Abstract:
A ferroelectric memory device of the present invention includes at least one memory cell which includes a semiconductor transistor and a ferroelectric capacitor, the ferroelectric memory device reading out data from each of the at least one memory cell and comparing the read data with a reference level signal to amplify a signal corresponding to the data read out from the at least one memory cell. The ferroelectric memory device further includes: at least one of an external voltage detection circuit for detecting a level of a voltage externally provided to the ferroelectric memory device and a temperature detection circuit for detecting an ambient temperature around the ferroelectric memory device; a reference signal generation circuit connected to the at least one of the external voltage detection circuit and the temperature detection circuit for outputting a potential based on an output from the at least one of the external voltage detection circuit and the temperature detection circuit; and a reference level adjustment section for adjusting the potential output from the reference signal generation circuit into a suitable reference level signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ferroelectric memory device and a method for generating a reference, level signal therefor. More particularly, the present invention relates to a 1T1C type ferroelectric memory, device and a method for generating a reference level signal therefor. 
     2. Description of the Related Art 
     A ferroelectric memory device includes a plurality of memory cells each of which includes a semiconductor transistor (or a switch) and a ferroelectric capacitor. The memory cells are selectively activated by selectively turning ON/OFF the semiconductor transistors. Information is stored in the memory device based on the polarity of the ferroelectric capacitor. A 1T1C type ferroelectric memory device includes a plurality of memory cells each of which includes a transistor, e.g., a MOS (Metal Oxide Semiconductor) transistor, and a ferroelectric capacitor. A potential output from a memory cell (i.e., information which has been stored in the ferroelectric capacitor of the memory cell) is compared with a reference level signal to amplify a signal corresponding to the data from the memory cell. 
     FIG. 7A illustrates, in a simplified manner, a reference level generation circuit  700  of a conventional ferroelectric memory device as disclosed in Japanese Laid-Open Publication No. 10-50075. Referring to FIG. 7A the reference level generation circuit  700  includes two reference cells  102  and  103 . The reference cells  102  and  103  are both connected to a RWL (reference word line) signal line. The reference cells  102  and  103  are also connected to bit lines  100   a  and  100   b , respectively. Each of the bit lines  100   a  and  100   b  crosses the RWL signal line and a BSH (bit line short) signal line. The reference level generation circuit  700  further includes a switch transistor  101 . The gate of the switch transistor  101  is connected to the BSH signal line, the source/drain of the switch transistor  101  are respectively connected to the bit lines  10   a  and  100   b  . The reference level generation circuit  700  uses two reference cells, each having the same structure as that of a memory cell in a ferroelectric memory device (not shown), for outputting an “L” level signal (data “ 0 ”) and an “H” level signal (data “1”), respectively. The two potentials are shorted with each other so as to generate an intermediate level between the “H” level and the “L” level, which is used as a reference level. 
     The generation of the reference level will be described with reference to a timing diagram shown in FIG.  7 B. Referring to FIG. 7B, first, a RWL signal is activated (indicated as the transition to the “H” level). Then, as illustrated in FIG. 7A, data obtained by inverting data “0” is output from the reference cell  102  to the bit line  100   a , and data obtained by inverting data “1” is output from the reference cell  103  to the bit line  100   b . While the RWL signal is activated, a BSH signal is activated (indicated as the transition to the “H” level). Thus, the reference level generation circuit  700  illustrated in FIG. 7A closes the switch transistor  101  so as to short the respective outputs from the reference cells  102  and  103  with each other, thereby setting the potential of each of the bit lines  100   a  and  100   b  to an intermediate level (reference level) between the “H” level and-the “L” level. After the reference level is generated, the reference level generation circuit  700  enables a sense amplifier (not shown) by activating an SAE (sense amp enable) signal (indicated as the transition to the “H” level) so as to compare the output from the selected memory cell with the reference level and amplify a signal corresponding to the output from the selected memory cell. 
     In this conventional example, each memory cell and each reference cell have the same structure, and a reference level is generated by shorting with each other the “H”level and the “L” level which are output from the two reference cells  102  and  103 , respectively. Therefore, the reference level is an intermediate level which is centered between the “H” level and the “L” level. However, this conventional example has a problem in that the reference cells  102  and  103 , each of which is a ferroelectric capacitor as that used in a memory cell, may deteriorate over time. Generally, a reference cell is accessed more often than a normal memory cell. Therefore, a memory device may become inoperable due to the deterioration of the reference cells even through the memory cells remain operable. This problem can be overcome by increasing the number of reference cells to be provided, which however undesirably increases the chip area. 
     In order to solve this problem, a ferroelectric memory device  850  having a reference level generation circuit  800  as illustrated in FIG. 8A has been proposed in the art. The reference level generation circuit  800  includes a reference signal generation circuit  107 , a capacitor  106  for storing a potential (level) output from the reference signal generation circuit  107 , and switch transistors  104  and  105  for controlling the capacitor  106  and the reference signal generation circuit  107 , respectively. In the reference level generation circuit  800 , the reference signal generation circuit  107  is connected to the source of the switch transistor  105 , and the drain of the switch transistor  105  is connected to the first electrode of the capacitor  106  and the source of the switch transistor  104 . The second electrode of the capacitor  106  is connected to a ground. The gates of the switch transistors  104  and  105  are connected to an RWL line and a PRC (pre-charge control) line, respectively. The drain of the switch transistor  104  is connected to a bit line  100 . The potential generated by the reference signal generation circuit  107  is charged into the capacitor  106 , and the capacitor  106  is shorted with the bit line  100 s. Thus, a potential (reference level) is generated onto the bit line  100   a  by virtue of the charge sharing between the capacitor  106  and the bit line  100   c . A sense amplifier  15  is connected to the bit line  100   c  and also to another bit line  11 , which is connected to a memory cell  12 . The memory cell  12  includes a semiconductor transistor  16  and a ferroelectric capacitor  17 . The source of the semiconductor transistor  16  is connected to the bit line  11 , the drain of the semiconductor transistor  16  is connected to a first electrode of the ferroelectric capacitor  17 , and the gate of the semiconductor transistor  16  is connected to a word line  13 . A second electrode of the ferroelectric capacitor  17  is connected to a plate line  14 . With such a configuration, the output from the selected memory cell  12  is compared with the reference level signal to amplify the signal corresponding to the output of the memory cell  12 . 
     The generation of the reference level by the reference level generation circuit  800  of the ferroelectric memory device  850  illustrated in FIG. 8A will be described with reference to a timing diagram shown in FIG.  8 B. Referring to FIG. 8B, a PRC signal is activated (indicated as the transition to the “H” level) to close the switch transistor  105  so that the capacitor  106  is charged by the reference signal generation circuit  107 . Then, the PRC signal is deactivated (indicated as the transition to the “L” level), after which RWL signal is activated (indicated as the transition to the “H” level) so as to close the switch transistor  104 . Thus, a reference level is generated onto the bit line  100   c  by virtue of the charge sharing between the capacitor  106  and the bit line  100   c . After the reference level is generated, the SAE signal is activated so as to enable the sense amplifier  15 . Thus, the output from the selected memory cell  12  is compared with the reference level signal to amplify the signal corresponding to the output of the memory cell  12 . 
     In the case of the reference level generation circuit  800  illustrated in FIG. 8A, the potential output from the reference signal generation circuit  107  is not at an intermediate level between the “H” level and the “L” level which is to be output onto the bit line  100   c . This is because, in this configuration, the potential output from the reference signal generation circuit  107  is not directly supplied to the bit line  100   c , and it is only necessary that the potential which is finally output onto the bit line  100   c  is at the intermediate level between the “H” level and the “L” level. Thus, the output from the reference signal generation circuit  107  is adjusted so that the potential appearing on the bit line  100   c  is at the intermediate level between the “H” level and the “L” level. 
     With such a configuration, the reference signal generation circuit  107  does not use a ferroelectric capacitor, thereby avoiding the problem associated with the first conventional example of FIG. 7A, i.e., the problem associated with the deterioration of reference cells. 
     In still another conventional example, FIG. 9A illustrates a ferroelectric memory device  950  having a reference level generation circuit  900 . The reference level generation circuit  900  includes a reference signal generation circuit  109 , a pulse generation circuit  110  and a capacitor  108 . Pulses are provided to a bit line  100   d , and the potential of the bit line  100   d  is boosted by the capacitance ratio of the capacitor  108 . The reference signal generation circuit  109  is connected to the pulse generation circuit  110 , and the pulse generation circuit  110  is connected to the first electrode of the capacitor  108 . The second electrode of the capacitor  108  is connected to the bit line  100   d . The memory cell  12  of FIG. 9A has the structure as described above in connection with the ferroelectric memory device  850  with reference to FIG.  8 A. In the configuration illustrated in FIG. 9A, the “H” level of the pulse used to boost the bit line  100   d  is determined based on the output from the reference signal generation circuit  109 . Thus, the output from the reference signal generation circuit  109  is adjusted so that the boosted potential appearing on the bit line  100   d  is at the intermediate level between the “H” level and the “L” level. 
     This operation will be described with reference to a timing diagram shown in FIG.  9 B. In FIG. 9B, “REF” denotes the output from the reference signal generation circuit  109 , based on which the potential of the “H” level of the pulse is determined. The timing of the “H” level of the pulse is determined by the pulse generation circuit  110 . Moreover, the transition of the SAE signal for the sense amplifier  15  is the same as described above with reference to FIGS. 7B and 8B. 
     In still another conventional example, FIG. 10A illustrates a ferroelectric memory device  1050  having a reference level generation circuit  1000 . In the reference level generation circuit  1000 , a reference signal generation circuit  112  is connected a bit line  100   e  via a switch transistor  111 . A PRRF (pre-charge reference) signal is input to the gate of the switch transistor  111 . In the reference signal generation circuit  112 , a reference level is generated by means of resistance division, or the like, without using a ferroelectric element, and the generated reference level (potential) is directly supplied to the bit line  100   e . The memory cell  12  of FIG. 10A has the structure as described above in connection with the ferroelectric memory device  850 . 
     This operation will be described with reference to a timing diagram shown in FIG.  10 B. Referring to FIG. 10B, the PRRF signal is activated (indicated as the transition to the “H” level) so as to close the switch transistor  111 . Thus, a reference level generated by the reference signal generation circuit  112  is supplied onto the bit line  10   e . After the reference level is generated, the SAE signal is activated (indicated as the transition to the “H” level) so as to enable the sense amplifier  15 . Thus, the output from the selected memory cell  12  is compared with the reference level signal to amplify the signal corresponding to the output of the memory cell  12 . 
     The problem associated with the first conventional reference signal generation circuit  700  shown in FIG. 7A, i.e., the problem associated with the deterioration of reference cells, can be avoided by employing these conventional reference level generation circuits  800 ,  900  and  1000  of the ferroelectric memory devices  850 ,  950  and  1050  illustrated in FIGS. 8A,  9 A and  10 A, respectively. 
     However, in these ferroelectric memory devices, the amount of data (or charge) which is output from a ferroelectric memory cell is influenced by various environmental factors such as the level of a voltage externally provided to the device (hereinafter, also referred to as the “externally provided voltage” or simply as the “external voltage”) or the ambient temperature around the device (hereinafter, referred to simply as the “ambient temperature”). 
     When the level of the externally provided voltage or the ambient temperature changes, the output of the reference signal generation circuit changes in response to the changes in the circuit characteristics due to the voltage or temperature changes. These changes in the output of the reference signal generation circuit are different from changes in the characteristics of a ferroelectric element due to voltage or temperature changes. Thus, since the conventional reference level generation circuits  800 ,  900  and  1000  of the ferroelectric memory devices  850 ,  950  and  1050  illustrated in FIGS. 8A,  9 A and  10 A, respectively, do not use a ferroelectric element, which is used in the memory cell  12  of a ferroelectric memory device, the generated reference level does not follow changes in the characteristics of the ferroelectric element due to changes in the environmental factors such as the voltage or the temperature. 
     SUMMARY OF THE INVENTION 
     According to one aspect of this invention, there is provided a ferroelectric memory device including at least one memory cell which includes a semiconductor transistor and a ferroelectric capacitor, the ferroelectric memory device reading out data from each of the at least one memory cell and comparing the read data with a reference level signal to amplify a signal corresponding to the data read out from the at least one memory cell. The ferroelectric memory device further includes: at least one of an external voltage detection circuit for detecting a level of a voltage externally provided to the ferroelectric memory device and a temperature detection circuit for detecting an ambient temperature around the ferroelectric memory device; a reference signal generation circuit connected to the at least one of the external voltage detection circuit and the temperature detection circuit for outputting a potential based on an output from the at least one of the external voltage detection circuit and the temperature detection circuit: and a reference level adjustment section for adjusting the potential output from the reference signal generation circuit into a suitable reference level signal. 
     In one embodiment of the invention, the ferroelectric memory device includes a plurality of the reference signal generation circuits. The at least one of the external voltage detection circuit and the temperature detection circuit selects an optimal one of the plurality of the reference signal generation circuits. 
     In one embodiment of the invention, the reference signal generation circuit does not include a ferroelectric element. 
     In one embodiment of the invention, the reference signal generation circuit includes a ferroelectric element. The ferroelectric element does not undergo polarization during the adjustment of the potential output from the reference signal generation circuit into a suitable reference level signal. 
     In one embodiment of the invention, the reference level adjustment section includes a capacitor for temporarily storing a potential. 
     In one embodiment of the invention, the reference level adjustment section further includes at least two switches. The reference signal generation circuit is connected to the capacitor via one of the two switches. The capacitor is connected to the memory cell via the other one of the two switches. 
     In one embodiment of the invention, the ferroelectric memory device further includes a pulse generation circuit for determining a timing of the signal generated by the reference signal generation circuit. 
     In one embodiment of the invention, the reference level adjustment section includes a capacitor between the pulse generation circuit and the memory cell. 
     In one embodiment of the invention, the reference level adjustment section includes a switch. 
     In one embodiment of the invention, the pulse generation circuit generates a pulse having an “H” level and a “L” level, the “H” level corresponding to a potential generated by the reference level generation circuit and the “L” level corresponding to VDD or another potential which is lower than the “H” level. 
     According to another aspect of this invention, there is provided a method for generating a reference level signal for use in a ferroelectric memory device, including at least one memory cell which includes a semiconductor transistor and a ferroelectric capacitor, the ferroelectric memory device reading out data from each of the at least one memory cell and comparing the read data with a reference level signal to amplify a signal corresponding to the data read out from the at least one memory cell. The method includes the steps of: detecting at least one of a level of a voltage externally provided to the ferroelectric memory device and an ambient temperature around the ferroelectric memory device; providing a potential based on at least one of the level of the externally provided voltage and the temperature; and adjusting the potential into a reference level signal having a potential level between an “H” level and an “L” level of the memory cell. 
     The functions of the present invention will now be described. 
     According to the present invention, a reference level is generated by a potential generation circuit, or the like, which is provided by way of a resistance division, without using a ferroelectric capacitor or any other ferroelectric element. Where a ferroelectric capacitor or any other ferroelectric element is used, the ferroelectric capacitor can be used as a path condenser for the reference signal generation circuit so as to generate a reference level without polarization of the ferroelectric element. Thus, it is possible to avoid the problem associated with the deterioration of reference cells as in the prior art. By the use of a ferroelectric element, it is possible to obtain a large capacitance for a small area. 
     It is possible to employ a single reference signal generation circuit capable of generating a plurality of different potentials. In such a case, when the level of the externally provided voltage or the ambient temperature changes, the potential output from the reference level generation circuit can be changed according to the outputs from a voltage detection circuit, a temperature detection circuit, and the like, so as to generate a reference level which is close to the intermediate level between the “H” level and the “L” level. Alternatively, it is possible to selectively activate one of a plurality of reference signal generation circuits which generate respectively different potentials so as to generate a reference level which is close to the intermediate level between the “H” level and the “L” level. Thus, the reference level can be controlled to follow changes in the characteristics of a ferroelectric element. 
     Thus, the invention described herein makes possible the advantages of (1) providing a ferroelectric memory device which is free of problems associated with deterioration in reference cells, and in which the reference level can be controlled to follow changes in the characteristics of a ferroelectric element due to changes in the environmental factors such as the level of the externally provided voltage or the ambient temperature; and (2) providing a method for generating a reference level signal for such a ferroelectric memory device. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates a circuit configuration for generating a reference level used in a ferroelectric memory device according to Embodiment 1 of the present invention; 
     FIG. 1B is a timing diagram illustrating the transitions of the various signals according to Embodiment 1 of the present invention; 
     FIG. 2A illustrates a circuit configuration for generating a reference level used in a ferroelectric memory device according to Embodiment 2 of the present invention; 
     FIG. 2B is a timing diagram illustrating the transitions of the various signals according to Embodiment 2 of the present invention; 
     FIG. 3A illustrates a circuit configuration for generating a reference level used in a ferroelectric memory device according to Embodiment 3 of the present invention; 
     FIG. 3B is a timing diagram illustrating the transitions of the various signals according to Embodiment 3 of the present invention: 
     FIG. 4A illustrates a circuit configuration for generating a reference level used in a ferroelectric memory device according to Embodiment 4 of the present invention; 
     FIG. 4B is a timing diagram illustrating the transitions of the various signals according to Embodiment 4 of the present invention: 
     FIG. 5A illustrates a circuit configuration for generating a reference level used in a ferroelectric memory device according to Embodiment 5 of the present invention; 
     FIG. 5B is a timing diagram illustrating the transitions of the various signals according to Embodiment 5 of the present invention; 
     FIG. 6A illustrates a circuit configuration for generating a reference level used in a ferroelectric memory device according to Embodiment 6 of the present invention; 
     FIG. 6B is a timing diagram illustrating the transitions of the various signals according to Embodiment 6 of the present invention; 
     FIG. 6C illustrates a resistance division circuit configuration for generating an intended potential by way of a resistance division; 
     FIG. 6D illustrates another resistance division circuit configuration which uses a capacitor; 
     FIG. 7A illustrates a circuit configuration for generating a reference level used in a conventional ferroelectric memory device; 
     FIG. 7B is a timing diagram illustrating the transitions of the various signals used in the conventional ferroelectric memory device of FIG. 7A; 
     FIG. 8A Illustrates a circuit configuration for generating a reference level used in another conventional ferroelectric memory device; 
     FIG. 8B is a timing diagram illustrating the transitions of the various signals used in the conventional ferroelectric memory device of FIG. 8A; 
     FIG. 9A illustrates a circuit configuration for generating a reference level used in another conventional ferroelectric memory device; 
     FIG. 9B is a timing diagram illustrating the transitions of the various signals used in the conventional ferroelectric memory device of FIG. 9A; 
     FIG. 10A illustrates a circuit configuration for generating a reference level used in another conventional ferroelectric memory device; and 
     FIG. 10B is a timing diagram illustrating the transitions of the various signals used in the conventional ferroelectric memory device of FIG.  10 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Various embodiments of the present invention will now be described with reference to the drawings. 
     Embodiment 1 
     FIG. 1A illustrates a ferroelectric memory device  150  according to Embodiment 1 of the present invention, including a reference level generation circuit  100  for generating a reference level. 
     The reference level generation circuit  100  of the ferroelectric memory device  150  includes a reference signal generation circuit  1  for generating a plurality of different potentials (levels), an external voltage detection circuit  2  for detecting a potential of externally provided voltage VDD, a temperature detection circuit  3  for detecting the ambient temperature, and a reference level adjustment section  9  for adjusting the potential generated by the reference signal generation circuit  1 . The reference signal generation circuit  1  may be a constant voltage generation circuit, or the like, which is provided by way of a resistance division or based on a band gap, without using a ferroelectric element. In this embodiment, the reference level adjustment section  9  of the reference level generation circuit  100  includes switch transistors  4  and  5  and a capacitor  6 . In the reference level generation circuit  100 , the external voltage detection circuit  2  detects the potential of externally provided voltage VDD. The external voltage detection circuit  2  and the temperature detection circuit  3  are each connected to the reference signal generation circuit  1 . The potential output from the reference signal generation circuit  1  is adjusted by the reference level adjustment section  9  so that the potential appearing on a bit line  7  is at the intermediate level between the “H” level and the “L” level. The output of the reference signal generation circuit  1  is connected to the source of the switch transistor  5  included in the reference level adjustment section  9 . The drain of the switch transistor  5  is connected to the first electrode of the capacitor  6  and to the source of the switch transistor  4 . The second electrode of the capacitor  6  is connected to the ground. The gates of the switch transistors  4  and  5  are connected to the RWL line and the PRC line, respectively. The drain of the switch transistor  4  is connected to the bit line  7 . The external voltage detection circuit  2  may be a circuit which compares an external voltage with the voltage generated by a constant voltage generation circuit. The temperature detection circuit  3  may be implemented by way of a resistance division using a plurality of resistors having different temperature coefficients. 
     The sense amplifier  15  connected to the bit line  7  is also connected to another bit line  11 , which is connected to the memory cell  12 . The memory cell  12  includes the semiconductor transistor  16  and the ferroelectric capacitor  17 . The source of the semiconductor transistor  16  is connected to the bit line  11 , the drain of the semiconductor transistor  16  is connected to the first electrode of the ferroelectric capacitor  17 , and the gate of the semiconductor transistor  16  is connected to the word line  13 . The second electrode of the ferroelectric capacitor  17  is connected to the plate line  14 . With such a configuration, the output from the selected memory cell  12  is compared with the reference level signal to amplify the signal corresponding to the output of the memory cell  12 . 
     In the conventional reference level generation circuit  800  illustrated in FIG. 8A, only a single constant level potential is output from the reference signal generation circuit  109 , whereby it is only possible to generate a constant level potential as the reference level. In contrast, in the reference level generation circuit  100  of the ferroelectric memory device  150  of this embodiment, a plurality of different potentials (levels) can be generated from the reference signal generation circuit  1  according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 . A level generated by the reference signal generation circuit  1  is charged into the capacitor  6 , and the capacitor  6  is shorted with the bit line  7 . Thus, a potential (reference level) is generated onto the bit line  7  by virtue of the charge sharing between the capacitor  6  and the bit line  7 . The output from the reference signal generation circuit  1  is adjusted so that the potential appearing on the bit line  7  is at the intermediate level between the “H” level and the “L” level. 
     FIG. 1B shows a timing chart illustrating transitions of various signals during this operation. The transitions of these signals are the same as those described above with reference to FIG.  8 B. Referring to the timing chart of FIG. 1B, the PRC signal is activated (indicated as the transition to the “H” level) to close the switch transistor  5  so that the capacitor  6  is charged by the reference signal generation circuit  1 . At this time, the reference signal generation circuit  1  outputs a potential (level) according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 . Then, the PRC signal is deactivated (indicated as the transition to the “L” level), after which the RWL signal is activated (indicated as the transition to the “H” level) so as to close the switch transistor  4 . Thus, a reference level is generated onto the bit line  7  by virtue of the charge sharing between the capacitor  6  and the capacitance of the bit line  7 . After the reference level is generated, the SAE signal is activated (indicated as the transition to the “H” level) so as to enable the sense amplifier  15 . Thus, the output signal from the selected memory cell  12  is compared with the reference level signal to amplify the signal corresponding to the output of the memory cell  12 . 
     With the configuration of this embodiment, even when the level of the externally provided voltage or the ambient temperature changes, it is possible to selectively output one of a plurality of different levels which can be output by the reference signal generation circuit  1  based on the control provided by the external voltage detection circuit  2  and the temperature detection circuit  3 . Moreover, the ferroelectric memory device of this embodiment is capable of generating a reference level without using any ferroelectric element and thus is free of the problem associated with the deterioration of reference cells. 
     Embodiment 2 
     FIG. 2A illustrates a ferroelectric memory device  250  according to Embodiment 2 of the present invention, including a reference level generation circuit  200  for generating a reference level. 
     The reference level generation circuit  200  of the ferroelectric memory device  250  includes reference signal generation circuits  1   a  to  1   c , the external voltage detection circuit  2 , the temperature detection circuit  3  and the reference level adjustment section  9 . As the reference level generation circuit  100  of Embodiment 1, the reference level adjustment section  9  includes the switch transistors  4  and  5  and the capacitor  6 . The reference level generation circuit  200  includes the multiple reference signal generation circuits  1   a  to  1   c  for generating respectively different potentials (levels), instead of the single reference signal generation circuit  1  of Embodiment 1 shown in FIG. 1A which is capable of generating a plurality of different potentials (levels). As the reference signal generation circuit  1  of Embodiment 1, each of the reference signal generation circuits  1   a  to  1   c  generates a potential (level) by way of a resistance division, or the like, without using a ferroelectric element. Other than the reference level generation circuit  200 , the ferroelectric memory device  250  has the same configuration as that of the ferroelectric memory device  150 . 
     In the reference level generation circuit  200 , the external voltage detection circuit  2  detects the potential of externally provided voltage VDD. Each of the external voltage detection circuit  2  and the temperature detection circuit  3  is connected to the reference signal generation circuits  1   a  to  1   c . Each of the reference signal generation circuits  1   a  to  1   c  is connected to the source of the switch transistor  5 . The drain of the switch transistor  5  is connected to the first electrode of the capacitor  6  and to the source of the switch transistor  4 . The second electrode of the capacitor  6  is connected to the ground. The gates of the switch transistors  4  and  5  are connected to the RWL line and the PRC line, respectively. The drain of the switch transistor  4  is connected to the bit line  7 . As described above, in the reference level generation circuit  200  of the ferroelectric memory device  250  of this embodiment, each of the external voltage detection circuit  2  and the temperature detection circuit  3  is connected to the reference signal generation circuits  1   a  to  1   c . Therefore, it is possible to generate a plurality of different potentials (levels) by selectively activating the reference signal generation circuits  1   a  to  1   c  according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 . A level generated by one of the reference signal generation circuits  1   a  to  1   c  is charged into the capacitor  6 , and the capacitor  6  Is shorted with the bit line  7 . Thus, a potential (reference level) is generated onto the bit line  7  by virtue of the charge sharing between the capacitor  6  and the bit line  7 . One of the reference signal generation circuits  1   a  to  1   c  is selectively activated so that the potential appearing on the bit line  7  is at the intermediate level between the “H” level and the “L” level. 
     FIG. 2B shows a timing chart illustrating transitions of various signals during this operation. The transitions of these signals are the same as those described above with reference to FIG.  8 B. Referring to the timing chart of FIG. 2B, the PRC signal is activated (indicated as the transition to the “H” level) to close the switch transistor  5  so that the capacitor  6  is charged by one of the reference signal generation circuits  1   a  to  1   c . At this time, one of the reference signal generation circuits  1   a  to  1   c  is selectively activated according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 , so that a potential (level) is output from the selected one of the reference signal generation circuits  1   a  to  1   c . Then, the PRC signal is deactivated (indicated as the transition to the “L” level), after which the RWL signal is activated (indicated as the transition to the “H” level) so as to close the switch transistor  4 . Thus, a reference level is generated onto the bit line  7  by virtue of the charge sharing between the capacitor  6  and the capacitance of the bit line  7 . After the reference level is generated, the SAE signal is activated (indicated as the transition to the “H” level) so as to enable the sense amplifier  15 . Thus, the output from the selected memory cell  12  is compared with the reference level signal to amplify the signal corresponding to the output of the memory cell  12 . 
     With the configuration of this embodiment, even when the level of the externally provided voltage or the ambient temperature changes, the plurality of different levels from the reference signal generation circuits  1   a  to  1   a  can be optimally selected and output based on the control provided by the external voltage detection circuit  2  and the temperature detection circuit  3 . Moreover, the ferroelectric memory device of this embodiment is capable of generating a reference level without using any ferroelectric element and thus is free of the problem associated with the deterioration of reference cells. 
     Embodiment 3 
     FIG. 3A illustrates a ferroelectric memory device  350  according to Embodiment 3 of the present invention, including a reference level generation circuit  300  for generating a reference level. 
     The reference level generation circuit  300  includes the reference signal generation circuit  1  (as that of the reference level generation circuit  100  of Embodiment 1) for generating a plurality of different potentials (levels) by way of a resistance division, or the like, without using a ferroelectric element, the external voltage detection circuit  2  for detecting a potential of externally provided voltage VDD, the temperature detection circuit  3  for detecting the ambient temperature, a pulse generation circuit  10  for determining the timing of signals output from the reference signal generation circuit  1 , and the reference level adjustment section  9  for adjusting the potential output from the pulse generation circuit  10 . In the reference level generation circuit  300 , the reference level adjustment section  9  includes a capacitor  8 . Each of the external voltage detection circuit  2  and the temperature detection circuit  3  is connected to the reference signal generation circuit  1 . The reference signal generation circuit  1  is connected to the pulse generation circuit  10 , which is connected to the first electrode of the capacitor  8 . The second electrode of the capacitor  8  is connected to the bit line  7 . Other than the reference level generation circuit  300 , the ferroelectric memory device  350  has the same configuration as that of the ferroelectric memory device  150 . 
     In the reference level generation circuit  300 , a pulse is provided to the bit line  7  from the pulse generation circuit  10 , and the potential (level) of the bit line  7  is boosted by coupling of the capacitor  8  to the bit line  7 . At this time, the “H” level of the pulse is adjusted by adjusting the output of the reference signal generation circuit  1  so that the boosted level appearing on the bit line  7  is at the intermediate level between the “H” level and the “L” level of the memory cell  12 . 
     In the conventional reference level generation circuit  900  illustrated in FIG. 9A, the “H” level of the pulse is set to one level. In contrast, in the reference level generation circuit  300  of the ferroelectric memory device  350  of this embodiment, it is possible to generate a plurality of different potentials (levels) from the reference signal generation circuit  1  according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 . Thus, it is possible to set the “H” level of the pulse to an optimal level. 
     FIG. 3B shows a timing chart illustrating transitions of various signals during this operation. The transitions of these signals are the same as those described above with reference to FIG.  9 B. In FIG. 3B, “REF” denotes the output from the reference signal generation circuit  1 , based on which the potential of the “H” level of the pulse is determined. At this time, the reference signal generation circuit  1  outputs a potential (level) according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 . The timing of the “H” level of the pulse is determined by the pulse generation circuit  10 . Moreover, the transition of the SAE signal for the sense amplifier  15  is the same as described above with reference to FIG.  9 B. 
     With the reference level generation circuit  300  of the ferroelectric memory device  350  of this embodiment, even when the level of the externally provided voltage or the ambient temperature changes, the plurality of different levels from the reference signal generation circuit  1  can be optimally selected and output based on the control provided by the external voltage detection circuit  2  and the temperature detection circuit  3 . Thus, it is possible to set the “H” level of the pulse to an optimal level. Moreover, the ferroelectric memory device of this embodiment is capable of generating a reference level without using any ferroelectric element and thus is free of the problem associated with the deterioration of reference cells. 
     Embodiment 4 
     FIG. 4A illustrates a ferroelectric memory device  450  according to Embodiment 4 of the present invention, including a reference level generation circuit  400  for generating a reference level. 
     The reference level generation circuit  400  of the ferroelectric memory device  450  includes the reference signal generation circuits  1   a  to  1   c , the external voltage detection circuit  2 , the temperature detection circuit  3 , the pulse generation circuit  10 , and the reference level adjustment section  9 . As the reference level generation circuit  300  of Embodiment 3, the reference level adjustment section  9  includes the capacitor  8 . The reference level generation circuit  400  includes the multiple reference signal generation circuits  1   a  to  1   c  for generating respectively different potentials (levels), instead of the single reference signal generation circuit  1  of Embodiment 3 shown in FIG. 3A which is capable of generating a plurality of different potentials (levels). Each of the external voltage detection circuit  2  and the temperature detection circuit  3  is connected to the reference signal generation circuits  1   a  to  1   c . Each of the reference signal generation circuits  1   a  to  1   c  is connected to the pulse generation circuit  10 , which is connected to the first electrode of the capacitor  8 . The second electrode of the capacitor  8  is connected to the bit line  7 . Other than the reference level generation circuit  400 , the ferroelectric memory device  450  has the same configuration as that of the ferroelectric memory device  150 . 
     In the reference level generation circuit  400 , a pulse is provided to the bit line  7  from the pulse generation circuit  10 , and the potential (level) of the bit line  7  Is boosted by coupling of the capacitor  8  to the bit line  7 . At this time, the “H” level of the pulse is adjusted by selectively activating one of the reference signal generation circuits  1   a  to  1   c  so that the boosted level appearing on the bit line  7  is at the intermediate level between the “H” level and the “L” level. 
     With the reference level generation circuit  400  of the ferroelectric memory device  450  of this embodiment, it is possible to generate a plurality of different potentials (levels) by selectively activating the reference signal generation circuits  1   a  to  1   c  according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 . Thus, it is possible to set the “H” level of the pulse to an optimal level. 
     FIG. 4B shows a timing chart illustrating transitions of various signals during this operation. The transitions of these signals are the same as those described above with reference to FIG.  9 B. In FIG. 4B, “REF” denotes the output from the selectively activated one of the reference signal generation circuits  1   a  to  1   c , based on which the potential of the “H” level of the pulse Is determined. At this timer the selectively activated one of the reference signal generation circuits  1   a  to  1   c  outputs a potential (level) according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 . The timing of the “H” level of the pulse is determined by the pulse generation circuit  10 . Moreover, the transition of the SAE signal for the sense amplifier  15  is the same as described above with reference to FIG.  9 B. 
     With the reference level generation circuit  400  of the ferroelectric memory device  450  of this embodiment, even when the level of the externally provided voltage or the ambient temperature changes, the plurality of different levels from the reference signal generation circuits  1   a  to  1   c  can be optimally selected and output based on the control provided by the external voltage detection circuit  2  and the temperature detection circuit  3 . Thus, it is possible to set the “H” level of the pulse to an optimal level. Moreover, the ferroelectric memory device of this embodiment is capable of generating a reference level without using any ferroelectric element and thus is free of the problem associated with the deterioration of reference cells. 
     Embodiment 5 
     FIG. 5A illustrates a ferroelectric memory device  550  according to Embodiment 5 of the present invention, including a reference level generation circuit  500  for generating a reference level. 
     The reference level generation circuit  500  includes the reference signal generation circuit  1  (as that of the reference level generation circuit  100  of Embodiment 1) for generating a plurality of different potentials (levels) by way of a resistance division, or the like, without using a ferroelectric element, the external voltage detection circuit  2  for detecting a potential of externally provided voltage VDD, the temperature detection circuit  3  for detecting the ambient temperature and the reference level adjustment section  9 . In the reference level generation circuit  500 , the reference level adjustment section  9  includes a switch transistor  21 . Each of the external voltage detection circuit  2  and the temperature detection circuit  3  is connected to the reference signal generation circuit  1 . The reference signal generation circuit  1  and the bit line  7  are connected to each other via the switch transistor  21 . The PRRF signal is input to the gate of the switch transistor  21 . Other than the reference level generation circuit  500 , the ferroelectric memory device  550  has the same configuration as that of the ferroelectric memory device  150 . 
     In the reference level generation circuit  500 , a reference level is directly provided from the reference signal generation circuit  1  to the bit line  7  via the switch transistor  21 . The provided level is the intermediate level between the “H” level and the “L” level of the memory cell  12 . 
     In the conventional reference level generation circuit  1000  illustrated in FIG. 10A, the reference level is fixed to one level. In contrast, in the reference level generation circuit  500  of the ferroelectric memory device  550  of this embodiment, it is possible to generate a plurality of different potentials (levels) from the reference signal generation circuit  1  according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 . 
     FIG. 5B shows a timing chart illustrating transitions of various signals during this operation. The transitions of these signals are the same as those described above with reference to FIG.  10 B. Referring to the timing chart of FIG. 5B, the PRRF signal is activated (indicated as the transition to the “H” level) to close the switch transistor  21  so that the reference level generated by the reference signal generation circuit  1  is provided to the bit line  7 . After the reference level is generated, the SAE signal is activated (indicated as the transition to the “H” level) so as to enable the sense amplifier  15 . Thus, the output from the selected memory cell  12  is compared with the reference level signal to amplify the signal corresponding to the output of the selected memory cell  12 . 
     With the reference level generation circuit  500  of the ferroelectric memory device  550  of this embodiment, even when the level of the externally provided voltage or the ambient temperature changes, the plurality of different levels from the reference signal generation circuit  1  can be optimally selected and output based on the control provided by the external voltage detection circuit  2  and the temperature detection circuit  3 . Moreover, the ferroelectric memory device of this embodiment is capable of generating a reference level without using any ferroelectric element and thus is free of the problem associated with the deterioration of reference cells. 
     Embodiment 6 
     FIG. 6A illustrates a ferroelectric memory device  650  according to Embodiment 6 of the present invention, including a reference level generation circuit  600  for generating a reference level. 
     The reference level generation circuit  600  of the ferroelectric memory device  650  includes the reference signal generation circuits  1   a  to  1   c , the external voltage detection circuit  2 , the temperature detection circuit  3  and the reference level adjustment section  9 . As the reference level generation circuit  500  of Embodiment 5, the reference level adjustment section  9  includes the switch transistor  21 . The reference level generation circuit  600  includes the multiple reference signal generation circuits  1   a  to  1   c  for generating respectively different potentials (levels), instead of the single reference signal generation circuit  1  of Embodiment 5 shown in FIG. 5A which is capable of generating a plurality of different potentials (levels). Each of the external voltage detection circuit  2  and the temperature detection circuit  3  is connected to the reference signal generation circuits  1   a  to  1   c . Each of the reference signal generation circuits  1   a  to  1   c  is connected to the bit line  7  via the switch transistor  21 . The PRRF signal is input to the gate of the switch transistor  21 . Other than the reference level generation circuit  600 , the ferroelectric memory device  650  has the same configuration as that of the ferroelectric memory device  150 . 
     In the reference level generation circuit  600 , a reference level is directly provided from selectively activated one of the reference signal generation circuits  1   a  to  1   c  to the bit line  7  via the switch transistor  21 . The provided level is the intermediate level between the “H” level and the “L” level of the memory cell  12 . 
     With the reference level generation circuit  600  of the ferroelectric memory device  650  of this embodiment, it is possible to generate a plurality of different potentials (levels) by selectively activating the reference signal generation circuits  1   a  to  1   c  according to the voltage and temperature changes detected respectively by the external voltage detection circuit  2  and the temperature detection circuit  3 . 
     FIG. 6B shows a timing chart illustrating transitions of various signals during this operation. The transitions of these signals are the same as those described above with reference to FIG.  10 B. Referring to the timing chart of FIG. 6B, PRRF signal is activated (indicated as the transition to the “H” level) to close the switch transistor  21  so that the reference level generated by the selectively activated one of the reference signal generation circuits  1   a  to  1   c  is provided to the bit line  7 . After the reference level is generated, the SAE signal is activated (indicated as the transition to the “H” level) so as to enable the sense amplifier  15 . Thus, the output from the selected memory cell  12  is compared with the reference level signal to amplify the signal corresponding to the output of the selected memory cell  12 . 
     With the reference level generation circuit  600  of the ferroelectric memory device  650  of this embodiment, even when the level of the externally provided voltage or the ambient temperature changes, the plurality of different levels from the reference signal generation circuits  1   a  to  1   c  can be optimally selected and output based on the control provided by the external voltage detection circuit  2  and the temperature detection circuit  3 . Moreover, the ferroelectric memory device of this embodiment is capable of generating a reference level without using any ferroelectric element and thus is free of the problem associated with the deterioration of reference cells. 
     FIG. 6C illustrates a resistance division circuit configuration used in the reference signal generation circuit  1  for generating an intended potential by way of a resistance division. Resistors  18  and  19  are serially connected to each other. The first electrode of the resistor  18  is grounded, and voltage Vcc is applied to the first electrode of the resistor  19 . In this configuration, the voltage between the resistors  18  and  19  can be used as a reference level so as to generate an intended potential. 
     Where a reference signal generation circuit uses a ferroelectric capacitor or any other ferroelectric element, the ferroelectric capacitor can be used as a path condenser for the reference level generation circuit so as to generate a reference level without polarization of the ferroelectric element. FIG. 6D illustrates another resistance division circuit configuration used in the reference signal generation circuit  1  which uses a capacitor. The configuration of FIG. 6D is obtained by additionally providing a capacitor  20  along the signal path in the configuration illustrated in FIG. 6C. A ferroelectric capacitor may be used for the capacitor, in which case it is possible to obtain a larger capacitance for a smaller area. In such a case, the ferroelectric capacitor will not undergo polarity inversion, whereby it is possible to avoid the problems associated with deterioration in reference cells. 
     When the reference level adjustment section  9  is intended to adjust potentials, signal VDD and the signal generated by the reference signal generation circuit  1  can be input to the reference level adjustment section  9  as the “H” signal and the “L” signal, respectively. Alternatively, the signal generated by the reference signal generation circuit  1  may be used as the “H” signal and the GND as the “L” signal. 
     While each of the above-described embodiments employs both the external voltage detection circuit  2  and the temperature detection circuit  3 , it should be apparent to those skilled in the art that the use of either one of these circuits would still be within the scope of the present invention. Moreover, while the embodiments have been described above with respect to a single memory cell, it should be understood that a memory device generally includes a plurality of memory cells. 
     As described above in detail, according to the present invention, it is possible to suitably generate a reference level for use in a memory device without using a ferroelectric capacitor or any other ferroelectric element. Where a ferroelectric capacitor or any other ferroelectric element is used, it is possible to generate a reference level without polarization of the ferroelectric element. Thus, it is possible to provide a ferroelectric memory device which is free of the problem in the prior art associated with the deterioration of reference cells due to repeated read/write operations. When the external voltage or the ambient temperature changes, the reference level can be controlled to follow changes in the characteristics of a ferroelectric element due to the changes in the level of the externally provided voltage or the ambient temperature. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.