MEMORY CIRCUIT WITH IMPROVED WORD LINE NOISE PREVENTING CIRCUITS

Abstract: A memory circuit provided with an improved word line noise preventing circuit is disclosed. The memory circuit is of the type having a pair of digit lines, a plurality of word lines intersecting with the digit lines, a plurality of memory cells, a sense amplifier coupled to the pair of digit lines and a plurality of noise preventing circuits provided for the word lines. The memory is featured in that the noise preventing circuits are disenabled during the period when the sense amplifier amplifis the voltage difference between the pair of digit lines.

Description: The present invention relates to a dynamic memory circuit employing field effect transistors, and more particularly to a word line noise preventing circuit for preventing word lines from being affected by noise or the like according to the preamble part of claim 1. Dynamic memories have been widely utilized in various fields because of large memory capacity and relatively low power consumption. In dynamic memories, word lines are driven in dynamic manner. Namely, all word lines are reset to the ground potential in reset (or precharge) period and in active period selected one word line is driven to a power voltage (Vcc) through a source-follower transistor while the remaining non-selected word lines are left in floating state. In other words, the non-­ selected lines are not connected to any voltage source in active period. Therefore, the non-selected word lines are easily affected by noise such as voltage change in digit lines. Particularly, rise of potential of digit lines having higher level (logic 1 level) to the power voltage Vcc by active pull up circuits raise the potential of the non-selected word lines. In this case, many memory cells which should not be selected are erroneously accessed so that a plurality of stored data are simultaneously read out to the each digit line, resulting in destruction of memory data stored in the memory cells. In order to avoid the above problem due to the fluctuation in the potentials of the non-selected word lines, word line noise preventing circuits are provided to the respective word lines. The word line noise preventing circuit maintains the potential of the non-­ selected word line at the ground potential during active period. Such a noise preventing circuit is disclosed in G3-A-2102 645 which shows a memory circuit according to the preamble part of claim 1. On the other hand, half the digit lines which have been charged to a precharge voltage such as the power voltage through reset period are discharged to the ground potential by amplifying operation of sense amplifiers. The discharge of the digit lines affects the potential of the substrate on which a memory is fabricated and potentials of memory cell capacitors through coupling therebetween so that the potentials of the substrate and the memory cell capacitors are lowered. Especially, in the memory cell storing logic "0" level, i.e. the ground potential, the potential of the memory cell capacitors is lowered from the ground potential to a negative potential, and in the case where such negative potential is lower than the ground potential which is applied to the non-selected word lines by, a threshold voltage of memory cell transistors, the memory cell transistors having gates coupled to the non-selected word lines become conducting. Thus, in addition to the memory cell transistors coupled to the selected word line, the memory cell transistors coupled to the non-selected word lines are erroneously, rendered conductive, which means in other words a plurality of data from a plurality of memory cells are read out on the same digit line at the same time. Accordingly, data stored in these memory cells are destroyed due to mutual interference. From WO-A-81/03570 noise preventing circuits are known in which a plurality of quiet line flip-flops are provided. These flip-flops share the same precharge transistor so that a higher integration density can be obtained. Due to this particular structure all flip-flops are disabled when one of the wordlines is once selected. Therefore, sufficient noise preventing function cannot be expected. In DE-A-33 07 756 a memory circuit having a plurality of pull-down circuits for wordline noise prevention is disclosed. In this circuit, the commonly connected source electrodes of transistors of the pull-down circuits are connected to a voltage line provided with a negative potential. However, it is necessary to provide a special voltage generating circuit for generating the negative potential in this known memory. It is therefore the object of the invention to provide a memory circuit which has a simple construction and which can operate stably without causing data destruction or malfunction. This object is achieved by a memory circuit having the features given in claim 1; the dependent claims are re­ lated to further developments of the invention. According to this features, the word line noise prevention circuits are disabled during the amplifying period by the sense amplifiers so that the clamping of the non-selected word lines is released. Accordingly, the non-selected word lines are allowed to be affected by the discharge of the digit lines so that the potential of the non-selected word lines are lowered below the reference voltage as well as the fall of the potentials of the memory cell capacitors. Therefore, the gate-source potential of the memory cell transistors connected to the non-selected word lines is not increased thereby to prevent the memory cell transistors coupled to the non-­ selected word lines from assuming conductive during the period when the sense amplifiers discharge the digit lines. Thus, data destruction can be effectively avoided. The memory circuit is further provided with a plurality of active pull-up circuits each coupled to each pair of digit lines for operatively raising the higher potential side digit line to the power voltage after the amplification by the sense amplifier, and the word line noise preventing circuit is enabled at least the period when the active pull-up circuit operates. The word line noise preventing circuit according to the present invention is composed of a first field effect transistor having a current path connected between a first node and a reference potential source, a series circuit of second and third field effect transistors connected in series between one of the word lines and the reference potential source, the first and second transistors forming a flip-flop circuit, and a precharge transistor for operatively precharging the first node, the third transistor controlling the noise preventing circuit between the enabled state and the disenabled state. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is a schematic circuit diagram showing a major part of a memory circuit according to a prior art; Fig. 2 is a timing chart showing operation of the memory circuit of Fig. 1; Fig. 3 is a schematic circuit diagram showing a major part of a memory circuit according to one embodiment of the present invention; Fig. 4 is a timing chart showing operation of the circuit in Fig. 3; and Fig. 5 is a schematic diagram showing another embodiment of the invention: DETAILED DESCRIPTION OF THE INVENTION: With reference to Figs. 1 and 2, a memory circuit according to a prior art is explained. Fig. 1 shows a major part of a conventional dynamic memory circuit. A pair of digit lines DL and DL are arranged in parallel, while a pair of dummy word lines DW₁ and DW₂ and a plurality of word lines WL₁ to WL n are intersecting with the digit lines DL and DL . Dummy cells DM consisting of FETs Q D1 and Q D2 and a capacitor C D are connected to the dummy word lines DW₁, DW₂ and the digit lines DL, DL while memory cells MC each composed of a cell FET Q M and a memory cell capacitor C S are connected to the word lines WL₁ to WL n and the digit lines DL and DL , in a known way. In Fig. 1, among many pairs of digit lines, one pair of digit lines DL and DL are representatively illustrated. FETs Q₁ and Q₂ form a flip-flop type sense amplifier. FETs Q₃ to Q₈ and capacitors C₁ and C₂ form an active pull-up circuit which raises a potential of the higher side digit line to a power voltage Vcc after the operation of the sense amplifier. FETs Q₉ to Q₁₁ form a precharge circuit for equally precharging the digit lines DL and DL to Vcc. FETs Q₁₂ and Q₁₃ are coupled between the digit lines DL, DL and a pair of bus lines I/O, I/O , respectively to make data-transfer therebetween when FETs Q₁₂ and Q₁₃ are rendered conductive by a column selection signal Yi. The word lines WL₁ to WL n are selected row decoders RD₁ to RD n , respectively. The row decoder RD₁ is composed of a source-follower FET Q₁₇ connected in series between a timing signal source RA and the word line WL₁, a NOR circuit having FETs Q D1 to Q DN receiving row address signals AX₀ ( AX₀ ) to AX n ( AX n ) and a precharge FET Q₁₄, and a transfer FET Q₁₆ for connecting an output of the NOR circuit to a gate of FET Q₁₅. Other word decoders WD₂ to WD n have the same structure except a combination of address signals AX₀, AX₀ to Ax n , AX n applied thereto. Word line noise preventing circuits NP₁ to NP n are connected to the word lines WL₁ to WL n , respectively. The word line noise preventing circuit NP₁ is composed of a pair of FETs Q₁₇ and Q₁₈ forming a flip-flop circuit, a drain (N2) of FET Q₁₈ being connected to the word line WL₁, and a precharge FET Q₁₉ for precharging a node N₁ to Vcc. A substrate bias circuit 10 is formed on the same semiconductor chip on which the memory circuit is fabricated. The circuit 10 receives an oscillation signal ø osc to generate a substrate bias (V SUB ) through a capacitor C B and a diode D B to be applied to the substrate SUB in a known way. Fig. 2 shows operation of the circuit of Fig. 1. At a time point t₁, the previous access cycle is terminated and a reset period is initiated. A reset precharge signal P is first raised to Vcc to render FETs Q₁₄ in each row decoder, Q₁₉ in each noise preventing circuit and Q D2 in each dummy memory cell. Thus, the dummy cells DM and the row decoders are reset for subsequent operations. Also in each noise preventing circuits, the node N₁ is charged to Vcc to render FET Q₁₈ conductive so that the word line connected to that noise preventing circuit is clamped to the ground potential. Subsequently, timing signals PDL₀ and PDL₁ are sequentially raised to Vcc so that the digit lines DL and DL are equally precharged to Vcc and the capacitors C₁ and C₂ are also precharged to Vcc through FETs Q₅ and Q₆. At a time t₂, the timing signals P, PDL₀ and PDL₁ are fallen to the ground potential to terminate the reset period and introduce an active period. Subsequently, in the active period, after the respective address signals AX₀ ( AX₀ ) to AX n ( AX n ) have established their logical states, a control signal XDT is raised to Vcc so that the gate potential of FET Q₁₅ is determined by the output of the NOR circuit in each row decoder. Then, after the gate potential of FET Q₁₅ has been determined, the signal XDT is reduced to an intermediate voltage which is lower than Vcc but higher than a threshold voltage of FET Q₁₆. Then the timing signal RA having Vcc is applied to the drain of FET Q₁₅ in each row decoder at a time t₃. In this instance, the signal RA is transmitted through the row decoder in which the gate potential of FET Q₁₅ is high in level to a selected word line (e.g. WL₁) to set it approximately at Vcc. In this instance, in the word line noise preventing circuit e.g. NP₁, since the current capability of FET Q₁₉ is set smaller than that of FET Q₁₅, the potential at the node N₂ is raised to discharge the node N1. Thus, FET Q₁₈ becomes non-conducting so that the selected word line e.g. WL₁ is raised to Vcc. While in the remaining noise preventing circuits (e.g. NP₂ to NP n ), the charge at the node N₁ is maintained without being discharged to keep FET Q₁₈ conductive. Thus, the non-­ selected word lines (e.g. WL₂ to WL n ) are clamped to the ground potential so that fluctuation in potential of the non-selected word lines can be effectively avoided. In response to this selection of word line WL₁, data stored the capacitor C S in the memory cell MC coupled to the selected word line is transferred onto the digit line e.g. DL . Also, the dummy word line e.g. DW₁ is simultaneously driven to Vcc by the decoder DWD₁ to connect the capacitor C D to the digit line DL so that a slight change in potential is applied to DL. The change caused by the dummy cell is of an intermediate degree between the cases caused by the memory cells storing "1" and "0". Subsequently, at a time t₄, a sense enable signal SE₁ is fallen from Vcc to the ground potential. In response to this, the sense amplifier (Q₁, Q₂) starts to discharge the lower potential side digit line (e.g. DL) without materially discharging the high potential side digit line (e.g. DL ), thus discriminating the pair of digit lines into binary logic levels. In this instance, the discharge of the lower potential side digit lines makes the substrate bias voltage V SUB and the potential V A at the output node A of the memory cell capacitor C S in each of memory cells lower through coupling between the digit lines and the substrate and the cell capacitors C S , as illustrated. Especially for those memory cells storing "0" level, the potential V A is fallen to a negative potential. While the non-selected word lines are kept at the ground potential as explained above. Accordingly, in the case where the fall of the potential V A at the node A is larger than a threshold voltage of the memory cell transistor Q M in the memory cells storing "0" level, the memory cell transistor Q M is forward-biased to take conductive state even though the word line connected thereto is of the non-selected (ground) level. Accordingly, a plurality of data stored in a plurality of memory cells are read out on to the same digit line in this case so that data of the memory cells are destroyed due to mutual interference among a plurality of memory cells. The above fall in potentials of the substrate and the memory cell capacitors gradually recovers to return to their initial potentials due to supply of charge to the substrate from the substrate bias circuit 10. Subsequently, at a time t₅, the active pull-up circuits start their operations in response to a rise of a timing signal SE 2. Particularly, in the case where DL and DL are at "0" and "1" level, respectively, FET Q₄ is conducting and hence the potential at a node N₃ is discharged to the ground potential so that FET Q₃ becomes non-conductive state, while FET Q₇ is not conductive so that the rise of SE 2 is superposed on the previously charged level (Vcc) at a node N₂ so that the potential at the node N₄ becomes above Vcc thereby to render FET Q₈ in the non-saturated conductive state. Accordingly, the potential of the higher potential digit line DL is raised to Vcc through FET Q₈. As explained above, the discharge of the lower potential digit lines affects the memory cells, resulting in malfunction. Referring to Figs. 3 and 4, a memory circuit according to one embodiment of the invention is described. In Figs. 3 and 4, elements corresponding to those in Figs. 1 and 2 are designated by the same references. This embodiment is achieved by employing word line noise preventing circuits NP₁′ to NP n ′ in place of the circuits NP₁ to NP n in Fig. 1. The word line noise preventing circuit NP₁′ is composed of a series connection of FETs Q₁₈ and Q₂₀ connected between the word line WL₁ the ground potential, FET Q₁₇ connected between the precharge node N₁ and the ground potential and forming a flip-flop with FET Q₁₇ and a precharge FET Q₁₉. A control signal ø N is applied to a gate of FET Q₂₀. When the signal ø N is high (Vcc) in level, EFT Q₂₀ is conductive thereby to enable the noise preventing circuit NP₁′. While ø N is low (GND) in level, FET Q₂₀ is non-conductive thereby to disenable the circuit NP₁′. The other noise preventing circuits NP₂′ to NP n ′ have the same construction as NP₁′. The signal ø N is rendered low in level when the sense amplifiers discharge the lower potential side digit lines so that the noise preventing circuits NP₁′ to NP n ′ are disenabled. Accordingly, the non-selected word lines are not clamped to the ground potential but laid floating state. After discharge of the digit lines by the sense amplifiers and during the period when the active pull-up circuits are enabled thereby to raise the higher potential side digit lines approximately to Vcc, the signal ø N is made high to enable the noise preventing circuits NP₁′ to NP n ′. Therefore, the non-selected word lines are clamped to the ground potential without being affected by the rise of the higher potential side digit lines by the active pull-up circuits. The operation of this embodiment is explained with reference to Fig. 4. The operation from the time point t1 to the time t4 is the same as that of the circuit of Fig. 1 except the noise preventing circuits. Namely, during this period from t1 to 64, the signal øN assumes the low level (GND) to render FET Q20 non-conductive in each of the noise preventing circuits. Therefore, the noise preventing circuits NPi t NPn′ are disenabled so that the non-selected word lines are laid in floating state. Subsequently, at a time t4, the signal SE1 is fallen from Vcc to the ground potential so that the sense amplifiers start their amplification operations, under the disenabled state of the noise preventing circuits. Accordingly, the lower potential side digit lines are discharged. In response to the discharge of the digit lines, the substrate bias voltage VSUB and the potential VA of the memory cell capacitors Cs storing "0" level are lowered to negative values, respectively. However, in this instance the noise preventing circuits NPi to NPn′ are disenabled and therefore, the potential of the non-selected word lines are also lowered to a negative potential due to coupling between the substrate and the word lines. Accordingly, even when the potential VA of the memory cell capacitor Cs is lowered to a negative value, the gate potential of the memory cell transistor QM coupled to the non-selected word line is simultaneously lowered to a negative value, as illustrated, and therefore, the memory cell transistor QM can be kept at non-conducting state. Accordingly, destruction of stored data and/or mutual interference among memory cells can be effectively avoided. Subsequently, after the potential of the non-selected word line WLN, the substrate bias voltage VSUB and the potential VA of the memory cell capacitor CS return to their predetermined values at a time t4′, the signal øN is raised to VCC thereby to enable the noise preventing circuits NPi t NPn′. Accordingly, the non-selected word lines are clamped to the ground potential. Then, at a time t5, the signal SE2 is raised to Vcc and the active pull-up circuits are enabled. Thus, the higher potential side digit lines are raised to Vcc by the pull-up circuit. The side of the higher potential side digit lines affect the non-selected word lines so as to raise their potentials. However, in this instance, the non-selected word lines are clamped to the ground potential by the noise preventing circuits, such affection can be effectively avoided. As to the waveform of øN, it is also possible to maintain it at high during the period from t1 to t4 as illustrated by dotted line. Fig. 5 shows another embodiment of the invention. This embodiment is featured in that FET Q20 receiving øN and the precharge FET Q19 in Fig. 3 are commonly provided as FETs Q20′ and Q19′ for a plurality of noise preventing circuits NP1" to NPn". According to this feature, the size of each noise preventing circuit can be reduced to enhance high density structure of a memory circuit.

Claims: 1. A memory circuit comprising at least a pair of digit lines (DL, DL ) a plurality of word lines (WL1-WLn) intersecting with said pair of digit lines, a plurality of memory cells (MC) coupled to said digit lines and said word lines, a word line selection circuit (RD1-RDn) for selecting one of said word lines, a sense amplifier (Q1, Q2) operatively discharging one line of said pair of digit lines during a first period (t4-t4′), a pull-up circuit (Q3-Q8) for operatively raising the potential of the other line of said pair of digit lines during a second period (t5-) after said first period, a plurality of word line noise preventing circuits (NP1-NPn), each of said noise preventing circuits operatively clamping the associated word line in the case when said associated word line is not selected, each of said noise preventing circuits including a first node (N2) coupled to the associated word line, a second node (N1), a first transistor (Q18) having a gate coupled to said second node, first means for coupling a source-drain path of said first transistor between said first node and a reference voltage terminal (GND), a second transistor (Q17) having a gate coupled to said first node and a source-drain path coupled between said second node and said reference voltage terminal, and a third transistor (Q19) coupled to said second node for operatively precharging said second node thereby to render said first transistor conductive, CHARACTERIZED in that said memory circuit further comprises control means (øN, Q20) for disenabling said noise preventing circuits during said first period thereby to isolate the respective first nodes from said reference voltage terminal and enabling said noise preventing circuits during said second period. 2. The memory circuit according to claim 1, CHARACTERIZED in that said control means includes a plurality of switch elements (Q20), each of said switch elements having a switched current path inserted in series with respect to the source-drain path of the first transistor (Q18) of each one of said word line noise preventing circuits. 3. The memory circuit according to claim 1, in which said memory circuit is formed on a semiconductor substrate (SUB), CHARACTERIZED in that said memory circuit further comprises a substrate bias circuit (10) for biasing said substrate at a predetermined potential.

Inventors: MORITA, YASUKAZU
Applicants: NEC CORPORATION
Application year: 1986
Main classification: G11C8
IPC codes: G11C8, G11C11