Static random access memory having Bi-CMOS construction
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor memory and, more particularly, to a random access memory of a static type having bipolar transistors and complementary MOS transistors. 
     Utilizing advantages of both a bipolar transistor circuit and a complementary MOS transistor circuit, it was disclosed in NIKKEI ELECTRONICS, Mar. 10, 1986, at page 199 to 217 to construct a static type random access memory by bipolar transistors and CMOS transistors. Such a memory is called &#34;Bi-CMOS&#34; SRAM. In the Bi-CMOS SRAM disclosed, peripheral circuits such as an address buffer, a decoder/driver, a sense amplifier and an output circuit are constructed by Bi-CMOS transistor circuits and a memory cell array is constructed by an N-channel MOS transistor circuit. As a result, an operation speed of each peripheral circuit is improved, while realizing a low power consumption and a high integration. 
     However, a memory cell array of the Bi-CMOS SRAM is the same as that of a conventional SRAM constructed only by MOS transistors, and therefore an operation speed of the cell array is not improved. More specifically, a great number of memory cells are connected in common to each pair of bit lines. For this reason, these bit lines are prolonged, so that they have a large stray capacitance. One of each pair of bit lines is discharged in accordance with the data stored in a selected memory cell, and the discharging of the bit line is carried out by a MOS transistor contained in the selected memory cell. However, the MOS transistors generally have a relatively low current ability. Moreover, the MOS transistor is formed in a fine pattern to decrease the size of the memory cell. Therefore, the current ability of the MOS transistor in the memory cell is very small, so that the bit line is discharged with a large time constant. As a result, the improvement in the operation speed of the Bi-CMOS SRAM is restricted. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a semiconductor memory in which an operation speed of a memory cell portion is improved. 
     Another object of the present invention is to provide a Bi-CMOS SRAM attaining a data read operation at a very high speed. 
     A semiconductor memory according to the present invention comprises a plurality of cell blocks, each of the cell blocks including a plurality of word lines, a pair of pre-bit lines, a plurality of memory cells each connected to one of the word lines and to the pre-bit lines, first, second and third nodes, a first transistor of a bipolar type having a base connected to one of the pre-bit lines and a collector-emitter current path connected between the first and third nodes, a second transistor of the bipolar type having a base connected to the other of the pre-bit lines and a collector-emitter current path connected between the second and third nodes, and a third transistor connected between the third node and a reference point; a pair of main-bit lines connected respectively to the first and second nodes of each of the cell blocks; and means responsive to a set of address signals for selecting one of the word lines in one of the cell blocks and for turning ON the third transistor in the one of the cell blocks. 
     According to the invention, a great number of memory cells arranged in the same column are divided into a plurality of cell blocks and the bit line is divided into the pre-bit line and the main-bit line. Accordingly, the number of memory cells connected in common to the pre-bit line is reduced, so that a stray capacitance of the pre-bit line is made small. The discharging of the pre-bit line is thereby carried out with a small time constant by a selected memory cell. All of the cell blocks in each column are connected to the main-bit lines whose capacitances are also reduced as compared conventional bit line structure due to the reduction in transistors connected thereto. Moreover, the first or second transistor of the bipolar type discharges one of the main-bit lines in response to a potential difference between the pre-bit lines. The bipolar transistor has a large current ability. Accordingly, the discharging of the main-bit line is carried out with a small time constant. As a result, a potential difference appears between the main-bit lines within a short time in response to the data stored in tee selected memory cell. The data read operation is thus performed at a high speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which 
     FIG. 1 is a block diagram showing a first embodiment of the present invention; 
     FIG. 2 is a timing chart representing a circuit operation of a memory shown in FIG. 1; 
     FIG. 3 is a block diagram showing a second embodiment of the present invention; 
     FIG. 4 is a timing chart representing a data write operation of a memory shown in FIG. 3; and 
     FIG. 5 is a block diagram representing a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to FIG. 1, a first embodiment of the present invention has a plurality of memory cells MC arranged in a known matrix form of N rows and M columns (N and M being integers) with N word lines in rows and M pairs of bit lines in columns. In this embodiment, N and M are 256, respectively. Accordingly, the memory shown in FIG. 1 has 65536 memory cells. The memory cells MC arranged in the same row are connected in common to one of 256 word lines WL 1  to WL 256 . The memory cells arranged in the same column are divided into 32 cell blocks CB 1-1 to CB 1-32 (CB M-1 to CB M-32). Therefore, each of cell blocks includes eight memory cells, but two of them MC1 and MC8 are representively illustrated in FIG. 1. The cell blocks CB 1-1 to CB 1-32 (CB M-1 to CB M-32) arranged in the same column are connected to a pair of main-bit lines MBL 1  and MBL 1  (MBL m  and MBL m ). Each cell block further includes a pair of pre-bit lines SBL and SBL, a first bipolar NPN transistor Q 10  having a base connected to a true one (SBL) of the pre-bit lines, a second bipolar NPN transistor Q 11  having a base connected to a complementary one (SBL) of the pre-bit lines and a collect-emitter current path connected in series of the collector-emitter current path between the main-bit lines MBL and MBL as shown, a third transistor of an N-channel MOS type connected between a node of emitters of the transistors Q 10  and Q 11  and a ground terminal, and two N-channel MOS transistors Q 7  and Q 8  each connected between each of the pre-bit lines SBL and SBL and a power terminal Vcc. The transistor Q 12  can be replaced by an NPN bipolar transistor and the transistors Q7 and Q8 can be replaced by resistors. Eight memory cells (MC1-MC8) in each cell block are connected in common to the pre-bit lines SBL and SBL. Each of the memory cells MC includes, as well known in the art, two N-channel MOS transistors Q 1  and Q 2  which are cross-coupled in a flip-flop form, two resistors R 1  and R 2  each operating as a load and two N-channel MOS transistors Q 3  and Q 4  each operating as a transfer gate. The resistors R 1  and R 2  can be replaced by a P-channel MOS transistor, respectively. 
     According to the present embodiment, each pair of main bit lines e.g. MSB1 and MSB1 are connected to collectors of only 32 transistors such as shown Q 10  Q 11 , respectively, although 256 memory cells are arranged in each column, and therefore capacitance of each bit line is greatly reduced as compared to the conventional bit line structure in which transistors of all the memory cells of the same column are connected to each bit line. Also, in each of cell blocks such as CB 1-1, only eight transistors (Q 3 , Q 4 ) are connected to each pre-bit line SBL, SBL and the capacitance of each pre-bit line is extremely small. It is favorable to fabricate the bit lines by a metal wiring layer formed on a semiconductor substrate via an insulating layer because such metal wiring formed on an insulating layer has a small stray capacitance. While the pre-bit lines SBL, SBL are advantageously formed by diffusion regions connected to sources or drains of the transistors Q 3 , Q 4  of the memory cells in view of a high density arrangement of the memory cells in each cell block. The cell blocks are arranged in M columns and 32 rows and each cell block is identified by the designation CB i-j  (i: address of columns, j: address of rows). Each word lines e.g. WL1 to WL8 are extending in parallel through M cell blocks of the same rows, e.g. CB 1-1 to CB M-1. M cell blocks of the same row are selected by a row group selection signal (X 1S  - X 32 ) For example, the cell blocks of the first rows (CB 1-1 to CB M-1) are selected by the row group selection signal X1S so that each pair of bipolar transistors Q 10 , Q 11  in the cell blocks CB 1-1 to CB M-1 are enabled by the transistor Q 12  rendered conductive. 
     The row group selection signals X 1S  to X 32S  are generated by decoding upper 5 bits of row address signals RA 1  to RA 5  by the row decoder 10. The row decoder 10 also decodes the remaining 3 bits of row address signals RA6 to RA8 to generate eight pre-selection signal and by performing AND logic operation of the eight pre-selection signals and each of the row group selection signals X 1S  to X 32S  generates selection signals X 1  to X 256  for 256 word lines WL 1  -WL 256 . For example, the signals X 1  to X 8  for selecting the word lines WL 1  to WL 8  extending through the cell blocks CB 1-1, CB 2-1 ... CB M-1 are generated by performing AND operation on the signal X 1S  and the eight pre-selection signals. 
     A data read operation will be described below with reference to FIG. 2. Valid row address signals RA 1  to RA 8  are supplied to row address terminals 10-1 to 10-8 at a time point T 0 . The row address buffer/decoder/driver 10 responds to the valid row address data and changes one of the selection signals X 1  to X 256  to a high level. Assuming that the signal X 1  is changed to the high level, the word line W 1  is energized. The word line WL 1  has a relatively large stray capacitance, but the energizing thereof is completed for a short time since the row address buffer/ decoder/driver 10 has a Bi-CMOS construction to present a large current ability. The transistors Q 3  and Q 4  in the memory cell MC are thereby turned ON. Assuming that this memory cell MC stores a data &#34;1&#34; and the transistors Q 1  and Q 2  take thereby a non-conducting state and a conducting state, respectively, the complementary pre-bit 
     line SBL is discharged by the transistor Q 2  via the transistor Q 4  to lower the potential thereof, whereas the potential of the true pre-bit line SBL is held at the high level. Since only eight memory cells are connected to the pre-bit lines SBL and SBL, the stray capacitance C SB  thereof is very small. Therefore, the discharging of the complementary pre-bit line SBL is carried out with a small time constant, as shown in FIG. 2. The row address buffer/decoder/driver 10 further changes one of group selection selection signals X 1S  to X 32S  to the high level by making use of a part of the valid row address signals, i.e., RA l  to RA 5 . Each of the selection signals X 1S  to X NS  is supplied to the gate of the transistor Q 12  in each of the cell blocks CB 1-1 to CB 1-32 (CB M-1 to CB M-32). Since the word line WL 1  is energized, the selection signal X 1S  is changed to the high level to select the cell block CB 1-1 (CB M-1), as shown in FIG. 2. The row address buffer/decoder/driver 10 generates the high level selection signal X 1S  after a little time delay from the generation of the high level selection signal X 1 . The transistor Q 12  in the cell block CB 1-1 (CB M-1) is thereby turned ON. The transistor Q 12  operates, when turned ON, as a current source of a differential amplifier composed of the bipolar transistors Q 10  and Q 11 . At this time, a potential difference has already occured between the bases of the transistors Q 10  and Q 11  by the lowering of the potential of the pre-bit line SBL. The transistor Q 10  is thereby turned ON to discharge electric charges of the stray capacitance C MB  of the true bit line MBL 1  along with the transistor Q 12 . If the selected memory cell stores a data &#34;0&#34;, the transistors Q 11  and Q 12  discharge the stray capacitor C MB  of the complementary main-bit line MBL 1 . The transistors Q 10  and Q 11  are bipolar transistors and hence have a very large current ability as compared to a MOS transistor. Since each cell block has only one current source transistor represented by Q 12 , this transistor can be formed with a relatively large size to produce a large current. As a result, the discharging of the true bit line MBL 1  is carried out rapidly, as shown in FIG. 2. When a bipolar transistor is employed as the transistor Q 12 , the discharging time period of the bit line MBL (MBL) is further shortened. 
     According to prior art, the memory cells MC arranged in the same column are connected in common to the main-bit lines MBL and MBL without transistors Q 10  and Q 11 . Accordingly, the discharging of the bit line MBL (MBL) is carried out by the MOS transistor Q 1  (Q 2 ) through the MOS transistor Q 3  (Q 4 ) and the column-selecting transistor The current ability of the MOS transistor is considerably smaller than the bipolar transistor when they are of the same size. Moreover, the transistors Q 1  to Q 4  constitute a memory cell MC and are therefore formed in a fine pattern to reduce the size of the memory cell. The current ability thereof is thus decreased more and more. As a result, the bit line MBL 1  is discharged with a very large time constant, as shown by a dotted line 200 in FIG. 2. 
     Turning back to the memory according to this embodiment, a column address buffer/decoder/driver 20 changes one of column selection signals Y 1  to Y 256  to the high level in response to valid column address signals CA 1  to CA 8  supplied to column address terminals 20-1 to 20-8. The column selection signals Y 1  to Y 256  are supplied to a column selection circuit 30. Assuming that the column address buffer/decoder/driver 20 produces the high level selection signal Y 1 , the column selection circuit 30 selects the first true and complementary main-bit lines MBL 1  and MBL 1  to transmit a potential difference therebetween to a sense amplifier/output circuit 40 of a Bi-CMOS construction. As a result, an output data &#34;1&#34; is produced from an output terminal 50 at a time point T 1 , as shown in FIG. 2. Thus, an a time point T 1  that is before the time points T 2  and T 3 , as shown in FIG. 2. An access time period of this memory is represented by AT 0  and is shorter than the period AT 1 . 
     Referring to FIG. 3, a third embodiment of the present invention further includes a data write circuit, in which the same constituents as those shown in FIG. 1 are denoted by like numerals to omit their further description. It should be noted that only one memory cell is shown to avoid the complexity of the drawing. In the memory cell MC, P-channel MOS transistors Q 5  and Q 6  are employed in place of the load resistors R1 and R2 of FIG. 1 and resistors R 11  and R 12  are employed in place of the MOS transistors Q 7  and Q 8  of FIG. 1. In a data read operation, a write-enable signal WE taking the high level is supplied via a terminal 55 to a read/write control circuit 60 of a Bi-CMOS construction which then produces a high level read activating signal RE and a low level write activating signal WE. The sense amplifier/output circuit 40 is activated by the high level signal RE and a data input circuit/write circuit 70 is inactivated by the low level signal WE. As a result, the above-mentioned data read operation is carried out. 
     In the data write operation, the control signal WE takes the low level, and the control circuit 60 produces a low level read activating signal RE and a high level write activating signal WE. The data input circuit/write circuit 70 is thereby activated and the sense amplifier/ output circuit 40 is inactivated. As shown in FIG. 4, the row address buffer/decoder/driver 10 responds to valid row address signals RA 1  to RA 8  and changes the first row selection signal X1 to the high level. The transistors Q 3  and Q 4  are thereby turned ON. Assuming that the memory cell MC has been storing the data &#34;1&#34;, the transistor Q 2  discharges the complementary sub-bit line BL through the transistor Q 4  to lower the potential thereof. It should be noted that the lowered potential of the bit line BL takes not a low level (i.e., a ground level), but an intermediate level between the ground level and the V cc  level, which is determined by the resistor R 12  and the conducting resistances of the transistors Q 2  and Q 4 . In the data write mode, the row address buffer/decoder/driver 10 receives the high level write activating signal WE and hence does not energize the second row selection signal X 1S  to hold it at the low level, as shown in FIG. 3. The transistor Q 12  is thereby maintained in the non-conducting state. The valid column address signals CA i  to CA j  cause the column address buffer/decoder/driver 30 to produce the high level column selection signal Y 1 . The column selection circuit 30 thereby connect the first cell block connected to the first pair of data lines DL 1  and true and complementary main-bit lines MBL 1  and MBL 1  to the data input circuit/write circuit. Thus, the circuit 70 responds to a valid input data D 1  supplied to an input terminal 80 and changes one of the main-bit lines MBL 1  and MBL 1  to the low (ground) level. In the case where the input data &#34;0&#34; is supplied, the true main-bit line MBL 1  is changed to the low level. The potential of true pre-bit line SBL is thereby lowered via the collector-base junction of the transistor Q 10  to such a potential that becomes lower than the potential of the complementary pre-bit line SBL, as shown in FIG. 5. Since the complementary bit line MBL 1  takes the high level, the collector-base junction of the transistor Q 11  is reverse-biased. When the potential of the true pre-bit line SBL becomes lower than that of the complementary pre-bit line SBL, the transistors Q 1  and Q 4   are turned ON and those Q 2  and Q 3  are turned OFF. The potential of the complementary pre-bit line SBL is thus increased to the Vcc level by the transistor Q 4 , as shown in FIG. 5. After the data writing, the bit lines MBL 1  and MBL 1  are precharged to the Vcc level to reverse-bias the collector-base junctions of the transistors Q 10  and Q 11 . The data written into the memory cell MC is thereby held. 
     With reference to FIG. 5, a Bi-CMOS SRAM according to a third embodiment employs column selection signals Y 1  to Y l  as control signals for turning on transistors Q 12  in cell blocks 90-1 to 90-l, respectively. Each of cell blocks 90-1 to 90-l includes 16 memory cells MC 1  to MC 16 , for example. The circuit construction of each cell block is similar to that of FIG. 1, and therefore the explanation thereof is omitted. The data read operation is also similar to FIG. 1. 
     The present invention is not limited to the abovementioned embodiment, but may be modified and changed without departing from the scope and spirit of the present invention.