Abstract:
A semiconductor memory device which includes at least two memory cell arrays, a sense amplifier shared by the memory cell arrays and at least two transfer gates connected respectively between each of the memory cell arrays and the sense amplifier. The semiconductor memory device further includes a first voltage supplier supplying a first voltage to the transfer gates, and a second voltage supplier supplying a second voltage to the transfer gates, with second voltage being higher than the first voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a semiconductor memory device and a testing method thereof, and more particularly, to a semiconductor memory device and a testing method thereof which device is tested by using a high voltage. 
     2. Description of the Related Art 
     In developing a semiconductor memory device, reducing a circuit scale has always been a concern, and thus conventional technologies have devised a semiconductor memory device having a plurality of memory cell arrays sharing a sense amplifier. The sense amplifier is generally referred to as a shared sense amplifier. 
     In the semiconductor memory device having the above-mentioned structure, two memory cell arrays, for example, share a sense amplifier. A gate, referred to as a bit-line transfer (BT) gate, is provided between each of the memory cell arrays and the sense amplifier. The opening and closing of each of the BT gates is controlled according to which of the memory cell arrays is targeted in reading data so that data is read selectively from either of the memory cell arrays into the sense amplifier. 
     Conventionally, the above-mentioned BT gates are controlled by a method referred to as “VPP driving method via VDD”. In the “VPP driving method via VDD”, the electric potentials of BT gate drive lines regulating the opening and closing of the BT gates are at the level of a power-source voltage VDD in a standby state, and then in reading or writing data, the level is driven to the level of a boosted voltage VPP or a ground voltage VSS. This method is effective in reducing an amount of electric power being consumed compared to a method of driving the BT gates continuously by the boosted voltage VPP. 
     However, in a wafer-level burn-in (WLBI) test, when applying a voltage of, for example, 4V to a bit line, a voltage higher than the 4V by a threshold voltage of transistors composing the BT gate has to be applied to gates of the transistors. Raising the level of the above-mentioned power-source voltage VDD to a level higher than the 4V imposes unnecessary stress on the sense amplifier supplied with the power-source voltage VDD and controlling circuits thereof, more specifically, transistors and other devices included in these circuits, the transistors and other devices having a short-length gate and driven by the internal power-source voltage. Hence, there is a problem that a WLBI test cannot be sufficiently performed. 
     It is noted that the “WLBI test” here means a test investigating stresses such as a leak between adjacent bit lines, or between opposing memory cells connected to a bit line leading to a sense amplifier. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide an improved and useful semiconductor memory device and a testing method thereof in which device and method the above-mentioned problems are eliminated. 
     A more specific object of the present invention is to provide a semiconductor memory device which device can be tested by using a high voltage with ease, and a testing method thereof which method can test a semiconductor memory device by using a high voltage with ease. 
     In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a semiconductor memory device comprising: 
     at least two memory cell arrays; 
     a sense amplifier shared by the memory cell arrays; 
     at least two transfer gates connected respectively between each of the memory cell arrays and the sense amplifier; 
     a first voltage supplier supplying a first voltage to the transfer gates; and 
     a second voltage supplier supplying a second voltage to the transfer gates, the second voltage being higher than the first voltage. 
     The present invention can prevent unnecessary stress from imposing on unintended circuits of the semiconductor memory device. 
     Additionally, the semiconductor memory device according to the present invention may have a normal-operation mode and a test mode testing a quality of the memory cell arrays, wherein the second voltage supplier may be activated only in the test mode. 
     According to the present invention, a test testing a quality of the memory cell arrays can be easily performed. 
     Additionally, in the semiconductor memory device according to the present invention, the first voltage supplier may supply the transfer gates with the first voltage, in a first state in the normal-operation mode, and may supply at least one of the transfer gates with one of voltages opening and closing the transfer gate, in a second state in the normal-operation mode. 
     According to the present invention, reading and writing data from/to the memory cell arrays can be performed easily. 
     Additionally, in the semiconductor memory device according to the present invention, the second voltage supplier may supply the second voltage to all of the transfer gates. 
     According to the present invention, the above-mentioned test can be performed to all of the memory cell arrays at the same time. 
     Specifically, in the semiconductor memory device according to the present invention, each of the transfer gates may include at least one transistor having a gate supplied with the first voltage and the second voltage. 
     In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention a method of testing a semiconductor memory device including at least two memory cell arrays, a sense amplifier shared by the memory cell arrays, and at least two transfer gates connected respectively between each of the memory cell arrays and the sense amplifier, the method comprising the step of: 
     supplying the transfer gates with a higher voltage in a test than a voltage supplied in a normal operation. 
     The testing method according to the present invention can prevent unnecessary stress from imposing on unintended circuits of the semiconductor memory device. 
     Additionally, in the method according to the present invention, the semiconductor memory device may have a test mode testing a quality of the memory cell arrays, and the step of supplying may include regulating the higher voltage supplied to the transfer gates according to a signal activated by the semiconductor memory device in the test mode. 
     According to the present invention, a test testing a quality of the memory cell arrays can be easily performed. 
     Additionally, the method according to the present invention may further comprise the steps of: 
     supplying the transfer gates with a first voltage, in a first state in the normal operation; and 
     supplying at least one of the transfer gates with one of second and third voltages opening and closing the transfer gate, in a second state in the normal operation. 
     According to the present invention, reading and writing data from/to the memory cell arrays can be performed easily. 
     Additionally, in the method according to the present invention, the step of supplying may supply the higher voltage to all of the transfer gates. 
     According to the present invention, the above-mentioned test can be performed to all of the memory cell arrays at the same time. 
     Specifically, in the method according to the present invention, each of the transfer gates may include at least one transistor, and the step of supplying may supply the higher voltage to a gate of the transistor. 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a structure of a semiconductor memory device according to an embodiment of the present invention; 
     FIG.  2 A and FIG. 2B are illustrations for explaining features of the semiconductor memory device shown in FIG. 1; 
     FIG. 3 is a first timing diagram indicating operations of the semiconductor memory device shown in FIG. 1; 
     FIG. 4 is a circuit diagram showing a structure of a BT gate drive circuit shown in FIG. 1; 
     FIG. 5 is a circuit diagram showing a structure of a precharge control circuit shown in FIG. 1; 
     FIG. 6 is a circuit diagram showing a structure of a VDD precharge circuit shown in FIG. 1; 
     FIG. 7 is a circuit diagram showing a structure of a VPP precharge circuit shown in FIG. 1; and 
     FIG. 8 is a second timing diagram indicating operations of the semiconductor memory device shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will now be given, with reference to the drawings, of embodiments according to the present invention. Elements in the drawings that are identical or equivalent are referenced by the same characters. 
     FIG. 1 is a block diagram showing a structure of a semiconductor memory device according to an embodiment of the present invention. As shown in FIG. 1, the semiconductor memory device according to the present embodiment comprises a first memory cell array  1  including bit lines BL 1 , a second memory cell array  3  including bit lines BL 2 , a BT (bit-line transfer) gate drive circuit  5 , a precharge control circuit  7 , a VDD precharge circuit  9 , a VPP precharge circuit (a second voltage supplier)  11 , sense amplifiers  13  and  15 , BT gate drive lines  17  and  19 , and BT gates BTGa and BTGb. The Bt gate BTGa includes N-channel MOS transistors NT 1  to NT 4 . The Bt gate BTGb includes N-channel MOS transistors NT 5  to NT 8 . It is noted that the BT (bit-line transfer) gate drive circuit  5  and the VDD precharge circuit  9  compose a first voltage supplier. 
     The first memory cell array  1  is connected to the sense amplifiers  13  and  15  via the BT gate BTGa. The second memory cell array  3  is connected to the sense amplifiers  13  and  15  via the BT gate BTGb. The BT gate drive circuit  5  is supplied with selective signals ss 0  and ss 1  and a column-selective signal cs. The BT gate drive circuit  5  supplies a BT gate drive signal GD 0  to gates of the N-channel MOS transistors NT 1  to NT 4  via the BT gate drive line  17 , and supplies a BT gate drive signal GD 1  to gates of the N-channel MOS transistors NT 5  to NT 8  via the BT gate drive line  19 . 
     The precharge control circuit  7  is connected to the BT gate drive circuit  5  and is supplied with control signals CR 0  and CR 1 . The precharge control circuit  7  supplies a precharge signal /PRE to the VDD precharge circuit  9  and the sense amplifiers  13  and  15 . 
     The VDD precharge circuit  9  and the VPP precharge circuit  11  are connected in parallel between the BT gate drive lines  17  and  19 , and are supplied with a mode-switchover signal wbiz. 
     The semiconductor memory device having the above-described structure has a normal-operation mode and a WLBI (wafer-level burn-in) test mode. The mode-switchover signal wbiz is at a low level in the normal-operation mode, and is at a high level in the WLBI test mode. 
     The semiconductor memory device according to the present embodiment is characterized by using the above-mentioned mode-switchover signal wbiz to precharge the BT gate drive lines  17  and  19  with a boosted voltage VPP so as to drive the BT gates BTGa and BTGb with the boosted voltage VPP in a WLBI test. 
     Specifically, in a conventional technology, only the VDD precharge circuit  9  is provided as a circuit pre-charging the BT gate drive lines  17  and  19 , as shown in FIG. 2A, the VDD precharge circuit  9  supplying a power-source voltage VDD (a first voltage) to the BT gate drive lines  17  and  19  according to the precharge signal/PRE supplied from the precharge control circuit  7 . However, in the semiconductor memory device according to the present embodiment, the VPP precharge circuit  11  is further provided between the BT gate drive lines  17  and  19 , as shown in FIG. 2B, so as to supply the boosted voltage VPP (a second voltage) to the BT gate drive lines  17  and  19  according to the mode-switchover signal wbiz. 
     It is noted that the power-source voltage VDD is supplied from an external power source, and that the boosted voltage VPP is generated by boosting the power-source voltage VDD in the semiconductor memory device. For example, the power-source voltage VDD is 4V, a threshold voltage of the N-channel MOS transistors NT 1  to NT 8  is 1V, and the boosted voltage VPP is 5V. 
     Hereinbelow, a brief description will be given, with reference to FIG. 3, of operations of the semiconductor memory device according to the present embodiment shown in FIG.  1 . FIG. 3 is a timing diagram indicating operations of the semiconductor memory device according to the present embodiment. First, the mode-switchover signal wbiz is at a low level in a normal-operation mode, as indicated by FIG.  3 -( d ), so that the VDD precharge circuit  9  is activated so as to precharge the BT gate drive lines  17  and  19  with the power-source voltage VDD, as indicated by FIG.  3 -( b ) and FIG.  3 -( c ). Then, when the mode-switchover signal wbiz is raised to a high level at a time T 1  into a WLBI-test mode, as indicated by FIG.  3 -( d ), the VPP precharge circuit  11  is activated so as to precharge the BT gate drive lines  17  and  19  with the boosted voltage VPP, as indicated by FIG.  3 -( b ) and FIG.  3 -( c ). 
     Further, at a time T 2  when a WLBI test is over, the mode-switchover signal wbiz is dropped to a low level into the normal-operation mode, as indicated by FIG.  3 -( d ). At this time, the VPP precharge circuit  11  is deactivated so that the BT gate drive lines  17  and  19  are pre-charged with the power-source voltage VDD, as indicated by FIG.  3 -( b ) and FIG.  3 -( c ). It should be noted that the precharge signal /PRE is at a low level all through the above-mentioned course, as indicated by FIG.  3 -( a ). 
     Hereinbelow, descriptions will be given, with reference to FIG. 4 to FIG. 7, of a specific structure of each of the circuits shown in FIG.  1 . FIG. 4 is a circuit diagram showing a structure of the BT gate drive circuit  5  shown in FIG.  1 . As shown in FIG. 4, the BT gate drive circuit  5  comprises inverting circuits  22  and  23 , N-channel MOS transistors NT 9  to NT 22 , and P-channel MOS transistors PT 1  to PT 14 . In this structure, the N-channel MOS transistors NT 9  and NT 10 , the P-channel MOS transistor PT 1 , an internal power-source voltage node Vii and a ground node compose a clocked inverter CINV 1 . Also, the N-channel MOS transistors NT 16  and NT 17 , the P-channel MOS transistor PT 8 , an internal power-source voltage node Vii, and a ground node compose a clocked inverter CINV 2 . 
     In the BT gate drive circuit  5  having the above-mentioned structure, only when the column-selective signal cs is activated to a high level, the clocked inverters CINV 1  and CINV 2  are activated so that the clocked inverter CINV 1  outputs an inverted signal of the selective signal ss 0 , and that the clocked inverter CINV 2  outputs an inverted signal of the selective signal ss 1 . 
     Subsequently, the BT gate drive circuit  5  generates the control signal CR 0  according to the selective signal ss 0 , the control signal CR 0  being supplied to the precharge control circuit  7 , and generates the control signal CR 1  according to the selective signal ss 1 , the control signal CR 1  being supplied to the precharge control circuit  7 . In addition, the BT gate drive circuit  5  generates the BT gate drive signals GD 0  and GD 1  according to the selective signals ss 0  and ss 1 . 
     FIG. 5 is a circuit diagram showing a structure of the precharge control circuit  7  shown in FIG.  1 . As shown in FIG. 5, the precharge control circuit  7  comprises N-channel MOS transistors NT 23  and NT 24  and P-channel MOS transistors PT 15  and PT 16 . In this structure, the N-channel MOS transistors NT 23  and NT 24 , the P-channel MOS transistor PT 15 , a node supplying the boosted voltage VPP, and a ground node compose a clocked inverter CINV 3 . 
     The precharge control circuit  7  having the above-mentioned structure is activated by the control signal CR 1  supplied from the BT gate drive circuit  5 , and generates and outputs the precharge signal /PRE according to the control signal CR 0  supplied from the BT gate drive circuit  5 . 
     FIG. 6 is a circuit diagram showing a structure of the VDD precharge circuit  9  shown in FIG.  1 . As shown in FIG. 6, the VDD precharge circuit  9  comprises inverting circuits  24  and  25 , an AND circuit  27 , and P-channel MOS transistors PT 17  and PT 18 . In this structure, the inverting circuit  24  is supplied from the precharge control circuit  7  with the precharge signal /PRE, and the inverting circuit  25  is supplied with the mode-switchover signal wbiz. The VDD precharge circuit  9  supplies the power-source voltage VDD to the BT gate drive lines  17  and  19 , only when the precharge signal /PRE is at a low level and the mode-switchover signal wbiz is at a low level. 
     FIG. 7 is a circuit diagram showing a structure of the VPP precharge circuit  11  shown in FIG.  1 . As shown in FIG. 7, the VPP precharge circuit  11  comprises an inverting circuit  26  and P-channel MOS transistors PT 19  and PT 20 . In this structure, the inverting circuit  26  is supplied with the mode-switchover signal wbiz. The VPP precharge circuit  11  supplies the boosted voltage VPP to the BT gate drive lines  17  and  19 , only when the mode-switchover signal wbiz is at a high level. 
     Hereinbelow, a more detailed description will be given, with reference to FIG. 8, of operations of the semiconductor memory device according to the present embodiment. FIG. 8 is a timing diagram indicating operations of the semiconductor memory device according to the present embodiment. First, the mode-switchover signal wbiz is at a low level in a normal-operation mode until a time T 3 , as indicated by FIG.  8 -( e ). In standby states from an initial state to a time T 1  and from a time T 2  to the time T 3 , the selective signal ss 0  is at a low level, as indicated by FIG.  8 -( a ). The selective signal ss 1  is always at a low level until a time T 4 , as indicated by FIG.  8 -( b ). 
     In the above-mentioned standby states, the precharge control circuit  7  supplies the VDD precharge circuit  9  with the precharge signal /PRE activated at a low level so that the VDD precharge circuit  9  precharges the BT gate drive lines  17  and  19  with the power-source voltage VDD in the above-mentioned periods corresponding to the standby states (i.e., in a first state in the normal-operation mode). 
     In the period from the time T 1  to the time T 2 , in order to read or write data (i.e., in a second state in the normal-operation mode), first the precharge signal /PRE generated in the precharge control circuit  7  is raised to a high level in this period so as to deactivate the VDD precharge circuit  9  shown in FIG.  6 . 
     When data is to be read or written from or to the second memory cell array  3 , the BT gate drive circuit  5  increases the voltage of the BT gate drive signal GD 1  from the level of the power-source voltage VDD to the level of the boosted voltage VPP, as indicated by FIG.  8 -( d ), and decreases the voltage of the BT gate drive signal GD 0  from the level of the power-source voltage VDD to the level of a ground voltage VSS, as indicated by FIG.  8 -( c ). Thereby, the N-channel MOS transistors NT 1  to NT 4  of the Bt gate BTGa are turned off, and the N-channel MOS transistors NT 5  to NT 8  of the Bt gate BTGb are turned on so as to selectively open the Bt gate BTGb. 
     Accordingly, in the period from the time T 1  to the time T 2 , only the second memory cell array  3  is connected to the sense amplifiers  13  and  15  so that data is read or written from or to the second memory cell array  3 . 
     Next, at the time T 3  transiting from the normal-operation mode to the WLBI test mode, the mode-switchover signal wbiz is raised from the low level to a high level, as indicated by FIG.  8 -( e ). Thereby, the VDD precharge circuit  9  shown in FIG. 6 is deactivated, and the VPP precharge circuit  11  shown in FIG. 7 is activated so as to increase the voltages of the BT gate drive signals GD 0  and GD 1  from the level of the power-source voltage VDD to the level of the boosted voltage VPP, as indicated by FIG.  8 -( c ) and FIG.  8 -( d ). 
     Thereby, the gates of the N-channel MOS transistors NT 1  to NT 8  composing the Bt gates BTGa and BTGb are supplied with the boosted voltage VPP for performing a WLBI test. 
     As described above, in the semiconductor memory device according to the present embodiment, the boosted voltage VPP can be supplied only to the gates of the N-channel MOS transistors NT 1  to NT 8  composing the Bt gates BTGa and BTGb in a WLBI test. This avoids imposing unnecessary stress on the sense amplifiers  13  and  15  and controlling circuits thereof, more specifically, transistors and other devices included in these circuits, the transistors and other devices having a short-length gate and driven by the power-source voltage VDD. 
     Therefore, the semiconductor memory device according to the present embodiment enables a WLBI test to be sufficiently performed without imposing stress on undesired parts. This prevents the semiconductor memory device from deteriorating unnecessarily through the WLBI test. 
     Additionally, since the boosted voltage VPP can be supplied to all of the gates of the N-channel MOS transistors NT 1  to NT 8  composing the Bt gates BTGa and BTGb, a WLBI test can be performed to the first memory cell array  1  and the second memory cell array  3  at the same time; thus a WLBI test can be performed effectively. 
     Besides, in a conventional technology, in order to supply the boosted voltage VPP selectively to the Bt gate BTGa or BTGb, a circuit is required to be provided especially for performing the selection. However, the semiconductor memory device according to the present embodiment does not require such a circuit, enabling a WLBI test to be performed easily. 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 2000-259579 filed on Aug. 29, 2000, the entire contents of which are hereby incorporated by reference.