Semiconductor Memory Device

The present invention concerns a Dynamic Random Access Memory (DRAM), and particularly, a circuit arrangement of bit lines therein.

Generally, a DRAM comprises a plurality of bit lines of a same length arranged in parallel with each other, and a plurality of flip-flop sense amplifiers connected with each of the bit line pairs. The memory cell comprises one transistor and one capacitor. Between each bit line and each word line is connected a memory cell, so that all the connected memory cells are arranged in a matrix of rows and columns. The circuit arrangement of the bit line pairs and the sense amplifiers conventionally appears in two forms. Namely, one form is called an open bit line arrangement wherein each of the sense amplifiers is disposed at the center of each bit line pair, while the other is called a folded bit line arrangement wherein each of the sense amplifiers is disposed at one end of each bit line pair. However, considering the equilibrium of the bit lines and the high density circuit arrangement of the memory cells, the folded bit line method is mainly used. Nowadays, as the memory cells in the DRAM are highly integrated, the space between the bit lines becomes narrower and the storage capacitor of the memory cells also becomes smaller. Hence, when a memory cell is accessed and a sense amplifier operates in corresponding to the bit line connected with the memory cell accessed, the mutual coupling capacitance between the bit line and its upper and lower adjacent bit lines may affect its normal operation.

For example, FIG. 1 exemplifies a conventional folded bit line circuit arrangement. The memory cells MC10-MC12 and MC20-MC22 are connected with the intersection points of the bit lines B0-B2 and B0-B2 and the word lines W1 and W2, and each of the bit line pairs B0--B0, B1--B1 and B2--B2 is connected, at each end, with corresponding one of the sense amplifiers SA0-SA2. Each of the memory cells MC10-MC12 and MC20-MC22 comprises a MOS transistor M and a storage capacitor C connected in series with the drain-source path of the transistor. Each of the drains of the MOS transistors is connected with one of the bit lines B0, B0, . . . B2 and B2, while each gate of the memory cells MC10-MC12 and MC20-MC22 is connected with one of the word lines W1 and W2. The other end of the storage capacitor is connected with a constant voltage Vp. It is assumed that the parasitic capacitance of each bit line is CB, the mutual coupling capacitance between the adjacent bit lines is CC, and the capacitance of the storage capacitor C is CS.

If the memory cells MC10-MC12 are chosen by the word line signal applied to the word line W1, electric charges stored into the storage capacitors of the memory cells are respectively transferred through the corresponding MOS the bit lines B0-B2, so that the voltage of each of the bit lines B0-B2 becomes higher or lower by the amount of ##EQU1## than the voltage of each of the other bit lines B0-B2, wherein VS is the voltage of the storage capacitor, and VBL is the bit line voltage prior to selecting the memory cell. If the memory cells MC10-MC12 cause the bit lines B0-B2 to have the voltage higher by .DELTA.VS than the voltage of the bit lines B0-B2, the sense amplifiers SA0-SA2 are activated so that the voltage of the bit lines B0, B1 and B2 having a low voltage of VS is lowered. Hence, the voltage of the bit line B1 is lowered due to the influence of the coupling capacitance CC resulting from the lowering of voltages of the adjacent bit lines B0 and B1. Such an adverse effect increases as narrowing of the space between the bit lines, resulting from increasing of the memory density. Further, if the capacity of the memory cell is reduced, the sense amplifier is liable to malfunction due to the coupling capacitance.

The object of the present invention is to provide a circuit arrangement whereby undesirable effect due to the coupling capacitance between bit lines can be reduced even in the high density memory capacity.

According to one aspect of the present invention, a semiconductor DRAM comprises:

a plurality of bit lines arranged parallel to each other;

a plurality of word lines intersecting said plurality of bit lines;

a plurality of upper sense amplifiers respectively connected to uppermost ends of each of odd numbered bit line pairs;

a plurality of lower sense amplifiers respectively connected to lowermost ends of each of even numbered bit line pairs;

a memory cell array having a plurality of memory cells arranged sequentially in a diagonal line within selected locations of a plurality of spacings formed by intersection of said bit lines and word lines, said memory cell being disposed at every fourth spacing in a row and a column;

first latching means for activating said upper sense amplifiers, said latching means being connected with said upper sense amplifiers; and

second latching means coupled with said lower sense amplifiers, said first latching means and said second latching means being alternately activated to each other.

In another aspect of the present invention, the semiconductor DRAM comprises:

a plurality of sense amplifiers arranged in rows and columns;

a plurality of bit line pairs each comprising a pair of bit lines coupled to each of said sense amplifiers in a column and extended toward opposite row-directions to each other, and an other pair of bit lines coupled to each of the sense amplifiers in an adjacent column and extended toward opposite row-direction to each other, said two pair of bit lines being disposed in parallel to each other, respectively;

a plurality of word lines arranged perpendicularly to said bit lines;

a plurality of memory cells each arranged sequentially in each row and column within selected locations of a plurality of spacings formed by intersection of said bit lines and word lines;

a plurality of latching means each connected with the sense amplifiers arranged in a same column; and

a plurality of dummy bit lines arranged in parallel with each of left and right outermost bit lines.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which:

FIG. 1 illustrates the circuit of a conventional DRAM;

FIG. 2 illustrates the circuit of a folded bit line DRAM according to the present invention; and

FIG. 3 illustrates the circuit of an open bit line DRAM according to the present invention.

Referring to FIG. 2, on the upper end of the circuit are a plurality of upper sense amplifiers 10U arranged in a row, while on the lower end are a plurality of lower sense amplifiers 10D arranged in a row. Each of the sense amplifiers comprises MOS transistors 12-15. Drains of the MOS transistors 12 and 14 are cross-connected with their gates through sensing nodes 16, 18, while sources of the transistors 12, 14 are connected with a source common node 11. Between the drains of the MOS transistors 12, 14 and the input/output lines I/OU, I/OU and I/OD, I/OD are respectively connected the source-drain paths of load MOS transistors 13, 15, while to the gates of the MOS transistors 13, 15 is applied the load signal .phi.s.

The source common nodes 11 of the sense amplifirs 10U are connected with the upper common line 24 connected with the drain of the MOS transistor 20 for driving the upper sense amplifiers 10U. The source of the MOS transistor 20 is grounded, and the gate of the transistor 20 receives the signal .phi.L to activate the upper sense amplifer. The sensing nodes 16 and 18 of the upper sense amplifiers 10U are respectively connected with the upper bit line (or column line) pairs UBL1 and UBL2, UBL1 and UBL2, . . . UBLK and UBLKextending to the lower direction. The end of the bit line pairs opposite to the sense amplifiers 10U is connected with the precharging means 30U for precharging the bit lines. The precharging means 30U comprises MOS transistors 32 and 34, the sources of which are respectively connected with the bit line pairs, and the drains of which are connected with a constant precharging voltage V1, and the gates of which are connected with the precharging signal P.

The source common node 11D of the lower sense amplifiers 10D having the same construction as the upper sense amplifiers 10U is connected with the drain of the MOS transistor 22 for driving the lower sense amplifiers 10D through the lower common line 26. The source and the gate of the MOS transistor 22 are respectively connected with the ground and the signal .phi.L having the opposite characteristic to the signal .phi.L. Hence, if the upper sense amplifiers 10U are operated by the signal .phi.L, the lower sense amplifiers 10D are not operated, or vice versa.

The sensing nodes 16D and 18D of the lower sense amplifiers 10D are respectively connected with the lower bit lines DBL1 and DBL1. . . DBLK and DBLK extending to the upper direction, arranged equidistantly between the upper bit lines UBL1 and UBL1 . . . UBLK and UBLK. The end of the lower bit line pairs DBL1 and DBL1 . . . DBLK and DBLK opposite to the lower sense amplifiers 10D is connected with a precharging means 30D having the same construction as the precharging means 30U. Between the precharging means 30U and 30D are arranged parallel word lines (or row lines) WL1-WL4N intersecting perpendicularly the bit lines UBL1 and DBL1 . . . UBLK and DBLK. With in every fourth intersecting spacing in the directions of rows and columns of the word and the bit lines are sequentially connected the memory cells M11-M4NK, respectively.

Before reading the data from a given memory cell, all the bit lines UBL1-DBLK are precharged with the precharge voltage V1 by the precharging means 30U and 30D. Completing the precharging operation, the word line is chosen in order to read out the data from a given memory cell. For example, the word line WL1 will be chosen to read the data from the memory cell M12. If the word line WL1 is chosen, the memory cells M11-M1K connected with the word line WL1 are chosen, and the charges stored into the storage capacitors of the memory cells M11-M1K are respectively transferred to the bit lines UBL1, UBL2, . . . UBLK. Hence, the bit lines UBL1, UBL2, . . . UBLK have the voltage somewhat higher or lower than the precharge voltage V1 due to the state of the received charges. Thereafter, if the signal .phi.L is applied to the gate of the MOS transistor 20, the upper sense amplifiers 10U are activated. If the signal .phi.S is applied to the gates of the MOS transistors 13, 15, the bit line voltage of a pair of the bit lines UBL1, UBL2 . . . UBLK and UBL1, UBL2 . . . UBLK is transferred to the input/output lines I/O and I/O. However, the lower sense amplifiers 10D are not activated due to the off state of the MOS transistor 22 whose gate receives the signal .phi.L having the opposite characteristic to the signal .phi.L. Thus, the lower bit lines DBL1 and DBL1, . . . DBLK and DBLK connected with the lower sense amplifiers 10D maintain the constant value of the precharge voltage V1. Hence, even if the sensing operation of the upper sense amplifiers 10U causes the upper bit lines UBL1 and UBL1, . . . UBLK and UBLK to change their voltage, there can be reduced considerably a danger to misread the data due to the coupling capacitance between each of the upper bit lines and its adjacent lower bit lines. Although heretofore is described a case that the upper sense amplifiers are operated by selecting an odd numbered word line, the similar result is obtained by selecting an even numbered word line.

Referring to FIG. 3 showing a circuit according to the open bit line arrangement in DRAM, the sense amplifiers 40U, 40M, 40D all have the same construction as the sense amplifier 10U of FIG. 2. The sense amplifiers 40U, 40M, 40D are equidistantly arranged in their respective columns. The sense amplifiers 40U, 40M, 40D are respectively connected, through lines 62, 64, 66 connected with the common source nodes 11, with drains of the MOS transistors 52, 54, 56 whose sources are grounded. The gate of the MOS transistor 54 is connected with the signal .phi.L to activate the sense amplifiers 40M, while the gates of the MOS transistors 52, 56 are connected with the signal .phi.L having the opposite characteristic to the signal .phi.L. Hence, if the sense amplifiers 40M are activated, the adjacent sense amplifiers 40U, 40D are not activated, or vice versa. The sensing nodes of the sense amplifiers 40M are respectively connected with the bit line pairs BLM1 and BLM1 . . . BLMK and BLMK of the same length extended oppositely to each other. Likewise, the sensing nodes of the sense amplifiers 40U, 40D are respectively connected with the bit line pairs BLU1 and BLU1 . . . BLUK and BLUK and BLD1 and BLD1 . . . BLDK and BLDK of the same length as the bit lines BLM1 and BLM1 . . . BLMK and BLMK extended oppositely to each other. Bit line groups each [BLUK, BLUK] and [BLM1, BLD . . . BLUK, BLDK] are equidistantly arranged in parallel with each other. Moreover, the dummy bit lines DBL are arranged equidistantly in parallel with the bit line groups [BLU1-BLUK] and [BLD1-BLDK] and supplied with a constant precharge voltage to reduce the capacitance coupling with their adjacent bit lines. The end of each bit line opposite to the sense amplifier is connected with the precharging means (not shown) for precharging the bit line with a given voltage. As shown in FIG. 3, memory cells are each connected between the intersecting points of the word lines [. . . W1N, W21-W2N, W31-W3N, W41 . . . ] and the bit lines. Namely, the memory cells are arranged so that all the bit lines connected with one of the sense amplifiers 40U, 40M, 40D can access the data in the memory cells if one word line is selected.

For example, it is assumed that the word line W32 is selected after all the bit lines are precharged. Then, the charges stored into the memory cells M321-M32K are transferred to the bit lines BLM1-BLMK. Thereafter, the signal .phi.L causes the MOS transistor 54 to be switched on, and the sense amplifiers 40M to perform the sensing operation. At this time, the sense amplifiers 40U, 40D adjacent to the sense amplifiers 40M are not activated by the signal .phi.L having the opposite characteristic to the signal .phi.L. Thus, the bit lines BLD1-BLDK adjacent to the bit lines BLM1-BLMK are at stand-by state, namely, respectively constant with the precharge voltage, thereby reducing undesirable effect that the data is misread due to the coupling capacitance during the sensing operation.

As described above, when any one bit line of the bit line pair connected with each sense amplifier is leveled down or up according to the sensing operation of the sense amplifier, the present invention has the adjacent bit line be on stand-by state, so that the coupling effect with the adjacent bit line will be reduced to provide a stable sensing operation.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that modifications in detail may be made without departing from the spirit and scope of the invention.