Abstract:
A voltage generator includes a bias signal generator generating first to fourth bias signals using a reference voltage, the first to fourth bias signals having different voltage levels. A driving signal generator receives the first and third bias signals to generate a pull-up signal in response to a voltage level of an output terminal and receiving the second and fourth bias signals to generate a pull-down signal in response to a voltage level of the output terminal. A voltage driver pulls up and pulls down a voltage level of the output terminal in response to the respective pull-up and pull-down signals. An auxiliary driving controller disables the pull-up signal when the voltage level of the output terminal is greater than that of the reference voltage and the pull-down signal when the voltage level of the output terminal is less than that of the reference voltage.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a voltage generator, and more particularly, to a voltage generator that can stably drive a bit line precharge voltage or cell plate voltage in a low power supply voltage condition and minimize a standby current and an operation current.  
       BACKGROUND  
       [0002]     Generally, semiconductor memory devices often have low drivability due to conditions related to process changes. In such a case, drivability of a voltage is also decreased, causing a large change in internal voltages. The change in the internal voltages results in erroneous operation of semiconductor memory devices.  
         [0003]     As semiconductor memory devices are being highly integrated, process conditions are also changing to a great extent. Thus, a core voltage decreases, and this decreasing core voltage leads to decrease in drivability of a bit line precharge voltage and a cell plate voltage.  
         [0004]      FIG. 1  is a circuit diagram of a conventional voltage generator designed to generate a bit line precharge voltage.  
         [0005]     The conventional voltage generator includes a core voltage controller  10  and a voltage driver  20 . The core voltage controller  10  includes a core voltage generation block  11 , a bias voltage generation block  12  and a gate voltage generation block  13 .  
         [0006]     The core voltage generation block  11  generates one half of a core voltage VCORE that becomes a reference voltage of a bit line precharge voltage VBLP or a cell plate voltage (not shown). The core voltage generation block  11  includes P-type channel metal-oxide semiconductor (PMOS) transistors P 1  and P 2  and resistors R 1  and R 2 . The PMOS transistors P 1  and P 2  and the resistors R 1  and R 2  are connected in series between a terminal of the core voltage VCORE and a terminal of a ground voltage VSS. A reference voltage VREF is generated by a voltage divider using resistance from a self-bias diode and resistance from lines.  
         [0007]     When a power supply voltage is supplied from an external source, the voltage divider illustrated in  FIG. 1  is used to generate a power voltage. However, when the power supply voltage is generated within an internal device, the reference voltage VREF can be generated through a reference voltage generator from another apparatus.  
         [0008]     The bias voltage generation block  12  generates bias voltages PBIAS and NBIAS using the reference voltage VREF. The bias voltage generation block  12  includes PMOS transistors P 3  to P 6  and N-type channel metal-oxide semiconductor (NMOS) transistors N 1  to N 6 . The PMOS transistor P 3  and the NMOS transistors N 1  and N 3  are connected in series between the terminal of the core voltage VCORE and the terminal of the ground voltage VSS, and thus, current consistently flows to the terminal of the ground voltage VSS. The reference voltage VREF is supplied to a gate of the PMOS transistor P 3 , and a gate and one terminal of the NMOS transistor N 1  are connected with each other, and the same connection is applied to the NMOS transistor N 3 .  
         [0009]     The PMOS transistor P 4  and the NMOS transistors N 2  and N 4  are connected in series between the terminal of the core voltage VCORE and the terminal of the ground voltage VSS, thereby being configured as in a current mirror circuit. Due to this configuration, current flows consistently to the terminal of the core voltage VCORE. A gate and one terminal of the PMOS transistor P 4  are connected with each other, and gates of the NMOS transistors N 1  and N 2  are connected with each other. A gate of the NMOS transistor N 3  is connected with a gate of the NMOS transistor N 4 . Due to this connection architecture, the same current flows to the NMOS transistors N 2  and N 4 .  
         [0010]     The PMOS transistor P 5  is connected between the terminal of the core voltage VCORE and an NMOS transistor N 7 . Gates of the PMOS transistors P 4  and P 5  are connected together, forming a current mirror circuit. The PMOS transistor P 6  is connected between the terminal of the core voltage VCORE and an NMOS transistor N 8 , and the bias voltage PBIAS is supplied to a gate of the PMOS transistor P 6 . The NMOS transistor N 5  is connected between the terminal of the ground voltage and the PMOS transistor P 7 , and the bias voltage NBIAS is supplied to a gate of the NMOS transistor N 5 . The NMOS transistor N 6  is connected between the terminal of the ground voltage and the PMOS transistor P 8 , and the bias voltage NBIAS is supplied to a gate of the NMOS transistor N 6 .  
         [0011]     The gate voltage generation block  13  includes the NMOS transistors N 7  and N 8  and PMOS transistors P 7  and P 8 . A gate voltage NGATE is supplied to gates of the NMOS transistors N 7  and N 8  that are connected with each other. A gate voltage PGATE is supplied to gates of the PMOS transistors P 7  and P 8  that are connected commonly with each other. That is, the NMOS transistors N 7  and N 8  and the PMOS transistors P 7  and P 8  are configured as a current mirror circuit. The gate voltage generation block  13  generates the gate voltages NGATE and PGATE. The gate voltage NGATE has a voltage level greater than the reference voltage VREF by a voltage level of a threshold voltage of the NMOS transistor N 7 . The gate voltage PGATE has a voltage level less than the reference voltage REF by a voltage level of a threshold voltage of the PMOS transistor P 7 .  
         [0012]     The voltage driver  20  includes a PMOS transistor P 9  and an NMOS transistor N 9 . The PMOS transistor P 9  and the NMOS transistor N 9  are connected in series between the terminal of the core voltage VCORE and the terminal of the ground voltage VSS. A Pull-up signal PDRV and a pull-down signal NDRV are supplied to respective gates of the PMOS transistor P 9  and the NMOS transistor N 9 . A bit line precharge voltage VBLP is output through a common terminal between the PMOS transistor P 9  and the NMOS transistor N 9 .  
         [0013]      FIG. 2  is a voltage waveform diagram of the conventional voltage generator illustrated in  FIG. 1 .  
         [0014]     The PMOS transistor P 6  operates due to a turn-on resistance whose value is close to a threshold voltage, thereby allowing current to flow consistently. Therefore, since the PMOS transistor P 6  operates usually all the time, the turn-on resistance is set high. As a voltage level of the bit line precharge voltage VBLP changes, the NMOS transistor N 8  operates like a source follower. Thus, the NMOS transistor N 8  operates rapidly.  
         [0015]     If a voltage level of the bit line precharge voltage VBLP decreases, voltage levels of the gate voltage NGATE of the NMOS transistor N 8  and the bit line precharge voltage VBLP increase. Thus, current flows rapidly to the NMOS transistor N 8 , and this rapid current flow causes the voltage level of the pull-up signal PDRV to decrease. As a result, the PMOS transistor P 9  turns on, resulting in increase in the voltage level of the bit line precharge voltage VBLP.  
         [0016]     The NMOS transistor N 6  operates due to a turn-on resistance whose value is close to the threshold voltage. Therefore, since the NMOS transistor N 6  operates usually all the time, the turn-on resistance is set high. As a voltage level of the bit line precharge voltage VBLP changes, the PMOS transistor P 8  operates like a source follower. Thus, the PMOS transistor P 8  operates rapidly.  
         [0017]     If a voltage level of the bit line precharge voltage VBLP increases, voltage levels of the gate voltage PGATE of the PMOS transistor P 8  and the bit line precharge voltage VBLP increase. Thus, current flows rapidly to the PMOS transistor P 8 , and this rapid current flow causes the voltage level of the pull-down signal NDRV to increase. As a result, the NMOS transistor N 9  turns on, resulting in decrease in the voltage level of the bit line precharge voltage VBLP.  
         [0018]     The conventional voltage generator is used to improve the drivability. The PMOS transistor P 9  and the NMOS transistor N 9 , having a very low threshold voltage, are included in the voltage driver  20  to increase the drivability of the last terminal. This configuration improves reading and writing operations in an active state; however, when in a precharge state, current is more likely to leak.  
         [0019]     In detail, if a threshold voltage level of the PMOS transistor P 9  is less than a target voltage level, a standby current is generated due to a large amount of the off-state leakage current. The standby current may result in negative effects. For instance, the standby current may be an issue in low power or mobile products.  
         [0020]     Therefore, if threshold voltages of the PMOS transistor P 9  and the NMOS transistor N 9  are lowered to secure an operation region of the last driver terminal, the drivability can be improved, but severe damage may arise in respect of the standby current.  
         [0021]     Also, if the bit line precharge voltage VBLP is not stable or the voltage generator operates during a standby mode, the PMOS transistor P 8  operates like a source follower. Thus, the voltage driver  20  turns on fast, and turns off slowly since a minimum amount of current is supplied to reduce the standby current.  
         [0022]     Accordingly, two points of turning on and turning off the last driver terminal are often mismatched. As a result, there may be a case that the PMOS transistor P 8  and the NMOS transistor N 9  turn on simultaneously, resulting in generation of a direct current.  
         [0023]     During the operation, the standby current and the direct current are likely to be generated. Thus, a ringing current may be generated during the standby mode and the operation mode, further decreasing the drivability of semiconductor memory devices.  
       SUMMARY OF THE INVENTION  
       [0024]     It is, therefore, an object of the present invention to provide a voltage generator that can stably drive a bit line precharge voltage or a cell plate voltage with a low power supply voltage and minimize a standby current and an operation current by placing PMOS and NMOS transistors that have a low threshold voltage at a driver terminal and controlling a voltage driver of the last terminal to turn on and turn off substantially for the same time.  
         [0025]     In accordance with an aspect of the present invention, there is provided a voltage generator, including: a bias signal generator generating first to fourth bias signals using a reference voltage having a voltage level that is one half of a power supply voltage, the first to fourth bias signals having a different voltage level, the first bias signal having a voltage level greater than that of the reference voltage by a predetermined voltage level, the second bias signal having a voltage level less than that of the reference voltage by a predetermined voltage level; a driving signal generator receiving the first and third bias signals to generate a pull-up signal in response to a voltage level of an output terminal and receiving the second and fourth bias signals to generate a pull-down signal in response to a voltage level of the output terminal; a voltage driver pulling up and pulling down a voltage level of the output terminal in response to the pull-up signal and the pull-down signal, respectively; and an auxiliary driving controller disabling the pull-up signal when the voltage level of the output terminal is greater than that of the reference voltage and the pull-down signal when the voltage level of the output terminal is less than that of the reference voltage in response to the first and second bias signals and the voltage level of the output terminal.  
         [0026]     In accordance with another aspect of the present invention, there is provided a voltage generator, including: a bias signal generator generating first to fourth bias signals using a reference voltage having a voltage level that is one half of a power supply voltage, the first to fourth bias signals having a different voltage level, the first bias signal having a voltage level greater than that of the reference voltage by a predetermined voltage level, the second bias signal having a voltage level less than that of the reference voltage by a predetermined voltage level; a driving signal generator receiving the first and third bias signals to generate a pull-up signal in response to a voltage level of an output terminal and receiving the second and fourth bias signals to generate a pull-down signal in response to a voltage level of the output terminal; a voltage driver pulling up and pulling down a voltage level of the output terminal in response to the pull-up signal and the pull-down signal, respectively; and an auxiliary driver supportively pulling up a voltage level of the output terminal when the voltage level of the output terminal is less than that of the reference voltage and pulling down a voltage level of the output terminal when the voltage level of the output terminal is greater than that of the reference voltage.  
         [0027]     In accordance with a further another aspect of the present invention, there is provided a voltage generator, including: a bias signal generator generating first to fourth bias signals using a reference voltage having a voltage level that is one half of that of a power supply voltage, the first to fourth bias signals having a different voltage level, the first bias signal having a voltage level greater than the reference voltage by a predetermined voltage level, the second bias signal has a voltage level less than the reference voltage by a predetermined voltage level; a driving signal generator receiving the first and third bias signals to generate a pull-up signal in response to a voltage level of an output terminal and receiving the second and fourth bias signals to generate a pull-down signal in response to a voltage level of the output terminal; a PMOS transistor pulling up a voltage level of the output terminal in response to the pull-up signal; an NMOS transistor pulling down a voltage level of the output terminal in response to the pull-down signal; a first multiplexer selectively supplying one of the power supply voltage and a voltage having a voltage level greater than that of the power supply voltage as a substrate bias voltage of the PMOS transistor in response to an active signal; and a second multiplexer selectively supplying one of a ground voltage and a voltage having a voltage level less than that of the ground voltage as a substrate bias voltage of the NMOS transistor in response to the active signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The above and other objects and features of the present invention will become better understood with respect to the following description of the exemplary embodiments given in conjunction with the accompanying drawings, in which:  
         [0029]      FIG. 1  is a circuit diagram of a conventional voltage generator;  
         [0030]      FIG. 2  is a voltage waveform diagram of the conventional voltage generator;  
         [0031]      FIG. 3  is a circuit diagram of a voltage generator in accordance with an embodiment of the present invention;  
         [0032]      FIG. 4  is a circuit diagram of a voltage generator in accordance with another embodiment of the present invention;  
         [0033]      FIG. 5  is a voltage waveform diagram of the voltage generator according to the embodiment of the present invention;  
         [0034]      FIG. 6  is a circuit diagram of a voltage generator in accordance with still another embodiment of the present invention;  
         [0035]      FIG. 7  illustrates a circuit diagram of a voltage generator in accordance with a further another embodiment of the present invention; and  
         [0036]      FIG. 8  illustrates an operation timing diagram of the voltage generator illustrated in  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]     A voltage generator in accordance with exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0038]      FIG. 3  illustrates a circuit diagram of a voltage generator in accordance with an embodiment of the present invention. Herein, like reference numerals denote like elements described in  FIG. 1 .  
         [0039]     The voltage generator includes a core voltage controller  10 , an auxiliary driving controller  100  and a voltage driver  110 . Since the core voltage controller  10  according to the present embodiment is substantially the same as the core voltage controller  10  described in  FIG. 1 , detailed description thereof will be omitted. However, for better understanding, the core voltage controller  10  in the present invention is be divided into two parts including a bias signal generator that generates four bias voltages PBIAS, NGATE, PGATE, and NBIAS and a driving signal generator that generates pull-up and pull-down signals PDRV and NDRV.  
         [0040]     The auxiliary driving controller  100  includes PMOS transistors P 10  to P 12  and NMOS transistors N 10  to N 12 . The PMOS transistor P 10  is connected between a terminal of a core voltage VCORE and the NMOS transistor N 10 . A gate of the PMOS transistor P 10  is connected with a gate of the PMOS transistor P 11 . The PMOS transistor P 11  is connected between the terminal of the core voltage VCORE and an output node A.  
         [0041]     The NMOS transistor N 10  is connected between the PMOS transistor P 10  and an output terminal for a bit line precharge voltage VBLP. The bias voltage NGATE is supplied to a gate of the NMOS transistor N 10 . The PMOS transistor P 12  is connected between the NMOS transistor N 11  and the output terminal for the bit line precharge voltage VBLP. The bias voltage PGATE is supplied to a gate of the PMOS transistor P 12 .  
         [0042]     The NMOS transistor N 11  is connected between a terminal of a ground voltage VSS and the PMOS transistor P 12 . A gate of the NMOS transistor N 11  is connected with a gate of the NMOS transistor N 12 . The NMOS transistor N 12  is connected between the terminal of the ground voltage VSS and an output node B.  
         [0043]     The voltage driver  110  includes a PMOS transistor P 13  and an NMOS transistor N 13 . The PMOS transistor P 13  and the NMOS transistor N 13  are connected in series between the terminal of the core voltage VCORE and the terminal of the ground voltage. The pull-up signal PDRV is supplied to a gate of the PMOS transistor P 13 , while the pull-down signal NDRV is supplied to a gate of the NMOS transistor N 13 . The bit line precharge voltage VBLP is output through a common terminal between the PMOS transistor P 13  and the NMOS transistor N 13 .  
         [0044]     Hereinafter, operation of the voltage generator according to the present invention will be described.  
         [0045]     First, the four bias signals PBIAS, NGATE, PGATE, and NBIAS have a different voltage level. More particularly, the bias signal NGATE has a voltage level greater than that of the reference voltage by a predetermined voltage level, and the bias signal PGATE has a voltage level less than that of the reference voltage by a predetermined voltage level. The bias voltage PBIAS has a voltage level close to a voltage difference between the core voltage VCORE and a threshold voltage of a PMOS transistor P 6 . The bias voltage PBIAS consistently supplies a gate voltage to the PMOS transistor P 6 , so that a consistent amount of current flows. Also, the bias voltage NBIAS has a voltage level close to an added voltage value of the ground voltage VSS and a threshold voltage of an NMOS transistor N 6 . The bias voltage NBIAS consistently supplies a gate voltage to the NMOS transistor N 6 , so that a consistent amount of current flows.  
         [0046]     As the bit line precharge voltage VBLP changes, an NMOS transistor N 8  operates fast using the bit line precharge voltage VBLP as a source. Also, as the bit line precharge voltage VBLP changes, an NMOS transistor N 8  operates fast using the bit line precharge voltage VBLP as a source. The PMOS transistor P 8  and the NMOS transistor N 8  that are configured in a source follower structure operate fast in response to a change in a voltage level of the bit line precharge voltage VBLP. As a result of this fast operation, the PMOS transistor P 13  and the NMOS transistor N 13  turn on or off.  
         [0047]     However, since a consistent amount of current flows to the NMOS transistor N 8  and the PMOS transistor P 8 , it takes long to turn on or off the PMOS transistor P 13  and the NMOS transistor N 13 , which are included in the last output terminal. Hence, when the bit line precharge voltage VBLP increases, a gate source voltage of the PMOS transistor P 8  also increases. Thus, a voltage level of the pull-down signal NDRV increases, and this increasing voltage level causes the NMOS transistor N 13  to turn on in order to decrease the voltage level of the bit line precharge voltage VBLP.  
         [0048]     At this time, a gate source voltage of the NMOS transistor N 10  that is configured in a source follower structure decreases, and a node AP has a voltage level close to a voltage difference between the core voltage VCORE and a threshold voltage of the NMOS transistor N 10 . Voltage levels of the gates of the PMOS transistors P 10  and P 11  to which current flows consistently are controlled according to the voltage level of the node AP. As a result of the voltage level control, a voltage level of the node A rapidly increases to a voltage level of the core voltage VCORE, thereby disallowing generation of a current path.  
         [0049]     The PMOS transistor P 12 , which is also configured in source follower structure, turns on more rapidly than the usual case, and thus, a voltage level of a node AC increases. According to the voltage level of the node AC, the NMOS transistors N 11  and N 12  turn on to decrease a voltage level of the node B. As a result, a current path is not generated.  
         [0050]     On the other hand, if the bit line precharge voltage VBLP decreases, a gate source voltage of the NMOS transistor N 8  increases. Thus, a voltage level of the pull-up signal decreases, triggering the PMOS transistor P 13  to turn on to increase a voltage level of the bit line precharge voltage VBLP.  
         [0051]     At this time, a gate source voltage of the PMOS transistor P 12  that is configured in a source follower structure decreases. Thus, a node AN has a voltage level close to an added voltage value of the ground voltage VSS and a threshold voltage of the PMOS transistor P 10 . Hence, voltage levels of the gates of the NMOS transistors N 11  and N 12  to which current flows consistently are controlled according to the voltage level of the node AN, so that the voltage level of the node B decreases rapidly to a voltage level of the ground voltage VSS. As a result, a current path is not generated.  
         [0052]     The NMOS transistor N 10  that is configured in a source follower structure turns on more rapidly than the usual case, and thus, a voltage level of the node AP decreases. The PMOS transistors P 10  and P 11  turn on according to the voltage level of the node AP to thereby increase the voltage level of the node A. As a result, a current path is not generated.  
         [0053]      FIG. 4  is a circuit diagram of a voltage generator in accordance with another embodiment of the present invention.  
         [0054]     The voltage generator includes a core voltage controller  10 , an auxiliary driving controller  200  and a voltage driver  210 . Since the core voltage controller  10  according to the present embodiment is substantially the same as the core voltage controller  10  described in  FIG. 1 , detailed description thereof will be omitted. However, for better understanding, the core voltage controller  10  in the present invention is be divided into two parts including a bias signal generator that generates four bias voltages PBIAS, NGATE, PGATE, and NBIAS and a driving signal generator that generates pull-up and pull-down signals PDRV and NDRV.  
         [0055]     The auxiliary driving controller  200  includes PMOS transistors P 14  to P 17 , NMOS transistors N 14  to N 17  and resistors R 3  and R 4 . The PMOS transistor P 14  is connected between a terminal of a core voltage VCORE and the NMOS transistor N 14 . A gate of the PMOS transistor P 14  is connected with a gate of the PMOS transistor P 15 . The PMOS transistor P 15  is connected between the terminal of the core voltage VCORE and the resistor R 3 .  
         [0056]     The NMOS transistor N 14  is connected between the PMOS transistor P 14  and an output terminal for a bit line precharge voltage VBLP. The bias voltage NGATE is supplied to a gate of the NMOS transistor N 14 . The resistor R 3  is connected between the PMOS transistor P 15  and a terminal of a ground voltage VSS. The NMOS transistor N 15  is connected between a node D and the terminal of the ground voltage VSS, and a gate of the NMOS transistor N 15  is connected with the resistor R 3 .  
         [0057]     The PMOS transistor P 16  is connected between the NMOS transistor N 16  and the output terminal for the bit line precharge voltage VBLP. The bias voltage PGATE is supplied to a gate of the PMOS transistor P 16 . The PMOS transistor P 17  is connected between the terminal of the core voltage VCORE and a node C, and a gate of the PMOS transistor P 17  is connected to the resistor R 4 . The resistor R 4  is connected between the terminal of the core voltage VCORE and the NMOS transistor N 17 .  
         [0058]     The NMOS transistor N 16  is connected between the terminal of a ground voltage VSS and the PMOS transistor P 16 . A gate of the NMOS transistor N 16  is connected with a gate of the NMOS transistor N 17 . The NMOS transistor N 17  is connected between the terminal of the ground voltage VSS and the resistor R 4 .  
         [0059]     The voltage driver  210  includes a PMOS transistor P 18  and an NMOS transistor N 18 . The PMOS transistor P 18  and the NMOS transistor N 18  are connected in series between the terminal of the core voltage VCORE and the terminal of the ground voltage. The pull-up signal PDRV is supplied to a gate of the PMOS transistor P 18 , while the pull-down signal NDRV is supplied to a gate of the NMOS transistor N 18 . The bit line precharge voltage VBLP is output through a common terminal between the PMOS transistor P 18  and the NMOS transistor N 18 .  
         [0060]     Hereinafter, operation of the voltage generator according to the present invention will be described.  
         [0061]     First, the four bias signals PBIAS, NGATE, PGATE and NBIAS have different voltage levels. More particularly, the bias signal NGATE has a voltage level greater than that of the reference voltage by a predetermined voltage level, and the bias signal PGATE has a voltage level less than that of the reference voltage by a predetermined voltage level.  
         [0062]     When the bit line precharge voltage VBLP increases, a gate source voltage of a PMOS transistor P 8  also increases. Thus, a voltage level of the pull-down signal NDRV increases, and this increasing voltage level causes the NMOS transistor N 18  to turn on to decrease a voltage level of the bit line precharge voltage VBLP.  
         [0063]     At this time, the PMOS transistor P 16 , which is configured in a source follower structure, turns on rapidly, resulting in increase of a voltage level of a node BN. According to the voltage level of the node BN, the NMOS transistors N 16  and N 17  turn on, triggering the PMOS transistor P 17  to turn on. Hence, a voltage level of the node C increases rapidly to the voltage level of the core voltage VCORE. As a result, a current path is not generated.  
         [0064]     The NMOS transistor N 14 , which is configured in a source follower structure, has a gate source voltage that becomes low. As a result, the NMOS transistor N 14  retains an ‘off’ state. At this point, the NMOS transistor N 14  makes a voltage level of a node BP increase via a bootstrapping operation. Subsequently, the PMOS transistors P 14  and P 15  retains an ‘off’ state, and this retained ‘off’ state makes the NMOS transistor N 15  remain turned off. Consequently, a current path is not generated.  
         [0065]     On the other hand, when the bit line precharge voltage VBLP decreases, a gate source voltage of the NMOS transistor N 8  increases. Therefore, the pull-up signal PDRV has a voltage level that is lowered. As a result, the PMOS transistor P 18  turns on to increase the voltage level of the bit line precharge voltage VBLP.  
         [0066]     At this time, a gate source voltage of the PMOS transistor decreases, and this decrease causes the node BN to have a voltage whose level decreases. As a result, the NMOS transistors N 16  and N 17  turn on to increase a gate voltage of the PMOS transistor P 17 , and this increasing gate voltage makes a voltage level of the node C increase. Consequently, current is not allowed to flow regardless of the voltage level of the bit line precharge voltage VBLP.  
         [0067]     The NMOS transistor N 14  turns on more rapidly than the usual case, and thus, a voltage level of the node BP decreases. Also, according to the voltage level of the node BP, the PMOS transistors P 14  and P 15  turn on, triggering a gate voltage of the NMOS transistor N 15  to increase. As a result, a voltage level of the node D decreases to a voltage level of the ground voltage VSS. Hence, a current path is not generated. In summary,  
         [0068]      FIG. 5  is a voltage waveform diagram of the voltage generators illustrated in  FIGS. 3 and 4 . As illustrated, a current path is not generated between those terminals for the bit line precharge voltage VBLP, the pull-up signal PDRV and the pull-down signal NDRV. Hence, the drivability of a semiconductor memory device can be improved.  
         [0069]      FIG. 6  is a circuit diagram of a voltage generator according to still another embodiment of the present invention.  
         [0070]     The voltage generator includes a core voltage controller  10 , an auxiliary driving controller  300  and a voltage driver  310 . Since the core voltage controller  10  according to the present embodiment is substantially the same as the core voltage controller  10  described in  FIG. 1 , detailed description thereof will be omitted. However, for better understanding, the core voltage controller  10  in the present invention is divided into two parts including a bias signal generator that generates four bias voltages PBIAS, NGATE, PGATE, and NBIAS and a driving signal generator that generates pull-up and pull-down signals PDRV and NDRV. The four bias signals PBIAS, NGATE, PGATE, and NBIAS have different voltage levels. More particularly, the bias signal NGATE has a voltage level greater than that of the reference voltage by a predetermined voltage level, and the bias signal PGATE has a voltage level less than that of the reference voltage by a predetermined voltage level.  
         [0071]     The auxiliary driving controller  300  includes an NMOS transistor N 19  and a PMOS transistor P 19 . The NMOS transistor N 19  and the PMOS transistor P 19  are connected in series between a terminal of a core voltage VCORE and a terminal of a ground voltage VSS. The bias voltages NGATE and PGATE are supplied to respective gates of the NMOS transistor N 19  and the PMOS transistor P 19 . A bit line precharge voltage VBLP is output through a common terminal between the NMOS transistor N 19  and the PMOS transistor P 19 .  
         [0072]     The voltage driver  310  includes a PMOS transistor P 20  and an NMOS transistor N 20 . The PMOS transistor P 20  and the NMOS transistor N 20  is connected in series between the terminal of the core voltage VCORE and the terminal of the ground voltage VSS. The pull-up and pull-down signals PDRV and NDRV are supplied to respective gates of the PMOS transistor P 20  and the NMOS transistor N 20 . The bit line precharge voltage VBLP is output through a common terminal between the PMOS transistor P 20  and the NMOS transistor N 20 .  
         [0073]     A direct current path is not generated by additionally placing the NMOS transistor N 19  that has the bias voltage NGATE as an input voltage and the bit line precharge voltage VBLP as a source, and the PMOS transistor P 19  that has the bias voltage PGATE as an input and the bit line precharge voltage VBLP as a source. As a result, the drivability of the voltage driver  310  can be improved.  
         [0074]      FIG. 7  is a circuit diagram of a voltage generator in accordance with a further another embodiment of the present invention.  
         [0075]     The voltage generator includes a core voltage controller  10 , a voltage driver  410  and an output controller  410 . Since the core voltage controller  10  according to the present embodiment is substantially the same as the core voltage controller  10  described in  FIG. 1 , detailed description thereof will be omitted. However, for better understanding, the core voltage controller  10  in the present embodiment is divided into two parts including a bias signal generator that generates four bias voltages PBIAS, NGATE, PGATE, and NBIAS and a driving signal generator that generates pull-up and pull-down signals PDRV and NDRV. The four bias signals PBIAS, NGATE, PGATE, and NBIAS have different voltage levels. More particularly, the bias signal NGATE has a voltage level greater than that of the reference voltage by a predetermined voltage level, and the second bias signal PGATE has a voltage level less than that of the reference voltage by a predetermined voltage level.  
         [0076]     The voltage driver  410  includes a PMOS transistor P 21  and an NMOS transistor N 21 . The PMOS transistor P 21  and the NMOS transistor N 21  are connected in series between a terminal of a core voltage VCORE and a terminal of a ground voltage VSS. The pull-up signal PDRV and the pull-down signal NDRV are supplied to respective gates of the PMOS transistor P 21  and the NMOS transistor N 21 . A bit line precharge voltage VBLP is output through a common terminal between the PMOS transistor P 21  and the NMOS transistor N 21 .  
         [0077]     The output controller  420  includes transfer gates T 1  to T 4 . The transfer gate T 1  outputs the core voltage VCORE to a bulk of the PMOS transistor P 21  depending on the states of control signals AA and BB. The transfer gate T 2  outputs a power supply voltage VDD to the bulk of the PMOS transistor P 21  depending on the states of the control signals AA and BB.  
         [0078]     The transfer gate T 3  outputs the ground voltage VSS to a bulk of the NMOS transistor N 21  depending on the states of the control signals AA and BB. The transfer gate T 4  outputs a back bias voltage VBB to the bulk of the NMOS transistor N 21  depending on the states of the control signals AA and BB.  
         [0079]     The control signal AA is a signal that is inverted from an active signal ACT by an inverter INV 1 . The control signal BB is a signal that is inverted from the control signal AA by an inverter INV 2 . The transfer gates T 1  and T 3  receive the control signal AA through the gate of the PMOS transistor P 21 , and the control signal BB through the gate of the NMOS transistor N 21 . The transfer gates T 2  and T 4  receive the control signal BB through the gate of the PMOS transistor P 21 , and the control signal AA through the NMOS transistor N 21 .  
         [0080]      FIG. 8  is an operational timing diagram of the voltage generator illustrated in  FIG. 7 .  
         [0081]     When the active signal ACT is enabled during an active operation mode ACTIVE PERIOD, the control signal AA has a logic low, while the control signal BB has a logic high. Thus, the transfer gates T 1  and T 3  turn on to supply the core voltage VCORE to the bulk of the PMOS transistor P 21  and the ground voltage VSS to the bulk of the NMOS transistor N 21 . Hence, during the active operation mode ACTIVE PERIOD, threshold voltage levels of the PMOS transistor P 21  and the NMOS transistor N 21  decrease, thereby improving the drivability of the semiconductor memory device.  
         [0082]     On the other hand, in the case of a standby mode, when the active signal ACT is disabled, the control signal AA has a logic high, while the control signal BB has a logic low. Thus, the transfer gates T 2  and T 4  turn on, triggering the supply of the power supply voltage VDD and the back bias voltage VBB to the bulk of the PMOS transistor P 21  and the bulk of the NMOS transistor N 21 , respectively. As a result, the threshold voltage levels of the PMOS transistor P 21  and the NMOS transistor N 21  increase, and thus, current is not likely to leak.  
         [0083]     In other words, the bulk bias of the PMOS transistor P 21  to which the core voltage VCORE is supplied as a source is controlled to be self-biased for the purpose of decreasing the threshold voltage level during the active operation mode ACTIVE PERIOD. During the standby mode STANDBY PERIOD, the back bias voltage VBB is supplied to the NMOS transistor N 21  of the voltage driver  400  to increase the threshold voltage level (i.e., to decrease the leakage current).  
         [0084]     According to various embodiments of the present invention, a bit line precharge voltage and a cell plate voltage can be supplied stably in the state of a low power supply voltage with a low core voltage, and at the same time, a standby current and an operation current can be minimized.  
         [0085]     During an active operation mode, controlling a threshold voltage level of the voltage driver contributes to an improvement on the drivability. During a standby mode, a path where current is likely to leak is not blocked, and thus, reliability of semiconductor memory devices can be enhanced.  
         [0086]     The present application contains subject matter related to the Korean patent application Nos. KR 2005-91587 and 2005-0118144, filed in the Korean Patent Office on Sep. 29, 2005, and Dec. 6, 2005, the entire contents of which being incorporated herein by reference.  
         [0087]     While the present invention has been described with respect to certain preferred 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.