Patent Publication Number: US-8125820-B2

Title: Semiconductor memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of PCT International Application PCT/JP2010/000643 filed on Feb. 3, 2010, which claims priority to Japanese Patent Application No. 2009-030146 filed on Feb. 12, 2009. The disclosures of these applications including the specifications, the drawings, and the claims are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to semiconductor memory devices, and more particularly, to techniques of controlling the potential of a bit line in a memory circuit. 
     Conventionally, there is a known technique of stepping down the potential of a bit line by driving an N-channel MOS (NMOS) transistor connected to the bit line using a pulse in order to improve the static noise margin (SNM) of a memory cell in a static random access memory (SRAM). Note that data is read out using a sense amplifier which detects a minute potential difference between a pair of bit lines (see M. Khellah et al., “Wordline &amp; Bitline Pulsing Schemes for Improving SRAM Cell Stability in Low-Vcc 65 nm CMOS Designs,” 2006 Symposium on VLSI Circuits, Digest of Technical Papers, pp. 12-13). 
     There is also a known technique of controlling the potential level of a signal line in a decoder circuit to drive a word line in a semiconductor memory device (see Japanese Patent Publication No. 2007-164922). 
     SUMMARY 
     In the aforementioned conventional bit line potential step-down technique, the potential of a bit line is highly likely to be excessively stepped down. When the bit line potential is excessively stepped down during read operation, a memory cell is erroneously written, so that data is corrupted. Moreover, because the NMOS transistor is connected to a bit line in order to step down the bit line, the possibility that the bit line potential is excessively stepped down is increased as the drive capability of the NMOS transistor is increased due to variations. Moreover, because a pulse signal for controlling the NMOS transistor is externally input through an IO block, the pulse signal is deformed, so that the pulse width changes or the like, which may be responsible for the bit line potential being excessively stepped down. 
     Moreover, in the aforementioned conventional bit line potential step-down technique, the stepped-down bit line potential reduces the drive capability of an access transistor in the SRAM memory cell, so that it takes a long time for the potential difference between a pair of bit lines to reach a predetermined value. Therefore, the SNM is improved, but the speed is reduced, which is a problem. 
     Moreover, in the aforementioned conventional word line drive technique, the amplitude of the potential of the decoder circuit is reduced, thereby increasing the speed and reducing the power. However, in the NMOS transistor, it takes a long time (precharge time) to increase the potential of a signal line to Vdd−Vtn where Vdd is the voltage of a power supply and Vtn is the threshold voltage of the NMOS transistor. 
     The detailed description describes implementations of a bit line potential control for semiconductor memory devices in which a potential control technique resistant to variations is used to reduce or prevent erroneous operation, such as erroneous write operation and the like, and improve the SNM, thereby achieving stable operation. 
     The detailed description also describes implementations of higher speed operation using a technique of allowing read operation using a low amplitude by stepping down the bit line potential. 
     A first example semiconductor memory device according to the present disclosure includes a memory array block including a plurality of memory cells arranged in a matrix, a plurality of bit lines including first bit lines provided with respect to columns of the memory cells, a plurality of first transistors configured to control potentials of the first bit lines, and a plurality of first logic gates configured to control the first transistors. A drain or a source of each of the first transistors is connected to an input of the corresponding first logic gate, and a gate of each of the first transistors is connected to an output of the corresponding first logic gate. 
     A second example semiconductor memory device according to the present disclosure includes a memory array block including a plurality of memory cells arranged in a matrix, a plurality of bit lines including first bit lines provided with respect to columns of the memory cells, a plurality of first transistors configured to control potentials of the first bit lines, a plurality of first capacitors each including two electrodes, one of the two electrodes being connected to the corresponding bit line, and a plurality of first logic gates configured to control the first capacitors. A gate of each of the first transistors is connected to an input of the corresponding first logic gate, and the other electrode of each of the first capacitors is connected to an output of the corresponding first logic gate. 
     As described above, the bit line potential may be stepped down via the first transistor. In this case, a signal is supplied via the same connect node to the drain or source of the first transistor and the input of the first logic gate, whereby the bit line potential is not excessively stepped down. Moreover, the bit line potential may be stepped down via the first capacitor. In this case, a signal is supplied via the same connect node to the drain or source of the first transistor and the input of the first logic gate, whereby the bit line potential is not excessively stepped down. 
     As a result, the bit line potential is not excessively stepped down, and therefore, erroneous operation, such as erroneous write operation and the like, can be reduced or prevented. At the same time, an improvement in the SNM which is ultimately intended by stepping down the bit line potential can be achieved. 
     A third example semiconductor memory device according to the present disclosure includes a memory array block including a plurality of memory cells arranged in a matrix, a plurality of bit lines including first bit lines provided with respect to columns of the memory cells, and a plurality of word lines including first word lines provided with respect to rows of the memory cells, an IO block connected to the first bit lines, a decoder block connected to the first word lines, and a control block provided adjacent to both the IO block and the decoder block. The decoder block includes a plurality of word drivers each including a first N-channel MOS transistor having a source connected to a first common node. The control block includes a second N-channel MOS transistor having a drain connected to the first common node, a first transistor configured to control a potential of the first common node, and a first logic gate configured to control the first transistor. A drain or a source of the first transistor is connected to an input of the first logic gate, and a gate of the first transistor is connected to an output of the first logic gate. 
     According to the present disclosure, the semiconductor memory device can perform stable operation by improving the SNM while reducing or preventing erroneous operation, such as erroneous write operation or the like. Moreover, read operation with a small amplitude is achieved by stepping down the bit line potential, resulting in higher-speed operation. 
     Higher-speed and higher-frequency operation can also be achieved by stepping down the potential of a signal line of a decoder circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example configuration of an SRAM block which is a semiconductor memory device according to an embodiment of the present disclosure. 
         FIG. 2  is a block diagram showing details of a memory array block and an IO block of  FIG. 1 . 
         FIG. 3  is a block diagram showing other details of the memory array block and the IO block of  FIG. 1 . 
         FIG. 4  is a circuit diagram showing a first example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 5  is a circuit diagram showing a second example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 6  is a diagram showing timings of  FIGS. 4 and 5 . 
         FIG. 7  is a circuit diagram showing a third example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 8  is a circuit diagram showing a fourth example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 9  is a diagram showing timings of  FIGS. 7 and 8 . 
         FIG. 10  is a circuit diagram showing a fifth example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 11  is a circuit diagram showing a sixth example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 12  is a diagram showing timings of  FIG. 11 . 
         FIG. 13  is a circuit diagram showing the configuration of  FIG. 8  in which a data read circuit is connected to only one of two bit lines. 
         FIG. 14  is a circuit diagram showing the configuration of  FIG. 11  in which a data read circuit is connected to only one of two bit lines. 
         FIG. 15  is a circuit diagram showing a seventh example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 16  is a diagram showing timings of  FIG. 15 . 
         FIG. 17  is a circuit diagram showing an eighth example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 18  is a diagram showing timings of  FIG. 17 . 
         FIG. 19  is a circuit diagram showing a ninth example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 20  is a diagram showing timings of  FIG. 19 . 
         FIG. 21  is a circuit diagram showing a tenth example detailed configuration of the IO block of  FIG. 2 . 
         FIG. 22  is a diagram showing timings of  FIG. 21 . 
         FIG. 23  is a diagram showing the configuration of the SRAM block of  FIG. 1  in which a first detection circuit is additionally provided. 
         FIG. 24  is a plan view schematically showing a first example layout configuration of a bit line potential control circuit of  FIG. 8 . 
         FIG. 25  is a plan view schematically showing a second example layout configuration of the bit line potential control circuit of  FIG. 8 . 
         FIG. 26  is a plan view schematically showing a third example layout configuration of the bit line potential control circuit of  FIG. 8 . 
         FIG. 27  is a plan view schematically showing a first example layout configuration of bit line potential control circuits of  FIGS. 15 and 19 . 
         FIG. 28  is a plan view schematically showing a second example layout configuration of the bit line potential control circuits of  FIGS. 15 and 19 . 
         FIG. 29  is a plan view schematically showing a third example layout configuration of the bit line potential control circuits of  FIGS. 15 and 19 . 
         FIG. 30  is a plan view schematically showing a first example layout configuration of a bit line potential control circuit of  FIG. 10 . 
         FIG. 31  is a plan view schematically showing a second example layout configuration of the bit line potential control circuit of  FIG. 10 . 
         FIG. 32  is a plan view schematically showing a first example layout configuration of a bit line potential control circuit of  FIG. 11 . 
         FIG. 33  is a plan view schematically showing a second example layout configuration of the bit line potential control circuit of  FIG. 11 . 
         FIG. 34  is a circuit diagram showing details of a decoder block and a control block in the SRAM block of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described in detail hereinafter with reference to the accompanying drawings. Note that like parts in IO blocks are indicated by like reference characters. 
       FIG. 1  shows an example configuration of an SRAM block which is a semiconductor memory device according to an embodiment of the present disclosure. The SRAM block of  FIG. 1  includes a memory array block  1  including a plurality of memory cells arranged in a matrix, an IO block  2  provided with respect to columns of the memory array block  1 , a decoder block  3  provided with respect to rows of the memory array block  1 , and a control block  4  provided adjacent to both the IO block  2  and the decoder block  3 . 
       FIG. 2  shows details of the memory array block  1  and the IO block  2  of  FIG. 1 . In  FIG. 2 , the memory array block  1  includes a plurality of memory cells (MEM)  5  arranged in a matrix, a plurality of bit lines including first bit lines BL and NBL provided with respect to columns of the memory cells  5 , a plurality of word lines including first word lines WL provided with respect to rows of the memory cells  5 . The IO block  2  is connected to the bit lines including the first bit lines BL and NBL provided with respect to the columns of the memory cells  5 . The IO block  2  includes first transistors TR 1 , a first logic gate LG 1 , and a second logic gate LG 2 . The first transistors TR 1  control the potentials of the first bit lines BL and NBL, and the first logic gate LG 1  controls the first transistors TR 1 . A signal is supplied through a first connect node CN 1  to the drain or source of each first transistor TR 1  and an input of the first logic gate LG 1 . An output of the first logic gate LG 1  is connected via a second connect node CN 2  to the gates of the first transistors TR 1 . The second logic gate LG 2  which supplies a signal to the first connect node CN 1  is provided inside the IO block  2 . The second logic gate LG 2  receives a precharge and potential control signal PCD. 
     With this configuration, timings are generated from the first connect node CN 1  in the IO block  2 , resulting in smaller variations in the decrease in the bit line potential. As a result, it is possible to reduce or prevent erroneous operation caused by an excessive decrease in the potential. Moreover, because the logic gate LG 2  which supplies a signal to the first connect node CN 1  is provided inside the IO block  2 , a timing deviation caused by waveform deformation at the first connect node CN 1  can be reduced. 
     In  FIG. 3 , the logic gate LG 2  which supplies a signal to the first connect node CN 1  is provided in the control block  4  outside the IO block  2 . This configuration can reduce the number of elements in the IO block  2 , resulting in a smaller area. 
       FIG. 4  shows a first example detailed configuration of the IO block  2  of  FIG. 2 . Each first transistor TR 1  includes an N-channel MOS (NMOS) transistor. The first logic gate LG 1  includes two inverters connected in cascade. A precharge circuit  10  is connected to the first bit lines BL and NBL. The precharge circuit  10  is controlled in accordance with a precharge signal PCH. 
       FIG. 5  shows a second example detailed configuration of the IO block  2  of  FIG. 2 . The precharge signal PCH of  FIG. 4  is also used as a precharge and potential control signal PCD. With this configuration, an interconnect dedicated to a precharge signal is not required, so that interconnect resources can be more easily ensured. 
       FIG. 6  shows timings of  FIGS. 4 and 5 . Here, three states will be described. 
     (i) When the signal PCD is at the “L” level, the first connect node CN 1  is at the “H” level and the second connect node CN 2  is at the “H” level, so that the first transistors TR 1  are turned on, and the “H” level of the first connect node CN 1  is propagated to the first bit lines BL and NBL. As a result, the first bit lines BL and NBL are precharged to the “H” level. 
     (ii) When the signal PCD is at the “H” level, the first connect node CN 1  is at the “L” level, and the second connect node CN 2  is at the “H” level, the first transistors TR 1  are turned on, and the “L” level of the first connect node CN 1  is propagated to the first bit lines BL and NBL. As a result, the potentials of the first bit lines BL and NBL are stepped down to the “L” level. 
     (iii) When the signal PCD is at the “H” level, the first connect node CN 1  is at the “L” level, and the second connect node CN 2  is at the “L” level, the first transistors TR 1  are turned off. As a result, the first bit lines BL and NBL are transitioned to a potential which is slightly lower than the “H” level (floating state). 
     Here, it is assumed that the precharge circuit  10  of  FIG. 4  or  5  is not provided. In the aforementioned condition (i), the precharge potential is Vdd−Vtn because the first transistors TR 1  each include an NMOS transistor. In (ii), the potentials of the first bit lines BL and NBL can be quickly stepped down because the first transistors TR 1  each include an NMOS transistor. In (iii), the potentials of the first bit lines BL and NBL are stepped down for a delay time corresponding to the first logic gate LG 1  before the first transistors TR 1  are turned off, so that the first bit lines BL and NBL are transitioned to the floating state. 
     Here, it is assumed that the precharge circuit  10  of  FIG. 4  or  5  is provided. In the aforementioned condition (i), the precharge circuit  10  can cause the precharge potential to increase to Vdd and, at the same time, can quickly performs precharge. 
       FIG. 7  shows a third example detailed configuration of the IO block  2  of  FIG. 2 . The first transistors TR 1  each include a P-channel MOS (PMOS) transistor, and the first logic gate LG 1  includes a single inverter. A precharge circuit  10  is connected to the first bit lines BL and NBL. The precharge circuit  10  is controlled in accordance with a precharge signal PCH. 
       FIG. 8  shows a fourth example detailed configuration of the IO block  2  of  FIG. 2 . The precharge signal PCH of  FIG. 7  is also used as a precharge and potential control signal PCD. With this configuration, an interconnect dedicated to a precharge signal is not required, so that interconnect resources can be more easily ensured. 
       FIG. 9  shows timings of  FIGS. 7 and 8 . Here, three states will be described. 
     (i) When the signal PCD is at the “L” level, the first connect node CN 1  is at the “H” level and the second connect node CN 2  is at the “L” level, so that the first transistors TR 1  are turned on, and the “H” level of the first connect node CN 1  is propagated to the first bit lines BL and NBL. As a result, the first bit lines BL and NBL are precharged to the “H” level. 
     (ii) When the signal PCD is at the “H” level, the first connect node CN 1  is at the “L” level, and the second connect node CN 2  is at the “L” level, the first transistors TR 1  are turned on, and the “L” level of the first connect node CN 1  is propagated to the first bit lines BL and NBL. As a result, the potentials of the first bit lines BL and NBL are stepped down to the “L” level. 
     (iii) When the signal PCD is at the “H” level, the first connect node CN 1  is at the “L” level, and the second connect node CN 2  is at the “H” level, the first transistors TR 1  are turned off. As a result, the first bit lines BL and NBL are transitioned to a potential which is slightly lower than the “H” level (floating state). 
     Here, it is assumed that the precharge circuit  10  of  FIG. 7  or  8  is not provided. In the aforementioned condition (i), the precharge potential is Vdd because the first transistors TR 1  each include a PMOS transistor. Because the transistors which control the precharge potential are connected via two stages to the first bit lines BL and NBL, the precharge speed is reduced. In (ii), because the first transistors TR 1  each include a PMOS transistor, it is possible to reduce or prevent erroneous operation caused by an excessive decrease in the potentials of the first bit lines BL and NBL. In (iii), the potentials of the first bit lines BL and NBL are stepped down for a delay time corresponding to the first logic gate LG 1  before the first transistors TR 1  are turned off, so that the first bit lines BL and NBL are transitioned to the floating state. 
     Here, it is assumed that the precharge circuit  10  of  FIG. 7  or  8  is provided. In the aforementioned condition (i), the precharge circuit  10  can quickly performs precharge. 
       FIG. 10  shows a fifth example detailed configuration of the IO block  2  of  FIG. 2 . In  FIG. 10 , second transistors TR 2  control the potentials of the first bit lines BL and NBL. First capacitors CAP 1  are connected to the first bit lines BL and NBL. A fourth logic gate LG 4  controls the first capacitors CAP 1 . A signal is supplied through a third connect node CN 3  to inputs of the second transistors TR 2  and the fourth logic gate LG 4 . An output of the fourth logic gate LG 4  is connected via a fourth connect node CN 4  to the first capacitors CAP 1 . A fifth logic gate LG 5  which supplies a signal to the third connect node CN 3  is provided inside the IO block  2 . Note that the fifth logic gate LG 5  may be provided outside the IO block  2 . 
     In this configuration, the first capacitor CAP 1  are used, and therefore, there is no dependence of transistor variations, particularly the threshold voltage Vt, whereby variations in the decrease in the potentials of the first bit lines BL and NBL can be reduced or prevented. 
       FIG. 11  shows a sixth example detailed configuration of the IO block  2  of  FIG. 2 . In addition to the configuration of  FIG. 10 , third transistors TR 3  are interposed between the first bit lines BL and NBL and the first capacitors CAP 1 . A sixth logic gate LG 6  controls the third transistors TR 3  via a fifth connect node CN 5 . The fourth logic gate LG 4  controls the sixth logic gate LG 6  and the first capacitors CAP 1 . 
     With this configuration, during read operation, the third transistors TR 3  are turned off, so that the capacitances of the first capacitors CAP 1  are not connected to the first bit lines BL and NBL, resulting in higher-speed operation than that of  FIG. 10 . 
       FIG. 12  shows timings of  FIG. 11 . Here, three states will be described. 
     (i) When the signal PCD is at the “H” level, the third connect node CN 3  is at the “L” level, so that the second transistors TR 2  are turned on. As a result, the first bit lines BL and NBL are precharged to the “H” level. 
     (ii) When the signal PCD is at the “L” level, the third connect node CN 3  is at the “H” level, and the fourth connect node CN 4  is at the “L” level, the second transistors TR 2  are turned off, and the “L” level of the fourth connect node CN 4  is propagated via the first capacitors CAP  1  to the first bit lines BL and NBL. As a result, the potentials of the first bit lines BL and NBL are stepped down to the “L” level. 
     (iii) When the signal PCD is at the “L” level, the third connect node CN 3  is at the “H” level, the fourth connect node CN 4  is at the “L” level, and the fifth connect node CN 5  is at the “H” level, the second transistors TR 2  are turned off and the third transistors TR 3  are also turned off. As a result, the first bit lines BL and NBL are transitioned to a potential which is slightly lower than the “H” level (floating state). 
     In the aforementioned condition (ii), the first capacitors CAP 1  are used to step down the first bit lines BL and NBL. Therefore, there is no dependence of transistor variations, particularly the threshold voltage Vt, whereby variations in the decrease in the potentials of the first bit lines BL and NBL can be reduced or prevented. In (iii), the third transistors TR 3  are turned off, so that the capacitances of the first capacitors CAP 1  are not connected to the first bit lines BL and NBL, whereby high-speed operation can be achieved without increasing the bit line capacitance. 
       FIGS. 13 and 14  show configurations which include a data read circuit  11  which is connected only to the first bit line BL, in addition to  FIGS. 8 and 11 , respectively. With this configuration, the first bit line BL can be operated with a small amplitude, resulting in higher-speed operation. 
       FIGS. 15 and 16  show a seventh example configuration of the IO block  2  of  FIG. 2 , and timings thereof, respectively. In  FIG. 15 , the first logic gate LG 1  includes a two-input NAND. A first internal signal RE is connected to one of the inputs of two-input NAND. The first internal signal RE is used to make a decision whether to perform read operation or write operation. 
       FIG. 16  shows states of the first bit lines BL and NBL in operation modes. During read operation, when the first internal signal RE is at the “H” level, the two-input NAND is equivalent to an inverter and performs the same operation as that of  FIGS. 8 and 9 . As a result, the potentials of the first bit lines BL and NBL are stepped down for a delay time of the first logic gate LG 1 . 
     During write operation, when the first internal signal RE is at the “L” level, the second connect node CN 2  is at the “H” level, so that the first transistors TR 1  are turned off. As a result, the potentials of the first bit lines BL and NBL are not stepped down, and therefore, remain at the “H” level. 
     With this configuration, the potentials of the first bit lines BL and NBL are not stepped down during write operation, whereby power for write operation can be reduced. 
       FIGS. 17 and 18  show an eighth example configuration of the IO block  2  of  FIG. 2 , and timings thereof, respectively. In  FIG. 17 , a sixth logic gate LG 6  includes a two-input NAND. A first internal signal RE is connected to one of the two inputs of the sixth logic gate LG 6 . The first internal signal RE is used to make a decision whether to perform read operation or write operation. 
       FIG. 18  shows states of the first bit lines BL and NBL in operation modes. During read operation, when the first internal signal RE is at the “H” level, the two-input NAND is equivalent to an inverter and performs the same operation as that of  FIGS. 11 and 12 . As a result, the potentials of the first bit lines BL and NBL are stepped down for a delay time of the sixth logic gate LG 6 . 
     During write operation, when the first internal signal RE is at the “L” level, the fifth connect node CN 5  is at the “H” level, so that the third transistors TR 3  are turned off. As a result, the potentials of the first bit lines BL and NBL are not stepped down, and therefore, remain at the “H” level. 
     With this configuration, the potentials of the first bit lines BL and NBL are not stepped down during write operation, whereby power for write operation can be reduced. 
       FIGS. 19 and 20  show a ninth example configuration of the IO block  2  of  FIG. 2 , and timings thereof, respectively. In  FIG. 19 , the first logic gate LG 1  includes a two-input NAND. A first external signal SIG is connected to one of the two inputs of the first logic gate LG 1 . The first external signal SIG is used to make a decision whether or not to step down the first bit lines BL and NBL. 
       FIG. 20  shows states of the first bit lines BL and NBL in potential modes. When the first external signal SIG is at the “H” level (on), the two-input NAND is equivalent to an inverter and performs the same operation as that of  FIGS. 8 and 9 . As a result, the potentials of the first bit lines BL and NBL are stepped down for a delay time of the first logic gate LG 1 . When the first external signal SIG is at the “L” level (off), a second connect node CN 2  is at the “H” level, so that the first transistors TR 1  are turned off. As a result, the potentials of the first bit lines BL and NBL are not stepped down, and remain at the “H” level. 
     With this configuration, for example, when there is the possibility that stepping down of the first bit lines BL and NBL may lead to erroneous read operation, the erroneous read operation can be reduced or prevented by turning off the first external signal SIG. 
       FIGS. 21 and 22  show a tenth example configuration of the IO block  2  of  FIG. 2 , and timings thereof, respectively. In  FIG. 21 , a sixth logic gate LG 6  includes a two-input NAND. A first external signal SIG is connected to one of the two inputs of the sixth logic gate LG 6 . The first external signal SIG is used to make a decision whether or not to step down the first bit lines BL and NBL. 
       FIG. 22  shows states of the first bit lines BL and NBL in potential modes. When the first external signal SIG is at the “H” level (on), the two-input NAND is equivalent to an inverter and performs the same operation as that of  FIGS. 11 and 12 . As a result, the potentials of the first bit lines BL and NBL are stepped down for a delay time of the sixth logic gate LG 6 . When the first external signal SIG is at the “L” level (off), a fifth connect node CN 5  is at the “H” level, so that third transistors TR 3  are turned off. As a result, the potentials of the first bit lines BL and NBL are not stepped down, and therefore, remain at the “H” level. 
     With this configuration, for example, when there is the possibility that stepping down of the first bit lines BL and NBL may lead to erroneous read operation, the erroneous read operation can be reduced or prevented by turning off the first external signal SIG. 
       FIG. 23  shows the example configuration of the SRAM block of  FIG. 1  in which a first detection circuit  6  which is additionally provided. In  FIG. 23 , the first detection circuit  6  detects process variations, voltage variations, and temperature. A first external signal SIG is an activation signal generated by the first detection circuit  6 . 
     In this configuration, the first detection circuit  6  which detects process variations, voltage variations, and temperature is provided. By inputting the result of the detection as the first external signal SIG to the first logic gate LG 1  or the sixth logic gate LG 6  of the IO block  2 , a decision can be made whether or not to step down the first bit lines BL and NBL. As a result, the first detection circuit  6  detects process variations, voltage variations, and temperature, and when there is the possibility that stepping down of the first bit lines BL and NBL may lead to erroneous read operation, turns off the first external signal SIG. 
       FIG. 24  schematically shows a first example layout configuration of the bit line potential control circuit of  FIG. 8 . Note that this example configuration does not include the precharge circuit  10 . In  FIG. 24 , the bit line potential control circuit includes a diffusion layer  100 , gate electrodes  101 , a first interconnect layer  102 , and a second interconnect layer  103 . In this configuration, the gate electrodes  101  on the diffusion layer  100  are substantially parallel to the first bit lines BL and NBL, whereby the area of the first interconnect layer  102  interposed between the second interconnect layer  103  used in the first bit lines BL and NBL and the diffusion layer  100  used in the first transistors TR 1  can be minimized. Therefore, the load capacitance of the first bit lines BL and NBL can be reduced, which contributes to a reduction in the power and an increase in the speed during driving of the first bit lines BL and NBL. 
       FIGS. 25 and 26  schematically show second and third example layout configurations of the bit line potential control circuit of  FIG. 8 . Note that, in these configurations, the precharge circuit  10  is not provided. In the configurations, when the gate electrodes on the diffusion layer of the memory cells  5  of  FIG. 2  are substantially perpendicular to the first bit lines BL and NBL, gate electrodes  101  on a diffusion layer  100  of  FIGS. 25 and 26  are arranged in the same direction as that of the gate electrodes of the memory cells  5 . Therefore, an impurity is implanted into the diffusion layer  100  immediately below the gate electrodes  101  in the same direction between the memory array block  1  and the IO block  2 , whereby variations caused by impurity implantation can be reduced. 
       FIG. 27  schematically shows a first example layer configuration of the bit line potential control circuits of  FIGS. 15 and 19 . Note that, in this configuration, the precharge circuit  10  is not provided. In the configuration, gate electrodes  101  on a diffusion layer  100  are substantially parallel to the first bit lines BL and NBL, whereby the area of a first interconnect layer  102  interposed between a second interconnect layer  103  used in the first bit lines BL and NBL and the diffusion layer  100  used in the first transistors TR 1  can be minimized. Therefore, the load capacitance of the first bit lines BL and NBL can be reduced, which contributes to a reduction in the power and an increase in the speed during driving of the first bit lines BL and NBL. 
       FIGS. 28 and 29  schematically show second and third example layer configurations of the bit line potential control circuits of  FIGS. 15 and 19 , respectively. Note that, in these configurations, the precharge circuit  10  is not provided. In the configurations, when the gate electrodes on the diffusion layer of the memory cells  5  of  FIG. 2  are substantially perpendicular to the first bit lines BL and NBL, gate electrodes  101  on a diffusion layer  100  of  FIGS. 28 and 29  are arranged in the same direction as that of the gate electrodes of the memory cells  5 . Therefore, an impurity is implanted into the diffusion layer  100  immediately below the gate electrodes  101  in the same direction between the memory array block  1  and the IO block  2 , whereby variations caused by impurity implantation can be reduced. 
       FIG. 30  schematically shows a first example layer configuration of the bit line potential control circuit of  FIG. 10 . In this configuration, gate electrodes  101  on a diffusion layer  100  are substantially parallel to the first bit lines BL and NBL, whereby the area of a first interconnect layer  102  interposed between a second interconnect layer  103  used in the first bit lines BL and NBL and the diffusion layer  100  used in the second transistors TR 2  can be minimized. Therefore, the load capacitance of the first bit lines BL and NBL can be reduced, which contributes to a reduction in the power and an increase in the speed during driving of the first bit lines BL and NBL. 
       FIG. 31  schematically shows a second example layer configuration of the bit line potential control circuit of  FIG. 10 . In this configuration, when the gate electrodes on the diffusion layer of the memory cells  5  of  FIG. 2  are substantially perpendicular to the first bit lines BL and NBL, gate electrodes  101  on a diffusion layer  100  of  FIG. 31  are arranged in the same direction as that of the gate electrodes of the memory cells  5 . Therefore, an impurity is implanted into the diffusion layer  100  immediately below the gate electrodes  101  in the same direction between the memory array block  1  and the IO block  2 , whereby variations caused by impurity implantation can be reduced. 
       FIG. 32  schematically shows a first example layer configuration of the bit line potential control circuit of  FIG. 11 . In this configuration, gate electrodes  101  on a diffusion layer  100  are substantially parallel to the first bit lines BL and NBL, whereby the area of a first interconnect layer  102  interposed between a second interconnect layer  103  used in the first bit lines BL and NBL and the diffusion layer  100  used in the second transistors TR 2  can be minimized. Therefore, the load capacitance of the first bit lines BL and NBL can be reduced, which contributes to a reduction in the power and an increase in the speed during driving of the first bit lines BL and NBL. 
       FIG. 33  schematically shows a second example layer configuration of the bit line potential control circuit of  FIG. 11 . In this configuration, when the gate electrodes on the diffusion layer of the memory cells  5  of  FIG. 2  are substantially perpendicular to the first bit lines BL and NBL, gate electrodes  101  on a diffusion layer  100  of  FIG. 33  are arranged in the same direction as that of the gate electrodes of the memory cells  5 . Therefore, an impurity is implanted into the diffusion layer  100  immediately below the gate electrodes  101  in the same direction between the memory array block  1  and the IO block  2 , whereby variations caused by impurity implantation can be reduced. 
       FIG. 34  shows details of the decoder block  3  and the control block  4  in the SRAM block of  FIG. 1 . In  FIG. 34 , a potential control circuit  7  is connected to source lines of NMOS transistors in the decoder block  3 . 
     Specifically, the decoder block  3  includes a plurality of word drivers  8 . MWL  126  and MWL 127  are signals input to the word drivers  8 , and WL 126  and WL 127  are signals output from the word drivers  8 . The word drivers  8  each include a first PMOS transistor P 1  having a source connected to a power supply Vdd, and a first NMOS transistor N 1  having a source connected to a first common node CCN. 
     The control block  4  includes a second NMOS transistor N 2  having a drain connected to the first common node CCN, and the potential control circuit  7 . The potential control circuit  7  includes a first transistor TR 11 , a first logic gate LG 11 , and a second logic gate LG 12 . The first transistor TR 11  controls the potential of the first common node CCN, and the first logic gate LG 11  controls the first transistor TR 11 . A signal is supplied through a first connect node CN 11  to the drain or source of the first transistor TR 11  and an input of the first logic gate LG 11 . An output of the first logic gate LG 11  is connected to the gate of the first transistor TR 11 . The second logic gate LG 12  which supplies a signal to the first connect node CN 11  receives a clock signal CLK. 
     With the configuration of  FIG. 34 , higher-speed operation can be performed by previously stepping down the source lines of the word drivers  8 , which are long interconnects. 
     Note that the present disclosure is not limited to the aforementioned embodiments, and various changes and modifications can be made without departing the spirit and scope of the present disclosure. Specifically, the positive and negative logics in the circuits can be changed as appropriate. In addition, although an SRAM is described as an example of the semiconductor memory device in each embodiment, ROMs, other non-volatile memories, and the like may be used. Moreover, the present disclosure is not limited to a one-port memory cell. A multi-port memory call may be used. In this case, a number of bit lines corresponding to the multiple ports are required. 
     As described above, the semiconductor memory device of the present disclosure can perform stable operation by improving the SNM while reducing or preventing erroneous operation, such as erroneous write operation or the like. Moreover, read operation with a small amplitude is achieved by stepping down the bit line potential, which contributes to higher-speed operation. The present disclosure is particularly useful for memories, such as SRAMs, ROMs, and the like, the increase of the speed of a decoder circuit provided in a memory, a cache memory for a microprocessor, and the like.