Abstract:
The semiconductor memory device includes: a memory cell including a capacitor having a charge storage node and a first MIS transistor and a second MIS transistor each having a source connected to the charge storage node; a first word line and a first bit line respectively connected to the gate and the drain of the first MIS transistor; a second word line and a second bit line respectively connected to the gate and the drain of the second MIS transistor; and a timer circuit for generating a periodic signal having a predetermined period. The first word line or the second word line is activated in response to the periodic signal.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a semiconductor memory device that stores desired data and also permits data write and read operation at high speed. 
   A dynamic random access memory (DRAM) device, for example, among various types of semiconductor memory devices, has been frequently used as a semiconductor memory device capable of recording and holding a large capacity of data. 
   In recent years, with the tendency of a finer design rule in the semiconductor process, the DRAM cell structure generally composed of one transistor and one capacitor has been complicated, and thus the process cost has become increasingly high. For this reason, in a so-called system LSI in which both a DRAM circuit and an arithmetic-logic circuit are fabricated, there is often used a DRAM cell having a simple planer structure in which a MOS transistor is used in place of the capacitor for reduction of the process cost. 
     FIG. 9  shows an exemplary configuration of a DRAM cell disclosed in U.S. Pat. No. 5,600,598, in which a MOS transistor is used as a capacitor. 
   As shown in  FIG. 9 , the conventional DRAM cell includes an access transistor  101  that is a first MOS transistor having its gate and drain connected to a word line WL and a bit line BL, respectively, and a charge storage transistor  102  that is a second MOS transistor for storing charge in its channel. The source and drain of the charge storage transistor  102  are connected to the source of the access transistor  101 , and the gate thereof is connected to a cell plate. 
   In the DRAM cell having the configuration described above, during write operation, for example, the word line WL is activated, and when the voltage value of the bit line BL is in a high level at this time, “1” is written in the channel of the charge storage transistor  102 . Contrarily, when the voltage value of the bit line BL is in a low level, “0” is written in the channel. 
   During read operation, by activation of the word line WL, charge stored in the channel of the charge storage transistor  102  is transferred to the precharged bit line BL, and the potential of the bit line BL is amplified with a sense amplifier connected to the bit line BL, so that data in the selected DRAM cell is read. 
   In recent years, further enhancement in performance has been demanded for LSI systems, and in this relation, it is requested to enhance the performance of semiconductor memory devices (memory blocks). DRAM cells are advantageously used when a large-capacity memory device is required because each DRAM cell is composed of a smaller number of elements compared with a SRAM cell. In DRAM cells, however, information (charge) stored in a capacitor disappears with time. Therefore, to hold the recorded data, it is necessary to execute so-called refresh operation, in which the data is read and rewritten repeatedly, before the data disappears. This requirement that refresh operation must be executed constantly during the running of the device is a factor of impairing usability of DRAM devices. 
   The number of times of the refresh operation did not present a big problem because a sufficient amount of charge was stored in conventional capacitors. However, with the recent tendency of finer memory cells and use of MOS transistors as capacitors, it has become increasingly difficult to secure a sufficient capacitance value for capacitors. As a result, more frequent refresh operation is required, and this disadvantageously impairs the operation of the system. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is eliminating the necessity of supplying a signal instructing refresh operation for DRAM cells from outside a DRAM circuit. 
   To attain the above object, according to the present invention, a DRAM cell includes a first transistor for external access to a charge storage capacitor and a second transistor for data refresh. Refresh operation for the DRAM cell is executed periodically with a periodic signal generated inside the DRAM circuit via the second transistor. 
   Specifically, the first semiconductor memory device of the present invention includes: a memory cell including a capacitor having a charge storage node, and a first MIS transistor and a second MIS transistor each having a first source/drain terminal connected to the charge storage node; a first word line and a first bit line respectively connected to a gate and a second source/drain terminal of the first MIS transistor; a second word line and a second bit line respectively connected to a gate and a second source/drain terminal of the second MIS transistor; and a timer circuit for generating a periodic signal having a predetermined period, wherein the first word line or the second word line is activated in response to the periodic signal. 
   In the first semiconductor memory device, the memory cell is subjected to refresh operation at the predetermined period with the periodic signal generated by the timer circuit provided internally. Accordingly, no supply of an external refresh signal is required, and thus the refresh operation (refresh signal) will no more adversely affect the operation of the memory device. 
   In the first semiconductor memory device of the present invention, the capacitor is preferably a MIS transistor kept in a conduction state at all times and is composed of a gate and a channel of the transistor. With this configuration, the memory cell is entirely composed of planer MIS transistors. This simplifies the manufacture and also enables high integration. 
   The second semiconductor memory device of the present invention includes: a plurality of memory cells each including a capacitor having a charge storage node, and a first MIS transistor and a second MIS transistor each having a first source/drain terminal connected to the charge storage node; a plurality of first word lines and a plurality of first bit lines respectively connected to gates and second source/drain terminals of the first MIS transistors; a plurality of second word lines and a plurality of second bit lines respectively connected to gates and second source/drain terminals of the second MIS transistors; a timer circuit for generating a periodic signal having a predetermined period, an access word line selection circuit for selectively activating the plurality of first word lines in response to an external access request; and a refresh word line selection circuit for selectively activating the plurality of second word lines at the predetermined period with the periodic signal. 
   In the second semiconductor memory device, the memory cells are subjected to refresh operation with the periodic signal from the timer circuit provided internally. Accordingly, no supply of a refresh signal from outside a memory block is required, and thus the refresh operation will no more adversely affect the operation of the memory device. 
   The second semiconductor device preferably further includes: a plurality of first sense amplifiers for data access connected to the respective first bit lines; and a plurality of second sense amplifiers for data refresh connected to the respective second bit lines, wherein the first sense amplifiers are activated in response to an external access request, and the second sense amplifiers are activated in response to the periodic signal. 
   In the second semiconductor memory device, preferably, the access word line selection circuit selectively activates the plurality of first word lines and the refresh word line selection circuit selectively activates the plurality of second word lines, both in synchronization with an external clock signal input externally. 
   In the case described above, an activation time period of the first word lines and an activation time period of the second word lines are preferably shifted in phase from each other. This ensures execution of the refresh operation without corrupting data held in the memory cell. 
   In particular, an activation time period of the first word lines and an activation time period of the second word lines are preferably set so as to be shifted in phase by a half period of a memory operation cycle. 
   The timer circuit is preferably a counter circuit for generating the periodic signal from the number of pulses of the external clock signal. 
   In the second semiconductor memory device, when an address value of a first word line selected from the plurality of first word lines is identical to an address value of a second word line selected from the plurality of second word lines at a same timing, the selected second word line is preferably disabled. This can prevent corruption of data held in the memory cell. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a semiconductor memory device of Embodiment 1 of the present invention. 
       FIG. 2  is a circuit diagram of an alteration of a memory cell of the semiconductor memory device of Embodiment 1 of the present invention. 
       FIG. 3  is a timing chart of a refresh word line activation signal and an external clock signal in the semiconductor memory device of Embodiment 1 of the present invention. 
       FIG. 4  is a timing chart of an access word line activation signal, a refresh word line activation signal, an external clock signal and a refresh clock signal in the semiconductor memory device of Embodiment 1 of the present invention. 
       FIGS. 5A and 5B  are a block diagram and a timing chart, respectively, for showing how to generate the refresh clock signal when a timer circuit is used in the semiconductor memory device of Embodiment 1 of the present invention. 
       FIG. 6  is a timing chart of an access word line activation signal, a refresh word line activation signal, an external clock signal and a refresh clock signal in an alteration of the semiconductor memory device of Embodiment 1 of the present invention. 
       FIG. 7  is a block diagram of a semiconductor memory device of Embodiment 2 of the present invention. 
       FIG. 8A  is a timing chart of an access word line activation signal, a refresh word line activation signal, an external clock signal and a refresh clock signal in the semiconductor memory device of Embodiment 2 of the present invention. 
       FIG. 8B  is a block diagram of an access word line selection circuit and a refresh work line selection circuit of the semiconductor memory device of Embodiment 2 of the present invention. 
       FIG. 9  is a circuit diagram of a conventional DRAM cell composed of only MOS transistors. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
   Embodiment 1 
     FIG. 1  shows a circuit configuration of a semiconductor memory device  10 A of Embodiment 1 of the present invention. 
   Referring to  FIG. 1 , the semiconductor memory device  10 A includes a plurality of memory cells  11  arranged in a matrix. Each of the memory cells  11  includes a capacitor  21  having a charge storage node  21   a , and first and second MOS transistors  22  and  23  having their sources connected to the charge storage node  21   a.    
   The gate of the first MOS transistor  22  is connected to an access word line WL, and the drain thereof is connected to an access bit line BL. The gate of the second MOS transistor  23  is connected to a refresh word line RWL, and the drain thereof is connected to a refresh bit line RBL. 
   The access bit lines BL are electrically connected to sense amplifiers  12  for external access, and the refresh bit lines RBL are electrically connected to sense amplifiers  13  for internal refresh. 
   The access word lines WL are electrically connected to an access word line selection circuit  14 , and the refresh word lines RWL are electrically connected to a refresh word line selection circuit  15 . 
   The access word line selection circuit  14  selects an access word line WL designated by an address signal Add input externally among the plurality of access word lines WL in synchronization with an external clock signal CLK input externally. 
   The refresh word line selection circuit  15  is connected to a counter  16  as a timer circuit. The counter  16  generates a refresh clock signal RCLK in synchronization with the external clock signal CLK and outputs the generated signal. The refresh word line selection circuit  15  therefore autonomously selects the plurality of refresh word lines RWL in synchronization with the refresh clock signal RCLK received from the counter  16 , to thereby activate the refresh word lines RWL periodically. 
   The capacitor  21  for data storage of each memory cell  11  may be replaced with a third MOS transistor  24  as shown in FIG.  2 . The third MOS transistor  24  has its source and drain connected to the sources of the first and second MOS transistors  22  and  23 , and is kept in the conduction state with its channel being formed at all times. In this case, the channel of the third MOS transistor  24  serves as the charge storage node. 
   The semiconductor memory device  10 A of Embodiment 1 may be formed as one semiconductor chip, or may be embedded in a system LSI together with a microprocessor (MPU) and a logic circuit. In the case of a system LSI, the semiconductor memory device  10 A serves as a memory circuit (memory block). 
   Hereinafter, the operation of the semiconductor memory device  10 A having the configuration described above will be described with reference to the relevant drawings. 
     FIG. 3  shows a timing chart of a refresh word line activating signal and an external clock signal in the semiconductor memory device of Embodiment 1. 
   Referring to  FIG. 3 , each refresh word line RWL connected to the gates of the second MOS transistors  23  of the memory cells  11  is activated at a refresh period Tref, a period according to the signal generated and output by the counter  16  that counts the number of clock cycles of the external clock signal CLK until a predetermined number is reached. 
     FIG. 4  is a timing chart of an access word line activation signal, the refresh word line activation signal, the external clock signal and the refresh clock signal in the semiconductor memory device of Embodiment 1. 
   Referring  FIG. 4 , during external access requested aperiodically from outside, such as data read and data write, an access word line WL and a sense amplifier  12  for external access connected to an access bit line BL, selected by the access word line selection circuit  14  according to the address signal Add input in synchronization with the external clock signal CLK, are activated to thereby enable input/output of data from/to outside, as described earlier. 
   The internal refresh operation is executed with the refresh clock signal RCLK output from the counter  16  to the refresh word line selection circuit  15  at an inter-pulse period Tp that corresponds to four cycles of the external clock signal CLK. In other words, the refresh word lines RWL and the refresh bit lines RBL selected by the refresh word line selection circuit  15  in synchronization with the refresh clock signal RCLK are sequentially activated. Assuming that the total number of the refresh word lines RWL is (n+1) (n is a positive integer), when the refresh word lines RWL have been sequentially activated from the first RWL( 0 ) through the final RWL(n), the refresh operation is repeated from the first refresh word line RWL( 0 ). The period of repetition of this refresh operation is the refresh period Tref. 
   As described above, the refresh operation is executed reliably with the refresh clock signal RCLK, which is generated based on the external clock signal CLK by the counter  16  provided inside the semiconductor memory device  10 A, before data held in each memory cell  11  disappears. In this way, disappearance of data in each memory cell  11  is prevented. Accordingly, an externally generated refresh signal, conventionally required, is no more necessary, and thus the operation of the semiconductor memory device  10 A will not be impaired by supply of the external refresh signal. Moreover, since the number of external signal lines can be reduced, further high integration can be attained. 
   Although the inter-pulse period Tp of the refresh clock signal RCLK was set to correspond to four cycles of the external clock signal CLK in Embodiment 1, it is not limited to this value. 
   As shown in  FIG. 5A , the refresh clock signal RCLK may be generated using a timer circuit  17 , which is a RC delay circuit composed of a resistance element and a capacitance element, for example, in place of the counter  16  counting the number of clock cycles. More precisely, as shown in  FIG. 5A , the timer circuit  17  and a refresh clock generation circuit  18  may be used in place of the counter  16 . The refresh clock generation circuit  18  receives a timer signal CT generated and output by the timer circuit  17  and the external clock signal CLK, and generates the refresh clock signal RCLK based on these signals. 
     FIG. 5B  shows a timing chart of the timer signal CT, the external clock signal CLK and the refresh clock signal RCLK. Referring to  FIG. 5B , the refresh clock signal RCLK is activated in synchronization with every first pulse of the external clock signal CLK after every rising of the timer signal CT. The refresh word lines RWL are sequentially activated with the activation of the refresh clock signal RCLK. 
   (Alteration of Embodiment 1) 
     FIG. 6  shows a timing chart of an access word line activation signal, a refresh word line activation signal, an external clock signal and a refresh clock signal in a semiconductor memory device of an alteration of Embodiment 1 of the present invention. 
   As shown in  FIG. 6 , in this alteration, also, the refresh clock signal RCLK is activated at the inter-pulse period Tp corresponding to four cycles of the external clock signal CLK. The difference of this alteration from Embodiment 1 is that the activation of the refresh word lines RWL is performed during non-activation of the access word lines WL. More specifically, the refresh word lines RWL are activated for time periods respectively shifted from the activation time periods of the access word lines WL by a half period of the memory operation cycle, that is, in the illustrated example, by a half of the period of the external clock signal CLK. 
   The access word lines WL are selected and activated according to the address signal Add received aperiodically from outside, and the refresh word lines RWL are activated at a period (inter-pulse period Tp) determined by the counter  16  provided internally, as described earlier. Therefore, there possibly occurs an event that an access word line WL and a refresh word line RWL happen to access a same memory cell  11 . In such an event, charge stored in the memory cell  11  is distributed both to the access bit line BL and the refresh bit line RBL. This reduces the voltage during initial read in both the sense amplifier  12  for external access and the sense amplifier  13  for internal refresh, and thus causes malfunction of the sense amplifiers  12  and  13 . 
   If the access to the access bit line BL is for write operation, the write operation of the access bit line BL will collide with the read amplifying operation of the refresh bit line RBL, causing malfunction. 
   The malfunction described above can be prevented reliably in the alteration of Embodiment 1 in which the activation time periods of the refresh word lines RWL and the refresh bit lines RBL are set to fall within the non-activation time period during which the access word lines WL are not activated. 
   To implement the operation of this alteration, the refresh word line selection circuit  15  may be configured to activate the refresh word lines RWL at the timing of falling of the received refresh clock signal RCLK, for example. 
   Embodiment 2 
     FIG. 7  shows a circuit configuration of a semiconductor memory device  10 B of Embodiment 2 of the present invention. In  FIG. 7 , the same components as those in  FIG. 1  are denoted by the same reference numerals, and the description thereof is omitted here. 
   As shown in  FIG. 7 , the semiconductor memory device  10 B of Embodiment 2 has a feature that an activation disable signal Dis disabling activation of the refresh word lines RWL is output from the access word line selection circuit  14  to the refresh word line selection circuit  15 . More specifically, as shown in the timing chart in  FIG. 8A , the timings of activation of the access word lines WL and the refresh word lines RWL are the same, and in an event that an access word line WL and a refresh word line RWL access a same memory cell, the activation of the refresh word line RWL is disabled or masked. 
     FIG. 8B  shows an exemplary circuit configuration for implementing the activation disable signal Dis and the masking of the refresh word line RWL with the activation disable signal Dis. 
   Referring to  FIG. 8B , the refresh word line selection circuit  15  includes a mask circuit  15   a  composed of an AND gate receiving the access word line activation signal (=activation disable signal Dis) in the inverted form. With this configuration, if the refresh word line selection circuit  15  selects a refresh word line RWL having an identical address value as an access word line WL currently selected and receiving a high-level activation signal, the selected refresh word line RWL is masked with the activation disable signal Dis and changed to a low-level non-activation signal. 
   As described above, in Embodiment 2, substantially the same effect as that obtained by the semiconductor memory device of Embodiment 1 is obtained. In addition, it is possible to avoid the occurrence that an access word line WL selected according to the address signal Add input externally and a refresh word line RWL selected autonomously by the refresh word line selection circuit  15  access a same memory cell at a same timing. Accordingly, malfunction in the memory cell  11  can be reliably prevented. 
   While the present invention has been described in preferred embodiments, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention.