Abstract:
A nonvolatile semiconductor memory device comprising a power source terminal and a P-channel MOS transistor. A low power-source voltage is applied to the terminal during a read period. The source of the P-channel MOS transistor is coupled to the power source terminal. The conduction of the MOS transistor is controlled by data-writing operation. The drain of the MOS transistor is connected by a node to a plurality of bit lines. The device further comprises a plurality of memory cells and a plurality of N-channel MOS transistor. The memory cells have double-gate structure, each having a source coupled to the ground and a drain coupled to the corresponding bit line. Each N-channel MOS transistor has a source and a drain connected to the ground and the corresponding bit line, respectively, for discharging the bit line. Each N-channel MOS transistor is rendered conductive temporarily when the supply of the high power source voltage to the power source terminal is started, whereby the potential of the corresponding bit line is decreased. The bit-line potential is decreased sufficiently since the P-channel MOS transistors have a conductance greater than that of any other transistor incorporated in the device.

Description:
This application is a continuation of application Ser. No. 409,307, filed Sept. 19, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor nonvolatile memory device, and more particularly, to a semiconductor nonvolatile memory device containing memory cells each of the double gate structure in which potentials on the bit lines are controlled. 
     2. Description of the Related Art 
     There has been known an EPROM containing nonvolatile memory cells each of the double gate structure. FIG. 1 shows an arrangement of a part of a typical EPROM which uses transistors of the floating gate type as nonvolatile memory cells. 
     In the figure, MC 11 , MC 12 , . . . , MC 1n ,..., MC mn  designate memory cells. The sources of those cells MC 11 , MC 12 , . . , MC 1n , . . . , MC mn  are all grounded. The gate electrodes of the memory cells MC 11 , MC 12  , . . . , MC 1n  are connected to a word line WL 1  ; The gate electrodes of the memory cells MC 21 , . . . , MC 2n , to a word line WL 2  ; The gate electrodes of the memory cells MC m1 , . . . , MC mn , to a word line WL m . These word lines WL l  to WL m  are coupled with a row decoder 12. The drains of the memory cells MC 11 , . . . , MC ln , MC 12 , . . . , MC m2 , . . . , MC ln , . . . , MC ln . . . , MC mn  are connected to bit lines BL l  to BL n , respectively. These bit lines BL l  to BL n  are connected to the drains of bit line select transistors of the P channel type BT l  to BT n . The gates of the transistors BT l  to BT n  are coupled with a column decoder 14. The sources of these transistors are connected together to a node 16. 
     The node 16 is coupled with the drain of a write select transistor 18. The gate electrode of the transistor 18 is coupled with a voltage shifter 20 and receives write data Din through the voltage shifter 20. The source of the transistor 18 is connected to a power source terminal 22. The node 16 is also connected to a sense amplifier 24. The amplifier 24 outputs data Dout through an output buffer 26. 
     The voltage shifter 20 is made up of P channel MOS transistors 28 and 30 and N channel MOS transistors 32 and 34, and these transistors are connected as shown. Reference numerals 22 and 36 are power source terminals. Power source voltage Vcc is applied to the terminal 36, and power voltage Vpp is applied to the terminal 22. In accordance with write data Din, that is inputted during a write period, the voltage shifter 20 produces at an output node 38 high potential Vpp or ground potential Vss. 
     In each of the memory cells MC ll  to MC mn , data is stored by utilizing a change of the threshold voltage, which is caused by injecting hot electrons into the floating gate. For example, a state of the memory cell when is injected with no electron corresponds to data &#34;1&#34;; a state of the cell when it is injected with electrons corresponds to data &#34;0&#34;. To inject electrons, a high potential is simultaneously applied to the drain and the gate of the memory cell. 
     Let us consider a case that data is written into the memory cell MC 11 . The word line WL 1  is selected by the row decoder 12, and its potential is set at a high potential for data write. The bit line select transistor BT 1  is made conductive by the output signal of the column decoder 14, to select the bit line BL 1 . To write data &#34;0&#34;, the transistor 18 is conductive, and a high potential of the power source voltage Vpp is applied from the power terminal 22 to the bit line BL 1 , through write select transistor 18 and the bit line select transistor BT 1 . Under this condition, the memory cell MC 11  is conductive to allow current to flow through the source-drain path. As a result, a voltage drops across the transistors 18 and BT 1 , so that the potential on the bit line BL 1  becomes lower than the potential of Vpp. This potential, however, is much higher than the bit line potential when the memory device is in a read mode (This voltage will be denoted as Vpp&#39; ). 
     Accordingly, the high potential is simultaneously applied to the gate and the drain in the memory cell MC 11 . Under the high potential applied, hot electrons are generated in a portion closer to the drain in the channel region between the source and drain. Those electrons are injected into the floating gate. In this way, data &#34;0&#34; is written into the memory cell. To write data &#34;1&#34;, the write select transistor 18 is made non-conductive. At this time, the high potential is not applied to the bit line BL 1 . Accordingly, the high potential is applied to the memory cell MC 11  alone. Only the low potential is applied to the drain. Under this condition, no electron is injected into the floating gate, and data &#34;1&#34; is retained. Also in the case of data &#34;0&#34; writing, the memory cell whose drain and gate are simultaneously set at high potential is only the memory cell MC 11 . In other words, no electron injection into the floating gate is performed in other memory cells. Accordingly, data is written into only the memory cell addressed. 
     Voltage Vpp applied to the terminal 22 is as low as power source voltage Vcc during the read period, but is much higher than voltage Vcc during the write period to write data into the memory cell. A change of the voltage at the terminal 22 is detected by a voltage detector (not shown), and the data write operation starts. 
     FIGS. 2A through 2D show a timing chart useful in explaining a data write operation in the EPROM shown in FIG. 1. In the charts, T1 between time points t 0  and t 1  indicates a normal read period. A write period lasts from time point t 1 . In the write period, T3 between time points t 2  and t 3  indicates a write permission period. A write inhibit period is indicated by T2 between time points t 1  and t 2 , and T4 lasting from time point t 4 . 
     In the operation of the write mode, at time point t 1 , the power source voltage Vpp level at the terminal 22 is changed from the low potential voltage Vcc level to the high potential voltage, as shown in FIG. 2A. After a predetermined time from the voltage change high potential Vpp (at time point t 2 ), a write control signal externally applied, such as a program signal PGM and a chip enable signal CE, are set in &#34;0&#34; level during a preset period of time (T3), as shown in FIG. 2B. In synchronism with the write control signal, a potential on the word line is changed from potential Vcc to high potential Vpp at time t 2 , as shown in FIG. 2C. At the same time, a potential on the bit line is set at the Vpp&#39; potential or the Vss potential in accordance with the contents of data, &#34;0&#34; or &#34;1&#34;, by the write select transistor 18, as shown in FIG. 2D. 
     It is assumed that the memory cell MC 11  is addressed in the EPROM of FIG. 1. During the write inhibit period T2 in FIGS. 2A through 2D, the voltage shifter 20 produces the high potential Vpp at the node 38. During this period, the write select transistor 18 is made nonconductive, to prohibit the application of the high potential to the bit line BL 1 . 
     Let us then consider a circuit operation of the EPROM when the voltage applied to the terminal 22 is changed from potential Vcc level to high potential, and the period T2 starts (at time t 1 ). 
     At the start of the period T2, the potential at node 38, like the potential at the terminal 22, will rise from potential Vc level to high potential level. The voltage shifter 20 is based on a feedback circuit, as shown in FIG. 1. Accordingly, the potential rise at the node 3 will possibly delay behind the potential rise at the power terminal 22. When the write data Din is set in &#34;0&#34; level, the N channel MOS transistor 34 is nonconductive and the P channel MOS transistor 30 is conductive. The node 38 is set at the potential Vcc of the terminal 22, through the transistor 30. 
     Under this condition, when the potential applied to the terminal 22 is changed from potential Vcc to potential Vpp, the node 38 is charged through the transistor 30 as indicated by a line &#34;a&#34; in FIG. 3A. Due to a time delay by a resistor component of the transistor 30 and a capacitor component associated with the node 38, an actual potential rise at the node 38 as indicated by a line &#34;b&#34; delays with respect to the potential rise represented by the line &#34;a&#34;, as shown in FIG. 3A. When a potential difference ΔV existing between lines &#34;a&#34; and &#34;b&#34; becomes larger than an absolute value |Vthp| of the threshold voltage of the P channel MOS transistor, the write select transistor 18 is conductive. A conduction period of this transistor is denoted as T10 in FIG. 3B. During this period, the bit line BL 1  selected by the addressing is charged to the potential Vpp through the transistor 18. The conventional memory device is not provided with a path to allow the bit line BL 1  to be discharged when a potential difference between the potential at the terminal 22 and the potential at the node 38 becomes smaller than the absolute value |Vthp|, and the write select transistor 18 is nonconductive again. Therefore, in such a situation, the bit line BLl is kept at the high potential for data write. 
     The column decoder 14 for generating a gate drive signal for the bit select line transistors BT l  to BT n  also employs a feedback arrangement (not shown) similar to that of the voltage shifter 20. Accordingly, the transistors BT 2  to BT n , which are coupled with the nonselect bit lines and should be nonconductive, will possibly be conductive for a short period of time, like the write select transistor 18. Thus, in the conventional memory device, all of the bit lines including the selected bit lines are charged to the high potential for data write. 
     Under this condition, however, data will never be written into the memory cell, because during the write inhibit period T2 shown in FIGS. 2A through 2D, the word line is at the potential Vcc. Afterwards, the write control signal drops to a &#34;0&#34; level, and the memory device enters the operation phase of the write permission period T3. As a result, the word line potential rises to the high potential Vpp. To write data &#34;1&#34;, the transistor 18 is rendered nonconductive. Therefore, properly speaking, the bit line potential should not be at the high potential when the EPROM is in a write mode. Actually, however, for the above reason, where the bit line has been charged to the high potential, when the word line is at the high potential, there is the possibility that data &#34;0&#34; is written into the memory. Thus, the conventional semiconductor nonvolatile memory device inherently involves such problem that the data different from the correct data is written into the memory cell, viz., an incorrect write occurs. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a semiconductor nonvolatile memory device capable of controlling the bit line potential, thereby to prevent the incorrect write. 
     According to an aspect of the present invention, there is provided a semiconductor nonvolatile memory device comprising: a power source terminal coupled for reception with a first power source voltage during a given read period and a second power source potential higher than the first power source during a given write period; a first MOS transistor of the P channel type having source, drain and gate electrodes, one of the source and drain being connected to the power source terminal, the first MOS transistor being conductive in responsive to the data write operation; at least one bit line connected at one end to the source or the drain of the first MOS transistor not connected to the power source terminal; at least one nonvolatile memory cell of the double gate structure having source, drain and gate electrodes, the source being coupled with a low potential and the drain being connected to the bit line; and at least one second MOS transistor of the N channel type having source, drain and gate electrodes, one of the source and drain being connected to the other end of the bit line, the source or the drain not connected to the other end of the bit line being connected to the low potential, when the second power source voltage is applied to at least the power source terminal, the second MOS transistor being rendered temporarily conductive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned aspects and other features of the present invention are explained in the following description, the taken in connection with the accompanying drawings wherein: 
     FIG. 1 is a circuit diagram of an EPROM of prior art which uses transistors of the floating gate type as nonvolatile memory cells; 
     FIGS. 2A through 2D form a timing chart useful in explaining a data write operation of the EPROM of FIG. 1, in which FIG. 2A shows a variation of a voltage at the power source terminal, FIG. 2B, a variation of a write control signal, FIG. 2C, a variation of a word line voltage, and FIG. 2D, a variation of a bit line voltage; 
     FIGS. 3A and 3B show waveforms of the voltage at the power source terminal and the bit line voltage, when a write select transistor in the EPROM is conductive; 
     FIG. 4 is a circuit diagram of an EPROM using transistors of the floating gate type as nonvolatile memory cells, which is an embodiment of the present invention; 
     FIGS. 5A through 5E form a timing chart useful in explaining a data write operation of the EPROM of FIG. 4, in which FIG. 5A shows a variation of a voltage at the power source terminal, FIG. 5B, a variation of a write control signal, FIG. 5C, a variation of a reset signal RSTI, FIG. 5D, a variation of an output enable signal, and FIG. 5E, a variation of a reset signal RST2; 
     FIGS. 6A and 6B show another timing chart useful in explaining the data write operation of the EPROM of FIG. 4, in which FIG. 6A shows a variation of a power source voltage and FIG. 6B shows a variation of a reset signal RST3; and 
     FIGS. 7, 8 and 9 show arrangements of a circuit for supplying the reset signals RST1 to RST3 supplied to a bit line discharge transistor in the EPROM of FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A specific embodiment of a semiconductor nonvolatile memory device according to the present invention will be described with reference to the accompanying drawings. 
     Reference is first made to FIG. 4 showing an arrangement of an EPROM using nonvolatile transistors of the floating gate type as the memory cells, into which the present invention is incorporated. 
     In the figure, memory cells MC 11 , MC 12 ,. . . , MC ln , . . . , MC mn , each consisting of a floating gate type transistor, are arrayed in a matrix fashion. The sources of those transistors are grounded. The gates of the memory cells of &#34;n&#34; linearly arrayed in a row are coupled with one of &#34;m&#34; word lines WL l  to WL m , which are driven by output signals of a row decoder 12. The drains of the memory cells of &#34;m&#34; linearly arrayed in a column are coupled with one of &#34;n&#34; bit lines BL l  to BL n . Those bit lines BL l  to BL n  are also coupled with the drains of bit line select transistors BT l  to BT n  as P channel MOS transistors, which are driven by output signals of a column decoder 14. The sources of those transistors BT l  to BT n  are connected together to a node 16. 
     The node 16 is further connected to the drain of a write select transistor 18 as a P channel MOS transistor. The source of the transistor 18 is coupled with a power source terminal 22. During a read period, a power source voltage Vcc of low potential is applied to the terminal 22. During a write period, a power source voltage Vpp of high potential is applied to the terminal 22. The gate of the transistor 18 is coupled for reception with a signal from an output node 38 of a voltage shifter 20 of the feedback type. 
     The node 16 is coupled with a sense amplifier 24, which senses data in the form of a potential at the node 16 during a read period. The data sensed by the amplifier 24 is outputted as read data Dout, through an output buffer 26. 
     The voltage shifter 20 is made up of P channel MOS transistors 28 and 30 and N channel MOS transistors 32 and 34, and these transistors are connected as shown. Reference numerals 22 and 36 are power source terminals. During a read period, power source voltage Vcc at low potential is coupled with the terminal 36. In accordance with write data Din, that is inputted during a write period, the voltage shifter 20 produces at an output node 38 high potential Vpp or ground potential Vss. During a read period, power source voltage Vcc of low potential is coupled with the terminal 36. During a write period, power voltage Vpp of high potential is coupled with the terminal 22. 
     The reason why the feedback arrangement is employed for the voltage shifter 20 will be given below. 
     Where the high potential Vpp appears at the power source terminal 22, if write data Din is set in &#34;1&#34; level, a P channel MOS transistor 30 in the voltage shifter 20 will not be conductive. The reason for this is that the potential of the &#34;1&#34; level of the write data Din is lower than the low potential Vcc. When write data Din of &#34;1&#34; level is applied to the voltage shifter 20, an N channel MOS transistor 34 is conductive and the node 3 is set at the low potential. The low potential at the node 38 makes a P channel MOS transistor 28 conductive. Through the transistor 28, the Vpp potential is applied to the gate of the transistor 30, rendering the transistor 30 nonconductive. An N channel MOS transistor 32 in the voltage shifter 20 prohibits the Vpp potential at the gate of the transistor 30, when the gate is set at that potential, from transferring to the input terminal Din. 
     The ends of the bit lines BL l  to BL n , which are not coupled with the bit line select transistors BT l  to BT n , are coupled with the drains of bit line discharge transistors BD l  to BD n  as N channel MOS transistors. The sources of those transistors BD l  to BD n  are all grounded. The gates of them are coupled together to a circuit point coupled for reception with a reset signal RST. 
     Conductance (gm value) of each of the bit line than that of any of the transistor 18, and the bit line select transistors BT l  to BT n , which are serially interposed between the power source terminal 22 and the respective bit lines. 
     The EPROM as mentioned above employs only one stage of the bit line select transistors, for simplicity. Practically, two or more stages of those transistors, that are connected in series, are used in accordance with the number of bit lines. These stages of the transistors are arranged in an inversed V whose peak is positioned at the node 16. An EPROM configured on the plurality-of-bit basis contains the same number of the FIG. 4 circuit arrangements as that of the number of bits of simultaneous data read and write. In this case, the number of the decoders, the row decoder 12 and the column decoder 14, remains unchanged. 
     The operation of the EPROM thus arranged will be described with reference to FIGS. 5A through 5E showing a timing chart. In the figure, Tll between time points t 10  to t 11  designates a read period. Period T 12  and the subsequent ones constitute a write period. The write period consists of a write permission period T13 (t 12  -t 13 ), write inhibition period T12 (t 11  -t 12 ) and T14 (t 13  -t 14 ), and write data verify period T15 (t 14  -t 15 ). During the verify period T15, immediately after data is written into a memory cell, the written data i read out from the cell and it is verified with reference to the original data or data before written. 
     To write data into the memory cells, as in the prior art EPROM, the power voltage Vcc supplied to the output terminal 22 is changed from the low potential voltage Vcc to the high potential voltage Vpp. 
     After a predetermined time from the voltage change from the potential Vcc to the potential Vpp, a write control signal externally applied, such as a program signal PGM and a chip enable signal CE, are set in &#34;0&#34; level during a preset period of time. In synchronism with the write control signal, a potential on the word line is changed from potential Vcc to potential Vpp. At the same time, a potential on the bit line is set at the Vpp&#39; potential or the Vss potential in accordance with the contents of data, &#34;0&#34; or &#34;1&#34;. 
     It is assumed that the memory cell MC 11  is addressed in the EPROM of FIG. 4, and that the potential at the terminal 22 is changed from potential Vcc to potential Vpp at time t 11 , as shown in FIG. 5A. The circuit operation of the EPROM when the write inhibit period T12 starts will be described with reference to FIGS. 5A through 5E. 
     At the start of the period T12, there is the possibility that where &#34;0&#34; write data Din is supplied, the bit lines BL l  to BL n  are charged through the transistor 18 to the high potential Vpp, as already mentioned. In this instance, however, when the the potential at the terminal 22 is changed, and supply of the high potential Vpp starts, a reset signal RST applied to the common gate of the bit line discharge transistors BD l  to BD n  is temporarily set in a &#34;1&#34; level. This pulse signal is denoted as RSTl in FIG. 5C. The &#34;1&#34; duration of the reset signal RSTl is substantially equal to the period T12. With the reset signal of a &#34;1&#34; level, the transistors BD l  to BD n  are all conductive, and the bit lines BL l  to BL n  charged up to the high potential Vpp are discharged through those transistors to ground. 
     At the instant that the write permission period T13 where the write control signal drops to a &#34;0&#34; level, starts, the reset signal RSTI drops to a &#34;0&#34; level, as shown in FIG. 5B. In turn, the transistors BD l  to BD n  becomes all nonconductive. Therefore, even when the word line potential is high, the incorrect data &#34;0&#34; will never be written into the memory cell into which no data needs to be stored. 
     During the period T13, the potential at the output node 38 of the voltage shifter 20 of a bit coupled for reception with data Din of &#34;1&#34; level, becomes &#34;0&#34; level (Vss). In turn, the write select transistor 18 of this bit is conductive, and the potential at the node 16 becomes high. Accordingly, in this bit, the correct &#34;0&#34; data is written. 
     The reset signal RSTl shown in FIG. 5C rises to a &#34;1&#34; level also during the subsequent period T14. The reason for this is that during the verify period T15, to read the data once written from the memory cell, the bit line must be set at the low potential for data read by the sense amplifier 24. Also during the period T14, the transistors BD l  to BD n  are all conductive to ground the bit lines BL l  to BL n . Afterwards, the bit line selected by the addressing is set at the low potential for data read by a load circuit (not shown) provided in the sense amplifier 24. The verify period T15 starts when an output enable signal OE externally applied is decreased to a &#34;0&#34; level (FIG. 5D). 
     The reset signal RSTI in FIG. 5C may be replaced by a reset signal RST2 as shown in FIG. 5E, if required. The &#34;1&#34; level duration of the signal RST2 is shorter than the period between time points t 11  and t 12 . At time t 11 , the potential at the terminal 22 is changed from potential Vcc to potential Vpp. At time t 12 , the write permission period T13 starts Allowing for the verify operation during the verify period T15, the reset signal RST2 shown in FIG. 5E may be shaped to rise again in waveform for a given period of time in the first half of the write inhibit period T13, which precedes to the verify period, as shown in FIG. 5E. 
     In a situation that the low potential power source voltage Vcc is externally applied and at the same time the write high potential power source voltage Vpp is applied to the power source terminal 22, the operation as shown in FIGS. 6A and 6B is performed. As shown in FIG. 6A, the voltage Vcc rises at time t 11  &#39;. In synchronism with the rise of the voltage Vcc, a reset signal RST3 rises to a &#34;1&#34; level and this state is continued for a given period. The voltage Vcc and the reset signal RST3 may be related in this way. 
     The reset signal RSTl shown in FIG. 5C may be generated by a logic arrangement as shown in FIG. 7. FIG. 7 shows a basic reset signal generating circuit for generating the reset signal RSTl. Signal c is a signal which remains at the high level when the Vpp terminal is at the high potential for writing data, not at power source voltage Vcc. Signal α and the output enable signal OE are input to the OR gate 40. The output of the OR gate 40 and the write control signal (PGM or CE) are input to the AND gate 42. The output of this AND gate 42 is used as reset signal RST1. 
     The logic circuit described above can produce an output which is at the &#34;1&#34; level at all times, except for the period (T13 in FIG. 5B) during which power source voltage Vpp is at the high level so that data can be written into the memory cell. The logic circuit thus arranged can produce a reset signal RSTl as shown in FIG. 5C. 
     A circuit arrangement shown in FIG. 8 may be used for generating the reset signal RST2 shown in FIG. 5E. The circuit, like the ordinary address transition detector, detects a potential rise to the power voltage Vpp. As shown, to obtain a desired reset signal RST2, the voltage Vpp is applied directly to one of the input terminals of an AND gate 48, and through to the other input thereof. 
     The reset signal RST3 shown in FIG. 6B may be generated by using a circuit to detect a potential rise to the voltage Vpp, viz., a called power-on circuit. Such a circuit may be realized by interconnecting a resistor 50, a capacitor 52 and an inverter 54 shown in FIG. 9. 
     In many EPROMs, verify reset transistors are coupled with the bit lines and through these transistors, the bit lines are discharged when the operation mode of the EPROM shifts from the write mode (period T13) to the verify mode (period T15). Accordingly, the bit line discharge transistors BD l  to BD n  in the EPROM of FIG. 4 may be replaced by the verify reset transistors. Where the verify reset transistors are used for the transistors BD l  to BD n , the reset signals RSTl to RST3 are applied through an OR gate to the gates of the verify reset transistors. 
     As seen from the foregoing description, the semiconductor nonvolatile memory device according to the present invention is arranged such that when the supply of a power source voltage of the high potential applied to the power source terminal starts, the N channel MOS transistor inserted between the bit lines and the low potential is made temporarily conductive to discharge the bit lines to the low potential. Therefore, the incorrect data will not be written mistakenly. 
     Incidentally, when the verify reset transistors are used in place of the bit line discharge transistors BD l  to BD n , thet conductance of each of those transistors must be satisfactorily larger than that of any of the transistors for write select and the bit line select.