Static-Type Random-Access Memory Device

(1) Field of the Invention

The present invention relates to a static-type random-access memory (RAM) device and more particularly to a static-type RAM device such as a metal-insulator semiconductor (MIS) memory device in which the amplitude of the data signal in a memory cell just after the writing of data thereinto is completed is increased and the stability of the data stored in the memory cell is increased.

(2) Description of the Prior Art

In a semiconductor memory device, it is possible for the data stored in each memory cell to sometimes be destroyed due to the irradiation of alpha rays thereinto or due to the application of noises thereto. The destruction of data occurs more frequently when the amplitude of the data stored in each memory cell, i.e., the voltage difference between the high level portion and the low level portion of each memory cell, is small. Therefore, it is necessary to increase the amplitude of the data stored in each memory cell as much as possible although the potential of the power source is limited to a predetermined value.

FIG. 1 is a circuit diagram of a conventional static-type RAM device (hereinafter referred to as a static RAM device). In FIG. 1, MC is a memory cell composed of four MIS transistors Q.sub.1 through Q.sub.4 and two resistors R.sub.1 and R.sub.2. BL and BL are bit lines, WL is a word line, and DB and DB are data buses. Q.sub.5 and Q.sub.6 are transistors constituting column-selecting transfer gates connected between the bit lines BL and BL and the data buses DB and DB, respectively. Q.sub.11 and Q.sub.12 are load transistors connected between the bit lines BL and BL and a power source V.sub.cc, and Q.sub.7 through Q.sub.10 are transistors constituting a write-in circuit WTC. RD is a row decoder, and RB is a row driver composed of transistors Q.sub.13 and Q.sub.14. One end of each of the data buses DB and DB is connected to a sense amplifier SA (not shown in the drawing). In FIG. 1, only one memory cell MC, only one pair of bit lines BL and BL, and only one word line WL are illustrated. In practice, however, a plurality of memory cells, bit line pairs, and word lines are arranged.

In the static RAM device of FIG. 1, when data is written into the memory cell MC, a row-selecting signal X having a high potential level obtained by inverting and amplifying the output of the row decoder RD, is applied to the word line WL, and a column-selecting signal Y having a high potential level is applied to the gate electrodes of the column-selecting transistors Q.sub.5 and Q.sub.6 by a column decoder (not shown). Thus, the transistors Q.sub.3 and Q.sub.4 of the memory cell MC and the column-selecting transistors Q.sub.5 and Q.sub.6 are all turned on. Therefore, the data buses DB and DB and the bit lines BL and BL are connected, respectively, and the bit lines BL and BL and the nodes A and B of the memory cell MC are connected, respectively. In this condition, a write-enable signal WE is rendered low, and write-in signals IN and IN are rendered, for example, low and high, respectively, and are applied to the write-in circuit WTC. In this condition, the transistors Q.sub.7 and Q.sub.10 of the write-in circuit WTC are turned on and the transistors Q.sub.8 and Q.sub.10 thereof are turned on, thereby rendering the potentials of the data buses DB and DB high and low, respectively, and rendering the potentials of the bit lines BL and BL high and low, respectively. Therefore, the transistor Q.sub.1 of the memory cell MC is turned on and the transistor Q.sub.2 of the memory cell MC is turned off, with the result that the potentials of the nodes A and B of the memory cell MC become low and high, respectively. FIG. 2 illustrates the waveforms at the signals of each node when the data "1" or "0", which is opposite to the data "0" or "1" which is initially stored in the memory cell MC, is written into the memory cell.

The potential of the high level portion of the memory cell into which the information has just been written, i.e., the potential of the above-mentioned node B, becomes lower than the power supply voltage V.sub.cc. As is illustrated in FIG. 2, when the write-in signal IN changes from a low level potential to a high level potential which is approximately equal to the power supply voltage V.sub.cc, the potential of the data bus DB rises from a low level potential to the potential V.sub.cc -V.sub.th (Q.sub.7), i.e., the potential which is attained by subtracting the threshold voltage V.sub.th (Q.sub.7) of the transistor Q.sub.7 from the power supply voltage V.sub.cc. The potential of the bit line BL becomes V.sub.cc -V.sub.th (Q.sub.6) when the threshold voltage V.sub.th (Q.sub.6) of the transistor Q.sub.6 is larger than the threshold voltage V.sub.th (Q.sub.7) and becomes V.sub.cc -V.sub.th (Q.sub.7) when V.sub.th (Q.sub.6) is smaller than V.sub.th (Q.sub.7). Therefore, the potential V.sub.B of the above-mentioned node B becomes

where V.sub.th (Q.sub.4, Q.sub.6, Q.sub.7) is the largest threshold voltage among the threshold voltages V.sub.th (Q.sub.4), V.sub.th (Q.sub.6), and V.sub.th (Q.sub.7) of the transistors Q.sub.4, Q.sub.6, and Q.sub.7, and it is assumed that the potentials of the high level signals of the word line WL, the write-in signals IN and IN, and the column-selecting signal Y all rise to approximately the power supply voltage V.sub.cc.

The potential V.sub.B of the node B, i.e., the node of the memory cell which is rendered high by the above-mentioned write-in operation, rises toward the power supply voltage V.sub.cc by the time constant R.multidot.C after the write-in operation is finished due to the supply of electric charges through the load resistor R.sub.2 of the memory cell. In this case, R=R.sub.1 =R.sub.2, and C is the stray capacitance of the node B. In a recent static RAM device, since the value of the resistance R is determined to be on the order of more than a gigohm in order to decrease power consumption, the time constant R.multidot.C is relatively large. Therefore, in the above-mentioned conventional static RAM device, the time interval from the completion of the writing of data into the memory cell to the time when the potential of the high level portion of the memory cell is almost equal to the power supply voltage V.sub.cc becomes long. Thus, in the conventional static RAM device, the stored data is often destroyed by external noises, alpha rays, and so on which are irradiated into the memory cell during the above-mentioned time interval.

In order to eliminate the above-mentioned problems of the conventional RAM device, in the present invention the potential of the word line in a static RAM device is temporarily pulled up to a potential level greater than the power supply voltage just after data is written into a memory cell.

It is an object of the present invention to increase the voltage level of a data signal written into a memory cell and to prevent destruction of the data stored in the memory cell as a result of external noises and so on.

According to the present invention, this object is attained by providing a static-type RAM device having static-type memory cells disposed at the intersecting portions of bit line pairs and word lines and including a bit-line pulling-up means for pulling up the potential of one of the bit lines of a selected bit line pair to a voltage which is approximately equal to a power supply voltage or which is larger than the power supply voltage and a word-line pulling-up meas for pulling up the potential of a selected word line to a voltage which is higher than the power supply voltage after data is written into a memory cell.

FIG. 1 is a block circuit diagram of a conventional MIS static-type RAM device;

FIG. 2 is a waveform diagram of the operation of the static-type RAM device of FIG. 1;

FIG. 3 is a block circuit diagram of a MIS static-type RAM device as an embodiment according to the present invention;

FIG. 4 is a waveform diagram of the operation of the static-type RAM device of FIG. 3; and

FIGS. 5A and 5B are block circuit diagrams of other embodiments according to the present invention.

The embodiments of the present invention will now be explained with reference to the drawings. FIG. 3 is a circuit diagram of a MIS static-type RAM device as an embodiment of the present invention. The static-type RAM device of FIG. 3 differs from the device of FIG. 1 in that, in the RAM device of FIG. 3, there is provided a high-voltage supplying circuit HVS which supplies power to row drivers RB connected to row decoders, transfer gates for selecting a column are connected between bit lines BL, BL and data buses DB, DB are constituted of parallel connections of a P-type transistor Q.sub.15 and an N-type transistor Q.sub.16 and a P-type transistor Q.sub.18 and an N-type transistor Q.sub.17, load transistors Q.sub.23 and Q.sub.24 of the bit lines BL and BL are P-type transistors, and a write-in circuit WTC' comprises complementary metal-insulator semiconductor (MIS)-type inverters. The high-voltage supplying circuit HVS comprises transistors Q.sub.25 through Q.sub.31, a capacitor C.sub.1, and a delay circuit DL.

In the static-type RAM device of FIG. 3, when data is written into a memory cell MC, a row-selecting signal X having a high potential level and which is an inverted signal of the output of a row decoder RD, is applied to a word line WL. At the same time, a column-selecting signal Y having a high potential level is applied to the N-type MIS transistors Q.sub.16 and Q.sub.17 and an inverted signal Y of the column-selecting signal Y is applied to the P-type transistors Q.sub.15 and Q.sub.18 in order to turn on the transfer gates so that a column is selected by a column decoder, etc. (not shown). A write-enable signal WE is rendered low, and write-in signals IN and IN are rendered low and high, respectively, and are applied to the write-in circuit WTC'. In this condition, the P-type transister Q.sub.19 is turned on by the write-in signal IN, which has a low potential level, and the potential of the data bus DB is pulled up to approximately the power supply voltage V.sub.cc, as is illustrated in FIG. 4. Since each of the transfer gates for selecting columns comprises a parallel connection of a P-type transistor and an N-type transistor and since each of the load transistors Q.sub.23 and Q.sub.24 is a P-type transistor, the potential of the bit line BL is also pulled up to approximately the power supply voltage V.sub.cc.

The inverted write-enable signal WE applied to the high-voltage supplying circuit HVS is inverted by a CMIS inverter comprising the transistors Q.sub.25 and Q.sub.26, and thereby the write-enable signal WE is produced and applied to the transistors Q.sub.28 and Q.sub.29. The inverted write-enable signal WE is delayed for a predetermined length of time by the delay circuit DL, which outputs a delayed signal WE.sub.d applied to the transistors Q.sub.27 and Q.sub.30. The transistors Q.sub.27 through Q.sub.30 comprise a NOR gate whose output (point D) becomes high only when both the write-enable signal WE and the delayed signal WE.sub.d are low because the transistors Q.sub.27 and Q.sub.28 are turned on and the transistors Q.sub.29 and Q.sub.30 are turned off in this condition. Therefore, when the inverted write-enable signal WE becomes high again, a pulse signal is generated at the point D, which has a time width corresponding to the delay time of the delay circuit DL. The transistor Q.sub.31 and capacitor C.sub. 1 form a clamping circuit. The transistor Q.sub.31 is turned on when the potential of the point D is low and charges the capacitor C.sub.1 so that the potential difference of the terminals thereof becomes approximately V.sub.cc. Therefore, when the potential of the point D becomes high, the transistor Q.sub.31 is turned off and a voltage, which is the sum of the potential of the point D and the voltage charge of the capacitor C.sub.1, i.e., a voltage which is higher than or equal to the power supply voltage V.sub.cc, is generated at a point E and is supplied to the row driver RB. Consequently, the potential X of the word line WL is temporarily pulled up to a potential level which is greater than or equal to the power supply voltage V.sub.cc after the writing in of data is completed. In this condition, since the potential of the bit line BL is pulled up to approximately the power supply voltage V.sub.cc, as was mentioned before, the potential of the node B in the memory cell MC is rapidly pulled up to approximately the power supply voltage V.sub.cc after the writing in of data is completed, thereby decreasing the possibility of destruction of the stored data by external noises and so on.

FIG. 5A is a MIS static-type RAM device which is another embodiment of the present invention. In the static-type RAM device of FIG. 5A, load transistor Q.sub.32 and Q.sub.33 of the bit lines BL and BL and transistors Q.sub.34 and Q.sub.35 of the transfer gates for selecting a column comprise N-type transistors each having a smaller threshold voltage than other enhancement transistors used in the RAM device and a write-in circuit WTC" comprise two inverter circuits each having a depletion-type load transistor Q.sub.36 or Q.sub.38, thereby pulling up the potential of the high level voltage of a bit line to approximately the power supply voltage V.sub.cc. The other portions of the device of FIG. 5A are the same as those of FIG. 3.

It is also possible to replace the load transistors Q.sub.32 and Q.sub.33 of the bit lines BL and BL shown in FIG. 5A with depletion-type transistors Q.sub.40 and Q.sub.41, as is illustrated in FIG. 5B.

As was mentioned above, according to the present invention, since the potential of the high level point of a memory cell is pulled up to approximately the power supply voltage V.sub.cc just after the writing in of data is completed, the possibility of destruction of stored data in each memory cell by external noises, the irradiation of alpha rays, and so on, is decreased, thereby making it possible to obtain a memory device having a high reliability.