Patent Publication Number: US-7215567-B2

Title: Ferroelectric memory device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a ferroelectric memory device serving as a nonvolatile memory for reading data without destructing the data in order to lengthen the service life for the number of read times of a memory cell and further accelerate the access time. 
   2. Description of the Related Art 
   In recent years, the significance of a nonvolatile memory in which data can be electrically written or whose data can be erased has been increased in the memory field. Though there are various types of nonvolatile memories, a ferroelectric memory is noticed from the viewpoints of high-speed performance, low-voltage characteristic, and low power consumption. There are various specific configurations of a ferroelectric memory as shown below. 
   A ferroelectric memory using a ferroelectric capacitor is known which defines two states in accordance with a remanent polarization state in a ferroelectric film. This capacitor detects an internal storage state of 1 or 0 by generating the internal polarization state of 1 or 0 in accordance with the way of applying two types of voltages equal to or higher than the coercive voltage of a ferroelectric thin film and having two different polarities to the ferroelectric capacitor, passing through a storage state due to remanent polarization, and applying voltage equal to or higher than the coercive electric field of the ferroelectric thin film to fetch electric charges.  FIGS. 57 ,  58 ,  59  are illustrations for explaining the ferroelectric memory. 
     FIG. 57  is a sectional view showing a structure of a ferroelectric capacitor. As shown in  FIG. 57 , the ferroelectric capacitor has a structure for holding a ferroelectric thin film  5740  constituted of an inorganic ferroelectric by a first terminal  5741  constituted of a metallic electrode and a second terminal  5742  constituted of a metallic electrode. 
     FIG. 58  shows a polarization-charge-applied-voltage characteristic of the ferroelectric capacitor shown in  FIG. 57 . In  FIG. 58 , curved lines passing through four characteristic points  5801 ,  5802 ,  5803 , and  5804  show characteristics of applied voltage V and internal polarization charge Q applied between the first terminal  5741  and the second terminal  5742  of the ferroelectric capacitor in  FIG. 57 . 
   The characteristic point  5801  shows a state of applying positive high voltage V to the second terminal  5742  from the first terminal  5741  and the characteristic point  5802  shows a state of applying positive high-voltage V to the first terminal  5741  from the second terminal  5742 . At the characteristic points  5801  and  5802 , internal polarization causes polarization reverse in positive and negative states. 
   When releasing a potential difference between the first terminal  5741  and the second terminal  5742  of the ferroelectric capacitor under the state of the characteristic point  5801  as 0, the internal polarization is kept as remanent polarization and becomes the state shown at the characteristic point  5804 . Moreover, when releasing a potential difference between the first terminal  5741  and the second terminal  5742  of the ferroelectric capacitor under the state of the characteristic point  5802  as 0, internal polarization is kept as remanent polarization and becomes the state shown at the characteristic point  5803 . 
   Thus, because the ferroelectric capacitor has the hysteresis characteristic shown in  FIG. 3 , it releases terminals at the both ends of a ferroelectric capacitor and has different remanent polarization depending on a previous state even if setting voltage to 0. The remanent polarization can store data by corresponding to the characteristic points  5803  and  5804 . 
   When applying voltage V (ΔVB) to the second terminal  5742  on the basis of the first terminal  5741  from a state in which the terminals at the both ends of the ferroelectric capacitor are released, the characteristic point moves to the characteristic point  5801 . In this case, when the previous state is at the characteristic point  5803 , electric charges of ΔQHB shown in  FIG. 58  are fetched, while when it is at the characteristic point  5804 , electric charges of ΔQLB are fetched. As shown in  FIG. 58 , because ΔQLB&lt;&lt;ΔQHB, it is possible to determine the previous state stored as remanent polarization as 1 or 0 in accordance with the fetched number of electric charges. 
   The circuit shown in  FIG. 59  is known as a specific circuit for performing the above operations. 
   As shown in  FIG. 59 , this circuit is constituted of a ferroelectric capacitor  5911  and an N-type insulating-gate field-effect transistor (hereafter referred to as MOSFET)  5912  and includes a word line (WL)  5913 , bit line (BL)  5914 , and plate line (PL)  5915 . In this case, the MOSFET is an abbreviation of a Metal-Oxide-Semiconductor Field-Effect Transistor. 
   More minutely, the word line  5913  is connected to the gate electrode of the MOSFET  5912 . The bit line  5914  is connected to an electrode serving as a source or drain of the MOSFET  5912 . Moreover, the plate line  5915  is connected to one end of the ferroelectric capacitor  5911 . The other end of the ferroelectric capacitor  5911  is connected to an electrode serving as a drain or source of the MOSFET  5912 . 
   The circuit in  FIG. 59  constituted as described above supplies potential to be applied to the ferroelectric capacitor  5911  to the bit line  5914  and plate line  5915  and turns on/off the MOSFET  5912  by the word line  5913  and thereby, performs the write operation and read operation of the above described electric charges. 
   In this case, the above method fetches electric charges when reading data. That is, because data is destructed, the above method is a method generally referred to as destructive read and an example of this method is disclosed in JP11-39882A (hereinafter referred to as Patent Document 1). 
   There is a method referred to as a nondestructive read in which data is not destructed when it is read. By improving a material of a ferroelectric thin film, there is a method for detecting a difference between output electric charges due to a difference between operating points when a weak voltage is applied to a ferroelectric capacitor due to a difference between inclinations of characteristics corresponding the characteristic points  6003  and  6004  shown in  FIG. 60  by assuming the polarization-charge-applied-voltage hysteresis characteristic to be asymmetric as shown in  FIG. 60 . Examples of this method are disclosed in JP2-198094A and JP5-82800A (hereinafter referred to as Patent Documents 2 and 3). 
   Moreover, as shown in  FIGS. 61 and 62 , there is a structure in which a ferroelectric thin film  6100  is formed at the gate portion of a field-effect transistor, voltage equal to or higher than the coercive voltage of the ferroelectric thin film  6100  is applied between a gate electrode  6101  and a substrate  6109  or to a source electrode  6102  and drain electrode  6103  to make the ferroelectric thin film  6100  generate polarization so as to store data in accordance with the state of remanent polarization even after the applied voltage is removed. This can detect a written polarization direction, that is, a difference of 1 or 0 because electric charges induced by the channel of a field-effect transistor depend on the remanent polarization and become a difference between threshold voltages, and flowing current values are different. 
   A field-effect transistor having a ferroelectric thin film at its gate portion may be referred to as MFSFET. In this case, the MFSFET is an abbreviation of a Metal-Ferroelectrics-Semiconductor Field-Effect Transistor. 
   In  FIG. 61 , potential 0 is applied to the gate electrode  6101  through the word line  6104  and a positive potential V equal to or higher than coercive voltage is applied to the source electrode  6102  and drain electrode  6103  through the first bit line  6105  and second bit line  6106 , and the ferroelectric thin film  6100  causes the polarization of a positive electrode at the gate side and the polarization of a negative electrode at the substrate side. 
   Moreover, in  FIG. 62 , positive potential V equal to or higher than coercive voltage is applied to the gate electrode  6101  through the word line  6104  and potential 0 is applied to the source electrode  6102  and drain electrode  6103  through the first bit line  6105  and second bit line  6106 , and the ferroelectric thin film  6100  causes negative-electrode polarization at the gate side and positive-electrode polarization at the substrate  6109  side. 
   When reading data, there is a method for detecting a difference between currents flowing through an MFSFET by using a difference between the remanent polarizations as a change in threshold voltages of the MFSFET. As an example of this, there is JP2002-543627A (hereinafter referred to as Patent Document 4). 
   SUMMARY OF THE INVENTION 
   However, the above conventional ferroelectric memories have the following problems respectively. 
   That is, in the case of the method for destruction-reading data described for  FIGS. 57 to 59  or shown in Patent Document 1, it is necessary to write deleted data again after reading the data. Therefore, because the write operation is performed after reading data, a cycle time becomes very long and a trouble occurs in high-speed read. 
   Because not only the above described but also write is performed for every read, write and data deletion are repeated and there is a problem that the service life as a ferroelectric material expires because 10 10  to 10 12  times are the limit of the number of rewriting times. Therefore, there is a problem that the material is insufficient from the viewpoints of quality and reliability as a static random access memory (hereafter referred to as SRAM) and dynamic random access memory (hereafter referred to as DRAM) from or in which data is frequently read or written at a high speed and its purposes are limited. 
   Moreover, as shown in Patent Documents 2 and 3, there is a method for preparing an asymmetric hysteresis characteristic of polarization-charge-applied-voltage of a ferroelectric thin film and reading data without destructing the data by using the asymmetric characteristic. However, it is not actually easy to obtain a material having a complex non-linear hysteresis characteristic shown in  FIG. 60  or a typical view in Patent Document 3 and there is a problem that a delay of development or a trouble in fabrication easily occurs. Moreover, there is a problem that a difference between output signals of 1 and 0 in reading is small and the configuration of a detection circuit is difficult. 
   The method for holding remanent polarization data of a ferroelectric thin film by having bulk of a silicon substrate, setting a ferroelectric thin film on the gate electrode of a field-effect transistor, applying voltage equal to or higher than the coercive voltage of the ferroelectric thin film to a substrate and a gate electrode shown in  FIG. 61  or  62  or Patent Document 4 has a following disadvantage. 
   That is, when arranging memory cells like a matrix and sharing word lines and bit lines between memory cells, voltage equal to or higher than coercive voltage may be applied to a memory cell at an unselected address. Therefore, efficient sharing is difficult and thus, there is a problem that an integration degree is lowered and the cost is easily increased. 
   Moreover, as described above, because voltage equal to or higher than coercive voltage may be applied to a memory cell at an unselected address, there is a disadvantage that the hysteresis characteristic of polarization-charge-applied voltage of a ferroelectric thin film of a conventional ferroelectric material is insufficient in angular characteristic and desired control cannot be made. 
   Furthermore, as shown in  FIGS. 61 and 62 , the ferroelectric thin film  6100  is located on the substrate  6109 . Therefore, this represents that a ferroelectric is crystallized on silicon (Si). However, it is generally difficult to directly grow inorganic ferroelectric crystal on silicon crystal and many crystal defects are left on the boundary. Therefore, an ideal MFS (Metal Ferroelectrics Semiconductor) structure is not formed. 
   Therefore, in  FIGS. 61 and 62 , the remanent polarization of the ferroelectric thin film  6100  is influenced due to the above crystal defects in the ferroelectric thin film  6100  and the substrate  6109  and there is a disadvantage that desired data disappears in a short period. 
   Therefore, the present invention solves the problem and its object is to provide a ferroelectric memory capable of realizing high-speed, high integration degree, and long service life and by using nondestructive read and a storage method or a control method for improving the integration degree as a memory, selecting a material of a ferroelectric thin film suitable for the method and configuration and capable of being substituted for an SRAM or DRAM and widely used. 
   To solve the above problem and achieve the object of the present invention, each invention is constituted as described below. 
   That is, a first invention at least includes a field-effect transistor having a ferroelectric thin film at its gate portion, a word line connected to the gate electrode of the field-effect transistor, a first bit line connected to a first electrode serving as a source or drain of the field-effect transistor, a second bit line connected to a second electrode serving as a drain or source of the field-effect transistor, a write circuit for applying voltage equal to or higher than the coercive electric field of the ferroelectric thin film between the first bit line and the word line at first write timing and applying voltage equal to or higher than the coercive electric field of the ferroelectric thin film between the second bit line and the word line at second write timing, and a read circuit for applying voltage equal to or lower than the coercive electric field of the ferroelectric thin film between the first bit line and the word line at first read timing to detect a current flowing between the first bit line and the second bit line and applying a voltage equal to or lower than the coercive electric field of the ferroelectric thin film between the second bit line and the word line at second read timing to detect a current flowing between the second bit line and the first bit line. 
   A second invention is at least provided with a memory cell group including a plurality of field-effect transistors set like a matrix and respectively having a ferroelectric thin film at its gate portion, a word line connected in common to each gate electrode of field-effect transistors arranged in the same row out of the above field-effect transistors, a first bit line to be connected in common to each first electrode serving as a source or drain of each of field-effect transistors arranged in the same column out of the above field-effect transistors, a second bit line to be connected in common to a second electrode serving as a drain or source of each of the field-effect transistors arranged in the same column out of the above field-effect transistors, a write circuit for applying voltage equal to or higher than the coercive electric field of the ferroelectric thin film between the first bit line and the word line and between the second bit line and the word line at different timings when writing data to a field-effect transistor at a selected address, a read circuit for applying voltage equal to or lower than the coercive electric field of the ferroelectric thin film between the first bit line and the word line at the first read timing to detect a current flowing between the first bit line and the second bit line and applying voltage equal to or lower than the coercive electric field of the ferroelectric thin film between the second bit line and the word line at second read timing different from the first read timing to detect a current flowing between the second bit line and the first bit line, when reading data from a field-effect transistor at a selected address and a selection and control circuit for selecting a field-effect transistor at a desired address when the write circuit performs the write operation and controlling voltage so that voltage equal to or higher than the coercive electric field of the ferroelectric thin film is applied between a word line and a bit line of the selected field-effect transistor and voltage equal to or lower than the coercive electric field of the ferroelectric thin film is applied to remaining word lines and bit lines other than the above word line and bit line, while for selecting a field-effect transistor at a desired address when the read circuit performs the read operation and controlling voltage so that voltage equal to or lower than the coercive electric field of the ferroelectric thin film is applied to the word lines and bit lines of all the field-effect transistors. 
   A third invention applies a predetermined voltage to the above word line, first bit line, and second bit line so that when reading data from the above field-effect transistor, the field-effect transistor for reading the data operates in a saturated region in the first or second invention. 
   A fourth invention is constituted in the second invention so that the field-effect transistor is constituted of an N-type field-effect transistor and it is assumed that voltage supplied from the selection and control circuit to the N-type field-effect transistor ranges between 0 and V, and coercive voltage corresponding to the coercive electric field of the ferroelectric thin film ranges between (1/2) V and V and when writing data 1 in an N-type field-effect transistor at a selected address, the selection and control circuit supplies potential V to a word line of the selected address, potential (1/2) V to a word line at an unselected address, potential 0 [V] to a bit line at a selected address and potential (1/2) V to a bit line at an unselected address and when writing data 0 in an N-type field-effect transistor at a selected address, the selection and control circuit supplies potential 0 [V] to a word line at a selected address, potential (1/2) V to a word line at an unselected address, potential V to a bit line at a selected address, and potential (1/2) V to a bit line at an unselected address. 
   A fifth invention is constituted in the second invention so that the field-effect transistor is constituted of an N-type field-effect transistor, it is assumed that voltage supplied from the selection and control circuit to the N-type field-effect transistor ranges between 0 and V, and coercive voltage corresponding to the coercive electric field of the ferroelectric thin film ranges between (1/3) and (2/3) V, and 
   when writing data 1 in an N-type field-effect transistor at a selected address, the selection and control circuit supplies potential V to a word line at a selected address, potential (1/3) V to a word line at an unselected address, potential (1/3) V to a bit line at a selected address, and potential (2/3) V to a bit line at an unselected address and moreover, when writing data 0 in an N-type field-effect transistor at a selected address, the selection and control circuit supplies potential 0 [V] to a word line at a selected address, potential (2/3) V to a word line at an unselected address, potential (2/3) V to a bit line at a selected address, and potential (1/3) V to a bit line at an unselected address. 
   A sixth invention is constituted in the second invention so that the field-effect transistor is constituted of an N-type field-effect transistor and it is assumed that voltage supplied from the selection and control circuit to the N-type field-effect transistor ranges between 0 and V and coercive voltage corresponding to the coercive electric field of the ferroelectric thin film ranges between (1/4) and (3/4)V and moreover, when writing data 1 in an N-type field-effect transistor at a selected address, the selection and control circuit supplies potential V to a word line at a selected address, potential (2/4) V to a word line at an unselected address, potential (1/4) V to a bit line at a selected address, and potential (3/4) V to a bit line at an unselected address and moreover, when writing data 0 in an N-type field-effect transistor at a selected address, the selection and control circuit supplies potential 0 [V] to a word line at a selected address, potential (2/4) V to a word line at an unselected address, potential (3/4) V to a bit line at a selected address, and potential (1/4) V to a bit line at an unselected address. 
   A seventh invention is constituted in the fourth invention so that when reading data from an N-type field-effect transistor at a selected address, the selection and control circuit supplies potential (1/2) V to a word line at a selected address, potential 0 [V] to a bit line, and potential (1/2) V to a bit line to be paired with the above bit line and moreover, supplies potential 0 [V] to a word line at an unselected address and unselected bit lines other than the paired bit lines. 
   An eighth invention is constituted in the fifth invention so that when reading data from an N-type field-effect transistor at a selected address, the selection and control circuit supplies potential (1/3) V to a word line at a selected address, potential 0 [V] to a bit line, and potential (1/3) V to a bit line to be paired with the above bit line and moreover, supplies potential 0 [V] to a word line at an unselected address and unselected bit lines other than the paired bit lines. 
   A ninth invention is constituted in the sixth invention so that when reading data from an N-type field-effect transistor at a selected address, the selection and control circuit supplies potential (1/4) V to a word line at a selected address, potential 0 [V] to a bit line, and potential (1/4) V to a bit line to be paired with the above bit line and moreover, supplies potential 0 [V] to a word line at an unselected address and unselected bit lines other than the paired bit lines. 
   A tenth invention is constituted in the second invention so that the field-effect transistor is constituted of a P-type field-effect transistor and it is assumed that voltage supplied from the selection and control circuit to the P-type field-effect transistor ranges between 0 and V and coercive voltage corresponding to the coercive electric field of the ferroelectric thin film ranges between (1/2) V and V and moreover, when writing data 1 in a P-type field-effect transistor at a selected address, the selection and control circuit supplies potential 0 [V] to a word line at a selected address, potential (1/2) V to a word line at an unselected address, potential V to a bit line at a selected address, and potential (1/2) V to a bit line at an unselected address, and when writing data 0 in a P-type field-effect transistor at a selected address, the selection and control circuit supplies potential V to a word line at a selected address, potential (1/2) V to a word line at an unselected address, potential 0 [V] to a bit line at a selected address, and potential (1/2) V to a bit line at an unselected address. 
   An eleventh invention is constituted in the second invention so that the field-effect transistor is constituted of a P-type field-effect transistor, it is assumed that voltage supplied from the selection and control circuit to the P-type field-effect transistor ranges between 0 and V and coercive voltage corresponding to the coercive electric field of the ferroelectric thin film ranges between (1/3) and (2/3) V and moreover, when writing data 1 in a P-type field-effect transistor at a selected address, the selection and control circuit supplies potential 0 [V] to a word line at a selected address, potential (2/3) V to a word line at an unselected address, potential (2/3) V to a bit line at a selected address, and potential (1/3) V to a bit line at an unselected address, and when writing data 0 in a P-type field-effect transistor at a selected address, the selection and control circuit supplies potential V to a word line at a selected address, potential (1/3) V to a word line at an unselected address, potential (1/3) V to a bit line at a selected address, and potential (2/3) V to a bit line at an unselected address. 
   A twelfth invention is constituted in the second invention so that the field-effect transistor is constituted of a P-type field-effect transistor, it is assumed that voltage supplied from the selection and control circuit to the P-type field-effect transistor ranges between 0 and V and coercive voltage corresponding to the coercive electric field of the ferroelectric thin film ranges between (1/4) V and (3/4) V, and moreover when writing data 1 in a P-type field-effect transistor at a selected address, the selection and control circuit supplies potential 0 [V] to a word line at a selected address, potential (2/4) V to a word line at an unselected address, potential (3/4) V to a bit line at a selected address, and potential (1/4) V to a bit line at an unselected address, and moreover when writing data 0 in a P-type field-effect transistor at a selected address, the selection and control circuit supplies potential V to a word line at a selected address, potential (2/4) V to a word line at an unselected address, potential (1/4) V to a bit line at a selected address, and potential (3/4) V to a bit line at an unselected address. 
   A thirteenth invention is constituted in the tenth invention so that when reading data from a P-type field-effect transistor at a selected address, the selection and control circuit supplies potential (1/2) V to a word line at a selected address, potential V to a bit line, and potential (1/2) V to a bit line to be paired with the above bit line, and moreover supplies potential V to a word line at an unselected address and unselected bit lines other than the paired bit lines. 
   A fourteenth invention is constituted in the eleventh invention so that when reading data from a P-type field-effect transistor at a selected address, the selection and control circuit supplies potential (2/3) V to a word line at a selected address, potential V to a bit line, and potential (2/3) V to a bit line to be paired with the above bit line and moreover, supplies potential V to a word line at an unselected address and unselected bit lines other than the paired bit lines. 
   A fifteenth invention is constituted in the twelfth invention so that when reading data from a P-type field-effect transistor at a selected address, the selection and control circuit supplies potential (3/4) V to a word line at a selected address, potential V to a bit line, and potential (3/4) V to a bit line to be paired with the above bit line and moreover, supplies potential V to a word line at an unselected address and unselected bit lines other than the paired bit lines. 
   In a sixteenth invention, a field-effect transistor having a ferroelectric thin film at the gate portion is channel-doped in any one of the first to fifteenth inventions. 
   In a seventeenth invention, the ferroelectric thin film is constituted of an inorganic ferroelectric in any one of the first to sixteenth inventions. 
   In an eighteenth invention, a ferroelectric thin film constituted of the inorganic ferroelectric is constituted of PZTN in the seventeenth invention. 
   In a nineteenth invention, the ferroelectric thin film is constituted of an organic ferroelectric in any one of the first to sixteenth inventions. 
   In a twentieth invention, a ferroelectric thin film constituted of the organic ferroelectric is constituted of PVDF, P(VDF/TrFE), or odd-number nylon such as nylon 7, or nylon 11 in the nineteenth invention. 
   As described above, according to the present invention having one of the above configurations, the threshold voltage of a MFSFET is changed due to remanent polarization of a ferroelectric thin film at the gate portion of the MFSFET and it can be detected as a current difference. Therefore, data read is nondestructive read and high-speed read can be made, and there is an advantage that the service life of a device is greatly lengthened. 
   Moreover, in the present invention, it is possible to store two-bit data in one MFSFET, share a word line or bit line by arranging memory cell group like a matrix, and layout at a high area efficiency is possible. Therefore, there is an advantage that it is possible to provide a high-integration, compact, and low-cost nonvolatile memory. 
   Moreover, the present invention is high-speed, long-service-life, and low-cost and at the same time, has such characteristics and features as original nonvolatile property, low voltage, and small power consumption of a ferroelectric memory. Therefore, not only the present invention is substituted for a general memory such as a nonvolatile memory, SRAM, or DRAM but also only one chip is used though a plurality of chips have been used so far. Therefore, there is an advantage that change to low cost, compactness, and lower power consumption is further progressed. 
   Furthermore, according to the present invention, by using as ferroelectric thin film at gate portion of MFSFET, PZTN of inorganic ferroelectric having a good angular characteristic in the polarization-charge-applied-voltage hysteresis characteristic, or PVDF, P(VDF/TrFE), or odd-number nylon such as nylon 7 or nylon 11 serving as organic ferroelectric for easily forming an ideal MFS structure, various characteristics are improved and there is an advantage that fabrication is easy and stability is increased. As a result, there are advantages that the quality reliability is improved and the fabrication cost is decreased. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing a first configuration of a memory cell used for a ferroelectric memory device of the present invention; 
       FIG. 2  is a sectional view showing a configuration of the cross section of an N-type MFSFET constituting the memory cell in  FIG. 1 ; 
       FIG. 3  is a characteristic diagram showing a hysteresis characteristic of polarization electric charge and applied voltage of a ferroelectric thin film of an N-type MFSFET; 
       FIG. 4  is an illustration showing an example of one polarization state in the ferroelectric thin film of an N-type MFSFET; 
       FIG. 5  is an illustration showing another example of one polarization state in the ferroelectric thin film of the N-type MFSFET; 
       FIG. 6  is an illustration showing an example of the other polarization state in the ferroelectric thin film of the N-type MFSFET; 
       FIG. 7  is an illustration showing another example of the other polarization state in the ferroelectric thin film of the N-type MFSFET; 
       FIG. 8  is an illustration showing a polarization state when data (0,0) is stored in the ferroelectric thin film of an N-type MFSFET; 
       FIG. 9  is an illustration showing a polarization state when data (0,1) is stored in the ferroelectric thin film of the N-type MFSFET; 
       FIG. 10  is an illustration showing a polarization state when data (1,0) is stored in the ferroelectric thin film of the N-type MFSFET; 
       FIG. 11  is an illustration showing a polarization state when data (1,1) is stored in the ferroelectric thin film of the N-type MFSFET; 
       FIG. 12  is a sectional view showing a state in an N-type MFSFET when operating the N-type MFSFET in a saturated region; 
       FIG. 13  is a sectional view for explanation when reading data from an N-type MFSFET; 
       FIG. 14  is a sectional view for another explanation when reading data from an N-type MFSFET; 
       FIG. 15  is a block diagram showing a memory cell group and its peripheral circuits used for a ferroelectric memory device of the present invention; 
       FIG. 16  is an illustration for potential control of various portions when writing data 1 in an N-type MFSFET at a selected address by using three types of potentials; 
       FIG. 17  is an illustration for potential control of various portions when writing data 1 in selected one of N-type MFSFETs arranged like a matrix by using three types of potentials; 
       FIG. 18  is an illustration for potential control of various portions when writing data 0 in an N-type MFSFET at a selected address by using three types of potentials; 
       FIG. 19  is an illustration for potential control of various portions when writing data 0 in selected one of N-type MFSFETs arranged like a matrix by using three types of potentials; 
       FIG. 20  is an illustration for potential control of various portions when reading one data from selected one of N-type MFSFETs arranged like a matrix by using three types of potentials; 
       FIG. 21  is an illustration for potential control of various portions when reading the other data in an N-type MFSFET arranged like an array by using three types of potentials; 
       FIG. 22  is an illustration for potential control of various portions when writing data 1 in an N-type MFSFET at a selected address by using four types of potentials; 
       FIG. 23  is an illustration for potential control of various portions when writing data 1 in selected one of N-type MFSFETs arranged like a matrix by using four types of potentials; 
       FIG. 24  is an illustration for potential control of various portions when writing data 0 in an N-type MFSFETs at a selected address by using four types of potentials; 
       FIG. 25  is an illustration for potential control of various portions when writing data 0 in selected one of N-type MFSFETs arranged like a matrix by using four types of potentials; 
       FIG. 26  is an illustration for potential control of various portions when reading one data from selected one of N-type MFSFETs arranged like a matrix by using four types of potentials; 
       FIG. 27  is an illustration for potential control of various portions when reading the other data from selected one of N-type MFSFETs arranged like a matrix by using four types of potentials; 
       FIG. 28  is an illustration for potential control of various portions when writing data 1 in an N-type MFSFET at a selected address by using five types of potentials; 
       FIG. 29  is an illustration for potential control of various portions when writing data 1 in selected one of N-type MFSFETs arranged like a matrix by using five types of potentials; 
       FIG. 30  is an illustration for potential control of various portions when writing data 0 in an N-type MFSFET at a selected address by using five types of potentials; 
       FIG. 31  is an illustration for potential control of various portions when writing data 0 in selected one of N-type MFSFETs arranged like a matrix by using five types of potentials; 
       FIG. 32  is an illustration for potential control of various portions when reading one data from selected one of N-type MFSFETs arranged like a matrix by using five types of potentials; 
       FIG. 33  is an illustration for potential control of various portions when reading the other data from selected one of N-type MFSFETs arranged like a matrix by using five types of potentials; 
       FIG. 34  is a circuit diagram showing a second configuration of a memory cell used for a ferroelectric memory device of the present invention; 
       FIG. 35  is a circuit diagram showing a configuration of a memory cell group constituted of a P-type MFSFET; 
       FIG. 36  is an illustration for potential control of various portions when writing data 1 in a P-type MFSFET at a selected address by using three types of potentials; 
       FIG. 37  is an illustration for potential control of various portions when writing data 1 in selected one of P-type MFSFETs arranged like a matrix by using three types of potentials; 
       FIG. 38  is an illustration for potential control of various portions when writing data 0 in a P-type MFSFET at a selected address by using three types of potentials; 
       FIG. 39  is an illustration for potential control of various portions when writing data 0 in selected one of P-type MFSFETs arranged like a matrix by using three types of potentials; 
       FIG. 40  is an illustration for potential control of various portions when reading one data from selected one of P-type MFSFETs arranged like a matrix by using three types of potentials; 
       FIG. 41  is an illustration for potential control of various portions when reading the other data from selected one of P-type MFSFETs arranged like a matrix by using three types of potentials; 
       FIG. 42  is an illustration for potential control of various portions when writing data 1 in a P-type MFSFET at a selected address by using four types of potentials; 
       FIG. 43  is an illustration for potential control of various portions when writing data 1 in selected one of P-type MFSFETs arranged like a matrix by using four types of potentials; 
       FIG. 44  is an illustration for potential control of various portions when writing data 0 in a P-type MFSFET at a selected address by using four types of potentials; 
       FIG. 45  is an illustration for potential control of various portions when writing data 0 in selected one of P-type MFSFETs arranged like a matrix by using four types of potentials; 
       FIG. 46  is an illustration for potential control of various portions when reading one data from selected one of P-type MFSFETs arranged like a matrix by using four types of potentials; 
       FIG. 47  is an illustration for potential control of various portions when reading the other data from selected one of P-type MFSFETs arranged like a matrix by using four types of potentials; 
       FIG. 48  is an illustration for potential control of various portions when writing data 1 in a P-type MFSFET at a selected address by using five types of potentials; 
       FIG. 49  is an illustration for potential control of various portions when writing data 1 in selected one of P-type MFSFETs arranged like a matrix by using five types of potentials; 
       FIG. 50  is an illustration for potential control of various portions when writing data 0 in a P-type MFSFET at a selected address by using five types of potentials; 
       FIG. 51  is an illustration for potential control of various portions when writing data 0 in selected one of P-type MFSFETs arranged like a matrix by using five types of potentials; 
       FIG. 52  is an illustration for potential control of various portions when reading one data from selected one of P-type MFSFETs arranged like a matrix by using five types of potentials; 
       FIG. 53  is an illustration for potential control of various portions when reading the other data from selected one of P-type MFSFETs arranged like a matrix by using five types of potentials; 
       FIG. 54  is a block diagram showing the general configuration outline of a ferroelectric memory device of the present invention; 
       FIG. 55  is a sectional view showing a second configuration of an MFSFET structure having a ferroelectric thin film used for a ferroelectric memory device of the present invention; 
       FIG. 56  is a sectional view showing a third configuration of an MFSFET structure having a ferroelectric thin film used for a ferroelectric memory device of the present invention; 
       FIG. 57  is a sectional view showing a structure of a ferroelectric capacitor used for a conventional ferroelectric memory device; 
       FIG. 58  is a characteristic diagram of a typical hysteresis characteristic of polarization electric charge and applied voltage of a ferroelectric thin film made of a generally-used material used for a conventional and the present invention&#39;s ferroelectric memory devices; 
       FIG. 59  is a circuit diagram sowing a configuration of a memory cell used for a conventional ferroelectric memory device; 
       FIG. 60  is a characteristic diagram showing characteristics of polarization electric charge and applied voltage of a ferroelectric capacitor used for a conventional nondestructive-read ferroelectric memory device; 
       FIG. 61  is an illustration showing a polarization state in a ferroelectric thin film of a conventional MFSFET; and 
       FIG. 62  is an illustration showing another polarization state in a ferroelectric thin film of a conventional MFSFET. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention are described below by referring to the accompanying drawings. 
   (Fist Configuration of Memory Cell of the Present Invention and its Operation Principle) 
     FIG. 1  is a circuit diagram of a first configuration of one memory cell (storage device) serving as the basic unit of a ferroelectric memory device of the present invention. First, the structure, operation principle, and control method of the one memory cell are described. 
   As shown in  FIG. 1 , this memory cell is constituted of an N-type MFSFET  100 , a word line  104 , first bit line  105 , and second bit line  106 . 
   The N-type MFSFET  100  has a gate electrode  101 , first electrode  102  constituted of N +  diffusion and serving as a source or drain, and second electrode  103  constituted of N +  diffusion and serving as a drain or source. Moreover, in the N-type MFSFET  100 , the word line  104  is connected to the gate electrode  101 , the first bit line  105  is connected to the first electrode  102 , and the second bit line  106  is connected to the second electrode  103 . In the N-type MFSFET  100  shown in  FIG. 1 , a ferroelectric thin film is embedded immediately below its gate electrode  101  and this state is shown in  FIG. 2 . 
     FIG. 2  is a sectional view obtained by cutting the channel portion of the N-type MFSFET shown in  FIG. 1  in the source-drain direction. 
   In  FIG. 2 , reference numeral  201  denotes a gate electrode made of metal and  202  denotes a first electrode constituted of N +  diffusion and serving as a source or a drain and  203  denotes a second electrode serving as a source or drain and constituted of N +  diffusion, and  209  denotes a silicon substrate. In this case, the gate electrode  201 , first electrode  202 , and second electrode  203  in  FIG. 2  correspond to the gate electrode  101 , first electrode  102 , and second electrode  103  in  FIG. 1 , respectively. 
   In  FIG. 2 , reference numeral  200  denotes a ferroelectric thin film formed of PZTN. When applying voltage to the both ends of the ferroelectric thin film  200  formed of PZTN superior in crystallinity, polarization is generated in the thin film  200 . The polarization once generated has a composition that it is not easily reversed and has a hysteresis characteristic having the good angular characteristic shown in  FIG. 3 . 
     FIG. 58  is a typical characteristic diagram of PZT or SBT having been used so far as a ferroelectric thin film. As a result of comparing  FIG. 3  with  FIG. 58 , it is found that the PZTN recently noticed has a hysteresis characteristic having an angular characteristic better than a conventional typical ferroelectric. 
   PZT is a generic name of Pb(Zr, Ti)O 3 , PZTN is a generic name of a substance in which a part of Ti of PZT is replaced with Nb, and SBT is a generic name of SrBi 2 Ta 2 O 9  or a composition close to it. Moreover, when using the ferroelectric thin film  200 , platinum (Pt) is generally used for the gate electrode  201 . 
   As shown in  FIG. 3 , when applying positive voltage equal to or higher than a coercive electric field to a ferroelectric thin film, operating point becomes the state of a characteristic point  311 . In this case, when removing applied voltage and releasing the film, operating point becomes a characteristic point  312  and a quantity corresponding to the intersection of the axis of ordinate is held as remanent polarization. Moreover, when applying negative voltage equal to or higher than coercive voltage, operating point moves to the state of a characteristic point  313 . Therefore, when removing applied voltage and releasing the film, operating point becomes a characteristic point  314  and a quantity corresponding to the intersection of the axis of ordinate is held as remanent polarization. 
   In  FIG. 1 , when applying voltage equal to or higher than a coercive voltage corresponding to the coercive electric field of the ferroelectric thin film  200  in  FIG. 2  to the word line  104  and the first bit line  105 , an electric field is formed between the gate electrode  201  and the first electrode  202  and the ferroelectric thin film  200  held by the electrodes  201  and  202  locally causes polarization at the inside of it. 
   For example, when the gate electrode  201  shown in  FIG. 2  has potential 0 and the first electrode  202  shown in  FIG. 2  has potential +V, the gate electrode- 401  side causes positive internal polarization and the first electrode- 402  side causes negative internal polarization nearby the first electrode  402  of a ferroelectric thin film  400  as shown in  FIG. 4 . However, when the gate electrode  201  shown in  FIG. 2  has potential +V and the first electrode  202  has potential 0, the gate electrode- 401  side causes negative internal polarization and the first electrode- 402  side causes positive internal polarization nearby the first electrode  402  of the ferroelectric thin film  400  as shown in  FIG. 5 . 
   Moreover, in  FIG. 1 , when applying voltage equal to or higher than coercive voltage corresponding to the coercive electric field of the ferroelectric thin film  200  in  FIG. 2  to the word line  104  and second bit line  103 , an electric field is formed between the gate electrode  201  and the second electrode  203  and the ferroelectric thin film  200  held by the both electrodes  201  and  203  locally causes polarization at the inside of it. 
   For example, when the gate electrode  201  shown in  FIG. 2  has potential 0 and the second electrode  203  shown in  FIG. 2  has potential +V, the gate electrode- 401  side causes positive internal polarization and the second electrode- 403  side causes negative internal polarization nearby the second electrode  403  of the ferroelectric thin film  400  as shown in  FIG. 6 . However, when the gate electrode  201  shown in  FIG. 2  has potential +V and the second electrode  203  shown in  FIG. 2  has potential 0, the gate electrode- 401  side causes negative internal polarization and the second electrode- 403  side causes positive internal polarization nearby the second electrode  403  of the ferroelectric thin film  400  as shown in  FIG. 7 . 
     FIGS. 4 ,  5 ,  6 , and  7  are sectional views obtained by cutting the channel portion of an N-type MFSFET having the ferroelectric thin film  400  at its gate portion in the source-drain direction and sectional views showing wirings and internal state of an MFSFET. Moreover, in  FIGS. 4 ,  5 ,  6 , and  7 , reference numeral  400  denotes a ferroelectric thin film,  401  denotes a gate electrode,  402  denotes a first electrode constituted of N+ diffusion and serving as a source or drain,  403  denotes a second electrode constituted of N+ diffusion and serving as a drain or source,  404  denotes a word line,  405  denotes a first bit line, and  406  denotes a second bit line. The word line  404 , first bit line  405 , and second bit line  406  in  FIGS. 4 ,  5 ,  6 , and  7  correspond to the word line  104 , first bit line  105 , and second bit line  106  in  FIG. 1 , respectively. 
   In  FIGS. 4 and 5 , polarization of the ferroelectric thin film  400  generated at the first electrode- 402  side is stored as remanent polarization even if it is released from voltage. The data storing state is used as a first data storing place shown by a broken line  107  in  FIG. 1 . Moreover, in  FIGS. 6 and 7 , polarization of the ferroelectric thin film  400  generated at the second electrode- 403  side is also stored as remanent polarization even if it is released from voltage. The data storing state is used as a second data storing place shown by a broken line  108  in  FIG. 1 . 
   As described above, the remanent polarization at the first electrode- 402  side and the remanent polarization at the second electrode- 403  side can be independently written when the ferroelectric thin film  400  is an insulator and the first electrode  402  is slightly separated from the second electrode  403  and are stored as remanent polarizations. 
     FIGS. 8 ,  9 ,  10 , and  11  are state diagrams showing states of storing storage remanent polarization data in two places in the ferroelectric thin film  400  in one MFSFET having the ferroelectric thin film  400  at its gate portion. 
   In  FIG. 8 , remanent polarization is stored at the first electrode- 402  side and the second electrode- 403  side of the ferroelectric thin film  400  and a positive polarization state is stored a the gate electrode- 401  side. In  FIG. 9 , remanent polarization is stored at the first electrode- 402  side and the second electrode- 403  side of the ferroelectric thin film  400 . Positive polarization state and negative polarization state are stored at the gate electrode- 401  side. 
   In  FIG. 10 , remanent polarization is stored at the first electrode- 402  side and second electrode- 403  side of the ferroelectric thin film  400  and negative and positive polarization states are stored at the gate electrode- 401  side, respectively. In  FIG. 11 , remanent polarization is stored at the first electrode- 402  side and second electrode- 403  side of the ferroelectric thin film  400  and a negative polarization state is stored at the gate electrode- 401  side. 
   As described above, one MFSFET can store four states of remanent polarization and store two-bit data. 
   When the ferroelectric thin film at the gate portion causes polarization, electric charges are induced in the channel region of the MFSFET. As shown in  FIGS. 5 ,  7 ,  9 ,  10 , and  11 , when positive polarization is present at the channel region side, electrons are induced in the channel region. This state is shown in each drawing as a group of dotted lines. The induced electrons form a channel or at least lower the threshold voltage of the MFSFET. Moreover, when negative polarization is present at the channel side, it raises the threshold voltage of the MFSFET. 
   As described above, polarization occurs in the ferroelectric thin film of the MFSFET and data values 1 and 0 can be stored in the form of remanent polarization. Though data values 1 and 0 are only arrangements on definition, the data values 1 and 0 are defined as described below in this specification. 
   That is, the data value 1 is a case in which the remanent polarization of a ferroelectric influences the threshold voltage of the MFSFET to lower the threshold voltage so that a channel region is easily formed. Moreover, the data value 0 is a case of raising the threshold voltage so that the channel region is not easily formed. 
   According to the above definition, stored data can be shown as (0,0) in the state of  FIG. 8 , (0,1) in the state of  FIG. 9 , (1,0) in the state of  FIG. 10  and (1,1) in the state of  FIG. 11 . 
   (Data Read) 
   Then, a method for reading the data accumulated in the ferroelectric thin film of the MFSFET as remanent polarization is described below. 
     FIG. 12  is a sectional view obtained by cutting the channel portion of an N-type MFSFET having the ferroelectric thin film  400  at its gate portion in the source-drain direction and shows a state of electric charges of a channel. 
   In  FIG. 12 , reference numeral  1200  denotes a ferroelectric thin film,  1201  denotes a gate electrode,  1202  denotes a first electrode serving as a source or drain and constituted of N +  diffusion, and  1203  denotes a second electrode serving as a drain or source and constituted of N +  diffusion. Potential 0 is supplied to the first electrode  1202  through a first bit line  1205 . Moreover, positive voltage V 1  equal to or lower than coercive voltage corresponding to the coercive electric field of the ferroelectric thin film  1200  is supplied to the gate electrode  1201  and second electrode  1203  through a word line  1204  and second bit line  1206 . In this case, the first electrode  1202  becomes a source and the potential VGS between the source and the gate is shown by the following expression (1).
 
 VGS=V 1−0= V 1  (1)
 
   Moreover, the potential VDS between the drain and the source is shown by the following expression (2).
 
 VDS=V 1−0= V 1  (2)
 
   Therefore, when assuming the threshold voltage of an N-type MFSFET in a state in which the ferroelectric thin film  1200  does not cause polarization as Vth and the Vth is positive, magnitudes of VDS and (VGS−Vth) are determined similarly to the case of a MOSFET serving as an insulating-gage field-effect transistor in order to determine a saturated or unsaturated operation region of the MFSFET. 
   VDS is equal to V 1  and VGS−Vth is equal to V 1 −Vth. Therefore, as long as the Vth is positive, VDS becomes larger than VGS−Vth and the N-type MFSFET performs a saturated-region operation. 
   Because there is a potential difference between a source and a drain, the potential of the surface of a channel changes from the source to the drain. When assuming this potential as VC, a channel is formed by induced carriers in a region meeting (VC−Vth)&gt;0. This channel is the region  1211  shown in  FIG. 12 . 
   The potential VC of the channel changes its value depending on a place as described above and (VC−Vth) increases toward the source side. Therefore, because the induced number of electric charges increases toward the source side, depth differs like the channel region  1211  in  FIG. 12  to show a difference of the number of electric charges induced in the channel. 
   Thus, when reaching the point  1212  where (VC−Vth) is equal to 0, electric charges for forming a channel are not induced. Moreover, when approaching to the drain side, a depletion layer region  1213  where no carrier is induced appears. 
   The current IDS flowing between the drain and the source of the MFSFET is shown by the following expression (3) because the saturated region operation is performed as described above by assuming the conductance constant of the MFSFET as β.
 
 IDS =(1/2)β( V 1 −Vth ) 2   (3)
 
   According to the above expression, the current IDS flowing through the saturated region is actually decided by a region in which a channel is formed by inducing electric charges at the source side and it is shown that the current IDS is hardly influenced by the state of the depletion layer side at the drain side. Therefore, the polarization state of the ferroelectric thin film  1200  at the source side influences the threshold voltage to control the magnitude of the flowing current value. When assuming that the threshold voltage changes by 
   ±ΔE in the positive direction and the negative direction of a polarization state, the current-value difference ΔIDS is shown by the following expression (4) when showing ΔE as IDS (ΔE) as a function of a current I. 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         IDS 
                       
                       = 
                       
                         
                           IDS 
                           ⁡ 
                           
                             ( 
                             
                               
                                 + 
                                 Δ 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               E 
                             
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                         - 
                         
                           IDS 
                           ⁡ 
                           
                             ( 
                             
                               
                                 - 
                                 Δ 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               E 
                             
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                       = 
                       
                         2 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           β 
                           · 
                           Δ 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           E 
                           · 
                           
                             ( 
                             
                               V1 
                               - 
                               Vth 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   Moreover, because V 1  is equal to or lower than coercive voltage and Vth is positive, when assuming that the absolute value of −ΔE is large, it is possible to set the difference between polarization states of the ferroelectric thin film  1200  to the difference between on and off of the MFSFET. 
   However, because various factors influence the above number of polarization electric charges or threshold voltage, the number of polarization electric charges or threshold voltage may not directly become an optimum point. 
   In this case, a difference between polarization states is forcibly set so as to become an actual operation point of on or off of a MOSFET by channel-doping the above MFSFET. Thus, data detection is securely performed and the configuration of a detection circuit is simplified. Thus, small size, reduced power consumption and high-speed operation are realized. 
   That is, by channel-doping the MFSFET and properly adjusting the threshold voltage of the MFSFET, it is possible to realize a configuration for easily detecting data because of bringing the MFSFET into an operation region in which distinction is clear because a difference between on and off of the MFSFET appears in accordance with a difference between remanent polarization directions. 
     FIG. 13  is a sectional view of an N-type MFSFET obtained by cutting the channel portion in the source-drain direction when reading data from the first data storing place and shows a state of electric charges of a channel. 
   In  FIG. 13 , potential 0 is supplied to the first electrode  1202  through the first bit line  1205  similarly to the case of  FIG. 12 . Moreover, positive potential V 1  equal to or lower than coercive voltage corresponding to the coercive electric field of the ferroelectric thin film  1200  is supplied to the gate electrode  1201  and second electrode  1203  through the word line  1204  and second bit line  1206 . In this case, the first electrode  1202  serves as a source electrode and the MFSFET operates in a saturated region. 
   Therefore, a remanent polarization state nearby the first data storing place  1207  in  FIG. 13  influences a threshold voltage nearby the source and becomes a conclusive factor for a difference between flowing currents. In this case, because the remanent polarization stage of the second data storing place  1208  is present on the depletion layer  1213 , the flowing current is hardly influenced. Therefore, by detecting the current value of the MFSFET in the state of  FIG. 13 , it is possible to detect the remanent polarization state of the first data storing place  1207 . 
     FIG. 14  is a sectional view of an N-type MFSFET obtained by cutting the channel portion of an N-type MFSFET in the source-drain direction when reading data from the second data storing place and shows a state of electric charges of the channel. 
   In  FIG. 14 , potential 0 is supplied to the second electrode  1203  through the second bit line  1206 . 
   Moreover, positive potential V 1  equal to or lower than coercive voltage corresponding to the coercive electric field of the ferroelectric thin film  1200  is supplied to the gate electrode  1201  and first electrode  1202  through the word line  1204  and first bit line  1205 . 
   In this case, the second electrode  1203  serves as a source electrode and the MFSFET operates in the saturated region. Therefore, the remanent polarization state nearby the second data storing place  1208  in  FIG. 14  influences the threshold voltage nearby the source and becomes a conclusive factor for a difference between flowing currents. In this case, because the remanent polarization state of the first data storing place  1207  is present on the depletion layer  1213 , flowing current is hardly influenced. Therefore, by detecting the current value of the MFSFET in the state of  FIG. 14 , it is possible to detect the remanent polarization state of the second data storing place  1208 . 
   A case about a simple memory cell is described above. In fact, however, it is necessary to use a configuration improving integration efficiency by arranging a plurality of memory cells like a matrix. The total arrangement and a way of controlling each signal line are described below. 
   (Memory Cell Group and its Peripheral Circuits) 
     FIG. 15  is an illustration showing a memory cell group constituted of a plurality of memory cells and a configuration of its peripheral circuits. 
   The memory cell group  1520  is constituted of N-type MFSFETs  1521 ,  1522 ,  1523 ,  1524 , and  1525  respectively having a ferroelectric thin film at its gate portion as shown in a broken line in  FIG. 15  and these N-type MFSFETs are arranged like a matrix. 
   In the case of the MFSFETs  1521 ,  1522 , and  1523  arranged in the same row, gate electrodes are connected to a common word line  1541 . Moreover, in the case of the MFSFETs  1521 ,  1524 , and  1525  arranged in the same column, first electrodes are connected to a common first bit line  1531  and second electrodes are connected to a common second bit line  1532 . 
   As shown in  FIG. 15 , peripheral circuits include bit-line selection and control circuits  1551  and  1552 , word-line selection and control circuit  1553 , write circuit  1554 , write/read circuit  1555 , and read circuit  1556 . 
   The bit-line selection and control circuits  1551  and  1552  and word-line selection and control circuit  1553  respectively select a word line and bit line to be connected with the memory cell group  1520  so that a desired memory cell in the memory cell group  1520  can be selected. 
   The write circuit  1554  writes data in the memory cell selected as described above in the memory cell group  1520 . The read circuit  1556  reads data from the memory cell selected as described above in the memory cell group  1520 . The write/read circuit  1555  performs the write operation by linking with the write circuit  1554  when the write circuit  1554  writes data in a memory cell and performs the read operation by linking with the read circuit  1556  when the read circuit  1556  read data from a memory cell. 
   In this case, when the write circuit  1554  performs the data write operation and potential necessary for data write is simply supplied to a word line or bit line of a memory cell at a selected address, voltage equal to or higher than a coercive voltage may be added to a memory cell at an unselected address because of the influence of the word line or bit line to which the potential is supplied and the remanent polarization of the memory cell at the unselected address may be rewritten. Therefore, as to be described later, it is necessary not only to control a word line or bit line at a selected address but also to properly and comprehensively control a word line or bit line corresponding to an unselected address. 
   Therefore, the word-line selection and control circuit  1553  and bit-line selection and control circuits  1551  and  1552  properly and comprehensively control word lines or bit lines of the total memory cell. Moreover, consideration for not applying voltage equal to or higher than coercive voltage to every memory cell when reading data is necessary. Therefore, the word-line selection and control circuit  1553  and bit-line selection and control circuits  1551  and  1552  are used. 
   Then, an embodiment of a control method of a word line and bit line considering not only a selected address but also an unselected address described above is described below. 
   Embodiment 1 of Control Method of Word Line and Bit Line 
     FIGS. 16 ,  17 ,  18 ,  19 ,  20 , and  21  are illustrations showing relations between word lines and bit lines for using three types of potentials for control by setting intermediate potential (1/2) V when the potential of power supply ranges between 0 [V] and V [V] (refer to  FIG. 16 ). 
   As shown in  FIG. 3 , when hysteresis characteristic sufficiently approaches to an angular type, coercive voltage is present between (1/2) V and V as an absolute value, and remanent polarization is stored even if voltage of approx. (1/2) V is applied to a ferroelectric thin film, the method shown in  FIGS. 16 to 21  can be used. 
     FIG. 16  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 1 in a memory cell at a selected address. In  FIG. 16 , the selected word line WA is set to potential V, the selected bit line BA is set to potential 0, the unselected word line WI is set to (1/2) V, and the unselected bit line BI is set to (1/2) V. In this case, when the potential V is applied to the ferroelectric thin film of the memory cell, data 1 is written but when ±(1/2) V or potential 0 is applied to the ferroelectric thin film, remanent polarization is kept. 
     FIG. 17  shows potentials of word lines and bit lines when writing data 1 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 17 , reference numeral  1760  denotes a memory cell at an address in which data 1 should be written,  1761  denotes the word line at a selected address, and  1762  denotes a bit line at a selected address. 
   Potential V is supplied to the word line  1761  at a selected address and potential 0 is supplied to the selected bit line  1762  and (1/2) V is supplied to all other word lines and bit lines at unselected addresses. In this case, voltage V is applied to only the ferroelectric thin film of the memory cell  1760  at a selected address and ±(1/2) V or potential 0 is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only the memory cell  1760  at a selected address and data 1 is written. However, because other memory cells at unselected addresses respectively have voltage equal to or lower than coercive voltage, memory cells at unselected addresses are not influenced even if the other memory cells have different values. 
     FIG. 18  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 0 in a memory cell at a selected address. In  FIG. 18 , the selected word line WA is set to potential 0, the selected bit line BA is set to potential V, the unselected word line WI is set to (1/2) V, and the unselected bit line BI is set to (1/2) V. In this case, when potential −V is applied to the ferroelectric thin film of the memory cell, data 0 is written but when ±(1/2) V or potential 0 is applied to the ferroelectric thin film, remanent polarization is kept. 
     FIG. 19  shows potentials of word lines and bit lines when writing data 0 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 19 , reference numeral  1760  denotes a memory call at an address in which data 0 should be written,  1761  denotes a word line at a selected address, and  1762  denotes a bit line at a selected address. 
   Potential 0 is supplied to the word line  1761  at a selected address and potential V is supplied to the bit line  1762  at a selected address and (1/2) V is supplied to all other word lines and bit lines at unselected addresses. In this case, −V is applied to only the ferroelectric thin film of the memory cell  1760  at a selected address and ±(1/2) V or potential 0 is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only the memory cell  1760  at a selected address and data 0 is written. Because other memory cells at unselected addresses respectively have voltage equal to or lower than the coercive voltage even if they have different values, memory cells at unselected addresses are not influenced. 
     FIG. 20  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading remanent polarization data from the first data storing place of the selected MFSFET. 
   In  FIG. 20 , reference numeral  2070  denotes an N-type MFSFET at a selected address,  2071  denotes a fist bit line connected to a first electrode serving as the source or drain of the MFSFET  2070 , and  2072  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  2070 . Reference numeral  2074  denotes a word line connected to the gate electrode of the MFSFET  2070  at the selected address. Reference numeral  2077  denotes the first data storing place of the MFSFET  2070 . 
   In this case, to read remanent polarization data from the first data storing place  2077  of the selected MFSFET  2070 , potential (1/2) V is supplied to the selected word line  2074 , potential 0 is supplied to the selected first bit line  2071 , and potential (1/2) V is supplied to the selected second bit line  2072 . 
   Then, the MFSFET  2070  is turned on and operates in a saturated region because it is an N-type MFSFET and potential (1/2) V is supplied to the gate electrode of it from the word line  2074 , potential 0 is supplied to the source electrode of it from the first bit line  2071 , and potential (1/2) V is supplied to the drain electrode of it from the second bit line  2072 . 
   Therefore, the current when the directionality of remanent polarization of the first data storing place  2077  located at the source side of the N-type MFSFET influences the threshold voltage of the MFSFET flows between the first bit line  2071  and the second bit line  2072 . Therefore, by detecting the current by the read circuit  1556  in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential 0 is supplied to the other unselected word line  2075  and unselected bit line  2073 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, because only voltage equal to or lower than coercive voltage of 0 [V] or (1/2) V is applied to ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
     FIG. 21  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading remanent polarization data from the second data storing place of a selected MFSFET. 
   In  FIG. 21 , reference numeral  2070  denotes an N-type MFSFET at a selected address,  2071  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  2070 , and  2072  denotes a second bit line connected to a second electrode serving as a drain or source of the MFSFET  2070 . Reference numeral  2074  denotes a word line connected to the gate electrode of the MFSFET  2070  at the selected address. Moreover, reference numeral  2078  denotes a second data storing place of the MFSFET  2070 . 
   In this case, to read reaming polarization data from the second data storing place  2078  of the selected MFSFET  2070 , potential (1/2) V is supplied to the selected word line  2074 , potential 0 is supplied to the selected second bit line  2072 , and potential (1/2) V is supplied to the selected first bit line  2071 . 
   Then, the MFSFET  2070  is turned on and operates in a saturated region because it is an N-type MFSFET and potential (1/2) V is supplied to the gate electrode of it from the word line  2074 , potential 0 is supplied to the source electrode of it from the second bit line  2072 , and potential (1/2) V is supplied to the drain electrode of it from the first bit line  2071 . 
   Therefore, the current when the directionality of the remanent polarization of the second data storing place  2078  located at the source side of the N-type MFSFET influences the threshold voltage of the MFSFET flows between the second bit line  2072  and the first bit line  2071 . Therefore, by detecting this current by the read circuit  1556  in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential 0 is supplied to the other unselected word lines  2075  and unselected bit lines  2073 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, because only voltage equal to or lower than coercive voltage of 0 [V] or (1/2) V is applied to ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
   In the above description, when reading data from a memory cell, the remanent polarization of a ferroelectric thin film is stored and the data in a memory cell is not destructed. Therefore, the present invention does not require rewrite of data which has been required for a conventional ferroelectric memory and a cycle required for rewrite of the data. 
   Embodiment 2 of Control Method of Word Line and Bit Line 
     FIGS. 22 ,  23 ,  24 ,  25 ,  26 , and  27  are illustrations showing relations between word lines and bit lines for using four types of potentials for control by setting two intermediate potentials (1/3) V and (2/3) V when the potential of power supply ranges between 0 [V] and V[V] (refer to  FIG. 22 ). 
   As shown in  FIG. 3 , when a hysteresis characteristic sufficiently approaches an angular type and coercive voltage ranges between 1/2V and V, the method described for the embodiment 1 of the control method of word line and bit line is preferable. However, the method shown in  FIGS. 22 to 27  can be used as a method suitable when it is necessary to improve the allowance for holding remanent polarization to coercive voltage and the safety by considering a ferroelectric material in which the angular characteristic of hysteresis is insufficient and fluctuation on fabrication and remanent polarization is stored even if voltage of 1/3V is applied to a ferroelectric thin film. 
     FIG. 22  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 1 in a memory cell at a selected address. In  FIG. 22 , the selected word line WA is set to potential V, the selected bit line BA is set to potential (1/3) V, the unselected word line WI is set to (1/3) V, and the unselected bit line BI is set to potential (2/3) V. In this case, when (2/3) V is applied to the ferroelectric thin film of a memory cell, data 1 is written and remanent polarization is kept in the ferroelectric thin film at ±(1/3) V or 0. 
     FIG. 23  shows potentials of each word line and each bit line when writing data 1 in only a memory cell at a selected address while a word line group and a bit line group are actually arranged like a matrix. In  FIG. 23 , reference numeral  2360  denotes a memory cell at an address in which data 1 should be written,  2361  denotes a word line at a selected address, and  2362  denotes a bit line at a selected address. 
   Potential V is supplied to the word line  2361  at a selected address and potential (1/3) V is supplied to the selected bit line  2362 , (1/3) V is supplied to other word lines at unselected addresses, and potential (2/3) V is supplied to all other bit lines at unselected addresses. In this case, voltage (2/3) V is applied to only the ferroelectric thin film of the memory cell  2360  at a selected address and voltage ±(1/3) V or 0 is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than a coercive voltage is applied to only the memory cell  2360  at a selected address, data 1 is written, memory cells at unselected addresses are not influenced because they respectively have voltage equal to or lower than coercive voltage even if they have different values. 
     FIG. 24  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 0 in a memory cell at a selected address. In  FIG. 24 , the selected word line WA is set to potential, the selected bit line BA is set to potential (2/3) V, unselected word line WI is set to (2/3) V, and unselected bit line BI is set to (1/3) V. In this case, when voltage −(2/3) V is applied to the ferroelectric thin film of a memory cell, data 0 is written and remanent polarization is kept in the ferroelectric thin film at voltage ±(1/3) V or 0. 
     FIG. 25  shows the potential of each word line and the potential of each bit line when writing data 0 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 25 , reference numeral  2360  denotes a memory cell at an address in which data 0 should be written,  2361  denotes a word line at a selected address, and  2362  denotes a bit line at a selected address. 
   Potential 0 is supplied to the word line  2361  at the selected address and potential (2/3) V is supplied to the selected bit line  2362 , and (2/3) V is supplied to all other word lines at unselected addresses and (1/3) V is supplied to all other bit lines at unselected addresses. In this case, voltage −(2/3) V is applied to only the ferroelectric thin film of the memory cell  2360  at the selected address and ±(1/3) V or potential 0 is applied to other ferroelectric thin films at the unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only the memory cell  2360  at the selected address and data 0 is written, but other memory cells at unselected addresses are not influenced because the memory cells respectively have voltage equal to or lower than coercive voltage even if they have different values. 
     FIG. 26  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading remanent polarization data from the first data storing place of a selected MFSFET. 
   In  FIG. 26 , reference numeral  2670  denotes an N-type MFSFET at a selected address,  2671  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  2670 , and  2672  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  2670 . Reference numeral  2674  denotes a word line connected to the gate electrode of the MFSFET  2670  at the selected address. Moreover, reference numeral  2677  denotes the first data storing place of the MFSFET  2670 . 
   In this case, to read the remanent polarization data from the first data storing place  2677  of the selected MFSFET  2670 , potential (1/3) V is supplied to the selected word line  2674 , potential 0 is supplied to the selected first bit line  2671 , and potential (1/3) V is supplied to the second bit line  2672  of a selection pair. 
   Then, the MFSFET  2670  is turned on and operates in a saturated region because it is an N-type MFSFET and potential (1/3) V is supplied to the gate electrode of it from the word line  2674 , potential 0 is supplied to the source electrode of it from the first bit line  2671 , and potential (1/3) V is supplied to the drain electrode of it from the second bit line  2672 . 
   Therefore, the current when the directionality of the remanent polarization of the first data storing place  2677  located at the source side of the N-type MFSFET influences the threshold voltage of the MFSFET flows between the first bit line  2671  and the second bit line  2672 . Then, by detecting the current by the read circuit  1556  in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential 0 is supplied to all other unselected word lines  2675  and unselected bit lines  2673 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, because only voltage equal to or lower than a coercive voltage of 0 [V] or (1/3) V is applied ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
     FIG. 27  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading remanent polarization data from the second data storing place of the selected MFSFET. 
   In  FIG. 27 , reference numeral  2670  denotes an N-type MFSFET at a selected address,  2671  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  2670 , and  2672  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  2670 . Reference numeral  2674  denotes a word line connected to the gate electrode of the MFSFET  2670  at the selected address. Moreover, reference numeral  2678  denotes the second data storing place of the MFSFET  2670 . 
   In this case, to read remanent polarization data from the second data storing place  2678  of the selected MFSFET  2670 , potential (1/3) V is supplied to the selected word line  2674 , potential 0 is supplied to the selected second bit line  2672 , and potential (1/3) V is supplied to the first bit line  2671  of a selection pair. 
   Then, the MFSFET  2670  is turned on and operates in a saturated region because it is an N-type MFSFET and potential (1/3) V is supplied to the gate electrode of it from the word line  2674 , potential 0 is supplied to the source electrode of it from the second bit line  2672 , and potential (1/3) V is supplied to the drain electrode of it from the first bit line  2671 . 
   Therefore, the current when the directionality of the remanent polarization of the second data storing place  2678  located at the source side of the N-type MFSFET influences the threshold voltage of the MFSFET flows between the second bit line  2672  and the first bit line  2671 . Therefore, by detecting the current by the read circuit  1556  in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential 0 is supplied to all other unselected word lines  2675  and unselected bit lines  2673 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, because only voltage equal to or lower than coercive voltage of 0 [V] or (1/3) V is applied to ferroelectric thin films of all MFSFETs, the remanent polarization data is kept. 
   Embodiment 3 of Control Method of Word Line and Bit Line 
     FIGS. 28 ,  29 ,  30 ,  31 ,  32 , and  33  are illustrations showing relations between word lines and bit lines for using five types of potentials for control by setting three intermediate potentials (1/4) V, (2/4) V, and (3/4) V when the potential of power supply ranges between 0 [V] and V[V] (refer to  FIG. 28 ). 
   In the case of the method of the embodiment 2 as the above-described control method of word line and bit line, it is necessary to change many unselected word lines present for write to (1/3) V or (2/3) V though they are unselected in accordance with the fact that data at a selected address is 1 or 0 and unnecessary current consumption due to a lot of charge and discharge currents and delay of write time may be caused. 
   Therefore, though write voltage and coercive voltage for keeping remanent polarization have the same allowance, the same potential of (2/4) V can be used for an unselected word line in writing. Therefore, a method suitable for smaller power consumption is described below by referring to  FIGS. 28 to 33 . 
     FIG. 28  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 1 in a memory cell at a selected address. In  FIG. 28 , the selected word line WA is set to potential V, selected bit line BA is set to potential (1/4) V, unselected word line WI is set to (2/4) V, and unselected bit line BI is set to (3/4) V. In this case, when voltage (3/4) V in an absolute value is applied to the ferroelectric thin film of a memory cell, data 1 is written and remanent polarization is kept in the ferroelectric thin film at potential ±(1/4) V or 0. 
     FIG. 29  shows the potential of each word line and the potential of each bit line when writing data 1 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 29 , reference numeral  2960  denotes a memory cell in which data 1 should be written,  2961  denotes a word line at a selected address, and  2962  denotes a bit line at a selected address. 
   In this case, potential V is supplied to the word line  2961  at the selected address and potential (1/4) V is supplied to the selected bit line  2962  and (2/4) V is supplied to other word lines at unselected addresses and (3/4) V is supplied to all other bit lines at unselected addresses. In this case, voltage (3/4) V is applied to only the ferroelectric thin film of the memory cell  2960  at a selected address and voltage ±(1/4) V is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only the memory cell  1760  at a selected address, data 1 is written but other memory cells at unselected addresses are not influenced because they respectively have voltage equal to or lower than coercive voltage even if they have different values. 
     FIG. 30  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 0 in a memory cell at a selected address. In  FIG. 30 , the selected word line WA is set to potential 0, the selected bit line BA is set to potential (3/4) V, the unselected word line WI is set to (2/4) V, and the unselected bit line BI is set to (1/4) V. In this case, when −(3/4) V is applied to the ferroelectric thin film of a memory cell, data 0 is written and remanent polarization is kept in the ferroelectric thin film at potential ±(1/4) V or potential 0. 
     FIG. 31  shows the potential of each word line and the potential of each bit line when writing data 0 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 31 , reference numeral  2960  denotes a memory cell at an address in which data 0 should be written,  2961  denotes a word line at a selected address, and  2962  denotes a bit line at a selected address. 
   In this case, potential 0 is supplied to the word line  2961  at the selected address and potential (3/4) V is supplied to the selected bit line  2962 , and (2/4) V is supplied to other word lines at unselected addresses and (1/4) V is supplied to all other bit lines at unselected addresses. In this case, voltage −(3/4) V is applied to only the ferroelectric thin film of the memory cell  2960  at the selected address and voltage ±(1/4) V is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only the memory cell  2960  at the selected address, data 0 is written but other memory cells at unselected addresses are not influenced because they respectively have voltage equal to or lower than coercive voltage even if they have different values. 
     FIG. 32  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading remanent polarization data from the first data storing place of a selected MFSFET. 
   In  FIG. 32 , reference numeral  3270  denotes an N-type MFSFET at a selected address,  3271  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  3270 , and  3272  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  3270 . Reference numeral  3274  denotes a word line connected to the gate electrode of the MFSFET  3270  at the selected address. Moreover, reference numeral  3277  denotes the first data storing place of the MFSFET  3270 . 
   In this case, to read remanent polarization data from the first data storing place  3277  of the selected MFSFET  3270 , potential (1/4) V is supplied to the selected word line  3274 , potential 0 is supplied to the selected first bit line  3271 , and potential (1/4) V is supplied to the selected second bit line  3272 . 
   Then, the MFSFET  3270  is turned on and operates in a saturated region because it is an N-type MFSFET and potential (1/4) V is supplied to the gate electrode of it from the word line  3274 , potential 0 is supplied to the source electrode of it from the first bit line  3271 , and potential (1/4) V is supplied to the drain electrode of it from the second bit line  3272 . 
   Therefore, the current when the directionality of remanent polarization of the first data storing place  3277  located at the source side of an N-type MFSFET influences the threshold voltage of the MFSFET flows between the fist bit line  3271  and the second bit line  3272 . Therefore, by detecting the current by the read circuit  1556  in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential 0 is supplied to all other unselected word lines  3275  and unselected bit lines  3273 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, because only voltage equal to or lower than coercive voltage of 0 [V] or (1/4) V is applied to ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
     FIG. 33  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading remanent polarization data from the second data storing place of a selected MFSFET. 
   In  FIG. 33 , reference numeral  3270  denotes an N-type MFSFET at a selected address,  3271  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  3270 , and  3272  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  3270 . Reference numeral  3274  denotes a word line connected to the gate electrode of the MFSFET  3270  at the selected address. Moreover, reference numeral  3278  denotes the second data storing place of the MFSFET  3270 . 
   In this case, to read remanent polarization from the second data storing place  3278  of the selected MFSFET  3270 , potential (1/4) V is supplied to the selected word line  3274 , potential 0 is supplied to the selected second bit line  3272 , and potential (1/4) V is supplied to the selected first bit line  3271 . 
   Then, the MFSFET  3270  is turned on and operates in a saturated region because it is an N-type MFSFET and potential (1/4) V is supplied to the gate electrode of it from the word line  3274 , potential 0 [V] is supplied to the source electrode of it from the second bit line  3272 , and potential (1/4) V is supplied to the drain electrode of it from the first bit line  3271 . 
   Therefore, the current when the directionality of the remanent polarization of the second data storing place  3278  located at the source side of the N-type MFSFET influences the threshold voltage of the MFSFET flows between the second bit line  3272  and the first bit line  3271 . Therefore, by detecting the current by the read circuit  1556  in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential 0 is supplied to all other unselected word lines  3275  and unselected bit lines  3273 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, only voltage of equal to or lower than coercive voltage of 0 [V] or (1/4) V is applied to ferroelectric thin films of all MFSFETs, the remanent polarization data is kept. 
   (Second Configuration of Memory Cell of the Present Invention) 
     FIG. 34  is a circuit diagram showing a configuration of a second example of a memory cell serving as the basic unit of a ferroelectric memory device of the present invention. 
   As shown in  FIG. 34 , this memory cell is constituted of a P-type MFSFET  3400 , word line  3404 , first bit line  3405 , and second bit line  3406 . That is, the memory cell is obtained by replacing the N-type MFSFET  100  of the memory cell shown in  FIG. 1  with the P-type MFSFET  3400 . 
   The P-type MFSFET  3400  is constituted of a gate electrode  3401 , a first electrode  3402  serving as a source or drain and constituted of P +  diffusion, and a second electrode  3403  serving as a drain or source and constituted of P +  diffusion. Moreover, in the P-type MFSFET  3400 , the word line  3404  is connected to the gate electrode  3401 , the first bit line  3405  is connected to the first electrode  3402 , and the second bit line  3406  is connected to the second electrode  3403 . In the P-type MFSFET  3400  in  FIG. 34 , a ferroelectric thin film is embedded immediately below the gate electrode  3401  and this point is the same as the case of the N-type MFSFET  100  shown in  FIG. 1 . 
   Thus, the memory cell shown in  FIG. 34  is obtained by replacing the N-type MFSFET  100  of the memory cell shown in  FIG. 1  with the P-type MFSFET  3400 . Therefore, the memory cell shown in  FIG. 34  basically has a configuration and function same as those of the memory cell shown in  FIG. 1  except that positives and negatives of a power supply and voltage to be controlled are reversed and can hold two-bit remanent polarization data. 
     FIG. 35  shows a circuit on which a memory cell group  3520  is formed by arranging unit memory cells shown in  FIG. 34  like a matrix. The memory  3520  shown in  FIG. 35  is obtained by replacing an N-type MFSFET with a P-type MFSFET in the memory cell group  1520  shown in  FIG. 15 . Even if replacing the N-type MFSFET with the P-type MFSFET, the configuration of an MFSFET, word line, first bit line, and second bit line is the same. Therefore, though peripheral circuits of the memory cell group  3520  are not illustrated, the same circuits as the peripheral circuits shown in  FIG. 15  are basically used. 
   Embodiment 4 of Control Method of Word Line and Bit Line 
     FIGS. 36 ,  37 ,  38 ,  39 ,  40 , and  41  show a case of replacing the MFSFET of a memory cell with the above-described P-type MFSFET and are drawings showing relations between word lines and bit lines for using three types of potentials for control by setting intermediate potential (1/2) V when the potential of power supply ranges between 0 [V] and V[V]. 
     FIG. 36  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 1 in a memory cell at a selected address. In  FIG. 36 , potential 0 is supplied to the selected word line WA, potential V is supplied to the selected bit line BA, (1/2) V is supplied to the unselected word line WI, and potential (1/2) V is supplied to the unselected bit line BI. In this case, when voltage −V is applied to the ferroelectric thin film of the memory cell, data 1 is written, and remanent polarization is kept when voltage ±(1/2) V or 0 is applied to the ferroelectric thin film. 
     FIG. 37  shows the potential of each word line and the potential of each bit line when writing data 1 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 37 , reference numeral  3760  denotes a memory cell in which data 1 should be written,  3761  denotes a word line at a selected address, and  3762  denotes a bit line at a selected address. 
   In this case, potential 0 is supplied to the word line  3761  at the selected address and potential V is supplied to the selected bit line  3762  and (1/2) V is supplied to word lines and bit lines at unselected addresses. In this case, voltage −V is applied to only the ferroelectric thin film of the memory cell  3760  at the selected address and voltage ±(1/2) V or 0 is applied to the ferroelectric thin film of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only the memory cell  3760  at the selected address, data 1 is written but data in memory cells at unselected addresses is not changed because they respectively have voltage equal to or lower than the coercive voltage even if they have different values. 
     FIG. 38  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 0 in a memory cell at a selected address. In  FIG. 38 , the selected word line WA is set to potential V, the selected bit line BA is set to potential 0, the unselected word line WI is set to (1/2) V, and the unselected bit line BI is set to (1/2) V. In this case, when voltage V is applied to the ferroelectric thin film of the memory cell, data 0 is written and remanent polarization is kept in the ferroelectric thin film when voltage ±(1/2) V or 0 is applied to the ferroelectric thin film. 
     FIG. 39  shows the potential of each word line and the potential of each bit line when writing data 0 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 39 , reference numeral  3760  denotes a memory cell at an address in which data 0 should be written,  3761  denotes a word line at a selected address, and  3762  denotes a bit line at a selected address. 
   In this case, potential V is supplied to the word line  3761  at the selected address and potential 0 is supplied to the selected bit line  3762  and (1/2) V is supplied to all other word lines and bit lines at unselected addresses. In this case, voltage V is applied to only the ferroelectric thin film of the memory cell  3760  at the selected address and voltage ±(1/2) V or 0 is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only a memory cell at a selected address, data 0 is written, and other memory cells at unselected addresses are not influenced because they respectively have voltage equal to or lower than coercive voltage even if they have different values. 
     FIG. 40  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading remanent polarization data from the first data storing place of the selected MFSFET. 
   In  FIG. 40 , reference numeral  4070  denotes a P-type MFSFET at a selected address,  4071  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  4070  and  4072  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  4070 . Reference numeral  4074  denotes a word line connected to the gate electrode of the MFSFET  4070  at the selected address. Reference numeral  4077  denotes a first data storing place of the MFSFET. 
   In this case, to read remanent polarization data from the fist data storing place  4077  of the selected MFSFET  4070 , potential (1/2) V is supplied to the selected word line  4074 , potential V is supplied to the selected first bit line  2071 , and potential (1/2) V is supplied to the selected second bit line  2072 . 
   Then, the MFSFET  4070  is turned on and operates in a saturated region because it is a P-type MFSFET and potential (1/2) V is supplied to the gate electrode of it from the word line  4074 , potential V is supplied to the source electrode of it from the first bit line  4071 , and potential (1/2) V is supplied to the drain electrode of it from the second bit line  2072 . 
   Therefore, the current when the directionality of the remanent polarization of the first data storing place  4077  located at the source side of the P-type MFSFET influences the threshold voltage of the MFSFET flows between the first bit line  4071  and the second bit line  4072 . Then, by detecting the current by the read circuit shown in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential V is supplied to all other unselected word lines  4075  and unselected bit lines  4073 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, because only voltage equal to or lower than coercive voltage of 0 [V] or (1/2) V is applied to ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
     FIG. 41  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading the remanent polarization data from the second data storing place of the selected MFSFET. 
   In  FIG. 41 , reference numeral  4070  denotes a P-type MFSFET at a selected address,  4071  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  4070 , and  4072  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  4070 . Reference numeral  4074  denotes a word line connected to the gate electrode of the MFSFET  4070  at the selected address. Moreover, reference numeral  4078  denotes a second data storing place of the MFSFET  4070 . 
   In this case, to read remanent polarization data from the second data storing place  4078  of the selected MFSFET  4070 , potential (1/2) V is supplied to the selected word line  4074 , potential V is supplied to the selected second bit line  4072 , and potential (1/2) V is supplied to the selected first bit line  4071 . 
   Then, the MFSFET  4070  is turned on and operates in a saturated region because it is a P-type MFSFET and potential (1/2) V is supplied to the gate electrode of it from the word line  4074 , potential V is supplied to the source electrode of it from the second bit line  4072 , and potential (1/2) V is supplied to the drain electrode of it from the first bit line  4071 . 
   Therefore, the current when the directionality of the remanent polarization of the second data storing place  4078  located at the source side of the P-type MFSFET influences the threshold voltage of the MFSFET flows between the second bit line  4072  and the first bit line  4071 . Therefore, by detecting the current by the read circuit shown in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential V is supplied to all other unselected word lines  4075  and unselected bit lines  4073 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, because only voltage equal to or lower than coercive voltage of 0 [V] or (1/2) V is applied to ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
   Embodiment 5 of Control Method of Word Line and Bit Line 
     FIGS. 42 ,  43 ,  44 ,  45 ,  46 , and  47  are illustrations showing relations between word lines and bit lines for using four types of potentials for control by setting two intermediate potentials (1/3) V and (2/3) V when replacing the MFSFET of a memory cell with the above-described P-type MFSFET and the potential of power supply ranges between 0 [V] and V[V]. 
     FIG. 42  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 1 in the address of a selected memory cell. In  FIG. 42 , the selected word line WA is set to potential 0, the selected bit line BA is set to potential (2/3) V, unselected word line WI is set to (2/3) V, and unselected bit line BI is set to (1/3) V. In this case, when voltage −(2/3) V is applied to the ferroelectric thin film of the memory cell, data 1 is written and remanent polarization is kept when voltage ±(1/3) V or 0 is applied to the ferroelectric thin film. 
     FIG. 43  shows the potential of each word line and the potential of each bit line when writing data 1 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 43 , reference numeral  4360  denotes a memory cell at an address in which data 1 should be written,  4361  denotes a word line at a selected address, and  4362  denotes a bit line at a selected address. 
   In this case, potential 0 is supplied to the word line  4361  at the selected address and potential (2/3) V is supplied to the selected bit line  4362 , and (2/3) V is supplied to other word lines at unselected addresses and (1/3) V is supplied to all other bit lines at unselected addresses. In this case, voltage −(2/3) V is applied to only the ferroelectric thin film of the memory cell  4360  at the selected address and voltage ±(1/3) V or 0 is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only the memory cell at the selected address, data 1 is written, but other memory cells at unselected addresses are not influenced because they respectively have voltage equal to or lower than coercive voltage even if they have different values. 
     FIG. 44  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 0 in a memory cell at a selected address. In  FIG. 44 , the selected word line WA is set to potential V, the selected bit line BA is set to potential (1/3) V, the unselected word line WI is set to (1/3) V and the unselected bit line BI is set to potential (2/3) V. In this case, when voltage (2/3) V is applied to the ferroelectric thin film of the memory cell, data 0 is written, and remanent polarization is kept when voltage ±(1/3) V or 0 is applied to the ferroelectric thin film. 
     FIG. 45  shows the potential of each word line and the potential of each bit line when writing data 0 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 45 , reference numeral  4360  denotes a memory cell at an address in which data 1 should be written,  4361  denotes a word line at a selected address, and  4362  denotes a bit line at a selected address. 
   In this case, potential V is supplied to the word line  4361  at the selected address and potential (1/3) V is supplied to the selected bit line  4362 , and (1/3) V is supplied to other word lines at unselected addresses and (2/3) V is supplied to all other bit lines at unselected addresses. In this case, voltage (2/3) V is applied to only the ferroelectric thin film of the memory cell  4360  at the selected address and voltage ±(1/3) V or 0 is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only a memory cell at a selected address, data 0 is written, but other memory cells at unselected addresses are not influenced because they respectively have voltage equal to or lower than coercive voltage even if they have different values. 
     FIG. 46  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading remanent polarization data from the first data storing place of the selected MFSFET. 
   In  FIG. 46 , reference numeral  4670  denotes a P-type MFSFET at a selected address,  4671  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  4670 , and  4672  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  4670 . Reference numeral  4674  denotes a word line connected to the gate electrode of the MFSFET  4670  at the selected address. Moreover, reference numeral  4677  denotes the first data storing place of the MFSFET  4670 . 
   In this case, to read remanent polarization data from the first data storing place  4677  of the selected MFSFET  4670 , potential (2/3) V is supplied to the selected word line  4674 , potential V is supplied to the selected first bit line  4671 , and potential (2/3) V is supplied to the selected second bit line  4672 . 
   Then, the MFSFET  4670  is turned on and operates in a saturated region because it is a P-type MFSFET and potential (2/3) V is supplied to the gate electrode of it from the word line  4674 , potential V is supplied to the source electrode of it from the first bit line  4671 , and potential (2/3) V is supplied to the drain electrode of it from the second bit line  4672 . 
   Therefore, the current when the directionality of the remanent polarization of the first data storing place  4677  located at the source side of the P-type MFSFET influences the threshold voltage of the MFSFET flows between the fist bit line  4671  and the second bit line  4672 . Therefore, by detecting the current by the read circuit shown in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential V is supplied to all other unselected word lines  4675  and unselected bit lines  4673 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, because only voltage equal to or lower than the coercive voltage of 0 [V] or (1/3) V is applied to ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
     FIG. 47  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected second bit line B 2 , and unselected bit line BI when reading the remanent polarization data from the second data storing place of the selected MFSFET. 
   In  FIG. 47 , reference numeral  4670  denotes a P-type MFSFET at a selected address,  4671  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  4670 , and  4672  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  4670 . Reference numeral  4674  denotes a word line connected to the gate electrode of the MFSFET  4670  at the selected address. Reference numeral  4678  denotes the second data storing place of the MFSFET  4670 . 
   In this case, to read remanent polarization data from the second data storing place  4678  of the MFSFET  4670 , potential (2/3) V is supplied to the selected word line  4674 , potential V is supplied to the selected second bit line  4672 , and potential (2/3) V is supplied to the selected first bit line  4671 . 
   Then, the MFSFET  4670  is turned on and operates in a saturated region because it is a P-type MFSFET and potential (2/3) V is supplied to the gate electrode of it from the word line  4674 , potential V is supplied to the source electrode of it from the second bit line  4672 , and potential (2/3) V is supplied to the drain electrode of it from the first bit line  4671 . 
   Therefore, the current when the directionality of the remanent polarization of the second data storing place  4678  located at the source side of the P-type MFSFET influences the threshold voltage of the MFSFET flows between the second bit line  4672  and the first bit line  4671 . Therefore, by detecting the current by the read circuit shown in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential V is supplied to other unselected word lines  4675  and unselected bit lines  4673 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, only voltage equal to or lower than coercive voltage 0 [V] or (1/3) V is applied to ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
   Embodiment 6 of Control Method of Word Line and Bit Line 
     FIGS. 48 ,  49 ,  50 ,  51 ,  52 , and  53  are illustrations showing relations between word lines and bit lines for using five types of potentials for control by setting three intermediate potentials (1/4) V, (2/4) V, and (3/4) V when the MFSFET of a memory cell is replaced with the above-described P-type MFSFET and potential of power supply ranges between 0[V} and V[V] (refer to  FIG. 48 ). 
     FIG. 48  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 1 in a memory cell at a selected address. In  FIG. 48 , the selected word line WA is set to potential 0, the selected bit line BA is set to potential (3/4) V, the unselected word line WI is set to potential (2/4) V, and the unselected bit line BI is set to potential (1/4) V. In this case, when voltage −(3/4) V is applied to the ferroelectric thin film of the memory cell, data 1 is written, and remanent polarization is kept when voltage ±(1/4) V or 0 is applied to the ferroelectric thin film. 
     FIG. 49  shows the potential of each word line and the potential of each bit line when writing data 1 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 49 , reference numeral  4960  denotes a memory cell at an address in which data 1 should be written,  4961  denotes a word line at a selected address, and  4962  denotes a bit line at a selected address. 
   In this case, potential 0 is supplied to the word line  4961  at a selected address and potential (3/4) V is supplied to the selected bit line  4962 , potential (2/4) V is supplied to other bit lines at unselected addresses, and potential (1/4) V is supplied to all other bit lines at unselected addresses. In this case, voltage −(3/4) V is applied to only the ferroelectric thin film of the memory cell  4960  at the selected address and voltage ±(1/4) V is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only the memory cell  4960  at a selected address, data 1 is written, and other memory cells at unselected addresses are not influenced because they respectively have voltage equal to or lower than coercive voltage even if they have different values. 
     FIG. 50  shows a relation between selected word line WA, unselected word line WI, selected bit line BA, and unselected bit line BI when writing data 0 in a memory cell at a selected address. In  FIG. 50 , the selected word line WA is set to potential V, the selected bit line BA is set to potential (1/4) V, the unselected word line WI is set to (2/4) V, and the unselected bit line BI is set to (3/4) V. In this case, when (3/4) V is applied to the ferroelectric thin film of the memory cell, data 0 is written and remanent polarization is kept when potential ±(1/4) V or 0 is applied to the ferroelectric thin film. 
     FIG. 51  shows the potential of each word line and the potential of each bit line when writing data 0 in only a memory cell at a selected address while word line group and bit line group are actually arranged like a matrix. In  FIG. 51 , reference numeral  4960  denotes a memory cell at an address in which data 0 should be written,  4961  denotes a word line at a selected address, and  4962  denotes a bit line at a selected address. 
   In this case, potential V is supplied to the word line  4961  at a selected address and potential (1/4) V is supplied to the selected bit line  4962  and potential (2/4) V is supplied to other word lines at unselected addresses and potential (3/4) V is supplied to all other bit lines at unselected addresses. In this case, voltage (3/4) V is applied to only the ferroelectric thin film of the memory cell  4960  at a selected address and voltage ±(1/4) V is applied to ferroelectric thin films of other memory cells at unselected addresses. 
   Therefore, voltage equal to or higher than coercive voltage is applied to only the memory cell  4960  at the selected address, data 0 is written, and other memory cells at unselected addresses are not influenced because they respectively have a voltage equal to or lower than the coercive voltage even if they have different values. 
     FIG. 52  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected-pair second bit line B 2 , and unselected bit line BI when reading remanent polarization data from the first data storing place of a selected MFSFET. 
   In  FIG. 52 , reference numeral  5270  denotes a P-type MFSFET at a selected address,  5271  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  5270 , and  5272  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  5270 . Reference numeral  5274  denotes a word line connected to the gate electrode of the MFSFET  5270  at the selected address. Moreover, reference numeral  5277  denotes the first data storing place of the MFSFET  5270 . 
   In this case, to read remanent polarization data from the first data storing place  5277  of the selected MFSFET  5270 , potential (3/4) V is supplied to the selected word line  5274 , potential V is supplied to the selected first bit line  5271 , and potential (3/4) V is supplied to the selected second bit line  5272 . 
   Then, the MFSFET  5270  is turned on and operates in a saturated region because it is a P-type MFSFET and potential (3/4) V is supplied to the gate electrode of it from the word line  5274 , potential V is supplied to the source electrode of it from the first bit line  5271 , and potential (3/4) V is supplied to the drain electrode of it from the second bit line  5272 . 
   Therefore, the current when the directionality of the first data storing place  5277  located at the source side of the P-type MFSFET influences the threshold voltage of the MFSFET flows between the first bit line  5271  and the second bit line  5272 . Therefore, by detecting the current by the read circuit shown in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnitude of the detected current. 
   In this case, potential V is supplied to all other unselected word lines  5275  and unselected bit lines  5273 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, because only voltage equal to or lower than coercive voltage of 0 [V] or (1/4) V is applied to ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
     FIG. 53  shows a relation between selected word line WA, unselected word line WI, selected first bit line B 1 , selected-pair second bit line B 2 , and unselected bit line BI when reading the remanent polarization data from the second data storing place of the selected MFSFET. 
   In  FIG. 53 , reference numeral  5270  denotes a P-type MFSFET at a selected address,  5271  denotes a first bit line connected to a first electrode serving as the source or drain of the MFSFET  5270 ,  5272  denotes a second bit line connected to a second electrode serving as the drain or source of the MFSFET  5270 . Reference numeral  5274  denotes a word line connected to the gate electrode of the MFSFET  5270  at the selected address. Moreover, reference numeral  5278  denotes the second data storing place of the MSFFET  5270 . 
   In this case, to read remanent polarization data from the second data storing place  5278  of the selected MFSFET  5270 , potential (3/4) V is supplied to the selected word line  5274 , potential V is supplied to the selected second bit line  5272 , and potential (3/4) V is supplied to the first bit line  5271  of a selection pair. 
   Then, the MFSFET  5270  is turned on and operates in a saturated region because it is a P-type MFSFET, potential (3/4) V is supplied to the gate electrode of it from the word line  5274 , potential V is supplied to the source electrode of it from the second bit line  5272 , and potential (3/4) V is supplied to the drain electrode of it from the first bit line  5271 . 
   Therefore, the current when the directionality of the remanent polarization of the second data storing place  5278  located at the source side of the P-type MFSFET influences the threshold voltage of the MFSFET flows between the second bit line  5272  and the first bit line  5271 . Therefore, by detecting the current by the read circuit shown in  FIG. 15 , it is possible to determine 1 or 0 of read data in accordance with the magnification of the detected current. 
   In this case, potential V is supplied to other unselected word lines  5275  and unselected bit lines  5273 . As a result, no current flows because other MFSFETs at unselected addresses are turned off. Moreover, only voltage equal to or lower than coercive voltage 0 [V] or (1/4) V is applied to ferroelectric thin films of all MFSFETs, remanent polarization data is kept. 
   (Configuration of Ferroelectric Memory Device) 
   Then, the outline of the general configuration of an embodiment of a ferroelectric memory device of the present invention using the memory cells and control circuit described for  FIGS. 1 to 53  is described below by referring to  FIG. 54 . 
   In this case,  FIG. 15  is a block diagram of a memory cell group and only its peripheral control circuits. However, in the case of an actual memory device, a circuit factor including other functions is necessary.  FIG. 54  is a block diagram showing the outline of the general configuration of a ferroelectric memory device including the portion. 
   As shown in  FIG. 54 , an embodiment of a ferroelectric memory device of the present invention is constituted of a memory cell group  5420 , bit-line selection and control circuit  5451 , word-line selection and control circuit  5453 , write control circuit  5454 , read control circuit  5456 , input/output circuit  5457 , power supply circuit  5458 , and total control circuit  5459 . 
   In the case of the memory cell group  5420 , memory cells shown in  FIG. 1  or  34  are arranged like a matrix like the memory cells of the memory cell group  1520  in  FIG. 15 . 
   The word-line selection and control circuit  5453  and bit-line selection and control circuit  5451  select a desire memory cell by selecting the word line, first bit line, and second bit line as shown in  FIG. 1  or  34  included in the memory cell group  1520 . 
   The write control circuit  5454  writes the data passing through the input/output circuit  5457  in a memory cell selected by the word-line selection and control circuit  5453  and bit-line selection and control circuit  5451  in accordance with an instruction of the total control circuit  5459 . 
   The read control circuit  5456  reads the data stored in a memory cell selected by the word-line selection and control circuit  5453  and bit-line selection and control circuit  5451  in accordance with an instruction of the total control circuit  5420  and outputs the data to the input/output circuit  5457 . 
   The power supply circuit  5458  generates a plurality of signals having different potentials necessary when the write control circuit  5454  writes data in a desired memory cell and generates a plurality of signals having different potentials necessary when the read control circuit  5456  reads data from a desired memory cell. The total control circuit  5459  controls, when reading or writing data, each portion in accordance with the operation. 
   The word-line selection and control circuit  5453 , bit-line selection and control circuit  5451 , write control circuit  5454 , and read control circuit  5456  control a word line, first bit line, and second bit line of a memory cell at a selected address when writing or reading data and control a word line, first bit line, and second bit line of a memory cell at an unselected address. 
   In this case, a part of the write circuit  1554  and a part of the write-read circuit  1555  shown in  FIG. 15  are included in the write control circuit  5454  in  FIG. 54 . Moreover, a part of the read circuit  1556  and a part of the write-read circuit  1555  shown in  FIG. 15  are included in the read control circuit  5456  in  FIG. 54 . 
   Moreover, in  FIGS. 15 and 54 , a field-effect transistor of a memory cell group uses a MFSFET having a ferroelectric for its gate portion. However, peripheral control circuits are respectively constituted of an insulating-gate field-effect transistor (MOSFET) in principle. 
   (Other Materials of Ferroelectric Thin Film) 
   As described above, a memory cell uses an MFSFET in which a ferroelectric thin film is embedded immediately below a gate electrode, its sectional structure is shown in  FIG. 2 , and a material of the ferroelectric thin film is inorganic ferroelectric such as PZTN, PZT, or SBT. 
   However, as shown in  FIG. 2 , the ferroelectric thin film  200  is formed on the silicon substrate  209 . Therefore, this crystallizes ferroelectric on silicon (Si). It is generally difficult to directly grow inorganic ferroelectric crystal on silicon crystal and an ideal MFS structure is not formed because many crystal defects are left on the boundary. As a result, in the ferroelectric thin film  200  and silicon substrate  209 , the remanent polarization of ferroelectric thin film  200  is influenced because of the crystal defects and a trouble is considered that desired data disappears in a short period. 
   Therefore, it is preferable to use not inorganic ferroelectric but organic ferroelectric as the material of the ferroelectric thin film  200 . Because organic ferroelectric is formed at a low temperature compared to inorganic ferroelectric, a layer including many defects at the interface with silicon is not formed but an ideal MFS structure is formed. 
   Therefore, in  FIG. 2 , when the ferroelectric thin film  200  is organic ferroelectric, it is possible to realize a memory cell of an MFSFET having a preferable-characteristic ferroelectric thin film at its gate portion. In this case, PVDF (poly(vinylidene fluoride), P(VDF/TrFE) (poly(vinylidene fluoride-trifluoroethylene), or odd-number nylon such as nylon 7 or nylon 11 is proper as the material of organic ferroelectric. 
   (Other Structure of MFSFET Having Ferroelectric Thin Film at its Gate Portion) 
   As described above, a memory cell uses an MFSFET in which a ferroelectric thin film is embedded immediately below a gate electrode and its sectional structure is shown in  FIG. 2 . However, it is also allowed to use the following structure. 
     FIG. 55  is a second structure of an MFSFET having a ferroelectric thin film at its gate portion. 
   In  FIG. 55 , reference numeral  5501  denotes agate electrode constituted of metal,  5502  denotes a first electrode serving as a source or drain and constituted of N +  diffusion, and  5503  denotes a second electrode serving as a drain or source and constituted of N +  diffusion. Reference numeral  5500  denotes a ferroelectric thin film and  5504  denotes a buffer layer constituted of a paraelectric thin film. 
     FIG. 2  is different from  FIG. 55  in that the buffer layer  5504  constituted of a paraelectric insulator is used in  FIG. 55 . The buffer layer  5504  is used because when the crystallinity of the ferroelectric thin film  5500  is incomplete, leak of electric charges occurs between the silicon substrate  209  and ferroelectric thin film  200  of an MFSFET in the case of the structure in  FIG. 2  and the holding characteristic of the remanent polarization of the ferroelectric thin film  200  may be deteriorated. 
   Therefore, as shown in  FIG. 55 , the buffer layer  5504  which is an insulator and in which a ferroelectric thin film is easily crystal-grown is set between the silicon substrate  5509  and the ferroelectric thin film  5500  so as to prevent the above characteristic from deteriorating. 
     FIG. 56  shows a third structure of an MFSFET having a ferroelectric thin film at its gate portion. 
   In  FIG. 56 , reference numeral  5601  denotes a gate electrode constituted of metal,  5602  denotes a first electrode serving as a source or drain and constituted of N +  diffusion, and  5603  denotes a second electrode serving as a drain or source and constituted of N +  diffusion. Reference numeral  5600  denotes a ferroelectric thin film,  5604  denotes a first buffer layer formed of paraelectric thin film placing emphasis on insulating property, and  5605  denotes a second buffer layer made of a material in which ferroelectric is easily crystallized. 
     FIG. 55  is different from  FIG. 56  in that the second buffer layer  5605  is used in  FIG. 56 . The function of a buffer layer is to easily realize the insulating property between a silicon substrate and a ferroelectric thin film and crystallization of the ferroelectric thin film. However, it may be difficult to realize two functions by one material. 
   Therefore, as shown in  FIG. 56 , the first buffer layer  5604  and second buffer layer  5605  are formed between the silicon substrate  5509  and the ferroelectric thin film  5500  so that their function is shared and can be compatible. 
   OTHERS 
   The present invention is not restricted to the above embodiment. Another embodiment is described below. 
   In the case of the embodiments 1, 2, and 3 respectively relating to a control method of word line and bit line, when reading data from a memory cell, potential differences between gate electrode and source electrode are set to (1/2) V, (1/3) V, and (1/4) V. However, when it is possible to read data independently of these potentials, it is also allowed to supply a lower potential. In this case, long-term reliability and service life of a device are improved. 
   Moreover, in  FIG. 2 , PZTN is shown as a preferable example as a material of an inorganic ferroelectric thin film. However, by using a control method suitable for the characteristic of a ferroelectric thin film, it is allowed to use a material other than PZTN. For example, it is allowed to use BLT (Bi 4X La X Ti 3 O 12 ), (Ba, Sr) TiO 3 , Bi 4 Ti 3 O 12 , or BaBiNb 2 O 9  in addition to PZT and SBT shown as ferroelectric. Moreover, there are infinite materials when the rate of composition is changed. Furthermore, it is allowed to use an object obtained by laminating materials in which the upper layer and lower layer of a ferroelectric thin film are different in composition. 
   Furthermore, it is also allowed to use Ta, Ti, or alloy of Pt and Ti other than the above-described platinum (Pt) as a metal film or a material of a metal film electrode. Furthermore, it is possible to use an oxide conductive film of RuO 2 , IrO 2 , SrRuO 3 , or RhO 2  according to circumstances. Not only electrical characteristic but also reliability of quality, easiness on fabrication, and fabrication cost are comprehensively studied on which material is selected among the above materials. 
   Moreover, in  FIGS. 2 ,  55 , and  56 , an MFSFET is described by using a type having a bulk substrate. Furthermore, an integrated circuit using a silicon-on-insulator substrate (SOI) having an embedded oxide film layer can be similarly applied. 
   A basic circuit configuration of the present invention having a ferroelectric thin film at its gate portion to store two-bit data in one transistor can be applied to an integrated circuit including an MFSFET or MOSFET using silicon. Moreover, in the case of a product having a specification allowing slow response speed, the basic circuit configuration can be applied to a TFT using an organic ferroelectric for its gate portion or the integrated circuit of an organic transistor.