Abstract:
In a semiconductor memory device including a plurality of word lines, a plurality of pairs of bit lines, a plurality of memory cells, each connected between one of the word lines and one of the bit lines, and a plurality of sense amplifiers for amplifying the difference in potential between the pair of the bit lines, a plurality of offset circuits, is provided, for applying an offset voltage independent of voltages at the bit lines, to at least one of the pair of the bit lines to reduce the difference in potential between the pair of the bit lines before the sense amplifiers are operated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory device such as a ferroelectric random access memory (FeRAM) device, and more particularly, to the improvement of a reliability test or a burn-in test of the semiconductor memory device. 
     2. Description of the Related Art 
     In a prior art semiconductor memory device including a plurality of word lines, a plurality of pairs of bit lines, a plurality of memory cells each connected between one of the word lines and one of the bit lines, and a plurality of sense amplifiers for amplifying the difference in potential between the pair of the bit line a plurality of offset circuits is provided for applying an offset voltage to at least one of the pair of the bit lines to reduce the difference in potential between the pair of the bit lines before the sense amplifiers are operated, thus carrying out a reliability test, i.e., a burn-in test (see: JP-A-11-149796). This will be explained later in detail. 
     In the above-described prior art semiconductor memory device, however, the offset voltage is greatly dependent upon the voltage at a bit line. As a result, if the offset voltage is too large, some of normal semiconductor memory devices will be deemed to be defective and scrapped. On the other hand, if the offset voltage is too small, some of defective semiconductor memory devices will pass. Thus, it is impossible to carry out a high reliability test. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a semiconductor memory device capable of carrying out a high reliability test. 
     According to the present invention, in a semiconductor memory device including a plurality of word lines, a plurality of pairs of bit lines, a plurality of memory cells each connected between one of the word lines and one of the bit lines, and a plurality of sense amplifiers for amplifying the difference in potential between the pair of the bit lines, a plurality of offset circuits is provided for applying an offset voltage independent of voltages at the bit lines to at least one of the pair of the bit lines, to reduce the difference in potential between the pair of the bit lines before the sense amplifiers are operated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description set forth below, as compared with the prior art, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a block circuit diagram illustrating a prior art FeRAM device; 
     FIG. 2 is a partially-detailed circuit diagram of the FeRAM device of FIG. 1; 
     FIGS. 3A through 3I are timing diagrams for explaining the normal operation of the FeRAM device of FIGS. 1 and 2; 
     FIGS. 4A through 4I are timing diagrams for explaining the test operation of the FeRAM device of FIGS. 1 and 2; 
     FIG. 5 is a graph showing the offset voltage in the FeRAM device of FIGS. 1 and 2; 
     FIG. 6 is a circuit diagram illustrating a first embodiment of the FeRAM device according to the present invention; 
     FIGS. 7A through 7I are timing diagrams for explaining the normal operation of the FeRAM device of FIG. 6; 
     FIG. 8 is a graph showing the offset voltage in the FeRAM device of FIG. 6; 
     FIG. 9 is a circuit diagram illustrating a second embodiment of the FeRAM device according to the present invention; 
     FIGS. 10A through 10I are timing diagrams for explaining the normal operation of the FeRAM device of FIG. 9; 
     FIG. 11 is a circuit diagram illustrating a third embodiment of the FeRAM device according to the present invention; 
     FIGS. 12A through 12I are timing diagrams for explaining the normal operation of the FeRAM device of FIG. 11; 
     FIG. 13 is a circuit diagram illustrating a modification of the FeRAM device of FIG. 6; 
     FIG. 14 is a circuit diagram illustrating a modification of the FeRAM device of FIG. 13; 
     FIGS. 15A and 15B are circuit diagrams of modifications of the offset circuit of FIG. 14; and 
     FIG. 16 is a circuit diagram illustrating a modification of the FeRAM device of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the description of the preferred embodiments, a prior art FeRAM device will be explained with reference to FIGS. 1,  2 ,  3 A through  3 I,  4 A through  4 I and  5 . 
     In FIG. 1, which illustrates a prior art FeRAM device (see JP-A-11-149796), reference numeral  1  designates a memory cell array including memory cells MCij (i=1, 2, . . . , n; j=1, 2, . . . , n) of one-transistor, one-ferroelectric capacitor type formed at intersections between word lines WL 1 , WL 2 , . . . , WLm, plate lines PL 1 , PL 2 , . . . , PLm and bits lines BL 1 , {overscore (BL 1 )}, BL 2 , {overscore (BL 2 )}, . . . , BLn, {overscore (BLn)}. For example, the memory cell MC 22  has a MOS transistor having a gate connected to the word line WL 2 , a drain connected to the bit line BL 2 , a source, and a ferroelectric capacitor connected between the source of the transistor and the plate line PL 2 . The word lines WL 1 , WL 2 , . . . , WLm and the plate lines PL 1 , PL 2 , PLm are controlled by an X decoder/plate decoder  2 . 
     Also, a dummy cell array  3  including dummy cells is connected to the bit lines BL 1 , {overscore (BL 1 )}, BL 2 , {overscore (BL 2 )}, . . . , BLn, {overscore (BLn)}. Each of the dummy cells has a similar configuration to those of the memory cells; however, the capacitance of a capacitor of each of the dummy cells is about half of that of the ferroelectric capacitor of the memory cell array  1 . The dummy cells of the dummy cell array  3  are connected to dummy word lines DWL and {overscore (DWL)} which are controlled by a dummy cell line decoder  4 . 
     Further, a precharging circuit  5  including precharging MOS transistor is connected to the bit lines BL 1 , {overscore (BL 1 )}, BL 2 , {overscore (BL 2 )}, . . . , BLn, {overscore (BLn)}. The precharging transistors of the precharging circuit  5  are connected to a recharging bit line PBL which is controlled by a bit line precharging circuit  6 . In this case, the bit lines BL 1 , {overscore (BL 1 )}, BL 2 , {overscore (BL 2 )}, . . . , BLn, {overscore (BLn)} are precharged by the bit line precharging circuit  6  at GND. 
     Additionally, a transfer gate circuit  7  including transfer gate Transistors are connected to the bit lines BL 1 , {overscore (BL 1 )}, BL 2 , {overscore (BL 2 )}, . . . , BLn, {overscore (BLn)}. The transfer gate transistors are connected to a transfer gate line TGL which is controlled by a transfer gate drive circuit  8 . 
     Further, sense amplifiers  91 ,  92 , . . . ,  9   n  are connected to the bit lines BL 1 , {overscore (BL 1 )}, BL 2 , {overscore (BL 2 )}, . . . , BLn, {overscore (BLn)} on the outside of the transfer gate circuit  7 . The sense amplifiers  91 ,  92 , . . . ,  9   n  are connected to sense enable lines SAP and SAN which are controlled by a sense amplifier drive circuit  10 . 
     In addition, offset circuits  111 ,  112 , . . . ,  11   n  are connected to the bit lines BL 1 , {overscore (BL 1 )}, BL 2 , {overscore (BL 2 )}, . . . , BLn, {overscore (BLn)}. The offset circuits  111 ,  112 , . . . ,  11   n  are connected to offset validation lines OV 1  and OV 2  which are controlled by an offset drive circuit  12 . 
     Y-selection transfer gates YST 1 , YST 2 , . . . , YSTn are connected between the bit lines BL 1 , {overscore (BL 1 )}, BL 2 , {overscore (BL 2 )}, . . . , BLn, {overscore (BLn)} and input/output buses IO and {overscore (IO)}. One of the Y-selection transfer gates YST 1 , YST 2 , . . . , YSTn is selected by Y-selection switch signals YSW 1 , YSW 2 , . . . , YSWn which are controlled by a Y decoder  13 . 
     The X decoder/plate decoder  2 , the dummy word line decoder  4 , the bit line precharging circuit  6 , the transfer gate drive circuit  8 , the sense amplifier drive circuit  10  and the offset drive circuit  12  are operated by receiving X address decode and control signals XP from an address predecoder  14 . On the other hand, the Y decoder  13  is operated by receiving Y address decode and control signals YP from the address predecoder  14 . Note that the address predecoder  14  receives address signals Ai, a read/write signal R/{overscore (W)}, a row address strobe signal {overscore (RAS)}, a column address strobe signals {overscore (CAS)} and the like. 
     In FIG. 2, which is a partially detailed circuit diagram of the FeRAM device of FIG. 1, the sense amplifier such as  92  is constructed by cross-coupled P-channel MOS transistors Q p1  and Q p2  connected between the bit lines BL 2  and BL 2  and the sense enable line SAP, and P-channel MOS transistors Q n1  and Q n2  connected between the bit lines BL 2  and BL 2  and the sense enable line SAN. 
     Also, the offset circuit such as  112  is constructed by cross-coupled N-channel MOS transistors Q n3  and Q n4  between the bit lines BL 2  and {overscore (BL 2 )}, and switching N-channel MOS transistors Q n5  and Q n6  connected between the drains of the transistors Q n3  and Q n4  and the bit lines BL 2  and {overscore (BL 2 )}. The transistors Q n5  and Q n6  are controlled by the voltages at the offset validation lines OV 1  and OV 2 , respectively. 
     The normal read operation of the FeRAM device of FIGS. 1 and 2 is explained with reference to FIGS. 3A through 3I, where data of the memory cell MC 22  is read out. 
     First, at time t 1 , as shown in FIG. 3A, the voltage at the precharging bit line PBL is changed from high (=V CC ) to low (=GND), thus entering a selection mode from a stand-by mode. As a result, the bit lines BL 2  and {overscore (BL 2 )} become in a floating state. In the stand-by mode, note that the bit lines BL 2  and {overscore (BL 2 )} are precharged at GND as shown in FIG.  3 I. 
     Next, at time t 2 , as shown in FIG. 3B, the voltage at the dummy word line DWL is changed from low (=GND) to high (=V CC +V th +α), while the voltage at the dummy word line DWL is unchanged. Here, V CC  designates a power supply voltage, V th  designates a threshold voltage of the N-channel MOS transistors, and α is a positive value. Therefore, when the voltage at the dummy word line DWL is V CC +V th +α, the switching transistors of the corresponding dummy cells are completely turned ON. Thus, as shown in FIG. 3I, the voltage at the bit line {overscore (BL 2 )} is pushed up to a reference voltage VREF in accordance with the capacitance of the dummy cell DC. 
     Simultaneously, as shown in FIG. 3C, the voltage at the word line WL 2  is changed from low (=GND) to high (=V CC +V th +α), while the voltages at the other word lines are unchanged. Similarly, as shown in FIG. 3D, the voltage at the plate line PL 2  is changed from low (=GND) to high (=V CC +V th +α), while the voltages at the other plate lines are unchanged. Thus, as shown in FIG. 3I, the voltage at the bit line BL 2  is pushed up to a read voltage V 2  in accordance with the capacitance of the memory cell MC 22 . Generally, if the capacitor of the memory cell MC 22  stores “0” (changed state), the voltage V 2  is higher than the reference voltage VREF, as shown in FIG.  3 I. 
     Next, at time t 3 , as shown in FIG. 3E, the voltage at the transfer gate line TGL is changed from low (=GND) to high (=V CC +V th +α). Also, as shown in FIG. 3F, the voltage at the sense enable line SAP is changed from high (=V CC ) to low (=GND) and the voltage at the sense enable line SAN is changed from low (=GND) to high (=V CC ), thus entering a sense mode. As a result, as shown in FIG. 3I, the difference in potential between the bit lines BL 2  and {overscore (BL 2 )} is enlarged by the operation of the sense amplifier  92 . 
     Next, at time t 4 , as shown in FIG. 3H, the Y-selection switch signal YSW 2  is changed from low (=GND) to high (=V CC ), while the other Y-selection switch signals are unchanged, thus entering a read mode. As a result, as shown in FIG. 3I, the voltages at the bit lines BL 2  and {overscore (BL 2 )} are transferred via the Y-selection transfer gate YST 2  to the input/output buses IO and {overscore (IO)}, respectively. 
     Note that, if the operation is a write operation, the voltages at the input/output buses IO and {overscore (IO)} are transferred via the Y-selection transfer gate YST 2  to the bit lines BL 2  and {overscore (BL 2 )}, respectively, as indicated by X in FIG.  3 I. 
     Next, at time t 5 , as shown in FIG. 3H, the voltage at the Y-selection switch signal YSW 2  returns from high to low, thus completing the read mode. In this state, as shown in FIG. 3I, the voltages at the bit lines BL 2  and {overscore (BL 2 )} are electrically separated from the input/output buses IO and {overscore (IO)}, and therefore, become in a floating state. 
     Next, at time t 6 , as shown in FIG. 3D, the voltage at the plate line PL 2  is changed from high to low, thus entering a rewrite (refresh) mode. That is, in this case, in the memory cell MC 22 , since the switching MOS transistor is still turned ON by the high voltage of the word line WL 2 , the charge at the bit line BL 2  is transferred via the switching MOS transistor to the node of the ferroelectric capacitor. Note that this rewrite operation can be surely carried out due to the activated sense amplifier  92 . 
     Next, at time t 7 , as shown in FIG. 3F, the voltage at the sense enable line SAP is changed from high to low and the voltage at the sense enable line SAN is changed from high to low, thus completing the sense mode as well as the rewrite mode. 
     Finally, at time t 8 , as shown in FIG. 3A, the voltage at the precharging bit line PBL is changed from high to low, thus returning from the selection mode to a stand-by mode. As a result, the word line WL 2  is again precharged to GND. 
     Note that the voltage at the transfer gate line TGL is changed from high to low around time t 8 , as shown in FIG.  3 E. 
     The test operation of the FeRAM device of FIGS. 1 and 2 is explained next with reference to FIGS. 4A and 4I, where the memory cell MC 22  is tested. 
     In a test operation for the memory cell MC 22 , at time t 2 ′, after time t 2  and before time t 3  the offset circuit  112  is operated to decrease the higher one of the voltages at the bit lines BL 2  and {overscore (BL 2 )}, which makes the read operation difficult. Therefore, if such a test operation is carried out before the shipping, reliability of the FeRAM device can be assured. 
     In more detail, as shown in FIG. 4I, if the voltage at the bit line BL 2  is higher than the voltage at the bit line {overscore (BL 2 )} before time t 2 ′, an ON current flowing through the transistor Q n1  is larger than an ON current flowing through the transistor Q n2 , and an ON current flowing through the transistor Q p1  is smaller than an ON current flowing through the transistor Q p2 . In this state, at time t 2 ′, the voltage at the offset validation line OV 2  is changed from low (=GND) to high (=V CC ) while the voltage at the offset validation line OV 1  is unchanged. Therefore, a current flowing through the transistors Q n4  and Q n6  is added to the current flowing through the transistor Q n2 , so that the voltage at the bit line BL 2  is decreased by an offset voltage ΔV. 
     In the Fe RAM device of FIGS. 1 and 2, however, the offset voltage ΔV is greatly dependent upon the voltage at the bit line such as {overscore (BL 2 )}. For example, the ON current flowing through the transistor Q n4  is dependent upon its source-to-gate voltage, i.e., the voltage at the bit line {overscore (BL 2 )}. That is, the higher the voltage at the bit line {overscore (BL 2 )}, the larger the ON current flowing through the transistor Q n4 . As a result, as shown in FIG. 5, the higher the voltage at the bit line {overscore (BL 2 )}, the larger the offset voltage ΔV. In FIG. 5, note that the transistors are manufactured by using a 0.55 μm gate length design. 
     Thus, in the FeRAM device of FIGS. 1 and 2, since the offset voltage greatly fluctuates, it is impossible to carry out a high reliability test. 
     In FIG. 6, which illustrates a first embodiment of the present invention, an offset circuit  61  is provided instead of the offset circuit  112  of FIG.  2 . Note that the same offset circuit  61  is connected to the bit lines other than the bit lines BL 2  and {overscore (BL 2 )}. The offset circuit  61  as well as the other offset circuits are controlled by an offset control circuit  62 . 
     The offset circuit  61  is constructed by a capacitor CD 1  connected to the bit line {overscore (BL 2 )} and a capacitor CD 2  connected to the bit line BL 2 . 
     The offset control circuit  62  is constructed by an OR circuit  621  for receiving the signals of the plate lines PL 1 , PL 3 , . . . , a NAND circuit  622  for receiving the output signal of the OR circuit  621  and a test signal TE at a test terminal, and a delay circuit  623  for delaying the output signal of the NAND circuit  622  to generate an offset control signal OPL 1 . Also, the offset control circuit  62  is constructed by an OR circuit  624  for receiving the signals of the plate lines PL 2 , PL 4 , . . . , a NAND circuit  625  for receiving the output signal of the OR circuit  624  and the test signal TE at the test terminal, and a delay circuit  626  for delaying the output signal of the NAND circuit  625  to generate an offset control signal OPL 2 . The offset control signals OPL 1  and OPL 2  are supplied to the capacitors CD 1  and CD 2  of the offset circuit  61 . 
     The test operation of the FeRAM device of FIG. 6 is explained next with reference to FIGS. 7A through 7I, where the memory cell MC 22  is tested and the test signal TE is “1” (high). 
     In a test operation for the memory cell MC 22 , the offset control circuit  62  is operated to generate an offset control signal OPL 2  at time t 2 ′ after time t 2  and before time t 3 , as shown in FIG.  7 F. That is, at time t 2 , as shown in FIG. 7D, when the voltage at the plate line PL 2  is changed from low to high, the output signal of the OR circuit  624  is changed from low to high, so that the output signal of the NAND circuit  625  is changed from high to low. As a result, after a delay time determined by the delay circuit  626 , the delayed output signal, i.e., the offset control signal OPL 2  is changed from high to low, as shown in FIG.  7 F. Thus, the voltage at the bit line BL 2  is decreased by an offset voltage ΔV 1  defined by 
     
       
         ΔV 1 =V 2 ·CD/(CD+CB)  (1)  
       
     
     where V 2  is the voltage at the bit line BL 2  immediately before the offset operation; 
     CD is a capacitance of the capacitor CD 2 ; and 
     CB is a parasitic capacitance of the bit line BL 2 . 
     Then, at time t 6 , as shown in FIG. 7D, when the voltage at the plate line PL 2  is changed from high to low, the output signal of the OR circuit  624  is changed from high to low, so that the output signal of the NAND circuit  625  is changed from low to high. As a result, after a delay time determined by the delay circuit  626 , the delayed output signal, i.e., the offset control signal OPL 2  is changed from low to high, as shown in FIG.  7 F. 
     In the first embodiment, the offset voltage ΔV 1  is not dependent upon the voltage at the bit line such as BL 2 . In other words, as shown in FIG. 8, even when the voltage at the bit line BL 2  is higher, the offset voltage ΔV 1  is unchanged. Thus, in the FeRAM device of FIG. 6, since the offset voltage hardly fluctuates, it is possible to carry out a high reliability test. 
     In FIG. 9, which illustrates a second embodiment of the present invention, an offset control circuit  62 ′ is provided instead of the offset control circuit  62  of FIG.  6 . 
     The offset control circuit  62 ′ is constructed by an OR circuit  621 ′ for receiving the signals of the plate lines PL 2 , PL 4 , . . . , a AND circuit  622 ′ for receiving the output signal of the OR circuit  621 ′ and the test signal TE, and a delay circuit  623 ′ for delaying the output signal of the AND circuit  622 ′ to generate an offset control signal OPL 1 . Also, the offset control circuit  62 ′ is constructed by an OR circuit  624 ′ for receiving the signals of the plate lines PL 1 , PL 3 , . . . , an AND circuit  625 ′ for receiving the output signal of the OR circuit  624 ′ and the test signal TE, and a delay circuit  626 ′ for delaying the output signal of the AND circuit  625 ′ to generate an offset control signal OPL 2 . 
     The test operation of the FeRAM device of FIG. 9 is explained next with reference to FIGS. 10A through 10I, where the memory cell MC 22  is tested and the test signal TE is “1” (high). 
     In a test operation for the memory cell MC 22 , the offset control circuit  62 ′ is operated to generate an offset control signal OPL 1  at time t 2 ′ after time t 2  and before time t 3 , as shown in FIG.  10 F. That is, at time t 2 , as shown in FIG. 10D, when the voltage at the plate line PL 2  is changed from low to high, the output signal of the OR circuit  621 ′ is changed from low to high, so that the output signal of the AND circuit  622 ′ is changed from low to high. As a result, after a delay time determined by the delay circuit  623 ′, the delayed output signal, i.e., offset control signal OP 1  is changed from low to high, as shown in FIG.  10 F. Thus, the voltage at the bit line {overscore (BL 2 )} is increased by an offset voltage ΔV 2  defined by 
     
       
         ΔV 2 =VREF·CD/(CD+CB)  (2)  
       
     
     where VREF is the voltage at the bit line {overscore (BL 2 )} immediately before the offset operation; 
     CD is a capacitance of the capacitor CD 1 ; and 
     CB is a parasitic capacitance of the bit line {overscore (BL 2 )}. 
     Then, at time t 6 , as shown in FIG. 10D, when the voltage at the plate line PL 2  is changed from high to low, the output signal of the OR circuit  621 ′ is changed from high to low, so that the output signal of the AND circuit  622 ′ is changed from high to low. As a result, after a delay time determined by the delay circuit  623 ′, the delayed output signal, i.e., the offset control signal OPL 1  is changed from high to low, as shown in FIG.  10 F. 
     Even in the second embodiment, the offset voltage ΔV 2  is not dependent upon the voltage at the bit line such as BL 2 . 
     Thus, in the FeRAM device of FIG. 9, since the offset voltage hardly fluctuates, it is possible to carry out a high reliability test. 
     In FIG. 11, which illustrates a third embodiment of the present invention, an offset control circuit  63  is provided instead of the offset control circuit  62  and  62 ′ of FIGS. 6 and 9. 
     The offset control circuit  63  is constructed by an OR circuit  631  for receiving the signals of the plate lines PL 1 , PL 2 , PL 3 , PL 4 , . . . , a delay circuit  632  for delaying the output signal of the NOR circuit  631 , a tri-state buffer circuit  633   a  for receiving the output signal of the delay circuit  632 , the test signal TE and an inverted signal of a flag signal FG to generate an offset control signal OPL 1 , a tri-state buffer circuit  633   b  for receiving the output signal of the delay circuit  632 , the test signal TE and the flag signal FG to generate an offset control signal OPL 2 , and a V CC /2 generating circuit  634  for receiving the output signal of the delay circuit  632  to make the offset control signals OPL 1  and OPL 2  be V CC /2 when all the signals of the plate signals PL 1 , PL 2 , PL 3 , PL 4 , . . . are low (=GND). 
     Note that the flag signal FG is a signal for determining an offset direction. That is, in a selection mode, when the voltages at the bit lines {overscore (BL 2 )} and BL 2  are high and low, respectively, the voltage of the flag signal FG is low (=GND). On the other hend, in a selection mode, when the voltages at the bit lines {overscore (BL 2 )} and BL 2  are low and high, respectively, the voltage of the flag signal FG is high (=V CC ). The flag signal FG is set in advance by the address predecoder  14  of FIG.  1 . 
     The test operation of the FeRAM device of FIG. 11 is explained next with reference to FIGS. 12A through 12I, where the memory cell MC 22  is tested under the test signal TE is “1” (high) and the flag signal FG is “1” (high). 
     In a test operation for the memory cell MC 22 , the offset control circuit  63  is operated to generate offset control signals OPL 1  and OPL 2  at time t 2 ′ after time t 2  and before time t 3 , as shown in FIG.  12 F. That is, at time t 2 , as shown in FIG. 12D, when the voltage at the plate line PL 2  is changed from low to high, the output signal of the OR circuit  631  is changed from low to high. As a result, after a delay time determined by the delay circuit  632 , the delayed output signal is changed from low to high. Therefore, the offset control signal OPL 1  is changed from V CC /2 to high (=V CC ), as shown in FIG.  12 F. Thus, the voltage at the bit line {overscore (BL 2 )} is increased by an offset voltage ΔV 2  defined by the formula (2). 
     Simultaneously, the offset control signal OPL 2  is changed from V CC /2 to low (=GND), as shown in FIG.  12 F. Thus, the voltage at the bit line BL 2  is decreased by an offset voltage ΔV 1  defined by the formula (1). 
     Then, at time t 6 , as shown in FIG. 12D, when the voltage at the plate line PL 2  is changed from high to low, the output signal of the OR circuit  631  is changed from high to low. As a result, after a delay time determined by the delay circuit  632 , the delayed output signal is changed from high to low. Therefore, the offset control signal OPL 1  is changed from high (=V CC ) to V CC /2, as shown in FIG.  12 F. Similarly, the offset control signal OPL 2  is changed from low(=GND) to V CC /2. 
     Even in the third embodiment, the offset voltages ΔV 1  and ΔV 2  are not dependent upon the voltage at the bit line such as BL 2 . 
     Thus, in the FeRAM device of FIG. 11, since the offset voltage hardly fluctuates, it is possible to carry out a high reliability test. 
     In FIG. 13, which illustrates a modification of the FeRAM device of FIG. 6, an offset circuit  61 ′ is provided instead of the offset circuit  61  of FIG. 6, and an offset validation circuit  13  is added to the FeRAM device of FIG.  6 . 
     In the offset circuit  61 ′ of FIG. 13, an N-channel MOS transistor Q 1  is connected between the bit line {overscore (BL 2 )} and the capacitor CD 1 , and an N-channel MOS transistor Q 2  is connected between the bit line BL 2  and the capacitor CD 2 . The transistors Q 1  and Q 2  are controlled by offset validation signals OC 1  and OC 2 , respectively, of the offset validation circuit  13 . That is, only when the offset circuit  61 ′ is required to be operated for a test mode, is the transistor Q 1  or Q 2  turned ON, so that the capacitor CD 1  or CD 2  is connected to the bit line BL 2  or BL 2 . Otherwise, the transistors Q 1  and Q 2  are turned OFF to electrically separate the capacitors CD 1  and CD 2  separated from the bit lines {overscore (BL 2 )} and BL 2 , so that the parasitic capacitance thereof can be substantially decreased. As a result, the normal operation speed is hardly affected by the presence of the capacitors CD 1  and CD 2 . 
     The offset validation circuit  13  is constructed by an OR circuit  131  for receiving the signals of the word lines WL 1 , WL 3 , . . . , an AND circuit  132  for receiving the output signals of the OR circuit  131  and the test signal TE, and a delay circuit  133  for delaying the output signal of the AND circuit  132  to generate an offset validation signal OC 1 . Also, the offset validation circuit  13  is constructed by an OR circuit  134  for receiving the signals of the word lines WL 2 , WL 4 , . . . , an AND circuit  135  for receiving the output signal of the OR circuit  134  and the test signal TE, and a delay circuit  136  for delaying the output signal of the AND circuit  135  to generate an offset validation signal OC 2 . 
     Thus, the offset validation signals OC 1  and OC 2  have similar waveforms to those of the word lines such as WL 1  and WL 2 , respectively. 
     Note that the modificaiton of FIG. 6 as illustrated in FIG. 13 can be applied to the second and third embodiments of the present invention as illustrated in FIGS. 9 and 11. 
     In FIG. 14, which illustrates a modification of the FeRAM device of FIG. 13, an offset circuit  61 ″ is provided instead of the offset circuit  61 ′ of FIG. 13, thus preventing the nodes between the transistors Q 1  and Q 2  and the capacitors CD 1  and CD 2  from being in a floating state when the offset circuit  61 ″ is not operated. 
     The offset circuit  61 ″ further includes N-channel MOS transistors Q 3  and Q 4  and inverters I 1  and I 2  in addition to the elements of the offset circuit  61 ′. In this case, the node between the transistors Q 3  and Q 4  is grounded, because an offset operation is carried out to pull down the voltage at the bit line {overscore (BL 2 )} or BL 2 . 
     The modification of FIG. 13 as illustrated in FIG. 14 can be applied to the second and third embodiments of the present invention as illustrated in FIGS. 9 and 11. In the second embodiment, V CC  is applied to the node between the transistors Q 3  and Q 4  as illustrated in FIG. 15A, because an offset operation is carried out to pull up the voltage at the bit line {overscore (BL 2 )} or BL 2 . On the other hand, in the third embodiment, V CC /2 is applied to the node between the transistors Q 3  and Q 4  as illustrated in FIG. 15B, because an offset operation is carried out to pull up or down the voltage at the bit line {overscore (BL 2 )} or BL 2 . 
     In the above-described embodiments, a one-transistor, one-capacitor (1T/LC) type operation system where access of one memory cell is associated with access of its corresponding dummy cell; however, the present invention can be applied to a two-transistor, two-capacitor (2T/2C) type operation where access of one memory cell is associated with access of another memory cell, as illustrated in FIG. 16, which is a modification of the FeRAM device of FIG.  13 . In FIG. 16, note that the dummy cells are removed. For example, if data “0” is written into the memory cell MC 12 , data “1” is written into the memory cell MC 22 . 
     Also, the present invention can be applied to other semiconductor memory devices than FeRAM devices. 
     As explained hereinabove, since the offset voltage hardly fluctuates, a high reliability test can be carried out.