Abstract:
The disclosure is a method of controlling operations in a static random access memory employing twin cells. After a wordline coupled to first and second cell transistors is conductive, a voltage difference between a first bitline, which is connected to a first cell capacitor through the first cell transistor, and a second bitline, which is connected to a second cell capacitor through the second cell transistor, is driven into a sense amplifier to be developed with amplification. An active wordline turns nonconductive when one of the bitline voltages accords with a predetermined reference voltage.

Description:
This application claims priority to Korean Patent Application No. 2002-1874, filed on Jan. 12, 2002. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to semiconductor memory, and more specifically, to static random access memories (SRAMs) employing twin memory cells as a storage unit. 
     2. Discussion of Related Art 
     Single-transistor dynamic random access memories are well known in the field of semiconductor memory devices. As shown in FIG. 1, a unit memory cell MC 1  or MC 2  comprises of one transistor TR and one capacitor C. A gate of the cell transistor TR is coupled to a wordline WL 1 , and a current channel of the cell transistor is formed between a bitline BL and a plate voltage terminal Vp that is connected to an electrode of the cell capacitor C. Another memory cell MC 2 , whose gate is coupled to another wordline WL 2 , forms its current channel between another bitline BLB (complementary to BL) and an electrode of the cell capacitor C. The bit line pair of BL and BLB are coupled to a sense amplifier  12 . 
     In the structure of the DRAM cell array, if the wordline WL 1  is selected, a cell data stored in the memory cell MC 1  is transferred to the bitline BL through a charge sharing effect. The bitlines BL and BLB that have been held on a precharge voltage, e.g., at one half of a power supply voltage, increase/decrease or decrease/increase according to the data stored in the memory cell MC 1  through the charge sharing. The voltage level of the bitline BL rises up to a high level if the data is “1” and falls down to a low level if the data is “0”. Then, the sense amplifier  12  detects and amplifies the voltage difference between the bitlines BL and BLB. 
     Leakage currents arising from the structural properties of the DRAM reduce the charge amount of the cell capacitor, resulting in a data loss. A refresh is needed periodically to prevent the data loss. A DRAM refresh time is determined with regarding the maximal term of maintaining data “1” in a memory cell. One method for achieving a low-power operation is associated with the extension of the refresh time (or refresh period). Lengthening a refresh time is possible by adopting a twin-cell structure in a memory cell array, as shown in FIGS. 2 and 3. FIGS. 2 and 3 show twin-cell arrays also applicable in SRAMs. 
     In FIG. 2, one twin-cell, as a SRAM cell unit, comprises two DRAM-type cells MC 1  and MC 2 . Each cell comprises one transistor TR and one capacitor C. While the cells MC 1  and MC 2  are respectively connected to the bitlines BL and BLB, which are coupled to a sense amplifier  22 , the cell gates are coupled to one wordline WL 1  in common. The cells MC 1  and MC 2  store a pair of complementary data bits “1” and “0”. 
     Another kind of the twin-cell, as shown in FIG. 3, is a modification of the twin-cell structure of FIG. 2 in an arrangement pattern. Bitlines BL 1 ˜BL 4  are alternately arranged in a column and a twin-cell is composed of two DRAM-type cells MC 1  and MC 2  (or MC 3  and MC 4 ) connected respectively to BL 1  and BL 3  (or BL 2  and BL 4 ). The bitlines BL 1  and BL 3 , as a pair, are coupled to a sense amplifier  32 , while BL 2  and BL 4  as another bitline pair are coupled to sense amplifier  34 . A wordline WL 1  is commonly coupled to gates of the cells MC 1  and MC 3  while a wordline WL 2  to gates of MC 2  and MC 4 . The cells in one twin-cell store a pair of complementary data bits logically. 
     In with a refresh of the twin-cell structure of FIG. 2, referring to FIG. 4A, the bitlines BL and BLB are set up to a precharge voltage VBL (0.5 AIVC) in response to a precharge signal PEQ before the wordline WL 1  becomes active. At this time, assuming that the cell is preparing to store data “1” in MC 1  and “0” in MC 2 , a cell node CN 1  between the cell transistor and capacitor in MC 1  maintains the power supply voltage level AIVC corresponding to data “1” while a cell node CN 2  between the cell transistor and capacitor in MC 2  hold a ground voltage level GND corresponding to data “0”. Once the wordline WL 1  is active, a charge sharing starts between MC 1  and BL and between MC 2  and BLB. As a result, a BL voltage increases by Vcsu, the voltage difference between the bitline voltage VBL (0.5 AIVC) to a shared-in voltage supplied from the cell MC 1  (CN 1 ), while BLB voltage decreases by Vcsd the voltage difference between VBL and a shared-out voltage discharged out of the cell MC 2  (CN 2 ). With a slight estrangement between BL and BLB, the sense amplifier  22  begins developing a voltage difference and thereby the bitline BL rises up to AIVC while the counter bitline BLB falls down to a ground voltage GND. At this time, the current voltage levels on BL and BLB are restored into the cell nodes CN 1  and CN 2 , respectively. After then, if the wordline becomes inactive, the bitlines BL and BLB are set on the precharge voltage in response to the precharge signal PEQ. 
     As another operational feature for the twin-cell SRAM employing a column selection line CSL, referring to FIG. 4B, the column selection line CSL becomes conductive when a voltage difference is sufficiently settled between the bitlines BL and BLB after a charge sharing therebetween. The active CSL electrically connects the bitlines BL and BLB to corresponding input/output lines. During the CSL activation, the input/output lines have a precharge voltage level of an internal power supply voltage (i.e., AIVC), and the bitline BLB and the cell node CN 2  are held at a clamping level. The bitline BL and the cell node CN 1  are not affected. After CSL becomes inactive, BLB and CN 2  fall to GND. Restored in the cell nodes, the bitline WL 1  is shut down and the bitlines BL and BLB return to the precharge voltage level VBL in response to the precharge signal PEQ. 
     As aforementioned, since the DRAM-type cells in the twin-cell unit store a pair of complementary data bits, a bitline voltage difference is twice as large as that of the single cell structure. As shown in FIGS. 4A and 4B, the sense amplifier ( 22 ,  32  or  34 ) can take the voltage differential value of 2Vcs because BL and BLB increase and decrease simultaneously, by the Vcs through the charge sharing operations. 
     Therefore, the twin-cell memory does not need to define a refresh cycle time or period, capable of a normal sensing operation even though a leakage current dissipate a voltage level of the cell node, holding data “1”, to be lower than the bitline precharge voltage VBL. Such a twin-cell memory has a longer refresh time (e.g., several seconds) than the traditional single cell memory typically about 100˜200 ms. Further, a stand-by current is reduced as compared to the single cell memory. The refresh operation is similar to a normal read operation except that sense data does not turn out of the memory chip. 
     An instance of a SRAM-type memory, as well as DRAMs, employing the twin-cell structure has been described in the publication, entitled “2M×16 bit uni-transistor random access memory”, for Product No. “K1S321615M” (May 2001), so called “UtRAM”. The UtRAM is constructed of the same internal structure as a traditional DRAM while using the same commands as SRAM, but conducts a refresh operation automatically by means of an internal control unit without an external refresh command. Thus, it performs a refresh operation for at least each cycle period, even during read/write commands. If the refresh operation is not performed for each cycle period, a refresh operation concurrent with read/write operations can result in loss of data stored in memory cells. As a result, the twin-cell SRAM needs to lengthen an access time or to degrade an operation speed relative to other single-cell DRAMs. 
     Therefore, a need exists for a system and method for a twin-cell memory device having a long refresh time and high speed of operation 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a twin-cell SRAM enhancing access time and speed in operation. 
     It is another object of the present invention to provide a method for controlling wordlines in a twin-cell SRAM with a shortened wordline activation time 
     It is further another object of the present invention to provide a method for conducting operations of reading, writing, and refreshing in a DRAM-type SRAM, with a shortened wordline activation time. 
     The present invention will be better understood from the following detailed description of the exemplary embodiment thereof taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described below in more detail, with reference to the accompanying drawings: 
     FIG. 1 illustrates a typical core structure of a memory cell array in a dynamic random access memory; 
     FIGS. 2 and 3 illustrate general core structures of twin-cell array RAMs; 
     FIG. 4A is a timing diagram of a refresh operation in the twin-cell array core shown in FIG. 2 or  3 ; 
     FIG. 4B is a timing diagram of a read operation in the twin-cell array core shown in FIG. 2 or  3 ; 
     FIG. 5 is a block diagram of a static RAM according to the invention; 
     FIG. 6 shows an overall operation timing in the static RAM shown in FIG. 5; 
     FIG. 7 is a flow chart of a refresh operation in the static RAM; 
     FIG. 8 is a timing diagram of the refresh operation; 
     FIG. 9 is a flow chart of a read operation in the static RAM; 
     FIG. 10 is a timing diagram of the read operation in the static RAM; and 
     FIG. 11 is a diagram comparing the conventional with the present one with respect to an access time in static SRAMs. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It should be understood that the description of the preferred embodiment is merely illustrative and that it should not be taken in a limiting sense. In the following detailed description, several specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these specific details 
     FIG. 5 illustrates a functional structure of a SRAM with twin-cells. A timing diagram related to the structure of FIG. 5 is shown in FIG. 6. A Twin-cell structure employed in a memory cell array  110  may be any one of those kinds shown in FIGS. 2 and 3, or others. According to an embodiment of the present invention, the memory cell array  110  adopts the twin-cell illustrated by FIG.  2 . The SRAM includes, in addition to the memory cell array  110 , a row selection circuit  120 , a sense-amp/column-gate (SA&amp;CG) block  140 , a column selection circuit  130 , a control block  150 , a reference column gate  160 , a reference wordline driver  170 , a signal generator  180 , a reference voltage generator  190 , a reference sense amplifier  200 , and a reference cell array RMC. 
     The row selection circuit  120  activates a wordline (or row) arranged in the memory cell array  110 . The SA&amp;CG block  140  detects and amplifies data stored in the twin-cell MC coupled to an activated wordline, in response to a sensing enable signal PLA provided from the control block  150  and turns the data into input/output lines I/Oi. The control block  150  generates the sensing enable signal PLA and a refresh enable signal RFen from control signals /CS, /OE, and /WE provided externally. 
     The reference cell RMC comprises a transistor and a capacitor. The transistor comprises a gate coupled to a reference wordline RWL, and a current channel connected between a reference bitline RBL and a reference cell node RCN 1 , wherein the RCN 1  is disposed between the reference cell transistor and capacitor. The reference cell RMC is designed to store data “1”. A pair of reference bitlines RBL and RBLB are connected to the reference column gate  160  that supplies the reference data “1” into the reference cell RMC. The complementary reference bitline RBLB is also coupled to another reference cell (not shown). The reference cell RMC can be implemented as a twin-cell structure or a single-cell structure. 
     The reference wordline RWL is coupled to the driver  170  that activates the reference wordline RWL in response to the refresh enable signal RFen provided from the control block  150 . At the time of the RWL activation, as shown in FIG. 6, a wordline in the memory cell array  110  becomes active by the row selection circuit  120 . The RWL activation varies voltage levels of the reference bitlines RBL and RBLB, such that the RBL voltage rises while the RBLB voltage drops. 
     The signal generator  180  activates the detection enable signal DETen in response to the refresh enable signal RFen and the sensing enable signal PLA. During an activation of the sensing enable signal PLA after the reference wordline RWL has turned on, the detection enable signal DETen goes up to a high level from a low level. FIG. 6 shows the detection enable signal DETen becomes active upon a sensing operation for the selected wordline in the memory cell array  110 . 
     The detection circuit  200 , coupled to the reference cell node RCN 1 , determines whether a current voltage level at the node RCN 1  is identical to the reference voltage VREF generated from the reference voltage generator  190 . The reference voltage VREF may be established on a low voltage of about 0.3 V through 0.4V, and can vary depending on manufacturing factors and loading capacity. If a voltage of the reference cell node RCN 1  is lower than the reference voltage VREF, the detection circuit  200  outputs a wordline disable signal WLdis of a low level, maintaining a current activation state of the wordline. Otherwise, if the RCN 1  voltage reaches the reference voltage VREF, the wordline disable signal WLdis is generated with a high level from the detection circuit  200 , making the wordline shut down. 
     After the wordline deactivation by the signal WLdis, the reference cell node RCN 1 , which has stored data “1”, is recharged up to a power supply voltage of the memory cell array, and then the reference wordline RWL becomes nonconductive. Subsequent to the deactivation of the reference wordline RWL, the detection enable signal DETen and the wordline disable signal WLdis are deactivated in series, terminating the refresh operation. 
     Refresh and read operations in a twin-cell SRAM are described with reference to FIGS. 7 through 10. FIGS. 7 and 8 show a refresh operation, while FIGS. 9 and 10 show a read operation. It would be understood that the timing features shown in FIG. 6 are also applicable to the following description, as an overall operation. For an appropriate explanation, it assumed the memory cell array  110  includes memory cells like MC 1  and MC 2 , for example, are constructed in the form of the twin-cell structure shown in FIG. 2, and the memory cell MC 1  stores data “1” while the memory cell MC 2  holds data “0”. Therefore, the cell node CN 1  of the memory cell MC 1  is at the power supply voltage level for use of the memory cell array while the cell node CN 2  of the memory cell MC 2  is at the ground voltage level GND. 
     As well known, an internal refresh operation in the twin-cell SRAM without an external command is similar to a read operation but it does not send detected data out of the chip. Referring to FIGS. 7 and 8, before a refresh operation, the bitlines BL and BLB are at the precharge voltage VBL (0.5 AVIC) during a precharge cycle. Beginning the refresh operation, a selected wordline WL 1  coupled to the memory cells MC 1  and MC 2  becomes conductive S 100 . As the cell capacitors C are electrically connected to the bitlines BL and BLB through the conductive memory cells MC 1  and MC 2 , a charge sharing operation automatically starts to make the bitlines BL and BLB respond to the charges of the capacitors respectively. Thus, the voltage of the bitline BL rises by Vcsu while the voltage of the bitline BLB drops by Vcsd. With a voltage difference between the bitlines enough to trigger a sense amplifier (e.g.,  22  of FIG. 2) in the SA&amp;CG block  140 , the sense amplifier develops (senses and amplifies) the voltage difference of the bitlines BL and BLB S 120 . Thus, the BL voltage BL goes up to the array power supply voltage AIVC while the BLB voltage down to the ground voltage GND. The voltage of the bitlines BL and BLB charges (e.g., refreshes) the corresponding cell nodes CN 1  and CN 2 . 
     It is determined whether the bitline voltage corresponding to high-level data “1” is equal to the reference voltage VREF S 140 . If the BL voltage is not equal to the reference voltage VREF, the wordline WL 1  will remain active, maintaining the restore condition. If the BL voltage reaches the reference voltage VREF, the wordline WL 1  becomes inactive (nonconductive) S 160 , terminating the restore operation. The reference voltage VREF is designed to level 80% of the power supply voltage AIVC, e.g., 0.3˜0.4V. 
     In the refresh operation, it can be seen that the selected wordline turns inactive even before the cell node CN 1  is recharged up to the power supply voltage AIVC. Such an early deactivation for the wordline shortens an activation time of wordline, and reduces the amount of refresh current or standby current, without adding to a requisite refresh cycle period. 
     Assuming, in a single-cell memory structure, a refresh failure occurs when a cell node voltage is AIVC−Vx (Vx is a certain voltage lower than AIVC), a refresh failure in a twin-cell structure can be generated at the voltage of 0.5AIVC−Vx or VBL−Vx. Thus, due to a leakage current, the twin-cell structure meets with a refresh failure when a cell node voltage is lowered by 0.5AIVC+Vx or VBL+Vx, while the single-cell structure falls when a cell node voltage is lowered by Vx from the power supply voltage AIVC. As a result, it is possible for the present twin-cell SRAM to possess an improved refresh cycle time (e.g., longer) together with reducing refresh current or standby current. Leakage current decreases exponentially with a decrease of a cell node voltage, and a refresh time increases in the twin-cell SRAM according to an embodiment of the present invention. 
     The sense amplifier can be constructed with PMOS transistors (not shown) to pull the BL voltage up to the power supply voltage AIVC and NMOS transistors (not shown) to pull the BLB voltage down to the ground voltage GND. Because the current drivability of a PMOS transistor is less than that of an NMOS of equal size, a pull-up operation for BL is faster than a pull-down operation for BLB. In the twin-cell structure, for the purpose of preventing a sensing failure when the cell node voltage drops by VBL+Vx, the cell node voltage corresponding to data “0” needs to return to the ground voltage GND first. During the development of the bitlines BL and BLB, as shown in FIG. 8, recharging the cell node (i.e., CN 1 ), which is to be set to the ground voltage GND, proceeds faster than recharging the counter cell node (i.e., CN 2 ), which is to be set to the power supply voltage AIVC. 
     As current leaks from a storage cell, the cell node voltage corresponding to data “1” degrades exponentially, a refresh time for recharging the cell node to the power supply voltage AIVC is similar to that for recharging it to a lower voltage than the power supply voltage (e.g., 0.3˜0.4V lower). The cell node of data “1” can be recharged at the end of the restore period relative to its beginning because of a smaller source-to-drain voltage difference of the PMOS transistor and a smaller charging current, resulting in a timing gap between the recharging times with AIVC and the voltage lower than AIVC by 0.3˜0.4V. However, since the cell node of data “1” is charged up to 80% of AIVC, which is enough to correspond to data “1”, while the cell node of data “0” fully drops to the ground voltage, the present twin-cell SRAM may shorten a wordline activation time without degrading refresh performance. 
     In reading data from a memory cell in the twin-cell SRAM, referring to FIGS. 9 and 10, the method for wordline activation and a restore operation, from steps S 200  to S 220  of FIG. 9, are similar to the aforementioned refresh operation (See FIG.  7 .). Completing the recharge operation for the cell nodes CN 1  and CN 2 , a column selection line CSL is conductive by a column selection signal supplied from the column selection circuit  130  (See FIG. 5.) turns on switching transistors (not shown) to connect the bitlines BL and BLB to their corresponding input/output lines at a step S 240 . As shown in FIG. 10, voltages of the bitline BLB and the cell node CN 2  are clamped on a specific voltage level while those of BL and CN 1  are not influenced. After an inactivation of the column selection line CSL, the selected wordline WL 1  turns nonconductive when the BLB voltage reaches the reference voltage VREF in a step S 260 . 
     The selected wordline WL 1  becomes inactive when the cell node CN 2  of the memory cell MC 2  storing data “0” is recharged to the ground voltage GND. During the read operation, the time for recharging the cell node corresponding to data “0” is later than that for recharging the cell node corresponding to data “1” up to 80% of AIVC and earlier than that for fully charging the cell node of data “1” up to AIVC. Therefore, such wordline deactivation at the time of charging the cell node of data “0” to the ground voltage can shorten the wordline activation. Writing data in the present twin-cell SRAM is similar with the read operation, but data is applied to the bitlines from the input/output lines through the switching transistors controlled by the column selection line CSL. 
     FIG. 11 illustrates the present wordline activation times of the refresh and read/write operations, T 2  and T 4 , are shorter than those of the conventional, T 1  and T 3 , by T 1  and T 2 , respectively. Thus, the access time in the present twin-cell SRAM is less than that of the prior art by T 1 +T 2 . 
     As described above, the present twin-cell SRAM shortens a wordline activation time either in a refresh operation or read/write operations, are reduces the amount of refresh current or standby current needed, because a wordline selected in operation turns inactive when a cell node voltage corresponding to data “1” reaches 80% of a array power supply voltage or when a cell node voltage corresponding to data “0” downs to a ground voltage. By means of the shortening of the wordline activation time, it is possible to reduce the whole access time including the refresh and read/write operations therein. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as described in the accompanying claims.