Abstract:
In a semiconductor memory device, a circuit for controlling a voltage level applied to a bit line isolation circuit preferably includes a memory cell connected between a cell bit line pair and a word line; a bit line pre-charge circuit; a sense amplifier bit line pre-charge circuit; a charge transfer circuit connected between the cell bit line pair and the sense amplifier bit line pair; a first sense amplifier circuit for amplifying a voltage of the sense amplifier bit line pair to a first voltage in response to a first control signal; and a second sense amplifier circuit for amplifying the voltage of the sense amplifier bit line pair to a second voltage in response to a second control signal. The combination of the two-stage sense amplifier ciruitry allows for the accurate determination of minimally-different logical voltage levels and minimized circuit area.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory device and a bit line sensing method. More particularly, the present invention relates to a semiconductor memory device that may be operated at a low power voltage. 
     2. Description of the Related Art 
     In a semiconductor memory device, a charge transfer pre-sensing (CTPS) function is typically used to improve a sensing margin of a lower power voltage. 
     FIG. 1 illustrates a circuit diagram of a conventional semiconductor memory device having a charge transfer pre-sensing scheme. This conventional semiconductor memory device includes a cell bit line pre-charge circuit  10 , a PMOS sense amplifier  12 , a sense amplifier bit line pre-charge circuit  16 , an NMOS sense amplifier  18 , first and second bit line isolation circuits  14  and  20 , respectively, and memory cells MCi and MCj. 
     The memory cell MCi represents a representative memory cell among memory cells arranged in a memory cell array block BLK 1  among n-number of memory cell array blocks BLK 1  to BLKn. Similarly, memory cell MCj represents a representative memory cell among memory cells arranged in the memory cell array block BLK 2 . A capacitor Cb 1  represents a cell bit line load capacitor, and a capacitor Csa represents a sense amplifier bit line load capacitor. 
     NMOS transistors N 1 , N 2  and N 3  of the cell bit line pre-charge circuit  10  are turned on in response to a control signal BLPRE having a logic “high” level to pre-charge the cell bit line pair BLcelli and BLBcelli to a voltage Vcca/2. PMOS transistors P 1  and P 2  of the PMOS sense amplifier  12  are turned on in response to a voltage of the cell bit line pair BLBcelli and BLcelli having a logic “low” level to amplify a voltage of the bit line pair BLcelli and BLBcelli having a logic “high” level to a voltage Vcca. NMOS transistors N 4  and N 5  of the first bit line isolation circuit  14  are turned on in response to a control signal SG 1  to electrically connect the cell bit line pair BLcelli and BLBcelli to the sense amplifier bit line pair BLsa and BLBsa, respectively. NMOS transistors N 6 , N 7  and N 8  of the sense amplifier bit line pre-charge circuit  16  are turned on in response to a control signal SAPRE having a logic “high” level to pre-charge the sense amplifier bit line pair BLsa and BLBsa to a voltage Vcca. NMOS transistors N 9  and N 10  of the NMOS sense amplifier  18  are turned on in response to a voltage of the sense amplifier bit line pair BLsa and BLBsa having a logic “low” level to make the voltage of the sense amplifier bit line pair BLsa and BLBsa a ground voltage. NMOS transistors N 11  and N 12  of the second bit line isolation circuit  20  are turned on in response to a control signal SG 2  to electrically connect the cell bit line pair BLcellj and BLBcellj to the sense amplifier bit line pair BLsa and BLBsa, respectively. 
     Operation of the semiconductor memory device of FIG. 1 is described below with reference to FIG. 2, in which a timing diagram illustrating an operation of the semiconductor memory device of FIG. 1 is shown. In FIG. 2, a data is read from the memory cell array block BLK, and a cell array voltage Vcca is 0.8 volts. 
     A pre-charging operation is initiated with the application of the control signals BLPRE, SAPRE, SG 1  and SG 2 . The activating voltage level for SG 1  and SG 2  is 0 volts. The NMOS transistors N 4  and N 5  are turned off to electrically separate the sense amplifier bit line pair BLsa and BLBsa from the cell MCi bit line pair BLcelli and BLBcelli, and the NMOS transistors N 11  and N 12  are turned off to electrically separate the sense amplifier bit line pair BLsa and BLBsa from the cell MCj bit line pair BLcelli and BLBcelli. The NMOS transistors N 1  to N 3  are turned on to pre-charge the cell bit line pair BLcelli and BLBcelli to a voltage Vcca/2 (=0.4 volts). The NMOS transistors N 6 , N 7  and N 8  are turned on to pre-charge the sense amplifier bit line pair BLsa and BLBsa to a voltage Vcca(1+γ) (=1.6 volts). 
     Then, when a read command is applied to select word line WLi, the NMOS transistor NM of the memory cell MCi connected to the word line WLi is turned on, and a charge sharing operation is performed between a capacitor C and the cell bit line pair BLcelli and BLBcelli. This causes a voltage difference ΔVBLcell to occur between the cell bit line pair. 
     When the control signal SG 1  is activated with a voltage of 0.9 volts, the NMOS transistors N 4  and N 5  are turned on and a charge transfer operation is performed between the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa. Hence, a voltage of the sense amplifier bit line pair BLsa and BLBsa is lowered, and a voltage difference occurs between the sense amplifier bit line pair BLsa and BLBsa, as governed by the equation: 
     
       
         Δ VBLsa (=Δ V ×( Cb   1 + Csa )/ Csa ).  [1] 
       
     
     As the voltage of the sense amplifier bit line pair BLsa and BLBsa decreases, the voltage of the cell bit line pair BLcelli and BLBcelli increases, and the voltage difference between the cell bit line pair BLcelli and BLBcelli, ΔVBLcelli, is steadily lowered to 0 volts. Thereafter, when the voltage difference ΔVBLsa occurs between the sense amplifier bit line pair BLsa and BLBsa, the PMOS sense amplifier  12  and the NMOS sense amplifier  18  operate to amplify the voltage of the cell bit line BLcelli and the sense amplifier bit line BLsa to Vcca, and the voltage of the cell bit line BLBcelli, and the sense amplifier bit line BLBsa to 0 volts. 
     However, the semiconductor memory device of FIG. 1 has a problem in that it is difficult to set a level of the control signal SG 1 , which is governed by the inequality [2]:          1   2        Vcca        (     1   +     1     1   +       Cb1   +   Cs     Cs           ≤     VSG1   -   Vth     ≤             Vcca        (     1   +   γ     )          Csa     +       (       1   2        Vcca     )        Cb1         Cb1   +   Cs   +   Csa       +   0.05                              
     where VSG 1  denotes a voltage of the control signal SG 1  and Vth denotes a threshold voltage of the NMOS transistors N 4  and N 5 . 
     A left member of the inequality is a lower limit of a voltage of the control signal SG 1 , and a right member of the inequality is an upper limit of a voltage of the control signal SG 1 . A voltage of the control signal SG 1  is at least higher by a threshold voltage Vth than a data having a logic “high” level, and is at least lower by a threshold voltage Vth than a data having a logic “low” level. That is, the left member and the right member of the inequality are the upper limit and the lower limit, respectively, when NMOS transistors N 4  and N 5  operate in a saturation region rather than in a linear region to perform a charge transfer operation. 
     In order to increase a voltage margin of the control signal SG 1 , the pre-charge voltage has to be greater than a voltage Vcca with γ having a value of about 1. Assume that the capacitor C has a capacitance of 20 fF, the capacitor Cb 1  has a capacitance of 120 fF, the voltage Vcca has a voltage of 0.8 volts, and a threshold voltage has a voltage of 0.4 volts. If the values are substituted in the inequality, the voltage VSG 1  of the control signal SG 1  is bounded by 0.85 volts or more and 0.95 volts or less. Thus, the voltage VSG 1  has a very small margin of 0.1 volts. Therefore, it is very difficult to accurately set a level of the control signal SG 1 . 
     In addition, the semiconductor memory device of FIG. 1 has a problem in that the PMOS sense amplifier  12  is not shared between the respective memory cell array blocks but is separately configured, thereby increasing a layout area size. Furthermore, since the sense amplifier bit line pair BLsa and BLBsa have to be pre-charged to be higher than a voltage Vcca during the pre-charge operation, power consumption is high. 
     FIG. 3 illustrates a circuit diagram of another conventional semiconductor memory device having a charge transfer pre-sensing scheme. The semiconductor memory device of FIG. 3 includes first and second bit line isolation circuits  30  and  38 , a pre-charge circuit  32 , a PMOS sense amplifier  34  and an NMOS sense amplifier  36 , and memory cells MCi and MCj. As in the memory device shown in FIG. 1, the memory cells MCi and MCj represent representative memory cells among memory cells arranged in a memory cell array block BLK 1  and BLK 2 , respectively, among n-number of memory cell array blocks BLK 1  to BLKn. Capacitor Cb 1  represents a cell bit line load capacitor, and capacitor Csa represents a sense amplifier bit line load capacitor. 
     NMOS transistors N 13  and N 14  of the first bit line isolation circuit  30  are turned on in response to a control signal Vot to electrically connect the cell bit line pair BLcelli and BLBcelli to the sense amplifier bit line pair BLsa and BLBsa. NMOS transistors N 15  to N 17  of the pre-charge circuit  32  are turned on in response to a control signal BLPRE to pre-charge the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa to a voltage VBL. When a sense amplifier enable signal SAP having a voltage level Vcc is applied, PMOS transistors P 3  and P 4  of the PMOS sense amplifier  34  are turned on in response to a signal of the sense amplifier bit line pair BLsa and BLBsa having a logic “low” level to amplify a signal of the sense amplifier bit line pair BLsa and BLBsa having a logic “high” level to a voltage Vcc. When a sense amplifier enable signal SAN having a voltage level of 0 volts is applied, NMOS transistors N 18  and N 19  of the NMOS sense amplifier  36  are turned on in response to a signal of the sense amplifier bit line pair BLsa and BLBsa having a logic “high” level to amplify a signal of the sense amplifier bit line pair BLsa and BLBsa having a logic “low” level to a voltage of 0 volts. NMOS transistors N 20  and N 21  of the second bit line isolation circuit  38  are turned on in response to a control signal SG 2  to electrically connect the cell bit line pair BLcellj and BLBcellj to the sense amplifier bit line pair BLsa and BLBsa. 
     Operation of the semiconductor memory device of FIG. 3 is described below with reference to the timing diagrams (A) and (B), as shown in FIG.  4 . In particular, in FIG. 4, (A) is a timing diagram that relates to the operation of a cell bit line pair portion of the semiconductor memory device of FIG. 3, and (B) is a timing diagram that relates to the operation of a sense amplifier bit line pair of the semiconductor memory device of FIG.  3 . In FIG. 4, a data is read from the memory cell array block BLK 1 . 
     During a time period t 1 , upon the application of a control signal Vot having a voltage level “Vcc+Vtn”, a control signal BLPRE having a logic “high” level, and a voltage VBL having a level Vcc/2, the NMOS transistors N 15 , N 16  and N 17  are turned on to pre-charge the sense amplifier bit line pair BLsa and BLBsa to a voltage Vcc/2. The NMOS transistors N 13  and N 14  are turned on to transfer charges from the sense amplifier bit line pair BLsa and BLBsa to the cell bit line pair BLcelli and BLBcelli, thereby pre-charging the cell bit line pair BLcelli and BLBcelli to a voltage Vcc/2. 
     During a time period t 2 , when the control signal Vot having a voltage level of 0 volts and the voltage VBL having a voltage level Vcc(1+γ) are applied, the NMOS transistors N 13  and N 14  are turned off, such that the sense amplifier bit line pair BLsa and BLBsa is pre-charged to a voltage Vcc(1+γ). During a time period t 3 , when a high voltage VPP is applied to the word line WLi, NMOS transistor NM of the memory cell MCi is turned on, such that a voltage difference ΔVBLcell occurs between the cell bit line pair BLcelli and BLBcelli. 
     During a time period t 4 , when the control signal Vot having a voltage level “β+Vtn” is applied, the NMOS transistors N 13  and N 14  are turned on. A voltage of the cell bit line pair BLcelli and BLBcelli is raised to a voltage β (=Vot−Vtn), such that a voltage difference between the cell bit line pair BLcelli and BLBcelli becomes 0 volts. A voltage of the sense amplifier bit line pair BLsa and BLBsa is steadily lowered, so that a voltage difference ΔVBLsa occurs. During a time period t 5 , when a voltage difference of the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa is lowered below a threshold voltage Vtn of the NMOS transistors N 13  and N 14 , the NMOS transistors N 13  and N 14  are turned off, thereby opening the connection between the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa. 
     During a time period t 6 , when the sense amplifier enable signals SAP and SAN, which have a voltage Vcc and a voltage of 0 volts respectively, are applied, the NMOS sense amplifier  34  and the PMOS sense amplifier  36  operate to amplify a voltage of the sense amplifier bit line pair BLsa and BLBsa to a voltage of Vcc and 0 volts, respectively. At this time, when the control signal Vot is transited to a voltage level “Vcc+Vtn”, the NMOS transistors N 13  and N 14  are turned on, and the voltage of the sense amplifier bit line pair BLsa and BLBsa is transferred to the cell bit line pair BLcelli and BLBcelli. Therefore, a voltage of the cell bit line pair BLcelli and BLBcelli is also amplified to a voltage of Vcc and 0 volts, respectively. 
     However, the semiconductor memory device of FIG. 3 has a layout area size smaller than that of FIG. 1, and the voltage level of the control signal Vot applied to the NMOS transistors of the bit line isolation circuit has to be varied from Vcc+Vtn to 0 volts, from 0 volts to β+Vtn, and from β+Vtn to Vcc+Vtn, and a voltage level of the voltage VBL also has to be varied from Vcc/2 to Vcc(1+γ), and from Vcc(1+γ) to Vcc/2. Disadvantageously, it is also very difficult to accurately control such voltage levels. 
     SUMMARY OF THE INVENTION 
     In an effort to overcome the problems described above, it is a feature of an embodiment of the present invention to provide a semiconductor memory device and a bit line sensing method capable of controlling a bit line sensing operation without increasing a layout area size. 
     A preferred embodiment of the present invention provides a semiconductor memory device including a memory cell connected between a cell bit line pair and a word line; a bit line pre-charge circuit connected to the cell bit line pair for pre-charging the cell bit line pair to a voltage lower than a first voltage in response to a cell bit line pre-charge control signal; a sense amplifier bit line pre-charge circuit connected to a sense amplifier bit line pair for pre-charging the sense amplifier bit line pair to the first voltage in response to a sense amplifier bit line pre-charge control signal; a charge transfer circuit connected between the cell bit line pair and the sense amplifier bit line pair for transferring charges to the cell bit line pair and the sense amplifier bit line pair in response to a control signal; a first sense amplifier circuit connected between the sense amplifier bit line pair for amplifying a voltage of the sense amplifier bit line pair to the first voltage; and a second sense amplifier circuit connected between the sense amplifier bit line pair for amplifying a voltage of the sense amplifier bit line pair to a second voltage in response to a sense amplifier enable signal. 
     The control signal may be maintained at the second voltage, and may be transited to the first voltage before the sense amplifier enable signal is generated after an active command is applied, and may be transited from the first voltage to a third voltage after the sense amplifier enable signal is generated. The control signal is generated by a signal generating circuit and a control signal, wherein the signal generating circuit preferably includes: a first delay circuit having the same line load as a word line enable signal for enabling the word line, and receiving and delaying the active command; a second delay circuit delaying an output signal of the first delay circuit by a first time to generate a first signal; and a third delay circuit delaying the output signal of the first delay circuit by a second time to generate a second signal, wherein the control signal generating circuit generates the control signal having the first voltage in response to the first signal, and generates the control signal having the third voltage in response to the second signal. 
     The voltage lower than the first voltage is preferably the first voltage/2. The first voltage may be an internal or an external power voltage. The second voltage may be a ground voltage. The third voltage may be a high voltage higher than the power voltage. 
     The sense amplifier bit line pre-charge circuit preferably includes first and second PMOS transistors serially connected between the sense amplifier bit line pair and having a common source receiving the first voltage and a gate receiving the sense amplifier bit line pre-charge control signal. The first sense amplifier circuit preferably includes third and fourth PMOS transistors serially connected between the sense amplifier bit line pair and having a gate connected to the sense amplifier bit line pair and a common source receiving the first voltage. The first sense amplifier circuit preferably includes third and fourth PMOS transistors serially connected between the sense amplifier bit line pair and having a gate connected to the sense amplifier bit line pair and a common source receiving the first voltage in response to the sense amplifier enable signal. The second sense amplifier circuit preferably includes first and second NMOS transistors serially connected between the sense amplifier bit line pair and having a gate connected to the sense amplifier bit line pair and a common source receiving the second voltage in response to the sense amplifier enable signal. 
     A preferred embodiment of the present invention also provides a bit line sensing method of a semiconductor memory device including separating a cell bit line pair from a sense amplifier bit line pair when a control signal of a first voltage is applied, pre-charging the cell bit line pair to a voltage lower than the first voltage, and pre-charging the sense amplifier bit line pair to the first voltage when a sense amplifier bit line pre-charge control signal is applied; generating a voltage difference between the cell bit line pair by enabling a word line and selecting a memory cell; generating a first voltage difference between the cell bit line pair by connecting the cell bit line pair to the sense amplifier bit line pair when the control signal having the second voltage is applied, and generating a second voltage difference between the sense amplifier bit line pair by reducing a voltage of the sense amplifier bit line pair; maintaining a voltage of the cell bit line pair and the sense amplifier bit line pair to a certain voltage; and amplifying the sense amplifier bit line pair to the first voltage and the second voltage by enabling the PMOS sense amplifier and the NMOS sense amplifier, and transferring a voltage of the sense amplifier bit line pair to the cell bit line pair when the control signal having the third voltage is applied. 
     The voltage lower than the first voltage may be the first voltage/2. The first voltage may be an internal power voltage or an external power voltage. The second voltage may be a ground voltage. The third voltage may be a high voltage higher the power voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be readily apparent to those of ordinary skill in the art upon review of the detailed description that follows when taken in conjunction with the accompanying drawings, in which like reference numerals denote like parts, and in which: 
     FIG. 1 illustrates a circuit diagram of a conventional semiconductor memory device having a charge transfer pre-sensing scheme; 
     FIG. 2 is a timing diagram illustrating an operation of the semiconductor memory device of FIG. 1; 
     FIG. 3 illustrates a circuit diagram of another conventional semiconductor memory device having a charge transfer pre-sensing scheme; 
     FIG. 4 illustrates timing diagrams (A) and (B), which illustrate an operation of the semiconductor memory device of FIG. 3; 
     FIG. 5 illustrates a circuit diagram of a semiconductor memory device according to the present invention; 
     FIG. 6 illustrates a circuit diagram of a control signal generating circuit for generating a control signal ISOi shown in FIG. 5, according to the present invention; 
     FIG. 7 illustrates a circuit diagram of a signal generating circuit for generating signals P 1  and P 2  shown in FIG. 6, according to the present invention; and 
     FIG. 8 illustrates timing diagrams (A) and (B), which illustrate an operation of the semiconductor memory device of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Korean Patent Application No. 2001-71144, filed Nov. 15, 2001, and entitled: “Semiconductor Memory Device and Bit Line Sensing Method Thereof,” is hereby incorporated by reference in its entirety. 
     Reference will now be made in detail to preferred embodiments of the present invention, an example of which is illustrated in the accompanying drawings. 
     FIG. 5 illustrates a circuit diagram of a semiconductor memory device according to the present invention. The semiconductor memory device of FIG. 5 preferably includes a cell bit line pre-charge circuit  40  connected between cell bit line pair BLcelli and BLBcelli, a sense amplifier bit line pre-charge circuit  44 , a PMOS sense amplifier  46  and an NMOS sense amplifier  48 , which are connected between sense amplifier bit line pair BLsa and BLBsa, and charge transfer circuits or bit line isolation circuits  42  and  50  connected between the sense amplifier bit line pair BLsa-BLBsa and the cell bit line pairs BLcelli-BLBcelli and BLcellj-BLBcellj, respectively. 
     Memory cells MCi and MCj represent representative memory cells among memory cells arranged in memory cell array blocks BLK 1  and BLK 2 , respectively, among n-number of memory cell array blocks BLK 1  to BLKn. A capacitor Cb 1  represents a cell bit line load capacitor, and a capacitor Csa represents a sense amplifier bit line load capacitor. 
     The cell bit line pre-charge circuit  40  includes NMOS transistors N 22 , N 23  and N 24 . The NMOS transistor N 22  is connected between the cell bit line pair BLcelli and BLBcelli and has a gate receiving a control signal BLPRE. The NMOS transistors N 23  and N 24  are serially connected between the cell bit line pair BLcelli and BLBcelli and have a gate receiving the control signal BLPRE and a common source receiving a pre-charge voltage Vcca/2. The sense amplifier bit line pre-charge circuit  44  has PMOS transistors P 5  and P 6 . The PMOS transistors P 5  and P 6  are serially connected between the sense amplifier bit line pair BLsa and BLBsa and have a gate receiving a control signal SAPRE and a common source receiving a first voltage Vcca. The PMOS sense amplifier  46  has PMOS transistors P 7  and P 8  serially connected between the sense amplifier bit line pair BLsa and BLBsa. The PMOS transistors P 7  and P 8  have a common source receiving the voltage from SAPRE, or an external power voltage, but preferably the first voltage Vcca. A gate of the PMOS transistor P 7  is connected to the sense amplifier bit line BLBsa, and a gate of the PMOS transistor P 8  is connected to the sense amplifier bit line BLsa. The NMOS sense amplifier  48  has NMOS transistors N 27  and N 28  serially connected between the sense amplifier bit line pair BLsa and BLBsa. The NMOS transistors N 27  and N 28  have a common source receiving a signal SAN. A gate of the NMOS transistor N 27  is connected to the sense amplifier bit line BLBsa, and a gate of the NMOS transistor N 28  is connected to the sense amplifier bit line BLsa. The bit line isolation circuit  42  includes NMOS transistors N 25  and N 26  connected between the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa, respectively. The bit line isolation circuit  50  includes NMOS transistors N 29  and N 30  connected between cell bit line pair BLcellj and BLBcellj and the sense amplifier bit line pair BLsa and BLBsa, respectively. Each of the memory cells MCi and MCJ includes an NMOS transistor NM having a gate connected to the word lines WLi and WLj and a drain connected to the cell bit line BLcelli and BLcellj, respectively, and a capacitor C connected between a source of the NMOS transistor NM and a ground voltage. 
     The NMOS transistors N 22 , N 23 , and N 24  of the cell bit line pre-charge circuit  40  are turned on in response to the control signal BLPRE having a logic “high” level to pre-charge the cell bit line pair BLcelli and BLBcelli to a voltage Vcca/2. The NMOS transistors N 25  and N 26  of the bit line isolation circuit  42  are turned on in response to a control signal ISOi to transfer charges to, and electrically connect, the cell bit line pair BLcelli and BLBcelli to the sense amplifier bit line pair BLsa and BLBsa. The PMOS transistors P 5  and P 6  of the sense amplifier bit line pre-charge circuit  44  are turned on in response to the control signal SAPRE having a logic “low” level to pre-charge the sense amplifier bit line pair BLsa and BLBsa to a first voltage Vcca. The PMOS transistors P 7  and P 8  of the PMOS sense amplifier  46  are turned on in response to a signal of the sense amplifier bit line pair BLsa and BLBsa having a logic “low” level to amplify a signal of the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa having a logic “high” level to a first voltage Vcca. When a sense amplifier enable signal SAN of 0 volts is applied, the NMOS transistors N 27  and N 28  of the NMOS sense amplifier  48  are turned on in response to a signal of the sense amplifier bit line pair BLsa and BLBsa to amplify a signal of the cell bit line pair BLcellj and BLBcelIj and the sense amplifier bit line pair BLsa and BLBsa having a logic “low” level to a second voltage of 0 volts. The NMOS transistors N 29  and N 30  of the bit line isolation circuit  50  are turned on in response to a control signal ISOj to transfer charges to, and electrically connect the cell bit line pair BLcellj and BLBcellj to the sense amplifier bit line pair BLsa and BLBsa. 
     In FIG. 5, the PMOS sense amplifier  46  is preferably configured to be directly connected to a first voltage Vcca. But, like the NMOS sense amplifier  48 , the PMOS sense amplifier  46  may be configured such that a first voltage Vcca is applied to the common source of the PMOS transistors P 7  and P 8 . In this case, when a sense amplifier enable signal having a first voltage level Vcca is applied, the PMOS transistors P 7  and P 8  are turned on in response to a signal of the sense amplifier bit line pair BLsa and BLBsa having a logic “low” level to amplify a signal of the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa having a logic “high” level to a first voltage Vcca. 
     FIG. 6 illustrates a circuit diagram of a control signal generating circuit for generating the control signal ISOi shown in FIG. 5 according to the present invention. The circuit of FIG. 6 preferably includes inverters I 1  to I 3 , an NMOS transistor N 31 , and a PMOS transistor P 9 . The inverters I 1  and I 2  buffer a control signal P 1  having a logic “high” level to generate a high third voltage Vpp. The inverter I 3  inverts a control signal P 2  having a logic “high” level to generate a second voltage 0V. The NMOS transistor N 31  receives a first voltage Vcca to generate the control signal ISOi in response to an output signal of the inverter I 2  having the high third voltage level Vpp. The PMOS transistor P 9  receives a high third voltage Vpp to generate the control signal ISOi in response to an output signal of the inverter I 3  having a ground voltage level (second voltage 0V). 
     FIG. 7 illustrates a circuit diagram of a signal generating circuit for generating the signals P 1  and P 2  shown in FIG. 6 according to the present invention. The signal generating circuit of FIG. 7 preferably includes a word line enable signal generating circuit  60 , and first, second and third delay circuits  62 ,  64  and  66 , respectively. The first delay circuit  62  is designed to have the same line load as the word line enable signal generating circuit  60 . The word line enable signal generating circuit  60  incorporates a predetermined time delay after reception and decoding of a row address RA in response to an inverted row address strobe signal RASB before generating a word line enable signal WL. 
     The first delay circuit  62  generates a signal RWL in response to the inverted row address strobe signal RASB at the same time that the word line enable signal WL is generated. The second delay circuit  64  delays the signal RWL by a first predetermined time to generate the signal P 1 . The third delay circuit  66  delays the signal RWL by a second predetermined time to generate the signal P 2 . Since the first delay circuit  62  preferably has a same configuration as the word line enable signal generating circuit  60 , the signals RWL, P 1  and P 2  may be generated with accurate timing. 
     Operation of the semiconductor memory device of FIG. 5 is described below with reference to FIG. 8, which illustrates timing diagrams (A) and (B), illustrating an operation of the semiconductor memory device of FIG.  5 . In particular, (A) is a timing diagram to describe an operation of a cell bit line pair portion, and (B) is a timing diagram to describe an operation of a sense amplifier bit line pair. In FIG. 8, data is read from the memory cell array block BLK 1 . 
     During a time period t 1 , when the control signal ISOi is maintained at the second voltage of 0 volts, the control signal BLPRE having a logic “high” level, and the control signal SAPRE having a logic “low” level are applied, the NMOS transistors N 25  and N 26  are turned off. The NMOS transistors N 22 , N 23  and N 24  are turned on to pre-charge the cell bit line pair BLcelli and BLBcelli to a voltage Vcca/2. The PMOS transistors P 5  and P 6  are turned on to pre-charge the sense amplifier bit line pair BLsa and BLBsa to a first voltage Vcca. 
     During a time period t 2 , when a high voltage Vpp is applied to the word line WLi, the NMOS transistor NM of the memory cell MCi is turned on to perform a charge sharing operation between the capacitor C and the cell bit line pair BLcelli and BLBcelli. Thus, a voltage difference occurs between the cell bit line pair BLcelli and BLBcelli. 
     During a time period t 3 , when the control signal ISOi of a first voltage Vcca is applied, the NMOS transistors N 25  and N 26  are turned on, so that a charge transfer occurs between the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa, and a voltage of the cell bit line pair BLcelli and BLBcelli is steadily raised to cause a voltage difference ΔVBLcelli, and a voltage of the sense amplifier bit line pair BLsa and BLBsa is steadily lowered to cause ΔVBLsa. 
     During a time period t 4 , when a voltage difference between the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa is lower than a threshold voltage of the NMOS transistors N 25  and N 26 , the NMOS transistors N 25  and N 26  are turned off. As a result, a connection between the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa is cut off, and charge transfer therebetween ceases. Consequently, the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa are maintained at a certain level. 
     During a time period t 5 , when the sense amplifier enable signal SAN of 0 volts is applied, the NMOS sense amplifier  46  and the PMOS sense amplifier  48  operate to amplify a voltage of the sense amplifier bit line pair BLsa and BLBsa to a voltage of 0 volts and a voltage Vcca, respectively. At this time, when the control signal ISOi is transited to a high third voltage level Vpp, the NMOS transistors N 25  and N 26  are turned on so that an amplified level of the sense amplifier bit line pair BLsa and BLBsa is transferred to the cell bit line pair BLcelli and BLBcelli. That is, during this time period, the cell bit line pair BLcelli and BLBcelli as well as the sense amplifier bit line pair BLsa and BLBsa is amplified. 
     The semiconductor memory device of the present invention is preferably configured to apply a voltage Vcca, which corresponds to an intermediate voltage level of the control signal ISOi, as a voltage for pre-charging the cell bit line pair BLcelli and BLBcelli and the sense amplifier bit line pair BLsa and BLBsa. However, the semiconductor memory device of the present invention may be configured to apply an external power voltage instead of the voltage Vcca. 
     As described above, the semiconductor memory device according to the present invention shares the PMOS sense amplifier and the NMOS sense amplifier between the memory cell array blocks to perform a charge transfer pre-sensing operation, and thus a layout area size can be significantly reduced. In addition, using the semiconductor memory device and the bit line sensing method according to the present invention, it is easier to control a level of the control signal applied to the bit line isolation circuit than by using conventional implementations. 
     A preferred embodiment of the present invention has been disclosed herein and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the invention as set forth in the following claims.