Patent Publication Number: US-7903479-B2

Title: Semiconductor memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. patent application Ser. No. 11/819,804 filed on Jun. 29, 2007 now U.S. Pat. No. 7,609,566 which claims priority of Korean patent application number 10-2007-0000409 filed on Jan. 03, 2007. The disclosure of each of the foregoing applications is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to semiconductor design technologies; and, more particularly, to a semiconductor memory device which is provided with an internal power generator capable of increasing a cell retention time during a self refresh mode and a refresh unit capable of minimizing power consumption by adjusting a refresh period depending on a level of an internal power. 
     Generally, basic operations of a semiconductor memory device include a write operation for storing data provided from outside and a read operation for outputting desired data stored therein upon request from the outside. In order to perform these write and read operations, the semiconductor memory device requires the capability of storing data from the outside. 
     In particular, since DRAM (Dynamic Random Access Memory) is a nonvolatile memory, data stored therein is lost after a fixed amount of time elapses. Therefore, a refresh operation is needed to fully restore data stored in memory cells before the fixed amount of time in order to prevent the loss of data. This refresh operation has priority over any other operations in DRAM. 
     Such a refresh operation is done at a fixed period that is closely concerned with a retention time of cell data. More details on this will be given below with reference to a circuit and a cross-sectional view of unit memory cell. 
       FIG. 1A  is a conceptual circuit diagram of unit memory cell of a conventional DRAM. For reference, the unit memory cell denotes a space where 1 bit data is stored. 
     Referring to  FIG. 1A , the unit memory cell is provided with a capacitor C 1  for storing data and an NMOS transistor NM 1  for accessing the capacitor. 
     To be more specific, a word line WL is connected to the gate end of the NMOS transistor NM 1 . And, a bit line BL is connected to one side active region (here, a drain end) of the NMOS transistor NM 1  and the cell capacitor to other side active region (here, a source end) thereof. 
       FIG. 1B  is a cross-sectional view of the unit memory cell of  FIG. 1A , and particularly shows a leakage current generated in the unit memory cell. 
     As shown in  FIG. 1B , although the transistor is turned off, a leakage current occurs in the cell capacitor (here, it is assumed that logic high data is stored in the cell capacitor). Two major factors of the leakage current are an off current I OFF  and a junction current I JUN . 
     Meanwhile, an internal power VBB with a negative electric potential lower than a ground voltage is generally connected to a bulk of the NMOS transistor in the unit memory cell shown in  FIG. 1A . By keeping the bulk bias low like this, the leakage current as shown in  FIG. 1B  is reduced by setting a threshold voltage of the transistor higher than that of a general NMOS. But, as the level of the internal power VBB lowers, the threshold voltage becomes higher to decrease off-current, while rather increasing the leakage current by elevation of junction-current. That is, the off-current and the junction-current constituting the leakage current have a trade-off relationship depending on the level of the internal power VBB. Therefore, in order to extend a retention time of cell data, it is important to find an optimal level of the internal power VBB making both the off-current and the junction-current smaller. 
     The following is a description for a conventional internal power generator for generating an internal power VBB to be applied to a bulk end of cell and a refresh unit for refreshing cell data. 
       FIG. 2  is a block diagram showing a configuration of a conventional internal power generator. 
     Referring to  FIG. 2 , the conventional internal power generator includes a charge pumping circuit  40  for negative-pumping an external power VDD to generate an internal power VBB with a level lower than the external power VDD, a reference voltage generator  50  for producing a reference voltage VINT_BB, a level sensor  10  for sensing a level of the internal power VBB on the basis of the reference voltage VINT_BB, an oscillator  20  for generating a period signal OSC in response to a sensing signal BBE of the level sensor  10 , and a pumping control signal generator  30  for controlling the operation of the charge pumping circuit  40  in response to the period signal OSC. 
     The reference voltage generator  50  is provided with a voltage generator  52  for generating a target voltage of the internal power VBB and a level shifter  54  for level-shifting an output voltage VREF of the voltage generator  52  to generate the reference voltage VINT_BB having a stable level regardless of the external power VDD (or power supply voltage). 
       FIG. 3  illustrates an internal circuit diagram of the level sensor  10  of  FIG. 2 . 
     Referring to  FIG. 3 , the level sensor  10  is composed of a voltage divider  12  for voltage-dividing a level difference between the ground voltage VSS and the reference voltage VINT_BB by a level difference between the ground voltage VSS and the internal power VBB, an inverter  14  for taking the reference voltage VINT_BB and the ground voltage VSS as driving powers and inverting and outputting an output voltage of the voltage divider  12 , a differential amplifier  16  taking an output voltage of the inverter  14  and an inverted voltage of the output of the inverter  14  as differential inputs, and an inverter I 1  for inverting an output voltage of the differential amplifier  16  to provide an inverted voltage as the sensing signal BBE. 
     Hereinafter, the operation of the internal power generator shown in  FIGS. 2 and 3  will be briefly described. 
     First, the level sensor  10  senses a level of a feedbacked internal power VBB on the basis of the reference voltage VINT_BB. At this time, when the level of the internal power VBB is higher than the reference voltage VINT_BB and the output voltage of the voltage divider  12  exceeds a logic threshold voltage of the inverter  14 , the sensing signal BBE is activated to a logic high level. 
     Then, the oscillator  20  is active by the sensing signal BBE to create the period signal OSC. In response to the period signal OSC, the pumping control signal generator  30  drives the charge pumping circuit  40 , which causes the level of the internal power VBB to drop. 
     When the level of the internal power VBB drops, the output voltage of the voltage divider  12  becomes lower than the logic threshold voltage of the inverter  14  which makes the sensing signal BBE inactivated to a logic low level. 
     Thus, the operations of the oscillator  20 , the pumping control signal generator  30  and the charge pumping circuit  40  are closed. 
     As described above, the internal power generator in the conventional semiconductor memory device is driven to maintain the internal power VBB in a level corresponding to the target level of the reference voltage. Here, as mentioned above, the level of the reference voltage is set such that the operation such as storage of data in write operation or restoration of data in read operation can be made within a designated time, while securing the retention time of data by reduction in leakage current. For reference, in order to secure the retention time of data, it is preferred that the level of the internal power VBB is as low as possible. But, if the level of the internal power VBB becomes lower, the threshold voltage becomes higher, which prolongs the operation time of data storage or restoration. 
     However, the conventional internal power generator is driven without considering IDD6 circumstance capable of securing a greater margin than an active mode as a driving time for restoration of data. That is, it was impossible to control the retention time of cell data to be extended for the refresh interval. 
     For reference, the IDD6 circumstance is a mode that is entered when the clock enable signal CLK is transited to a logic low level and again stores all cells by performing 8K number of times of refreshes for 64 ms. 
       FIG. 4  is a block diagram showing a configuration of a refresh unit included in the conventional semiconductor memory device. 
     Referring to  FIG. 4 , the conventional refresh unit includes a mode input/output controller  60  for accepting a clock enable signal CKE and an auto refresh command AREF_CMD and generating an internal auto refresh signal AREFP, a self refresh entry signal SREF_EN and a self refresh escape signal SREF_EXP, a refresh interval signal generator  70  for generating a self refresh interval signal SREF notifying a self refresh interval by using the internal auto refresh signal AREFP, the self refresh entry signal SREF_EN and the self refresh escape signal SREF_EXP, a refresh period signal generator  80  for periodically outputting a period-pulse signal PL_FLG during activation of the self refresh interval signal SREF, an internal refresh signal generator  90  for activating an internal refresh signal REFP in response to the internal auto refresh signal AREFP and the period-pulse signal PL_FLG, and an internal address counter  95  for increasing a row address by one bit unit in response to the internal refresh signal REFP to output an internal address RCNTI[0:N]. 
     For reference, the clock enable signal CKE is a signal representing whether a clock synchronizing the operation of the semiconductor memory device is valid or not. Thus, if only the clock enable signal CKE is inactivated, the semiconductor memory device enters a power-down mode for minimizing its own power consumption. 
       FIG. 5  shows an internal circuit diagram of the refresh period signal generator  80  of  FIG. 4 . 
     Referring to  FIG. 5 , the refresh period signal generator  80  includes an oscillator  82  which has an inverter chain and is active upon activation of the self refresh interval signal SREF to generate a signal OSC_OUT at regular intervals, and a pulse generator  84  for producing the period-pulse signal PL_FLG of pulse type from the output signal OSC_OUT of the oscillator  82 . 
     In brief operation, first of all, when the self refresh interval signal SREF is activated to a logic high level, the oscillator  82  generates the signal OSC_OUT at a regular interval. Here, the regular intervals are determined based on the voltage levels of signals applied to the gate ends of NMOS transistors and PMOS transistors constituting the inverter chain. Next, the pulse generator  84  senses a rising edge of the output signal OSC_OUT of the oscillator  82 , and generates the period-pulse signal PL_FLG of pulse type. 
       FIG. 6  presents an operational waveform diagram of the refresh unit according to the prior art shown in  FIGS. 4 and 5 . 
     As shown in  FIG. 6 , the clock enable signal CKE is first transited to a logic low level and at the same time the auto refresh command AREF_CMD is activated. Then, the mode input/output controller  60  activates the self refresh entry signal SREF_EN in response to the logic level transition of the clock enable signal CKE and activates the internal auto refresh signal AREFP in response to the auto refresh command AR. 
     Next, the internal refresh signal generator  90  generates the internal refresh signal REFP in response to the internal auto refresh signal AREFP. In succession, the internal address generator  95  increases the row address by one bit unit whenever the internal refresh signal REFP is activated, to output the internal address RCNTI[0:N]. 
     Further, the refresh interval signal generator  70  activates the self refresh interval signal SREF in response to activation of the internal auto refresh signal AREFP and the self refresh entry signal SREF_EN, wherein this activation is maintained until the self refresh escape signal SREF_EXP is applied. 
     Subsequently, the refresh period signal generator  80  periodically activates the period-pulse signal PL_FLG during the activation of the self refresh interval signal SREF. And then, the internal refresh signal generator  90  activates a new internal refresh signal REFP of pulse type whenever the period-pulse signal PL_FLG is applied. Lastly, the internal address generator  95  increases the row address by one bit unit whenever the internal refresh signal REFP is activated, to output the internal address RCNTI[0:N]. 
     For reference, the internal refresh signal REFP is applied to each bank, which makes a word line corresponding to the internal address RCNTI[0:N] active to perform self refresh. 
     Meanwhile, the refresh period by the refresh unit in the conventional semiconductor memory device is determined by the period of the period-pulse signal PL_FLG. The period-pulse signal PL_FLG is generated at a regular period, regardless of the level of the internal power VBB. Therefore, although the level of the internal power VBB is optimized so that the retention time is reduced, it is unlikely to reflect the above. This reduces the number of times of refresh and thus cannot decrease power consumption. 
     Therefore, the conventional semiconductor memory device does not adjust the level of the internal power under the self refresh mode, thereby making it impossible to adjust the retention time of cell data. Also, the refresh unit cannot be driven appropriately according to the retention time. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are directed to provide a semiconductor memory device for reducing a current consumption by adjusting a refresh period according to a bulk voltage level. 
     In accordance with an aspect of the present invention, there is provided a semiconductor memory device including a charge pumping circuit, a level sensor, an oscillator, and a pumping control signal generator. The charge pumping circuit performs a negative-pumping operation to an external power in order to generate an internal voltage having a level lower than the external power. The level sensor senses a level of the internal voltage corresponding to a level of an adjusted reference voltage during a refresh mode. The oscillator generates a period signal in response to a sensing signal of the level sensor. The pumping control signal generator controls the operation of the charge pumping circuit in response to the period signal. 
     In accordance with another aspect of the present invention, there is provide driving method of a semiconductor memory device including performing a negative-pumping operation to an external power in order to generate a bulk voltage of memory cell; sensing a level of the bulk voltage in response to a reference voltage; and controlling the negative-pumping operation so that the bulk voltage has a level corresponding to the reference voltage. The reference voltage have different voltage levels for a self refresh mode and a normal mode. 
     In accordance with a further another aspect of the present invention, there is provided a semiconductor memory device including a refresh exit/entry controller and a control signal generator. The refresh exit/entry controller receives a clock enable signal and an auto refresh command and generates a self refresh interval signal notifying that the current operation is in a self refresh mode. The control signal generator periodically generates an internal refresh signal and an internal address for refresh driving during activation of the self refresh interval signal. An activation period of the internal refresh signal is adjusted according to a level of a bulk voltage applied to a memory cell. 
     In accordance with still another aspect of the present invention, there is provided a driving method of a semiconductor memory device, including operating the semiconductor memory device in a self refresh mode by activating a self refresh interval signal in response to a clock enable signal and an auto refresh command; and generating an internal refresh signal and an internal address for controlling refresh driving at predetermined intervals during an activation of the self refresh interval signal. The self refresh interval signal notifies that a current operation is performed in the self refresh mode. The predetermined intervals are adjusted according to a level of a bulk voltage applied to a memory cell. 
     In accordance with still another aspect of the present invention, there is provided a semiconductor memory device, including an internal power generator, a refresh unit, a level controller, a period adjustor. The internal power generator performs a negative-pumping operation to an external power in order to generate a bulk voltage to be applied to a bulk end of cell. The refresh unit generates an internal refresh signal for refresh driving at regular intervals during a self refresh mode. The level controller controls a level of the bulk voltage in the self refresh mode. The period adjustor adjusts the regular intervals when the level controller is driven. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a conceptual circuit diagram of unit memory cell of a conventional DRAM. 
         FIG. 1B  is a cross-sectional view of the unit memory cell of  FIG. 1A , and particularly shows a leakage current generated in the unit memory cell. 
         FIG. 2  is a block diagram showing a configuration of a conventional internal power generator. 
         FIG. 3  is an internal circuit diagram of the level sensor of  FIG. 2 . 
         FIG. 4  is a block diagram showing a configuration of a refresh unit in a conventional semiconductor memory device. 
         FIG. 5  is an internal circuit diagram of the refresh period signal generator of  FIG. 4 . 
         FIG. 6  is an operational waveform diagram of the refresh unit according to the prior art shown in  FIGS. 4 and 5 . 
         FIG. 7  is a block diagram illustrating a configuration of a semiconductor memory device in accordance with a preferred embodiment of the present invention. 
         FIG. 8  is an internal circuit diagram of the internal voltage generator of  FIG. 7 . 
         FIG. 9  is an internal circuit diagram of the level sensor of  FIG. 8 . 
         FIG. 10  is an internal circuit diagram of the refresh unit of  FIG. 7 . 
         FIG. 11  is an internal circuit diagram of the refresh period signal generator of  FIG. 10 . 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that the invention can be easily carried out by those skilled in the art. 
       FIG. 7  is a block diagram illustrating a configuration of a semiconductor memory device in accordance with a preferred embodiment of the present invention. 
     Referring to  FIG. 7 , the inventive semiconductor memory device includes an internal power generator  100  for negative-pumping an external power to generate an internal power VBB to be applied to a bulk end of cell, a refresh unit  500  for generating an internal refresh signal REFP for refresh driving at regular intervals during a self refresh mode, a level adjustor  240  for adjusting a reference level of the internal power VBB in the self refresh mode, and a period adjustor  722  for adjusting an interval of the internal refresh signal when the level adjustor  240  is driven. 
     As stated above, the semiconductor memory device of the invention additionally includes the level adjustor  240  adjusts the level of the internal power VBB during the self refresh mode, and adjusts the level of the internal power VBB so that a minimum leakage current is generated from memory cell during the self refresh mode. Further, the semiconductor memory device additionally includes the period adjustor  722  for adjusting an execution interval of refresh by sensing the level adjustment of the internal power VBB, to thereby reduce the number of times of refresh performed during the self refresh mode. Thus, IDD6, that is, power consumed during the self refresh mode can be reduced. 
     The following is a detailed description for an internal circuit diagram of each block with reference to the drawings. 
       FIG. 8  is an internal circuit diagram of the internal voltage generator  100  of  FIG. 7 . 
     Referring to  FIG. 8 , the internal power generator  100  is provided with a level sensor  200  for sensing a level of the internal power VBB corresponding to a level of an adjusted reference voltage VINT_BB during the refresh mode, and a power supplier  300  for negative-pumping an external power VDD under the control of the level sensor  200  to generate the internal power VBB having a level lower than the external power VDD. 
     More specifically, the power supplier  300  is composed of a charge pumping circuit  360  for negative-pumping the external power VDD to generate the internal power VBB having a level lower than the external power VDD, an oscillator  320  for generating a period signal OSC in response to a sensing signal BBE of the level sensor  200 , and a pumping control signal generator  340  for controlling the operation of the charge pumping circuit  360  in response to the period signal OSC. 
     For reference, the internal power generator  100  is further provided with a reference voltage generator  400  for generating the reference voltage VINT_BB having a stable level regardless of the external power VDD. The reference voltage generator  400  is composed of a voltage generator  420  for producing a target voltage of the internal power VBB and a level shifter  440  for level-shifting an output voltage of the voltage generator  420  to create the reference voltage VINT_BB having a stable level regardless of the external power (power supply voltage) VDD. 
       FIG. 9  is an internal circuit diagram of the level sensor  200  of  FIG. 8 . 
     Referring to  FIG. 9 , the level sensor  200  is provided with a feedback circuit  220  for providing a level of the internal power VBB on the basis of the reference voltage VINT_BB as a feedback voltage, a level controller  40  for controlling a level of the feedback voltage during the self refresh mode, and a differential amplifier  260  for taking the feedback voltage and its inverted feedback voltage as differential inputs to output the sensing signal BBE. 
     The level controller  240  is composed of an NAND gate ND 1  taking a self refresh interval signal SREF notifying that the current operation is in the self refresh mode and a first level adjustment signal VBB_UPP, an inverter I 1  for inverting an output signal of the NAND gate ND 1  to provide an up-control signal CRT_UP, an NAND gate ND 2  taking the self refresh interval signal SREF and a second level adjustment signal VBB_DN, and an inverter I 2  for inverting an output signal of the NAND gate ND 2  to output a down-control signal CRT_DN. 
     The feedback circuit  220  is composed of first to fourth PMOS transistors whose each gate takes a ground voltage VSS and which are connected in series between the reference voltage VINT_BB and an output node, fifth to eighth PMOS transistors whose each gate takes the internal voltage VBB and which are connected in series between the output node and the ground voltage, a first NMOS transistor NM 2  whose gate accepts the down-control signal CRT_DN and which is connected in parallel with the first PMOS transistor, and a second NMOS transistor NM 3  whose gate receives the up-control signal CRT_UP and which is connected in parallel with the eighth PMOS transistor. 
     Now, the operation of the level sensor  200  will be briefly described. 
     First, in the self refresh mode, when the self refresh interval signal SREF notifying that the current operation is in the self refresh mode is activated, the level controller  240  is active to output any activated one of the first and the second level control signals VBB_UP and VBB_DN as the up- or down-control signal CTR_UP or CTR_DN. Here, if the first level adjustment signal VPP_UP is assumed to be set, the up-control signal CRT_UP is activated. 
     Then, the second NMOS transistor NM 3  in the feedback circuit  220  is active by the up-control signal CRT_UP, and therefore, although the level of the internal power VBB is the same, the level of the feedback voltage taken at the output end drops. Thus, the point of time at which the sensing signal BBE is activated is higher than that in the normal mode, so that the level of the internal power VBB rises. 
     Meanwhile, when the down-control signal CTR_DN is activated, the first NMOS transistor NM 2  in the feedback circuit  220  is active and the level of the feedback voltage at the output end rises. Thus, the level of the internal power VBB becomes lower than that in the normal mode. 
     As mentioned above, the internal power generator  100  including the level sensor  200  adjusts the level of the internal power VBB based on the level adjustment signals VBB_UP and VBB_DN during the refresh mode, and then outputs an adjusted internal power. That is, leakage current can be minimized by adjusting the level of the internal power VBB. 
     More concretely, in the leakage current generated in the memory cell, an off-current or junction-current has relatively large importance depending on the properties of product. In case an off-current is great, a retention time of cell data can be extended by lowering the level of the internal power VBB through the down-control signal CTR_DN during the refresh. Similarly, in case a junction-current is great, a retention time of cell data can be prolonged by elevating the level of the internal power VBB through the up-control signal CTR_UP. 
     For reference, the level adjustment signals VBB_UP and VBB_DN are set in the test step of wafer level so that the leakage current can be minimized. After this test procedure, the level-adjustment signals VBB_UP and VBB_DN are set and applied through fuse option or metal option. 
     As described above, the internal power generator adjusts the level of the internal power VBB during the self refresh mode, thereby decreasing leakage current of cell data. Accordingly, the retention time is prolonged. 
     Meanwhile, if the retention time of data is increased, the number of times of refresh can be decreased during the self refresh mode, thereby reducing IDD6 power consumption. 
     The following is a description for the refresh unit  500  capable of adjusting the period of the self refresh on the basis of the adjustment of the internal power VBB. For reference, the period adjustor  722  shown in  FIG. 7  may be included in the refresh unit. 
       FIG. 10  is an internal circuit diagram of the refresh unit  500  depicted in  FIG. 7 . 
     Referring to  FIG. 10 , the refresh unit  500  includes a refresh exit/entry controller  600  for generating the self refresh interval signal SREF notifying that the current operation is in the self refresh mode and the internal auto refresh signal AREFP in response to the clock enable signal CKE and the auto refresh command AREF_CMD, and a control signal generator  700  and  800  for periodically generating an internal refresh signal REFP and an internal address RCNTI[0:N] during activation of the self refresh interval signal SREF, wherein an activation period of the internal refresh signal REFP is varied upon variation of the level of the internal power VBB applied to a memory cell. 
     To be more specific, the refresh exit/entry controller  600  is provided with a mode input/output controller  620  for taking the clock enable signal CKE and the auto refresh signal AREF_CMD and generating the internal auto refresh signal AREFP, a self refresh entry signal SREF_EN and a self refresh escape signal SREF_EXP, and a refresh interval signal generator  640  for generating the self refresh interval signal SREF notifying a self refresh interval based on the internal auto refresh signal AREFP, the self refresh entry signal SREF_EN and the self refresh escape signal SREF_EXP. 
     The control signal generator  700  and  800  is provided with a refresh period signal generator  720  for periodically outputting the period pulse signal PL_FLG during activation of the self refresh interval signal SREF of which period is adjusted depending on the level of the internal power, an internal refresh signal generator  740  for activating the internal refresh signal REFP in response to the internal auto refresh signal AREFP and the period pulse signal PL_FLG, and an internal address counter  800  for increasing a row address by one bit unit in response to the internal refresh signal REFP to output the internal address RCNTI[0:N]. 
     As mentioned early, in case there is a variation of the level of the internal power VBB determining the retention time of cell data, the refresh unit  500  of the invention reflects the above and adjusts the period of the internal refresh signal REFP. Thus, the period can be delayed by the retention time of cell data, thereby reducing IDD6 power consumption. 
       FIG. 11  is an internal circuit diagram of the refresh period signal generator  720  of  FIG. 10 . 
     Referring to  FIG. 11 , the refresh period signal generator  720  is constituted by the period adjustor  722  for generating a period adjustment signal in response to the first and the second level adjustment signals VBB_UP and VBB_DN, an oscillator  724  for adjusting the period of the period signal OSC_OUT during activation of the self refresh interval signal SREF in response to the period adjustment signal, and a pulse generator  726  for generating the period-pulse signal PL_FLG of pulse type from the period signal OSC_OUT. 
     More specifically, the period adjustor  722  has a NOR gate NR 1  for taking the first and the second level adjustment signals VBB_UP and VBB_DN and outputting the period adjustment signal. 
     The oscillator  724  is provided with a driving voltage supplier  724   a  for adjusting a level of a driving voltage in response to the period adjustment signal to supply an adjusted driving voltage and an inverter chain  724   b  for producing the period signal OSC_OUT with a period corresponding to the level of the driving voltage during activation of the self refresh interval signal SREF. 
     The driving voltage supplier  724   a  is composed of a plurality of resistors coupled in series between the external voltage VDD and the ground voltage VSS and an NMOS transistor NM 4  whose gate receives the period adjustment signal and which is coupled in parallel with one of the plurality of resistors, wherein divided voltages are provided as a first and a second driving voltages. 
     The pulse generator  726  is composed of a delay circuit  726   a  for delaying the period signal OSC_OUT, an inverter  13  for inverting an output signal of the delay circuit  726   a , an NAND gate ND 3  taking an output signal of the inverter I 3  and the period signal OSC_OUT, and an inverter I 4  for inverting an output signal of the NAND gate ND 3  to provide the period pulse signal PL_FLG. 
     Now, the operation of the refresh period signal generator  720  will be briefly explained. 
     First, when the first and the second level adjustment signals VBB_UP and VBB_DN are all inactivated, the period adjustment signal is inactivated to a logic high level. Thus, the resistor, which is coupled in parallel with the NMOS transistor NM 4  that is under the control of the period adjustment signal in the driving voltage supplier  724   a , does not affect the total resistance value. 
     On the other hand, when one of the first and the second level adjustment signals VBB_UP and VBB_DN is activated, the period adjustor  722  activates the period adjustment signal to a logic low level. Then, the NMOS transistor NM 4  in the driving voltage supplier  724   a  is turned off, which increases the total resistance value by the resistor coupled in parallel with it. Thus, the amount of current supplied from the driving voltage supplier  724   a  is decreased and the period of the period signal OSC_OUT generated by the inverter chain  724   b  is extended. In succession, the period of the period pulse signal PL_FLG created by the pulse generator  726  is also prolonged. 
     As such, the increase in period of the period pulse signal PL_FLG means an increase in interval of the internal refresh signal REFP generated in response to it, and a small number of times of refresh is performed. 
     In other words, the level adjustment signals VBB_UP and VBB_DN are signals that are set to adjust the level of the internal power VBB so that the retention time of cell data is prolonged. Thus, the refresh unit  500  of the invention including the refresh period signal generator  720  senses the state that the internal power VBB is adjusted by the level adjustment signals VBB_UP and VBB_DOWN and the retention time of cell data is prolonged, and then extends the refresh period. The number of times of refresh executed during the self refresh mode is reduced, which decreases IDD6 power consumption. 
     Therefore, the semiconductor memory device of the invention set forth above adjusts the level of the internal power VBB so that the retention time of cell data is prolonged during the self refresh mode where only the refresh operation is conducted at regular intervals without any commands from outside. Further, in case the level of the internal power VBB is adjusted, the refresh interval is extended, thereby decreasing IDD6 power consumption. 
     As a result, the present invention adjusts the level of the internal power VBB (bulk voltage applied to cell) so that the retention time of cell data is prolonged during the self refresh mode and thus extends the refresh interval, thereby decreasing power being consumed during the self refresh mode. 
     While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.