Abstract:
A non-volatile semiconductor device stores multi-value information of at least two bits in one memory cell. A source region and a drain region serve as diffusion regions. A first channel region and a second channel region are placed between the source region and the drain region. A first gate electrode is arranged over the first channel region and the drain region. A second gate electrode is arranged over the second channel region and the source region. The first channel region stores a first threshold value while the second channel region stores a second threshold value different from the first threshold value.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a non-volatile semiconductor device, and in particular, to a non-volatile semiconductor device for inputting and outputting multi-value information for one memory cell. 
     Referring to FIG.  1  through FIG. 3, description will be made about a structure of a related non-volatile memory cell. 
     Generally, when multi-value information of one bit or more is stored for a non-volatile memory cell, many kinds of threshold values are prepared for a channel portion of a transistor. 
     During a reading operation, the kind of the threshold value given to the memory cell is detected, and the multi-value information is obtained by logically calculating the detected result. 
     Referring to FIG. 4, description will be made about an example of a reading operation when 2-bit information is stored for one memory cell. 
     Four kinds of threshold values are prepared as VT 0 , VT 1 , VT 2 , and VT 3  to store the 2-bit information for the memory cell. 
     On the other hand, setting values of gate voltages are defined as VWD 1 , VWD 2  and VWD 3 , respectively. In this event, the relationship between the gate voltages and the threshold values is defined as the following equation (1). 
     
       
         0 &lt;VT   0 &lt; VWD   1 &lt; VT   1 &lt; VWD   2 &lt; VT   2 &lt; VWD   3 &lt; VT   3   (1) 
       
     
     Further, a sense-amplifier circuit detects an ON state when a current flows through the memory cell while it detects an OFF state when no current flows through the memory cell. 
     Now, it is assumed that the threshold value of the memory cell to be read is defined as VT 1 . 
     First, the gate voltage is set to VWD 1  in a first reading operation. At this time, the memory cell is not in a conductive state because the threshold value of the memory cell is equal to VT 1 . Accordingly, the judgement becomes OFF. 
     Subsequently, the gate voltage is set to VWD 2  in a second reading operation. Consequently, the gate voltage reaches a higher level than the threshold value VT 1  in the memory cell, and the memory cell is put into the conductive state. As a result, the judgement becomes ON. 
     Next, the gate voltage is set to VWD 3  in a third reading operation. In this case, the judgement also becomes ON. 
     The above-mentioned judgement results of the first to third reading operations are logically calculated to determine an output data. 
     As illustrated in FIG. 5, output information (OUT DATA) is assigned for the threshold value of each memory cell. Thereby, 2-bits information is stored for one memory cell. 
     Similarly, description will be made about such a case that 4-bits information is stored for one memory cell with reference to FIGS. 6 and 7. 
     When 4-bits output information is stored, the number of the threshold values of the memory cell is equal to  16 , and the reading number (namely, the gate switching number) is equal to 15. 
     In general, when the conventional non-volatile memory has the multi-value n, the necessary threshold number (NTV) and the gate voltage switching number (GNV) are represented by the following equations, respectively. 
     
       
           NTV=n   2   (2) 
       
     
     
       
           GNW=NTV −1  (3) 
       
     
     More specifically, when the output information to be stored for one memory cell is changed from 2 bits to 4 bits, the threshold number is changed from 4 to 16. At the same time, the reading number is changed from 3 to 15. 
     To this end, it is actually impossible to store further more output information in the conventional non-volatile memory cell. 
     Moreover, when ON bits continuously appears along the same word line (gate) direction in the memory cell illustrated in FIGS. 1 through 3, a current (IL 1 ) inevitably flows in a direction of an adjacent cell even when a selective memory has an OFF bit. 
     In this event, the ON bit corresponds to a cell which can flow the current while the OFF bits corresponds to a cell which can not flow the current. 
     To solve such a problem, a pre-charge technique is necessary to read out the OFF bit. Consequently, the conventional non-volatile semiconductor device must have a complex logic circuit for the pre-charge. As a result, the number of devices constituting the apparatus becomes high. 
     In addition, even the pre-charge is carried out, the current (IL 1 ) is not completely eliminated. This phenomenon prevents an accurate operation of the sense-amplifier circuit for detecting a fine current. 
     Further, the threshold value is set by implanting ions into the memory cell illustrated in FIG. 1, and the number of diffusion layers (BN) for which the sense amplifier circuit charges by the information (threshold value) of the adjacent cell with respect to the selective cell is different. Thereby, the difficulty of transient design inevitably increases. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide to a non-volatile semiconductor device which is capable of reducing the switching number of a gate voltage by decreasing the necessary number of threshold values when a multi-value information is read out from the non-volatile memory cell. 
     It is another object of this invention to provide a non-volatile semiconductor device in which a pre-charge technique is not required. 
     It is still another object of this invention to provide a non-volatile semiconductor device which is capable of reducing the difficulty of transient design of the non-volatile semiconductor device using a multi-value technique. 
     According to this invention, a non-volatile semiconductor device stores multi-value information of at least two bits in one memory cell. 
     In this case, a source region and a drain region serve as diffusion regions. Further, a first channel region and a second channel region are placed between the source region and the drain region. 
     A first gate electrode is arranged over the first channel region and the drain region. A second gate electrode is arranged over the second channel region and the source region. 
     With such a structure, the first channel region stores a first threshold value while the second channel region stores a second threshold value different from the first threshold value. 
     In this event, the first and second threshold values may be independently given to the first and second channel regions by implanting ions. 
     Herein, the first and second threshold values are independently given to the first and second channel regions in a order to produce combinations of the first and second threshold values as the multi-value information. 
     According to this invention, a non-volatile semiconductor memory device stores multi-value information of at least two bits in one memory cell. 
     In this case, a plurality of first word lines are placed in a horizontal direction while a plurality of diffusion layers are placed in a perpendicular direction for the first word lines. 
     Further, a plurality of second word lines are alternately placed so as to cover the diffusion layers. 
     A first channel region is arranged under the first word line and between adjacent diffusion lines and stores a first threshold value. 
     On the other hand, a second channel region is arranged under the second word line and between the adjacent diffusion lines and stores a second threshold value different from the first threshold value. 
     Moreover, a sense amplifier is coupled to at least one of the diffusion layers and produces combinations of the first and second threshold values as the multi-value information. 
     With this structure, each of the diffusion lines has a first width while each of the second word lines has a second width. 
     Under such a circumstance, the second width may be wider than the first width. 
     Further, a ground is preferably coupled to at least one of the diffusion lines. 
     In this event, the diffusion line serves as any one of a source region and a drain region while the first word line serves as a first gate electrode over the drain region and the first channel region. Further, the second word line serves as a second gate electrode over the source region and the second channel region. 
     The first and second threshold values may be independently given to the first and second channel regions by implanting ions. 
     The sense amplifier gives a voltage for any one of the first and second gate electrodes such that the corresponding channel region is always in a conductive state irrelevant of the stored threshold value, and produces the combinations of the first and second threshold values as the multi-value information by gradually changing a voltage of the other gate electrode. 
     For example, the sense amplifier gives the voltage for the first gate electrode such that the first channel region is always in a conductive state irrelevant of the first threshold value, and judges the second threshold value stored in the second channel region by gradually changing the voltage given to the second gate electrode. 
     Further, the sense amplifier gives the voltage for the second gate electrode such that the second channel region is always in a conductive state irrelevant of the second threshold value, and judges the first threshold value stored in the first channel region by gradually changing the voltage given to the first gate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the conventional memory cell portion; 
     FIG. 2 is a cross sectional view, taken along B-B′′ line in FIG. 1; 
     FIG. 3 is a circuit diagram of the memory cell portion illustrated in FIG. 1; 
     FIG. 4 is a diagram for explaining an operation for reading out 2-bits information from the conventional memory cell portion; 
     FIG. 5 is a diagram for explaining a corresponding relationship between reading results and output values when 2-bits information is read out from the conventional memory cell portion; 
     FIG. 6 is a diagram for explaining an operation for reading out 4-bits information from the conventional memory cell portion; 
     FIG. 7 is a diagram for explaining a corresponding relationship between reading results and output values when 4-bits information is read out from the conventional memory cell portion; 
     FIG. 8 is a diagram showing a structure of a memory cell portion according to an embodiment of this invention; 
     FIG. 9 is a cross sectional view, taken along A-A′ line in FIG. 8; 
     FIG. 10 is an enlarged diagram of a dot-line portion in FIG. 9; 
     FIG. 11 is a circuit diagram showing a memory cell portion illustrated in FIG. 8; 
     FIG. 12 is a diagram for explaining a state in which a threshold value is given to a memory cell portion illustrated in FIG. 10; 
     FIG. 13 is a diagram for explaining an operation of WORD  1  and WORD  2  when 2-bits information is read out from a memory cell portion of this invention; 
     FIG. 14 is a diagram for explaining a corresponding relationship between combination of threshold values of CH 1  and CH 2  and output values when 2-bit information is read out from a memory cell portion of this invention; 
     FIG. 15 is a diagram for explaining an operation of WORD  1  and WORD  2  when 4-bits information is read out from a memory cell portion of this invention; and 
     FIG. 16 is a diagram for explaining a corresponding relationship between combination of threshold values of CH 1  and CH 2  and output values when 4-bit information is read out from a memory cell portion of this invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to FIG. 8, description will be made about a structure of a memory cell according to an embodiment of this invention. 
     Herein, it is to be noted that BN represents a diffusion layer serving as a source region or a drain region of a transistor. First gate electrodes WORD  1  are formed in a perpendicular direction for BN so as to cross with a plurality of BNs. 
     Further, second gate electrodes WORD 2 T and WORD 2 B are formed directly over the diffusion layer serving as the source region of the memory cell. Moreover, the second gate electrode has a width wider than BN, and are placed so as to cover BN. 
     In FIG. 10, the source of the memory cell corresponds to BN 1 , and the second gate electrode WORD  2 T is formed directly over BN 1  and on a second channel region (CH 2 ). 
     Further, the first gate electrode WORD  1  is formed directly over BN 0  serving as the drain of the memory cell and on a first channel region (CH 1 ). 
     In this case, ions may be implanted for the channel regions CH 1  and CH 2  placed between BN 0  and BN 1  to independently obtain the threshold values. 
     Subsequently, description will be made about the operation * of the memory cell when 2-bits information is stored for one memory cell with reference to FIG.  13 . 
     Now, the threshold value of CH 1  is defined as VT 0  (0V&lt;VT 0 &lt;½VCC) while the threshold value of CH 2  is defined as VT 1  (½VCC&lt;VT 1 &lt;VCC). 
     The sense amplifier circuit judges as an ON state when the current flows from BN 0  to BN 1  during the reading operation while it judges as an OFF state when no current flows. 
     In this case, the operation of WORD  1  and WORD  2  is shown in FIG. 13 when the memory cell illustrated in FIG. 12 is read out. Herein, a vertical axis represents a voltage while an abscissa axis represents time. 
     In FIG. 13, the WORD  1  is set to a VCC level during a first reading time. Under this circumstance, each of VT 0  and VT 1  has a lower level than VCC. Consequently, the region of CH 1  is in a conductive state. 
     In this condition, the level of WORD 2 T is set to ½VCC. In this event, no current flows from BN 0  towards BN 1  because the threshold value of the CH 2  is equal to VT 1 . In consequence, the sense amplifier judges as the OFF state. 
     During a second reading time, the WORD  1  is changed to the level of ½VCC, and the WORD 2 T is changed to the level of VCC. Like the first reading time, the CH 2  region is in a conductive state because the threshold value of CH 2  is lower than VCC. 
     In this event, the threshold value of CH 1  is equal to VT 0 , and is lower than ½VCC. Therefore, the CH 1  region is in the conductive state. As a result, the current flows from BN 0  towards NB 1 , and the judgement becomes the ON state. 
     Although description has been so for made about the WORD 2 T as an example, the operation is the same when the WORD 2 B is used. During the reading operation time, the voltage is applied to any one of WORD  2 T and WORD 2 B. 
     In FIG. 14, reading results are shown when the threshold values of CH 1  and CH 2  are combined. As illustrated in FIG. 14, when the output data (OUT DATA) is assigned for the combination of the judgement results of the first and second reading operations, 2-bits data can be stored for one memor cell. 
     Subsequently, description will be made about the operation of the embodiment when 4-bits information is stored for one memory cell. 
     The four kinds of threshold value are set for CH 1  and CH 2  such that the information stored for one memory cell is changed from 2 bits to 4 bits. Further, the reading operation number is set to 6. 
     Under such a circumstance, the threshold values to be set for CH 1  and CH 2  are defined VT 0 , VT 1 , VT 2 ,and VT 3  while the voltages of WORD I, WORD 2 T,and WORD 2 B are defined as VWDO, VWD 1 , VWD 2 ,and VWD 3 . 
     In this case, the relationship between the threshold values and the gate voltages is represented by the following quotation (4). 
     
       
         0 &lt;VT   0 &lt; VWD   0 &lt; VT   1 &lt; VWD   1 &lt; VT   2 &lt; VWD   2 &lt; VT   3 &lt; VWD   3   (4) 
       
     
     In this event, the reading operation will be explained with reference to FIG. 15 when the threshold value of CH 1  is defined as VT 1  and the threshold value of CH 2  is defined as VT 2 . 
     In FIG. 15, the level of WORD  1  is represented by a dot line while the level of WORD 2 T (or WORD 2 B) is represented by a solid line. 
     First, the level of WORD  1  is set to VWD 3  between the first reading time and the third reading time. The CH 1  portion is in the conductive state between the first reading time and the third reading time because VWD 3  has a level higher than VT 3 . 
     In this state, the level of WORD 2 T is gradually changed. During the first reading time, WORD 2 T is set to the level of VWD 0 . Now, the threshold value of CH 2  is higher than VWD 0 . Consequently, no current flows through the memory cell, and the judgement is kept to the OFF state. 
     During the second reading time, the level of WORD 2 T is set to VWD 1 . In this event, no current flows through the memory cell, and the judgement becomes the OFF state. 
     During the third reading time, the level of WORD 2 T is set to VWD 2 . In this case, the threshold value of CH 2  is lower than VWD 2 . In consequence, the current flows through the memory cell, and the judgement becomes the ON state. 
     Subsequently, the level of WORD 2 T is set to VWD 3  during the fourth reading time and the sixth reading time. 
     The CH 2  region is in the conductive state between the fourth reading time and the sixth reading time because the threshold value of CH 2  is equal to VT 2   
     During the fourth reading time, the level of WORD  1  is set to VWD 2 . The threshold value of CH 1  is equal to VT 1 . Consequently, the current flows through the memory cell, and the judgement becomes the ON state. 
     Next, during the fifth reading time, the level of WORD  1  is set to VWD  1 . In this case, the gate voltage is higher than the threshold value of CH 1 . Thereby, the current flows through the memory cell, and the judgement becomes ON state. 
     Subsequently, the level of WORD  1  is set to VWD 0  during the sixth reading time, no current flows through the memory cell because the CH 1  portion is cut off. Therefore, the judgement becomes the OFF state during the sixth reading time. 
     Thus, the level of the WORD  1  is fixed to VWD 3  between the first reading time and the third reading time, and the level of WORD  2 T is gradually changed. 
     Conversely, the level of WORD 2 T is fixed to VWD 3  between the fourth reading time and the sixth reading time, and the level of WORD  1  is gradually changed. 
     As illustrated in FIG. 16, the output data is assigned for the judgement results during the first through sixth reading times. Thereby, the 4-bits data is stored for one memory cell. 
     As mentioned above, when the memory cell is used in this embodiment, the number (NTV) of the threshold values and the voltage switching number (GNW), which must be set to the memory cell having the multi-value n, are represented by the following equations (5) and (6). 
     
       
           NTV=n   (5) 
       
     
     
       
           GNW =( NTV −1)×2  (6) 
       
     
     Thus, the number (kinds) of the threshold values can be reduced as compared the conventional case when many informations are stored for the one memory cell according to this invention. 
     Further, the information can be read out with the gate voltage switching number smaller than the conventional case. Consequently, a high-speed operation is possible. As a result, as the multi-value number is higher, it is more advantageous. 
     Moreover, the memory cell can be electrically cut off from the adjacent memory cell during a reading operation. In consequence, the circuit device, such as the pre-charge circuit and the GND selector can be eliminated or omitted. 
     In addition, the current pass towards the adjacent cell portion is always cut off. Thereby, the charge current does not flow for the diffusion layer capacitor except for the diffusion layer selected via the adjacent memory cell. Consequently, the memory cell can always and stably be operated. 
     While this invention has so far been disclosed in conduction with several embodiments thereof, it will be readily possible for those skilled in the art to put this invention into practice in various other manners.