Abstract:
A semiconductor memory device includes isolation circuits disconnecting cell arrays from sense amplifiers, and isolation signal generating circuits generating isolation signals that control the isolation circuits. The isolation signal generating circuits are hierarchically divided into main isolation signal generating circuits and sub isolation signal generating circuits. The sub isolation signal generating circuits generate sub isolation signals having a first potential on a high-potential side. The main isolation signal generating circuits generate main isolation signals having a second potential on the high-potential side, the second potential being lower than the first potential.

Description:
This is a Division of application Ser. No. 09/789,514 filed Feb. 22, 2001, (which in turn is a Continuation Application of U.S. Pat. No. 6,529,439). The disclosure of the prior application(s) is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to semiconductor memory devices, and more particularly, to a semiconductor memory device equipped with an isolation signal generating circuit for controlling electrical isolation between a cell array and a sense amplifier. The present invention is also concerned with a semiconductor memory device of a hierarchical arrangement of word drivers for driving word lines. 
     2. Description of the Related Art 
     First, a first conventional semiconductor memory device will be described with reference to FIGS. 1 and 2. 
     FIG. 1 is a circuit diagram of a part of a DRAM (Dynamic Random Access Memory), which is the first conventional semiconductor memory device. The DRAM shown in FIG. 1 employs a shared sense amplifier formation in which a sense amplifier is shared by neighboring cell arrays. The DRAM also employs a hierarchical arrangement of word drivers so that each word driver includes a main word driver and a sub word driver. 
     The DRAM includes cell arrays  1 - 9 , main word drivers  10 - 12 , and sub word drivers  13 - 24 . The main word drivers  10 - 12  drive main word lines. The sub word drivers receive output signals of the main word drivers  10 - 12  and sub word selection signals that are output by a sub word selection signal generating circuit (not illustrated for the sake of simplicity), and select involved memory cells. 
     Column decoders  25 - 27  decode a column address signal and select columns based on decoding. Sense amplifier parts  28 - 39  are shared by neighboring cell arrays in a wiring direction of bit lines, and include a sense amplifier and a bit line precharge circuit. 
     The DRAM includes row block selecting signal lines  40 - 45 , and a timing signal line  46 . The DRAM includes isolation signal generating circuits  47 - 52 , which receive the row block selecting signals and a timing signal, and thus generate isolation signals for controlling electrical connection/disconnection between the neighboring cell arrays and the sense amplifier parts in the direction in which the bit lines run. 
     The DRAM includes isolation signal lines  53 - 58 , and isolation circuits  59 - 76 . The isolation signal lines  53 - 58  carry isolation signals respectively output by the isolation signal generating circuits  47 - 52 . The isolation circuits  59 - 76  respectively have, as switching elements, isolation transistors, which are turned ON/OFF by the isolation signals in order to control electrical connection and disconnection between the cell arrays and sense amplifiers which are adjacent in the direction in which the bit lines run. 
     In the DRAM thus configured, a read/write operation enables the cell arrays connected to selected main word lines, sub word drivers and sense amplifier parts associated with the selected main word lines. Further, the isolation transistors of the isolation circuits involved in the above read/write operation. 
     FIG. 2 is a circuit diagram illustrating an isolation operation performed in the DRAM that is described as the first conventional semiconductor memory device in the present specification. For example, when the word lines driven by the main word driver  11  are selected, the cell arrays  4 - 6 , the sub word drivers  17 - 20 , and the sense amplifier parts  31 - 36  are enabled within the circuit part shown in FIG. 2, while the isolation transistors of the isolation circuits  62 - 64  and  71 - 73  are turned OFF. The isolation transistors of the isolation circuits  59 - 61 ,  65 - 70 , and  74 - 76  are maintained in the ON state. 
     Thus, electrical connections are made between the cell array  4  and the sense amplifier parts  31  and  34 , between the cell array  5  and the sense amplifier parts  32  and  35 , and between the cell array  6  and the sense amplifier parts  33  and  36 . In contrast, the cell arrays  1 ,  2 ,  3 ,  7 ,  8  and  9  are respectively disconnected from the sense amplifier parts  31 ,  32 ,  33 ,  34 ,  35  and  36 . 
     A description will be given, with reference to FIGS. 3 through 5, of a second conventional DRAM, which employs the shared amplifier formation. The DRAM also employs a hierarchical arrangement of word drivers so that each word driver includes a main word driver and a sub word driver. Further, each isolation signal generating circuit is hierarchically arranged so as to include a main isolation signal generating circuit and a sub isolation signal generating circuit. 
     Referring to FIG. 3, the second conventional DRAM includes row block selection signal lines  77 - 82 , a timing signal line  83 , main isolation signal generating circuits  84 - 89 , and main isolation signal lines  90 - 95 . The main isolation signal generating circuits  84 - 89  receive the row block selection signal lines  77 - 82  and a timing signal carried over the timing signal line  83 , and generate resultant main isolation signals, which are transferred over the main isolation signal lines  90 - 95 . 
     The second conventional DRAM includes column block selection signal lines  96 - 101 , sub isolation signal generating circuits  102 - 137 , and sub isolation signal lines  138 - 155 . The sub isolation signal generating circuits  102 - 137  receive the main isolation signals and column block selection signals carried over the column block selection signal lines  96 - 101 , and generate resultant sub isolation signals, which are transferred over the sub isolation signal lines  138 - 155 . 
     The second conventional DRAM includes sub word drivers  156 - 173 , and has the same cell arrays  1 - 9 , main word drivers  10 - 12 , column decoders  25 - 27 , the sense amplifier parts  28 - 39 , and isolation circuits  59 - 76  as those of the first conventional DRAM. 
     In the second conventional DRAM, a read/write operation enables the cell arrays connected to selected main word lines, sub word drivers and sense amplifier parts associated with the selected main word lines. Further, the isolation transistors of the isolation circuits involved in the above read/write operation. 
     FIG. 4 is a circuit diagram illustrating an isolation operation performed in the second conventional. For example, when memory cells in the cell array  5  are selected, the cell array  5 , the sub word drivers  164  and  165 , the sub isolation signal generating circuits  110 ,  111 ,  128  and  129 , and the sense amplifier parts  32  and  35  are enabled within the circuit part shown in FIG.  4 . 
     Thus, the isolation transistors of the isolation circuits  63  and  72  are turned OFF, while those of the isolation circuits  59 - 62 ,  64 - 71 , and  73 - 76  are maintained in the ON state. Thus, connections of the cell array  5  with the sense amplifier parts  32  and  35  are made, while the cell arrays  2  and  8  are disconnected from the sense amplifier parts  32  and  35 , respectively. 
     FIG. 5 is a circuit diagram of a configuration of the main isolation signal generating circuits and the sub isolation signal generating circuits employed in the second conventional DRAM. A main isolation signal generating circuit  174  includes a NAND circuit  175 , an inverter  176 , PMOS (P-channel Metal Oxide Semiconductor) transistors  177 - 179 , and NMOS (N-channel MOS) transistors  180 - 182 . 
     A sub isolation signal generating circuit  183  includes PMOS transistors  184 - 187 , and NMOS transistors  188 - 191 . A symbol VPP denotes a boosted voltage obtained by boosting a power supply voltage supplied from the outside of the DRAM. A symbol VSS denotes a ground potential. 
     A description will be given, with reference to FIGS. 6 through 8, of a third conventional DRAM. 
     FIG. 6 illustrates a layout of a core part of the third conventional DRAM, which employs the shared sense amplifier formation and the hierarchical arrangements of the word drivers and isolation signal generating circuits. Further, the third conventional DRAM includes a hierarchical arrangement of sub word selection signal generating circuits (¼ signal generating circuits) so that each of the circuits is made up of a main sub-word selection signal generating circuit (main ¼ signal generating circuit) and a sub sub-word selection signal generating circuit (sub ¼ signal generating circuit). 
     The layout shown in FIG. 6 includes a core part  192 , cell areas  193 - 196 , main word driver areas  197  and  198 , sub word driver areas  199 - 204 , sense amplifier areas  205 - 210 , MS cross areas  211 - 213 , and SS cross areas  215 - 223 . The cell areas  193 - 196  include cell arrays. The main word driver areas  197  and  198  include main word drivers. The sub word driver areas  199 - 204  include sub word drivers. The sense amplifier areas  205 - 210  include sense amplifiers. The MS cross areas  211 - 213  include main sub-word selection signal generating circuits and main isolation signal generating circuits. The SS cross areas  215 - 223  include sub sub-word selection signal generating circuits, sub isolation signal generating circuits, and bit line precharge signal generating circuits. 
     FIG. 7 is a circuit diagram of a part of the core part of the third conventional DRAM. There are illustrated address signal lines  224 , main word drivers  225 - 227 , sub word drivers  228 - 247 , main sub-word selection signal generating circuits  248 - 251 , sub sub-word selection signal generating circuits  252 - 259 , main isolation signal generating circuits  260  and  261 , sub isolation signal generating circuits  262 - 265 , and bit line precharge signal generating circuits  266  and  267 . 
     Arrows of broken lines denote signal lines that carry a signal. This signal has an amplitude having the maximum level (corresponding to a power supply potential on the high-potential side) corresponding to a step-down voltage VII obtained by internally stepping down the power supply voltage VDD supplied from the outside of the DRAM, and the minimum level (corresponding to a power supply potential on the low-potential side) corresponding to the ground potential VSS. Arrows of solid lines denote signal lines that carry the following signal. This signal has an amplitude having the maximum level (corresponding to a power supply potential on the high-potential side) corresponding to the boosted voltage VPP, and the minimum level (low-potential) corresponding to the ground level VSS. 
     More particularly, the address signal lines  224  described by broken-line arrows carry the step-down potential VII that is the power supply potential on the high-potential side and is obtained by stepping down the external power supply voltage VDD. The following signal lines by solid-line arrows carry the boosted potential VPP that is the power supply potential on the low-potential side: the main word lines extending from the main word drivers  225 - 227 , the main sub-word selection signal lines extending from the main sub-word selection signal generating circuits  248 - 251 , the main isolation signal lines extending from the main isolation signal generating circuits  260  and  261 , the sub isolation signal lines extending from the sub isolation signal generating circuits  262 - 265 , and bit line precharge signal lines extending from the bit line precharge signal generating circuits  266  and  267 . 
     FIG. 8 is a circuit diagram of a circuit configuration arranged in the SS cross area in the core part of the third conventional DRAM. Referring to FIG. 8, there are illustrated sub isolation signal generating circuits  268  and  269 , each of which circuit includes PMOS transistors  270  and  271 , NMOS transistors  272  and  273 , and inverters  274  and  275 . There are also illustrated a bit line precharge signal generating circuit  276  composed of a NOR circuit  277  and an inverter  278 . A sub sub-word selection signal generating circuit  279  is made up of a PMOS transistor  280 , NMOS transistors  281  and  282 , and inverters  283 - 285 . A sub word driver  286  is made up of PMOS transistor  287  and NMOS transistors  288  and  289 . 
     However, the second conventional DRAM shown in FIG. 3 has a disadvantage in that the boosted potential VPP is used as the power supply potential of the main isolation signals on the high-potential side and the sub isolation signals. This consumes an increased amount of power. 
     The third conventional DRAM shown in FIG. 7 (FIG. 6) has a disadvantage in that the potential VPP is used as the power supply potential of the main sub-word selection signals on the high-potential side and sub sub-word selection signals. This increases power consumption. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a semiconductor memory device in which the above disadvantages are eliminated. 
     A more specific object of the present invention is to provide a semiconductor memory device equipped with a hierarchical arrangement of isolation circuits controlling electrical isolation between cell arrays and sense amplifiers, wherein reduced power can be consumed. 
     Another object of the present invention is to provide a semiconductor memory device equipped with a hierarchical arrangement of sub word selection signal generating circuits, wherein reduced power can be consumed. 
     The above objects of the present invention are achieved by a semiconductor memory device comprising: isolation circuits disconnecting cell arrays from sense amplifiers; and isolation signal generating circuits generating isolation signals that control the isolation circuits, the isolation signal generating circuits being hierarchically divided into main isolation signal generating circuits and sub isolation signal generating circuits, the sub isolation signal generating circuits generating sub isolation signals having a first potential on a high-potential side, the main isolation signal generating circuits generating main isolation signals having a second potential on the high-potential side, the second potential being lower than the first potential. 
     According to the above semiconductor memory device, the potential of only the sub isolation signals on the high-potential side is the first potential, while the potential of the main isolation signals on the high potential side is the second potential lower than the first potential. It is therefore possible to reduce the number of signal lines via which the first potential serving as a high-potential side power supply potential is supplied. 
     The above objects of the present invention are also achieved by a semiconductor memory device comprising: main word drivers that drive word lines and are hierarchically divided into main word drivers and sub word drivers; and sub word selection signal generating circuits that select sub word lines and are hierarchically divided into main sub-word selection signal generating circuits and sub sub-word selection signal generating circuits, the sub sub-word selection signal generating circuits generating sub sub-word selection signals having a first potential on a high-potential side, the main sub-word selection signal generating circuits generating main sub-word selection signals having a second potential on the high-potential side, the second potential being lower than the first potential. 
     According to the above semiconductor memory device, the potential of only the sub sub-word selection signals on the high-potential side is the first potential, while the potential of the main sub-word selection signals on the high-potential side is the second potential lower than the first potential. It is therefore possible to reduce the number of signal lines via which the first potential serving as a high-potential side power supply potential is supplied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a circuit diagram of a part of a first conventional DRAM; 
     FIG. 2 is a circuit diagram for explaining an isolating operation of the first conventional DRAM; 
     FIG. 3 is a circuit diagram of a second conventional DRAM; 
     FIG. 4 is a circuit diagram for explaining an isolating operation of the second conventional DRAM; 
     FIG. 5 is a circuit diagram of a main isolation signal generating circuit and a sub isolation signal generating circuit, both being provided in the second conventional DRAM; 
     FIG. 6 is a diagram of a layout of a core part of a third conventional DRAM; 
     FIG. 7 is a circuit diagram of a part of the core part of the third conventional DRAM; 
     FIG. 8 is a circuit diagram of a circuit configuration arranged in an SS cross area of the core part of the third conventional DRAM; 
     FIG. 9 is a circuit diagram of a first embodiment of the present invention; 
     FIG. 10 is a circuit diagram of a circuit configuration of a main isolation signal generating circuit and a sub isolation signal generating circuit, both being provided in the first embodiment of the present invention; 
     FIG. 11 is a circuit diagram of another circuit configuration of the sub isolation signal generating circuit employed in the first embodiment of the present invention; 
     FIG. 12 is a circuit diagram of a part of a core part of a second embodiment of the present invention; and 
     FIG. 13 is a circuit diagram of a circuit configuration arranged in an SS cross area of the core part of the second embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will be given, with reference to FIGS. 9 through 11, of a first embodiment of the present invention. 
     FIG. 9 is a circuit diagram of a DRAM according to a first embodiment of the present invention. The DRAM includes main isolation signal generating circuits  290 - 295  having different circuit configurations from those of the main isolation signal generating circuits  84 - 89 . The DRAM also includes sub isolation signal generating circuits  296 - 331  having different circuit configurations from those of the sub isolation signal generating circuits  102 - 137 . 
     That is, the DRAM according to the first embodiment of the present invention is provided newly with main isolation signal generating circuits  290 - 295  configured differently from those shown in FIG.  3  and sub isolation signal generating circuits  296 - 331  configured differently from those shown in FIG.  3 . The other parts of the DRAM according to the first embodiment of the present invention are configured in the same manner as those of the DRAM shown in FIG.  3 . 
     In the first embodiment of the present invention, the main isolation signal generating circuits  290 - 295  are configured in such a way as to generate a main isolation signal that swings between the external power supply potential VDD (or the step-down power supply potential VII) and the ground potential VSS. The sub isolation signal generating circuits  296 - 331  are configured in such a way as to generate a sub isolation signal that swings between the boosted power supply potential VPP and the ground potential VSS. 
     FIG. 10 is a circuit diagram of a circuit configuration of the main isolation signal generating circuits and the sub isolation signal generating circuits employed in the first embodiment of the present invention. Referring to FIG. 10, there is illustrated a main isolation signal generating circuit  332 , which includes a NAND circuit  333 . The main isolation signal generating circuit  332  receives a row block selection signal RBS 1  and a timing signal TM, and generates a main isolation signal MIS 1  therefrom. The NAND circuit  333  has a CMOS structure in which the power supply potential VII (that may be VDD) and the ground potential VSS serve as the power supply potentials on the high-potential and low-potential sides, respectively. 
     The DRAM includes a sub isolation signal generating circuit  334 , which receives the main isolation signal MIS 1  and a column block selection signal CBS, and generates a sub isolation signal SIS 1 . The circuit  334  includes a logic circuit of a NOR circuit, which performs a NOR operation on the main isolation signal MIS 1  and the column block selection signal CBS. The NOR circuit has a CMOS structure in which the power supply potential VII and the ground potential VSS serve as the power supply potentials on the high-potential and low-potential sides, respectively. 
     The DRAM includes a level conversion circuit  336 , which converts the level of the output of the NOR circuit  335  to thus produce the sub isolation signal SIS 1 . The circuit  336  includes an inverter  337  that inverts the output of the NOR circuit  335 , PMOS transistors  338  and  339 , and NMOS transistors  340  and  341 . The inverter  337  is a CMOS structure, which receives the power supply potential VII on the high-potential side and the ground potential VSS on the low-potential side. The sources of the PMOS transistors  338  and  339  are connected to a VPP line via which the boosted potential VPP is supplied. 
     A main isolation signal generating circuit  342  receives a row block selection signal RBS 2  and the timing signal TM, and thus generates a main isolation signal MIS 2 . The circuit  342  includes a NAND circuit  343  of a CMOS structure, which receives the power supply potential VII on the high-potential side and the ground potential VSS on the low-potential side. 
     A sub isolation signal generating circuit  344  receives the main isolation signal MIS 2  and the column block selection signal CBS, and thus generates a sub isolation signal SIS 2 . A NOR circuit  345  performs a NOR operation on the main isolation signal MIS 2  and the column block selection signal CBS. The NOR circuit  345  has a CMOS structure, which receives the power supply potential VII on the high-potential side and the ground potential VSS on the low-potential side. 
     A level conversion circuit  346  converts the output of the NOR circuit  345  to thus produce the sub isolation signal SIS 2 . The circuit  346  includes an inverter  347  inverting the output of the NOR circuit  345 , PMOS transistors  348  and  349 , and NMOS transistors  350  and  351 . The inverter  347  has a CMOS structure, which receives the power supply potential VII on the high-potential side and the ground potential VSS on the low-potential side. The sources of the PMOS transistors  348  and  349  are connected to the VPP line via which the boosted potential VPP is supplied. 
     The DRAM includes cell arrays  352  and  353 , and a sense amplifier part that is shared by the cell arrays  352  and  353 . NMOS transistors  355  through  357  form a bit line precharge circuit that precharges the bit lines BL and /BL to a precharge potential Vpr. A symbol PR is a bit line precharge signal, which controls ON/OFF of the NMOS transistors  355  through  357 . 
     A sense amplifier  358  includes PMOS transistors  359  and  360  and NMOS transistors  361  and  362 . A PMOS transistor  363  is turned ON/OFF by a sense amplifier driving signal PSA, and supplies the sense amplifier  358  with the power supply potential VII. An NMOS transistor is turned ON/OFF by another sense amplifier driving signal NSA, and supplies the sense amplifier  358  with the ground potential VSS. 
     NMOS transistors  365  and  366  are NMOS transistors that forms a column selection circuit. A symbol CL is a column selection signal, which controls ON/OFF of the NMOS transistors  365  and  366 . Symbols DB and /DB are data bus lines. 
     An isolation circuit  367  includes NMOS transistors  368  and  369  serving as isolation transistors that are turned ON/OFF by the sub isolation signal SIS 1 . An isolation circuit  370  includes NMOS transistors  371  and  372  serving as isolation transistors that are turned ON/OFF by the sub isolation signal SIS 2 . 
     When the cell array  352  is activated, the row block selection signal RBS 1  and RBS 2  are respectively set at VSS and VII, and the timing signal TM is set at VII, the column block signal CBS being set at VSS. 
     Thus, the main isolation signal MIS 1  is set at VII and the sub isolation signal generating circuit  334  operates as follows: the output of the NOR circuit  335  is equal to VSS; the output of the inverter  337  is VII; the NMOS transistor  340  is OFF; the NMOS transistor  341  is ON; the PMOS transistor  338  is ON; the PMOS transistor  339  is OFF; and the sub isolation signal SIS 1  becomes equal to VPP. The NMOS transistors  368  and  369  in the isolation circuit  367  are turned ON. 
     The main isolation signal MIS 2  is set at VSS, and the sub isolation signal generating circuit  344  operates as follows: the output of the NOR circuit  345  is VII; the output of the inverter  347  is VSS; the NMOS transistor  350  is ON; the NMOS transistor  351  is OFF; the PMOS transistor  348  is OFF; the PMOS transistor  349  is ON; and the sub isolation signal SIS 2  becomes equal to VSS. The NMOS transistors  371  and  372  in the isolation circuit  370  are turned OFF. As a result of the operation, the cell array  352  and the sense amplifier part  354  are electrically isolated, and the cell array  353  and the sense amplifier part  354  are electrically isolated. 
     In contrast, when the cell array  353  is activated, the row block selection signals RBS 1  and RBS 2  are respectively set at VII and VSS, and the timing signal TM is set at VII, the column selection signal CBS being set at VSS. 
     Thus, the main isolation signal MIS 1  is set at VSS and the sub isolation signal generating circuit  334  operates as follows: the output of the NOR circuit  335  is equal to VII; the output of the inverter  337  is VSS; the NMOS transistor  340  is ON; the NMOS transistor  341  is OFF; the PMOS transistor  338  is OFF; the PMOS transistor  339  is ON; and the sub isolation signal SIS 1  becomes equal to VSS. The NMOS transistors  368  and  369  in the isolation circuit  367  are turned OFF. 
     The main isolation signal MIS 2  is set at VII, and the sub isolation signal generating circuit  344  operates as follows: the output of the NOR circuit  345  is VSS; the output of the inverter  347  is VII; the NMOS transistor  350  is OFF; the NMOS transistor  351  is ON; the PMOS transistor  348  is ON; the PMOS transistor  349  is OFF; and the sub isolation signal SIS 2  becomes equal to VPP. The NMOS transistors  371  and  372  in the isolation circuit  370  are turned ON. As a result of the operation, the cell array  353  and the sense amplifier part  354  are electrically connected, while the cell array  352  and the sense amplifier part  354  are electrically isolated. 
     FIG. 11 is a circuit diagram of another circuit configuration of the sub isolation signal generating circuit employed in the first embodiment of the present invention. The circuit includes a level conversion circuit  373  that&#39;s converts the level of the main isolation signal MIS into another level. The circuit  373  includes an inverter  374  inverting the main isolation signal MIS, PMOS transistors  375  and  376 , and NMOS transistors  377  and  378 . The inverter  374  has a CMOS structure that receives the power supply potential VII on the high-potential side and the ground potential VSS on the low-potential side. The sources of the PMOS transistors  375  and  376  are connected to the VPP line via which the boosted potential VPP is supplied. 
     A level conversion circuit  379  converts the level of the column block selection signal CBS, and includes an inverter  380  inverting the column block selection signal CBS, PMOS transistors  381  and  382 , and NMOS transistors  383  and  384 . The inverter  380  has a CMOS structure that receives the external power supply potential VII on the high-potential side and the ground potential VSS on the low-potential side. The sources of the PMOS transistors  381  and  382  are connected to the VPP line via which the boosted potential VPP is supplied. 
     A NOR circuit  385  performs a NOR operation on the outputs of the level conversion circuits  373  and  379 . The NOR circuit  385  and the inverter  386  each have a CMOS structure that receives the boosted voltage VPP on the high-potential side and the ground potential VSS on the low-potential side. 
     When the isolation transistors are turned OFF by the sub isolation signal SIS, the main isolation signal MIS is set at VSS, and the column block selection signal CBS is set at VSS. 
     Thus, the level conversion circuit  373  operates as follows: the output of the inverter  374  is VII, the NMOS transistor  377  is OFF, the NMOS transistor  378  is ON, the PMOS transistor  375  is ON, the PMOS transistor  376  is OFF, and the output of the level conversion circuit  373  is VSS. 
     The level conversion circuit  379  operates as follows: the output of the inverter  380  is VII, the NMOS transistor  384  is ON, the NMOS transistor  383  is OFF, the PMOS transistor  382  is OFF, the PMOS transistor  389  is ON, and the output of the level conversion circuit  379  is VSS. 
     Thus, in the above case, the output of the NOR circuit  385  is VPP, the output of the inverter  386 , namely, the sub isolation signal SIS is VSS. As a result, the isolation transistors controlled by the sub isolation signal are turned OFF. 
     In contrast, when the isolation transistors controlled by the sub isolation signal SIS are turned ON, the main isolation signal MIS is set at VII. 
     Thus, the level conversion circuit  373  operates as follows: the output of the inverter  374  is VSS, the NMOS transistor  377  is ON, the NMOS transistor  378  is OFF, the PMOS transistor  375  is OFF, the PMOS transistor  376  is ON, and the output of the level conversion circuit  373  becomes equal to VPP. 
     Thus, in this case, the output of the NOR circuit  385  becomes equal to VSS, and the output of the inverter  386 , namely, the sub isolation signal SIS becomes equal to VPP. As a result, the isolation transistors controlled by the sub isolation signal SIS are turned ON. 
     As described above, according to the first embodiment of the present invention, only the high-potential-side potential of the sub isolation signal is set at the boosted potential VPP, while the high-potential-side potential of the main isolation signal is set at the step-down potential VII. It is therefore possible to reduce the number of signal lines that have the potential VPP on the high-potential side. 
     This results in reduction of consumption power of semiconductor devices that has the shared sense amplifier system and a hierarchical configuration in which the word drivers are hierarchically divided into the word drivers and sub word drivers and the isolation signal generating circuits are hierarchically divided into the main isolation signal generating circuits and the sub isolation signal generating circuits. 
     In a case where the boosted voltage VPP is needed for the potential of the bit line precharge signal PR on the high-potential side in the sub-isolation signal generating circuits configured as shown in FIG. 11, the sub isolation signal SIS can be used to generate the bit line precharge signal. The main isolation circuits may be configured so as to generate the main isolation signals that swing between the step-down potential VII obtained by stepping down the external power supply voltage VII and the ground potential VSS. 
     A description will be given, with reference to FIGS. 12 and 13, of a second embodiment of the present invention. 
     FIG. 12 is a circuit diagram of a part of a core part of the second embodiment of the present invention. In FIG. 12, main sub-word selection signal generating circuits  387 - 390  have a different circuit configuration from that of the main sub-word selection signal generating circuits  248 - 251  shown in FIG.  7 . Main isolation signal generating circuits  391  and  392  have a different circuit configuration from that of the corresponding circuits  260  and  261  shown in FIG.  7 . Sub isolation signal generating circuits  393 - 396  have a different circuit configuration from that of the corresponding circuits  262 - 265  shown in FIG.  7 . 
     In short, the second embodiment of the present invention substitutes the main sub-word selection signal generating circuits  387 - 390 , the main isolation signal generating circuits  391  and  392 , and the sub isolation signal generating circuits  393 - 396  for the corresponding circuits shown in FIG. 7, while the other parts of the second embodiment are the same as those of the DRAM shown in FIG.  7 . 
     In the second embodiment, the main sub-word selection signal generating circuits are configured so as to generate main sub-word selection signals that swing between the step-down potential VII on the high-potential side and the ground potential VSS on the low-potential side. The main isolation signal generating circuits are configured so as to generate main isolation signals that swing between the step-down potential VII on the high-potential side and the ground potential VSS on the low-potential side. The sub isolation signal generating circuits are configured so as to generate sub isolation signals that swing between the boosted potential VPP on the high-potential side and the ground potential VSS on the low-potential side in response to the main isolation signal that swings the step-down potential VII on the high-potential side and the ground potential VSS on the low-potential side. 
     Referring to FIG. 12, broken lines with arrows denote signal lines that carry signals that swing between the step-down potential VII on the high-potential side and the ground potential VSS on the low-potential side. Solid lines with arrows denote signals that carry signals that swing between the boosted potential VPP and the ground potential VSS. 
     More particularly, the signal lines swinging between the step-down potential VII and the ground potential VSS are used for the address signal lines  224 , the main sub-word selection signal lines that are the output lines of the main sub-word selection signal generating circuits  387 - 390 , and the main isolation signal lines that are the output lines of the main isolation signal generating circuits  391  and  392 . The signal lines swinging between the boosted potential VPP and the ground potential are used for the main word lines that are the output lines of the main word drivers  225 - 227 , the sub sub-word selection signal lines that are the output lines of the sub sub-word selection signal generating circuits  252 - 259 , the sub isolation signal lines that are the output lines of the sub isolation signal generating circuits  393 - 396 , and the bit-line precharge signal lines that are the output lines of the bit line precharge signal generating circuits  266  and  267 . 
     FIG. 13 is a circuit diagram of a circuit arranged in the SS cross areas of the core part in the second embodiment of the present invention. The circuit includes sub isolation signal generating circuits  397  and  398 , a bit line precharge signal generating circuit  399 , a sub sub-word selection signal generating circuit  400 , and a sub word driver  401 . The bit line precharge signal generating circuit  399 , the sub sub-word line signal generating circuit  400  and the sub word driver  401  are the same as the bit line precharge signal generating circuit  276 , the sub sub-word selection signal generating circuit  279 , and the sub word driver  286  shown in FIG.  8 . 
     The sub isolation signal generating circuit  397  includes an input circuit part  402 , which includes a PMOS transistor  403 , and NMOS transistors  404  and  405 . The input circuit part  402  operates with the step-down potential VII on the high-potential side and the ground potential VSS on the low-potential side. A PMOS transistor  406  serves as a level stabilization element, and the source thereof is connected to the VII line via which the step-down potential is supplied. 
     A level conversion circuit  407  converts the potential of the output of the input circuit part  402  on the high-potential side from the step-down potential VII to the boosted potential VPP. The circuit  407  includes an inverter  408  inverting the output of the input circuit part  402 , PMOS transistors  409  and  410 , and NMOS transistors  411  and  412 . The inverter  408  operates with the step-down potential VII on the high-potential side and the ground potential VSS on the low-potential side. The sources of the PMOS transistors  409  and  410  are connected to the VPP line via which the boosted potential is supplied. Inverters  413  and  414  operate with the boosted potential VPP on the on the high-potential side and the ground potential on the low-potential side. 
     According to the second embodiment of the present invention, only the potential of the sub sub-word selection signals on the high-potential side is set at the boosted potential VPP, while the potential of the main sub-word selection signals on the high-potential side is set at the step-down potential. It is therefore possible to reduce the number of signal lines that have the boosted potential VPP on the high potential side. 
     This results in reduction of consumption power of semiconductor devices that has the shared sense amplifier system and a hierarchical configuration in which the word drivers are hierarchically divided into the word drivers and sub word drivers and the isolation signal generating circuits are hierarchically divided into the main isolation signal generating circuits and the sub isolation signal generating circuits.