Abstract:
A semiconductor memory device includes: a bit line; a reference voltage generating circuit; a first transistor whose drain or source region is connected to the bit line, a voltage generated in the reference voltage generating circuit being applied to a gate region of the first transistor; and a memory cell connected to the first transistor at least via the bit line, wherein the reference voltage generating circuit includes: a second transistor connected to the first transistor in a source-follower connection; and at least one first element having an electrical resistance for controlling a current flowing the second transistor.

Description:
BACKGROUND OF THE INVENTION 
     1. Filed of the Invention 
     The present invention to a semiconductor memory device, and more specifically the present invention relates to a semiconductor memory device so configured as to generate a precharge voltage in the case where a precharge method is used for memory cell reading. 
     2. Description of the Related Art 
     For a memory cell reading method in semiconductor memory devices such as MROMs (Mask Read Only Memory), precharge methods have been conventionally proposed. An example will be described below in which a MROM is read by a precharge method. According to the precharge method, a bit line is connected to a memory cell transistor, which is either an ON transistor or an OFF transistor, wherein the bit is first charged to a precharge level for a certain period of time (hereinafter, this time is referred to as a precharge period). After the charging is completed, a sense circuit detects the discharge waveform of the bit line and determines whether the memory cell transistor connected to the bit line is an ON transistor or an OFF transistor. 
     Memory cell transistors are fabricated by, e.g., an ion injection method, and for making them ON transistors or OFF transistors, for example, the following two methods are used: 
     In one method, the threshold voltages of transistors are changed so as to form ON transistors which turn to the ON state by applying a voltage VI to their gates and OFF transistors which turn to the OFF state by applying the same voltage V 1  to their gates. In the other method, the threshold voltage is not changed. OFF transistors are formed by physically forming an electrical isolation between their sources and drains. For ON transistors, transistors in which the current flows between their sources and drains are used. 
     FIG. 5 shows a connection example between a precharge circuit and a memory cell transistor in a conventional semiconductor memory device  200 . According to the semiconductor memory device  200  in FIG. 5, a precharge circuit includes: a reference voltage generating circuit  30  for generating a reference voltage for charging a bit line  11 ; and a charge transistor N 1  formed of an Nch (N-channel) transistor. An inverter INV 1  is used in the reference voltage generating circuit  30 . The output of the inverter INV 1  is applied to the gate of the charge transistor N 1  and the drain of the charge transistor N 1  is connected to a supply voltage VCC. The source of the charge transistor N 1  is connected to the input terminal of the inverter INV 1 . The reference voltage generating circuit  30  including the inverter INV 1  and a charge transistor N 1  together form a feed back bias circuit. The reference voltage generated by the feedback bias circuit is determined by controlling the inversion voltage of the inverter INV 1 . 
     The drain of the Nch transistor NTR 1  is connected via the bit line  11  to the source of the charge transistor N 1 . The drain of a memory cell transistor M 1  is connected to the source of the Nch transistor NTR 1 . and the source of the memory cell transistor M 1  is connected to the drain of the Nch transistor NTR 2 . A sense circuit  20  is connected to the bit line  11  between the charge transistor N 1  and the Nch transistor NTR 1 . 
     The case will now be described where the memory cell transistor M 1  of such a semiconductor memory device is an OFF transistor (i.e., no current flows between the source and drain even if the gate voltage is in the H (High) level) and the gate voltages input to the respective gates of the Nch transistor NTR 1 , the memory cell transistor M 1 , and the Nch transistor NTR 2  are all in the H level. If the bit line  11  connected to the source of the charge transistor N 1  is initially in the L (Low) level, the L level is input to the input of the inverter INV 1  of the reference voltage generating circuit  30 . When the input of the inverter INV 1  turns to the L level, the output of the inverter INV 1  turns to the H level, and then the H level is input to the gate of the charge transistor N 1 . The supply voltage VCC, which is connected to the drain of the charge transistor N 1 , is then applied to the bit line  11 . 
     Since the memory transistor M 1  is an OFF transistor, no current path to the GND is provided, and the bit line  11  is charged through the charge transistor N 1 . 
     When the bit line  11  is charged to a voltage exceeding the inversion voltage of the inverter INV 1 , the output of the inverter INV 1  turns to the L level and the charge transistor N 1  turns to the OFF state. The potential of the bit line  11  then decreases through a spontaneous discharge or the like, and when the potential of the bit line  11  becomes lower than the inversion voltage of the inverter INV 1 , the output of the inverter INV 1  turns again to the H level and the N 1  transistor turns to the ON state so as to start the charging of the bit line  11 . 
     By repeating such operations, the bit line  11  is stabilized at a predetermined voltage close to the inversion voltage of the inverter INV 1 , i.e., the precharge voltage. 
     In the case where the memory cell transistor M 1  is an ON transistor, and the respective gates of the Nch transistor NTR 1 , memory cell transistor N 1 , and the Nch transistor NTR 2  are all in the H level, the bit line  11  is connected to GND via the Nch transistor NTR 1 , the memory cell transistor M 1 , and the Nch transistor NTR 2 . 
     When the performance of the charge transistor N 1  is low (i.e., the ON resistance is high), the voltage of the bit line  11  can be differentiated between the case where the memory cell transistor M 1  is an ON transistor and the case where the memory cell transistor M 1  is an OFF transistor. The sense circuit  20  reads the voltage of the bit line  11  and determines whether or not the memory cell transistor M 1  is an ON transistor or an OFF transistor. 
     In recent years, however, the size of memory cells has been significantly reduced and thus the performance (i.e., the current driving performance) of memory cell transistor M 1  has declined (i.e., the ON resistance has been higher). This causes the difference of the potentials to become small between of the bit line  11  connected to an ON transistor and the bit line  11  connected to an OFF transistor, and therefore, the sense circuit  20  may not be able to read the difference if the current driving performance of the charge transistor N 1  is high. On the other hand, if the performance of the charge transistor N 1  is low, the time required to charge the bit line  11  to the precharge voltage is extended, which may prevent a faster reading of the memory cells when the parasitic capacitance of the bit line  11  is large. 
     In order to solve these problems, a semiconductor memory device  300  shown in FIG. 6 has been proposed. Throughout the drawings, like components are denoted by like reference numerals. The semiconductor memory device  300  has the same configuration as the semiconductor memory device  200  in FIG. 5, except that a high performance (i.e., a low ON resistance) Nch charge transistor N 2  is connected to the Nch charge transistor N 1  and an Nch transistor NTR 0  is connected between the Nch charge transistor N 2  and the bit line  11 . The gate of the Nch charge transistor N 2  is connected to the gate of the Nch charge transistor N 1 , and the drain of the Nch charge transistor N 2  is connected to the supply voltage VCC. The source of the Nch charge transistor N 2  is connected to the drain of the Nch transistor NTR 0  and the source of the Nch transistor NTR 0  is connected to the bit line  11 . 
     The Nch transistor NTR 0  is turned ON/OFF by signals generated in a circuit generally known as an ATD circuit (address transition detection circuit). The period while the Nch transistor NTR 0  is ON corresponds to the precharge period, during which the bit line  11  is charged. 
     According to the semiconductor memory device  300 , the bit line  11  is charged quickly during the precharge period by the Nch charge transistor N 2 . When the precharge period ends, the Nch transistor NTR 0  turns OFF, and no current flows from the Nch charge transistor N 2  to the bit line  11 . After the completion of the precharge period, the sense circuit  20  reads the bit line  11  in a manner similar to that of the semiconductor memory device  200  in FIG.  5 . 
     The semiconductor memory device  300  in FIG. 6 can provide a faster reading by the sense circuit  20  because it is possible to make the potential difference large between the ON state and the OFF state of the memory cell transistors of the bit lines  11 . Japanese Laid-open Publication No. 3-30193 discloses a similar semiconductor memory device which is capable of a fast reading. 
     A reference voltage generating circuit  30 ′ shown in FIG. 7 employs a NOR circuit NOR 1  instead of the inverter INV 1  of the reference voltage generating circuit  30  of the semiconductor memory device  300  in FIG.  6 . One of the two inputs of the NOR circuit NOR 1  has a feed back structure similar to that of the inverter INV 1  in FIGS. 5 and 6. A switching signal CEB for switching between the stand-by (wait) state and the operation state is applied to the other input of the NOR circuit NOR 1 . 
     In this case, when the switching signal CEB is in the L level, the NOR circuit NOR 1  functions similar to the inverter INV 1  in FIGS. 5 and 6 so as to generate a charge voltage (i.e., the precharge voltage). When the switching signal CEB is in the H level, the NOR circuit NOR 1  outputs the L level, causing the reference voltage generating circuit  30 ′ to be in the stand-by state. Thus, there is no possibility that the voltage of the bit line  11  may increase. 
     According to the semiconductor memory devices  200  and  300  in FIGS. 5 and 6, and the reference voltage genera ting circuit  30 ′ in FIGS. 7, a through current flows in the inverter INV 1  (FIGS. 5 and 6 ) or the NOR circuit NOR 1  (FIG.  7 ), even if inverter INV 1  or the NOR circuit NOR 1  is in the CMOS configuration. This is because a predetermined reference voltage is applied to the inverter INV 1  or the NOR circuit NOR 1  while the inverter INV 1  or the NOR circuit NOR 1  generates the reference voltage. Therefore, in the case of a high performance MROM where a lot of information is simultaneously read from the memory cells, a plurality of such precharge circuits are required. In such a case, the through current flows in each of the plurality of precharge circuits. This requires a large operation current for the semiconductor memory device. 
     In order to reduce the operation current in a high performance MROM requiring a plurality of precharge circuits, a reference voltage generating circuit  30 ″ as shown in FIG. 8 has been proposed, in which a serial circuit including resistors R 1  and R 2  is used. The output potential of the reference voltage generating circuit  30 ″ is the same as the potential at the contact point between the resistors R 1  and R 2 . In order to further reduce the operation current of the semiconductor memory device, a similar resistor may be connected to the gate of the Nch charge transistor N 1  (FIG.  5 ), or the gates of the Nch charge transistor N 1  and the Nch charge transistor N 2  (FIG.  6 ). Furthermore, one or a few similar resistors may be connected to each of the charge transistors. 
     According to the device in FIG. 8, by increasing the resistance value of the serial circuit including the resistors R 1  and R 2 , the through current flowing between the supply voltage VCC and GND can be reduced. Furthermore, by connecting one or a few similar resistors to each of the charge transistors, the operation current of the charge transistors as a whole can be reduced. 
     According to the device in FIG. 8, however, reducing the through current or providing a few resistor-separation circuits extends the time required for charging the gate of the Nch charge transistor N 1 . Specifically, by reducing the through current, the charge current is also reduced and therefore the time required for charging the gate of the Nch charge transistor N 1  is extended. As a result, in the case of an MROM having a stand-by function, the access to the memory cells slows down after the memory device is released from the stand-by state. This may cause the problem that a significant reduction of the through current during the stand-by state is impossible. Therefore, even when the memory device is in the stand-by state, a stand-by current corresponding to the quantity of the through current is present. Accordingly, either in the precharge circuit employing a feedback device or in the precharge circuit employing resistor separation, it is difficult to reduce the stand-by current simultaneously with a reduction of the operation current. 
     SUMMARY OF THE INVENTION 
     According to one aspect of this invention, there is provided a semiconductor memory device, including: a bit line; a reference voltage generating circuit; a first transistor whose drain or source region is connected to the bit line, a voltage generated in the reference voltage generating circuit being applied to a gate region of the first transistor; and a memory cell connected to the first transistor at least via the bit line, wherein the reference voltage generating circuit includes: a second transistor connected to the first transistor in a source-follower connection; and at least one first element having an electrical resistance for controlling a current flowing the second transistor. 
     In one embodiment of the invention, the semiconductor memory device further includes: a third transistor whose gate region is connected to the gate region of the fist transistor; a fourth transistor having a source region and a drain region, in which one of the source region or the drain region is connected to either the drain region or the source region of the third transistor, the other of the drain region or the source region of the fourth transistor being connected to the bit line. 
     In another embodiment of the invention, an ON resistance of the third transistor is lower than an ON resistance of the first transistor. 
     In still another embodiment of the invention, the second transistor is an Nch transistor. 
     In still another embodiment of the invention, the at least one first element is a transistor. 
     In still another embodiment of the invention, the at least one first element is a resistor. 
     In still another embodiment of the invention, the reference voltage generating circuit further includes: a inversion voltage generating circuit connected to the gate region of the second transistor; and a fifth transistor whose gate region is supplied with an output of the inversion voltage generating circuit, either of the drain and source region of the fifth transistor being connected to the gate region of the first transistor, and the other of the drain and source region being grounded. 
     Thus, the invention described herein makes possible the advantages of providing a semiconductor memory device having a precharge circuit which is capable of reducing the operation current and the stand-by current at the same time. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating an example of a semiconductor memory device according to the present invention. 
     FIG. 2 is a partial circuit diagram illustrating another example of a semiconductor memory device according to the present invention. 
     FIG. 3 is a partial circuit diagram illustrating still another example of a semiconductor memory device according to the present invention. 
     FIG. 4 is a partial circuit diagram illustrating still another example of a semiconductor memory device according to the present invention. 
     FIG. 5 is a circuit diagram illustrating an example of a conventional semiconductor memory device. 
     FIG. 6 is a partial circuit diagram illustrating another example of a conventional semiconductor memory device. 
     FIG. 7 is a partial circuit diagram illustrating still another example of a conventional semiconductor memory device. 
     FIG. 8 is a partial circuit diagram illustrating still another example of a conventional semiconductor memory device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the attached drawings, preferred embodiments of the present invention will now be described in detail. FIG. 1 is a circuit diagram showing an example of the semiconductor memory device according to the present invention. A semiconductor memory device  100  includes a reference voltage generating circuit  10  having a high performance (i.e., low ON resistance) Nch transistor HN 1  which is connected in a source-follower connection, a low performance (i.e., high ON resistance) Nch charge transistor N 1 , and a high performance (i.e., low ON resistance) Nch charge transistor N 2 . The Nch charge transistor N 1  is supplied with the output of the reference voltage generating circuit  10 . The gate of the Nch transistor HN 1  is supplied with a switching signal CE which switches between the stand-by state and the operation state. The drain of the Nch transistor HN 1  is connected to a supply voltage VCC. 
     The source of the Nch transistor HN 1  is connected to the source of a low performance (high ON resistance) Pch (P-channel) transistor HP 1 , and the drain of the Pch transistor HP 1  is connected to the source of a low performance Pch transistor HP 2 . The contact point between the source of the Nch transistor HN 1  and the source of the Pch transistor HP 1  is connected to the gates of the Nch charge transistors N 1  and N 2 . The connection manner described above, in which the source of the Nch transistor HN 1  is connected to the gate of the Nch charge transistor N 1  (and to the gate of the Nch charge transistor N 2 ), is called a source-follower connection. According to the source-follower connection shown in FIG. 1, when the switching signal CE is applied to the gate of the Nch transistor HN 1 , a signal is output from the source of the Nch transistor HN 1  to the gates of the Nch charge transistors N 1  and N 2  in the same phase as that of the switching signal CE as if the signal follows the switching signal CE. 
     The drain and the gate of the Pch transistor HP 1  are connected to each other. The drain of the Pch transistor HP 2  is connected to GND, and the drain and the gate of the Pch transistor HP 2  are connected to each other. 
     The switching signal CE applied to the gate of the Nch transistor HN 1  is also applied to the inverter INV 1 , and the output of the inverter INV 1  is applied to the gate of the Nch transistor HN 2 . The drain of the Nch transistor HN 2  is connected to the source of the Pch transistor HP 1 , the gate of the Nch charge transistor N 1 , and the gate of the Nch charge transistor N 2 . The gates of the Nch charge transistors N 1  and N 2  are in the L level during the stand-by state. The source of the Nch transistor HN 2  is connected to GND. 
     The drain of the Nch charge transistor N 1  is connected to the supply voltage VCC. The bit line  11  connects the source of the Nch charge transistor N 1  and the drain of an Nch transistor NTR 1 , and a sense circuit  20  is connected therebetween. The source of the Nch transistor NTR 1  is connected to the drain of the memory cell transistor M 1 . The drain of an Nch transistor NTR 2  is connected to the source of the memory cell transistor M 1 . The source of the Nch transistor NTR 2  is connected to GND. 
     The gate of the charge transistor N 1  is connected to the gate of the Nch charge transistor N 2 , and the source of the Nch charge transistor N 2  is connected to the drain of the Nch transistor NTR 0 . The drain of the Nch charge transistor N 2  is connected to the supply voltage VCC. 
     The ON-OFF control of the Nch transistor NTR 0  may be performed by signals generated in an ATD circuit (address transition detection circuit) and the like. The source of the Nch transistor NTR 0  is connected to the bit line  11 . 
     According to the semiconductor memory device  100  having such a configuration, the Nch transistor HN 1  in the reference voltage generating circuit  10  is connected in a source-follower connection. Therefore, when the CE signal is in the L level (i.e., in the stand-by state), the Nch transistor HN 1 , the Nch charge transistor N 1 , and the Nch charge transistor N 2  are all turned OFF, whereby no through current flows in the reference voltage generating circuit  10  and therefore the stand-by current of the entire semiconductor memory device is reduced. 
     On the other hand, when the CE signal turns from the L level to the H level (i.e., turns from the stand-by state to the read state), the through current of the reference voltage generating circuit  10  flows only as a minute current from the Nch transistor HN 1  via the Pch transistors HP 1  and HP 2 , and the current for charging the bit line  11  flows to the Nch charge transistor N 1  at a high speed, whereby the bit line  11  is quickly charged. In addition, since the Nch transistor HN 1  is an N-type transistor, there is no possibility for the charge potential of the bit line  11  to reach the level of the supply voltage VCC. Therefore, the charge potential of the bit line  11  does not unnecessarily increase. 
     Furthermore, according to the semiconductor memory device  100 , high ON resistance Pch transistors HP 1  and HP 2  are serially connected between the gate of the charge transistor N 1  and GND. The gate and drain of each of the Pch transistors HP 1  and HP 2  are connected to each other. Therefore, the resistance between the gate of the Nch charge transistor N 1  and GND is high and no large current constantly flows into the bit line  11 . 
     If the voltage at the gate of the Nch charge transistor N 1  increases over a desired voltage for some reason, the current flows to GND through the pair of Pch transistors HP 1  and HP 2 . Therefore, the voltage of the charge transistor N 1  is lowered. 
     Although in the above example, a pair of Pch transistors HP 1  and HP 2  are connected between the source of the source-follower connected Nch transistor HN 1  and GND, three or more low performance Pch transistors may be alternatively connected therebetween. 
     Furthermore, according to the semiconductor memory device  100  of the present example, an Nch charge transistor N 2  has a high driving performance assisting the charging of the bit line  11 , and an Nch transistor NTR 0  which turns ON during the precharge period is provided between the source of the Nch charge transistor N 2  and the bit line  11 . Therefore, the current driving performance of the Nch charge transistor N 1  can be made low even if the current driving performance of the memory cell transistor M 1  is low, whereby the potential difference is made large between the bit line  11  connected to an ON transistor and the bit line  11  connected to an OFF transistor. As a result, it is possible for the sense circuit  20  to quickly read the potential of the bit line  11  so as to determine the ON/OFF state of the memory cell transistor M 1 . 
     Generally, when the temperature decreases, the resistance decreases. This causes the source/drain current within a transistor to increase, and thus the operation current to increase. According to the semiconductor memory device  100  of the present invention, however, the voltage is generated based on the threshold value of the Nch transistor HN 1 . Therefore, when the temperature decreases, the threshold value of the Nch transistor HN 1  becomes higher, and the charge level of the bit line  11  becomes lower. Under a low temperature, the line resistance also decreases, and therefore the information stored in the memory cell can be read even if the charge level of the bit line  11  is low. In addition, by lowering the charge level, the operation current is also reduced. 
     FIG. 2 shows a charge voltage generating circuit  10 ′ in another example of a semiconductor memory device  100  according to the present invention. The charge voltage generating circuit  10 ′ in FIG. 2 is similar to the charge voltage generating circuit  10  in FIG. 1 except that the charge voltage generating circuit  10 ′ in FIG. 2 includes a low performance (high ON resistance) Pch transistor HP 3  instead of the pair of Pch transistors HP 1  and HP 2  of the charge voltage generating circuit  10  in FIG.  1 . According to the charge voltage generating circuit  10 ′ in FIG. 2, the quantity of the flowing current is only that corresponding to the performance of the Pch transistor HP 3 , and therefore the operation current is reduced. This is because the performance of the Pch transistor HP 3  is low. 
     FIG. 3 shows a charge voltage generating circuit  10 ″ in still another example of a semiconductor memory device  100  according to the present invention. The charge voltage generating circuit  10 ″ in FIG. 3 is similar to the charge voltage generating circuit  10 ′ in FIG. 2 except that the charge voltage generating circuit  10 ″ in FIG. 3 includes an Nch transistor N 3  instead of the Pch transistor HP 3  of the charge voltage generating circuit  10 ′ in FIG.  2 . According to the charge voltage generating circuit  10 ″ in FIG. 3, a current flowing in the device is larger than that in the charge voltage generating circuit  10 ′ in FIG.  2 . but as compared to a conventional semiconductor memory device, both the operation current and the stand-by current are reduced. 
     The charge voltage generating circuit  10 ′″ in FIG. 4 includes a resistor R instead of the Nch transistor N 3  of the charge voltage generating circuit  10 ″ in FIG.  3 . This configuration also reduces both the operation current and the stand-by current as compared to a conventional semiconductor memory device. 
     In the above examples, only one pair of Pch transistors HP 1  and HP 2  (FIG.  1 ), one Pch transistor HP 3  (FIG.  2 ), one Nch transistor N 3  (FIG.  3 ), and one resistor R (FIG. 4) are provided on the chip. Alternatively, a plurality of such elements (or pairs of elements) can be provided on the chip. Furthermore, the transistor used in the present invention is not limited to the Nch transistor. Other appropriate elements such as Pch transistors and bipolar transistors can also be used. 
     As described above, according to the semiconductor memory device of the present invention, the chip area does not increase even though the semiconductor memory device is so configured as to precharge the bit line by the precharge circuit. In addition, even in a high performance device with a multiple bit capability, the operation current can be reduced. If the semiconductor memory device incorporates a stand-by function, the stand-by current can be reduced. Furthermore, the effect of reducing the temperature dependence of the operation current is also achieved. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.