Static type semiconductor memory device
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a static type semiconductor memory device, and more particularly to a semiconductor memory device in which a part of a word line and a part of a bit line can be selected at the same time in order to decrease the power consumption of the semiconductor memory device. 
     2. Description of the Prior Art 
     In recent years, the memory capacity of a semiconductor memory device has become larger and larger. When the memory capacity of a static type semiconductor memory device becomes large, a load current of each bit line becomes large and the stray capacitance of each bit line increases, so that the operating speed of the memory device becomes slow. 
     FIG. 1A is a schematic block diagram of a conventional static type RAM device and FIG. 1B is a partial circuit diagram of the RAM device. In these drawings, MCA designates a memory cell array having static type memory cells MC 0 ,0 ; . . . ; MC N-1 ,0 ; . . . which are disposed in a matrix of N rows and M columns. For example, when a word line X 0  is selected by a word address decoder WD and a bit line or bit line pair Y 0  and Y 0  is selected by a column decoder CD, a memory cell MC 0 ,0 disposed on a cross point of the word line X 0  and the bit line Y 0  is selected. Each of the memory cells, for example MC 0 ,0, comprises MIS transistors Q 3  through Q 6  and load resistors R 1  and R 2 . Only one of the cross coupled transistors Q 5  and Q 6  is turned on by this means, and the other is turned off according to the information stored in the memory cell MC.sub. 0,0. When the word line X 0  is selected and the potential of the word line X 0  becomes, for example, high, the transfer transistors Q 3  and Q 4  are turned on. If the transistor Q 5  is turned on, a current flows from a voltage source V DD   through an MIS bit line load transistor Q 1  of the bit line Y 0 , the transistor Q 3  and the transistor Q 5  to another voltage source V SS . In this condition, the transistor Q 6  is turned off and no current flows through an MIS load transistor Q 2  of the bit line Y 0 . Accordingly, there exists a potential difference between the bit lines Y 0  and Y 0 . A sense amplifier, which is not shown in the drawings, detects the potential difference and outputs the information stored in the memory cell MC 0 ,0. In FIG. 1B, MIS transistors Q 7  and Q 8  of the column decoder CD connect the selected bit line pair Y 0  and Y 0  to the sense amplifier under the control of the output signal from a NOR gate &#34;NOR&#34; which receives column address signals AC 0  through AC m-1 , where 2 m+1  =M. 
     Concerning the power consumption of the above-mentioned static type RAM device, more than 60% of the total power consumption is consumed by the memory cell array portion and the remainder is consumed by the peripheral circuit portion of the memory cell array portion. With the increase in the memory capacity, the ratio of the electric power consumed by the memory cell array portion becomes larger and larger, but the electric power consumed by the peripheral circuit portion does not increase much. Of the electric power consumed by the memory cell portion, most of the power consumption is caused by the bit line current flowing at the access time, i.e., at the time the read out or the write in of information is effected, and the electric current necessary for holding the information stored in the memory cells is very small. Therefore, it is essential to decrease the power consumption of the memory cell array portion, especially to decrease the bit line current, in order to decrease the power consumption of the memory device. 
     In order to decrease the bit line current, it is possible to divide each of the word lines into two half sections and to select only one of the half sections to which the selected memory cell is connected. In such a structure, another one of the half sections of the selected word line is not selected and, therefore, the total current flowing from the bit lines through the memory cell to the ground can be reduced approximately by one half. However, in such a memory device, the length of each of the bit lines is the same as that of the memory device of FIG. 1, and the stray capacitance of each of the bit lines becomes very large when the memory capacity is increased. Therefore, it is necessary to increase the charge current flowing from the transistors Q 1  and Q 2  to the bit lines, so that the power consumption becomes large when the memory capacity is increased. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to reduce the power consumption of a static type semiconductor RAM device having a large memory capacity. 
     It is another object of the present invention to increase the read out and write-in speed of the static type semiconductor RAM device having a large memory capacity. 
     According to the present invention, there is provided a static type semiconductor memory device which includes a cell array having a plurality of memory cells disposed at cross points of a plurality of word lines and a plurality of bits lines. In the device access to a selected memory cell is effected by selecting a word line connected to the selected memory cell and by selecting a bit line connected to the selected memory cell according to input address signals. The device also includes a plurality of cell array blocks which are formed by dividing the cell array in a direction parallel to the word lines and in a direction parallel to the bit lines. Each cell array block includes divided word lines and divided bit lines formed by dividing the word lines and the bit lines respectively. The memory device additionally includes a word decoder section and a column decoder section which, respectively, select only a divided word line and a divided bit line of said cell array block containing the selected memory cell according to the input address signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A and FIG. 1B are block circuit diagrams illustrating a conventional static type semiconductor RAM device; 
     FIG. 2 is a block circuit diagram illustrating a static type semiconductor memory device as a first embodiment of the present invention; 
     FIG. 3 is a block circuit diagram illustrating a static type semiconductor memory device as a second embodiment of the present invention; and 
     FIGS. 4A and 4B are circuit diagrams illustrating a detailed structure of the memory device of FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated schematically in FIG. 2, in a semiconductor RAM device as a first embodiment of the present invention, a memory cell array is divided into four cell array blocks BK 0  through BK 3 , and word lines X 0  through X N-1  and bit lines Y 0  through Y M-1  are respectively divided into two sections at the central portions thereof. In FIG. 2, WL and WL&#39; designate divided word line sections of each of the word lines X 0  through X N-1 , and BL and BL&#39; designate divided bit line sections of each of the bit lines Y 0  and Y M-1 . WD and WD&#39; are word decoders disposed between the cell array blocks BK 0  and BK 3  and the cell array blocks BK 1  and BK 2 . The word decoder WD selects one of the divided word lines WL of the cell array block BK 0  when a selecting signal WD 0  for the cell array block BK 0  is applied to the word decoder WD, and selects another one of the divided word lines WL&#39; of the cell array BK 3  when a selecting signal WD 3  for the cell array block BK 3  is applied to the word decoder WD. The word decoder WD&#39; selects one of the divided word lines WL of the cell array block BK 1  when a selecting signal WD 1  is applied thereto and selects one of the divided word lines WL&#39; of the cell array block BK 2  when a selecting signal WD 2  is applied thereto. One of N/2 word lines of each of the cell array blocks is selected by n-1 bit address signals which are applied to the word decoders WD and WD&#39; and which are not shown in FIG. 2, where 2 n+1  =N. The above-mentioned selecting signals WD 0  through WD 3  are formed from two bit address signals, each of which is also applied to the word decoder WD and WD&#39; respectively. Therefore, the word lines X 0  through X N-1  are selected by n bit address signals. 
     In FIG. 2, CD&#39; is a unified column decoder which selects one of the divided bit line BL or BL&#39; of the bit lines Y 0  through Y M-1 . Input address signals to the column decoder CD&#39; include the inverted signals WD 0  through WD 3  of the aforementioned selecting signals WD 0  through WD 3  so that the column decoder CD&#39; can select one of the divided bit lines from one of the cell array blocks BK 0  through BK 3  which is selected by the word decoder WD or WD&#39;. Since the column decoder CD&#39; selects one of the divided bit lines by applying a selecting pulse of a negative polarity, the inverted signals WD 0  through WD 3  are applied to the column decoder CD&#39;. If a column decoder, which selects the divided bit line by applying a selecting pulse of a positive polarity, is used, non-inverted signals WD 0  through WD 3  are applied to the column decoder. In FIG. 2, SA 0  through SA 3  designate sense amplifiers connected to the cell array blocks BK 0  through BK 3 , respectively. 
     FIG. 3 illustrates a static type RAM device as another embodiment of the present invention. The RAM device of FIG. 3 comprises four cell array blocks BK 0  and through BK 3 , word decoders WD and WD&#39;, column decoders CD and CD&#39;, AND gates AG 0  through AG N-1  and AG&#39; 0  through AG&#39; N-1 , NOR gates NG 0  through NG M-1  and NG&#39; 0  through NG&#39; M-1 , gate transistors Q 37  through Q 44  comprising a word block selector WBS and gate transistors Q 45  through Q 48  constituting column block selector or decoder CBD. 
     The RAM device of FIG. 3 has substantially the same structure as that of the memory device of FIG. 2 and except that the RAM device of FIG. 3 uses only one sense amplifier which is connected to the gate transistors Q 45 , Q 46  and Q 47  and Q 48  and which is not shown in FIG. 3. Each of the cell array blocks BK 0  and BK 3  has (N/2)×(M/2) bit memory cells arranged in a matrix of N/2 rows and M/2 columns, and selected by selecting signals WD 0  through WD 3 , respectively. For example, when the selecting signal WD 0  becomes high, the word decoder WD selects one of divided word lines X 0  through X.sub.(N-2)/2 of the cell array block BK 0  via the AND gates AG 0  through AG.sub.(N-2)/2 and the column decoder CD selects one of divided bit lines Y 0  through Y.sub.(M-2)/2 of the same cell array block BK 0  via the NOR gates NG 0  through NG.sub.(M-2)/2. Therefore, when the selecting signal WD 0  is high, one of memory cells of the cell array block BK 0  is selected, and, for example, a read out signal from the selected memory cell is transferred through data buses DB 0  and DB 0  and the gate transistors Q 37  and Q 38  which are turned on by the selecting signal WD 0  and through the gate transistors Q 45  and Q 46  to the sense amplifier (not shown in the drawing). The read out of information from each of the memory cells of the other cell array blocks BK 1  through BK 3  is effected in a similar manner to the read out of information from one of the memory cells of the cell array block BK 0  which is mentioned above. The gate transistors Q 45  and Q 46  and Q 47  and Q 48  comprise a column block selector or decoder CBD and select one of the groups of the cell array blocks each comprising the cell array blocks BK 0  and BK 1  or BK 3  and BK 2 . The gate transistors Q 45  and Q 46  are controlled, for example, by the most significant bit AC m-1  of the column address signal and the gate transistors Q 47  and Q 48   are controlled by the inverted signal AC m-1  of the most significant bit of the column address signal. 
     FIG. 4A is a partial detailed circuit diagram of the RAM device of FIG. 3. As illustrated in FIG. 4A, each of the memory cells of the cell array blocks BK 0  through BK 3  comprises mainly a flip-flop circuit. For example, the memory cell MC 0 ,0 of the cell array block BK 0  comprises a pair of cross coupled MIS transistors Q 5  and Q 6 , load resistors R 1  and R 2  connected between the drain electrode of the transistors Q 5  and a voltage source V CC  and between the drain electrode of the transistor Q 6  and the voltage source V CC  respectively. The memory cell further comprises a pair of MIS transfer transistors Q 3  and Q 4  connected between the drain electrode of the transistor Q 6  and a divided bit line Y 0  and between the drain electrode of the transistor Q 6  and a divided bit line Y 0 . The gate electrodes of the transistors Q 3  and Q 4  are connected to a divided word line X 0 . In this manner, N/2 one bit memory cells MC 0 ,0 through MC.sub.(N-2)/2,0 of the cell array block BK 0  are connected between a pair of divided bit lines Y 0  and Y 0  and to the divided word lines X 0  through X.sub.(N-2)/2. The divided bit lines Y 0  and Y 0  are connected to the voltage source V CC  through bit line load transistors Q 1  and Q 2 , respectively. The divided bit lines Y 0  and Y 0  are connected to the data buses DB 0  and DB 0 , respectively, through MIS transistors Q 7  and Q 8 . These transistors Q 7  and Q 8  are turned on and off by a bit drive signal supplied from a NOR gate of a column decoder CD comprising MIS transistors Q 111  through Q 11 (m-1), through a MIS transistor Q 10  which is controlled by the aforementioned selecting signal WD 0 . The MIS transistors Q 111  through Q 11 (m-1) are turned on and off by m-1 bit column address signals AC 0  through AC m-2 , respectively which are supplied from column address buffers (not shown in the drawing). A depletion type MIS transistor Q 9  is a load transistor for the MIS transistor Q 10  and for the MIS transistors Q 111  through Q 11 (m-1) comprising the NOR gate. 
     The divided bit lines Y&#39; 0  and Y&#39; 0  of the cell array block BK 1  are connected to the data buses DB 1  and DB 1  through MIS transistors Q 14  and Q 15 . The MIS transistors Q 14  and Q 15  are also controlled by the NOR gate &#34;NOR&#34; through a MIS transistor Q 13  which is controlled by the selecting signal WD 1 . The bit lines Y&#39; 0  and Y&#39; 0  are connected to the voltage source V CC  through load transistors Q 17  and Q 18 , respectively. N/2 one bit memory cells MC N/2 ,0 through MC N-1 ,0 are connected between the divided bit lines Y&#39; 0  and Y&#39; 0  and to the divided word lines X N/2  through X N-1 . 
     The structure of the other cell array blocks BK 3  and BK 2  and the other column decoder CD&#39; are substantially the same as those of the cell array blocks BK 0  and BK 1  and the column decoder CD. 
     The data buses DB 0  and DB 0 , and, DB 1  and DB 1  are commonly connected to the gate transistors Q 45  and Q 46  of the column block decoder CBD through the gate transistors Q 37  and Q 38  of the word block selector WBS and through the gate transistors Q 39  and Q 40  of the word block selector WBS, respectively. Similarly, the data buses DB 3  and DB 3 , and, DB 2  and DB 2  are commonly connected to the gate transistors Q 48  and Q 47  of the column block decoder CBD through the gate transistors Q 41  and Q 42  of the word block selector WBS and through the gate transistors Q 43  and Q 44  of the word block selector WBS, respectively. The column block selector CBD is connected to a sense amplifier SA and a buffer amplifier BA and to a write-in buffer WB comprising an input amplifier and three NAND gates G1 through G3. 
     The selecting signals WD 0  through WD 3  are formed by NAND gates NGW 0  through NGW 3  respectively to which the address signals A n-1 , A n-1 , A m-1  and A m-1  are applied, as illustrated in FIG. 4B. 
     The operation of the circuit of FIGS. 4A and 4B will now be explained. When the read-out of information from the memory cell, for example, MC 00  is effected, the potential level of the divided word line X 0  is caused to be high and the transistors Q 3  and Q 4  are turned on. In this case, the column address signals AC 0  through AC m-2  are all low and the selecting signal WD 0  is high. Therefore, the transistors Q 10 , Q 37  and Q 38  are all turned on and the level of the output potential of the NOR gate &#34;NOR&#34; is high, so that the transistors Q 7  and Q 8  are both turned on. In this case, since the column address signal AC m-1  is high, the transistors Q 45  and Q 46  are turned on. Therefore, the potential difference between the drain electrodes of the transistors Q 5  and Q 6  of the memory cell MC 0 ,0 is transferred to the sense amplifier SA and the buffer amplifier BA and the read-out data D out  corresponding to the potential difference is outputted. 
     When the write-in of information to the same memory cell is effected, the potential level of the word line X 0  is caused to be high, the selecting signal WD 0  is caused to be high and the column address signals AC 0  through AC m-1  are all caused to be low. Therefore, a write-in signal from the write-in buffer WB is transferred to the memory cell MC 0 ,0 and the write-in of information is effected. 
     In these operations, only one divided word line X 0  becomes high and all the other divided word lines of the cell array block BK 0  and of the cell array blocks BK 1  through BK 3  are low. Therefore, the load current flowing from the bit lines through the memory cells to the voltage source V SS  (from example, ground) can be decreased to half that of the conventional memory device of FIG. 1. Moreover, since the gate transistors Q 14  and Q 15  are in a turned off condition, the stray capacitance of the bit line pair Y 0  and Y 0  can be decreased to half that of the conventional memory device of FIG. 1. Therefore, the transconductance gm of each of the load transistors Q 1  and Q 2  can be decreased to half that of the conventional memory device, i.e., the charge current of each of the bit lines can be decreased to half that of the conventional memory device without reducing the charging speed of each of the bit lines. As a result, the power consumption of the above-mentioned memory device can be decreased to a quarter of that of the conventional memory device of FIG. 1. 
     In the above embodiments, the memory cell array is divided into four blocks. However, it should be noted that the memory cell array can be divided into a larger number of blocks, and it is possible to decrease the power consumption of the memory device even more. 
     Therefore, according to the present invention, it is possible to decrease the power consumption of the static type RAM device having a large memory capacity without reducing the operation speed thereof.