds to a write word line WWL in  FIG. 2 . The fifth wiring in  FIG. 1A-1  corresponds to a read word line RWL in  FIG. 2 . Note that a bit line BL in  FIG. 2  corresponds to both the second wiring and the third wiring in  FIG. 1A-1 . 
     In  FIG. 2 , the bit line BL is connected to a multiplexer (MUX)  180 . The multiplexer  180  is also connected to a write and read control signal line WREN, an input signal line BL_OS, and a read circuit input signal line RS, as well as the bit line BL. The write and read control signal line WREN transmits a signal for changing signal path in the multiplexer. The input signal line BL_OS transmits data signal which is written to the memory cell  170 . The read circuit input signal line RS transmits signal from a read circuit. 
     In addition, in  FIG. 2 , the source line SL is connected to the multiplexer  182 . The multiplexer  182  is connected to the write and read control signal line WREN, the input signal line BL_OS, and a second source line SL 2 , as well as the source line SL. The second source line SL 2  transmits a signal which is applied to the source line SL when data is read. 
     The multiplexer  180  and the multiplexer  182  have a function of changing signal paths in accordance with the write and read control signal line WREN. The multiplexer is also referred to as a selector. 
     Writing, holding, and reading of data are performed similarly to the case of  FIGS. 1A-1 ,  1 A- 2 , and  1 B. Here, as one example, the case where either a power supply potential VDD or a reference potential GND is applied to the node FG will be described. Note that data “1” is held when the power supply potential VDD is applied to the node FG, whereas data “0” is held when the reference potential GND is applied to the node FG. Further, a potential of the second source line SL 2  is set to GND. 
     Specific write operation will be described. A potential of the read word line RWL connected to the memory cell  170  is set to GND and a potential of the write word line WWL connected to the memory cell  170  is set to VDD, whereby the memory cell  170  is selected. 
     Then, signal paths in the multiplexer  180  and the multiplexer  182  are changed so that data of the input signal line BL_OS to be written may be transmitted to the bit line BL through the multiplexer  180  and to the source line SL through the multiplexer  182 . In other words, a signal for such operation is input to the write and read control signal line WREN to change the signal paths in the multiplexer  180  and the multiplexer  182 . 
     The operation is characterized in that the potential of the bit line BL and the potential of the source line SL are made equal to each other when data is written. This can reduce power consumption due to current flowing in the bit line BL and the source line SL even when a potential at which the first transistor  160  is on is applied to the node FG. The above multiplexer is one of the circuits which enable such operation. From the viewpoint of the potential of the source line SL, the operation can be regarded as a change in potential of the source line SL. Thus, the circuit having such a function can be generally referred to as a “potential change circuit.” Note that the “potential change circuit” is not limited to a multiplexer. 
     Note that in this specification, the term “equal” describing potentials includes “substantially equal.” The technical idea of the disclosed invention lies in that the potential difference between the bit line BL and the source line SL is sufficiently reduced and thus current flowing in the bit line BL and the source line SL is suppressed. Consequently, “substantially equal” potentials include, for example, potentials which enable power consumption to be sufficiently reduced (to one hundredth or less) compared to the case where the potential of the source line SL is fixed to GND. In addition, potential deviation due to wire resistance or the like are reasonably acceptable. 
     Note that in order to write data “0” to the memory cell  170 , GND is applied to the input signal line BL_OS, whereas in order to write data “1” to the memory cell  170 , a potential VDD+α is applied to the input signal line BL_OS, taking voltage drop in the transistor  162  into consideration. 
     Data is held by setting the potentials of the read word line RWL and the write word line WWL to a potential lower than GND by VDD (hereinafter, the potential is referred to as VL). 
     When the potential of the read word line RWL is decreased from GND to VL, the potential in the node FG is decreased by VDD (=GND−VL) due to capacitive coupling with the capacitor  164 . Accordingly, when VDD corresponding to data “1” has been applied to the node FG, the potential in the node FG is decreased by VDD to be GND, whereas when GND corresponding to data “0” has been applied to the node FG, the potential in the node FG is decreased by VDD to be VL (=GND−VDD). 
     Since VL is applied to the write word line WWL, the transistor  162  is turned off regardless of whether data “1” or data “0” has been written. Since the off-state current of the transistor  162  is significantly small, the charge in the gate electrode of the transistor  160  is held for a long period. 
     Data is read out by setting the potential of the read word line RWL to GND and the potential of the write word line WWL to VL. 
     When the potential of the read word line RWL is increased from VL to GND, the potential in the node FG is increased by VDD (=GND−VL) due to capacitive coupling with the capacitor  164 . Accordingly, when VDD corresponding to data “1” has been applied to the node FG, the potential in the node FG is increased by VDD to be VDD, whereas when GND corresponding to data “0” has been applied to the node FG, the potential in the node FG is increased by VDD to be GND (VL+VDD). 
     Note that to read out data, the signal path in the multiplexer  180  is changed so that the potential of the bit line BL may be transmitted to the read circuit input signal line RS through the multiplexer  180 . In other words, a signal for such operation is input to the write and read control signal line WREN to change the signal path in the multiplexer  180 . 
     In addition, the signal path in the multiplexer  182  is changed so that the source line SL is connected to the second source line SL 2  through the multiplexer  182 . In other words, a signal for such operation is input to the write and read control signal line WREN to change the signal path in the multiplexer  182 . Thus, GND is applied to the source line SL. 
     By the above reading operation, in the case where data “1” is written to the memory cell  170 , the transistor  160  is turned on and the potential of the bit line BL is decreased; whereas in the case where data “0” is written to the memory cell  170 , the transistor  160  is turned off and the potential of the bit line BL at the beginning of the reading operation is kept or the potential of the bit line BL is increased. Note that it depends on the circuit connected to the read circuit input signal line RS whether the potential of the bit line BL is kept or increased. 
     As described above, in the semiconductor device having the circuit structure illustrated in  FIG. 2 , current flowing in the source line SL and the bit line BL during the data writing operation can be suppressed. Consequently, power consumption during the data writing operation can be reduced. Note that the technical idea of the disclosed invention lies in that a potential difference between the bit line BL and the source line SL is sufficiently reduced, and thus current flowing in the bit line BL and the source line SL is suppressed. The circuit structure is not limited to the above one as long as the technical idea can be applied. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures and methods described in the other embodiments. 
     Embodiment 2 
     In this embodiment, an application example of the semiconductor device described in the above embodiment will be described. Specifically, an example of a semiconductor device in which the semiconductor devices described in the above embodiment are arranged in a matrix will be described with reference to  FIGS. 3 and 4 . 
       FIG. 3  illustrates an example of a circuit diagram of a semiconductor device including m×n memory cells  170 . 
     The semiconductor device according to this embodiment includes m (m is an integral greater than or equal to 2) write word lines WWLs, m read word lines RWLs, n (n is an integral greater than or equal to 2) bit lines BLs, n source lines SLs, a memory cell array in which the memory cells  170  are arranged in a matrix of m rows (in a vertical direction)×n columns (in a horizontal direction), multiplexers  180  connected to respective n bit lines BLs, multiplexers  182  connected to respective n source lines SLs, a first driver circuit  190 , and a second driver circuit  192 . Here, the structures described in the above embodiment (the structure illustrated in  FIG. 2 ) are applied to the memory cells  170 , the multiplexers  180 , and the multiplexers  182 . That is, the basic structure and operation are similar to those in  FIG. 2 . 
     Output signal lines BL_OUT, the read circuit input signal lines RS, and the write and read control signal line WREN are connected to the first driver circuit  190 . Here, the output signal line BL_OUT is a line which outputs data from the memory cell. 
     A chip-enable bar signal line CEB, an address selection signal line A, a write and read control signal line WREN, the write word lines WWLs, and the read word lines RWLs are connected to the second driver circuit  192 . Here, the chip-enable bar signal line CEB is a line which transmits a signal which activates the second driver circuit  192 . For example, when the potential of the chip-enable bar signal line CEB is low, the second driver circuit  192  is activated and outputs a high potential. When the potential of the chip-enable bar signal line CEB is high, the second driver circuit  192  outputs a low potential. The address selection signal line A is a line which transmits a signal for selecting a row address. 
       FIG. 4  is an example of a timing diagram of the semiconductor device in  FIG. 3 . CEB, WREN, and the like in the timing diagram denote the lines to which the potentials in the timing diagram are applied. Lines having a similar function are distinguished by “ — 1”, “ — 2”, and the like added to the end of their names. Note that for the sake of simplicity, the semiconductor device described here is an example where the memory cells  170  are arranged in 2 (rows)×2 (columns), but the disclosed invention is not limited to this. 
     The timing diagram in  FIG. 4  shows the relation of the potentials of the lines in the case where data “1” is written to all the memory cells (Writing  1 ), then the written data in all the memory cells is read out (Reading  1 ), then data “1” is written to the memory cells in the first row and the first column and in the second row and the second column while data “0” is written to the memory cells in the first row and the second column and in the second row and the first column (Writing  2 ), and then the written data in all the memory cells is read out (Reading  2 ). 
     In Writing  1 , the CEB has a low potential only when the A has a high potential. The WREN has a high potential to connect the BL_OS to the BL and the SL, thereby allowing writing into the memory cell. The second driver circuit  192  outputs a row selection signal corresponding to the potential of the A, to the RWL and the WWL. Here, when the A has a low potential, a signal which selects the first row is output whereas when the A has a high potential, a signal which selects the second row is output. The RWL and the WWL of the selected row have a high potential. 
     In Writing  1 , to write data “1” to all the memory cells, the BL_OS_ 1  and the BL_OS_ 2  have a high potential in accordance with the timing of the row selection. Note that the signal input periods of the BL_OS_ 1  and the BL_OS_ 2  are set to be longer than the signal input period of the RWL and the WWL. This is because writing failure (here, writing of data “0”) to the memory cell would occur if the signal input periods of the BL_OS_ 1  and the BL_OS_ 2  were shorter. Note that the potentials of the BL_OUT_ 1  and the BL_OUT_ 2  do not matter (may be high or may be low) in writing. 
     In Reading  1 , the CEB has a low potential only when the A has a high potential. The WREN has a low potential to connect the RS and the BL, and the SL 2  and the SL, thereby allowing reading out of the memory cell. The second driver circuit  192  outputs a row selection signal corresponding to the potential of the A, to the RWL and the WWL. Here, when the A has a low potential, a signal which selects the first row is output whereas when the A has a high potential, a signal which selects the second row is output. The RWL of the selected row has a high potential. The WWL has a low potential regardless whether the row is selected or not. 
     Through the above operation, the potential corresponding to the data held in the memory cells in the selected rows is applied to the BL_OUT_ 1  and the BL_OUT_ 2 . Note that the potentials of the BL_OS_ 1  and the BL_OS_ 2  do not matter in reading. 
     The relation among the potentials of the lines in Writing  2  is similar to that in Writing  1 . Note that the BL_OS_ 1  and the BL_OS_ 2  have a high potential or a low potential in accordance with the timing of the row selection in order to write data “1” into the memory cells in the first row and the first column and in the second row and the second column and in order to write data “0” into the memory cells in the first row and the second column and in the second row and the first column. 
     The relation among the potentials of the lines in Reading  2  is similar to that in Reading  1 . It is confirmed that the potential corresponding to the data held in the memory cells in the selected rows is applied to the BL_OUT_ 1  and the BL_OUT_ 2 . 
     Note that in a holding period, the CEB has a high potential and is disabled. Since reading or writing is not performed in the holding period, the other signals do not matter. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures and methods described in the other embodiments. 
     Embodiment 3 
     In this embodiment, a structure and a manufacturing method of a semiconductor device according to an embodiment of the disclosed invention will be described with reference to  FIGS. 5A and 5B ,  FIGS. 6A to 6D ,  FIGS. 7A to 7D ,  FIGS. 8 to 8D , and  FIGS. 9A to 9C . 
     &lt;Cross-sectional Structure and Planar Structure of Semiconductor Device&gt; 
       FIGS. 5A and 5B  illustrate an example of a structure of a semiconductor device.  FIG. 5A  illustrates a cross section of the semiconductor device, and  FIG. 5B  illustrates a plan view of the semiconductor device. Here,  FIG. 5A  illustrates a cross section taken along line A 1 -A 2  and line B 1 -B 2  in  FIG. 5B . The semiconductor device illustrated in  FIGS. 5A and 5B  includes the transistor  160  including a first semiconductor material in a lower portion, and the transistor  162  including a second semiconductor material in an upper portion. Here, the first semiconductor material and the second semiconductor material are preferably different from each other. For example, the first semiconductor material may be a semiconductor material which is not an oxide semiconductor and the second semiconductor material may be an oxide semiconductor. The semiconductor material which is not an oxide semiconductor may be, for example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like, and a single crystal semiconductor is preferably used. Further, an organic semiconductor material may be used. A transistor including such a semiconductor material can easily operate at high speed. On the other hand, a transistor including an oxide semiconductor can hold charge for a long time because of its characteristics. 
     Although both of the transistors are n-channel transistors in this description, it should be appreciated that p-channel transistors can be used. Since the technical feature of the disclosed invention lies in the use of a semiconductor material like an oxide semiconductor, which can sufficiently reduce off-state current, for the transistor  162  in order to hold data, it is not necessary to limit specific conditions such as a structure, material, or the like of the semiconductor device to those given here. 
     The transistor  160  in  FIGS. 5A and 5B  includes a channel formation region  116  provided in a substrate  100  including a semiconductor material (e.g., silicon), impurity regions  120  provided so that the channel formation region  116  is sandwiched therebetween, metal compound regions  124  in contact with the impurity regions  120 , a gate insulating layer  108  provided over the channel formation region  116 , and a gate electrode  110  provided over the gate insulating layer  108 . Note that a transistor whose source electrode and drain electrode are not illustrated in a drawing may be referred to as a transistor for the sake of convenience. Further, in such a case, in description of a connection of a transistor, a source region and a source electrode may be collectively referred to as a source electrode and a drain region and a drain electrode may be collectively referred to as a drain electrode. That is, in this specification, the term “source electrode” may include a source region and the term “drain electrode” may include a drain region. 
     An electrode  126  is connected to part of the metal compound region  124  of the transistor  160 . Here, the electrode  126  serves as a source electrode or a drain electrode of the transistor  160 . Further, the substrate  100  is provided with an element isolation insulating layer  106  which surrounds the transistor  160 . An insulating layer  128  and an insulating layer  130  are provided over the transistor  160 . Note that for high integration, it is preferable that, as in  FIGS. 5A and 5B , the transistor  160  does not have a sidewall insulating layer. On the other hand, when the characteristics of the transistor  160  have priority, the sidewall insulating layer may be formed on a side surface of the gate electrode  110  and the impurity region  120  may include a region having a different impurity concentration in a region overlapping with the sidewall insulating layer. 
     The transistor  162  in  FIGS. 5A and 5B  includes a source or drain electrode  142   a  and a source or drain electrode  142   b  which are provided over the insulating layer  130 ; an oxide semiconductor layer  144  which is electrically connected to the source or drain electrode  142   a  and the source or drain electrode  142   b ; a gate insulating layer  146  which covers the source or drain electrode  142   a , the source or drain electrode  142   b , and the oxide semiconductor layer  144 ; a gate electrode  148   a  which is provided over the gate insulating layer  146  so as to overlap with the oxide semiconductor layer  144 ; an insulating layer  143   a  which overlaps with part of the gate electrode  148   a  and is in a region between the source or drain electrode  142   a  and the oxide semiconductor layer  144 ; and an insulating layer  143   b  which overlaps with part of the gate electrode  148   a  and is in a region between the source or drain electrode  142   b  and the oxide semiconductor layer  144 . Note that the insulating layer  143   a  and the insulating layer  143   b  are preferably provided in order to reduce the capacitance between the source or drain electrode and the gate electrode, but they may be omitted. 
     Here, it is preferable that the oxide semiconductor layer  144  be highly purified by sufficient removal of impurities such as hydrogen and/or sufficient supply of oxygen. Specifically, the concentration of hydrogen in the oxide semiconductor layer  144  is lower than or equal to 5×10 19  atoms/cm 3 , preferably lower than or equal to 5×10 18  atoms/cm 3 , more preferably lower than or equal to 5×10 17  atoms/cm 3 , for example. Note that the concentration of hydrogen in the oxide semiconductor layer  144  is measured by secondary ion mass spectrometry (SIMS). Thus, in the oxide semiconductor layer  144  in which the hydrogen concentration is sufficiently reduced so that the oxide semiconductor layer is highly purified and in which defect states in the energy gap due to oxygen deficiency are reduced by sufficient supply of oxygen, the carrier concentration can be easily reduced. For example, the carrier concentration is lower than 1×10 12 /cm 3 , preferably lower than 1×10 11 /cm 3 , more preferably lower than 1.45×10 10 /cm 3 . In addition, the off-state current of the transistor can be easily reduced. For example, the off-state current (here, current per micrometer (μm) of channel width) at room temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10 −21  A) or less, preferably 10 zA or less. With the use of such an i-type (intrinsic) or substantially i-type oxide semiconductor, the transistor  162  which has significantly excellent off-state current characteristics can be easily obtained. 
     Note that although the transistor  162  in  FIGS. 5A and 5B  includes the oxide semiconductor layer  144  which is processed into an island shape in order to suppress leakage current between elements which is caused due to miniaturization, the oxide semiconductor layer  144  which is not processed into an island shape may be employed. In the case where the oxide semiconductor layer is not processed into an island shape, contamination of the oxide semiconductor layer  144  due to etching in the process can be prevented. 
     The capacitor  164  in  FIGS. 5A and 5B  includes the source or drain electrode  142   a , the oxide semiconductor layer  144 , the gate insulating layer  146 , and an electrode  148   b . The source or drain electrode  142   a  serves as one electrode of the capacitor  164 , and the electrode  148   b  serves as the other electrode of the capacitor  164 . 
     Note that in the capacitor  164  in  FIGS. 5A and 5B , the oxide semiconductor layer  144  and the gate insulating layer  146  are stacked, whereby insulation between the source or drain electrode  142   a  and the electrode  148   b  can be adequately ensured. Needless to say, in order to provide sufficient capacitance, the oxide semiconductor layer  144  in the capacitor  164  may be omitted. Alternatively, an insulating layer formed at the same time as the insulating layer  143   a  may be included in the capacitor  164 . Further alternatively, the capacitor  164  may be omitted if not necessary. 
     Note that in the transistor  162  and the capacitor  164 , end portions of the source or drain electrode  142   a  and the source or drain electrode  142   b  are preferably tapered. When the end portions of the source or drain electrode  142   a  and the source or drain electrode  142   b  are tapered, the coverage with the oxide semiconductor layer  144  can be improved and a breaking thereof can be prevented. Here, the taper angle is 30° to 60° inclusive, for example. Note that the taper angle is a tilt angle formed between a side surface and a bottom surface of a layer having a tapered shape (e.g., the source or drain electrode  142   a ) when the layer is seen in a direction perpendicular to a cross-sectional plane (i.e., the plane perpendicular to the surface of a substrate). 
     In this embodiment, the transistor  162  and the capacitor  164  are provided so as to overlap with the transistor  160 . By employing such a planar layout, high integration is possible. For example, when F is the minimum feature size, the area occupied by a memory cell can be 15 F 2  to 25 F 2  inclusive. 
     An insulating layer  150  is provided over the transistor  162  and the capacitor  164 , and an insulating layer  152  is provided over the insulating layer  150 . In an opening formed in the gate insulating layer  146 , the insulating layer  150 , and the insulating layer  152 , an electrode  154  is provided. Over the insulating layer  152 , a wiring  156  connected to the electrode  154  is formed. Note that although the metal compound region  124 , the source or drain electrode  142   b , and the wiring  156  are connected to each other via the electrode  126  and the electrode  154  in  FIGS. 5A and 5B , the disclosed invention is not limited thereto. For example, the source or drain electrode  142   b  may be in direct contact with the metal compound region  124 , or the wiring  156  may be in direct contact with the source or drain electrode  142   b.    
     Note that in  FIGS. 5A and 5B , the electrode  126  connecting the metal compound region  124  and the source or drain electrode  142   b  and the electrode  154  connecting the source or drain electrode  142   b  and the wiring  156  overlap with each other. In other words, a region where the electrode  126  serving as a source or drain electrode of the transistor  160  and the source or drain electrode  142   b  of the transistor  162  are in contact with each other overlaps with a region where the source or drain electrode  142   b  of the transistor  162  and the wiring  156  connecting one memory cell to another memory cell are in contact with each other. With such a layout, the degree of integration can be increased. 
     &lt;Method for Manufacturing Semiconductor Device&gt; 
     Next, an example of a method for manufacturing the semiconductor device will be described. First, a method for manufacturing the transistor  160  in the lower portion will be described below with reference to  FIGS. 6A to 6D  and  7 A to  7 D, and then a method for manufacturing the transistor  162  and the capacitor in the upper portion will be described with reference to  FIGS. 8A to 8D  and  FIGS. 9A to 9C . 
     &lt;Method for Manufacturing Transistor in Lower Portion&gt; 
     First, the substrate  100  including a semiconductor material is prepared (see  FIG. 6A ). As the substrate  100  including a semiconductor material, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like; a compound semiconductor substrate made of silicon germanium or the like; an SOI substrate; or the like can be used. Here, an example where a single crystal silicon substrate is used as the substrate  100  including a semiconductor material is described. Note that although the term “SOI substrate” generally means a substrate in which a silicon semiconductor layer is provided on an insulating surface, the term “SOI substrate” in this specification and the like also includes a substrate in which a semiconductor layer including a material which is not silicon is provided on an insulating surface. That is, the semiconductor layer included in the “SOI substrate” is not limited to a silicon semiconductor layer. Moreover, the SOI substrate also includes a substrate having a structure in which a semiconductor layer is provided over an insulating substrate such as a glass substrate with an insulating layer interposed therebetween. 
     As the substrate  100  including a semiconductor material, a single crystal semiconductor substrate of silicon or the like is particularly preferable because high-speed reading operation of the semiconductor device can be realized. 
     A protective layer  102  serving as a mask for forming an element isolation insulating layer is formed over the substrate  100  (see  FIG. 6A ). As the protective layer  102 , an insulating layer formed using a material such as silicon oxide, silicon nitride, or silicon oxynitride can be used, for example. Note that before or after this step, an impurity element imparting n-type conductivity or p-type conductivity may be added to the substrate  100  in order to control the threshold voltage of the transistor. When the semiconductor material in the substrate  100  is silicon, phosphorus, arsenic, or the like can be used as the impurity imparting n-type conductivity, and boron, aluminum, gallium, or the like can be used as the impurity imparting p-type conductivity. 
     Then, part of the substrate  100  in a region that is not covered with the protective layer  102  (in an exposed region) is removed by etching using the protective layer  102  as a mask. Thus, a semiconductor region  104  isolated from the other semiconductor regions is formed (see  FIG. 6B ). As the etching, dry etching is preferably performed, but wet etching may be performed. An etching gas and an etchant can be selected as appropriate in accordance with a material to be etched. 
     Then, an insulating layer is formed so as to cover the semiconductor region  104 , and the insulating layer in a region overlapping with the semiconductor region  104  is selectively removed, whereby the element isolation insulating layer  106  is formed (see  FIG. 6C ). The insulating layer is formed using silicon oxide, silicon nitride, silicon oxynitride, or the like. For removing the insulating layer, any of etching treatment, polishing treatment such as CMP (chemical mechanical polishing), and the like can be employed. Note that the protective layer  102  is removed after the formation of the semiconductor region  104  or after the formation of the element isolation insulating layer  106 . 
     Note that the element isolation insulating layer  106  may be formed by, instead of selectively removing an insulating layer, implanting oxygen to form an insulating region, for example. 
     Then, an insulating layer is formed on a surface of the semiconductor region  104 , and a layer including a conductive material is formed over the insulating layer. 
     The insulating layer is to be a gate insulating layer later, and can be formed by heat treatment (thermal oxidation treatment, thermal nitridation treatment, or the like) on the surface of the semiconductor region  104 , for example. High-density plasma treatment may be employed in stead of heat treatment. The high-density plasma treatment can be performed using, for example, a mixed gas of a rare gas such as He, Ar, Kr, or Xe, oxygen, nitrogen oxide, ammonia, nitrogen, or hydrogen. Needless to say, the insulating layer may be formed by a CVD method, a sputtering method, or the like. The insulating layer preferably has a single-layer structure or stacked structure including silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, aluminum oxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)), hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)) to which nitrogen is added, hafnium aluminate (HfAl x O y  (x&gt;0, y&gt;0)) to which nitrogen is added, and the like. The insulating layer can have a thickness of, for example, 1 nm to 100 nm inclusive, preferably 10 nm to 50 nm inclusive. 
     The layer including a conductive material can be formed using a metal material such as aluminum, copper, titanium, tantalum, or tungsten. Further, the layer including a conductive material may be formed using a semiconductor material such as polycrystalline silicon. There is no particular limitation on the method for forming the layer including a conductive material, and any of a variety of film formation methods such as an evaporation method, a CVD method, a sputtering method, and a spin coating method can be employed. Note that in this embodiment, an example where the layer including a conductive material is formed using a metal material is described. 
     After that, the insulating layer and the layer including a conductive material are selectively etched, whereby the gate insulating layer  108  and the gate electrode  110  are formed (see  FIG. 6C ). 
     Then, phosphorus (P), arsenic (As), or the like is added to the semiconductor region  104 , whereby the channel formation region  116  and the impurity region  120  are formed (see  FIG. 6D ). Note that phosphorus or arsenic is added here in order to form an n-channel transistor; an impurity element such as boron (B) or aluminum (Al) may be added when a p-channel transistor is formed. Here, the concentration of the impurity which is added can be set as appropriate; the concentration is preferably increased when the semiconductor element is highly miniaturized. 
     Note that a sidewall insulating layer may be formed around the gate electrode  110  and an impurity region to which an impurity element is added at a different concentration may be formed. 
     Then, a metal layer  122  is formed so as to cover the gate electrode  110 , the impurity region  120 , and the like (see  FIG. 7A ). Any of a variety of film formation methods such as a vacuum evaporation method, a sputtering method, and a spin coating method can be employed for forming the metal layer  122 . It is preferable that the metal layer  122  be formed using a metal material that reacts with a semiconductor material included in the semiconductor region  104  to form a low-resistance metal compound. Examples of such a metal material include titanium, tantalum, tungsten, nickel, cobalt, and platinum. 
     Then, heat treatment is performed so that the metal layer  122  may react with the semiconductor material. Thus, the metal compound region  124  which is in contact with the impurity region  120  is formed (see  FIG. 7A ). Note that when the gate electrode  110  is formed using polycrystalline silicon or the like, a metal compound region is also formed in a region of the gate electrode  110  which is in contact with the metal layer  122 . 
     As the heat treatment, irradiation with a flash lamp can be employed, for example. Although it should be appreciated that another heat treatment method may be used, a method by which the heat treatment can be achieved in an extremely short time is preferably used in order to improve the controllability of chemical reaction for formation of the metal compound. Note that the metal compound region is formed by the reaction of the metal material and the semiconductor material and has sufficiently high conductivity. The formation of the metal compound region can sufficiently reduce electric resistance and improve element characteristics. Note that the metal layer  122  is removed after the metal compound region  124  is formed. 
     Then, the electrode  126  is formed in a region in contact with part of the metal compound region  124  (see  FIG. 7B ). The electrode  126  is formed by, for example, forming a layer including a conductive material and then selectively etching the layer. The layer including a conductive material can be formed using a metal material such as aluminum, copper, titanium, tantalum, or tungsten. Further, the layer including a conductive material may be formed using a semiconductor material such as polycrystalline silicon. There is no particular limitation on the method for forming the layer including a conductive material, and any of a variety of film formation methods such as an evaporation method, a CVD method, a sputtering method, and a spin coating method can be employed. 
     Alternatively, the electrode  126  can be formed by forming an opening reaching the metal compound region  124  in the insulating layer  128  and the insulating layer  130  after the formation of the insulating layer  128  and the insulating layer  130 , and then by filling the opening. 
     In such a case, for example, it is possible to employ a method in which a thin titanium film is formed by a PVD method and a thin titanium nitride film is formed by a CVD method in a region including the opening, and then a tungsten film is formed so as to fill the opening. Here, the titanium film formed by a PVD method has a function of reducing an oxide film (such as a native oxide film) over which the titanium film is to be formed, and thereby lowering contact resistance with lower electrodes or the like (the metal compound regions  124 , here). The titanium nitride film formed after the formation of the titanium film has a barrier function of suppressing diffusion of the conductive material. A copper film may be formed by a plating method after the formation of the barrier film of titanium, titanium nitride, or the like. 
     Then, the insulating layer  128  and the insulating layer  130  are formed so as to cover the components formed in the above steps (see  FIG. 7C ). The insulating layers  128  and  130  can be formed using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, or aluminum oxide. In particular, a material with a low dielectric constant (a low-k material) is preferably used for the insulating layer  128  and the insulating layer  130 , because capacitance due to overlap of electrodes and/or wirings can be sufficiently reduced. Note that the insulating layer  128  and the insulating layer  130  may be a porous insulating layer formed using any of those materials. A porous insulating layer has a lower dielectric constant than an insulating layer with high density, and thus allows a further reduction in capacitance generated by electrodes and/or wirings. Further, the insulating layer  128  and the insulating layer  130  can be formed using an organic insulating material such as polyimide or acrylic. Note that although a stacked structure of the insulating layer  128  and the insulating layer  130  is employed here, one embodiment of the disclosed invention is not limited to this. A single-layer structure or a stacked structure including three or more layers can be used. 
     Through the above steps, the transistor  160  is formed with the use of the substrate  100  including a semiconductor material (see  FIG. 7C ). The transistor  160  can operate at high speed. By using this transistor as a read transistor, data can be read out at high speed. 
     Then, as a pretreatment for the formation of the transistor  162  and the capacitor  164 , CMP treatment is performed on the insulating layer  128  and the insulating layer  130  to expose the upper surfaces of the gate electrode  110  and the electrode  126  (see  FIG. 7D ). As the treatment for exposing the upper surfaces of the gate electrode  110  and the electrode  126 , etching treatment may be employed as an alternative to (or in combination with) CMP treatment. Note that it is preferable to planarize the surface of the insulating layer  128  and the insulating layer  130  as much as possible in order to improve the characteristics of the transistor  162 . 
     Note that a step of forming an electrode, a wiring, a semiconductor layer, an insulating layer, or the like may be further included before and after each of the above steps. For example, the wiring may have a multi-layer structure including a stacked structure of an insulating layer and a conductive layer to provide a highly integrated semiconductor device. 
     &lt;Method for Manufacturing Transistor in Upper Portion&gt; 
     Then, a conductive layer is formed over the gate electrode  110 , the electrode  126 , the insulating layer  128 , the insulating layer  130 , and the like and is selectively etched, whereby the source or drain electrode  142   a  and the source or drain electrode  142   b  are formed (see  FIG. 8A ). 
     The conductive layer can be formed by a PVD method typified by a sputtering method or a CVD method such as a plasma CVD method. As a material for the conductive layer, an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy including any of these elements as a component; or the like can be used. Any of manganese, magnesium, zirconium, beryllium, neodymium, scandium, or a material including any of these in combination may be used. 
     The conductive layer may have a single-layer structure or a stacked structure including two or more layers. For example, a single-layer structure of a titanium film or a titanium nitride film, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked, and the like can be given. Note that in the case where the conductive layer has a single-layer structure of a titanium film or a titanium nitride film, there is an advantage that the conductive layer is easily processed into the source or drain electrode  142   a  and the source or drain electrode  142   b  having tapered shapes. 
     Alternatively, the conductive layer may be formed using a conductive metal oxide. The conductive metal oxide can be indium oxide (In 2 O 3 ), tin oxide (SnO 2 ), zinc oxide (ZnO), an indium oxide-tin oxide alloy (In 2 O 3 —SnO 2 , which may be abbreviated to ITO), an indium oxide-zinc oxide alloy (In 2 O 3 —ZnO), or any of these metal oxide materials including silicon or silicon oxide. 
     The conductive layer is preferably etched so that end portions of the source or drain electrode  142   a  and the source or drain electrode  142   b  may be tapered. Here, the taper angle is preferably 30° to 60° inclusive, for example. When the source or drain electrode  142   a  and the source or drain electrode  142   b  are etched to have tapered end portions, the coverage with the gate insulating layer  146  formed later can be improved and breaking thereof can be prevented. 
     The channel length (L) of the transistor in the upper portion is determined by the distance between a lower end portion of the source or drain electrode  142   a  and a lower end portion of the source or drain electrode  142   b . Note that in light exposure for forming a mask for a transistor with a channel length (L) of less than 25 nm, it is preferable to use extreme ultraviolet rays whose wavelength is as short as several nanometers to several tens of nanometers. The resolution of light exposure with extreme ultraviolet rays is high and the depth of focus is large. Accordingly, the channel length (L) of the transistor formed later can be 10 nm to 1000 nm (1 μm) inclusive, whereby an operation speed of a circuit can be increased. Moreover, power consumption of the semiconductor device can be reduced by miniaturization. 
     Note that an insulating layer serving as a base may be provided over the insulating layer  128  and the insulating layer  130 . The insulating layer can be formed by a PVD method, a CVD method, or the like. 
     Then, the insulating layer  143   a  and the insulating layer  143   b  are formed over the source or drain electrode  142   a  and the source or drain electrode  142   b , respectively (see  FIG. 8B ). The insulating layer  143   a  and the insulating layer  143   b  can be formed by forming an insulating layer so as to cover the source or drain electrode  142   a  and the source or drain electrode  142   b  and then selectively etching the insulating layer. The insulating layer  143   a  and the insulating layer  143   b  are formed to overlap with part of a gate electrode formed later. By forming those insulating layers, the capacitance between the gate electrode and the source or drain electrode can be reduced. 
     The insulating layers  143   a  and  143   b  can be formed using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, or aluminum oxide. In particular, a material with a low dielectric constant (a low-k material) is preferably used for the insulating layer  143   a  and the insulating layer  143   b , because the capacitance between the gate electrode and the source or drain electrode can be sufficiently reduced. Note that the insulating layer  143   a  and the insulating layer  143   b  may be a porous insulating layer formed using any of those materials. A porous insulating layer has a lower dielectric constant than an insulating layer with high density, and thus allows a further reduction in capacitance between the gate electrode and the source or drain electrode. 
     Note that although the insulating layers  143   a  and  143   b  are preferably provided for reduction in capacitance between the gate electrode and the source or drain electrode, the insulating layers may be omitted. 
     Then, after an oxide semiconductor layer is formed so as to cover the source or drain electrode  142   a  and the source or drain electrode  142   b , the oxide semiconductor layer is selectively etched, whereby the oxide semiconductor layer  144  is formed (see  FIG. 8C ). 
     The oxide semiconductor layer can be formed using an In—Sn—Ga—Zn—O-based oxide semiconductor which is four-component metal oxide; 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, or a Sn—Al—Zn—O-based oxide semiconductor which are three-component metal oxide; 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, or an In—Mg—O-based oxide semiconductor which are two-component metal oxide; an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, a Zn—O-based oxide semiconductor; or the like. 
     In particular, an In—Ga—Zn—O-based oxide semiconductor material has sufficiently high resistance without an electric field and thus the off-state current can be sufficiently reduced. In addition, the In—Ga—Zn—O-based oxide semiconductor material has high field-effect mobility and thus is suitable as a semiconductor material used for a semiconductor device. 
     As a typical example of the In—Ga—Zn—O-based oxide semiconductor material, the one represented by InGaO 3 (ZnO) m  (m&gt;0) is given. In addition, an oxide semiconductor material in which Ga is replaced by M and which is represented by InMO 3 (ZnO) m  (m&gt;0) can also be given. Here, M denotes one or more metal elements selected from gallium (Ga), aluminum (Al), iron (Fe), nickel (Ni), manganese (Mn), cobalt (Co), and the like. For example, M can be Ga, Ga and Al, Ga and Fe, Ga and Ni, Ga and Mn, Ga and Co, or the like. Note that the above compositions are merely examples given in accordance with a crystal structure. 
     As a target used for forming the oxide semiconductor layer by a sputtering method, a target having a composition ratio expressed by the equation In:Ga:Zn=1:x:y (x is 0 or more, and y is 0.5 to 5 inclusive) is preferable. For example, a target having a composition ratio expressed by the equation In 2 O 3 :Ga 2 O 3 :ZnO=1:1:2 [molar ratio], or the like can be used. Alternatively, a target having a composition ratio expressed by the equation In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [molar ratio], a target having a composition ratio expressed by the equation In 2 O 3 :Ga 2 O 3 :ZnO=1:1:4 [molar ratio], or a target having a composition ratio expressed by the equation In 2 O 3 :ZnO=1:2 [molar ratio] can be used. 
     In this embodiment, an oxide semiconductor layer having an amorphous structure is formed by a sputtering method using an In—Ga—Zn—O-based metal oxide target. 
     The relative density of the metal oxide in the metal oxide target is 80% or higher, preferably 95% or higher, more preferably 99.9% or higher. With the use of the metal oxide target with high relative density, the oxide semiconductor layer can be formed to have a dense structure. 
     The atmosphere in which the oxide semiconductor layer is formed is preferably a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere including a rare gas (typically argon) and oxygen. Specifically, for example, an atmosphere of a high-purity gas is preferable from which impurities such as hydrogen, water, hydroxyl, or hydride are removed so that the concentration thereof is reduced to 1 ppm or less (preferably to 10 ppb or less). 
     In the formation of the oxide semiconductor layer, for example, an object to be processed is held in a process chamber that is kept under reduced pressure and the object is heated so that the temperature of the object becomes higher than or equal to 100° C. and lower than 550° C., preferably 200° C. to 400° C. inclusive. Alternatively, the temperature of the object in the formation of the oxide semiconductor layer may be room temperature (15° C. to 35° C. inclusive). Moisture in the process chamber is removed, a sputtering gas from which hydrogen, water, and the like are removed is introduced, and the above target is used, and thus the oxide semiconductor layer is formed. By forming the oxide semiconductor layer while heating the object, impurities in the oxide semiconductor layer can be reduced. In addition, damage by sputtering can be reduced. In order to remove moisture in the process chamber, an entrapment vacuum pump is preferably used. For example, a cryopump, an ion pump, a titanium sublimation pump, or the like can be used. Further, a turbo pump provided with a cold trap may be used. By evacuation with the cryopump or the like, hydrogen, water, and the like can be removed from the process chamber, whereby the impurity concentration of the oxide semiconductor layer can be reduced. 
     The oxide semiconductor layer can be formed under the following conditions, for example: the distance between the object and the target is 170 mm, the pressure is 0.4 Pa, the direct current (DC) power is 0.5 kW, and the atmosphere is an oxygen (oxygen: 100%) atmosphere, an argon (argon: 100%) atmosphere, or a mixed atmosphere including oxygen and argon. Note that a pulsed direct current (DC) power source is preferable because powdery substances (also referred to as particles or dust) formed at the time of film formation can be reduced and the film thickness can be uniform. The thickness of the oxide semiconductor layer is 1 nm to 50 nm inclusive, preferably 1 nm to 30 nm inclusive, more preferably 1 nm to 10 nm inclusive. With the oxide semiconductor layer having such a thickness, a short-channel effect which occurs along with miniaturization can be suppressed. Note that the appropriate thickness differs depending on the oxide semiconductor material to be used, the intended use of the semiconductor device, or the like; therefore, the thickness can also be determined in accordance with the material, the intended use, or the like. 
     Note that before the oxide semiconductor layer is formed by a sputtering method, a substance attached to a surface where the oxide semiconductor layer is to be formed (e.g., a surface of the insulating layer  130 ) is preferably removed by reverse sputtering in which an argon gas is introduced to generate plasma. Here, the reverse sputtering refers to a method in which ions collide with a surface of the substrate to be processed in order to modify the surface, in contrast to normal sputtering in which ions collide with a sputtering target. As a method for making ions collide with a surface to be processed, a method is given for example where high-frequency voltage is applied to the surface to be processed in an argon atmosphere and plasma is generated in the vicinity of the object to be processed. Note that the atmosphere may be nitrogen, helium, oxygen, or the like instead of an argon atmosphere. 
     After that, heat treatment (first heat treatment) is preferably performed on the oxide semiconductor layer. By the first heat treatment, excessive hydrogen (including water and hydroxyl) in the oxide semiconductor layer can be removed, the structure of the oxide semiconductor layer can be improved, and defect states in the energy gap can be reduced. The temperature of the first heat treatment is, for example, higher than or equal to 300° C. and lower than 550° C., or 400° C. to 500° C. inclusive. 
     The heat treatment can be performed in such a manner that, for example, the object to be processed is introduced into an electric furnace provided with a resistance heating element or the like and heated at 450° C. for one hour in a nitrogen atmosphere. During the heat treatment, the oxide semiconductor layer is not exposed to air to prevent entry of water and hydrogen. 
     The heat treatment apparatus is not limited to the electric furnace and may be an apparatus for heating an object by thermal conduction or thermal radiation from a medium such as a heated gas. For example, a rapid thermal annealing (RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA) 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 gas, an inert gas that does not react with the object by heat treatment, for example, nitrogen or a rare gas such as argon is used. 
     For example, as the first heat treatment, GRTA treatment may be performed in which the object is put in a heated inert gas atmosphere and heated for several minutes, and then taken out of the inert gas atmosphere. GRTA treatment enables high-temperature heat treatment for a short time. Moreover, GRTA treatment enables treatment at a temperature which exceeds the upper temperature limit. Note that the inert gas may be changed to a gas including oxygen during the treatment. This is because defect states in the energy gap due to oxygen vacancies can be reduced by performing the first heat treatment in an atmosphere including oxygen. 
     Note that as the inert gas atmosphere, an atmosphere that includes nitrogen or a rare gas (such as helium, neon, or argon) as a main component and does not include water, hydrogen, or the like is preferably used. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into the heat treatment apparatus is higher than or equal to 6N (99.9999%), preferably higher than or equal to 7N (99.99999%) (that is, the concentration of an impurity is lower than or equal to 1 ppm, preferably lower than or equal to 0.1 ppm). 
     In any case, impurities are reduced by the first heat treatment so that the i-type (intrinsic) or substantially i-type oxide semiconductor layer is obtained. Accordingly, a transistor having significantly excellent characteristics can be realized. 
     The above heat treatment (first heat treatment) has an effect of removing hydrogen, water, and the like and thus can be referred to as dehydration treatment, dehydrogenation treatment, or the like. The dehydration treatment or the dehydrogenation treatment can also be performed at the following timing: after the formation of the oxide semiconductor layer, after the formation of the gate insulating layer, after the formation of the gate electrode, or the like. Such dehydration treatment or dehydrogenation treatment may be performed once or more times. 
     The etching of the oxide semiconductor layer may be performed before or after the heat treatment. In addition, although dry etching is favorable in terms of miniaturization of elements, wet etching may be used. An etching gas and an etchant can be selected as appropriate in accordance with a material to be etched. Note that in the case where a problem of leakage current and the like between elements does not occur, the oxide semiconductor layer may be used without being etched into an island shape. 
     Then, the gate insulating layer  146  is formed in contact with the oxide semiconductor layer  144 . After that, over the gate insulating layer  146 , the gate electrode  148   a  is formed in a region overlapping with the oxide semiconductor layer  144  and the electrode  148   b  is formed in a region overlapping with the source or drain electrode  142   a  (see  FIG. 8D ). 
     The gate insulating layer  146  can be formed by a CVD method, a sputtering method, or the like. The gate insulating layer  146  preferably includes silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, tantalum oxide, hafnium oxide, yttrium oxide, hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)), hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)) to which nitrogen is added, hafnium aluminate (HfAl x O y  (x&gt;0, y&gt;0)) to which nitrogen is added, or the like. The gate insulating layer  146  may have a single-layer structure or a stacked structure. There is no particular limitation on the thickness of the gate insulating layer  146 ; in the case where the semiconductor device is miniaturized, the gate insulating layer  146  is preferably thin in order to ensure the operation of the transistor. For example, in the case of using silicon oxide, the thickness can be 1 nm to 100 nm inclusive, preferably 10 nm to and 50 nm inclusive. 
     When the gate insulating layer is thin as in the above description, a problem of gate leakage due to a tunneling effect or the like is caused. In order to solve the problem of gate leakage, it is preferable that the gate insulating layer  146  be formed using a high dielectric constant (high-k) material such as hafnium oxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)), hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)) to which nitrogen is added, or hafnium aluminate (HfAl x O y  (x&gt;0, y&gt;0)) to which nitrogen is added. With the use of a high-k material for the gate insulating layer  146 , the thickness of the gate insulating layer  146  can be increased to prevent gate leakage and at the same time electrical characteristics can be maintained. Note that a stacked structure of a film including a high-k material and a film including any of silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, and the like may be employed. 
     After the gate insulating layer  146  is formed, second heat treatment is preferably performed in an inert gas atmosphere or an oxygen atmosphere. The temperature of the heat treatment is 200° C. to 450° C. inclusive, preferably 250° C. to 350° C. inclusive. For example, the heat treatment may be performed at 250° C. for one hour in a nitrogen atmosphere. The second heat treatment can reduce variation in electric characteristics of the transistors. Moreover, in the case where the gate insulating layer  146  includes oxygen, oxygen can be supplied to the oxide semiconductor layer  144  to fill oxygen vacancies in the oxide semiconductor layer  144 , whereby an i-type (intrinsic) or substantially i-type oxide semiconductor layer can be formed. 
     Note that the second heat treatment is performed after the gate insulating layer  146  is formed in this embodiment; however, the timing of the second heat treatment is not limited to this. For example, the second heat treatment may be performed after the gate electrode is formed. Alternatively, the first heat treatment and the second heat treatment may be successively performed, the first heat treatment may double as the second heat treatment, or the second heat treatment may double as the first heat treatment. 
     As described above, at least one of the first heat treatment and the second heat treatment is employed, whereby impurities which are not the main components of the oxide semiconductor are excluded as much as possible and the oxide semiconductor layer  144  can be highly purified. 
     The gate electrode  148   a  and the electrode  148   b  can be formed by forming a conductive layer over the gate insulating layer  146  and then selectively etching the conductive layer. The conductive layer to be processed into the gate electrode  148   a  and the electrode  148   b  can be formed by a PVD method typified by a sputtering method or a CVD method such as a plasma CVD method. The details are similar to those of the source or drain electrode  142   a  or the like; thus, description thereof can be referred to. 
     Then, the insulating layer  150  and the insulating layer  152  are formed over the gate insulating layer  146 , the gate electrode  148   a , and the electrode  148   b  (see  FIG. 9A ). The insulating layer  150  and the insulating layer  152  can be formed by a PVD method, a CVD method, or the like. The insulating layer  150  and the insulating layer  152  can be formed using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, or aluminum oxide. 
     Note that for the insulating layer  150  and the insulating layer  152 , a material with a low dielectric constant may be preferably used or a structure with a low dielectric constant (e.g., a porous structure) may be preferably employed. This is because by reducing the dielectric constant of the insulating layer  150  and the insulating layer  152 , capacitance between wirings and electrodes can be reduced, which will increase operation speed. 
     Note that although a stacked structure of the insulating layer  150  and the insulating layer  152  is employed in this embodiment, one embodiment of the disclosed invention is not limited to this. A single-layer structure or a stacked structure including three or more layers can be used. Alternatively, the insulating layer may be omitted. 
     Note that the insulating layer  152  is preferably formed so as to have a planarized surface. By forming the insulating layer  152  having a planarized surface, an electrode, a wiring, or the like can be favorably formed over the insulating layer  152  even in the case where the semiconductor device is miniaturized, for example. The insulating layer  152  can be planarized by a method such as chemical mechanical polishing (CMP). 
     Then, an opening reaching the source or drain electrode  142   b  is formed in the gate insulating layer  146 , the insulating layer  150 , and the insulating layer  152  (see  FIG. 9B ). The opening is formed by selective etching using a mask or the like. 
     Here, the opening is preferably formed in a region overlapping with the electrode  126 . By forming the opening in the region, the element area can be prevented from increasing due to a contact region of the electrodes. That is, the degree of integration of the semiconductor device can be increased. 
     Then, the electrode  154  is formed in the opening and the wiring  156  in contact with the electrode  154  is formed over the insulating layer  152  (see  FIG. 9C ). 
     The electrode  154  can be formed in such a manner that, for example, a conductive layer is formed by a PVD method, a CVD method, or the like in a region including the opening and then part of the conductive layer is removed by etching treatment, CMP, or the like. 
     For a specific example, it is possible to employ a method in which a thin titanium film is formed by a PVD method in a region including the opening, a thin titanium nitride film is formed by a CVD method, and then a tungsten film is formed so as to fill the opening. Here, the titanium film formed by a PVD method has a function of reducing an oxide film (such as a native oxide film) over which the titanium film is to be formed, and thereby lowering contact resistance with lower electrodes or the like (the source or drain electrode  142   b , here). The titanium nitride film formed after the formation of the titanium film has a barrier function of suppressing diffusion of the conductive material. A copper film may be formed by a plating method after the formation of the barrier film of titanium, titanium nitride, or the like. 
     Note that in the case where the electrode  154  is formed by removing part of the conductive layer, it is preferable that a surface of the electrode  154  be processed to be flat. For example, when a thin titanium film or a thin titanium nitride film is formed in a region including the opening and then a tungsten film is formed so as to fill the opening, an unnecessary portion of the tungsten film, titanium film, titanium nitride film, or the like can be removed and the planarity of the surface of the electrode  154  can be improved by subsequent CMP treatment. Further, such treatment for improving the planarity can planarize the entire surface including the surface of the electrode  154 . By planarizing the entire surface including the surface of the electrode  154 , an electrode, a wiring, an insulating layer, a semiconductor layer, and the like can be favorably formed in later steps. 
     The wiring  156  is formed by forming a conductive layer by a PVD method typified by a sputtering method or a CVD method such as a plasma CVD method and then patterning the conductive layer. As a material for the conductive layer, an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy including any of these elements as a component; or the like can be used. Any of manganese, magnesium, zirconium, beryllium, neodymium, scandium, or a material including any of these in combination may be used. The details are similar to those of the source or drain electrode  142   a  or the like. 
     Through the above steps, the transistor  162  including the highly purified oxide semiconductor layer  144  and the capacitor  164  are completed (see  FIG. 9C ). 
     In the transistor  162  described in this embodiment, the oxide semiconductor layer  144  is highly purified and thus the hydrogen concentration thereof is lower than or equal to 5×10 19  atoms/cm 3 , preferably lower than or equal to 5×10 18  atoms/cm 3 , more preferably lower than or equal to 5×10 17  atoms/cm 3 . The carrier density of the oxide semiconductor layer  144  is sufficiently low (e.g., lower than 1×10 12 /cm 3 , preferably 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 ). Accordingly, the off-state current is also sufficiently small. For example, the off-state current (here, current per micrometer (μm) of channel width) of the transistor  162  at room temperature (25° C.) is smaller than or equal to 100 zA (1 zA (zeptoampere) is 1×10 −21  A), preferably smaller than or equal to 10 zA. 
     With the use of the highly purified intrinsic oxide semiconductor layer  144 , the off-state current of the transistor can be sufficiently reduced easily. Further, with the use of such a transistor, a semiconductor device capable of holding stored data for an extremely long time can be obtained. 
     Further, by forming the electrode  126  and the electrode  154  so as to overlap with each other, the element area can be prevented from increasing due to a contact region of the electrodes. Thus, higher integration can be realized. Further, in the semiconductor device in this embodiment, wirings can be shared to realize a semiconductor device having a sufficiently increased integration degree. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures and methods described in the other embodiments. 
     Embodiment 4 
     In this embodiment, the case where the semiconductor device described in any of the above embodiments is applied to an electronic device will be described with reference to  FIGS. 10A to 10F . In this embodiment, the case where the above semiconductor device is applied to an electronic device such as a computer, a mobile phone (also referred to as a mobile telephone or a mobile telephone device), a portable information terminal (including a portable game console, an audio player, and the like), a digital camera, a digital video camera, an electronic paper, or a television device (also referred to as a television or a television receiver) will be described. 
       FIG. 10A  is a notebook personal computer including a housing  701 , a housing  702 , a display portion  703 , a keyboard  704 , and the like. The semiconductor device described in any of the above embodiments is provided in at least one of the housing  701  and the housing  702 . Consequently, the notebook personal computer can perform writing and reading data at high speed and store data for a long time with sufficiently reduced power consumption. 
       FIG. 10B  is a portable information terminal (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  for operating the portable information terminal or the like is also provided. The semiconductor device described in any of the above embodiments is provided in the main body  711 . Consequently, the portable information terminal can perform writing and reading of data at high speed and store data for a long time with sufficiently reduced power consumption. 
       FIG. 10C  is an e-book reader  720  mounting an electronic paper. The e-book reader has two housings, 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 housing  721  and the housing  723  are connected by a hinge  737  and can be opened and closed along the hinge  737 . Further, the housing  721  is provided with a power switch  731 , operation keys  733 , a speaker  735 , and the like. At least one of the housing  721  and the housing  723  is provided with the semiconductor device described in any of the above embodiments. Consequently, the e-book reader can perform writing and reading of data at high speed and store data for a long time with sufficiently reduced power consumption. 
       FIG. 10D  is a mobile phone including two housings, a housing  740  and a housing  741 . Further, the housing  740  and the housing  741  in a state where they are developed as illustrated in  FIG. 10D  can be slid 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 , an operation key  745 , 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  that charges the mobile phone, an external memory slot  750 , and the like. In addition, an antenna is incorporated in the housing  741 . At least one of the housing  740  and the housing  741  is provided with the semiconductor device described in any of the above embodiments. Consequently, the mobile phone can perform writing and reading of data at high speed and store data for a long time with sufficiently reduced power consumption. 
       FIG. 10E  is 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 any of the above embodiments is provided in the main body  761 . Consequently, the digital camera can perform writing and reading of data at high speed and store data for a long time with sufficiently reduced power consumption. 
       FIG. 10F  is 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 by an operation switch of the housing  771  or a remote control  780 . The semiconductor device described in any of the above embodiments is mounted in the housing  771  and the remote control  780 . Consequently, the television device can perform writing and reading of data at high speed and store data for a long time with sufficiently reduced power consumption. 
     Thus, the semiconductor device according to any of the above embodiments is mounted in the electronic devices described in this embodiment. Accordingly, electronic devices with low power consumption can be realized. 
     Example 1 
     In this example, measurement results of the off-state current of a transistor including a purified oxide semiconductor will be described. 
     First, a transistor with a channel width W of 1 m, which was sufficiently large, was prepared in consideration of the fact that off-state current of a transistor including a purified oxide semiconductor is extremely small, and the off-state current was measured.  FIG. 11  shows the results obtained by the measurement of the off-state current of the transistor with a channel width W of 1 m. In  FIG. 11 , the horizontal axis represents a gate voltage VG and the vertical axis represents a drain current ID. In the case where the drain voltage VD is +1 V or +10 V and the gate voltage VG is within the range of −5 V to −20 V, the off-state current of the transistor was found to be smaller than or equal to 1×10 −12  A, which is the detection limit. In addition, it was found that the off-state current (here, current per micrometer (μm) of channel width) of the transistor is smaller than or equal to 1 aA/μm (1×10 −18  A/μm). 
     Described next are the results obtained by more accurately measuring the off-state current of the transistor including a purified oxide semiconductor. As described above, the off-state current of the transistor including a purified oxide semiconductor was found to be smaller than or equal to 1×10 −12  A, which is the detection limit of the measurement equipment. Here, the results obtained by measuring a more accurate off-state current value (the value smaller than or equal to the detection limit of the measurement equipment in the above measurement) with the use of an element for characteristic evaluation will be described. 
     First, the element for characteristic evaluation which was used in a method for measuring current will be described with reference to  FIG. 12 . 
     In the element for characteristic evaluation in  FIG. 12 , three measurement systems  800  are connected in parallel. The measurement system  800  includes a capacitor  802 , a transistor  804 , a transistor  805 , a transistor  806 , and a transistor  808 . A transistor including a purified oxide semiconductor was used as each of the transistor  804  and the transistor  808 . 
     In the measurement system  800 , one of a source terminal and a drain terminal of the transistor  804 , one terminal of the capacitor  802 , and one of a source terminal and a drain terminal of the transistor  805  are connected to a power source (for supplying V 2 ). The other of the source terminal and the drain terminal of the transistor  804 , one of a source terminal and a drain terminal of the transistor  808 , the other terminal of the capacitor  802 , and a gate terminal of the transistor  805  are connected to one another. The other of the source terminal and the drain terminal of the transistor  808 , one of a source terminal and a drain terminal of the transistor  806 , and a gate terminal of the transistor  806  are connected to a power source (for supplying V 1 ). The other of the source terminal and the drain terminal of the transistor  805  and the other of the source terminal and the drain terminal of the transistor  806  are connected to each other and serve as an output terminal. 
     A potential Vext_b 2  for controlling whether the transistor  804  is turned on or off is applied to a gate terminal of the transistor  804 . A potential Vext_b 1  for controlling whether the transistor  808  is turned on or off is applied to the gate terminal of the transistor  808 . A potential Vout is output from the output terminal. 
     Next, a method for measuring current with the use of the element for characteristic evaluation will be described. 
     First, an initialization period in which a potential difference is applied to measure the off-state current will be briefly described. In the initialization period, the potential Vext_b 1  for turning on the transistor  808  is input to the gate terminal of the transistor  808 , and a potential V 1  is applied to a node A which is a node connected to the other of the source terminal and the drain terminal of the transistor  804  (i.e., the node connected to the one of the source terminal and the drain terminal of the transistor  808 , the other terminal of the capacitor  802 , and the gate terminal of the transistor  805 ). Here, the potential V 1  is, for example, a high potential. The transistor  804  is kept in an off state. 
     After that, the potential Vext_b 1  for turning off the transistor  808  is input to the gate terminal of the transistor  808 , whereby the transistor  808  is turned off. After the transistor  808  is turned off, the potential V 1  is set to a low potential. The transistor  804  is still in an off state. The potential V 2  is set to the same potential as the potential V 1 . Thus, the initialization period is completed. When the initialization period is completed, a potential difference is generated between the node A and the one of the source electrode and drain electrode of the transistor  804 , and a potential difference is also generated between the node A and the other of the source electrode and the drain electrode of the transistor  808 . Accordingly, a small amount of charge flows through the transistor  804  and the transistor  808 . In other words, off-state current is generated. 
     Next, a measurement period of the off-state current will be briefly described. In the measurement period, the potential of the one of the source terminal and the drain terminal of the transistor  804  (i.e., the potential V 2 ) and the potential of the other of the source terminal and the drain terminal of the transistor  808  (i.e., the potential V 1 ) are fixed to a low potential. On the other hand, the potential of the node A is not fixed (the node A is in a floating state) in the measurement period. Accordingly, charge flows through the transistor  804 , and the amount of charge held in the node A changes over time. Further, as the amount of charge held in the node A changes, the potential of the node A changes. In addition, the output potential Vout of the output terminal also changes. 
       FIG. 13  shows details (the timing diagram) of the relation between potentials in the initialization period in which the potential difference is generated and in the following measurement period. 
     In the initialization period, first, the potential Vext_b 2  is set to a potential (a high potential) at which the transistor  804  is turned on. Thus, the potential of the node A comes to be V 2 , that is, a low potential (VSS). Note that it is not essential to apply the low potential (VSS) to the node A. After that, the potential Vext_b 2  is set to a potential (a low potential) at which the transistor  804  is turned off, whereby the transistor  804  is turned off. Then, the potential Vext_b 1  is set to a potential (a high potential) at which the transistor  808  is turned on. Thus, the potential of the node A comes to be V 1 , that is, a high potential (VDD). After that, the potential Vext_b 1  is set to a potential at which the transistor  808  is turned off. Thus, the node A is brought into a floating state and the initialization period is completed. 
     In the following measurement period, the potential V 1  and the potential V 2  are individually set to potentials at which charge flows to or from the node A. Here, the potential V 1  and the potential V 2  are set to the low potential (VSS). Note that at the timing of measuring the output potential Vout, it is necessary to operate an output circuit; thus, V 1  is set to the high potential (VDD) temporarily in some cases. Note that the period in which V 1  is the high potential (VDD) is set to be short so that the measurement is not effected. 
     When a potential difference is generated and the measurement period is started as described above, the amount of charge held in the node A changes over time and the potential of the node A changes accordingly. This means that the potential of the gate terminal of the transistor  805  changes and the output potential Vout of the output terminal also changes over time. 
     A method for calculating the off-state current based on the obtained output potential Vout will be described below. 
     The relation between the potential V A  of the node A and the output potential Vout is obtained before the off-state current is calculated, whereby the potential V A  of the node A can be obtained based on the output potential Vout. From the relation described above, the potential V A  of the node A can be expressed as a function of the output potential Vout by the following equation.
 
 V   A   =F ( V out)  [Equation 1]
 
     Charge Q A  of the node A is expressed by the following equation, with the potential V A  of the node A, capacitance C A  connected to the node A, and a constant (const). Here, the capacitance C A  connected to the node A is the sum of the capacitance of the capacitor  802  and other capacitance.
 
 Q   A   =C   A   V   A ±const  [Equation 2]
 
     Since current I A  of the node A is the time derivative of charge flowing to the node A (or charge flowing from the node A), the current I A  of the node A is expressed by the following equation. 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       A 
                     
                     ≡ 
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           Q 
                           A 
                         
                       
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         t 
                       
                     
                   
                   = 
                   
                     
                       
                         C 
                         A 
                         • 
                       
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         F 
                         ⁡ 
                         
                           ( 
                           Vout 
                           ) 
                         
                       
                     
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       t 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     Thus, the current I A  of the node A can be obtained from the capacitance C A  connected to the node A and the output potential Vout of the output terminal. 
     By the method described above, leakage current (off-state current) flowing between a source and a drain of a transistor which is in an off state can be calculated. 
     In this example, the transistor  804 , the transistor  805 , the transistor  806 , and the transistor  808  with a channel length L of 10 μm and a channel width W of 50 μm were formed using a purified oxide semiconductor. In the measurement systems  800  arranged in parallel, capacitance values of the capacitors  802  were 100 fF, 1 pF, and 3 pF. 
     Note that in the measurement according to this example, VDD was 5 V and VSS was 0 V. In the measurement period, Vout was measured while the potential V 1  was basically set to VSS and changed to VDD only for 100 milliseconds every 10 seconds to 300 seconds. Further, Δt of calculation of current I flowing through the element was approximately 30000 seconds. 
       FIG. 14  shows the relation between the output potential Vout and elapsed time Time in the current measurement. As is seen in  FIG. 14 , the potential changes over time. 
       FIG. 15  shows the off-state current at room temperature (25° C.) calculated based on the above current measurement.  FIG. 15  shows the relation between the source-drain voltage V and the off-state current I. According to  FIG. 15 , the off-state current is approximately 40 zA/μm when the source-drain voltage is 4 V. In addition, the off-state current is smaller than or equal to 10 zA/μm when the source-drain voltage is 3.1 V. Note that 1 zA is equivalent to 10 −21  A. 
     Further,  FIG. 16  shows the off-state current in an environment at a temperature of 85° C., which was calculated based on the above current measurement.  FIG. 16  shows the relation between the source-drain voltage V and the off-state current I in an environment at 85° C. According to  FIG. 16 , the off-state current is smaller than or equal to 100 zA/μm when the source-drain voltage is 3.1 V. 
     According to this example, it was confirmed that the off-state current is sufficiently small in a transistor including a purified oxide semiconductor. 
     Example 2 
     The number of times data can be rewritten in the semiconductor device according to one embodiment of the disclosed invention was examined. In this example, the examination results will be described with reference to  FIG. 17 . 
     A semiconductor device used for the examination is the semiconductor device having the circuit structure in  FIG. 1A-1 . Here, an oxide semiconductor was used in a transistor corresponding to the transistor  162 , and a capacitor with a capacitance value of 0.33 pF was used as a capacitor corresponding to the capacitor  164 . 
     The examination was performed by comparing the initial memory window width and the memory window width after storing and writing of data were repeated a predetermined number of times. Data was stored and written by applying 0 V or 5 V to a wiring corresponding to the third wiring in  FIG. 1A-1  and applying 0 V or 5 V to a wiring corresponding to the fourth wiring. When the potential of the wiring corresponding to the fourth wiring is 0 V, the transistor (the write transistor) corresponding to the transistor  162  is in an off state; thus, a potential applied to the node FG is held. When the potential of the wiring corresponding to the fourth wiring is 5 V, the transistor corresponding to the transistor  162  is in an on state; thus, a potential of the wiring corresponding to the third wiring is applied to the node FG. 
     The memory window width is one of the indicators of the memory device characteristics. Here, the memory window width refers to the shift amount ΔVcg in curves (Vcg-ID curves) between different memory states, which show the relation between the potential Vcg of a wiring corresponding to the fifth wiring and the drain current ID of a transistor (a read transistor) corresponding to the transistor  160 . The different memory states are a state where 0 V is applied to the node FG (hereinafter referred to as a Low state) and a state where 5 V is applied to the node FG (hereinafter referred to as a High state). That is, the memory window width can be obtained by sweeping the potential Vcg in the Low state and in the High state. 
       FIG. 17  shows the examination results of the initial memory window width and the memory window width after writing was performed 1×10 9  times. Note that in  FIG. 17 , the horizontal axis represents Vcg (V) and the vertical axis represents ID (A). According to  FIG. 17 , the memory window width is not changed by 1×10 9  times of data writing, which means that the semiconductor device does not deteriorate at least during the period in which data is written 1×10 9  times. 
     As described above, a semiconductor device according to one embodiment of the disclosed invention has very high rewriting durability and its characteristics do not change even when data is held and written 10 9  times. That is, it can be said that a semiconductor device having excellent reliability is realized according to one embodiment of the disclosed invention. 
     This application is based on Japanese Patent Application serial no. 2010-028820 filed with Japan Patent Office on Feb. 12, 2010, the entire contents of which are hereby incorporated by reference. 
     EXPLANATION OF REFERENCE 
       100 : substrate,  102 : protective layer,  104 : semiconductor region,  106 : element isolation insulating layer,  108 : gate insulating layer,  110 : gate electrode,  116 : channel formation region,  120 : impurity region,  122 : metal layer,  124 : metal compound region,  126 : electrode,  128 : insulating layer,  130 : insulating layer,  142   a : source or drain electrode,  142   b : source or drain electrode,  144 : oxide semiconductor layer,  143   a : insulating layer,  143   b : insulating layer,  146 : gate insulating layer,  148   a : gate electrode,  148   b : electrode,  150 : insulating layer,  152 : insulating layer,  154 : electrode,  156 : wiring,  160 : transistor,  162 : transistor,  164 : capacitor,  170 : memory cell,  180 : multiplexer,  182 : multiplexer,  190 : first driver circuit,  192 : second driver circuit,  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,  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 control,  800 : measurement system,  802 : capacitor,  804 : transistor,  805 : transistor,  806 : transistor,  808 : transistor.