SEMICONDUCTOR MEMORY DEVICE

Abstract: A semiconductor memory device includesa plurality of memory cells (Mll to M44) arranged in rows and columns, a plurality of bit lines (BLl to BL4), each coupled to the memory cells forming one column, and a MOS transistor (QX) connected between a power supply terminal (Vpp) and one end of every bit line (BLl to BL4). The device further includes at least one test memory cell (DML; DMl to DM4) coupled in series with the MOS transistor (QX), thus forming a series circuit connected between the power supply terminal (Vpp) and the ground.

Description: The present invention relates to a semiconductor memory device, and more particularly, to a MOS type programmable read-only memory (ROM). Generally, each memory cell of the MOS type programmable ROM is a double-gate, transistor cell which has a floating gate l4 electrically isolated from other elements of the cell, as is illustrated in Fig. l. The transistor cell comprises p-type semiconductor substrate ll, n-type impurity regions l2 and l3 formed in the surface of substrate ll, floating gate l4 formed on an oxide film (not shown) formed on the surface of substrate ll, and control gate l5 formed on an oxide film (not shown) formed on floating gate l4. N-type impurity regions l2 and l3 function as a source region and a drain region, respectively, and set apart for a predetermined distance. Floating gate l4 is located above that portion of substrate ll separating regions l2 and l3. Control gate l5 is positioned above floating gate l4. In order to write data into the memory cell, a high voltage is applied between control gate l5 and drain region l3, thereby imparting great energy to the electrons moving between drain region l3 and source region l2. The energized electrons move into floating gate l4. In other words, the electrons are injected into floating gate l4. The speed of injecting the electrons into floating gate l4 has been raised by reducing the size of the memory cell, or by reducing the thickness of the oxide films formed on and under gate l4. The time required for writing data into the memory cell has therefore been shortened. However, the memory cell of the structure shown in Fig. l is disadvantageous. The electrons can move from floating gate l4 through the oxide film formed on or under this gate, when data is written into any other memory cell connected to the same word gate line or the same bit line. This is because, when the data is written in the other memory cell, a high voltage is inevitably applied to the control gate or drain region of the first memory cell. This undesired phenomenon will be explained in greater detail, with reference to Fig. 2. Fig. 2 shows a semiconductor memory device comprising word lines WLl and WL2, bit lines BLl, and BL2, and memory cells Mll, Ml2, M2l and M22 arranged in rows and columns. The control gates of memory cells Mll and Ml2, forming the first row, are coupled to word line WLl. The control gates of memory cells M2l and M22, forming the second row, are connected to word line WL2. The drains of memory cells Mll and M2l, forming the first column, are coupled to bit line BLl. The drains of memory cells Ml2 and M22, forming the second column, are coupled to bit line BL2. The memory device further comprises selection transistors Ql, Q2 ... Qn. These transistors Ql, Q2 ... Qn are connected at one end to bit lines BLl, BL2 ... BLn, respectively, and at the other end to writing-voltage terminal Vpp by load transistor QD. Transistor QD is controlled by control signal D. Now, assume that data is first written into memory cell Mll and then into memory cell 2l. First, a high voltage is applied between word line WLl and bit line BLl. All other word lines and all other bit lines are set at 0V. As a result, the data is written into memory cell Mll. Then, the high voltage is applied between word line WLl and bit line BL2, thereby writing data into memory cell M2l. This high voltage is applied to the control gate of memory cell Mll although the data need not be written in memory cell Mll. Consequently, the electrons may drain from the floating gate of memory cell Mll through the oxide film formed on the floating gate, under the influence of the electric field generated between the control gate and floating gate of memory cell Mll, unless the oxide film between the floating and control gates of memory cell Mll has a sufficient insulating strength. Assume that data is first written into memory cell Mll and then in cell Mll. Consequently, the electrons may move from the floating gate of cell Mll to the drain of cell Mll unless the oxide film formed between the drain and floating gate has a sufficient insulating strength. In either case, the data cannot be successfully written in memory cell ll. The longer the time for writing data in each memory cell, the more frequently the data cannot be written. Therefore, MOS type programmable ROMs are tested before they are delivered to the customers, by applying data-writing voltage to them for the time specified in the test manual, although the time actually required for writing data into the memory cells can be much shorter. This test procedure must be performed in order not to deliver MOS type programmable ROMs which have failed to store the data, due to the undersired phenomenon that electrons drain out of the floating gates of some memory cells. It has been proposed that a so-called "gate-stress circuit" or a so-called "drain-stress circuit" be used in order to check the phenomenon effectively. The gate-stress circuit responds to a special control signal supplied from an external device, and applies a high voltage to all word gate lines of the MOS type programmable ROM and also resets all bit lines of the ROM at 0V. After electrons have been injected into the floating gates of all memory cells of the ROM within a short data-writing time, the gate-stress circuit is operated to set the floating gates of the memory cells at 0V and to set the control gates of the memory cells at a high voltage level. Hence, it can be quickly determined whether or not electrons have drained out of the floating gates of the memory cells due to the insufficient insulating strength of the oxide film interposed between the floating gates, on the one hand, and the control gates, on the other. The drain-stress circuit applies a high voltage to all bit lines of the MOS type programmable ROM, in response to a control signal supplied from an external device. When the circuit applies the high voltage to the drain lines, all word lines are set at 0V. As a result, it can be determined whether or not electrons have drained from the floating gates of the memory cells due to the insufficient insulating strength of the oxide film provided between the floating gates, one the one hand, and the control gates, on the other. As has been stated, when the drain-stress circuit applies the high voltage to the drain lines, the control gates of all memory cells are set at 0V. Therefore, none of the cells are turned on, and the voltage applied to the drain lines is inevitably higher than the voltage which will be applied to the drain lines to write data in the cells. In the case of the memory device shown in Fig. 2, a high voltage is applied to the drain of memory cell Mll after data has been written in memory cell Mll, in order to write data in memory cell Ml2. At this time, memory cell Ml2 is turned on since a high voltage is applied to its control gate. The drain voltage of memory cell Ml2 is divided by the ratio of the resistance of load transistor QD to the resistance of memory cell M12. Here arises a problem. Due to the use of the drain-stress circuit, none of the memory cells are turned on and all cells are tested with a drain voltage higher than the data-writing drain voltage. It is therefore possible that the memory device is regarded as storing erroneous data, even if it stores the correct data when operated under the normal condition. Drain and gate stress circuits are mentioned in the article "Fast Programmable 256K Read Only Memory With On-Chip Test Circuits" by s. Atsumi et. al, IEEE Transactions on Electronic Devices, vol. ED-32, No. 2, February 1985, p. 506. It is reported that the circuits drastically reduce the test time for gate oxide leakage current in unselected cells during programming. Accordingly, the object of the present invention is to provide a semiconductor memory device which can be subjected to drain-stress test under the conditions substantially the same as those under which it is used. This object is achieved by a semiconductor memory device comprising a plurality of memory cells of the floating gate transistor type arranged in rows and columns; a plurality of bit lines, each coupled to the drains of the floating gate transistors of the memory cells forming one column; a plurality of word lines, each coupled to control gates of the memory cells forming one row; input buffer means for receiving address signals; selection means for selectively energizing said bit lines and said word lines in response to the output signals of said input buffer means; a power supply terminal; a reference potential terminal (ground); and switching means coupled at one end to said power supply terminal and at the other end to said bit lines, thereby to selectively couple said power supply terminal to said bit lines, characterized by further comprising a test circuit having at least one test memory cell having the same structure as said memory cells and coupled between the bit line and the reference potential terminal, thus forming in combination with said switching means (19) a series circuit connected between said power supply terminal and said reference potential terminal (ground); and control means for turning on said test memory cell while said memory cells are undergoing a drain-stress test. During the drain-stress test, the test memory cell is turned on. Hence, all memory cells are put under the conditions substantially the same as those under which data is written in the memory cells. The drain-stress test can, therefore, be readily conducted on the memory cells. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. l schematically shows a memory cell; Fig. 2 is a circuit diagram of a conventional, semiconductor memory device which can be subjected to a drain-stress test; Fig. 3 is a circuit diagram of a semiconductor memory device according to the invention, which can be reliably subjected to a drain-stress test; and Fig. 4 is a circuit diagram of another semiconductor memory device according to the invention. One embodiment of the invention, i.e., a semiconductor memory device, will now be described with reference to Fig. 3. For brebity, this device is a l6-bit memory. Nonetheless, the memory capacity of the device is not limited to this particular value. As is shown in Fig. 3, a memory cell array MCA has memory cells Mll to M44 arranged in rows and columns. The control gates of memory cells Mll, M2l, M3l, and M4l, forming the first row, are coupled to word line WLl. The control gates of memory cells Ml2, M22, M32 and M42, forming the second row, are coupled to word line WL2. Those of cells Ml3, M23, M33 and M43, forming the third row, are coupled to word line WL3. And those of cells Ml4, M24, M34, and M44, forming the fourth or last row, are coupled to word line WL4. The drains of memory cells Mll, M2l, M3l, and M4l, forming the first column, are coupled to bit line BLl. The drains of memory cells M2l, M22, M23, M24, forming the second column, are coupled to bit line BL3. And the drains of memory cells M4l, M42, M43, and M44, forming the fourth or last column, are coupled to bit line BL4. Address signals Addl to Add4 supplied to address input terminals Al to A4 are input to input buffers l6₁ to l6₄. Two outputs BOl and BO2 of input buffer l6₁ are supplied to Y-decoder l7. The outputs of input buffers l6₃ and l6₄, and output BO2 are supplied to X-decoder l8. during the ordinary use of the memory device, Y-decoder l7 decodes the output BOl of input buffer l6₁ and the output of input buffer l6₂, whereby one of word lines WLl to WL4 is selected also during the ordinary use of the device, X-decoder l8 decodes the outputs of input buffers l6₃ and l6₄, thus turning on one of transistors Ql to Q4 for selecting the bit lines, whereby one of bit lines BLl to BL4 is selected. As a result, one memory cell Mij (i = l to 4, j = l to 4) is selected from the memory cell array MCA. Transistors Ql to Q4 are connected at one end to a connection node. A data-writing voltage Vpp is applied to the connection node of these transistors Ql to Q4 through write/non-write switching circuit l9. Circuit l9 includes MOS transistor QX which is turned on and off by write-control signal WCS supplied from an external device. Test memory cell DM, having the same structure as memory cells Mll to M44, is connected between the ground and the connection node of transistors Ql to Q4. Memory cell DM is turned on and off by the output of switch circuit 2l. Switch circuit 2l is controlled by output BO2 of input buffer l6₁ to determine whether or not voltage Vpp should be applied to test memory cell DM. Output BO2 of input buffer l6₁ is supplied to both Y-decoder l7 and X-decoder l8, as has been described above. Hence, when output BO2 is at the high level, all bit lines BLl to BL4 and all word lines WLl and WL2 are energized. These transistors WQl and WQ2 form a word line-driving circuit for applying data-writing voltage Vpp to any selected word line. Like MOS transistor QX of switching circuit l9, transistors WQl and WQ2 may be controlled by signal WCS. Input buffer l6₁ has two threshold voltages, for example, 2.5V and 5.5V. Each of address signals Addl to Add4 can be at one low level and two high levels. The low level is about 0V. The first high level ranges from 3.0V to 5.0V, and the second high level is over 6.0V. When address signal Add1 supplied to address input terminal A1 is at the low level, the output BO1 of input buffer 16₁ is at the "0" level. When address signal Add1 is at the first high level, the output BO1 is at the "1" level. When signal Add1 is at the second high level, the output BO2 of input buffer 16₁ is at the "H" level. Table 1 shows the relationship between the input and output of input buffer 16₁. Output BO1 , which is an inversion of output BO1 and can be used in place of output BO1, is also shown in this table. Input Level (Add1) Signal Level BO1 ( BO1 ) BO2 Mode Low level 0 0 (1) 0 Output First high level 1 1 (0) 0 Output Second high level H' * (*) H Cell select In Table 1, the "0" level is nearly equal to 0V, the "1" level is a high level close to 5V, the "H'" level is a high level over 6V, and "H" level is higher than the low level close to 0V. The mark "*" means that the level is either "0" or "1", in accordance with the circuit design. X-decoder 18 can produce four outputs 1X to 4X in the data-writing mode. (In the data-writing mode, address signal Add1 supplied to address input terminal A1 can be at the "H'" level.) Y-decoder 17 can generate four outputs 1Y to 4Y in the data-writing mode, too. Table 2 shows the relationship between address signals Add1, Add3 and Add4, on one hand, and the outputs 1X to 4X of X-decoder 18, on the other. Table 3 shows the relationship between address signals Add1 and Add2, on one hand, and the outputs 1Y to 4Y of Y-decoder 17, on the other. Add3 Add4 Add1 1X 2X 3X 4X 0 0 0 H 0 0 0 0 1 0 0 H 0 0 1 0 0 0 0 H 0 1 1 0 0 0 0 H 0 0 H' H H H H 0 1 H' H H H H 1 0 H' H H H H 1 1 H' H H H H In Table 2, it is also possible to use address signal Add1 of "1" instead of "0". Add1 Add2 1Y 2Y 3Y 4Y 0 0 H 0 0 0 0 1 0 H 0 0 1 0 0 0 H 0 1 1 0 0 0 H H' 0 0 0 0 0 H' 1 0 0 0 0 When signal Add1 supplied to address input terminal A1 is set at the "H" level after the ordinary data-writing, all outputs 1X to 4X supplied to bit lines BL1 to BL4 are set at the "H" level. Simultaneously, the potentials of all word lines WL1 to WL4, i.e., the outputs 1Y to 4Y of Y-decoder 17, are 0V. The memory device is, therefore, subjected to the drain-stress test. In this condition, switch circuit 21 is turned on by the output BO2 of input buffer l6₁, which is at the "H" level. As a result, data-writing voltage Vpp is applied to the control gate of test memory cell DM. When write control signal WCS is supplied to write/non-write switching circuit l9 in this condition, voltage Vpp is applied to the drain of cell in this condition, voltage Vpp is applied to the drain of memory cell DM through MOS transistor QX. Now that voltage Vpp is applied to the drain and control gate of test memory cell DM, memory cell DM is turned on. The voltage applied to the drains of memory cell Mll to M44 has the value obtained by dividing voltage Vpp by the ratio of the resistance of transistor QX to the resistance of cell DM, said value being the ordinary data-writing voltage. That is, the memory device can be tested under the conditions substantially the same as those under which it is used. Hence, there is no risk that the device is regarded as storing erroneous data even if it stores the correct data when operated under the normal condition. Fig. 4 shows another embodiment of the present invention. In this embodiment, test memory cells DMl to DM4 are connected to bit lines BLl to BL4, whereas, in the embodiment of Fig. 3, only one test memory cell DM is provided at the output block. In Fig. 4, the same numerals and symbols are used to designate the same elements as those shown in Fig. 3, which will not be described in detail. Test memory cells DMl to DM4 are turned on by an output BO2 at the "H" level when the memory device (Fig. 4) is put to the drain-stress test. Therefore, the potential of each of bit lines BLl to BL4 is set at the value obtained by dividing data-writing voltage Vpp by the ratio of the resistance of MOS transistor QX to the resistance of the test memory cell coupled to the bit line. The memory device shown in Fig. 4 performs, in principle, the same operation as the device of Fig. 3, and hence achieve the same advantage. The present invention is not limited to the embodiments described above. For instance, switch circuit 2l can be dispensed with in the embodiment shown in Fig. 3, and the output BO2 of input buffer l6₁ can be used to control test memory cell DM. Moreover, switch circuit 2l (Fig. 3) can be used in the embodiment shown in Fig. 4, in order to control test memory cells DMl to DM4.

Claims: A semiconductor memory device comprising a plurality of memory cells (M11 to M44) of the floating gate transistor type arranged in rows and columns; a plurality of bit lines (BL1 to BL4), each coupled to the drains of floating gate transistors of the memory cells forming one column; a plurality of word lines (WL1 to WL4), each coupled to control gates of the memory cells forming one row; input buffer means (16₁ to 16₄) for receiving address signals; selection means (17 and 18) for selectively energizing said bit lines (BL1 to BL4) and said word lines (WL1 to WL4) in response to the output signals of said input buffer means (16₁ to 16₄); a power supply terminal (Vpp); a reference potential terminal (ground); and switching means (19) coupled at one end to said power supply terminal (Vpp) and at the other end to said bit lines (BL1 to BL4), thereby to selectively couple said power supply terminal (Vpp) to said bit lines (BL1 to BL4), characterized by further comprising a test circuit having at least one test memory cell (DM;DM1 to DM4) having the same structure as said memory cells and coupled between the bit line and the reference potential terminal thus forming in combination with said switching means (19) a series circuit connected between said power supply terminal (Vpp) and said reference potential terminal (ground); and control means (16₁ and 21) for turning on said test memory cell (DM; DM1 to DM4) while said memory cells (M11 to M44) are undergoing a drain-stress test. A semiconductor memory device according to claim 1, characterized in that said switching means (19) comprises a MOS transistor (QX). A semiconductor memory device according to claim 1 or 2, characterized in that said input buffer means (16₁ to 16₄) has a plurality of input buffers (16₁ to 16₄), a specified one of which turns on said test memory cell (DM; DM1 to DM4) upon receipt of an address signal at a level higher than a predetermined level. A semiconductor memory device according to claim 1, characterized in that said test circuit includes a memory cell (DM) coupled between the other end of said switching means (19) and said reference potential terminal (ground), and having a control gate coupled to said control means (16₁ and 21). A semiconductor memory device according to claim 1, 2 or 3, characterized in that said test circuit includes a plurality of test memory cells (DM1 to DM4) coupled at one end to said bit lines (BL1 to BL4), and at the other end to said reference potential terminal (ground). A semiconductor memory device according to claim 3 or 4, characterized in that said control means comprises said specified input buffer (16₁), and a switch (21) coupled between said power supply terminal (Vpp) and the control gate of said test memory cell (DM), said switch (21) being turned on and off by the output (BO2) of said specified input buffer (16₁).

Inventors: OKUDA, TAIZO C/O PATENT DIVISION, SAITO, SHINJI C/O PATENT DIVISION, SHISHIKURA, NOBUO C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1987
Main classification: G11C29
IPC codes: G11C16, G11C17, G11C29