Sense amplifier circuit of a nonvolatile semiconductor memory device

Abstract: Disclosed is a sense amplifier circuit of nonvolatile semiconductor memory device with NAND structured cells, comprising a bit line isolation section (12) located between a pair of bit lines (BLi,BLj) connected to a memory cell array (100) and a pair of sub-bit lines (SBLi,SBLj) connected to an input/output gate circuit (200), a latch type voltage controlled current source (13) having n-channel MOS transistors (Q21,Q22) connected to the sub-bit lines (SBLi,SBLj), and a switching section (14) connected between the voltage-controlled current source (13) and a signal line (VSA), wherein the bit lines (BLi,BLj) are electrically and completely isolated from the sub-bit lines (SBLi,SBLj) by means of the bit line isolation section (12) receiving an isolation control signal (ISA) during the sensing operation. With the circuit, the sensing operation is not affected by a bit line load impedance, and thereby the sensing speed is improved in addition to the reduction of a peak current along the data path therein.

Description: The present invention relates to a sense amplifier circuit of nonvolatile semiconductor memory device, and particularly but not exclusively to a sense amplifier circuit in a flash electrically erasable and programmable read-only memory (hereinafter referred to as "EEPROM") with NAND structured cells. Hard disk, which is mainly used as an auxiliary memory device in a battery operated computer system such as a portable microcomputer of a notebook size, occupies relatively too large an area of the system. For this reason, the designers of such a computer system are highly interested in the development of EEPROMs that are to be of a high density and an excellent performance,not taking up too much space. A small size of memory cells is a key point of this problem. Thus, an EEPROM with NAND structured cells is devised so as to make it possible to reduce the number of the selection transistors and the contact holes to a bit line per a cell. An improved structure of this NAND EEPROM and the art for erasing and programming the same EEPROM are introduced under the title of "A NAND STRUCTURED CELL WITH A NEW PROGRAMMING TECHNOLOGY FOR HIGHLY RELIABLE 5V-ONLY FLASH EEPROM" on pages 129-130 of the Symposium on VLSI Technology published in 1990. The NAND EEPROM reported in the paper has an operation mode to simultaneously erase the information programmed in the memory cells, which EPPROM is usually called a flash EEPROM or a flash memory. EP-A-0618589 also discloses a NAND, EEPROM device and in particular discloses a semiconductor nonvolatile memory device which adopts a differential type sensing system comprising first and second bit lines connected in parallel with respect to a sense amplifier, comprising a first memory cell which is connected to a word line and a first bit line, a second memory cell which is connected to a word line common with said first memory cell and, connected to said second bit line, and a means for holding the potential of either bit line of said first and second bit lines at a first potential at the time of a predetermined operation, and, setting the potential of the other bit line at a second potential made to have a difference from said first potential for a predetermined time. In a commonly used flash memory, zero voltage is applied to a gate terminal, i.e. a word line, of selected cell transistor during the read operation therein. The bit line connected the selected cell transistor is maintained at an initial precharge level in case that the selected cell transistor is programmed to have a positive threshold voltage. On the contrary, if the cell transistor is programmed to have a negative threshold voltage, the bit line is discharged to ground level. The sense amplifier used in this flash memory can sense and amplify a very small difference between the level on the bit line and the reference bit line, the bit line being maintained to a precharge level or changed to a ground voltage level during the read operation thereof. Fig. 1 illustrates an example of a prior art sense amplifier circuit used in a NAND flash EEPROM with folded bit line architecture, wherein a sense amplifier circuit 1 is located between a cell array 100 and an input/output gate circuit 200. The cell array 100 includes a reference cell array for providing a reference voltage and the input/output gate circuit 200 is provided to transmit the data amplified by the sense amplifier circuit 1 to the input/output lines. In the sense amplifier circuit 1, a bit line isolation section 2 is located between the bit lines BLi and BLj connected to the cell array 100 and the sub-bit lines SBLi and SBLj connected to the input/output gate circuit 200. The bit line isolation section 2 has the n-channel metal oxide semiconductor field effect transistors Q1 and Q2 for selectively isolating the bit lines BLi and BLj from the sub-bit lines SBLi and SBLj. The sub-bit lines BLi and SBLj are connected to an n-latch sense amplifier section 3 having n-channel MOS transistors Q3 and Q4, a p-latch sense amplifier section 4 having p-channel MOS transistors Q5 and Q6, a precharging section 5 having p-channel MOS transistors Q7 and Q8, and an equalizing section 6 having a p-channel MOS transistor Q9, as shown in Fig. 1. To describe how this sense amplifier circuit operates, it is assumed that, for convenient description, a voltage developed in an erased cell which is selected is applied to a bit line BLi and a read reference voltage obtained from a reference cell which is selected is applied to a bit line BLj. Fig. 2 is a timing chart illustrating a sensing operation of the sense amplifier circuit. In Fig. 2, the bit lines BLi and BLj and the latch amplifier driving lines LA and /LA are precharged to a source voltage V cc , and the sub-bit lines SBLi and SBLj are also precharged to V cc by a precharge control signal /ΦEQ. When the voltages of the bit lines BLi and BLj sufficiently develop according to a cell selection, an isolation control signal ISO of pulse waveform is generated. With this signal ISO, the isolation transistors Q1 and Q2 are turned on and the voltage of the n-latch sense amplifier driving line /LA decreases to the level of ground voltage (GND), or 0V. To prevent the voltage drop of the bit lines by the isolation transistors Q1 and Q2, the isolation control signal ISO of high level has to be maintained over V cc + 2V tn , where V tn indicates the threshold voltage of the n-channel MOS transistor. When the isolation transistors Q1 and Q2 are turned on, the voltages of sub-bit lines are changed owing to the charge sharing between the bit lines BLi and BLj and the sub-bit lines SBLi and SBLj. As the voltage of the sub-bit line SBLi which is in charge sharing with the bit line BLi electrically connected to the erased cells are decreased relatively quickly, the transistors Q3 and Q6 are turned on. As a result, the voltage of the sub-bit line SBLi falls down to ground level by the latch amplifier driving lines LA and /LA, and that of the sub-bit line SBLj pulls up to V cc level. The very small difference between potential level on the bit lines BLi and BLj is sensed and amplified by the sense amplifier circuit 1 and transmitted to the input/output gate circuit 200. Hence, when the above-described sense amplifier circuit 1 performs a sensing operation, there may be some limits that the isolation control signal ISO is a pulse waveform and its high level is to be controlled over V cc + 2V tn . Additionally, the sense amplifier circuit 1 is affected by the bit line load in the cell array, which causes the lowering of sensing speed and the increase in peak current. The wording "bit line load" means the load impedance along the data path (i.e., bit lines) at the side of the cell array. Only the normal sensing operation is performed in the sense amplifier circuit 1, but the inverted sensing operation where the data memorized in a selected cell is inversely outputted can not be performed in the circuit, which limits the design of the input/output circuit. Fig. 3 shows another conventional sense amplifier circuit used in a NAND flash EEPROM with folded bit line architecture. In Fig. 3, the EEPROM has a cell array 100 which is located between a precharging section 7 having p-channel MOS transistors Q10 and Q11 and an equalizing section 8 having an n-channel MOS transistor Q12. Between bit lines BLi and BLj and sub-bit lines SBLi and SBLj, a bit line isolation section 9 having n-channel MOS transistors Q13 and Q14 is connected. Electrical connection of the bit lines to the sub-bit lines is selectively performed by the bit line isolation section 9. The sub-bit lines SBLi and SBLj are commonly connected to an n-latch sense amplifier section 10 having two n-channel MOS transistors Q15 and Q16 and a p-latch sense amplifier section 11 having two p-channel MOS transistors Q17 and Q18. Drain terminals of the transistors Q15 and Q16 are commonly connected to a latch amplifier driving line V sal and the sources of the transistors Q17 and Q18 are commonly connected to a latch amplifier driving line V sah . Fig. 4 is a timing chart showing the sensing operation in the read mode of the sense amplifier shown in Fig. 3. With reference to Fig. 4, a voltage of the bit line BLi develops in accordance with the state of a cell which is selected by a word line signal WL. Then, if the selected cell is an off-cell, or a programmed cell, the cell transistor selected thus has positive threshold voltage. Accordingly, the selected cell transistor is turned off when a word line selection signal of zero volts is applied to the gate thereof, and the voltage of the bit line BLi is maintained to the precharge level. However, if the selected cell is an on-cell, or an erased cell, the selected cell transistor has negative threshold voltage to be turned on even though a word line selection signal of zero volts is applied to the gate thereof. As a result, the voltage of the bit line BLi is lower than that of the bit line BLj connected to a reference cell. And then, isolation control signals ISOi and ISOj of predetermined pulse duration are generated and therefore the isolation transistors Q13 and Q14 are turned on. While the isolation control signals ISOi and ISOj of pulse waveform are activated, the voltage of n-latch sense amplifier driving line V sal is lowered from a precharge level of high level to a low level and that of p-latch sense amplifier driving line V sah is increased from a precharge level of low level to a high level. Thus, the data of the bit line BLi are detected by the sense amplifier sections 10 and 11. It may be known, therefore, that the minute difference between level on the bit lines BLi and BLj is amplified by the sense amplifier circuit 1 and transmitted to the input/output gate circuit 200. Fig. 5 is a timing chart illustrating the sensing operation of the sense amplifier circuit in program verification and inhibition modes of the NAND flash memory shown in Fig. 3. Herein, the program verification means that, when a negative threshold voltage of a selected on-cell shifts to a positive threshold voltage, a programming operation continues to be performed until the threshold voltage detected from the on-cell becomes a preset threshold voltage. The program inhibition means that unselected cells are not programmed during the programming operation. In order that the on-cell connected to the selected bit line BLi is not programmed during the programming operation, the voltage of the sub-bit line SBLi corresponding to the selected bit line BLi has to be maintained to V cc . As shown in Fig. 5, the isolation control signal ISOj is activated under the condition that the sub-bit lines SBLi and SBLj are precharged respectively to V cc and GND level (zero Volt) by an externally applied data signal from the I/O gate circuit 200 during the program inhibition, and then the unselected bit line BLj connected to the reference cell is discharged to have GND level. At this time, the latch driving signals V sal and V sah are inactivated. Thus, in the following read operation, because the voltage of the sub-line SBLj is previously set to GND level even though a voltage of the bit line BLi connected to the on-cell is lowered to low level, the voltage of the sub-line SBLi is maintained to V cc level. Also, in the programming operation of on-cell, or erased cell, connected to the selected bit line BLi, the sub-bit lines SBLi and SBLj are precharged respectively to GND and V cc level by the externally applied data signal, and the same operation as in the program verification is carried out. As the voltage of the sub-bit line SBLi shifts from GND level to V cc level in the following read operation of cells, the program inhibition is effective and then the programming operation is automatically terminated. During a sensing operation of bit line, there are changes in the sensing margin and the decoupling margin between the bit lines and the sub-bit lines according to the process parameters and the overlapping degree of the isolation control signals ISOi and ISOj and the latch driving signals V sal and V sah . Moreover, since the isolation transistor Q14 connected to the bit line BLj has to be previously turned on during the program verification and inhibition, there are problems that an additional control logic is required for the sense amplifier circuit and the time required for the program verification is prolonged. Therefore, it is an aim of embodiments of the invention to provide a sense amplifier circuit of a nonvolatile semiconductor memory device with folded bit line architecture, in which a stable sensing operation may be performed without using any bit line isolation signal of pulse waveform. It is another aim of embodiments of the invention to provide a sense amplifier circuit which is not affected by the load impedance of the bit line connected to memory cells in a nonvolatile semiconductor memory device with folded bit line architecture. It is a further aim of embodiments of the invention to provide a sense amplifier circuit of low power consumption. It is another aim of embodiments of the invention to provide a sense amplifier circuit for a flash memory device which is capable not only of a normal sensing operation but also of inverted sensing operation. It is still further an aim of embodiments of the invention to provide a sense amplifier circuit for a flash memory device which is suitable to minimize the changes in decoupling and sensing margins during a sensing operation of bit lines. It is another aim of embodiments of the invention to provide a sense amplifier circuit for a flash memory device which is capable of carrying out a high speed sensing operation. It is still another aim of embodiments of the invention to provide a sense amplifier circuit for a flash memory device which is suitable for program verification and inhibition. According to one aspect of the present invention there is provided a nonvoltatile semiconductor memory device having a sense amplifier circuit and a NAND structured memory cell array for storing information and a reference cell array for providing a reference voltage, said circuit comprising a pair of bit lines comprising a first and second bit line connected to said NAND structured memory cell array and said reference cell array, respectively, a pair of sub-bit lines comprising a first and second sub-line corresponding to said bit lines, respectively, bit line isolating means for electrically and selectively isolating said bit lines from said sub-bit lines in response to an externally applied isolation control signal during a precharging period and a sensing period, means for precharging and equalizing said sub-bit lines during a precharging period, a signal line for receiving an externally applied voltage signal, a voltage-controlled current source having first and second current paths connected to said first and second sub-bit lines respectively, whereby a current amount flowing through said second current path is adjusted in accordance with a voltage level of said first bit line and a current amount flowing through said first current path is adjusted in accordance with a voltage level of the said second bit line, switching means for electrically and selectively connecting at least one of said first and second current paths to said signal line in response to a switching control signal, and amplifier means for amplifying voltage levels of said sub-bit lines to first and second predetermined voltage levels, respectively when a potential difference between, said sub-bit lines is more than a predetermined voltage. For a better understanding of the present invention and as to how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: Fig. 1 is a circuit diagram showing a prior art sense amplifier for a nonvolatile semiconductor memory device; Fig. 2 is a timing chart showing the sensing operation of the sense amplifier shown in Fig. 1; Fig. 3 is a circuit diagram showing another prior art sense amplifier for a nonvolatile semiconductor memory device; Fig. 4 is a timing chart showing the sensing operation in a read mode of the sense amplifier shown in Fig. 3; Fig. 5 is a timing chart showing the sensing operation in program verification and inhibition modes of the sense amplifier shown in Fig. 3; Fig. 6 is a circuit diagram showing the construction of a sense amplifier for a nonvolatile semiconductor memory device in accordance with an embodiment of the present invention; Fig. 7 is a timing chart showing a normal sensing operation of the sense amplifier shown in Fig. 6; Fig. 8 is a timing chart showing an inverted sensing operation of the sense amplifier shown in Fig. 6; Fig. 9 is a diagram showing waveforms of the bit line voltage as a result of simulation of the normal sensing operation of the sense amplifier shown in Fig. 6; Fig. 10 is a diagram showing waveforms of the bit line voltage as a result of simulation of the inverted sensing operation of the sense amplifier shown in Fig. 6; Fig. 11 is a circuit diagram showing the construction of a sense amplifier for a nonvolatile semiconductor memory device in accordance with another embodiment of the present invention; Fig. 12 is a timing chart showing the sensing operation in a read mode of the sense amplifier shown in Fig. 11; Fig. 13 is a timing chart showing the sensing operation in program verification and inhibition modes of the sense amplifier shown in Fig. 11; Fig. 14 is a circuit diagram showing the construction of a sense amplifier for a nonvolatile semiconductor memory device in accordance with a further embodiment of the present invention; Fig. 15 is a timing chart showing the sensing operation in a read mode of the sense amplifier shown in Fig. 14; and Fig. 16 is a timing chart showing the sensing operation in program verification and inhibition modes of the sense amplifier of Fig. 14. For convenient description, assuming that a voltage developed in an erased cell which is selected is applied to a bit line BLi and a read reference voltage obtained from a reference cell which is selected is applied to a bit line BLj. Example 1 Fig. 6 shows a sense amplifier circuit for NAND flash memory device with folded bit line architecture according to a preferred embodiment of the present invention. In Fig. 6, the sense amplifier circuit is located between an input/output gate circuit 200 and a cell array 100 which includes a NAND structured memory cell array for storing input data and a reference cell array for providing a reference voltage to bit lines. The sense amplifier circuit has a bit line isolating section 12 which is constituted by two n-channel MOS transistors Q19 and Q20 and located between bit lines BLi and BLj and sub-bit lines SBLi and SBLj. The bit lines BLi and BLj are connected to the cell array 100 and the sub-bit lines SBLi and SBLj are connected to the input/output gate circuit 200. To gates of the transistors Q19 and Q20, an isolation control signal is applied. The respective bit lines BLi and BLj are electrically and selectively isolated from the sub-bit lines SBLi and SBLj by the bit line isolating section 12. The sub-bit lines SBLi and SBLj are connected to a voltage-controlled current source 13 of latch structure having n-channel MOS transistors Q21 and Q22, and a switching section 14 having n-channel MOS transistors Q23 and Q24 is connected between the voltage-controlled current source 13 and a signal line VSA. Drain and source of the transistor Q21 are connected respectively to the sub-bit line SBLi and drain of the transistor Q21, and gate of the transistor Q21 is commonly connected to the bit line BLj and drain of the isolation transistor Q20. Drain and source of the transistor Q22 are connected respectively to the sub-bit line SBLj and drain of the transistor Q24, and gate thereof is commonly connected to the bit line BLi and drain of the isolation transistor Q19. Sources of the transistors Q23 and Q24 of the switching portion 14 are commonly connected to the signal line VSA and gates thereof receive a switching control signal ΦSA of pulse waveform. Additionally, the sub-bit lines SBLi and SBLj are connected to an n-latch sense amplifier section 15 having n-channel MOS transistors Q25 and Q26, a p-latch sense amplifier section 16 having p-channel MOS transistors Q27 and Q28, a precharging section 17 having n-channel MOS transistors Q29 and Q30, and an equalizing section 18 comprising n-channel MOS transistor Q31. Referring to Fig. 7, the bit lines BLi and BLj are precharged to V cc level, and the sub-bit lines SBLi and SBLj are also precharged to V cc level by the precharging section 17 and equalizing section 18 while the precharging and equalizing control signal ΦEQ is maintained to the level of V cc +2V tn , where V tn represents the threshold voltage of n-channel MOS transistor. During this sensing operation, the voltages of the signal line VSA and the p-latch sense amplifier driving line LA are precharged respectively to GND and V cc level, and the n-latch sense amplifier driving line /LA is maintained to the level of V cc before the switching control signal ΦSA of a pulse are generated. In this condition, a memory cell is selected by a word line signal WL. According to the state of the selected memory cell, for example on-cell or off-cell, the respective voltages of the bit lines develop. Then, when there is a potential difference between the bit lines BLi and BLj, that is, if the voltage of the bit line BLi gets lower than that of the bit line BLj, the switching control signal ΦSA is activated. As a result, a sensing operation for detecting the potential difference therebetween is started. If the switching control signal ΦSA is shifted to high level, all the transistors Q23 and Q24 are turned on and the sub-bit line SBLi is discharged faster than the sub-bit line SBLj by the transistor Q21, because the voltage of the bit line BLj is higher than that of the bit line BLi. As a result, the voltage of SBLi is lower than that of SBLj. The voltage of the n-latch sense amplifier driving line /LA shifts from V cc to GND level and that of the p-latch sense amplifier driving line LA is maintained to the level of V cc , so that the transistors Q25 and Q28 are turned on and the voltages of the sub-bit lines SBLi and SBLj is shifted respectively to GND and V cc . In such manner, the minute potential difference between the bit lines BLi and BLj is amplified by the sense amplifier circuit. As shown again in Fig. 7, the sensing operation is performed under the condition that the signal line VSA is precharged to GND level and the sub-bit lines SBLi and SBLj are precharged to the level of V cc . It is understood, for example, in Fig. 7 that the sub-bit lines SBLi and SBLj are developed respectively to GND and V cc in case of the on-cell data sensing operation in which the bit lines BLi and BLj are developed respectively to GND level and the reference voltage V ref . As known in the above description, the data having the same as the phase of the cell data is accessed by the sense amplifier circuit embodying the present invention. In the first embodiment, the switching control signal ΦSA has a pulse waveform as shown in Fig. 7 so as not to continuously form a DC current path from the sub-bit lines SBLi and SBLj through the voltage-controlled current source 13 and the switching section 14 to the GND (that is the signal line VSA) by developed bit lines BLi and BLj after substantial sensing operation. Also, because the isolation control signal ISO is inactivated and maintained to GND level during the sensing operation, the bit lines BLi and BLj are electrically and completely isolated from the sense amplifier circuit, so that the sensing operation is not affected at all by the bit line load. As a result, the sensing speed is further improved and the peak current is reduced. In Fig. 8, a timing chart for the inverted sensing operation of the sense amplifier circuit is illustrated. As shown in Fig. 8, the bit lines BLi and BLj are precharged to level of V cc . While the precharging and equalizing control signal ΦEQ is maintained to V cc + 2V tn , the sub-bit lines SBLi and SBLj are precharged to GND level by the precharging section 17 and equalizing section 18. During such inverted read operation, the signal line VSA and the p-latch sense amplifier driving line LA are maintained respectively to V cc and GND level, and the both latch amplifier driving lines LA and /LA continue to be maintained to level of GND before the switching control signal ΦSA is generated as a pulse, so that a memory cell is selected by the word line signal WL. As a potential difference between the bit lines BLi and BLj is generated, the switching control signal ΦSA is activated. If the switching control signal ΦSA is shifted from low level to high level, the transistors Q23 and Q24 are turned on and the sensing operation is started. Because the voltage of the bit line BLj then is higher than that of the bit line BLi, the sub-bit line SBLi is charged faster than SBLj. As a result, the voltage of SBLi is higher than that of SBLj. At this time, the voltage of the p-latch sense amplifier driving line LA also is shifted from GND level to V cc and that of the n-latch sense amplifier driving line /LA continues to be maintained to level of GND, so that the transistors Q26 and Q27 of the voltage-controlled current sources 15 and 16 are turned on. Thus, the voltages of the sub-bit lines SBLi and SBLj are developed respectively to V cc and GND level and a very small difference between potential level on the bit lines BLi and BLj is sensed and amplified by the sense amplifier circuit. As above mentioned, the sensing operation is performed under the condition that the signal line VSA is precharged to level of V cc and the sub-bit lines SBLi and SBLj are precharged to GND, as shown in Fig. 8. As the sub-bit lines SBLi and SBLj are developed respectively to GND and V cc level in case of the sensing of on-cell data in which the bit lines BLi and BLj is developed respectively to GND level and the reference voltage V ref . As can be seen in the above description, the data which is in the opposite phase to the cell data may be accessed by the sense amplifier circuit according to the first embodiment of the present invention. With reference to Figs. 9 and 10, voltage waveforms of the bit lines are shown as a result of the simulation of the normal and the inverted sensing operation in the sense amplifier circuit of the invention. The simulation is performed under the condition that the power voltage V cc is 3.8V and the temperature is about -5 °C. The sense amplifier circuit and the cell array stage connected through the bit lines BLi and BLj are electrically isolated during the above sensing operation, so that the sensing speed can be further faster improved and a peak current hardly flows through the sense amplifier circuit. As described above, the sense amplifier circuit according to the embodiment of the present invention may carry out a stable sensing operation at a further higher speed without using a bit line isolation signal of pulse waveform to the bit line isolation section. Also a peak current flowing through the sense amplifier circuit is larger reduced. Because both the normal and the inverted sensing operations can be performed, designing a nonvolatile memory device is not limited. Example 2 Fig. 11 shows a sense amplifier circuit for NAND flash memory device with folded bit line architecture according to a second embodiment of the present invention. Referring to Fig. 11, the sense amplifier circuit is located between an input/output gate circuit 200 and a cell array 100 having a memory cell array and a reference cell array. The circuit has a bit line isolation section 21 which is constituted by n-channel MOS transistors Q35 and Q36 and located between bit lines BLi and BLj and sub-bit lines SBLi and SBLj. The bit lines BLi and BLj are connected to the cell array 100 and the sub-bit lines SBLi and SBLj are connected to the input/output gate circuit 200. In the circuit of Fig. 11, to the bit lines BLi and BLj, a precharging section 19 having p-channel MOS transistors Q32 and Q33, and an equalizing section 20 having an n-channel MOS transistor Q34 are connected. To the sub-bit lines SBLi and SBLj, a voltage-controlled current source 22 having n-channel MOS transistors Q37 and Q38 is connected. Between the voltage-controlled current source 22 and a ground (or, a potential lower than V cc ), a switching section 23 is connected. The switching section 23 is constituted by two n-channel MOS transistors Q39 and Q40. In the voltage-controlled current source 22, drain and source of the transistor Q37 are connected respectively to the sub-bit line SBLi and drain of the transistor Q39, and the gate thereof is commonly to the bit line BLj and drain of the isolation transistor Q36. Drain and source of the transistor Q38 are connected respectively to the sub-bit line SBLj and drain of the transistor Q40, and the gate thereof is commonly to the bit line BLi and the drain of the isolation transistor Q35. Sources of the transistors Q39 and Q40 are commonly connected to a ground or a terminal for receiving a voltage lower than V cc level, and the gates thereof receive switching control signals ΦSAi and ΦSAj of pulse waveform. To the sub-bit lines SBLi and SBLj, an n-latch sense amplifier section 24 having n-channel MOS transistors Q41 and Q42, and a p-latch sense amplifier section 25 having p-channel MOS transistors Q43 and Q44 are commonly connected. The sensing operation of the sense amplifier circuit of Fig. 11 will be described hereinafter with reference to a timing chart of Fig. 12. As shown in Fig. 12, the bit lines BLi and BLj are precharged to level of V cc while a word line signal WL is at low level. Then, the isolation control signals ISOi and ISOj and the p-latch sense amplifier driving line V sah are maintained to ground level, and the n-latch sense amplifier driving line V sal is maintained to level of V cc . In this state, a memory cell is selected by the shift of the word line signal WL to high level. In Fig. 12, each of bit line BLi and BLj is developed and a very small potential difference is generated between them. The switching control signals ΦSAi and ΦSAj are activated, and the p-latch sense amplifier driving line V sah and the n-latch sense amplifier driving line V sal are shifted from precharge levels respectively to high level of V cc and low level of a ground. At this time, the read operation for sensing a potential difference between the bit lines BLi and BLj is started. On the other hand, when each of the switching control signals ΦSAi and ΦSAj is shifted to high level, all the transistors Q39 and Q40 are turned on. If a memory cell which is selected by the word line signal WL is an on-cell, namely an erased cell, the current amount flowing through the transistor Q37 becomes more than that flowing through the transistor Q38 . This is because the bit line BLj has higher potential than the bit line BLi. Accordingly, the sub-bit line SBLi is discharged faster than SBLj, so that the potential of SBLi is lower than that of SBLj. The transistors Q41 and Q44 then are turned on by the latch driving lines V sal and V sah , resulting in the development of voltages of the sub-bit lines SBLi and SBLj respectively to GND and V cc . As a result, the minute potential difference between the bit lines BLi and BLj is sensed and amplified by the sense amplifier circuit. On the contrary, if the selected memory cell is an off-cell, namely a programmed cell, the current amount flowing through the transistor Q38 becomes more than that flowing through the transistor Q37 . This is because the bit line BLi has higher potential than the bit line BLj. Accordingly, the sub-bit line SBLj is discharged faster than SBLi, so that the potential of SBLj is lower than that of SBLi. The sub-bit lines SBLj and SBLi change respectively to GND and V cc . As a result, the minute potential difference between the bit lines BLi and BLj is sensed and amplified by the sense amplifier circuit. Similarly to the first embodiment, the switching control signals ΦSAi, ΦSAj have pulse waveforms as shown in Fig. 12 so as not to continuously form a DC current path from the precharged sub-bit lines SBLi and SBLj through the voltage-controlled current source 22 and the switching section 23 to the GND (that is the signal line VSA) by developed bit lines BLi and BLj after substantial sensing operation. Also, because the isolation control signals ISOi and ISOj are inactivated and maintained to GND level during the sensing operation, the bit lines BLi and BLj are electrically and completely decoupled with the sense amplifier circuit, so that the stable sensing operation is performed with higher sensing speed and the reduction of current consumption. Fig. 13 is a timing chart illustrating the sensing operation of the sense amplifier circuit of Fig. 11 for program verification and inhibition. Referring to Fig. 13, in order that the on-cell connected to the selected bit line BLi is not programmed, the sub-bit line SBLi corresponding to the bit line BLi has to be maintained to V cc level. During the program inhibition, the sub-bit lines SBLi and SBLj are precharged respectively to V cc and GND when the latch sense amplifier sections 24 and 25 hold the external data signals from the I/O gate circuit 200. Then, the isolation control signals ISOi and ISOj are inactivated, and therefore the read operation, or the sensing operation is performed. At this time, with the activation of only the switching control signal ΦSAj , only the transistor Q40 in the switching section 23 is turned on. The unselected sub-bit line SBLj the is precharged to GND level and the latch driving signals V sal and V sah are at active sate. On the contrary, the driving signal V sal of the n-latch sense amplifier section 24 is temporarily at inactive state and changes to be at active state when the switching control signal ΦSAj is activated. During the following read operation, because the sub-bit line SBLj is maintained to GND level, the potential of the sub-bit line SBLj continues to be maintained to Vcc level during the following sensing operation, even though that of the selected bit line BLi is developed to low level. Therefore, the sub-bit line SBLi continues to be maintained to V cc level for the program inhibition of the selected on-cell. The programming operation of on-cells (that is, erased cells) will be described hereinafter. Assuming that the on-cells are connected to the selected bit line BLi and the selected bit line BLi is maintained to GND level during the programming operation. The sub-bit lines SBLi and SBLj are precharged respectively to GND and V cc by the external data signals from the I/O gate circuit 200, and then the same operation as in the program verification well-known in the art is carried out. If the selected cell is sufficiently programmed during the programming operation, that is, the bit line BLj is higher than a reference voltage, the conductivity of the transistor Q38 is higher than that of the transistor Q37. In other words, when the switching control signal ΦSAj is at active state under the sufficiently programmed condition, a current flowing from the sub-bit line SBLj through the transistor Q38 of the latch amplifier 22 to the ground is rapidly increased in amount thereof. The sub-bit line SBLj then are sufficiently discharged to GND level. Thus, if the sub-bit line SBLi is shifted from GND level to V cc level by the latch sense amplifier sections 24 and 25, the sense amplifier circuit is in the program inhibition and the programming operation for the selected on-cell is automatically terminated. As described above, so as to carry out the program verification and inhibition, because there is no need for previously making the isolation transistor Q36 connected to the reference bit line conductive, it is not necessary to prepare an additional control logic for turning on the isolation transistor Q36. Compared with the conventional sense amplifier, the memory device in which the sense amplifier circuit is embodied may have a further simplified construction and save time required for the program verification and inhabitation. On the other hand, only the transistor Q31 of the bit line isolation section 21 is turned on so as to transmit the potential of the sub-bit line SBLi precharged by the external data signal to the bit line BLi, and only the precharged transistor Q33 is turned on for the program inhibition of the cell connected to the unselected bit line BLj, whereby the bit line BLj is precharged to V cc level. As described above, the sense amplifier circuit according to the second embodiment may perform a stable sensing operation without providing bit line isolation signals of pulse waveforms to the bit line isolation section. Also the time required for the program verification and inhibition can be reduced. Example 3 Fig. 14 shows a sense amplifier circuit for NAND flash memory device with folded bit line architecture according a third embodiment of the present invention. The sense amplifier circuit of Fig. 14 has the same construction as that of Fig. 11 except that an additional voltage-controlled current source 30 having n-channel MOS transistors Q45 and Q46 is located between the switching section 23 and a signal line V san . Component elements having similar functions to the component elements of the sense amplifier circuit of the second embodiment (shown in Fig. 11) are indicated by the same reference numerals, and description thereof are omitted. Referring to Fig. 14, drains of the transistors Q45 and Q46 are connected to each other, and sources thereof are also mutually connected. The sources of the transistors Q39 and Q40 are connected to the drains of the transistors Q45 and Q46, and the sources thereof are commonly connected to the signal line V san . The gates of the transistors Q45 and Q46 are connected respectively to the bit lines BLi and BLj. As shown in Fig. 15, a timing chart is illustrated to describe the sensing operation of the sense amplifier circuit during the read operation. The memory cell is selected according to the shift of the word line signal to high level after its precharging and equalizing operation, and a potential difference between the bit lines BLi and BLj is generated according to the state of the selected cells as shown in Fig. 15. And then, the switching control signals ΦSAi and ΦSAj are at active state, and the p-latch sense amplifier driving line V sah and the n-latch sense amplifier driving line V sal are shifted from precharge levels respectively to high level and low level. The read operation sensing a very small difference between level on the bit lines BLi and BLj is started. During the sensing operation, the sufficient gate bias is applied to the transistors Q45 and Q46 of the voltage-controlled current source 30 by the signal line V san which continues to be maintained to level of GND, and the transistors Q39 and Q40 are turned on when the switching control signals ΦSAi and ΦSAj are shifted to high level. The rest of the read operation in the sense amplifier circuit is the same as that in the secondary embodiment and its description is omitted. Similarly to the second embodiment, the switching control signals ΦSAi, ΦSAj have pulse waveforms as shown in Fig. 12 so as not to continuously form a DC current path from the sub-bit lines SBLi and SBLj through the voltage-controlled current source 22 and the switching section 23 to the ground by developed bit lines BLi and BLj during the substantial sensing operation. Also, because the isolation control signals ISOi and ISOj are inactivated and maintained to GND level during the sensing operation, the bit lines BLi and BLj are electrically and completely decoupled with the sub-bit lines SBLi and SBLj, so that the stable sensing operation can be carried out at a higher sensing speed and the current consumption can be reduced. Fig. 16 is a timing chart illustrating the sensing operation of the sense amplifier circuit of Fig. 14 for program verification and inhibition. Referring to Fig. 16, in order that the on-cell connected to the selected bit line BLi is not programmed, a read operation is performed by the external data signal from the I/O gate circuit 200 under the condition that the sub-bit lines SBLi and SBLj are precharged respectively to V cc level and ground level. Then, to the signal line V san , the voltage slightly higher than the rest of the reference voltage V ref subtracted by threshold voltages of the transistor Q45 or Q46 (V tn ) is applied, so that the transistors Q45 and Q46 respectively connected to the bit lines BLi and BLj through the gates thereof are both turned off. As a result, the sub-bit lines SBLi and SBLj continue to be maintained to precharge levels, that is, respectively V cc and GND level, and thereby the programming operation for the selected on-cell is inhibited. On the contrary, the driving signal V sah of the p-latch sense amplifier section is temporarily inactivated and changes to be activated when the switching control signal ΦSAi is at active state as shown in Fig. 16. During the following programming operation for the on-cell (namely, erased cell) connected to the selected bit line BLi, the same operation as in the program verification mode is performed after the sub-bit lines SBLi and SBLj are precharged respectively to GND and V cc by the external data signal from the I/O gate circuit 200. If the selected cell is sufficiently programmed and the bit line BLi has the potential of more than V san + V tn , the transistor Q45 is turned on. If the selected cell is sufficiently programmed and the switching control signal ΦSAj is at active state, the sub-bit line SBLj is sufficiently discharged to be maintained to ground level. Then, the program inhibition for the selected on-cell is performed and the programming operation is automatically terminated when the sub-bit line SBLi is shifted from GND to V cc by the latch sense amplifier sections 24 and 25.

Claims: A nonvoltatile semiconductor memory device having a sense amplifier circuit and a NAND structured memory cell array for storing information and a reference cell array for providing a reference voltage, said circuit comprising: a pair of bit lines comprising a first (BL i ) and second (BL j ) bit line connected to said NAND structured memory cell array and said reference cell array, respectively; a pair of sub-bit lines comprising a first (SBL i ) and second (SBL j ) sub-bit line corresponding to said bit lines, respectively; bit line isolating means (12;21) for electrically and selectively isolating said bit lines (BL ij ) from said sub-bit lines (SBL ij ) in response to an externally applied isolation control signal (1S0) during a precharging period and a sensing period; means (17;18;19;20) for precharging and equalizing said sub-bit lines during a precharging period; a signal line (VSA;V san ) for receiving an externally applied voltage signal; a voltage-controlled current source having first and second current paths connected to said first and second sub-bit lines respectively, whereby a current amount flowing through said second current path is adjusted in accordance with a voltage level of said first bit line and a current amount flowing through said first current path is adjusted in accordance with a voltage level of the said second bit line; switching means for electrically and selectively connecting at least one of said first and second current paths to said signal line in response to a switching control signal; and amplifier means for amplifying voltage levels of said sub-bit lines to first and second predetermined voltage levels, respectively when a potential difference between said sub-bit lines is more than a predetermined voltage. The nonvolatile semiconductor memory device as defined in claim 1, wherein said voltage-controlled current source (13;22) comprises a first n-channel MOS transistor (Q21;Q37) having a source-drain channel connected to one of said sub-bit lines (SBL i ) and a gate connected to the other of said bit lines (BL j ), and a second n-channel MOS transistor (Q22,Q38) having a source-drain channel connected to the other of said sub-bit lines (SBL j ) and a gate connected to the other one of said bit lines (BL i ). The nonvolatile semiconductor memory device as defined in any of claims 1 to 2, wherein said switching means comprises a first MOS transistor (Q23;Q39) having a source-drain channel connected between said signal line and said source-drain channel of said first n-channel MOS transistor (Q21;Q37) and a gate for receiving said switching control signal (ΦSA;ΦISA i ), and a second MOS transistor (Q24;Q40) having a source-drain channel connected between said signal line and said source-drain channel of said second n-channel MOS transistor (Q22;Q38) and a gate for receiving said switching control signal (ΦSA;ΦISA). The nonvolatile semiconductor memory device as defined in any preceding claim, wherein said switching control signal is a pulse signal having a pulse duration occurring after the starting of a sensing operation. The nonvolatile semiconductor memory device as defined in claim 3 or claim 4 when appended to claim 3, wherein said amplifier means comprises a third n-channel MOS transistor (Q25;Q41) having a source-drain channel connected between a first driving line ( LA ; V sal ) for receiving a first driving voltage signal and the first sub-bit line (SBL i ) and a gate connected to the second said sub-bit line (SBL j ), a fourth n-channel MOS transistor (Q26; Q42) having a source-drain channel connected between the first driving line ( LA ; V sal ) and said second sub-bit lines (SBL j ) and a gate connected to the first said sub-bit line (SBL i ), a first p-channel MOS transistor (Q27;Q43) having a source-drain channel connected between a second driving line (LA; V sah ) for receiving a second driving voltage signal and the first said sub-bit line (SBL i ) and a gate connected to the second said sub-bit line (SBL j ) and a second p-channel MOS transistor (Q28;Q44) having a source-drain channel connected between said second driving line (LA;V sah ) and the second said sub-bit line (SBL j ) and a gate connected to the first said sub-bit line (SBL i ). The nonvolatile semiconductor memory device as defined in any preceding claim, wherein said precharging and equalizing means comprises means for precharging each of said sub-bit lines to a source voltage level. The nonvolatile semiconductor memory device as defined in any preceding claim, wherein said externally applied voltage signal is maintained to a ground voltage level during said precharging and sensing period. The nonvolatile semiconductor memory device as defined in claim 5 or claim 6 or 7 when appended to claim 5, wherein said first driving voltage signal is maintained to a source voltage level during said precharging period and to a ground voltage level during said sensing period, and wherein said second driving voltage signal is maintained to said source voltage level during said precharging and sensing period. The nonvolatile semiconductor memory device as defined in any of claims 1 to 5, wherein said precharging and equalizing means comprises means for precharging each of said sub-bit lines to said ground voltage level. The nonvolatile semiconductor memory device as defined in claim 5 or claim 9 when appended to claim 5, wherein said externally applied voltage signal is maintained to said source voltage level during said precharging and sensing period. The nonvolatile semiconductor memory device as defined in claim 5, or claim 9 or 10 when appended to claim 5, wherein said first driving voltage signal is maintained to said ground voltage level during said precharging and sensing period, said second driving voltage signal is maintained to said ground voltage level during said precharging period and to said source voltage level during said sensing period. The nonvolatile semiconductor memory device as defined in claim 1 further comprising: means for precharging and equalizing said bit lines during a precharging period; and a constant voltage source for supplying the signal line with a constant voltage signal; whereby the pair of sub-bit lines (SBL ij ) are precharged by externally applied data signals; and the switching means connects said first and second current paths to said constant voltage source in response to first (ΦISA i ) and second (ΦISA j ) switching control signals, respectively. The nonvolatile semiconductor memory device is defined in claim 12, wherein the constant voltage signal is maintained to a ground voltage level during the precharging and sensing period. The nonvolatile semiconductor memory device as defined in claim 12 or claim 13, wherein the first driving voltage signal is maintained to a source voltage level during the precharging period and to a ground voltage level during the sensing period, and wherein the second driving voltage signal is maintained to the ground voltage level during the precharging period and to the source voltage level during the sensing period. The nonvolatile semiconductor memory device as defined in any of claims 12 to 14, further comprising another voltage-controlled current source having third and fourth current paths connected in series between said switching means and said constant voltage source and in parallel with each other, for adjusting a current amount flowing through said third current path in accordance with a voltage level of the one of said bit lines during the sensing period and adjusting a current amount flowing through said fourth current path in accordance with a voltage level of the other of said bit lines. The nonvolatile semiconductor memory device as defined in any one of claims 12 to 15, wherein each of the first and second switching control signals is a pulse signal having a pulse duration occurring after the starting of a sensing operation for data read. The nonvolatile semiconductor memory device as defined in any one of claims 12 to 16, wherein the first driving voltage signal is maintained to the ground voltage level during the precharging and sensing period for program verification and inhibition, and the second driving voltage signal is maintained to the source voltage level during the precharging period. The nonvolatile semiconductor memory device as defined in claim 15, wherein the first switching control signal is maintained to the ground voltage level during the precharging and sensing period for program verification and inhibition, and the second driving voltage signal is a pulse signal having a pulse duration occurring after the starting of a sensing operation for program verification and inhibition. The nonvolatile semiconductor memory device as defined in claim 12 or any claim appended thereto, wherein said bit line insulating means comprises means for electrically connecting the one of said bit lines to the one of said sub-bit lines in response to the activation of the second switching control signal during a programming operation of an on-cell connected to the one of said bit lines.

Inventors: JUNG TAE-SUNG, KIM MYUNG-JAE, JUNG, TAE-SUNG, KIM, MYUNG-JAE
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1996
Main classification: G11C7
IPC codes: G11C7, G11C16, G11C17