Memory Device

The present invention relates to a memory device, and particularly to a semiconductor memory device (IC memory) fabricated on a semiconductor substrate.

The degree of integration and operation speed of an IC memory have been increasing. In the IC memories, a polycrystalline silicon layer are widely employed for forming word lines because they are suitable for small-sized, fine patterning and enable self-aligning between semiconductor regions and silicon gates. However, a sheet resistance of the polycrystalline silicon is larger than conventional metal material. Accordingly between the both ends of word lines formed of the polycrystalline silicon, a relatively large impeadance inevitably exists, and hence a large time constant (i.e. delay) which is formed by the impedance and capacitance of the word line is resultantly produced between the both ends of the word line. This delay in the word line occupies a large part of a total delay in the whole memory circuit and occupies 20.about.30% of an access time. Thus, the improvement in the access time cannot be expected in view of the structure of the word line.

It is an object of the present invention to provide a memory device operable at a high speed.

It is another object of the present invention to provide a memory device having large memory capacity.

A memory device according to the present invention comprises a plurality of word lines, a plurality of data lines, a plurality of memory cells arranged at the respective intersections of the word lines and data lines, a word decoder circuit having a plurality of selection outputs, a plurality of first word driver circuits respectively receiving one of the selection outputs for providing a portion of the associated word line with a voltage capable introducing therein a selection level, a plurality detection circuits respectively coupled to another portion of the associated word line for detecting a change toward the selection level therein to produce a detection signal, and a plurality of second word driver circuits respectively responsive to the detection signal from the associated detection for providing another portion of the associated word line with a voltage capable of introducing the selection level.

In the above memory device, the driving ability of the second word drive circuits is favorably set larger than the first word drive circuits.

The detection circuit may be made of an inverter circuit having an input coupled to another portion of the word line and the second word drive circuit may be made of an insulated-gate field-effect transistor (IGFET) coupled between a power supply and another portion of the word line, a gate of which is supplied with the detection signal.

According to the present invention, the word line can be provided with the voltage for introducing the selection level at a plurality of different portions (i.e. "a portion" and "another portion") thereof and hence as a whole the selected word line can be quickly turned to the selection level without influence of its time constant. As a result, high speed operation can be achieved.

Furthermore, the driving ability of the first word drive circuits may be small and hence IGFETs forming them can be made small size. As a whole, the memory device according to the present invention can be with high integration.

FIG. 1 is a block diagram showing a conventional memory device;

FIG. 2 is a circuit diagram showing a memory cell;

FIG. 3 is a circuit diagram showing a word line of FIG. 1;

FIG. 4 is a wave form diagram showing changes in level of the word line in FIG. 3;

FIG. 5 is a block diagram showing a memory device according to a first embodiment of the present invention;

FIG. 6 is a circuit diagram showing a characteristic part of a memory according to a second embodiment of the present invention;

FIG. 7 is a waveform diagram showing an operation of the circuit in FIG. 6;

FIG. 8 is a circuit diagram showing a characteristic part of a memory device according to a third embodiment of the present invention;

FIG. 9 is a circuit diagram showing a characteristic part of a memory device according to a fourth embodiment of the present invention;

FIG. 10 is a waveform diagram showing an operation of the circuit in FIG. 9;

FIG. 11 is a circuit diagram showing a memory device according to a fourth embodiment of the present invention;

FIG. 12 is a circuit diagram showing a memory device according to fifth embodiment;

FIG. 13 is a block diagram showing a major part of a memory device according to a sixth embodiment of the present invention; and

FIG. 14 is a waveform diagram showing an operation of the circuit of FIG. 11.

First, with reference to FIGS. 1 to 4, a semiconductor memory device according to the prior art will be described.

A whole structure of the memory device is shown in FIG. 1. A row address inverter buffer 11 receives a plurality of row address signals AR.sub.l .about.AR.sub.n to produce their true and complementary address signals. A row decoder 12 receives the true and complementary address signals and makes one of its n pieces of outputs at a selection level. A plurality of word driver circuits WD.sub.l .about.WD.sub.k are respectively coupled between a plurality of word lines WL.sub.l .about.WL.sub.k and the outputs of the decoder 12. The word driver circuits WD.sub.l .about.WD.sub.k selectively drive one of the word line between the selection level and the nonselection level. A plurality pairs of digit lines D.sub.1, D.sub.2 . . . D.sub.n, D.sub.n intersect the word lines WL.sub.l .about.WL.sub.k. At the respective intersection of the word lines and the digit line pairs, a plurality of memory cells MC.sub.ll .about.MC.sub.kn and arranged. Digit data sense circuits SA.sub.l .about.SA.sub.n amplify data read on the digit line pairs. A column address inverter buffer 13 receives a plurality of column address input signals AC.sub.l .about.AC.sub.m and produces their true and complementary address signals. A column decoder 14 receives the address signals from the column address inverter buffer 13 and makes one of its selection outputs Y.sub.l .about.Y.sub.n at the selection level. True and complementary read data on the digit line pairs are transferred to a pair of bus lines DB and DB through a plurality pairs of transfer gate transistors QY.sub.l, QY.sub.l ' . . . QY.sub.n, QY.sub.n '. An input/output circuit 15 coupled to the bus lines DB and DB is controlled by a write enable signal W between a read mode and a write mode and outputs read-out data at an input output common terminal I/O or transfers write data applied to the terminal I/O to the bus lines DB and DB.

FIG. 2 shows a structure of the memory cells (MC.sub.ll .about.MC.sub.kn) shown in FIG. 1.

The memory cells P-channel IGFETs Q.sub.1 and Q.sub.3, and N-channel IGFETs Q.sub.2, Q.sub.4, Q.sub.5 and Q.sub.6. IGFETs Q.sub.1 to Q.sub.4 form a flip-flop circuit as a bi-stable circuit. IGFETs Q.sub.5 and Q.sub.6 have gates coupled to the word line (e.g. WL.sub.l) and transfer a true and a complementary output signals of the flip-flop circuit to the pair of digit lines (e.g. D.sub.1 and D.sub.2).

With reference to FIGS. 3 and 4, a problem in the word line of the prior art memory device will be described.

In FIG. 3, the word line WL.sub.l is usually made of a porycrystalline silicon having a sheet resistance and has a relatively large resistance R in its longitudinal direction. Also large capacitance C including its stray capacitance and load capacitance is attached to the word line WL.sub.l. Thus, the word line WL.sub.l inevitable has a large time constant (R.times.C) in its longitudinal direction, and hence access to the memory cell arranged near a remote end W.sub.e of is delayed with a large extent.

Waveforms of a near end W.sub.o to the word driver circuit WD.sub.l and the remote end W.sub.e are shown in FIG. 4. In this case, as a power voltage, 5 V is employed. Waveform V.sub.wo shows a voltage change at the near end W.sub.o while waveform V.sub.we show a voltage change at the remote end W.sub.e. As is clear from FIG. 4, rise of voltage (V.sub.we) at the remote end W.sub.e is delayed from the rise of V.sub.wo bt about 30 nano sec, which occupies a large part in access time. As a result, high speed operation cannot be expected.

Now with reference to FIG. 5, a first embodiment of the present invention will be described.

In FIG. 5, word level detection circuits WDT.sub.l .about.WDT.sub.n are coupled to the respective word lines at their remote end. The word level detection circuits detect an initiation of a change from a nonselection level to a selection level in the respective word lines. If the initiation is detected, the word level detection circuit (WDT.sub.l) produces a detection signal DRS and supplies it to an associated select word drive circuit WDR.sub.l or WDR.sub.n coupled to the remote end of the word line (WL.sub.l, WL.sub.n). The select word circuit WDR.sub.l or WDR.sub.n provides the associated word line with the selection level.

Thus, the word line to be selected is forcibly made at the selection level without the time delay near the remote ends. In this connection, the portion of the word lines to which the word level detection circuit and the select word drive circuit is not limited to the remote end.

With reference to FIGS. 6 and 7, a second embodiment of the present will be described in detail.

In FIG. 6, the word drive circuit WD.sub.l comprises P-channel IGFETs Q.sub.10 and Q.sub.11 and an N-channel Q.sub.12. IGFETs Q.sub.11 and Q.sub.12 form an inverter circuit receiving a decode signal from a NAND gate NAG forming the decoder 12. IGFET Q.sub.10 receives a chip enable signal CE at its gate and enables the inverter in response to the row level of CE. An output of the word drive circuit WD.sub.l is applied to a portion of the word line WL.sub.l (in this case the near end W.sub.o). The word level detection circuit WDT.sub.l is made of an inverter including P-channel IGFET Q.sub.14 and an N-channel IGFET Q.sub.15, whose gates are coupled to a portion of the word line which is different from the portion coupling to the word drive circuit. The select word drive circuit WDR.sub.l is made of a P-channel IGFET Q.sub.13 coupled between the power voltage V.sub.DD and a portion of the word line (in this case the remote end W.sub.e) at its drain and source and receiving an output ND of the circuit WDT.sub.l at its gate.

A selection operation of the circuits shown in will be explained by referring to FIG. 7.

A waveform V'.sub.we show a voltage change at the remote end W.sub.e of the word line WL.sub.l. At time=0, the word drive circuit WD.sub.l starts to supply the end W.sub.o with the selection level i.e. V.sub.cc. From this time point, the level V'.sub.we starts to rise in the similar curve as in FIG. 4. At this time point t.sub.l the level of V'.sub.we exceeds a logic threshold level V.sub.th of the inverter including IGFETs Q.sub.14 and Q.sub.15 so that the output signal ND starts to fall in level. At the same time, the IGFET Q.sub.13 conducts. Therefore, the level V'.sub.we of the remote end rapidly rises towards the high level (selection level) of 5 V.

Thus, the word line is quickly made at the selection level. As is clear from the figure, the word line can be set at the selective level within about 20 nano sec as a whole and high speed operation is obtained. Here, the threshold level V.sub.th is optionally set by the ratio of gm between IGFETs Q.sub.14 and Q.sub.15.

FIG. 8 shows a third embodiment of the present invention. This embodiment includes a word level detection circuit WPR.sub.l ' made of a P-channel IGFET Q.sub.51 and an N-channel IGFET Q.sub.52 and a selection word drive circuit WDT.sub.l ' made of an N-channel IGFET Q.sub.53. In this embodiment, the level of a non-selected word line (WL.sub.l) is at V.sub.cc level and the level of a selected word line is at ground potential. Thus, the operations of the second embodiment are self-explanatory because they can be analogized by inverting the polarity of the second embodiment.

FIG. 9 shows a fourth embodiment of the present invention, in which a reset structure made of is added to the selection word drive circuit WDT.sub.l ".

FIG. 10 is a timing chart showing the operations of the circuit of FIG. 9, in which waveforms V.sub.wo, V.sub.we represent the level at the portions W.sub.o and W.sub.e respectively.

In FIG. 9, the word level detection circuit WDR.sub.l " includes a P-channel IGFET Q.sub.61 and an N-channel IGFET Q.sub.62 and has the same structure as the circuit WDR.sub.l in FIG. 6. The IGFETs Q.sub.64 and Q.sub.65 have their respective gates supplied with reset signals P so that the IGFET Q.sub.64 is rendered conductive whereas the IGFET Q.sub.65 is rendered nonconductive when the reset signal P is at a low level to introduce an active period. Thus, this embodiment operates similarly to the first embodiment during the active period. On the contrary, when the reset signals P is turned a high level to introduce a reset period, the IGFET Q.sub.64 is rendered nonconductive whereas the IGFET Q.sub.65 is rendered conductive so that the remote end we promptly is turned to a low level, i.e., so that the selected word line is brought into its reset condition. Simultaneously with this, the IGFET Q.sub.61 is rendered conductive whereas the transistor Q.sub.62 is rendered nonconductive so that the output signal takes a high level. Namely, the circuits WDT.sub.l " and WDR.sub.l " are brought into reset condition.

FIG. 11 shows a first embodiment of the present invention. In this embodiment, the word level detection circuit WDT.sub.l '" is similarly structured as in WDT.sub.l in FIG. 6. A select word drive circuit WDR.sub.l '" is composed of a P-channel IGFET Q.sub.71 and an N-channel IGFET Q.sub.72. A word enable timing signal WE which is produced within the memory device in response to the chip enable signal is applied to a drain of IGFET Q.sub.71. IGFET Q.sub.72 is used to reset the level of the word line in response to the precharge signal P.

The foregoing description is directed to the case, in which the circuits, WDT.sub.l, WDT.sub.l ' . . . and WDR.sub.l, WDR.sub.l ' . . . according to the present invention is disposed at the opposite position to that of the decoder circuit and the word drive circuit with respect to the word line with a view to reducing the signal delay especially in case the word line has a high wiring impedance. However, the effects of the present invention are not limited to the above cases.

A sixth embodiment of the present invention will be described with reference to FIG. 12. In this embodiment, the word level detection circuit WDR.sub.l is disposed at a middle portion W.sub.m of the word line WL.sub.l. With respect to one word line WL.sub.l, two select word drive circuits WDT.sub.11 and WDT.sub.12 are disposed respectively at the middle portion W.sub.m and the remote end W.sub.e of the word line. The word level detection circuit WDR.sub.l and the select word drive circuits WDT.sub.11 and WDT.sub.12 may be constructed similarly in the preceding embodiments.

Another embodiment attaining the effects of the present invention will be described in the following.

FIG. 13 is a circuit diagram showing a seventh embodiment of the present invention, and FIG. 14 is a timing chart of the same, in which a wave form V.sub.we shows the level at the portion W.sub.e of the word line. In this embodiment, the word level detection circuit WDR.sub.l "" are disposed at the near end W.sub.o of the word line WL.sub.l. The circuits WDT.sub.l "" and WDR.sub.l "" are substantially the same as the circuits WDT.sub.l "' and WDR.sub.l "' in FIG. 11 except that the chip enable signal CE is applied to a drain of a P-channel IGFET Q.sub.81 in place of the WE. When the output Ns of the decoder circuit 12 is turned to a low level and the chip the chip enable signal CE is turned to the a low level, IGFETs Q.sub.85 and Q.sub.86 are rendered conductive so that the selected word line WL.sub.l is started to rise up to the final level V.sub.cc. Here, when the selected word line WL reaches a certain level, the word level detection circuits WDT.sub.l "" detects that level to start its operation. If the driving capacity of the IGFET Q.sub.81 in the select word drive circuit WDR.sub.l "" is preset larger than those of IGFETs Q.sub.85 and Q.sub.86 in the word drive circuit WD.sub.l, the selected word line WL.sub.l has its rising gradient steepened at a time t.sub.o, as shown in FIG. 14, so that the arrival time at the final time is shortened in comparison with the case, in which the select word line drive circuit WDR.sub.l "" is not added. On the contrary, in case the afore-mentioned arrival time is made the same, the dimensions of the IGFETs Q.sub.85 and Q.sub.86 can be reduced together with the load capacity of the signal CE.

Although the foregoing description is directed to the memory circuit, it is needless to say that the gist of the present invention should not be restricted thereto but can be extended to a general application, in which the signal of a signal line is intended to be self-amplified.