Abstract:
In order to latch and store a word line reset level voltage (negative voltage) which is set during reset operation, a word line driver includes PMOS transistors and NMOS transistors. The word line driver further includes a stress-reducing PMOS transistor and an NMOS transistor, and also a word line bias control circuit which controls and activates a supply bias during setting of a word line, start of resetting, and a reset period.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of PCT International Application PCT/JP2009/000756 filed on Feb. 23, 2009, which claims priority to Japanese Patent Application No. 2008-300972 filed on Nov. 26, 2008. The disclosures of these applications including the specifications, the drawings, and the claims are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to semiconductor memory devices, and more particularly, to semiconductor memory devices having negative boost word line drivers for causing the levels of word lines to have a negative voltage when the device is reset. 
     In recent years, for embedded dynamic random access memory (DRAM), there has been particularly a demand for higher integration which allows system-on-chip (SOC) with low cost. The area of a memory device is dominated by a memory array. Therefore, in order to achieve higher integration, attempts have been made to reduce the area of each memory cell by applying microfabrication techniques to memory cell transistors, or memory cell capacitors using high-k insulating film. 
     In DRAM, in order to fully restore a voltage having a bit line amplitude in a memory cell capacitor, a transistor having a thick film is used as the memory cell transistor because a voltage higher than the threshold voltage (Vth) needs to be applied to the gate. At the same time, the memory cell transistor needs to have a low leakage characteristic in order to hold data of the memory cell capacitor. Therefore, the threshold voltage of the memory cell transistor is set to a high value, and the back bias effect is large, and therefore, the set level of word lines is a voltage which is higher, by 1 V or more, than the high voltage of bit lines. 
     On the other hand, the gate length of the memory cell transistor needs to be decreased in order to reduce the size of the memory cell transistor. Also, the thickness of the memory cell transistor needs to be decreased in order to reduce a leakage current which is caused by the short channel effect. In this case, a voltage at the word line set level needs to be decreased in terms of reliability, and at the same time, the threshold voltage Vth needs to be decreased in order to ensure the restoration level of the memory cell capacitor, and the leakage current needs to be prevented from increasing. 
     To solve the above problems, an architecture called a negative boost word line scheme is known. In this technique, satisfactory characteristics required for the memory cell transistor may be obtained by modifying the reset level of word lines from a ground voltage to a negative boost voltage level. 
     When the negative boost word line scheme is introduced, a word line driver which can shift the level of a signal having a bit line voltage amplitude in both the positive direction and the negative direction is additionally required. As examples of such a negative boost word line driver, various level shift circuits are proposed in Japanese Patent Publication No. H07-307091. 
     Japanese Patent Publication No. H08-63964 describes a configuration in which an address decoding function is provided, and a level shift voltage in both the positive direction and the negative direction can be applied to a word line using a small number of elements. An example of this conventional word line driver is shown in  FIG. 7 . In  FIG. 7 , reference characters  100   a  and  100   b  indicate word line drivers, reference characters XA, XB, WD&lt; 0 &gt;, and WD&lt; 1 &gt; indicate word line select address signals, a reference character /STWD indicates a word line reset control signal, reference characters WL&lt; 0 &gt; and WL&lt; 1 &gt; indicate word lines, a reference character BL indicates a bit line, reference characters /AD and Node 1  indicate internal nodes, a reference character Vdd indicates the high voltage of the bit line, a reference character Vss indicates the low voltage of the bit line, a reference character Vpp indicates the set level voltage of the word lines, a reference character Vw indicates the reset level voltage of the word lines, a reference character Vcp indicates the plate voltage of a memory cell, reference characters QN 1 , QN 2 , QAN 1 , and QAN 2  indicate NMOS transistors, reference characters QP 1 -QP 3 , QAP 1 , and QAP 2  indicate PMOS transistors, a reference character QC indicates a memory cell transistor, and a reference character C indicates a memory cell capacitor. 
     Operation of the word line driver  100   a  thus configured will be described with reference to a timing diagram shown in  FIG. 8 . Initially, in a reset state before timing t 10 , the word line reset control signal /STWD is low, so that the level of the internal node Node 1  is Vpp, and the word line WL&lt; 0 &gt; is at the reset level Vw. The word line WL&lt; 1 &gt; driven by the adjacent word line driver  100   b  is similarly held at the reset level Vw (not shown). Thereafter, at timing t 10 , a portion of an address input to the word line driver  100   a  is selected (logic high), and the internal node /AD goes low. At timing t 11 , WD&lt; 0 &gt; which is a superimposition of a word line activate signal and an address signal goes high, and at the same time, the word line reset control signal /STWD goes high, so that the level of the internal node Node 1  goes low, and the word line WL&lt; 0 &gt; goes to the set level Vpp. Because WD&lt; 1 &gt; is low (non-selected state), the adjacent word line driver  100   b  remains in the reset state although the internal node /AD connected commonly to the word line drivers  100   a  and  100   b  is low. Thereafter, at timing t 12 , WD&lt; 0 &gt; goes low, and the word line reset control signal /STWD goes low, so that the internal node Node 1  goes high, and the voltage of the word line WL&lt; 0 &gt; returns to the reset level Vw. Thereafter, at timing t 13 , the word line select address signals XA and XB and the internal node /AD return to their original states. Here, t 10  and t 11 , or t 12  and t 13 , may be the same timing. 
     In the above configuration, |Vw| is applied between the gate and source of the NMOS transistor QN 1  when the word line WL&lt; 0 &gt; is selected, and therefore, a leakage current is likely to flow from the power supply Vpp to the power supply Vw. Therefore, the threshold voltage of the NMOS transistor QN 1  is set to be higher than those of transistors around the transistor QN 1 . In the case of embedded DRAM, the transistor QN 1  is typically formed using the same gate oxide film of a high-voltage transistor of a logic circuit or the memory cell transistor QC, whereby the number of processes is decreased to reduce the cost. However, in the case of the configuration including the negative boost word line driver, for example, when the memory device is reset, the source-to-drain voltage of the PMOS transistor QP 1 , the gate-to-source voltage of the NMOS transistor QN 1 , and the gate-to-source voltage of the PMOS transistor QP 2  are all Vpp+|Vw| (&gt; Vpp), and therefore, the reliability is likely to be degraded due to voltage stress. 
     To solve this problem, there is, for example, a known technique of reducing the voltage stress by inserting a diode transistor, as described in, for example, Japanese Patent Publication Nos. 2001-297583 and H11-283369. 
     Moreover, the negative boost word line driver may suffer from an increase in power consumption and current noise of the negative boost power supply. Because the negative boost voltage is generated using a charge pump having low power supply efficiency, power consumption is likely to increase if a discharged current from a word line increases. In addition, it takes a long time to cause a changed voltage level to return to the set level. As a result, the word line reset level changes, so that the leakage current of the memory cell transistor increases, and therefore, the charge holding characteristic is degraded. 
     To solve this problem, there is, for example, a known technique of, when a word line is reset, temporarily discharging the word line to a ground voltage, and thereafter, discharging the word line to the negative boost power supply, thereby reducing the discharged current to the negative boost power supply to reduce power consumption and improve voltage stability, as described in, for example, Japanese Patent Publication Nos. H10-241361 and 2002-352580. 
     SUMMARY 
     However, there is a strong demand for a reduction in cost which is achieved by a reduction in area. Therefore, a negative boost word line driver including a smaller number of parts than that of Japanese Patent Publication No. H08-63964 is preferable. Moreover, if such a negative boost word line driver is provided with a configuration for reducing the voltage stress, such as those described in Japanese Patent Publication Nos. 2001-297583 and H11-283369, high-speed word line drive is also required. Moreover, if the configuration of Japanese Patent Publication No. H10-241361 or 2002-352580 is used to reduce the power consumption and noise of the negative boost power supply, an additional NMOS transistor is required for each word line driver, resulting in considerable area penalties. 
     The present disclosure describes implementations of a negative boost word line driver with a smaller area, higher speed, higher accuracy, and lower power consumption. 
     An example semiconductor memory device of the present disclosure includes word line drivers configured to be selected based on a plurality of word line select address signals. Each of the word line drivers has a set level which is a first voltage, and a reset level which is a second voltage. A latch is formed in each of the word line drivers only when a corresponding word line is not selected, and the latch is used to hold the voltage of the word line at the reset level. As a result, the number of parts constituting the word line drivers the number of which is the same as the number of the word lines can be reduced, whereby the word line driver has a smaller area. 
     The present disclosure provides a negative boost word line driver with a smaller area, higher speed, higher accuracy, and lower power consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of word line drivers in a semiconductor memory device according to a first embodiment of the present disclosure. 
         FIG. 2  is a timing diagram of the word line drivers in the semiconductor memory device of  FIG. 1 . 
         FIG. 3  is a circuit diagram of word line drivers in a semiconductor memory device according to a second embodiment of the present disclosure. 
         FIG. 4  is a timing diagram of the word line drivers in the semiconductor memory device of  FIG. 3 . 
         FIG. 5  is a circuit diagram of word line drivers in a semiconductor memory device according to a third embodiment of the present disclosure. 
         FIG. 6  is a timing diagram of the word line drivers in the semiconductor memory device of  FIG. 5 . 
         FIG. 7  is a circuit diagram of word line drivers in a conventional semiconductor memory device. 
         FIG. 8  is a timing diagram of the word line drivers in the semiconductor memory device of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     &lt;&lt;First Embodiment&gt;&gt; 
     A first embodiment of the present disclosure will be described hereinafter with reference to the accompanying drawings. 
       FIG. 1  is a circuit diagram of word line drivers in a semiconductor memory device according to the first embodiment of the present disclosure. Reference characters  110   a  and  110   b  indicate word line drivers, reference characters XA, XB, WDI&lt; 0 &gt;, WDI&lt; 1 &gt;, WD&lt; 0 &gt;, and WD&lt; 1 &gt; indicate word line select address signals, reference characters WL&lt; 0 &gt; and WL&lt; 1 &gt; indicate word lines, a reference character BL indicates a bit line, reference characters /AD and Node 1  indicate internal nodes, a reference character RESET indicates a power supply activation flag signal, a reference character Vss indicates a bit line low voltage (ground voltage), a reference character Vpp indicates a word line set level voltage (positive boosted voltage), a reference character Vw indicates a word line reset level voltage (negative voltage), a reference character Vcp indicates a memory cell plate voltage, reference characters QN 1 , QN 2 , QAN 1 , and QAN 2  indicate NMOS transistors, reference characters QP 1 , QP 2 , QAP 1 , and QAP 2  indicate PMOS transistors, a reference character QC indicates a memory cell transistor, a reference character C indicates a memory cell capacitor, and reference characters OR 1  and OR 2  indicate OR circuits. The semiconductor memory device of  FIG. 1  is different from the conventional example of  FIG. 7  in the following points: the word line reset control signal /STWD and the PMOS transistor QP 3  are removed; the word line select address signals WD&lt; 0 &gt; and WD&lt; 1 &gt; are selected in synchronization with the power supply activation flag signal RESET by the added OR circuits OR 1  and OR 2 ; the high voltages of the word line select address signals XA, XB, WDI&lt; 0 &gt;, WDI&lt; 1 &gt;, WD&lt; 0 &gt;, and WD&lt; 1 &gt; are boosted from the bit line high level Vdd to the word line set level Vpp before being supplied; and the word line set level voltage Vpp is also applied to a NAND circuit which is composed of the NMOS transistors QAN 1  and QAN 2  and the PMOS transistors QAP 1  and QAP 2 . 
     More specifically, in the word line driver  110   a , the source, gate, and drain of the PMOS transistor QP 1  are connected to the word line set level voltage Vpp, the internal node Node 1 , and the word line WL&lt; 0 &gt;, respectively. The drain, gate, and source of the NMOS transistor QN 1  are connected to the word line WL&lt; 0 &gt;, the internal node Node 1 , and the word line reset level voltage Vw, respectively. The source, gate, and drain of the PMOS transistor QP 2  are connected to the word line set level voltage Vpp, the word line WL&lt; 0 &gt;, and the internal node Node 1 , respectively. The gate, source, and drain of the NMOS transistor QN 2  are connected to the word line select address signal WD&lt; 0 &gt;, the internal node /AD, and the internal node Node 1 , respectively. In the word line driver  110   a , only when the word line WL&lt; 0 &gt; is not selected, the PMOS transistor QP 1  in the off state, the NMOS transistor QN 1  in the on state, and the PMOS transistor QP 2  in the on state form a latch which holds the voltage of the word line WL&lt; 0 &gt; at the reset level Vw. 
     Operation of the circuit thus configured will be described with reference to a timing diagram shown in  FIG. 2 . The timing diagram of  FIG. 2  is different from the timing diagram of the word line driver in the conventional semiconductor memory device of  FIG. 8  in the following points: during a power supply activation period or a predetermined period of time after activation of the power supply, i.e., between timings t 0  and t 1 , the power supply activation flag signal RESET is enabled, and in response to this, the word line select address signals XA and XB go logic low (non-selected state), and the word line select address signals WD&lt; 0 &gt; and WD&lt; 1 &gt; go logic high (selected state), whereby all the word lines WL&lt; 0 &gt; and WL&lt; 1 &gt; temporarily go to the reset level Vw; and at word line reset timing t 4 , word line reset timing information is superimposed on one or both of the word line select address signals XA and XB, and the internal nodes /AD and Node 1  go high, whereby the word line WL&lt; 0 &gt; is discharged to the reset level Vw, and at delayed timing t 5 , the word line select address signal WD&lt; 0 &gt; goes low. Note that timings t 2  and t 3  of  FIG. 2  correspond to timings t 10  and t 11  of  FIG. 8 , respectively. 
     According to this embodiment, the word line the reset level Vw which is set during reset operation is latched and stored, whereby the word line reset control signal /STWD and the PMOS transistor QP 3  which are required in the conventional configuration are removed, so that it is possible to reduce the number of parts constituting the word line drivers  110   a  and  110   b  the number of which is the same as the number of the word lines, and therefore, the area can be reduced. 
     Also, according to this embodiment, WD&lt; 0 &gt; and WD&lt; 1 &gt; which are a portion of the plurality of word line select address signals go to the logic (high) which resets all the word lines during a predetermined period of time that the power supply is activated. Therefore, the initial outputs of all the word line drivers during the power supply activation period can be reliably caused to be in the reset state, which is then held by a latch included in each of the word line drivers  110   a  and  110   b . As a result, it is possible to reduce or prevent erroneous operation that the word line drivers  110   a  and  110   b  are initially multiply selected. 
     Also, according to this embodiment, the NMOS transistor QN 2  has an address decoding function, and therefore, it is not necessary to provide NAND circuits for decoding an address in one-to-one correspondence with the word line drivers  110   a  and  110   b , whereby the area can be reduced. 
     Note that only when the word line is set, the gate-to-source voltage of the NMOS transistor QN 1  is |Vw|, and therefore, a leakage current flows from the power supply Vpp to the power supply Vw. To reduce or prevent this, the threshold voltage of the NMOS transistor QN 1  may be set to be higher than the threshold voltages of the other transistors QP 1 , QP 2 , and QN 2 , or another NMOS transistor may be connected in series between the NMOS transistor QN 1  and the power supply Vw to provide the back bias effect. 
     &lt;&lt;Second Embodiment&lt;&lt; 
       FIG. 3  is a circuit diagram of word line drivers in a semiconductor memory device according to a second embodiment of the present disclosure. The word line drivers of  FIG. 3  are different from the word line drivers in the semiconductor memory device of the first embodiment of the present disclosure of  FIG. 1  in the following points: a PMOS transistor QP 4  and an NMOS transistor QN 3  for reducing stress are added in the word line drivers  120   a  and  120   b ; and a word line bias control circuit  200  including PMOS transistors QP 5  and QP 6  is added. Reference characters ACT and /ACT indicate word line bias control signals, and a reference character Node 2  indicates an internal node. 
     More specifically, in the word line driver  120   a , the source, gate, and drain of the PMOS transistor QP 1  are connected to the internal node Node 2 , the internal node Node 1 , and the word line WL&lt; 0 &gt;. The drain of the NMOS transistor QN 1  is connected to the word line WL&lt; 0 &gt;, the gate of the NMOS transistor QN 1  is connected via the stress-reducing NMOS transistor QN 3  to the internal node Node 1 , and the source of the NMOS transistor QN 1  is connected to the word line reset level voltage Vw. The gate of the stress-reducing NMOS transistor QN 3  is connected to the word line set level voltage Vpp. The source of the PMOS transistor QP 2  is connected to the word line set level voltage Vpp, the gate of the PMOS transistor QP 2  is connected via the stress-reducing PMOS transistor QP 4  to the word line WL&lt; 0 &gt;, and the drain of the PMOS transistor QP 2  is connected to the internal node Node 1 . 
     The gate of the stress-reducing PMOS transistor QP 4  is connected to the ground voltage Vss. The gate, source, and drain of the NMOS transistor QN 2  are connected to the word line select address signal WD&lt; 0 &gt;, the internal node /AD, and the internal node Node 1 , respectively. 
     On the other hand, in the word line bias control circuit  200 , the source, gate, and drain of the PMOS transistor QP 5  are connected to the word line set level voltage Vpp, the word line bias control signal /ACT, and the internal node Node 2 , respectively. The source, gate, and drain of the PMOS transistor QP 6  are connected to the bit line high voltage Vdd, the word line bias control signal ACT, and the internal node Node 2 , respectively. 
     Operation of the circuit thus configured will be described with reference to a timing diagram shown in  FIG. 4 . The timing diagram of  FIG. 4  is different from the timing diagram of the word line driver in the semiconductor memory device of the first embodiment of the present disclosure of  FIG. 2  in that at timing t 6 , the word line bias control signal ACT goes high and the inverted signal /ACT goes low, so that the internal node Node 2  is controlled to the word line set level voltage Vpp, and thereafter, at timing t 4 , the word line bias control signal ACT goes low and the inverted signal /ACT goes high, so that the internal node Node 2  is controlled to the bit line high voltage Vdd. 
     According to this embodiment, the NMOS transistor QN 3  can reduce the gate-to-source voltage of the NMOS transistor QN 1  in the word line reset state from Vpp+|Vw| to Vpp−Vth_n3+|Vw|, where Vth_n3 is the threshold voltage of the NMOS transistor QN 3 . Similarly, the PMOS transistor QP 4  can reduce the gate-to-source voltage of the PMOS transistor QP 2  in the word line reset state to Vpp−Vth_p4+|Vw|, where Vth_p4 is the threshold voltage of the PMOS transistor QP 4 . As a result, the reliability of the NMOS transistor QN 1  and the PMOS transistor QP 2  can be improved. 
     Also, according to this embodiment, the sources of the PMOS transistors QP 1  in the word line drivers  120   a  and  120   b  are connected commonly to the internal node Node 2 , and the word line bias control circuit  200  is provided which switches a voltage supplied to the internal node Node 2  between Vdd and Vpp. As a result, the source-to-drain voltage of the PMOS transistor QP 1  in the word line reset state can be reduced from the conventional Vpp+|Vw| to Vdd+|Vw|, whereby the reliability can be improved. Moreover, by reducing the source-to-drain voltage of the PMOS transistor QP 1 , a leakage current flowing into the power supply Vw can be reduced, and therefore, a highly stable retention characteristic and a reduction in power consumption can be simultaneously achieved. 
     Because the internal node Node 2  is shared by the word line drivers  120   a  and  120   b , the load capacitance of the internal node Node 2  is large. However, because the internal node Node 2  in the word line reset state is at the bit line high voltage Vdd, the load capacitance of the internal node Node 2  can be quickly charged to the word line set level voltage Vpp by the word line driver set timing t 3 , resulting in high-speed word line set operation. 
     The word line bias control circuit  200  is shared by the word line drivers  120   a  and  120   b , and therefore, area penalties are substantially negligible. 
     Note that the present disclosure is not limited to the above embodiment. Alternatively, for example, the internal node Node 2  in the word line reset state may be at an intermediate voltage between the word line set level voltage Vpp and the ground voltage Vss, e.g., a voltage which is lower by the transistor threshold voltage than the word line set level voltage Vpp, etc. 
     &lt;&lt;Third Embodiment&gt;&gt; 
       FIG. 5  is a circuit diagram of word line drivers in a semiconductor memory device according to a third embodiment of the present disclosure. The circuit diagram of  FIG. 5  is different from the circuit diagram of the word line driver in the semiconductor memory device of the second embodiment of the present disclosure of  FIG. 3  in that an NMOS transistor QN 4  is added to a word line bias control circuit  210 . Reference characters /ACT 1 , ACT 2 , and DISC indicate word line bias control signals. 
     More specifically, in the word line bias control circuit  210 , the source, gate, and drain of the PMOS transistor QP 5  are connected to the word line set level voltage Vpp, the word line bias control signal /ACT 1 , and the internal node Node 2 , respectively. The source, gate, and drain of the PMOS transistor QP 6  are connected to the bit line high voltage Vdd, the word line bias control signal ACT 2 , and the internal node Node 2 , respectively. The drain, gate, and source of the NMOS transistor QN 4  are connected to the internal node Node 2 , the word line bias control signal DISC, and the ground voltage Vss, respectively. 
     Operation of the circuit thus configured will be described with reference to a timing diagram shown in  FIG. 6 . The timing diagram of  FIG. 6  is different from the timing diagram of the word line driver in the semiconductor memory device of the second embodiment of the present disclosure of  FIG. 4  in that, at timing t 4 , the word line bias control signals /ACT 1  and DISC go high, so that the PMOS transistor QP 1  is turned on and the level of the internal node Node 2  goes to the ground voltage Vss, and at timing t 7 , the internal nodes /AD and Node 1  go high, so that the PMOS transistor QP 1  is turned off, and the word line bias control signal ACT 2  goes low, so that the internal node Node 2  is charged to the bit line high voltage Vdd. 
     According to this embodiment, not only the transistor reliability can be improved as in the second embodiment of the present disclosure, but also when the word lines are reset, a portion of the charge on the word line WL&lt; 0 &gt; is discharged to the power supply Vss via the PMOS transistor QP 1  and the NMOS transistor QN 4 , and thereafter, the remaining charge on the word line WL&lt; 0 &gt; is discharged to the power supply Vw (negative boost power supply), whereby current noise to the power supply Vw can be reduced. The reduction in the current noise can improve the charge holding characteristic of memory cells, and can also reduce power consumption in the negative boost voltage generation circuit which employs a charge pump, which has low efficiency, and therefore, power consumption in the memory chip can also be reduced. 
     In addition, the NMOS transistor QN 4  for discharging the word lines is shared by the word line drivers  120   a  and  120   b , whereby the area can be reduced without increasing the number of parts in the word line drivers  120   a  and  120   b  the number of which is the same as the number of the word lines. 
     Note that when charge on the word lines is discharged to the power supply Vss, the word line voltage cannot be set to be lower than the absolute value of the threshold voltage of the PMOS transistor QP 1 . However, if the substrate and the source (i.e., the internal node Node 2 ) of the PMOS transistor QP 1  are connected together, the substrate bias effect can be reduced, and therefore, the word line voltage can be reduced to a lower voltage, whereby the current noise to the power supply Vw can be further reduced. 
     The first to third embodiments are only for illustrative purposes, and the present disclosure is not limited to the first to third embodiments. Various changes and modifications can be made without departing the spirit and scope of the present disclosure. 
     The semiconductor memory device including the negative boost word line drivers of the present disclosure is useful as a semiconductor memory device with a smaller area, higher reliability, and lower power consumption.