Abstract:
A semiconductor memory device has plural memory cell blocks, each including memory cells storing data therein. A data bus and switching circuits transfer data from the memory cells to the data bus in response to a first logic level signal applied thereto. Column lines each have first and second ends. Each column line is connected to the corresponding switching circuit in each of the memory cell blocks. A column decoder, coupled to the first end of the column lines, provides the first logic level signal to one of the column lines upon the memory cell blocks being accessed. Potential setting circuits are coupled to the second end of the column lines, and preliminarily set the respective column lines to be in a predetermined potential so that each switching circuit is inactive prior to the column decoder providing the first logic level signal. All the memory cells in an array can be prevented from becoming inoperative even if a column line is broken.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the right of priority under 35 U.S.C. 119, of Japanese Patent Application Serial No. 03-341234, filed on Dec. 24, 1991. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory device capable of preventing a failure in memory cells, which is caused by the breaking of a column address selection signal line. 
     2. Description of the Related Art 
     A semiconductor memory device such as a Dynamic Random Access Memory (DRAM) has a memory cell array comprised of a plurality of sub arrays. Each of the sub arrays has a plurality of word lines, a plurality of bit line pairs disposed so as to intersect the corresponding word lines, and a plurality of column address selection signal lines (column lines) arranged in parallel to the bit line pairs respectively. Memory cells are respectively connected to points where the word lines intersect the bit line pairs, and arranged in matrix form. The memory cell array is electrically connected with a row address decoder array and a column address decoder array. 
     A DRAM, for example, is disclosed in the Japanese Laid-Open Patent Publication No. 60-136087, published on Jul. 19th, 1985. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a semiconductor memory device wherein when an arbitrary column line is broken, memory cells disposed in a memory cell array can be prevented from becoming inoperative. 
     In order to achieve the above object, a memory of a first invention of the present application comprises a plurality of memory cell blocks each including a plurality of memory cells each storing data therein, a data bus and a plurality of switching circuits respectively transferring data from the memory cells to the data bus in response to a first logic level signal applied thereto; a plurality of column lines each having first and second ends thereof, the column lines each being connected to the corresponding switching circuit in each of the memory cell blocks; a column decoder, coupled to the first end of the column lines, providing the first logic level signal to one of the column lines upon the memory cell blocks being accessed; and a plurality of potential setting circuits, coupled to the second end of the column lines, preliminarily settling the respective column lines to be in a predetermined potential so that each switching circuits is inactive prior to the column decoder providing the first logic level signal. 
     With the above arrangement, all the memory cells disposed in a memory cell array can be prevented from becoming inoperative even if an arbitrary column line is broken. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the inventions it is believed that the invention, the objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is formed by FIG. 1A and FIG. 1B and shows a block diagram of the semiconductor memory device constructed in accordance with the preferred embodiment of the invention; 
     FIG. 2 is a first partial chart to explain the discharging operation of the clamping circuit shown in FIG. 1; 
     FIG. 3 is a second partial chart to explain the discharging operation of the clamping circuit shown in FIG. 1; 
     FIG. 4 is a diagram illustrating another embodiment of the semiconductor memory device according to the present invention; and 
     FIG. 5 is a partial cross-section of the diffusion layer N +   shown in FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A semiconductor memory device, for example that is a DRAM, according to one embodiment of the present invention, is shown in FIG. 1. 
     The semiconductor memory device has a memory cell array 1 comprising a plurality of sub arrays 1 k  (k=1, 2, . . . , 1). A row address decoder array 2 and a column address decoder array 3 are electrically connected to the memory cell array 1. The row address decoder array 2 comprises a row address decoder for selecting one of word lines WL j  in response to a row address signal and a redundant row address decoder for selecting a pair of redundant word lines so as to determine a row used as an alternative to a defective or faulty row. The column address decoder array 3 comprises a column address decoder for selecting one of column lines CL i  (i =1, 2, . . . , m) in response to a column address signal and a redundant column address decoder for selecting a redundant column line so as to determine a column used as an alternative to a defective or faulty column. The column lines CL i  are electrically connected to their corresponding nodes N 1-i  (i=1, 2, . . . , m). The nodes N 1-i  are electrically connected to elements disposed within the column address decoder, for example, their corresponding output terminals of inverters 3 i  (i=1, 2, . . . , n) in the present embodiment. 
     The respective sub arrays 1 k  comprise a plurality of word lines WL j  (j=1, 2, . . . , n), a plurality of bit lines pairs BL i  a, BL i  b (i=1, 2, . . . , m) disposed so as to intersect the corresponding word lines WL j , a plurality of column address selection signal lines (hereinafter called &#34;column lines&#34;) CL i  (i=1, 2, . . . , m) disposed in parallel to the bit line pairs BL i  a, BL i  b, sense amplifier enable signal lines SLN, SLP arranged in parallel to the word lines WL j , and complementary data buses DB a , DB b . For example, WL n-1 , WL n  of the word lines WL j  are used as a pair of redundant word lines. For example, BL m  a, BL m  b of the bit line pairs BL i  a, BL i  b are used as redundant bit lines. Further, for example, CL m  of the column lines CL i  is used as a redundant column line. 
     Memory cells 10 ij  (i=1, 2, . . . , m, j=1, 2, . . . , n) are respectively connected to points where the word lines WL j  and the bit lines BL i  intersect, and arranged in matrix form. Each of the memory cells 10 ij  has a charge transfer N-channel MOS transistor (Metal Oxide Semiconductor) (hereinafter called an &#34;NMOS&#34;) 11 and a charge storage capacitor 12. The drain electrodes of the NMOSs 11 are electrically connected to the corresponding bit lines BL i  a or BL i  b, and the gate electrodes thereof are electrically connected to the corresponding word lines WL j . Further, each of the source electrodes of the NMOSs 11 is electrically connected via the capacitor 12 to its corresponding fixed potential or voltage HVCC whose voltage level is equal to half that of a power source potential or voltage VCC. 
     Sense amplifiers 20 i  (i=1, 2, . . . , m) for detecting differences in voltages between the respective bit line pairs and amplifying the so-detected differences are electrically connected between the respective bit line pairs BL i  a and BL i  b respectively. When the sense amplifier enable signal lines SLP and SLN are brought to &#34;H&#34; and &#34;L&#34; levels respectively, the respective sense amplifiers 20 i  are activated to amplify the differences in voltages between the respective bit line pairs. Each of the sense amplifiers 20 i  comprises a pair of NMOSs 21, 22 and a pair of P-channel MOS transistors (hereinafter called &#34;PMOSs&#34;) 23, 24. The respective source electrodes of the NMOSs 21, 22 are electrically connected to the sense amplifier enable signal line SLN. The gate electrodes of the NMOSs 21 are electrically connected to the bit lines BL i  b respectively, whereas the drain electrodes thereof are electrically connected to the bit lines BL i  a respectively. On the other hand, the gate electrodes of the NMOSs 22 are electrically connected to the corresponding bit lines BL i  a and the respective drain electrodes thereof are electrically connected to the corresponding bit lines BL i  b. The respective source electrodes of the PMOSs 23, 24 are electrically connected to the sense amplifier enable signal line SLP. The gate electrodes of the PMOSs 23 are electrically connected to the bit lines BL i  b respectively, whereas the drain electrodes thereof are electrically connected to the bit lines BL i  a respectively. On the other hand, the gate electrodes of the PMOSs 24 are electrically connected to the bit lines BL i  a respectively and the drain electrodes thereof are electrically connected to the bit lines BL i  b respectively. 
     Transfer gates 30 i  a, 30 i  b (i=1, 2, . . . , m) which are to be controlled by the column lines CL i , are electrically connected between the bit line pairs BL i  a, BL i  b and the data buses DB a , DB b . The respective transfer gates 30 i  a, 30 i  b are comprised of NMOSs. The drain electrodes of the transfer gates 30 i  a are electrically connected to the bit lines BL i  a respectively, whereas the respective source electrodes thereof are electrically connected to the data bus DB a . The drain electrodes of the transfer gates 30 i  b are electrically connected to the bit lines BL i  b respectively and the respective source electrodes thereof are electrically connected to the data bus DB b . The gate electrodes of the transfer gates 30 i  a, 30 i  b are electrically connected in common to the column lines CL i  respectively. 
     Clamping transistors NMOSs 40 i  (i=1, 2, . . . , m) capable of clamping the column lines CL i  to &#34;L&#34; levels respectively, are electrically connected to their corresponding nodes N 2-i  (i=1, 2, . . . , m), spaced farthest away from the column address decoder array 3, of the column lines CL i  (i=1, 2, . . . , m) in the sub arrays 1 k . In fact, the probability of breaking or disconnection of a single column line at several points is very low. It may be thus considered that the clamping transistors NMOSs 40 i  are electrically connected to only the corresponding column lines CL i  in the sub array 1 1 . 
     The drain electrodes of the clamping transistors NMOSs 40 i  are electrically connected to the column lines CL i  respectively and the source electrodes thereof are electrically connected to a ground electrode of a ground potential level. Further, the gate electrodes of the clamping transistors NMOSs 40 i  are electrically connected in common to a column-line clamping signal line CCL. The column-line clamping signal line CCL is supplied with a column-line clamping signal CC. In the present embodiment, the clamping transistors are NMOS types but may be P-channel transistors (PMOS types). 
     FIG. 2 illustrates the waveform of the column-line clamping signal CC. In response to a row address strobe signal RAS supplied to the row address decoder from the outside, the column-line clamping signal CC is brought to an &#34;L&#34; level upon reading and writing of data (when RAS is &#34;L&#34; in level). When, on the other hand, the column-line clamping signal CC is brought to an &#34;H&#34; level upon standing ready to read and write the data (when RAS is &#34;H&#34; in level). 
     A description will be made below of a data reading operation effected by other sub array 1 1  disposed between the column address decoder array 3 and the sub array 1 1 , for example, when a column line CL 2  is electrically disconnected, with reference to FIGS. 1 and 3. The operation of reading of data from a memory cell 10 1 ,3 shown in FIG. 1, will be described below as an illustrative example. A column line CL 2  * shown in FIG. 1 indicates the state of being electrically disconnected from others. 
     Data &#34;1&#34; is first written into the memory cell 10 1 ,3 and data &#34;0&#34; is then written into a memory cell 10 2 , 3. 
     Before the respective sub arrays 1 k  start to read data, that is, when the strobe signal RAS is &#34;H&#34; in level, the column-line clamping signal CC is rendered &#34;H&#34; in level. Therefore, the clamping transistor NMOS 40 2  is turned ON, thereby clamping the column line CL 2  to the &#34;L&#34; level. 
     When the respective sub arrays 1 k  initiate data reading operations, i.e., when the strobe signal RAS is &#34;L&#34; in level, the column-line clamping signal CC is brought to the &#34;L&#34; level, with the result that the clamping transistor NMOSs 40 i  are turned OFF. Thereafter, the word line WL 3 , which has been selected by the row address decoder, is rendered &#34;H&#34; in level. The NMOSs 11 of the memory cells 10 1 ,3, . . . , 10 m ,3 electrically connected to the word line WL 3 , are turned ON, so that data which have been stored in the memory cells 10 1 ,3, . . . 10 m ,3, are transmitted to the bit lines BL i  a (i=1, 2, . . . , m). Next, when the sense amplifier enable signal lines SLP and SLN are brought to the &#34;H&#34; and &#34;L&#34; levels respectively, the respective sense amplifiers 20 i  (i=1, 2, . . . , m) are activated. As a result, the differences in voltages between the respective bit line pairs BL i  a and BL i  b (i=1, 2, . . . , m) are detected and amplified. 
     When the column line CL 1  which has been selected by the column address decoder is then rendered &#34;H&#34; in level, the transfer gates 30 1  a, 30 1  b are turned ON. When the transfer gates 30 1  a, 30 1  b are turned ON, the pair of bit lines BL 1  a, BL 1  b and the data buses DB a , DB b  are allowed to conduct, thereby outputting data &#34;1&#34; read at the pair of bit lines BL 1  a, BL 1  b to the data buses DB a , DB b  respectively. 
     Even if the column line CL 2  * is brought to a floating state by disconnection with its potential or voltage level remaining at an &#34;H&#34; level, the column line CL 2  * is reliably clamped to the &#34;L&#34; level by the clamping transistor NMOS 40 2 . Therefore, the data stored in the non-selected memory cell 10 2 ,3 is not erroneously transferred to the data buses DB a , DB b . Only the data which has been stored in the memory cell 10 1 ,3, is accurately read and supplied to the data buses DB a , DB b . 
     Another embodiment of the present invention will be described below with reference to FIG. 4. The same elements of structure as those employed in the sub arrays of the semiconductor memory device shown in FIG. 1 are identified by the same reference numerals. 
     In the sub arrays, the column lines CL i  are respectively connected via the nodes N 2-i  (i=1, 2, . . . , m) to N type regions in reversely-biased PN junctions, e.g., N +   diffusion layers 50 i  (i=1, 2, . . . , m) formed in P-type substrates or P wells, which are used as an alternative to the clamping transistors NMOS 40 i  shown in FIG. 1. The column lines CL i  and the N +   diffusion layers 50 i  are electrically connected to one another for each sub array. 
     FIG. 5 is a cross-sectional view showing the manner of connection of a column line CL m-1  to an N +   diffusion layer 50 m-1 . 
     An N +   diffusion layer 50 m-1  is formed within a negative-biased P-type substrate (or a P well). An insulative layer 61 is fabricated on the N +   diffusion layer 50 m-1  and the column line CL m-1  is formed on the insulative layer 61. The column line CL m-1  and the N +   diffusion layer 50 m-1  are electrically connected to each other via a through hole 62 of the insulative layer 61. A PN junction is formed of the P type substrate 60 and the N +   diffusion layer 50 m-1 . 
     Even when a column line CL 2  * is brought to a floating state by disconnection with its potential or voltage level being maintained at an &#34;H&#34; level in the same manner as described above even in this case, electric charges held on the column line CL 2  * are gradually discharged through the PN junction. Therefore, the non-selected column line CL 2  * is not brought to a high potential. As a result, data which has been stored in a non-selected memory cell 10 2 ,3, is not transferred to data buses DB a , DB b  by mistake. Only data which has been stored in a memory cell 10 1 ,3, is accurately read so as to be supplied to the data buses DB a , DB b . 
     The P-type substrate 60 of the PN junction is brought to a negative bias potential. Therefore, when normally-operated column lines CL i  are successively selected by the column address decoder and brought to &#34;H&#34; levels, the PN junction is biased in the reverse direction. Thus, the selected normally-operated column lines CL i , which are &#34;H&#34; in level, are not immediately reduced to &#34;L&#34; levels through the PN junction. 
     In this case, the column lines CL i  are electrically connected to the N +   diffusion layer 50 i  respectively. Alternatively, the column lines CL i  may be connected to other corresponding N-type regions of a reversely-biased PN junction. 
     According to the present invention, as has been described above, even if an arbitrary column line is broken, all the memory cells in a memory cell array can be reliably prevented from becoming inoperative. 
     The present invention has been described above specifically by the preferred embodiments. It will however be understood that the invention is not limited to the specific examples described. Many changes of the illustrated embodiments and other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is thus intended that all the changes or the embodiments be considered as exemplarily only, with a true scope and spirit of the invention being covered by the following claims.