Abstract:
A non-volatile semiconductor device having an improved write voltage application circuit, of the type having a plurality of non-volatile memory elements each coupled to a row line and a column line, and a write voltage application circuit provided for each row line for operatively applying a regulated amount of a write current to the row line in a write state. The write voltage application circuit includes a P-channel MIS transistor which is adapted to take a conductive state of a large resistance at least in a write state, for regulating the amount of the write current.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a non-volatile semiconductor memory device made of semiconductor elements, and particularly to a write circuit for applying a high voltage to a memory cell. 
     Non-volatile semiconductor memory devices such as Electrically Programmable Read Only Memories (EPROMs), memory contents of which are erasable by ultra-violet rays, have been widely utilized in many kinds of electronic systems. As a memory cell of such EPROMs, an MIS transistor having a stacked gate structure has been mainly employed. The above type MIS transistor has a floating gate located above a channel region via an insulating layer and a control gate located above the floating gate via an insulating layer. The control gate of each MIS transistor is connected to a word line while a drain thereof is connected to a digit line in a matrix form. The memory state of each MIS transistor corresponds to a value of a threshold voltage which is determined by electric charge accumulated in its floating gate. Upon writing to an MIS transistor memory cell of the above type, potentials at its control gate and drain are raised with its source held at a ground potential so that avalanche breakdown is caused at the drain and hot electrons are injected to the floating gate, whereby a threshold voltage of the memory cell transistor is changed. 
     A conventional write voltage supply circuit for applying a high write voltage V pp  to a control gate of a memory cell transistor in writing is comprised of a series circuit of a switching MIS transistor and a current limiting MIS transistor of a depletion type. Through this series circuit, the high write voltage V pp  is applied to a control gate of a memory cell MIS transistor to be written. The purpose of using the current limiting MIS transistor is to limit a value of a current flowing the V pp  voltage to the ground potential. 
     However, the current limiting transistor is of a depletion type and therefore, steps in manufacturing the memory is inevitably increased, thus raising cost and reducing yield in manufacturing. In addition, the current limiting transistor must have a relatively large resistance and hence a channel length of the current limiting transistor must be large. This has increased the size of a semiconductor chip on which a memory device is fabricated. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a non-volatile memory device which can be easily fabricated. 
     It is another object of the present invention to provide a non-volatile memory device having a large memory capacity and a high density structure. 
     The non-volatile semiconductor memory device according to the present invention is of the type having a plurality of non-volatile memory cells each coupled to a row line and a column line, a row selection circuit for selecting one of the row lines, electric charges on the non-selected row lines being discharged to a reference potential, and a plurality of write voltage application circuits each provided for each row line to apply thereto a write voltage, and is featured in that the write voltage circuit includes a P-channel MIS transistor which is adapted to assume a shallow conductive state at least in a write state and a current flowing through the above P-channel MIS transistor is applied to the row line. 
     In order to obtain the shallow conductive state of the above P-channel MIS transistor, the gate potential of the P-channel MIS transistor is adjusted at a voltage which is lower than the write voltage V pp  by 2 to 3 volts. 
     According to the present invention, usage of a depletion MIS transistor in each write voltage application circuit is effectively avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of a non-volatile memory device according to prior art; 
     FIG. 2 is a schematic circuit diagram showing a major part of a non-volatile memory device according to the present invention; 
     FIG. 3 is a timing chart showing the operation of the memory of FIG. 2; 
     FIG. 4 is a schematic circuit diagram showing a practical memory array according to the present invention; and 
     FIG. 5 is a schematic circuit diagram showing a constant voltage generating circuit used in the memory of FIGS. 3 and 4. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a major part including a write voltage applying circuit of a non-volatile memory device according to the prior art will be described. 
     Although a plurality of memory cells are arranged in a matrix form in a practical product, one memory cell and a writing scheme for this memory cell are illustrated in FIG. 1 for easier understanding. 
     The memory cell transistor M 11  is a so-called floating gate MIS transistor. Its floating gate is left unconnected to any circuit line while its control gate is connected to a word line X 1 . A source of the transistor is connected to a ground potential while a drain of the transistor M 11  is connected to a digit line B 11  to which drains of other memory cell transistors in the same column (not shown) are connected. The digit line B 11  is connected to a load transistor Q L  (not shown) and an input of an output amplifier (not shown) through a column selection circuit 13 which is selected by a column selection signal Y i . A row driver circuit 11 is comprised of a P-channel MIS transistor Q 11 , an N-channel MIS transistor Q 12 , and an N-channel depletion type MIS transistor Q 14  which form an inverter circuit receiving a row decoder signal D 11 . Throughout the explanation, MIS transistors which are not referred to as depletion MIS transistors are enhancement MIS transistors. A write voltage supply circuit 12 is composed of an N-channel MIS transistor Q 13  and an N-channel depletion MIS transistor Q 15  connected in series between a write voltage V pp  and the word line X 1 . In a write operation for the memory cell transistor M 11 , a control signal C 12  is made high to make the transistor Q 13  conductive, while a control signal C 11  is rendered low to put the transistor Q 14  in a high resistance state and the row decoder signal D 11  is made high thereby to make the transistor Q 12  non-conductive. Therefore, a potential at the word line X 1  is put at nearly the write voltage V pp  through the transistors Q 13  and Q 15 . The write voltage V pp  is higher than a power voltage V cc . Typically, V cc  is about 5 V and V pp  is about 20 V. The column selection circuit 13 connects the digit line to V pp  by a selected level of the column decoder signal Y i . Thus, hot-electrons are injected to the floating gate of the transistor M 11  to raise the threshold voltage of the transistor M 11 . In the case where the memory cell transistor M 11  is not selected in a write operation, the row decoder signal D 11  assumes a low level to render the transistor Q 12  conductive so that the potential at the word line X 1  is made nearly the ground potential. In this instance, the transistor Q 15  serves as a current limiter for suppressing an electric current flowing to the ground through the transistor Q 12  from V pp . However, the transistor Q 15  is of a depletion type and hence manufacturing of the memory device has been complex and prolonged. Furthermore, the transistor Q 15  must have a large on-resistance and therefore, the transistor Q 15  is required to have a long channel region, resulting in larger a chip size. 
     Referring to FIG. 2, a major part of the non-volatile semiconductor memory according to the present invention will be described. 
     Similarly to FIG. 1, one memory cell transistor M 21  is representatively illustrated with a write scheme for the memory cell transistor M 21 . A write voltage circuit 22 is comprised of a P-channel MIS transistor Q 24  and a P-channel MIS transistor Q 25  connected in series between the write voltage V pp  and the word line X 2  connected to the control gate of the cell transistor M 21 . A word driver circuit 21 includes a CMOS type inverter composed of a P-channel MIS transistor Q 21  and an N-channel MIS transistor Q 22  receiving a word decoder signal D 21 . An output of the inverter is coupled to the word line X 2  through a source-drain path of an N-channel MIS transistor Q 23  receiving a predetermined potential C 21  at its gate. A source of the memory cell transistor M 21  is connected to the ground potential while its drain is connected to a digit line B 21  which is connected to a common node N 3  of a read-write selection switch SW via a column selection circuit 24 controlled by a digit decoder signal Y i . The switch SW connects the common node N 3  to the write voltage V pp  via a contact N 1  in a write state and to a node N 2  to which the power voltage V cc  is applied through an N-channel MIS transistor Q 26  serving as a load element in a read state. An input of an output amplifief 25 is connected to the node N 2 . In this arrangement, the significant feature resides in the write voltage apply circuit 22 made of P-channel MIS transistors Q 24  and Q 25 . A voltage V 0  generated by a voltage generator 23 is applied to the gate of the transistor Q 24 . This voltage V 0  has a value slightly lower than V pp  so that the transistor Q 24  assumes a shallow conductive state. In other words, the transistor Q 24  takes a conductive state, but its on-resistance is relatively large. Accordingly, the transistor Q 24  serves as a current limitter of a current flowing into the ground via the transistor Q 22  in a write state. The transistor Q 25  serves as a switch and assumes a conductive state in a write operation and a non-conductive state in a read operation in response to a control signal C 22 . In a write state, the transistor Q 23  takes a high impedance because the word line X 2  becomes V pp  which is higher than C 21 , i.e., V cc  thereby isolates the word line X 2  from V cc . FIG. 5 shows an example of a circuit of the voltage generator 23. A P-channel MIS transistor Q 31 , N-channel depletion MIS transistor Q 33  and a N-channel MIS transistor Q 34  is connected in series between the write power voltage V pp  and the ground potential. A P-channel MIS transistor Q 32  is connected in parallel with the transistor Q 31 . A control signal C 22  of the opposite phase with respect to the control signal C 22  is applied to the gates of the transistors Q 32  and Q 34 . In a read state, the control signal C 22  assumes the ground potential so that the transistor Q 32  takes a conductive state while the transistor Q 34  takes a non-conductive state. Accordingly, the output voltage V 0  is approximately equal to Vpp, while in a write state, the control signal C 22  assumes a high level close to V pp  so that the transistor Q 34  is conductive and the transistor Q 32  is non-conductive. As a result, the voltage V 0  takes an intermediate value &#34;Vpp-α&#34; in which α is usually 2 to 3 volts for the case of Vpp=20 V. 
     Thus, the transistor Q 24  of the circuit takes a conductive state in a write state and a non-conductive state in a read state in response to the value of V 0 . 
     FIG. 4 shows waveforms in operations of the memory shown in FIGS. 3 and 5. 
     T 1  shows a shows a read period in which a period T 1-1  shows the case where the memory cell transistor M 21  is not selected. During the period T 1-1 , the row decoder signal D 21  is at high to make the transistor Q 22  conductive. The potential C 21  is kept at V cc  throughtout the whole operation, while the control signal C 22  and the voltage V 0  are approximately at Vpp, so that the transistors Q 24  and Q 25  are non-conductive. Accordingly, the word line X 2  is at low in level. A period T 1-2  shows the case where the cell transistor M 21  is selected in a read cycle. In this instance, the row decoder signal D 21  is the ground potential and the transistor Q 21  is conducting, while the transistors Q 24  and Q 25  are still non-conducting. Therefore, the word line X 2  is raised in potential close to V cc  via the transistors Q 21  and Q 23 , while the digit line B 21  is connected to the node N 2  via the circuit 24, and the switch SW. 
     A write cycle is shown as T 2 . A period T 2-1  shows the case where the cell transistor M 21  is not selected. During the period T 2-1 , the control signal C 22  is approximately at the ground potential and the voltage V 0   is at the intermediate level of (Vpp-α) so that the transistors Q 24  and Q 25  are conductive. However, the transistor Q 22  is conductive in response to the high level of D 21 . Therefore, the word line X 2  is made approximately at the ground level through Q 22  and Q 23 . In this instance, although there is a current path from Vpp to the ground level via Q 24  and Q 25 , the transistor Q 24  operates to limit the current flowing therethrough. Thus, the potential of the word line X 2  is effectively set at the ground potential and the power consumption due to this current is also reduced. 
     A period T 2-2  is the case where the cell transistor M 21  is selected. In this instance, the row decoder signal D 21  is at the low level to make the transistor Q 22  non-conductive and the transistor Q 21  conductive. Therefore, the potential of the word line is raised to Vpp via the transistors Q 24  and Q 25 , achieving the selection of the word line X 2 . During this period T 2-2 , Vpp is also applied to the drain of M 21  via the circuit 24 and the switch SW. 
     FIG. 4 shows a practical layout where a plurality of memory cell transistors are arranged in a matrix form. In FIG. 4, the portions corresponding to those in FIG. 2 are designated by similar references. Each consecutive four word lines, e.g. X 11  to X 14  and their word driving schemes are classified into blocks BL l  to BL m . Each of the blocks BL l  to BL m  has the same structure and the block BL l  is representatively illustrated. Main row decoders RD l  to RD m  receiving row address signals a O  to a i , a O  to a i  provided for the blocks BL l  to BL m . Each of the main row decoders RD l  to RD m  selects its corresponding block, e.g. BL l . For example, the low level of the output D 1  of the decoder RD l  selects the block BL l . A NOR type row address driver 21&#39;-1 receives the output D 1  and a sub row decoded signal AX 1  and select the word line when both of D 1  and AX 1  are low. Other row address drivers 21&#39;-2 to 21&#39;-4 have the same structure as 21&#39;-1 except different sub row decoded signals AX 2 , AX 3  and AX 4  are applied. The signals AX 1  to AX 4  are the signals obtained by decoding two bits of row address signals which are not applied to the decoders RD l  to RD m . The write voltage application circuits 23&#39;-1 to 23&#39;-4 are connected between the write voltage Vpp and the word lines X 11  to X 14  in the block BL l . The signal C 22  and the voltage V 0  are applied to all the write voltage circuits in common. Although not shown in FIG. 4, column selection scheme such as the circuits 24, switch SW, the transistor Q 26  and the amplifier 25 in FIG. 2 is provided in a known way. 
     As has been described above, according to the present invention, a non-volatile semiconductor memory having a high density structure can be fabricated without using depletion MIS transistors.