Abstract:
A method is provided to increase the speed of a non-volatile memory transistor by increasing the read channel current in the non-volatile memory transistor. This increase in speed is accomplished without increasing the V CC  voltage supply source or decreasing the channel length of the non-volatile memory transistor. The increase in read channel current is accomplished by applying a low voltage to the substrate region of the non-volatile memory transistor, while grounding the source of the non-volatile memory transistor. If the non-volatile memory transistor is located in an array, the low voltage is applied to the sources and drains of non-volatile memory transistors on unselected bit lines to inhibit junction leakage channel current from these unselected non-volatile memory transistors.

Description:
FIELD OF THE INVENTION 
     The present invention relates to non-volatile memory devices, such as electrically or ultra-violet erasable and electrically programmable memory cells, and particularly to a method for increasing read current in non-volatile memory devices. 
     DISCUSSION OF RELATED ART 
     An electrically erasable and programmable read-only memory (EEPROM) cell or a UV erasable and electrically programmable read-only memory (EPROM) cell consists of a single transistor incorporating a floating gate ( 1 T cell). For programmable logic applications, an EEPROM cell can include both an access transistor and a floating gate storage transistor. In general, EPROM cells and EEPROM cells will be referred to as non-volatile memory cells (or transistors). In many memory and programmable logic applications, both complementary-symmetry metal-oxide-semiconductor (CMOS) transistors and non-volatile memory cells are used. 
     Advances in technology demand increased performance in smaller packages. As a result, a trend has emerged towards scaling down components. Such scaling includes a reduction in power consumed by the components as well as a physical reduction in component size. Historically, the minimum feature sizes of transistors have been scaled down from 0.5 microns, to 0.35 microns to 0.25 microns to 0.18 microns. It is expected that this trend will continue to provide sub-0.18 micron transistors. 
     To scale down CMOS transistors, scaling is performed in both the vertical dimension and the horizontal dimensions. To scale down a CMOS transistor in the vertical dimension, the gate oxide thickness is reduced. Thus, for 0.5, 0.35, 0.25 and 0.18 micron processes, CMOS gate oxide thickness are 120, 70, 50, and 30 Angstroms, respectively. To scale down a CMOS transistor in the horizontal dimensions, both the length and width of the transistors must shrink proportionally. If the CMOS transistor channel length is reduced without reducing the gate oxide thickness, the CMOS transistor will exhibit punch through. Punch through occurs when the source depletion layer and the drain depletion layer touch each other. As the minimum feature size of transistors is scaled down, a reduction in the nominal operating voltage (i.e., the V CC  supply voltage) is required. For example, for 0.5, 0.35, 0.25 and 0.18 micron technologies, the nominal V CC  supply voltages have been 5 Volts, 3.3 Volts, 2.5 Volts and 1.8 Volts, respectively. 
     In CMOS transistors, power is proportional to both the V CC  supply voltage and channel current. Because transistor speed directly depends on the channel current, most efforts for reducing power in devices utilizing CMOS transistors focus on power supply scaling. Scaling down the V CC  power supply voltage causes the speed of a scaled down transistor to stay the same or increase, but proportionally reduces power consumption. 
     Non-volatile memory cells cannot be scaled down by the same factor as CMOS transistors. More specifically, the vertical dimensions of a non-volatile memory cell cannot be scaled down by the same factor, because the thickness of the tunneling oxide and the thickness of the insulation layer between the floating gate and the control gate must be maintained for purposes of data retention and endurance. Because the vertical dimensions of the non-volatile memory cell cannot be significantly reduced, the horizontal dimensions (i.e., the cell channel) cannot be significantly reduced. 
     The read channel current in a non-volatile memory transistor is defined as the current through the non-volatile memory transistor during a read operation. The read channel current in a non-volatile memory transistor is proportional to the square of the V CC  supply voltage divided by the channel length. A reduction in the V CC  supply voltage without a corresponding reduction in the channel length of the non-volatile memory transistor results in a lowered read channel current. The lower read channel current causes a significant degradation of the speed of a non-volatile memory transistor. 
     It would be desirable to increase the channel current in a non-volatile memory transistor that is operating at a reduced V CC  voltage supply source without providing a corresponding reduction in transistor channel length. 
     Current methods for increasing the read channel current in non-volatile memory transistors include increasing the gate voltage, increasing the drain voltage, and increasing the coupling ratio of the non-volatile memory transistor. 
     The first method for increasing channel current during a read operation involves increasing the voltage applied to the gate of the non-volatile memory transistor. As described above, the cause of the lowered channel current is the lowered V CC  voltage supply source. During a read operation, this V CC  voltage supply source may be pumped to a larger voltage, thereby increasing the voltage applied to the gate of the non-volatile memory transistor. However, the use of a charge pump requires additional delay required to pump up the gate voltage and also requires additional space on the integrated circuit for the charge pump circuitry. 
     The second method for increasing channel current during a read operation involves increasing the voltage applied to the drain of the non-volatile memory transistor. The channel current is proportional to the electrical field between the source and the drain. Therefore, increasing the drain voltage causes the electrical field to increase, thereby increasing the channel current. However, the increased voltage on the drain causes an increase in read disturb of the non-volatile memory transistor. Read disturb occurs when read conditions cause hot electron injection from the channel region into the floating gate, thereby disturbing the contents of the non-volatile memory transistor. This read disturb typically limits the maximum drain voltage applied to the non-volatile memory transistor during a read operation to less than 2.0 Volts. 
     The third method of increasing the channel current during a read operation is to increase the coupling ratio between the control gate and the floating gate of the non-volatile memory transistor. The voltage of the floating gate controls the channel current during a read operation. This voltage on the floating gate is a function of the coupling ratio of the non-volatile memory transistor. The coupling ratio is proportional to the relative areas of the control gate and the floating gate. The coupling ratio is the percentage of the voltage applied to the control gate (i.e. the V CC  voltage supply source) that is transmitted to the floating gate. However, to increase the coupling ratio of a non-volatile memory transistor, the cell size of the non-volatile memory transistor has to increase, thereby requiring more space on the integrated circuit. 
     As described above, current methods for increasing read channel current in non-volatile memory transistors operating at a lowered voltage require a larger cell size or increase the possibility of read disturb. It would be desirable to increase the read channel current in a non-volatile memory transistor without more space on the integrated circuit or increasing the possibility of read disturb. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention describes a method to increase the speed of a non-volatile memory transistor by increasing the read channel current in the non-volatile memory transistor. This increase in speed is accomplished without increasing the V CC  voltage supply source or decreasing the channel length of the non-volatile memory transistor. 
     The increase in read channel current is accomplished by applying a low voltage to the substrate region of the non-volatile memory transistor, while grounding the source of the non-volatile memory transistor. If the non-volatile memory transistor is located in an array, the low voltage is applied to the source and drain of non-volatile memory transistors on unselected bit lines to inhibit junction leakage current from these unselected non-volatile memory transistors. 
     The design can be simplified by applying zero Volts to the source and drain of non-volatile memory transistors on unselected bit lines. However, this will result in higher substrate leakage. 
     The present invention will be more fully understood in view of the following description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of an n-channel non-volatile memory transistor, which is biased in accordance with one embodiment of the present invention. 
     FIG. 2 is a circuit diagram of a 2×2 array of non-volatile memory transistors, each of which is identical to the non-volatile memory transistor of FIG.  1 . 
     FIG. 3 is a circuit diagram of a 2×2 array of two-transistor non-volatile memory cells, which are biased in accordance with another embodiment of the present invention. 
     FIG. 4 is a graph of read channel current vs. drain voltage for different substrate biases, wherein the V CC  supply voltage is 3.3 Volts. 
     FIG. 5 is a graph of read channel current vs. drain voltage for different substrate biases, wherein the V CC  supply voltage is 2.5 Volts. 
     FIG. 6 is a graph of read channel current vs. drain voltage for different substrate biases, wherein the V CC  supply voltage is 1.8 Volts. 
     FIG. 7 is an isometric view of a 2×2 array of silicon-on-insulator (SOI) non-volatile memory transistors, which are biased in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a cross sectional view of an n-channel non-volatile memory transistor  100 , which is biased in accordance with one embodiment of the present invention. Non-volatile memory transistor  100  includes p-type substrate  101 , n-well  102 , p-well  103 , N-type source region  104 , N-type drain region  105 , channel region  106 , P+contact region  107 , N+contact region  108 , gate electrode  110  and sidewall spacers  121 - 122 . P-type channel region  106  is located between regions  104  and  105 . P+contact region  107  is formed in p-well  103 , and N+contact region  108  is formed in n-well  102 . Gate electrode  110  includes tunnel oxide layer  111 , floating gate  112 , oxide-nitride-oxide (ONO) layer  113  and control gate  114 . 
     The channel current (I) of non-volatile memory transistor  100  is defined by Equation (1) below.              I   =     k        w   L            ɛ   o       t   ox              (       V   CC     -     V   t       )     2               (   1   )                                
     In Equation (1), k is a constant, w is the channel width of transistor  100 , L is the channel length of transistor  100 , ε o  is the permittivity of channel region  106 , t ox  is the thickness of tunnel oxide layer  111 , V CC  is the supply voltage, and V t  is the threshold voltage of transistor  100 . 
     As shown in Equation (1), the channel current (I) can be increased by minimizing the threshold voltage V t  of transistor  100 . Therefore, the channel current can be maximized during a read operation by minimizing the dynamic threshold voltage V t  during a read operation. Equation (2) defines the factors affecting the threshold voltage V t . 
     
       
         
           
             
               
                 
                   
                     V 
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                       V 
                       
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                     + 
                     
                       γ 
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                             2 
                              
                             
                               φ 
                               p 
                             
                           
                           - 
                           
                             V 
                             BS 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
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     In Equation (2), V t0  is the initial threshold voltage of transistor 100, γ is the body effect, φ p  is potential in the p-type semiconductor, and V BS  is the voltage between p-well  103  (body) and source region  104 . As shown by Equation (2), the threshold voltage V t  can be minimized by causing V BS  to approach 2φ p . 
     Body effect γ is defined by Equation (3).              γ   =         t   OX            2        ɛ   S          qN   A             ɛ   o               (   3   )                                
     In Equation (3), t ox  is the thickness of tunnel oxide layer  111 , ε o  is the permittivity of tunnel oxide layer  111 , ε s  is the permittivity of p-well  103 , q is elementary charge (i.e. the charge carried by a single electron) stored by floating gate  112  and N A  is doping concentration of channel region  106 . As defined by Equation (3), the magnitude of the body effect γ is directly proportional to the thickness of tunnel oxide layer  111 . Thus, as the thickness of tunnel oxide layer  111  increases, the body effect similarly increases. For example, in a particular embodiment, tunnel oxide layer  111  has a thickness of 0.1 nm and transistor  100  has a body effect of 0.5 V ½ . 
     In accordance with the present invention, the V BS  voltage between p-well  103  and source region  104  is maximized during a read operation by grounding source region  104  and applying a positive voltage (e.g., 0.7 Volts) to p-well  103 . Under these conditions, the V BS  voltage between source region  104  and p-well  103  has a magnitude of 0.7 volts. In a particular embodiment, φ p  has a value of 0.35. Under these conditions, the radical in Equation (2) has a value of 0 Volts. A body effect of 0.5 and an initial threshold voltage V t0  of 0.58 Volts produces a dynamic threshold voltage V t  equal to 0.58 Volts. This is significantly less than the threshold voltage V t  of 1.0 Volt, which would result if V BS  were equal to 0 Volts. As illustrated by Equation (1), the lower threshold voltage V t  advantageously results in a higher channel current (I), without requiring a change in channel length or width. 
     Note that Equation (2) defines the threshold voltage V t  as seen from the first polysilicon (poly 1 ) layer. For a non-volatile memory cell having a second polysilicon (poly 2 ) layer, the threshold voltage V t  as seen from the poly 2  layer is approximated by: 
     
       
           V   t (poly 2 )= V   t (poly 1 )/α, 
       
     
     where α is the coupling ratio. In the above example, if the coupling ratio is 0.6, then V t (poly 1 )=1.0 Volt and V t (poly 2 )=1.67 Volt when V BS  is equal to 0 Volts. With V BS =0.7 Volt, V t (poly 1 )=0.58 Volt and V t (poly 2 )=0.97 Volt. Therefore, applying V BS  of 0.7 Volts can impact the threshold voltage V t  and read current of a non-volatile memory cell having both poly 1  and poly 2  layers more than a CMOS transistor or a non-volatile memory cell having only a poly 1  layer. 
     FIG. 2 is a circuit diagram of a 2×2 array of non-volatile memory transistors  100 A,  100 B,  100 C and  100 D, each of which is identical to non-volatile memory transistor  100  (FIG.  1 ). In the described embodiment, all of non-volatile memory transistors  100 A- 100 D are located in the same p-well  103 . Although a 2×2 array is described, one of ordinary skill can easily expand this array to have other sizes in view of the following disclosure. The array includes word line  201 , which is coupled to the control gates of transistors  100 A and  100 B, and word line  202 , which is coupled to the control gates of transistors  100 C and  100 D. The array further includes a first source bit line  211  that is coupled to the sources of transistors  100 A and  100 C, and a second source bit line  212  that is coupled to the sources of transistors  100 B and  100 D. The array additionally includes a first drain bit line  221 , that is coupled to the drains of transistors  100 A and  100 C, and a second drain bit line  222 , which is coupled to the drains of transistors  100 B and  100 D. A V sub  control circuit  250  is coupled to p-well  103 . 
     To read the contents of non-volatile memory transistor  100 A, a read voltage equal to the V CC  supply voltage is applied to word line  201 , the drain bit line  221  is held at 1.5 Volts and the source bit line  211  is grounded. In addition, V sub  control circuit applies a voltage of 0.7 Volts to p-well  103 . Under these conditions, read current will flow through transistor  100 A if this transistor is erased (i.e., stores no negative charge on floating gate  112 ). Conversely, no read current will flow if transistor  100 A is programmed (i.e., stores negative charge on floating gate  112 ). As described above, applying a voltage of 0.7 Volts to p-well  103  advantageously increases read current through transistor  100 A. 
     Also during a read of non-volatile memory transistor  100 A, a voltage of 0 Volts is applied to word line  202 . As a result, non-volatile memory transistors  100 C and  100 D are turned off, such that these transistors are not subjected to read conditions. 
     Also during a read of non-volatile memory transistor  100 A, the unselected source bit line  212  and unselected drain bit line  222  receive the same 0.7 Volt pulse that is applied to p-well  103 . As a result, the source and drain junctions of the memory cells in an unselected column have zero bias, thereby preventing source/drain junction leakage current in the unselected non-volatile memory transistors  100 B and  100 D. Because the source and drain of non-volatile memory transistor  100 B are held at the same voltage, this transistor  100 B is not subjected to a read (or disturb) condition. 
     The present invention can also be practiced using two-transistor non-volatile memory cells. FIG. 3 is a circuit diagram of a 2×2 array of two-transistor non-volatile memory cells  300 A- 300 D used in another embodiment of the present invention. Each of non-volatile memory cells  300 A- 300 D includes an access transistor  301 A- 301 D and a storage (floating gate) transistor  302 A- 302 D. Each of storage transistors  302 A- 302 D is substantially identical to non-volatile memory transistor  100  (FIG.  1 ). In the described embodiment, all of non-volatile memory cells  300 A- 300 D are located in the same p-well  103 . Although a 2×2 array is described, one of ordinary skill can easily expand this array to have other sizes in view of the following disclosure. The array includes word line  311 , which is coupled to the control gates of storage transistors  302 A and  302 B; word line  312 , which is coupled to the gates of access transistors  301 A and  301 B; word line  313 , which is coupled to the gates of access transistors  301 C and  301 D; and word line  314 , which is coupled to the control gates of storage transistors  302 C and  302 D. The array further includes a first source bit line  321  that is coupled to the sources of storage transistors  302 A and  302 C, and a second source bit line  322  that is coupled to the sources of storage transistors  302 B and  302 D. The array additionally includes a first drain bit line  331 , that is coupled to the drains of access transistors  301 A and  301 C, and a second drain bit line  332 , which is coupled to the drains of access transistors  301 B and  301 D. V sub  control circuit  250  is coupled to p-well  103 . 
     To read the contents of non-volatile memory cell  300 A, a read voltage equal to the V CC  supply voltage is applied to word lines  311  and  312 , the drain bit line  331  is held at 1.0 Volts and the source bit line  321  is grounded. In addition, V sub  control circuit  250  applies a voltage of 0.7 Volts to p-well  103 . Under these conditions, read current will flow through storage transistor  302 A if this transistor is erased (i.e., stores no negative charge on its floating gate). Conversely, no read current will flow if transistor  302 A is programmed (i.e., stores negative charge on its floating gate). As described above, applying a voltage of 0.7 Volts to the p-well advantageously increases read current through transistor  302 A. 
     Also during a read of non-volatile memory cell  300 A, a voltage of 0 Volts is applied to word lines  313  and  314 . As a result, access transistors  301 C and  301 D are turned off, such that storage transistors  302 C and  302 D are not subjected to read conditions. 
     Also during a read of non-volatile memory cell  300 A, the unselected source bit line  322  and the unselected drain bit line  332  are coupled to receive the same 0.7 Volt pulse that is applied to p-well  103 . This prevents leakage current in the unselected storage transistors  302 B and  302 D. Because the source and drain of storage transistor  302 B are held at the same voltage, this transistor  302 B is not subjected to a read (or disturb) condition. 
     FIG. 4 is a graph of read channel current (I) vs. drain voltage V d  of storage transistor  302 A. In the graph of FIG. 4, the V CC  supply voltage is 3.3 Volts. The different lines in FIG. 4 represent substrate bias voltages V BS  of 0 Volts, 0.2 Volts, 0.4 Volts, 0.6 Volts, 0.8 Volts and 1.0 Volts. Note that for a drain voltage V d  of 1.0 Volts, the read channel current increases from a value of 159.7 micro-amps with no substrate bias, to a value of 210.9 micro-amps with a substrate bias of 1.0 Volts. The read channel current is therefore increased by 32% in a 3.3 Volt system. 
     FIG. 5 is another graph of read channel current (I) vs. drain voltage V d  of storage transistor  302 A. In the graph of FIG. 5, the V CC  supply voltage is 2.5 Volts. The different lines in FIG. 5 represent substrate bias voltages V BS  of 0 Volts, 0.2 Volts, 0.4 Volts, 0.6 Volts, 0.8 Volts and 1.0 Volts. Note that for a drain voltage V d  of 1.0 Volts, the read channel current increases from a value of 109.1 micro-amps with no substrate bias, to a value of 167.5 micro-amps with a substrate bias of 1.0 Volts. The read channel current is therefore increased by 54% in a 2.5 Volt system. 
     FIG. 6 is another graph of read channel current (I) vs. drain voltage V d  of storage transistor  302 A. In the graph of FIG. 6, the V CC  supply voltage is 1.8 Volts. The different lines in FIG. 6 represent substrate bias voltages V BS  of 0 Volts, 0.2 Volts, 0.4 Volts, 0.6 Volts, 0.8 Volts and 1.0 Volts. Note that for a drain voltage V d  of 1.0 Volts, the read channel current increases from a value of 56.0 micro-amps with no substrate bias, to a value of 119.9 micro-amps with a substrate bias of 1.0 Volts. The read channel current is therefore increased by 114% in a 1.8 Volt system. The present invention therefore significantly enhances the read channel current at low V CC  supply voltages. 
     The present invention can also be practiced using silicon-on-insulator (SOI) non-volatile memory transistors. FIG. 7 is an isometric view of a 2×2 array of SOI non-volatile memory transistors  700 A- 700 D used in another embodiment of the present invention. SOI non-volatile memory transistors  700 A- 700 D are formed over insulating layer  701  on p-type silicon islands  702 A- 702 D, respectively. Although silicon islands  702 A- 702 D are shown without any insulating material between these islands, it is understood that an insulating material (e.g., silicon oxide) is typically located between these islands. The construction of non-volatile memory transistors  700 A- 700 D within silicon islands  702 A- 702 D is substantially identical to the structure of non-volatile memory transistor  100  (FIG.  1 ). Word lines  711 - 712 , source bit lines  721 - 722  and drain bit lines  731 - 732  connect transistors  700 A- 700 D in the same manner as transistors  100 A- 100 D (FIG.  2 ). However, line  741  connects p-type silicon islands  702 A and  702 C. Similarly, line  742  connects p-type silicon islands  702 B and  702 D. As a result, the substrates of the transistors in each column can be biased independently. Thus, V sub1  control circuit  751  is coupled to bias line  741 , and V sub2  control circuit  752  is coupled to bias line  742 . In the array of FIG. 7, only the transistors in the column being read will have substrates biased to 0.7 Volts. The substrates of the transistors in the other column will be biased at 0 Volts. 
     Thus, to read the contents of non-volatile memory transistor  700 A, a read voltage equal to the V CC  supply voltage is applied to word line  711 , the drain bit line  731  is held at 1.5 Volts, the source bit line  721  is grounded, and substrate bias line  741  is held at 0.7 Volts (by V sub1  control circuit  751 ). Under these conditions, read current will flow through transistor  700 A if this transistor is erased. Conversely, no read current will flow if this transistor  700 A is programmed. 
     During a read of non-volatile memory transistor  700 A, a voltage of 0 Volts is applied to word line  712 . As a result, non-volatile memory transistors  700 C and  700 D are turned off, such that these transistors are not subjected to a read condition. 
     Also during a read of non-volatile memory transistor  700 A, the substrate bias line  742  associated with the non-selected column is held at 0 Volts (by V sub2  control circuit  752 ). Similarly, the unselected source bit line  722  and the unselected drain bit line  732  are coupled to receive a voltage of 0 Volts. This prevents leakage current in the unselected non-volatile memory transistors  700 B and  700 D. Additionally, because the source and drain of transistor  700 B are held at the same voltage, transistor  700 B is not subject to read conditions. 
     Although the invention has been described in connection with the present embodiment, it is understood that this invention is not limited to the embodiment disclosed, but is capable of various modifications which would be apparent to a person skilled in the art. For example, the conductivity types of the various regions can be reversed, such that p-channel transistors are used rather than n-channel transistors. In such embodiments, the substrate would be an n-well that is biased with a negative voltage during a read operation. In addition, although the present invention has been described with a non-volatile memory array having an AND configuration, the non-volatile memory array can have other configurations (e.g., a NOR configuration) in other embodiments. Thus, the invention is limited only by the following claims.