Semiconductor memory device having redundant memory cells

The described embodiments of the present invention provide a method in which the circuit configuration of redundancy circuitry in a random access memory can be simplified and the setting operation of the address of the defective memory cell is also simplified. In one described embodiment, the redundant circuit includes a fuse decoder (11), which functions as the address-generating circuit for the address of the defective memory cell, and a latch circuit (21). A write operation to the defective memory cell on the write port containing the fuse decoder (11) causes the address of the defective cell to be stored in the latch circuit. Each input/output port, except the input port using the fuse decoder, includes a comparator (22) for comparing the address for an operation on the respective port to the address stored in the latch circuit. A timing logic circuit (23) responds to a coincident signal generated by the comparator by providing signals which enable access to the redundant memory cell rather that the defective memory cell.

This invention concerns a type of semiconductor memory device. More 
specifically, this invention concerns a type of selecting circuit of the 
redundant memory cell in the multi-port memory device having a redundant 
memory cell. 
PRIOR ART 
In order to increase the yield of the semiconductor memory device, a 
preparatory memory cell (redundant memory cell or preparatory memory cell) 
is formed in the semiconductor memory device; if the intrinsic memory cell 
becomes defective, the aforementioned redundant memory cell can be used as 
the save memory cell for the defective memory cell. This scheme is now 
widely applied. In this case, the semiconductor memory device should 
contain a circuit for selecting the redundant memory cell in addition to 
the redundant memory cell itself. 
The aforementioned redundant circuit is formed row and column respectively. 
In the case when the defective bit address is decoded by a redundant 
decoder, the address of the intrinsic memory cell and that of the 
redundant memory cell are exchanged with each other. 
FIG. 6 shows the configuration of the redundant circuit. FIG. 6 (A) shows 
the configuration of the write port, while FIG. 6 (B) shows the 
configuration of the read port. 
As far as write port (1A) is concerned, in fuse decoder (11A), there are a 
plurality of fuses (not shown in the figure) formed by polysilicon. These 
fuses can be fused by a laser beam, etc., corresponding to the bit address 
(column) to be saved. 
With the write operation, write column address signal WY from write column 
counter (10) is input to fuse decoder (11A); if the value of the write 
column address signal is coincident with the address defined by the laser 
fusion operation, timing logic circuit (11B) is activated, MOS transistor 
(11C) which functions as a gate circuit is energized (turned on), and 
redundant memory cell (20), connected on its tip, is activated. 
For the fuses (not shown in the figure) in fuse decoder (21A) in read port 
(2A), the fusion operation can also be carried out by laser in the same 
way as the fuses in fuse decoder (11A) in said write port (1A). 
Consequently, in read port (2A), too, when read column signal RY is 
generated from read column counter (30), fuse decoder (21A), timing logic 
circuit (21B), and MOS transistor (21C) perform the same operation as 
above, and the information stored in said redundant memory cell (20) can 
be read out. 
As shown in FIG. 6, a plurality of fuses is formed in fuse decoder (11A) in 
write port (1A), and in fuse decoder (21A) in read port (2A), 
respectively. A laser fusion operation is carried out to ensure that these 
fuses are fused so that they correspond to the same address; hence, it is 
necessary to form fuses for two sets or multiple sets with a number equal 
to the number of ports. But the space for forming the fuses is large. This 
is a problem. 
In particular, as the memory capacity of the overall semiconductor memory 
device is increased, the amount of fuses also increases, the area of the 
fuse formation region increases, and the In addition, for the field memory 
which has many input/output ports, as a set of the aforementioned fuse 
decoders is to be set up for each port, there is a large number of fuses; 
thus the aforementioned problem is exacerbated. 
On the other hand, for the address of the same save memory cell, it is 
necessary to carry out the laser fusion treatment for each port, and a lot 
of time is needed to carry out the address-setting operation for the 
various fuse decoders. This is also a disadvantage. 
OBJECT OF THE INVENTION 
An object of the present invention is to increase the integration level of 
the multi-port semiconductor memory device having a redundant circuit, to 
prevent errors in setting the address, and to improve the processing 
capability of the operation. 
SUMMARY OF THE INVENTION 
In order to solve the aforementioned problems, this invention provides a 
type of semiconductor memory device characterized by the following facts: 
the semiconductor memory device has a redundant memory cell as well as an 
input port and output port; when the intrinsic memory cell is defective, 
the aforementioned redundant memory cell is used as the memory cell for 
saving the defective memory cell. In this semiconductor memory device, 
there is an address-generating circuit which generates the address of the 
defective memory cell, and an address storage circuit that stores the 
address from the aforementioned address-generating circuit. For each of 
the aforementioned ports, there is an address comparison circuit which 
compares the address stored in the aforementioned address storage circuit 
and the access address, and which sends out a coincident signal when the 
aforementioned addresses are coincident; and a gate circuit which is 
connected to the aforementioned redundant memory cell, which is energized 
in response to the aforementioned coincident signal, and which energizes 
the aforementioned redundant memory cell. 
The aforementioned address-generating circuit has a plurality of fuses. The 
fuses are fused beforehand according to the prescribed bit address. It is 
also possible to arrange a latch circuit in the aforementioned 
address-generating circuit and to set the aforementioned address in the 
latch circuit. 
The address-generating circuit generates the address of the defective 
memory cell. The method for generating this address may be the same as 
that used in the conventional method, such as fusion of fuses or 
presetting the aforementioned address in the latch circuit and then 
generating the aforementioned address using the latch circuit. 
The address storage circuit stores the address from the address-generating 
circuit. The initial timing operation when the power source is turned on 
for the semiconductor memory device is taken as the storage timing. 
The address comparison circuit receives a storage address from the address 
storage circuit and the address for memory access as inputs, and compares 
them with each other. If they are found to coincide with each other, a 
coincident signal is output. As a response to the address coincident 
signal from the address comparison circuit, the gate circuit is energized. 
As the redundant circuit is connected to the gate circuit, as a response 
to the aforementioned energizing state, the redundant memory cell is 
energized. 
In the aforementioned access operation, there is an access for writing and 
one for reading. In the aforementioned energizing operation, there is also 
the write operation and the read operation. 
As can be seen from the aforementioned configuration, each port contains no 
conventional fuse decoder. Consequently, it is possible to prevent a 
decrease in the integration level due to the need of setting a large 
number of fuses. In addition, as a single common address-generating 
circuit is set for the defective memory cells, the address-setting 
operation becomes simpler, and there are less errors caused by the need 
for setting multiple fuse decoders. 
In the address-generating circuit for the defective memory cell, it is 
possible to use fuses in the same way as the conventional scheme. However, 
as there is no need to set the fuses for each port, the integration level 
can still be increased even when the fuses are used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The redundant circuit configuration in a first embodiment of the 
semiconductor memory device of this invention may be explained with 
reference to FIG. 1. FIG. 1 shows the circuit configuration of the portion 
related to the redundant circuit of this invention. The redundant circuit 
in this embodiment is the redundant circuit in a semiconductor memory 
device having one write port and a plurality of read ports (2-3). The 
intrinsic memory cells and the other circuit configuration needed for 
memory access are omitted here. 
This redundant circuit has write column counter (10) which generates write 
column address signal WY in the write operation, fuse decoder (11), timing 
logic circuit (12), gate circuit (13) consists of MOS transistors, and 
redundant memory cell (20). 
Fuse decoder (11), timing logic circuit (12) and gate circuit (13) are set 
in the write port. 
There is a plurality of gate circuits (4 in this embodiment) driven by 
timing logic circuit (12), in parallel to each other, and corresponding to 
the bit configuration of the semiconductor memory device. 
In read port (2), there is latch circuit (21) that functions as the address 
memory circuit, address comparison circuit (22), timing logic circuit 
(23), and gate circuit (24) consists of MOS transistors. Gate circuit (24) 
is connected to said redundant memory cell (20). In addition, read column 
counter (30) that generates read column address signal RY in the read 
operation is connected to address comparison circuit (22). The circuit 
configuration of read port (3), with the internal circuit details not 
shown is almost identical to the circuit configuration of read port (2); 
they share latch circuit (21). 
In fuse decoder (11), there is a plurality of fuses with a number equal to 
the settable number of addresses in the memory cell. These fuses are made 
of polysilicon formed in the semiconductor device. These fuses are fused 
beforehand by laser so as to set the address of the defective memory cell. 
For example, it is possible to use the laser to fuse the corresponding 
fuses beforehand. 
In the following, the write operation of the redundant circuit shown in 
FIG. 1 will be explained. 
In the write operation, write column address signal WY is generated from 
write column counter (10). This write column address signal WY is applied 
to fuse decoder (11). In fuse decoder (11), if the aforementioned write 
column address signal is coincident with the write column address defined 
by the fuses fused by laser beam, an address coincident signal is output. 
Corresponding to this address coincident signal, timing logic circuit (12) 
turns on corresponding gate circuit (13). 
Redundant memory cell (20) is connected to gate circuit (13). Instead of 
the defective memory cell as the memory cell corresponding to the 
aforementioned write column address, data is written into redundant memory 
cell (20). 
In the write operation into said redundant memory cell (20), repair address 
latch signal WRDE is output from timing circuit (12). 
This repair address latch signal WRDE is applied on clock terminal CK of 
latch circuit (21) in read port (2), and the write column address signal 
WY from write column counter (10) is latched in latch circuit (21). 
In the following, the read operation will be explained. 
In the read operation, read column address signal RY is generated from read 
column counter (30). This read column address signal RY is applied to 
address comparison circuit (22) and compared with the address stored in 
latch circuit (21). When these addresses are coincident, address 
comparison circuit (22) outputs an address coincident signal to timing 
logic circuit (23). In the same way as in said timing logic circuit (12), 
timing logic circuit (23) turns on corresponding gate circuit (24), 
connected to redundant memory cell (20). In this way, the read operation 
is carried out from redundant memory cell (20). 
For redundant memory cell (20), after the write operation is carried out, 
the read operation of the content of the memory cell is carried out. In 
the initial write operation after start-up of the semiconductor memory 
device, the address of the defective memory cell is automatically stored 
in latch circuit (21) in read port (2), and the address set in fuse 
decoder (11) is used to perform the read operation for said redundant 
memory cell (20). 
In this embodiment, it is only necessary to set fuses in fuse decoder (11) 
in write port (1); there is no need to set them in read ports (2) and (3). 
In the case when the number of write ports is increased, fuse decoder (11) 
is only set in a certain write port, such as the first write port. For the 
other write ports, it is only necessary to set the circuits corresponding 
to address comparison circuit (22), timing logic circuit (23), and gate 
circuit (24) in read port (2); for the defective memory cell's address, 
the output address of circuit (21) is input. The write column address 
signal WY from the write column counter in the write port is applied to 
the address comparison circuit corresponding to address comparison circuit 
(22) in the write port; the aforementioned address comparison is carried 
out for the write column address signal. 
FIG. 2 shows the detailed circuit configuration of write port (1) and read 
port (2) of the redundant circuit shown in FIG. 1. Write column counter 
(10), gate circuit (13), redundant memory cell (20), and read column 
counter (30) shown in FIG. 1 are not shown in FIG. 2. 
From write column counter (10), write column address signal WY [is sent], 
in this circuit, 16-bit write column address signals WY0-WY7, *WY0-*WY7 
(where * indicates signal inversion; the same applies to the following) 
are input to fuse decoder (11), which has 16 first-group fuses FA (1)-FA 
(16). These fuses FA (1)-FA (16) are formed by the polysilicon layer of 
the semiconductor device. In addition, these fuses FA (1)-FA (16) are 
fused by laser beam beforehand, in correspondence to the address of the 
defective memory cell. 
In addition, fuse decoder (11) also has AND-gates (111)-(114), and MOS 
transistors (115)-(118) which are connected to the outputs of said 
AND-gates (111)-(114). For each of AND-gates (111)-(114) and transistors 
(115)-(118), 16 fuses FA (1)-FA (16), that is, the number corresponding to 
the address of the defective memory cell, are set in parallel. 
In order to boost the voltage of node NA of the power source voltage line 
of transistors (115)-(118), charge-up circuit (119) is set. 
Timing logic circuit (12) has delay-type flip-flop DFF (121), AND-gate 
(122), second-group fuses FB (1)-FB (4), 4 first-group transistors 
(131)-(134), 4 second-group transistors (135)-(138), and transistor gates 
(13A)-(13D) consist of serial invertor circuits (139)-(142). The tips of 
these transistor gates (13A)-(13D) are connected to the gate circuits, 
respectively. 
One of the second-group fuses, FB (1), connected to the redundant memory 
cell used, is fused by laser beam beforehand. 
Latch circuit (21) in read port (2) has 8 [DFFs], DFF (211)-DFF (212), 
which store write column address signals WY0-WY7, delay circuit (213), and 
4 [DFFs], DFF (214)-DFF (217), that store output signals WRD0-WRD3 from 
timing logic circuit (12). In addition, DFF (214)-DFF (217) are circuits 
used for storing the fusion state of second-group fuses FB (1)-FB (4) in 
timing logic circuit (12). 
Address comparison circuit (22) has 8 Exclusive-OR (EOR) [sic; XOR] gates 
(221)-(222), and NOR-gate (223). 
Timing logic circuit (23) consists of DFF (231), AND-gate (232), and 4 
parallel AND-gates (243)-(246). The tips of these AND-gates are connected 
to gate circuits. Applied on one input terminal of these AND-gates is the 
Q-output of DFF (214)-DFF (217) which store signals WRD0-WRD3 that show 
the fusion state of the fuses of timing logic circuit (12); hence, gate 
circuit (24) is connected to the same redundant memory cell as the 
redundant memory cell connected to gate circuit (13). 
The write operation of the redundant circuit in FIG. 2 can be explained 
with reference to the write timing chart in FIG. 3. 
In this embodiment, the memory cell of bit address N is replaced by the 
redundant memory cell. For this purpose, laser fusion operation is carried 
out for first-group fuses FA (1)-FA (16) corresponding to address N of 
fuse decoder (11). 
Corresponding to write clock WCK, the write column address as shown by 
write column address signals WY0-WY7 from write column counter (10) is 
renewed. 
Inverted and delayed write clock *WCKD, which is delayed by a predetermined 
time from write clock WCK and then inverted, is applied to AND-gates 
(111)-(114) in fuse decoder (11). In the case when the outputs of 
AND-gates (111)-(114) indicate address N, the potential at node NA is not 
on the "low" level as indicated by the broken-line data, but is on the 
"high" level. This indicates that the write column address from write 
column counter (10) and the setting address that indicates the fusion 
state of the fuses are coincident. Hence, fuse decoder (11) also functions 
as an address coincident circuit. 
The aforementioned level signal of node NA is applied on DFF (121) via 
charge-up circuit (119), and is latched corresponding to write clock WCK. 
The Q-output of DFF (121) is applied on AND-gate (122); at the next timing 
of write clock WCK, it is output as repair address latch signal WRDE from 
AND-gate (122). 
In this embodiment, second-group fuse FB (1) in timing logic circuit (12) 
is fused by laser. Consequently, only write data WRD0 is on the "high" 
level, and data write is carried out into the corresponding memory cell in 
the redundant memory cell by write data WRD0. 
With reference to the read timing chart shown in FIG. 5, the read operation 
of the redundant circuit in FIG. 2 can be explained. 
Before the read operation is carried out, in the initial state of the 
semiconductor memory device, the "low" level read clear signal REDCR is 
output from a memory control circuit not shown in the figure, and DFFs 
(211)-(212) which store the values of write column address signals WY0-WY7 
in write circuit (21) are reset. 
In the initial operation of the semiconductor memory device, before the 
read operation, the aforementioned write operation is carried out. In the 
aforementioned write operation, when repair address latch signal WRDE is 
generated, the values of write column address signals WY0-WY7 are stored 
in DFF (211)-(212). In addition, repair address latch signal WRDE is 
delayed by delay circuit (213); corresponding to the delayed repair 
address latch signal C, write data WRD0 from said timing logic circuit 
(12) is set in DFF (214) in latch (21). 
After the aforementioned initial setting operation, the usual read 
operation is carried out. 
In the read operation, read column address signals RY10-RY17 are output 
from read column counter (30). EOR-gates (221)-(222) in address comparison 
circuit (22) compare addresses B0-B7 stored as address N in DFF 
(211)-(212) in latch circuit (21) and read column address signals 
RY10-RY17 from read column counter (30). When the values of read column 
address signals RY10-RY17 are coincident with address N, address 
comparison circuit (22) outputs an address coincident signal via NOR-gate 
(223). 
By means of read clock R1CK, this address coincident signal is latched in 
DFF (231); at the next timing of read clock R1CK, it is output from 
AND-gate (231) as a timing signal with a "high" level. 
As explained above, only the Q-output of DFF (214) is on the "high" level; 
hence, the output of AND-gate (243) becomes "high" level, and the read 
operation is carried out for the data from the redundant memory cell, 
which is the same as the redundant memory cell assigned by said timing 
logic circuit (12). 
For read port (3), the same operation as described above is also carried 
out. However, there are no parts in read port (3) that correspond to DFF 
(211)-(212), and DFF (214)-(217), and those in read port (2) are used in a 
shared form. 
As explained above, common first-group fuses FA (1)-FA (16) and 
second-group fuses FB (1)-FB (4) are set in write port (1). As explained 
above, the fuses are fused by laser beam. In this way, in the read 
operation for read ports (2) and (3), the read operation can be performed 
from the redundant memory cell, which is identical to the write redundant 
memory cell. 
The first-group fuses FA (1)-FA (16) and/or the second-group fuses FB 
(1)-FB (4) shown in FIG. 2 may be replaced by other address-generating 
means than fuses. For example, instead of fuses, it is possible to set up 
flip-flops and to set the aforementioned address into the flip-flops from 
the memory control circuit in the initial operation of the semiconductor 
memory device. In this case, there is no need to perform laser fusion, and 
it is possible to change the address of the defective memory cell to be 
substituted by the redundant memory cell even after the address of the 
defective memory cell is already set. That is, the freedom in the 
utilization of the redundant memory cell can be enlarged. 
FIG. 5 shows the circuit configuration of a second embodiment of the 
redundant circuit of this invention. 
According to this circuit configuration, in the case when there are a 
plurality of input/output ports as in the case of field memory, an 
address-generating circuit (40) which generates the address of the 
defective memory cell for the write ports and the read ports is arranged 
commonly. In this specific example, read ports (5), (7), and write ports 
(6), (8) are shown. 
The address-setting operation for address-generating circuit (40) may be 
carried out using the fuses as described above or using latch circuits. 
In the initial operation of the semiconductor memory device, an active 
signal ACT is output from a memory control circuit (not shown in the 
figure) to address-generating circuit (40). Corresponding to this active 
signal ACT, the address stored in address-generating circuit (40) is 
output. According to the memory control signal MEMCNT which is the delay 
signal of the active signal ACT, the address which are output from 
address-generating circuit (40) are stored in latch circuit (51). 
The circuit configurations in read ports (5), (7) and write ports (6), (8) 
are virtually identical to the circuit configuration in read port (3) as 
shown in FIG. 2; the column address is applied from the various column 
counters. 
In addition to the aforementioned schemes, this invention may also be 
implemented using various other modifications. For example, the various 
parts shown in FIG. 2, such as fuse decoder (11), gate circuit (13), and 
other circuit configurations may be replaced by other circuit 
configurations that can perform the same functions as the above. 
The above description was made with respect to the case of the saving of 
one defective memory cell. However, it is also possible to save a 
plurality of defective memory cells, just by setting a plurality of the 
redundant circuits the same as the aforementioned type. 
As explained above, according to this invention, independent of the number 
of input/output ports, it is acceptable to set a single address-generating 
circuit for the defective memory cell in the semiconductor memory device. 
Consequently, the circuit configuration of the redundant circuit is 
simplified, and the integration level of the semiconductor memory device 
can be increased. 
In addition, as only one address-generating circuit is set, the address 
setting operation of the defective memory cell can be carried out in a 
simple way, the operation efficiency can thus be increased. There is also 
no problem in error setting of the addresses when a plurality of addresses 
are set.