Abstract:
The disclosure is a nonvolatile semiconductor memory including bitlines connected to memory cells and high-voltage specified NMOS transistors connecting the bitlines to sense amplifiers. Column selection signals applied to the NMOS transistors are established at a voltage higher than a power supply voltage during a read operation. In the case of power supply voltage drain, the invention prevents degradation of the drivability of the NMOS transistors.

Description:
This application claims priority from Korean Patent Application No. 2001-52058, filed on Aug. 28, 2001, the contents of which are herein incorporated by this reference in their entirety. 
     TECHNICAL FIELD 
     The present invention is generally concerned with semiconductor memory devices and, more specifically, with nonvolatile semiconductor memory devices with the operation modes of erasing, programming, and reading, using a voltage higher than a power supply voltage. 
     BACKGROUND OF THE INVENTION 
     Referring to FIG. 1, as a kind of nonvolatile semiconductor memory devices, a NOR-type flash memory includes a memory cell array  10 , a row (X) decoder  20 , a column gate circuit  30 , a column (Y) decoder  40 , and sense amplifiers/write drivers (SA/WD) block  50 , in general. 
     The NOR-type memory cell array  10  comprises of plural memory cells coupled to wordlines WL and bitlines BL in a matrix pattern. Each memory cell, as shown in FIG. 2, is constructed of a stacked gate type for example, being made of source and drain regions formed in a P-type semiconductor substrate  2 ,  3  and  4 , a floating gate  6  isolated from the source and drain regions through an oxide film  7  thinner than 100 Å, and a control gate  8  formed over the floating gate  6  with an interlayer oxide film  9  interposed therebetween. The NOR-type flash memory has a multiplicity of bulk regions, isolated from each other, in which the memory cells are formed. Therefore, memory cells in the same bulk region are erased simultaneously in the unit of bulk, so referred to as a “sector” that for example covers the storage capacity of 64 Kb. 
     Returning to FIG. 1, a row decoder  20  selects one of wordlines WL 1 ˜WLi in response to a row address, and a column gate circuit  30  selects a part of bitlines BL 1 ˜BLj in response to column selection signals Y 1 ˜Yn provided from a column decoder  40 . The selected bitlines are connected to the SA/WD block  50 . The column gate circuit  30  is constructed of high-voltage specific NMOS transistors, T 11 ˜Tn 1 , T 12 ˜Tn 2 , . . . , T 1 m˜Tnm, which are connected to the bitlines BL 1 ˜BLj each of which corresponds to a group of the n-numbered transistors. The SA/WD block  50  senses data from a selected memory cell through its corresponding bitline during a read operation while it drives data into a selected memory cell during a program operation. 
     The following Table 1 shows voltage biasing states for performing relevant operations in the NOR-type flash memory. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Operation Mode 
                 Vg 
                 Vd 
                 Vs 
                 Vb 
               
               
                   
               
             
             
               
                 Programming 
                 +10 V 
                 +5 V˜+6 V 
                 0 V 
                 0 V 
               
               
                 Erasing 
                 −10 V 
                 Floating 
                 Floating 
                 +6 V  
               
               
                 Erase Repair 
                  +3 V 
                 +5 V˜+6 V 
                 0 V 
                 0 V 
               
               
                 Read 
                 +4.5 V  
                 +1 V 
                 0 V 
                 0 V 
               
               
                   
               
             
          
         
       
     
     Programming a memory cell involves hot electron injection by which a ground voltage (i.e., 0V) is applied to the source and substrate, a high voltage (e.g., +10V) to the control gate, and an appropriate positive voltage (e.g., +5˜6V) to the drain region. The high positive voltage Vg, applied to control gates of memory cell transistors, is supplied from the row decoder  20 . The positive voltage to the drain region, Vd, is supplied from the write driver  50  through the column gate circuit  30  in which a positive voltage of +5V˜+6V is applied to gates of the selected NMOS transistors among T 11 ˜Tnm. With the voltage biases, electrons (or negative charges) accumulate in the floating gate, resulting in an elevation of the transistor&#39;s threshold voltage. A programmed memory cell has a threshold voltage of +6V˜+7V, being detected as an “off-cell” when read. 
     Erasing the memory cells involves the Fowler-Nordheim (F-N) tunneling effect. A high (-potential) negative voltage of about −10V is applied to gates of memory cells while an appropriate positive voltage of about +5V biases the substrate (or bulk) of the memory cells. The drain region of the memory cell is in a floating state (or a high-impedance state) in order to maximize an erasing effect. The high negative voltage applied to the control gate of the memory cell is supplied from the row decoder  20 . Under the condition of voltage-biasing to erase the memory cells, a strong electric field of 6˜7 MV/cm over the oxide film  7  between the floating gate  6  and the substrate  2  induces the F-N tunneling, thus reducing a threshold voltage of the memory cell. The erased memory cell is detected as an “on-cell”. 
     Reading a memory cell to distinguish a current state of the memory cell is achieved by applying an appropriate positive voltage of about +1V to the drain region  4 , applying a positive voltage of about +4.5V to the control gate through a selected wordline, and applying 0V to the source region. The drain voltage (Vd) is supplied from the sense amplifier of the SA/WD block  50  through the column gate circuit  30 , and the gate voltage (Vg) is supplied from the row decoder  20 . If a selected memory cell has been programmed, there is no current flow through the programmed memory cell because its threshold voltage was set higher. Therefore, a voltage on a corresponding bitline increases and the sense amplifier detects the memory cell as an off-cell. On the other hand, if a selected memory cell has been erased, a current flows from the source region to the drain region, and a decreased voltage on a corresponding bitline lets the sense amplifier detect the memory cell as an on-cell. 
     As a high voltage beyond 5V is applied to the drain region during the program operation, the column gate circuit  30  employs high-voltage specified NMOS transistors (hereinafter, referred to as “HVNMOS transistors”) T 11 ˜Tnm in order to transfer the high voltage to the memory cells in full rate. Such a HVNMOS transistor is operative at a higher voltage than a power supply voltage, with a thick gate oxide film by which its threshold voltage is about +3V higher than a normal NMOS transistor having a threshold voltage of +0.5˜+0.7V. 
     During a read operation, the voltage level around the power supply voltage, e.g., 3˜5V, is established on a gate of the HVNMOS transistor in order to transfer the drain voltage of 1V to the drain region of the selected memory cell. However, if the power supply voltage becomes lower, the current drivability of the HVNMOS transistor degrades and accordingly the reading speed decreases. As a result, high-speed operation of the NOR flash memory device is impeded. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a nonvolatile memory device maintaining higher reading speed for memory cells which using a lower power supply voltage. 
     Features and advantages of embodiments the invention will be more fully described by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention, and many of the attendant advantages thereof, will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 is a schematic diagram showing a structure of a traditional nonvolatile semiconductor memory device; 
     FIG. 2 is a sectional schematic diagram of a memory cell of the device shown in FIG. 1; 
     FIG. 3 is a schematic block diagram showing a nonvolatile semiconductor memory device according to the invention, including a structure of a memory cell array, voltage generators, and switch circuits; 
     FIG. 4 is a circuit diagram of a first high-voltage generator shown in FIG. 3; 
     FIG. 5 is a circuit diagram of a second high-voltage generator shown in FIG. 3; 
     FIG. 6 is a circuit diagram of a voltage booster shown in FIG. 3; 
     FIG. 7 is a circuit diagram of a first switch circuit shown in FIG. 3; and 
     FIG. 8 is a circuit diagram of a second switch circuit shown in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description for purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without the specific details. In other instances, well-known systems are shown in diagrammatic or block diagram form in order not to obscure the present invention. 
     Referring to FIG. 3, a NOR flash memory  100  according to the present invention includes a memory cell array  110 , a row decoder (X-DEC)  120 , a column gate circuit  130 , a column decoder (Y-DEC)  140 , a sense-amp/write-driver (SA/WD) block  150 , a read voltage generator  160 , a program voltage generator  170 , a voltage booster  180 , a wordline voltage switch circuit  190 , and a column-gating voltage switch circuit  200 . The memory cell array  110  is composed of a plurality of stacked gate memory cells MC (shown in FIG. 2) coupled to wordlines WL 1 ˜WLi, bitlines BL 1 ˜BLj, and a common source line SL. The wordlines WL 1 ˜WLi are connected to the row decoder  120  that selects one of the wordlines and supplies a wordline voltage thereto. The bitlines BL 1 ˜BLj are connected to the column gate circuit  130  that connects a portion of the bitlines to the SA/WD block  150  in response to column selection signals Y 1 ˜Yn provided from the column decoder  140 . The column gate circuit  130  is composed of HVNMOS transistors T 11 ˜Tnm each of which is connected between a corresponding bitline and the SA/WD block  150 . Each HVNMOS transistor is operable in a high-voltage condition, with a threshold voltage of about 3V. The HVNMOS transistors T 11 ˜Tnm are arrayed in groups of in transistors connected to data lines DL 1 ˜DLm connected, in turn, to the SA/WD block  150 . 
     The read voltage generator  160  generates a read voltage VPP 1  in response to a read signal RD, and the program voltage generator  170  generates a program voltage VPP 2  in response to a program signal PGM. The read voltage VPP 1  is applied to a selected wordline as the wordline voltage V WL  during a read operation, while the program voltage VPP 2  is applied to a selected wordline as the wordline voltage V WL  during a program operation, commonly through the switch circuit  190  and the row decoder  120 . The switch circuit  190  selectively transfers one of the read voltage VPP 1  and the program voltage VPP 2  in response to the program signal PGM. The row decoder  120  transfers the wordline voltage V WL , which is selected from a read voltage VPP 1  or the program voltage VPP 2 , in response to address information. The program voltage VPP 2  is also provided to the column-gating voltage switch circuit  200 . The voltage booster  180  generates a column-gating voltage VPP 3  in response to a boosting signal PBST. The column-gating voltage switch circuit  200  transfers one of the program voltage VPP 2  and the column gating voltage VPP 3  alternatively in response to the program signal PGM and the boosting signal PBST. 
     The read signal RD activates a read operation mode while the program signal PGM activates a program operation mode. The read voltage VPP 1 , the program voltage VPP 2 , and the column-gating voltage VPP 3 , all higher than a power supply voltage (VCC), are established at about 4.5 V, 10 V, and VCC+Vth (Vth is a threshold voltage of the HVNMOS transistor), respectively. 
     Referring now to FIG. 4, the read voltage generator  160  comprises an oscillation controller  162 , an oscillator  164 , and a charge pump  166 . The oscillation controller  162  comprises a differential amplifier DA 1  constructed of PMOS transistors M 1  and M 2  and NMOS transistors M 3 ˜M 5 , resistors R 1  and R 2  serially connected between VPP 1  and a ground voltage GND, a NAND gate G 1  receiving an output of the differential amplifier DA 1  and the read signal RD, and an inverter INV 1  converting an output of the NAND gate G 1  into an oscillation enable signal OSCenr. In the differential amplifier DA 1 , a gate of the NMOS transistor M 4  is coupled to a voltage node N 1  between the resistors R 1  and R 2 , and a gate of the NMOS transistor M 3  is coupled to a reference voltage VREF 1 . A gate of the NMOS transistor M 5  that connects the differential amplifier DA 1  to the ground voltage is coupled to the read signal RD so as to control an activation of the differential amplifier DA 1 . The oscillation controller  162  determines whether a present level of the read voltage VPP 1  reaches a predetermined voltage level defined by the reference voltage VREF 1 . If VPP 1  is lower than VREF 1 , the oscillation enable signal OSCenr is active with a high level. To the contrary, if VPP 1  is higher than VREF 1 , OSCenr is inactive with a low level. 
     The oscillator  164  is formed of inverters INV 2 —INV 4 , capacitors C 1  and C 2 , and a NAND gate G 2 , generating a pumping clock OSCr in response to OSCenr of a high level. The pumping clock OSCr is a signal oscillating with a predetermined cycle period. When OSCenr is a low level, the oscillator  164  does not generate the pumping clock OSCr. 
     The charge pump  166  is composed of inverters INV 5  and INV 6 , capacitors CP 1 ˜CPn, and PMOS transistors PTR 1 ˜PTRn+1. The pumping clock OSCr is applied to the capacitors CP 1 ˜CPn in turn with the order of even and odd, so that the read voltage VPP 1  is charged up by the serial action of the pumping chain along the oscillation of the pumping clock OSCr. 
     Referring to FIG. 5, the program voltage generator  170  comprises an oscillation controller  172 , an oscillator  174 , and a charge pump  176 . Program voltage generator  170  is similar to the read voltage generator  160  except that resistors R 3  and R 4  (corresponding to R 1  and R 2 ) connected between the program voltage VPP 2  and the ground voltage GND and an NMOS transistor M 10  (corresponding to M 5 ) responds to the program signal PGM. The oscillation controller  172  determines whether a present level of the program voltage VPP 2  reaches a predetermined voltage level defined by the reference voltage VREF 2 . If VPP 2  is lower than VREF 2 , the oscillation enable signal OSCenp is active with a high level. To the contrary, if VPP 2  is higher than VREF 2 , OSCenp is inactive with a low level. 
     The oscillator  174  generates a pumping clock OSCp in response to OSCenp of a high level. The pumping clock OSCp is a signal oscillating with a predetermined cycle period. When OSCenp is a low level, the oscillator  174  does not generate the pumping clock OSCp. The program voltage VPP 2  generated from the charge pump  176  is gradually charged up to the predetermined level, i.e., 10V, with the oscillation of the pumping clock OSCp. 
     Referring to FIG. 6, in the voltage booster  180 , the boosting signal PBST is applied to a boosting capacitor C 3  through inverters INV 8  and INV 9  (comprising serially connected PMOS transistors M 11 , M 12 ) serially connected. The other electrode of the capacitor C 3  is connected to an output terminal N 3  from which the column-gating voltage VPP 3  is generated. Between VCC and the output terminal N 3  is connected a PMOS transistor M 13 , and between the output terminal N 3  and the ground voltage GND is connected a PMOS transistors M 14  and an NMOS transistor M 15 . A gate of the PMOS transistor M 13  is coupled to a common drain node of the transistors M 14  and M 15  whose gates are coupled to an output node N 4  of a level shifter LS 1 . The level shifter LS 1  is formed of PMOS transistors M 16  and M 17 , NMOS transistors M 18  and M 19 , and an inverter INV 10 . Sources of the PMOS transistors M 16  and M 17  are connected to the output terminal N 3 . The boosting signal PBST is applied directly to a gate of the NMOS transistor M 18  and indirectly to a gate of the NMOS transistor M 19  through the inverter  10 . When the boosting signal PBST is a low level, VPP 3  is set at VCC by the PMOS transistor M 13  that maintains a conductive state because the output node N 4  is a high level. When the boosting signal PBST rises to a high level, VPP 3  is charged up to the voltage level of VCC+Vth in accordance with a coupling ratio at the output terminal N 3 . 
     Turning now to FIG. 7, the wordline voltage switch circuit  190  employs HVMOS transistors M 20 ˜M 39  to transfer the high voltages such as VPP 1  and VPP 2 . It is formed of an inverter INV 11  converting the read signal RD to its complementary logic level, a level shifter LS 2  connected between VPP 1  and GND and responding to the read signal RD, a level shifter LS 3  connected between VPP 1  and GND and responding also to the read signal RD, HVPMOS transistors M 23  and M 24  connected in series between the VPP 1  and an output terminal N 5  from which the wordline voltage VWL is output, an inverter INV 12  converting the program signal PGM to its complementary logic level, a level shifter LS 4  connected between VPP 2  and GND and responding to the program signal PGM, a level shifter LS 5  connected between VPP 2  and GND and responding to the program signal PGM, and HVPMOS transistors M 34  and M 35  connected in series between the VPP 2  and the output terminal N 5 . Gates of the HVPMOS transistors M 23  and M 24  are coupled to output nodes N 6  and N 7  of the level shifters LS 2  and LS 3 , respectively. Gates of the HVPMOS transistors M 34  and M 35  are coupled to output nodes N 8  and N 9  of the level shifters LS 4  and LS 5 , respectively. 
     When the read voltage RD is active with a high level, the HVPMOS transistors M 23  and M 24  are turned on respectively by the level shifters LS 2  and LS 3 . Then, the read voltage VPP 1  is transferred to the row decoder  120  through the conductive HVPMOS transistors M 23  and M 24  as the wordline voltage V WL . During this time, as the program signal PGM is inactive with a low level, VPP 2  cannot affect the wordline voltage V WL  because the HVPMOS transistors M 34  and M 35  are in non-conductive states. In contrast, when the program voltage PGM is active with a high level, the HVPMOS transistors M 34  and M 35  are turned on respectively by the level shifters LS 4  and LS 5 . Then, the program voltage VPP 2  is transferred to the row decoder  120  through the conductive HVPMOS transistors M 34  and M 35  as the wordline voltage V WL . During this time, as the read signal RD is inactive with a low level, VPP 1  cannot affect the wordline voltage V WL  because the HVPMOS transistors M 23  and M 24  are in non-conductive states. 
     Referring to FIG. 8, the column-gating voltage switch circuit  200  also employs HVMOS transistors M 40 ˜M 59  to transfer the high-voltage VPP 2  or VPP 3  to the column-gating voltage V YG , being constructed similarly to that of the wordline voltage switch circuit  190  of FIG.  7 . In other words, it is formed of an inverter IWV 13  converting the program signal PGM to its complementary logic level, a level shifter LS 6  connected between VPP 2  and GND and responding to the program signal RD, a level shifter LS 7  connected between VPP 2  and GND and responding to the program signal PGM, HVPMOS transistors M 44  and M 45  connected in series between the VPP 2  and an output terminal N 10  from which the column-gating voltage V YG  is output, an inverter INV 14  converting the boosting signal PBST to its complementary logic level, a level shifter LS 8  connected between VPP 3  and GND and responding to the boosting signal PBST, a level shifter LS 9  connected between VPP 3  and GND and responding to the boosting signal PBST, and HVPMOS transistors M 54  and M 55  connected in series between the VPP 3  and the output terminal N 10 . 
     Gates of the HVPMOS transistors M 44  and M 45  are coupled to output nodes N 11  and N 12  of the level shifters LS 6  and LS 7 , respectively. Gates of the HVPMOS transistors M 54  and M 55  are coupled to output nodes N 13  and N 14  of the level shifters LS 8  and LS 9 , respectively. 
     When the program signal PGM is active with a high level, the HVPMOS transistors M 44  and M 45  are turned on each by the level shifters LS 6  and LS 7 . Then, the program voltage VPP 2  is transferred to the row decoder  120  through the conductive HVPMOS transistors M 44  and M 45  as the column-gating voltage V YG . During this time, as the boosting signal PBST is inactive with a low level, VPP 3  cannot affect the column-gating voltage V YG  because the HVPMOS transistors M 54  and M 55  are in non-conductive states. In contrast, when the boosting voltage PBST is active with a high level, the HVPMOS transistors M 54  and M 55  are turned on respectively by the level shifters LS 8  and LS 9 . Then, VPP 3  is transferred to the row decoder  120  through the conductive HVPMOS transistors M 54  and M 55  as the column-gating voltage V YG . During this time, as the program signal PGM is inactive with a low level, VPP 2  cannot affect the column-gating voltage V YG  because the HVPMOS transistors M 44  and M 45  are in non-conductive states. 
     Next will be described the overall operation for supplying and switching the wordline voltage V WL  and the column-gating voltage V YG  by reference with FIGS. 3 through 8. Before programming, it will be understood that an erasure operation in the memory cell array  110  shown in FIG. 3 is carried out by applying the voltage of −10V to wordlines belonging to a page or a sector of the memory cell array. 
     At the beginning of a program operation mode, the program signal PGM is set to a high level while the read signal RD and the boosting signal PBST are set at low levels. The high-level program signal PGM activates the program voltage generator  170  to produce a program voltage VPP 2  of 10V. Then, the switch circuit  190  transfers VPP 2  into the row decoder  120  as the wordline voltage V WL  in response to the program signal PGM while the switch circuit  200  transfers VPP 2  to the column decoder  140  as the column-gating voltage V YG  in response to the program signal PGM. 
     The row decoder  120  selects a wordline (e.g., WL 1 ) in response to a corresponding row address assigned to the wordline. The wordline voltage V WL  of VPP 2  is applied to the selected memory cells through the selected wordline. At the same time, the column decoder  140  selectively activates a column selection signal (e.g., Y 1 ) in response to a column address assigned thereto. The column-gating voltage V YG  of VPP 2  is applied to gates of the column gates T 11 , T 12 , . . . , and T 1 m. Then, selected bitlines (e.g., BL 1 , BL 4 , . . . ) are connected to the write drivers in the SA/WD block  150  and charged up to the drain voltage of 5˜6V for programming through the conductive column gates by the active column selection signal. The column-gating voltage of VPP 2  applied to the column gates enables transfer of the drain voltage to the bitlines without voltage loss from the level of 5˜6V, for a successful programming result. As a result, the selected memory cell MC biased by the program voltage VPP 2  is programmed such that negative charges (i.e., electrons) migrate to the floating gate, increasing the threshold voltage. 
     In a read operation, both the read signal RD and the boosting signal PBST are active with high levels while the program signal PGM is held at a low level. The read signal RD activates the read voltage generator  160  to create the read voltage VPP 1  of 4.5 V while the boosting signal PBST does the voltage booster  180  to generate the boosting voltage VPP 3  of VCC+Vth. Then, the switch circuit  190  transfers VPP 1  to the row decoder  120  as the wordline voltage V WL  in response to the read signal RD while the switch circuit  200  transfers VPP 3  to the column decoder  140  as the column-gating voltage V YG  in response to the boosting signal PBST. 
     The row decoder  120  selects a wordline (e.g., WL 1 ) in response to a corresponding row address assigned to the wordline. The wordline voltage V WL  of VPP 1  is applied to the control gates of the selected memory cells through the selected wordline. At the same time, the column decoder  140  selectively activates a column selection signal (e.g., Y 1 ) in response to a column address assigned thereto. The column-gating voltage V YG  of VPP 3  is applied to gates of the column gates T 11 , T 12 , . . . , and T 1 m. Then, selected bitlines (e.g., BL 1 , BL 4 , . . . ) are connected to the sense amplifiers in the SA/WD block  150 . As a result, the sense amplifiers detect the present states of the selected memory cells MC by sensing voltages on the bitlines. The column-gating voltage of VPP 3  applied to the column gates, VCC+Vth, prevents voltage drops of the bitline voltages to be detected in the sense amplifiers. During the read operation, the programmed memory cell retains a non-conductive state, due to a higher threshold voltage, for the wordline voltage V WL  of VPP 1 . Concurrently, a voltage on a bitline connected to the memory cell increases to cause the sense amplifier to determine that the memory cell is programmed. 
     As aforementioned, the column gates connect the bitlines to the sense amplifier or the write drivers in response to the high voltages in order to transfer voltages at higher speed during a read operation or a programming operation. The high voltages applied to the control electrodes of the NVNMOS transistors as column gates are advantageous to prevent a voltage loss of the bitline voltage or the drain voltage. The high voltages VPP 1 , VPP 2 , and VPP 3  are generated from the generators  160 ,  170 , and  180 , respectively, by means of charge pumps. Thus, even while the power supply voltage VCC becomes lower, the high voltages can be established and maintained. As a result, operation of the NOR flash memory of the present invention is not degraded with increased operating speed during a read operation or a programming operation because the drivability of the HVNMOS-type column gates is enhanced by the high voltages provided therein. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as described in the accompanying claims.