Semiconductor Memory Having Barrier Transistors Connected Between Sense and Restore Circuits

1. Field of the Invention

This invention relates to a semiconductor memory. In particular, the invention relates to the connection of the bit lines and the restore circuits in a memory which has barrier transistors between the bit lines and the sense amplifiers.

2. Description of the Related Art

As shown in FIG. 1, in conventional memories of this type, for instance the dynamic RAM (random access memory), barrier transistors T1 and T2 formed by N channel MOS transistors are inserted at one end of the bit line pair BL and BL of each column in the memory cell array 3. Restore circuit 1 and synchronized sense amplifier 2 are connected to the bit line pair section on the opposite side of the memory cell array 3 using barrier transistors T1 and T2 as a reference. This sense amplifier 2 operates the sense amplification and latch action by receiving synchronization signal .phi.1. A dummy cell circuit 4 and precharge equalizer circuit 5 are connected in series to the other end of the bit line pair BL and BL.

As is well known, for reading the memory cell data in a dynamic RAM, sense amplifier 2 reads the slight difference in potential which is generated in the bit line pair BL and BL by amplifying it and determines data "1" or "0". However, a problem arises in that the more the integration of dynamic RAMs progresses, the greater becomes the load capacity of bit lines BL and BL and the longer the sensing time becomes. Therefore, to counteract this, a resistance component due to the barrier transistors T1 and T2 is inserted between bit lines BL and BL and sense amplifier 2 so that latching of the bit line potential by sense amplifier 2 is made faster.

A barrier control signal .phi.T is applied to the gates of barrier transistors T1 and T2. The potentials shown in the following Table are used for this signal .phi.T according to the bit line precharge system.

Here, VT is the threshold voltage of barrier transistors T1 and T2. Accordingly, when restoring, it is necessary to make the bit line potential Vcc. Thus, at this time, in order to set barrier transistors T1 and T2 to the ON state, each system requires a greater potential than (Vcc+VT) as the .phi.T potential. In this case, in the Vss precharge system and the Vcc/2 system, it is necessary to step up the potential so that .phi.T is .gtoreq.Vcc+VT during restoring, which is different from other periods.

In the conventional memories mentioned above, a voltage step-up circuit (not shown in the FIG. is required as a memory peripheral circuit in order to generate the control signal .phi.T. This leads to accompanying problems in that the design and construction are more complicated, the area occupied on the memory clip surface is increased and, consequently, the area of the memory chip is increased.

An object of this invention is to provide a semiconductor memory in which the gate potential of the barrier transistors is constant, and to eliminate the need for a step-up circuit for the barrier transistor control signal. Thus, the design can be simplified, the circuit can be more easily constructed and the required chip surface area can be decreased.

The semiconductor memory of this invention has the characteristic of connecting restore circuits directly to a bit line pair by inserting MOS barrier transistors between the bit line sense amplifiers and the bit line pair and applying a constant potential to the gates of the barrier transistors.

In this way, there is no loss of efficiency from the insertion of the barrier transistors and therefore a step-up circuit for the barrier transistor control signal is no longer required.

FIG. 1 is a circuit diagram showing a part of a conventional dynamic RAM.

FIG. 2 is a circuit diagram showing a part of a Vcc/2 precharge system dynamic RAM of an embodiment of this invention.

An embodiment of this invention is explained in detail below with reference to drawings.

FIG. 2 shows the circuit connections of restore circuit 1, barrier transistors T1 and T2 and synchronized sense amplifier 2 for the bit line pair BL and BL which correspond to 1 column in the memory cell array 3 of, for instance, a Vcc/2 precharge system dynamic RAM. The device of FIG. 1 differs from the circuit connections described above in two major respects. The first difference is that restore circuit 1 is directly connected to bit line pair BL and BL (that is to say, taking barrier transistors T1 and T2 as a reference, it is connected on the memory cell side). The second point is that a constant potential, for instance power source potential Vcc, is applied to the gates of barrier transistors T1 and T2. The same symbols as appear in FIG. 1 are used for the other components shown in FIG. 2.

This embodiment shows the case of a CMOS (complementary insulated gate) memory. Sense amplifier 2 is connected in such a way that two N channel enhancement MOS transistors N1 and N2, form a flipflop circuit. Thus, the operating and non-operating states are controlled by N channel enhancement MOS transistor N2 whose switching is in turn controlled by sense amplifier control signal .phi.1. Also, N channel enhancement MOS transistors are used for barrier transistors T1 and T2. Moreover, restore circuit 1 is connected in such a way that two P channel enhancement MOS transistors P.sub.1 and P.sub.2 form a flipflop circuit. Thus, the operating and non-operating states are controlled by the restore control signal.

The following are explanations of the operation of the above circuits.

(1) When precharge operates, since gate potential Vcc of barrier transistors T1 and T2 is higher than bit line precharge potential Vcc/2+threshold voltage VT of barrier transistors T1 and T2, barrier transistors T1 and T2 are in the ON states. Therefore no interference occurs to the bit line precharge operation through the precharge circuit 5.

(2) When the memory cell data is read, sense amplifier control signal .phi.1 becomes high level and sense amplifier 2 carries out sense amplification of the potential difference generated between bit line pair BL and BL. This is the difference between the potential generated in one bit line BL through the holding charge of the memory cell 20 selected in accordance with the address input and the potential generated in this bit line BL by a selected dummy cell 22 which is connected to the other bit line BL. Next, restore circuit 1 operates and restores the bit line pair. Since restore circuit 1 is installed directly on bit line pair BL and BL, it is possible to step-up the bit line potential sufficiently to the required level of Vcc potential, even though the gate potential of barrier transistors T1 and T2 is only Vcc potential.

(3) When writing data to a memory cell, sense amplifier 2 is operated and the high/low relationship of the potentials of bit lines BL and BL is determined according to the write input data from the terminals 10 and 11. In this case, even if the selected memory data is reversed from "1" to "0" or from "0" to "1", when the potential relationship of bit lines BL and BL is reversed by sense amplifier 2, this bit line potential passes through barrier transistors T1 and T2 and reverses restore circuit 1. When it is possible to step up the bit line potential to Vcc potential by restore circuit 1.

Since, in the above memory, the gate potential of barrier transistors T1 and T2 can be set at Vcc potential, a barrier transistor control signal step-up circuit is no longer required. Therefore circuit design and pattern design are much simpler, circuit construction is easier and chip surface area can be decreased.

Moreover, by connecting restore circuit 1 directly to the bit line pair, since the load capacity of sense amplifier 2 is reduced at least by the capacity of restore circuit 1 compared with conventional examples, the sensing time of sense amplifier 2 becomes shorter (sensing becomes faster) because it is almost proportional to the load capacity.

Also, the resistance from restore circuit 1 to the far end of bit pair BL and BL in conventional circuits is large, since the resistance of the bit line itself and the resistance component of barrier transistors T1 and T2 are connected in series. However, in this embodiment this resistance component is only the resistance of the bit line. Therefore, because the restore time of the bit line is almost proportional to the above resistance, the restore time decreases in this embodiment in comparison to in conventional examples. As a result, it becomes possible to speed up the operation, particularly when data writing, and a speed-up of memory operation can be achieved.

This invention is not confined to the above embodiment. The same effect as in the embodiment can also be obtained even when the sense amplifier is constructed using P channel MOS transistors. In this case P channel MOS transistors are used for the barrier transistors and Vss potential (earth potential) is applied to their gates. Also, selection of an appropriate active level for the restore control signal makes restoring possible irrespective of whether P channel transistors or N channel transistors are used. This applies to all memories, whether they be NMOS memories, PMOS memories or CMOS memories.

As described above, when a semiconductor memory of this invention is used, a step-up circuit for the barrier transistor control signal is no longer required, because the restore circuit is connected directly to the bit line by providing barrier transistors between the sense amplifier and the restore circuit and applying a constant potential (Vcc potential or Vss potential) on their gates. Therefore, simplification of design and circuit construction can be achieved and the chip surface area can be made smaller. Furthermore, since sensing and restoring can be speeded up, this invention is suitable for high integration and large storage memories.

Various modifications and variations could be made to the invention without departing from the scope or spirit of the invention.