Semiconductor Memory Device Having Non-Volatile Memory Cells Connected in Series

1. Field of the Invention

The present invention relates to a semiconductor memory device having non-volatile memory cells capable of being electrically programmed and erased. More particularly, this invention is concerned with a NAND-type flash memory having a configuration such that a plurality of non-volatile memory cells are connected in series in units of an information storage region having a given size.

2. Description of the Related Art

In recent years, numerous types of flash memories in which information can be electrically programmed and the whole or part of the information can be electrically erased have been developed as mainstay non-volatile semiconductor memory devices. Above all, a NAND-type flash memory is suitable for a large-capacity file system because <1> although the speed of random access is low, the speed of sequential access is high, <2> writing can be carried out in units of a page, that is, a word line (normally, 264 bytes calculated by adding 8 redundant bytes to 256 bytes or 528 bytes calculated by adding 16 redundant bytes to 512 bytes), <3> that erasure can be carried out in units of a smaller block (normally, 16 pages) compared with a general flash memory referred to as a NOR-type flash memory, <4> that the time required for rewriting data is short, and <5> that since the area of a memory cell is small, a large storage capacity can be attained readily. Markets for this kind of NAND-type flash memory are predicted to expand in the future. The NAND-type flash memory is now attracting attention.

FIG. 1 shows part of the configuration of a memory cell array in a NAND-type flash memory that is an example of a prior art. In the drawing, there are shown word lines WL.sub.1 to WL.sub.16 arranged in units of one block (16 pages), and bit lines BL.sub.1, BL.sub.2, etc. Sixteen non-volatile memory cells (memory cell transistors) (Q1M.sub.1 to Q1M.sub.16 or Q2M.sub.1 to Q2M.sub.16) for storing information are connected in series on each bit line (BL.sub.1 or BL.sub.2) in units of one block, thus constituting one string of memory cells. One terminal of each string of memory cells Q1M.sub.1 to Q1M.sub.16 or Q2M.sub.1 to Q2M.sub.16 is connected with the bit line BL.sub.1, or BL.sub.2 via a selection transistor Q1A.sub.1 or Q2A.sub.1 that responds to the potential on a selection signal line SL.sub.1. The other terminal of each string of memory cells is grounded via a selection transistor Q1A.sub.2 or Q2A.sub.2 that responds to the potential on a selection signal line SL.sub.2.

For storing information in a memory cell, the information is stored as data "1" or data "0." Specifically, data "1" is stored by positively charging the floating gate of the memory cell (that is, by setting the threshold voltage of the memory cell transistor into a negative voltage). Data "0" is stored by negatively charging the floating gate of the memory cell (that is, by setting the threshold voltage of the memory cell transistor to a positive voltage).

For reading information from a memory cell, first, the potentials on the selection signal lines SL.sub.1 and SL.sub.2 are driven high, and the selection transistors Q1A.sub.1, Q2A.sub.1, Q1A.sub.2, and Q2A.sub.2 are all turned on. Consequently, the strings of memory cell Q1M.sub.1 to Q1M.sub.16 and Q2M.sub.1 to Q2M.sub.16 are connected on the bit lines BL.sub.1 and BL.sub.2 and to the ground. Thereafter, 0 V is applied to the control gate of a memory cell from which information should be read (that is, a selected word line), and a high-level voltage is applied to the control gates of the other memory cells (that is, non-selected word lines).

Assuming that information stored in a selected memory cell (=memory cell transistor) is data "1," even when the voltage of the control gate is 0 V, the memory cell transistor is turned on. A cell current flows into an associated bit line. By contrast, assuming that information stored in a selected memory cell is data "0," since the threshold voltage of the memory cell transistor is positive, when the voltage of the control gate is 0 V, the memory cell transistor is turned off. No cell current flows into the bit line. Meanwhile, a high-level voltage is applied to all the control gates of non-selected memory cells. Irrespective of stored information, all the non-selected memory cell transistors are on. A cell current flows into the bit line.

In other words, when information is read, a sense amplifier is used to detect whether or not a selected memory cell causes a cell current to flow into an associated bit line. Thereby, it can be determined whether information stored in the selected memory cell is data "1" or data "0."

As mentioned above, in the NAND-type flash memory, writing or reading is carried out in units of a page (=word line), and erasure is carried out in units of a block (=a plurality of pages). Since a plurality of memory cells are connected in series on each bit line in units of a block, if even one bit location (=one memory cell) within one block or one page is faulty, the operations of memory cells other than the faulty memory cell are affected adversely.

For example, in the configuration shown in FIG. 1, assume that one (Q1M.sub.2) of the 16 memory cells (=memory cell transistors) Q1M.sub.1 to Q1M.sub.16 connected in series on the bit line BL.sub.1 is brought to a non-functional state. In this case, when information is read from the nonfunctional memory cell Q1M.sub.2, since the memory cell transistor is off, no cell current flows into the bit line BL.sub.1. Irrespective of original stored information, data "0" is identified.

When an attempt is made to read information from any of the other memory cells Q1M.sub.1 and Q1M.sub.3 to Q1M.sub.16 contained in the same string of memory cells as the broken memory cell Q1M.sub.2, since the broken memory cell Q1M.sub.2 is off, a current flow is cut off. Even in this case, irrespective of original stored information, data "0" is identified.

A block including a faulty memory cell is a block judged as an unavailable block at the time of delivery of a product, and is, for example, referred to as an "invalid block." After a fault is discovered, access to such an invalid block is inhibited. This is because, as mentioned above, since a NAND-type flash memory is programmed or read in units of a page and erased in units of a block, access to an invalid block involves a string of memory cells in which a fault occurs.

As mentioned above, the NAND-type flash memory has a problem that since the NAND-type flash memory includes strings of memory cell each having a plurality of memory cells connected in series, when one bit location (=one memory cell) in a string of memory cells fails, even if the other memory cells contained in the string of memory cells functions normally, the whole block containing the defective memory cell becomes unavailable. In other words, a majority of normal memory cells other than a minority of defective memory cells (which may be one defective memory cell) among all the memory cells arranged in a limited information storage region are wasted. From the viewpoint of effective utilization of memory cells, there is room for improvement.

An object of the present invention is to provide a non-volatile semiconductor memory device in which, even if an invalid block occurs, the whole of the block need not be made unavailable but memory cells can be utilized effectively as a whole.

According to the present invention, there is provided a non-volatile semiconductor memory device including: a string of memory cells provided for units of a predetermined information storage region and having a plurality of rewritable non-volatile memory cells connected in series, said string of memory cells being divided into a plurality of groups of memory cells; a plurality of switching elements each connected in parallel with a corresponding one of said divided groups of memory cells; and control means for controlling on/off operations of said plurality of switching elements.

According to the configuration of a non-volatile semiconductor memory device of the present invention, if any memory cell contained in a string of memory cells connected in series fails, only a switching element associated with a group of memory cells containing the memory cell is turned on by a control means. Thus, the group of memory cells is bypassed, and the use of the string of memory cells can be continued.

In other words, the bypassed group of memory cells within a string of memory cells is made unavailable, but the remaining group of memory cells is available. Compared with the prior art in which a string of memory cells in question (a block containing the string of memory cells) is made entirely unavailable, memory cells can be utilized effectively as a whole.

Other objects and features of the present invention will be described hereinafter in detail by way of preferred embodiments with reference to the accompanying drawings, in which:

FIG. 1 is a circuit diagram showing part of the configuration of a memory cell array in an NAND-type flash memory which is an example of a prior art;

FIG. 2 is a block diagram schematically showing the overall configuration of an NAND-type flash memory in accordance with an embodiment of the present invention;

FIG. 3 is a circuit diagram showing part of the configuration of a memory cell array shown in FIG. 2;

FIG. 4 is a circuit diagram showing an example of the circuitry of a switching circuit shown in FIG. 2; and

FIG. 5 is a circuit diagram showing part of the configuration of a memory cell array in another embodiment of the present invention.

FIG. 2 schematically shows the overall configuration of a NAND-type flash memory in accordance with an embodiment of the present invention.

In the drawing, there are shown input/output terminals I/O1 to I/O8 for fetching an operation command or an address signal and for writing data into the inside of the device or outputting data read from a memory cell inside the device to outside. Also shown are control signals WPX, WEX, CEX, ALE, CLE, and REX for controlling the operations of the device. Specifically, the control signals WPX, WEX, CEX, ALE, CLE, and REX are a write-protect signal, write-enable signal, chip-enable signal, address latch-enable signal, command latch-enable signal, and read-enable signal respectively. There are also shown a high-potential supply voltage Vcc (normally, 5 V or 3.3 V) and a low-potential supply voltage Vss (normally 0 V). These voltages are supplied to circuits inside the device.

Also shown is a memory cell array 1 having rewritable non-volatile memory cells (memory cell transistors) arranged along a plurality of word lines WL and a plurality of bit lines BL which are laid out in the form of a matrix. In this embodiment, 16 memory cells are connected in series on each bit line in units of one block (16 pages), thus constituting a string of memory cells. In the memory cell array 1, a plurality of selection signal lines SL are arranged in a direction in which they become parallel to the word lines.

Also shown are an operation logic control circuit 2 for controlling the operations of the circuits inside the device in response to the control signals (WPX, WEX, CEX, ALE, CLE, and REX), an input/output control circuit 3 for controlling input or output of various signals or data through the input/output terminals I/O1 to I/O8, a command register 4 for temporarily storing an operation command input via the input/output control circuit 3, an address register 5 for temporarily storing an address signal input via the input/output control circuit 3, and a data register 6 for temporarily storing external writing data input via the input/output control circuit 3, and data read from a memory cell.

Also shown are a row address buffer 7 for buffering a row address contained in an address signal input via the address register 5, a column address buffer 8 for buffering a column address contained in the address signal input via the address register 5, a row decoder 9 for decoding the row address sent from the row address buffer 7 and selecting any of a plurality of word lines WL, a column decoder 10 for decoding the column address sent from the column address buffer 8 and selecting any of a plurality of column lines CL, a column gate 11 that when any column line is selected, connects a bit line corresponding to the column line with an associated data line (not shown), and sense amplifier (S/A) circuits 12 each sensing and amplifying data read onto each data line.

Also shown is a high-voltage generating circuit 13 for generating a high voltage employed inside the device (a high voltage used to write data, a high voltage used to erase data, or a high voltage used for verification to check if writing or erasure has been performed fully on a memory cell during data writing or erasure). A generated high voltage is supplied to a selected word line via the row decoder 9, to the S/A circuit 12 associated with a selected bit line, and to the memory cells within the memory cell array 1.

The input/output control circuit 3, data register 6, row decoder 9, column decoder 10, S/A circuits 12, and high-voltage generating circuit 13 function under the control of the operation logic control circuit 2.

The write-protect signal WPX is used to forcibly disable writing or erasure. Specifically, although the signal WPX is driven high for use, when an input signal is uncertain because, for example, a power supply is broken, the signal WPX is driven low in order to reset the operation of the high-voltage generating circuit 13. Stored data is thus protected from an unexpected operation. The write-enable signal WEX is used to fetch various signals or data into the inside of the device through the input/output terminals I/O1 to I/O8. The chip-enable signal CEX is used to specify a mode of the device. Specifically, when the signal CEX is driven high during reading, the device is brought to a standby mode in which a power consumption is low. When writing or erasure is in progress, either the high-level or low-level state is permitted. The address latch-enable signal ALE is used to control fetching of an address signal into the address register 5 and fetching of writing data into the data register 6. Specifically, when the signal ALE is driven high at the leading or trailing edge of the write-enable signal WEX, data is fetched as an address signal into the address register 5 through the input/output terminals I/O1 to I/O8. The command latch-enable signal CLE is used to control fetching of an operation command into the command register 4. That is to say, when the signal CLE is driven high at the leading ortrailing edge of the write-enable signal WEX, data is fetched as an operation command into the command register 4 through the input/output terminals I/O1 to I/O8. The read-enable signal REX is used to output data read from memory cells serially to outside. Specifically, when the signal REX is driven low, after a given period of time elapses, data to be output through the input/output terminals I/O1 to I/O8 is finalized. When the signal REX is driven high, an internal column address counter (not shown) is incremented by one.

The foregoing components and operations are substantially identical to those in a normal NAND-type flash memory.

The NAND-type flash memory of this embodiment includes: in addition to the foregoing components, an invalidated address memorizing circuit 20 in which if any memory cell of each string of memory cells having 16 memory cells connected in series fails, the address of the memory cell is stored using, for example, a fuse; a comparing circuit 21 for comparing a row address input via the row address buffer 7 (that is, a row address designated externally) with the address stored in the invalidated address memorizing circuit 20; a decoder 22 for selecting a selection signal line within the memory cell array 1 on which a selection transistor (not shown) that should be driven is connected; and a switching circuit 23 for connecting a selection signal line selected by the decoder 22 according to the result of comparison performed by the comparing circuit 21.

The comparing circuit 21 compares an externally-designated address with an address stored in the invalidated address memorizing circuit 20 (that is, the address of any memory cell of a string of memory cells in which a fault occurs). If both the addresses disagree with each other, a low-level signal is output. If both the addresses agree with each other, a high-level signal is output.

When an invalidated address is stored in the invalidated address memorizing circuit 20, whether or not a memory cell is faulty is detected during, for example, a verification check accompanying data erasure or a parity check accompanying data reading.

The FIG. 3 shows part of the configuration of the memory cell array 1.

The fundamental components are identical to those in the one of the prior art shown in FIG. 1. That is to say, 16 non-volatile memory cells Q1M.sub.1 to Q1M.sub.16 or Q2M.sub.1 to Q2M.sub.16 are connected in series on each bit line BL.sub.1 or BL.sub.2 in units of one block (16 pages=16 word lines WL.sub.1 to WL.sub.16), thus constituting one string of memory cells. One terminal of each of the strings of memory cells Q1M.sub.1 to Q1M.sub.16 and Q2M.sub.1 to Q2M.sub.16 is connected with a bit line BL.sub.1 or BL.sub.2, respectively, via selection transistors Q1A.sub.1 and Q2A.sub.1, respectively, which respond to the potential on a selection signal line SL11. The other terminals thereof are grounded via selection transistors Q1A.sub.2 and Q2A.sub.2 respectively which respond to the potential on a selection signal line SL.sub.21.

In this embodiment, in addition to the foregoing components, selection transistors Q1B.sub.1 and Q2B.sub.1, that respond to the potential on a selection signal line SL.sub.12, are connected between dividing points P.sub.11 and P.sub.21 at which the strings of memory cells are each divided into two groups of memory cells, and the bit lines BL.sub.1 and BL.sub.2, and selection transistors Q1B.sub.2 and Q2B.sub.2 that respond to the potential on the selection signal line SL.sub.22 are connected between the dividing points P.sub.11 and P.sub.21 and the ground. In short, the selection transistors Q1B.sub.1, Q1B.sub.2, Q2B.sub.1, and Q2B.sub.2 are connected in parallel with associated groups of memory cells in order to bypass the groups of divided memory cells.

In the foregoing configuration, when the selection transistor connected in series with each group of divided memory cells or the selection transistor connected in parallel therewith are driven over the respective selection signal lines so that when one of the selection transistors is on, the other transistor is off. Taking for instance a group of m memory cells Q1M.sub.1 to Q1M.sub.m connected on the bit line BL.sub.1, the selection transistors Q1A.sub.1 and Q1B.sub.1 are controlled according to the voltage levels of the associated selection signal lines SL.sub.11 and SL.sub.12 so that when the selection transistor Q1A.sub.1 is on (or off), the selection transistor Q1B.sub.1 is off (or on).

The aforesaid switching circuit 23 is included for controlling the voltage levels of the selection signal lines. FIG. 4 shows an example of the circuitry of the switching circuit 23.

As shown in FIG. 4, the switching circuit 23 includes a p-channel transistor QP that is connected between an output terminal of the selection transistor decoder and the selection signal line SL.sub.11 and is turned on or off in response to an output of the comparing circuit, and an n-channel transistor QN that is connected between the output terminal of the selection transistor decoder and the selection signal line SL.sub.12 and turned on or off in response to an output of the comparing circuit.

In the foregoing circuitry, when an output of the comparing circuit is low (that is, an externally-designated address disagrees with an address stored in the invalidated address memorizing circuit 20), the n-channel transistor QN is turned off and the p-channel transistor QP is turned on. The selection signal line SL.sub.12 is therefore disconnected from the selection transistor decoder, and an output of the selection transistor decoder is placed on the selection signal line SL.sub.11. Consequently, the selection transistors (Q1A.sub.1 and Q2A.sub.1 in FIG. 3) connected on the selection signal line SL.sub.11 are turned on, while the selection transistors (Q1B.sub.1 and Q2B.sub.1 in FIG. 3) connected on the selection signal line SL.sub.12 are turned off.

In contrast, when an output of the comparing circuit is high (that is, an externally-designated address agrees with an address stored in the invalidated address memorizing circuit 20), the p-channel transistor QP is turned off and the n-channel transistor QN is turned on. The selection signal line SL.sub.11 is therefore disconnected from the selection transistor decoder, and an output of the selection transistor decoder is placed on the selection signal line SL.sub.12. Consequently, the selection transistors (Q1B.sub.1 and Q2B.sub.1 in FIG. 3) connected on the selection signal line SL.sub.12 are turned on, while the selection transistors (Q1A.sub.1 and Q2A.sub.1 in FIG. 3) connected on the selection signal line SL.sub.11 are turned off.

In the example shown in FIG. 4, for brevity's sake, only the components relevant to the selection signal lines SL.sub.11 and SL.sub.12 are shown. The components relevant to the other selection signal lines SL.sub.21 and SL.sub.22 can be realized using the same circuitry.

Next, the operation and effect of the NAND-type flash memory in accordance with this embodiment will be described with reference to FIG. 3. For brevity's sake, only the circuitry relevant to the bit line BL.sub.1 will be discussed as an example.

In the circuitry, for example, assume that any memory cell of a group of memory cells (m memory cells Q1M.sub.1 to Q1M.sub.m) located between the dividing point P.sub.11 and the bit line BL.sub.1 fails.

The prior art (See-FIG. 1) has the drawback that if one bit location (one memory cell) among memory cells connected in series is faulty, the whole of the block containing the defective memory cell is unavailable.

In contrast, according to this embodiment, when the selection transistor Q1A.sub.1 is turned off and the selection transistor Q1B.sub.1 is turned on, the group of memory cells Q1M.sub.1 to Q1M.sub.m in which a fault occurs is disconnected from the bit line BL.sub.1. Besides, the bit line BL.sub.1 can be electrically connected with the dividing point P.sub.11 via the selection transistor Q1B.sub.1. In this case, the selection transistor Q1A.sub.2 remains on and the selection transistor Q1B.sub.2 remains off.

The bit line BL.sub.1 is thus connected with the dividing point P.sub.11 while bypassing the group of memory cells Q1M.sub.1 to Q1M.sub.m in which a fault occurs. The other group of memory cells Q1M.sub.m+1 to Q1M.sub.16 that functions normally can be kept available as it is. In other words, the group of memory cells Q1M.sub.1 to Q1M.sub.m, which are bypassed, out of the string of memory cells Q1M.sub.1 to Q1M.sub.16 is made unavailable, while the group of remaining memory cells Q1M.sub.m+1 to Q1M.sub.16 are available. The memory cells can therefore be utilized effectively as a whole.

Likewise, if a fault occurs in any memory cell of the group of memory cells Q1M.sub.m+1 to Q1M.sub.16 located between the dividing point P.sub.11 and ground, the selection transistor Q1A.sub.2 is turned off and the selection transistor Q1B.sub.2 is turned on. Thereby, the group of memory cells Q1M.sub.m+1 to Q1M.sub.16 in which a fault occurs is bypassed, and the other group of memory cells Q1M.sub.1 to Q1M.sub.m that functions normally is kept available as it is. In this case, the selection transistor Q1A.sub.1 remains on and the selection transistor Q1B.sub.1 remains off.

As mentioned above, according to the configuration of the NAND-type flash memory of this embodiment, even if an invalid block that is made unavailable because a fault is detected, connection switching of memory cells is controlled so that a group of memory cells containing a faulty memory cell is bypassed. Consequently, although bypassed pages are unavailable, a remaining majority of pages are available. Compared with the prior art in which the whole block becomes unavailable, the memory cells can be utilized effectively as a whole.

In the aforesaid embodiment, the description has been made by taking for instance the example of a configuration in which one string of memory cells is divided into two groups of memory cells. The number of groups of memory cells into which one string of memory cells is divided is not limited to two. For example, one string of memory cells may be divided into three groups of memory cells. FIG. 5 shows an example of a configuration in which one string of memory cells is divided into three groups of memory cells.

An embodiment shown in FIG. 5 is identical to the embodiment shown in FIG. 3 in terms of the fundamental configuration (arrangement of memory cells). In this embodiment, since one string of memory cells is divided into three groups of memory cells, there are differences described below from the embodiment shown in FIG. 3.

To begin with, there are two dividing points P.sub.11 and P.sub.12 (P.sub.21 and P.sub.22) for dividing each string of memory cells Q1M.sub.1 to Q1M.sub.16 (Q2M.sub.1 to Q2M.sub.16). A selection transistor Q1B.sub.1 (Q2B.sub.1) responding to the potential on a selection signal line SL.sub.12 is connected between a bit line BL.sub.1 (BL.sub.2) and one dividing point P.sub.11 (P.sub.21). A selection transistor Q1B.sub.2 (Q2B.sub.2) responding to the potential on a selection signal line SL.sub.22 is connected between one dividing point P.sub.11 (P.sub.21) and the other dividing point P.sub.12 (P.sub.22). A selection transistor Q1B.sub.3 (Q2B.sub.3) responding to the potential on a selection signal line SL.sub.32 is connected between the other dividing point P.sub.12 (P.sub.22) and ground. In short, the selection transistors Q1B.sub.1, Q1B.sub.2, Q1B.sub.3, Q2B.sub.1, Q2B.sub.2, and Q2B.sub.3 are connected in parallel with associated groups of memory cells in order to bypass the groups of divided memory cells.

Moreover, selection transistors Q1A.sub.1, Q1A.sub.2, Q1A.sub.3, Q2A.sub.1, Q2A.sub.2, Q2A.sub.3 are connected in series with the groups of memory cells. Herein, the selection transistor Q1A.sub.1 (Q2A.sub.1) responds to the potential on the selection signal line SL.sub.11, the selection transistor Q1A.sub.2 (Q2A.sub.2) responds to the potential on the selection signal line SL.sub.21, and the selection transistor Q1A.sub.3 (Q2A.sub.3) responds to the potential on the selection signal line SL.sub.31.

As in the embodiment shown in FIG. 3, the selection transistor connected in series with each group of memory cells and the selection transistor connected in parallel therewith are driven over the associated selection signal lines so that when one of the selection transistors is on, the other thereof is off. Taking for instance a group of m memory cells Q1M.sub.1 to Q1M.sub.m connected on the bit line BL.sub.1, the selection transistors Q1A.sub.1 and Q1B.sub.1 are controlled according to the voltage levels of the associated selection signal lines SL.sub.11 and SL.sub.12 so that when the selection transistor Q1A.sub.1 is on (or off), the selection transistor Q1B.sub.1 is off (or on). For controlling the voltage levels of the selection signal lines, a switching circuit, such as, for example, the one shown in FIG. 4, is employed.

The operation and effect exerted by the circuitry of this embodiment will be easily inferred from the description of the operation and effect exerted by the circuitry in the embodiment shown in FIG. 3. The description of the operation and effect will therefore be omitted.

However, in this embodiment, if a fault occurs in a group of memory cells located between the bit line BL.sub.1 (BL.sub.2) and dividing point P.sub.11 (P.sub.21), the selection transistor Q1A.sub.1 (Q2A.sub.1) associated with the group of memory cells is turned off, and the selection transistor Q1B.sub.1 (Q2B.sub.1) associated therewith is turned on. At this time, the selection transistors Q1A.sub.2 and Q1A.sub.3 (Q2A.sub.2 and Q2A.sub.3) associated with the other groups of memory cells remain on, and the selection transistors Q1B.sub.2 and Q1B.sub.3 (Q2B.sub.2 and Q2B.sub.3) associated therewith remain off. If a fault occurs in a group of memory cells located between two dividing points P.sub.11 (P.sub.21) and P.sub.12 (P.sub.22), the selection transistor Q1A.sub.2 (Q2A.sub.2) associated with the group of memory cells is turned off and the selection transistor Q1B.sub.2 (Q2B.sub.2) associated therewith is turned on. At this time, the selection transistors Q1A.sub.1 and Q1A.sub.3 (Q2A.sub.1 and Q2A.sub.3) associated with the other groups of memory cells remain on and the selection transistors Q1B.sub.1 and Q1B.sub.3 (Q2B.sub.1 and Q2B.sub.3) associated therewith remain off. If a fault occurs in a group of memory cells located between the dividing point P.sub.12 (P.sub.22) and ground, the selection transistor Q1A.sub.3 (Q2A.sub.3) associated with the group of memory cells is turned off and the selection transistor Q1B.sub.3 (Q2B.sub.3) associated therewith is turned on. At this time, the selection transistors Q1A.sub.1 and Q1A.sub.2 (Q2A.sub.1 and Q2A.sub.2) associated with the other groups of memory cells remain on and the selection transistors Q1B.sub.1 and Q1B.sub.2 (Q2B.sub.1 and Q2B.sub.2) associated therewith remain off.