Patent Publication Number: US-2023154523-A1

Title: Memory cell driver circuit

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a driver circuit for operating memory cells, in particular for reading, programming, and erasing memory cells for semiconductor memory devices, such as non-volatile memories. 
     BACKGROUND OF THE INVENTION 
     Memory circuits commonly use an external voltage source. However, the memory cell requires various voltages other than the external voltage in certain circumstances. The memory circuit internally generates a voltage higher than the external voltage for programming or erasing stored information in the memory cells to meet such voltage needs. 
     Specifically, various voltages in embedded flash memory circuits are applied to the memory cell through word lines for the program, read, and erase operations. A high-level voltage is applied to the memory cells for programming or erasing purposes, and a low-level voltage is applied to read the memory cells. Generally, a high programming voltage equivalent to the three to four times a standard power supply voltage (i.e., 7.5V˜10 V) is used. The supply of the high voltage to the memory cells requires the switching operation of a set of PMOS transistors. On the other hand, the supplying of the low reading voltages has been implemented through a switching operation of the NMOS transistors. 
     However, driving the low voltage with NMOS transistors to read the memory cell has a technical limit. The NMOS transistors lack the capacity for driving sufficient ranges of the voltages to determine if the sensed memory cell is programmed. Therefore, the reference voltage brought by the NMOS transistors tends to fail to reach a targeted supply voltage level (VDD) for accurate verification of the memory cell. Rather, a voltage level lower than the VDD voltage, a voltage equivalent to the VDD voltage minus a threshold voltage of the NMOS transistor, had to be applied to the memory cell. Thus, applying the broad range of voltages for an accurate reading of the memory cell is challenging. 
     In view of the state of the art outlined in the foregoing, the problem that the Applicant has faced has been how to supply the targeted level of the reference voltage to the memory cell to read. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a driver circuit for operating a memory cell adapted to be coupled to at least one memory cell through a respective output node, the driver circuit including: a first circuit for supplying the memory cell with a first read reference voltage through the output node; a second circuit for supplying the memory cell with a second read reference voltage through the output node; and a third circuit for controlling an operation of the second circuit, wherein a range of the second read reference voltage at the output node at the output node is wider than a range of the first read reference voltage at the output node during a read operation on the memory cell. 
     According to another aspect of the present invention, the first circuit includes stacked PMOS transistors and stacked NMOS transistors in series in which a common region of a pair of the PMOS and NMOS transistors is connected to the output node coupled to the memory cell; the PMOS transistors are adapted to provide a high voltage charging path between one of connected voltage supply nodes and the output node for programming or erasing the memory cell; and the NMOS transistors are adapted to provide a high voltage discharging path between the output node and one of the connected voltage supply nodes. 
     According to another aspect of the present invention, the NMOS transistors are adapted to provide a low voltage discharging path between the output node and the one of the connected voltage supply nodes. 
     According to another aspect of the present invention, the NMOS transistors further include a pair of NMOS transistors in parallel connected to respective voltage supply nodes. 
     According to another aspect of the present invention, one of the pair of NMOS transistors is adapted to be activated for charging the output node to a low-level voltage during the read operation of the memory cell and the other one of the pair of NMOS transistors is adapted to be activated for discharging the output node to a ground potential. 
     According to another aspect of the present invention, the second circuit includes a plurality of PMOS transistors stacked in series between a connected voltage supply node and the output node, each having a gate terminal coupled to a respective gate control node connected to the third circuit. 
     According to another aspect of the present invention, the second voltage at the output node is between a ground potential and a supply voltage at the voltage supply node coupled to the second circuit. 
     According to another aspect of the present invention, the third circuit includes one or more transistors for applying gate control signals to the PMOS transistors in the second circuit based on an external input signal received. 
     According to another aspect of the present invention, a level of the supply voltage at the voltage supply node is a nominal tolerable voltage stress level of the PMOS transistors based on an external input signal received. 
     According to another aspect of the present invention, the third circuit sets the gate control voltages for the plurality of PMOS transistors at a ground potential to deliver the second voltage to the output node during the read operation on the memory cell. 
     According to another aspect of the present invention, the third circuit is adapted to apply predefined different gate control voltages to deactivate the respective PMOS transistors when the output node is being charged to the high-level voltage through the first circuit. 
     According to another aspect of the present invention, the third circuit is adapted to apply a gate control voltage identical to the high-level voltage at the output node to the PMOS transistor adjacent to the output node when the output node is being charged to the high-level voltage through the first circuit. 
     According to another aspect of the present invention, the third circuit is adapted to apply the gate control voltages to the respective PMOS transistors, wherein the gate control voltages decrease in magnitude from the high-level voltage by a ratio defined by a number of the PMOS transistors. 
     According to another aspect of the present invention, the third circuit is adapted to selectively deactivate the PMOS transistors when the output node is being discharged to a ground level through the stacked NMOS transistors in the first circuit. 
     According to still another aspect of the present invention, an information processing system that includes at least one memory device, the memory devices including a driver circuit for driving a memory cell, the driver circuit including: a first circuit for charging the memory cell to a first voltage through a connected output node by applying one of voltages from voltage supply nodes connected; a second circuit for charging the memory cell to a second voltage through the output node by applying a voltage from a voltage supply node connected; and a third circuit for controlling an operation of the second circuit, wherein a range of the second voltage at the output node is greater than a range of the first voltage at the output node during a read operation on the memory cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of the present invention will become apparent to those skilled in the art from the following description regarding the drawings. Understanding that the drawings depict only typical embodiments of the invention and are not, therefore, to be considered limiting in scope, the invention will be described with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG.  1    is a functional block of a driver circuit according to the present invention; 
         FIG.  2    is a schematic form of the driver circuit of  FIG.  1    according to a preferred embodiment of the present invention; 
         FIG.  3    shows a charging operation of the driver circuit of  FIG.  1    for reading the memory cell according to one embodiment of the present invention; 
         FIG.  4    shows a discharging operation of the driver circuit of  FIG.  1    according to one embodiment of the present invention; 
         FIG.  5    shows a charging operation of the driver circuit of  FIG.  1    for programming or erasing the memory cell according to one embodiment of the present invention; 
         FIG.  6    shows a discharging operation of the driver circuit of  FIG.  1    according to another embodiment of the present invention; and 
         FIGS.  7 A and  7 B  are graphical diagrams illustrating distribution profiles of resulting cell threshold voltages of the embedded flash memory device without and with the present invention, respectively. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail hereinbelow with reference to the attached drawings. 
       FIG.  1    shows a block diagram of a driver circuit including a pair of voltage supply circuits for programming, reading, or erasing operation of memory devices. 
     The driver circuit  101 , for example, includes a circuit node  112  that is connected to a respective word line of a memory device  114  including one or more memory cell arrays (not shown). A memory cell in the memory device  114  can be programmed by injecting electrical charge into its floating gate to increase the cell threshold voltage therefore. 
     A voltage generator  102 , for example, includes a voltage pump (not shown) for generating and transmitting various high voltage levels to both a control circuit  104  and a CMOS transfer circuit  106  via a voltage node VPP connected. For programming, erasing, or reading the memory cell, the voltage generator  102  generates a voltage ranging between ground potential and a high voltage value defined as approximately a voltage potential equal to a supply voltage (VDD) multiplied by various ratios, e.g., four times VDD voltage. The value of the ratios is set based on the use of circuit node  112  in response to the different operations on the memory cell as described in  FIGS.  3  to  6   . 
     In an exemplary but not-limitative embodiment of the present invention, the control circuit  104  includes at least one transistor and is adapted to operate the PMOS transfer circuit  108  to drive the circuit node. The control circuit  104  is also adapted to apply different control signals to the PMOS transfer circuit  108  to reduce expected voltage stress on the PMOS transfer circuit  108  during the programming or erasing operation. The control circuit  104  can be further adapted to selectively operate transistors in the PMOS transfer circuit  108  to prevent an electrical leakage from the circuit node  112  when the circuit node  112  is discharged to a ground potential level. 
     The CMOS transfer circuit  106  includes a plurality of PMOS and NMOS transistors and charges the memory device  114  to the high voltage via the circuit node  112  upon receiving the high voltage from the voltage generator  102  for programming or erasing the memory cell. The CMOS transfer circuit  106  is further adapted to discharge the memory cell to a potential ground level for idle operation. For reading the memory cell, the CMOS transfer circuit  106  is adapted to apply a voltage to the circuit node  112  based on a voltage at a VRD 1  node generated by a voltage source  110 . 
     In an exemplary but not-limitative embodiment of the present invention, PMOS transfer circuit  108  includes a plurality of PMOS transistors to drive the circuit node. Upon receiving control signals of the control circuit  104 , the PMOS transfer circuit  108  applies a read reference voltage to the circuit node  112  to determine whether the memory cell is in a programmed state or not (erased). 
     A voltage source  110  is configured to generate various voltage levels between a ground potential and a standard supply voltage VDD (e.g., 2.5 volts). The voltage source  110  supplies the same or different voltages to the CMOS transfer circuit  106  and the PMOS transfer circuit  108  via VRD 1  and VRD 2  nodes, respectively. 
     For the operations of the memory cell, the circuit node  112  transmits voltages generated by the CMOS transfer circuit  106  and/or the PMOS transfer circuit  108  to the memory cell. For example, the circuit node  112  receives currents from the activated CMOS transfer circuit  106  and currents from the activated. PMOS transistor circuit  108 . Thus, a voltage at the circuit node  112  is responsive to the currents received. 
     The memory device  114  is adapted to include the memory cell array in which the memory cells are connected to respective circuit nodes  112 . Each of the memory cells has a programmed threshold voltage range and an erased voltage range, as shown in  FIGS.  7 A and  7 B . 
     Circuits and methods for implementing charge and discharge the circuit node  112  are described in reference to FIGS. 2 - 6 . The circuits and methods set forth in more detail may be implemented within a single integrated circuit die or may be implemented in a multiple-chip module. 
       FIG.  2    is a schematic form of the driver circuit of  FIG.  1    according to a preferred embodiment of the present invention. 
     In an exemplary but non-limitative embodiment of the present invention, the CMOS transfer circuit  106  includes nine stacked P and N-type metal oxide semiconductor transistors. In the CMOS transfer circuit  106 , the four P-type metal oxide semiconductor (PMOS) transistors CM 1 , CM 2 , CM 3  and CM 4  are arranged to connect the voltage node VPP to the circuit node  112 . 
     The CM 1 , CM 2 , CM 3  and CM 4  are adapted to receive gate signals through their gate terminals coupled to the nodes N 1 , N 2 , N 3  and N 4 , respectively. 
     The CM 1  has a source coupled to the VPP node having a voltage ranging from the VDD voltage to a voltage of four times the VDD voltage, a drain coupled to a source of the CM 2 , and the gate coupled to a node N 1  to provide a current path through CM 1 . The CM 2  has a source connected to the drain of the CM 1 , a drain coupled to a source of the transistor CM 3 , and a gate coupled to a node N 2  to provide a current path between the CM 1  and CM 3 . The CM 3  has a source connected to the drain of the CM 2 , a drain coupled to a source of the transistor CM 4 , and a gate coupled to a node N 3  to provide a current path between the CM 2  and CM 4 . The CM 4  has a source connected to the drain of the CM 3 , a gate coupled to a node N 4  to provide a current path through the transistor CM 4 , and a drain coupled to the circuit node  112 . 
     In the CMOS transfer circuit  106 , the five N-type metal oxide semiconductor (NMOS) transistors CM 5 , CM 6 , CM 7 , CM 8  and CM 9  are arranged to connect the circuit node  112  to either a ground node VSS and the VRD 1  node. 
     The CM 5  has a drain connected to both the drain of the CM 4  and the circuit node  112 , a source coupled to a drain of the CM 6 , and a gate coupled to the node N 4  to provide a charge path through the CM 5 , The CM 6  has a drain coupled to the source of the CM 5 , a source coupled to a drain of the CM 7 , and a gate coupled to a node N 5  to provide a charge path through the CM 6 . The CM 7  has a drain coupled to the source of the CM 6 , a source coupled to the drains of the CM 8  and CM 9 , and a gate coupled to a node N 6  to provide a charge path through the CM 7 . The CM 8  has a drain coupled to the source of the CM 7 , a source coupled to the ground node VSS, and a gate coupled to a node N 7  to provide a charge path through the CM 8 . The CM 9  has the drain coupled to the source of the CM 7 , a source coupled to the VRD 1  node that can have a voltage level between ground and the supply voltage VDD, and a gate coupled to a gate node N 8  to provide a charge path through the CM 9 . 
     The PMOS transfer circuit  108  is arranged between the VRD 2  node and the circuit node  112 . The VRD 2  node can have a voltage level between ground and the standard supply voltage VDD (e.g. 2.5 volts). The VRD 1  node coupled to the CMOS transfer circuit  106  and the VRD 2  node coupled to the PMOS transistor circuit  108  can have the same or different voltage. 
     In one embodiment, PMOS transistor circuit  108  includes four stacked PMOS transistors PM 1 , PM 2  PM 3  and PM 4 . 
     The PM 1  has a source coupled to the VRD 2  node, a drain coupled to a source of the PM 2 , and a gate coupled to a node N 12  to provide a charge path through the PM 1 . The PM 2  between the PM 1  and PM 3  has a source connected to a drain of the PM 1 , a drain connected to a source of the PM 3 , and a gate coupled to a node N 11  to provide a charge path through the PM 2 . The PM 3  between the PM 2  and PM 4  has a source connected to a drain of the PM 2 , a drain connected to a source of the PM 4 , and a gate coupled to a node N 10  to provide a charge path through the PM 3 . The PM 4  between the PM 3  and the circuit node 112  has a source connected to a drain of the PM 3 , a drain connected to the circuit node  112 , and a gate coupled to a node N 9  to provide a charge path through the PM 4 . 
     In one embodiment, when the VRD 2  node has a voltage equivalent to a supply voltage VDD, the activated PM 1 , PM 2 , PM 3  and PM 4  allow charge flows from the VRD 2  node to the circuit node  112  that is set be charged up to a level predefined. The results of the activation or deactivation of the PM 1 , PM 2 , PM 3  and PM 4  in providing the charge path are described in detail with reference to  FIGS.  3 - 6   . 
     The control circuit  104  is coupled to the PM 1 , PM 2 , PM 3  and PM 4  via the respective nodes N 9 , N 10 , N 11  and N 12  to transmit gate control signals based on received external input signals via SW 1  and SW 2 . The control circuit  104  can include one or more MOS transistors for controlling the PMOS transistors based on input signals received. 
       FIG.  3    shows a charging operation of the driver circuit of  FIG.  1    to read the memory cell according to one embodiment of the present invention. In  FIG.  3   , paths A and B are provided to supply a read reference voltage of which the range can be about from GND to VDD voltage to the circuit node  112 . In an exemplary embodiment, during the reading operation of the memory cell, the circuit node  112  can be charged to a targeted supply voltage level VDD by the path B. 
     The paths A and B are effective by activation of the selected transistors in the CMOS and PMOS transfer circuits  106  and  108 , respectively. 
     The path A is effective when a voltage at the VRD 1  node is less than the VDD voltage minus threshold voltage that is needed to turn the transistors CM 5 , CM 6 , CM 7  and CM 9  in the CMOS transfer Circuit  106 . The CM 5 , CM 6 , CM 7  and CM 9  are activated by the VDD voltage at their gates via the coupled nodes N 4 , N 5 , N 6  and N 8 , respectively. The path A enables the supply of current from the VRD 1  node to the circuit node  112  via the activated CM 5 , CM 6 , CM 7  and CM 9 . As a result of this current through the path A, the circuit node  112  can be charged to a voltage level of which the range is nearly from ground level (GND) to supply voltage VDD minus the threshold voltage. The deactivated CM 8  prevents unwanted leakage of the charges flowing out of the path A to the ground node VSS. 
     The path B is effective when a voltage at the VRD 2  node is greater than a threshold voltage that is needed to turn the transistors PM 1 , PM 2 , PM 3  and PM 4  in the PMOS transfer circuit  108 . A current flow on the PM 1 , PM 2 , PM 3  and PM 4  to the circuit node  112  when these transistors are activated with ground voltage (GND) on the respective gates from the control circuit  104 . The control circuit  104  is coupled to the PM 1 , PM 2 , PM 3  and PM 4  via the respective nodes N 9 , N 10 , N 11  and N 12  to transmit the GND signals based on received external input signals of VDD and GND via SW 1  and SW 2 , respectively. 
     As a result of this current through the path B, the circuit node  112  can be charged up to nearly the supply voltage level VDD at the VRD 2  node. Therefore, a higher read reference voltage to accurately verify the programmed memory cell can be supplied to the circuit node  112  through the path B compared to when the circuit node  112  is charged by the path A alone. 
     In an exemplary embodiment, when the path A, path B, or the paths A and B are effective for driving the memory cell to the read reference level, no other paths are provided for charging the circuit node  112 . Further, unwanted leakage current paths from the circuit node  112  are prevented. For example, a path between the VPP node to the circuit node  112  does not carry currents from the VPP node to the circuit node  112  by turning off the CM 1 . The CM 1  is turned off by having a gate coupled to the node N 1  to receive VDD and a source coupled to the VPP node to receive the VDD. The voltage at the gate of the CM 2  is ground. The CM 3  and CM 4  are turned off by receiving the VDD through their gates such that (i) the current from the VPP node is prevented and (ii) unwanted leakage current paths from the circuit node that is charged is prevented. 
       FIG.  4    shows a discharging operation of the driver circuit of  FIG.  1    according to one embodiment of the present invention. 
     A path C is provided by the CM 5 , CM 6 , CM 7  and CM 8  between the circuit node  112  and the ground node VSS. The path C enables the current flow from the circuit node  112  to the VSS node when the CM 5 , CM 6 , CM 7  and CM 8  have a gate voltage VDD via the nodes N 4 , N 5 , N 6  and N 7 , respectively. 
     When the path C is effective, no other paths are provided. A path between the circuit node  112  to the VPP node is inoperative by turning off the CM 1  and CM 4 . The CM 1 , CM 3  and CM 4  are turned off as their gates receive the VDD voltage, rendering a magnitude of respective gate-to-source voltages less than a magnitude of the threshold voltage that is needed to turn the CM 1  and CM 4  on. Also, a path between the circuit node  112  and the VRD 2  node is inoperative by turning off the PM 1 . The PM 1  is turned off when its gate receives the VDD voltage from the control circuit  104 , rendering a magnitude of the respective gate-source voltage less than a magnitude of the threshold voltage that is needed to turn the PM 1  on. 
     The control circuit  104  is coupled to the PM 1 , PM 2 , PM 3  and PM 4  via the respective nodes N 9 , N 10 , N 11  and N 12  to transmit the VDD, GND, GND, GND signals based on received external input signals of GND via. SW 1  and SW 2 . The CM 9  is deactivated to prevent unwanted loss of charge from the path C when its gate terminal receives a ground voltage at the node N 8 .  FIG.  4    is merely illustrative, and it should be noted that the selective operations of the transistors can be various to implement inoperative paths for discharging the circuit node  112  for operating the memory cells. 
       FIG.  5    shows a charging operation of the driver circuit of  FIG.  1    to program or erase the memory cell according to one embodiment of the present invention. 
     The path D between the VPP node and the circuit node  112  supplies a high voltage to the circuit node  112  to program or write data in the memory cell. The path D is effective by the activated CM 1 , CM 2 , CM 3  and CM 4 . 
     Given that these transistor&#39;s voltage tolerance limit is about a VDD voltage level, a gate voltage of three times the VDD to the CM 1 , CM 2 , CM 3  and CM 4  reduces the voltage stress on those transistors within its nominal operating voltage of VDD (=4×VDD−3×VDD) while the path D is effective. In other words, the stacked CM 1 , CM 2 , CM 3  and CM 4  each can operate at its nominal voltage level of VDD (Gate-Source Voltage) for providing the path D by receiving the gate input voltage, a voltage of three times VDD, respectively. 
     When the path D is effective, unwanted leakage currents from the circuit node  112  are prevented. A path leading to the VRD 2  node from the circuit node  112  is inoperative by turning off the PMOS transfer circuit  108 . 
     For instance, when the path D is effective, a path through the PM 4  is inoperative as the PM 4 &#39;s gate voltage of 4×VDD at the node N 9  places a magnitude of the gate-source voltage of the PM 4  not greater than a Vth (threshold voltage) of the PM 4 . 
     Also, when the path D is effective, a path through the PM 3  is inoperative as the PM 3 &#39;s gate voltage of 3×VDD at the node N 10  drives the common source/drain region between the PM 3  and PM 4  to about a voltage of 3×VDD plus a Vth of the PM 3 . This gate voltage places a magnitude of the gate-source voltage of the PM 3  not being greater than the PM 3  threshold voltage. Subsequently, the source/drain region&#39;s driven voltage reduces voltage stress to be applied to the drain-source of the PM 4  to be not higher than about VDD level (a nominal tolerable voltage stress level). 
     Also, when the path D is effective, a path through the PM 2  is inoperative as the PM 2 &#39;s gate voltage of 2×VDD at the node N 11  drives the common source/drain region between the PM 2  and PM 3  to about a voltage of 2×VDD plus a threshold voltage Vth of the PM 2 . This gate voltage places a magnitude of the gate-source voltage of the PM 2  not being greater than the PM 2  threshold voltage. Subsequently, the source/drain region&#39;s driven voltage reduces voltage stress to be applied to a drain-source of the PM 3  to be not higher than about the VDD level. 
     When the path D is effective, a path through the PM 1  is inoperative as the PM 1 &#39;s gate voltage of VDD at the node N 12  drives the common source/drain region between PM 1  and PM 2  voltage to about VDD plus Vth of the PM 1 . This gate voltage places a magnitude of the gate-source voltage of the PM 1  not being greater than the PM 1  threshold voltage. Subsequently, the source/drain region&#39;s driven voltage reduces voltage stress to be applied to a drain-source of the PM 2  to be not higher than about the VDD level. 
     The control circuit  104  is coupled to the PM 1 , PM 2 , PM 3  and PM 4  via the respective nodes N 9 , N 10 , N 11  and N 12  to transmit the VDD, 2×VDD, 3×VDD, 4×VDD signals based on received external input signals of GND and VDD via SW 1  and SW 2 . 
     Also, when the path D is effective, a path between the circuit node  112  and the ground node VSS blocks leakage currents from the circuit node  112  by the deactivated NMOS transistors of CMOS transfer circuit. 
     In one embodiment, when the path D is effective, a path through the CM 8  is inoperative as the CM 8  has a gate-source voltage not greater than a threshold voltage of the CM 8  by having a ground gate voltage through the N 7 . 
     Also, when the path D is effective, a path through the CM 7  is inoperative as the CM 7 &#39;s gate voltage at the N 6  drives the common source/drain region between CM 7  and CM 8  voltage to about VDD minus Vth of the CM 7 . This gate voltage places a magnitude of the gate-source voltage of the CM 7  not being greater than the CM 7  threshold voltage. Subsequently, the source/drain region&#39;s driven voltage reduces voltage stress to be applied to a drain-source of the CM 8  to be not higher than about the VDD level. 
     Also, when the path D is effective, a path through the CM 6  is inoperative as the CM 6 &#39;s gate voltage at the N 5  drives the common source/drain region between CM 6  and CM 7  voltage to about 2×VDD minus Vth of the CM 6 . This gate voltage places a magnitude of the gate-source voltage of the CM 6  not being greater than the CM 6  threshold voltage. Subsequently, the source/drain region&#39;s driven voltage reduces voltage stress to be applied to a drain-source of the CM 7  to be not higher than about the VDD level. 
     Also, when the path D is effective, a path through the CM 5  is inoperative as the CM 5 &#39;s gate voltage at the N 4  drives the common source/drain region between CM 5  and CM 6  voltage to about 3×VDD minus Vth of the CM 5 . This gate voltage places a magnitude of the gate-source voltage of the CM 5  not being greater than the CM 5  threshold voltage. Subsequently, the source/drain region&#39;s driven voltage reduces voltage stress to be applied to a drain-source of the CM 6  to be not higher than about the VDD level. 
       FIG.  6    shows a discharging operation of the driver circuit of  FIG.  1    according to another embodiment of the present invention. 
     During the discharging operation, a path E between the circuit node  112  and a ground node VSS is provided by activating the CMOS transistor circuit  112 . The VSS node is set to a ground voltage GND for enabling the charge flow from the circuit node  112  to the VSS when the CM 5 , CM 6 , CM 7  and CM 8  have the gate voltage VDD via the nodes N 4 , N 5 , N 6  and N 7 , respectively. 
     When the path E is effective, no other paths are provided. A path through the CM 1  is inoperative when the CM 1 &#39;s gate receives a high voltage 4×VDD, rendering the gate-source voltage magnitude through the coupled node N 1  not higher than the threshold voltage that is needed to turn the PM 1  on. 
     When the path E is effective, a path through the CM 2  is inoperative when the CM 2 &#39;s gate voltage from the N 2  drives the common source/drain region between CM 1  and CM 2  voltage to about 3×VDD voltage plus Vth of the CM 2 . This gate voltage places a magnitude of the gate-source voltage of the PM 2  not being greater than the threshold voltage to turn on the CM 2 . Subsequently, the source/drain region&#39;s driven voltage reduces voltage stress to be applied to a drain-source of the CM 1  to be not higher than about VDD voltage level (a nominal tolerable voltage stress level). 
     When the path E is effective, a path through the CM 3  is inoperative as the CM 3 &#39;s gate voltage from the N 3  drives the common source/drain region between CM 2  and CM 3  to about 2×VDD plus Vth of the CM 3 . This gate voltage places a magnitude of the gate-source voltage of the PM 3  not being greater than the threshold voltage to turn on the CM 3 . Subsequently, the source/drain region&#39;s driven voltage reduces voltage stress to be applied to a drain-source of the CM 2  to be not higher than about the VDD level. 
     When the path E is effective, a path through the CM 4  is inoperative as the CM 4 &#39;s gate voltage from the N 4  drives the common source/drain region between CM 3  and CM 4  to about VDD plus Vth of the CM 4 . This gate voltage places a magnitude of the gate-source voltage of the CM 4  not being greater than the threshold voltage to turn on the CM 4 . Subsequently, the source/drain region&#39;s driven voltage reduces voltage stress to be applied to a drain-source of the CM 3  to be not higher than about the VDD level. 
     It also needs to be noted that the gate voltages of the cascaded PMOS transistors in the CMOS transfer circuit  106  can be reduced by a predefined ratio depending on the number of cascaded transistors coupled to a high voltage supply node. As a result, the voltage stress on each transistor CM 1 , CM 2 , CM 3 , and CM 4  can be avoided or substantially reduced. 
     A path between the VRD 2  node and the circuit node  112  does not carry a current from the circuit node 112  by turning off the PM 1 . The PM 1  is turned off when the PM 1 &#39;s gate receives the VDD voltage from the control circuit  104 , placing a magnitude of the gate-source voltage through the coupled node N 12  not higher than the threshold voltage that is needed to turn the PM 1  on. 
     The unwanted leakage current from the discharging path is prevented. For instance, current leakage through the path between the circuit node  112  and the VRD 2  node is prevented by turning off the PM 1 . The PM 1  is turned off because the magnitude of the gate-source voltage of the PM 1  is not higher than the threshold voltage level to turn the PM 1 . 
     The control circuit  104  is coupled to the PM 1 , PM 2 , PM 3  and PM 4  via the respective nodes N 9 , N 10 , N 11  and N 12  to transmit the VDD, GND, GND, GND signals based on received external input signals of GND via. SW 1  and SW 2 . 
     To summarize, the proposed circuits enable switching the circuit node  112  between VDD and GND during the low voltage switching mode as shown in  FIGS.  3  and  4   , while the circuits operate switching the circuit node  112  between 4×VDD voltage and GND addressing the high voltage stress issue stated above during the high voltage switching mode, as shown in  FIGS.  5  and  6   . 
       FIGS.  7 A and  7 B  are graphic diagrams illustrating distribution profiles of cell threshold voltages in the embedded flash memory device. 
     The circuit node  112  in the proposed circuits can be connected to the embedded flash memory and provide a suitable read reference voltage (VRD 1  or VRD 2 , collectively VRD) level to the embedded flash memory cell. The flash memory stores data in the form of a cell threshold voltage, the lowest voltage at which the flash memory cell can be switched on. 
     During a read operation to the cell, the cell in an “erased” state is turned on by having a cell threshold voltage less than a read reference voltage (VRD) that is applied to the output circuit node  112 . In contrast, the cell in a programmed state is turned off by having a cell threshold voltage greater than the read reference voltage (VRD). Due to the process and voltage variation, memory cell threshold voltage of “E” and “P” states can have a voltage window in which the cell&#39;s threshold voltage lies. 
     Referring to  FIG.  7 A , the threshold voltage distribution “E” can span from a certain negative voltage to a minimum possible VRD to verify the cell threshold level in “E” state voltage distribution. Also, the threshold voltage distribution “P” can span from a maximum possible VRD to verify the cell threshold level in “P” state voltage distribution to a certain positive voltage. Thus, to determine with high accuracy whether the cell is erased or programmed, a large voltage gap of the cell threshold voltage between the erased and programmed states is preferred, since read reference voltage level at the middle of the gap can safely determine whether it is higher or less than the cell threshold voltage considering the wide distribution of the cell threshold voltage. 
     The proposed circuit in this disclosure can provide a wider range of VRD between GND and VDD voltage compared to a conventional circuit having a range of VRD between GND and VDD-Vth (transistor threshold voltage) as shown in  FIG.  7 A . Thus, as illustrated in  FIG.  7 B , a larger voltage gap of the cell threshold voltage between the erased and programmed states can be enabled, resulting in a much more reliable embedded flash memory cell connected to the proposed circuit by having a larger gap between “E” and “P” states. As a result, embedded flash memory lifetime can be extended significantly by adopting the proposed high voltage switching circuit connected to the embedded flash memory cell.