Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to U.S. Provisional Patent Application Ser. No. 61/341,086, filed on Mar. 25, 2010, assigned to the same assignee as the present invention, and incorporated herein by reference in its entirety. 
    
    
     RELATED PATENT APPLICATIONS 
     U.S. Provisional patent application Ser. No. 12/378,036, filed on Feb. 10, 2009, assigned to the same assignee as the present invention, and incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a single-polycrystalline logic-process-compatible integrated circuit memory. More particularly this invention relates to a single-polycrystalline silicon electrically erasable programmable floating gate memory device that comprises either PMOS or NMOS transistors. 
     2. Description of Related Art 
     In the semiconductor industry, generally, there are two important types of CMOS memories. One type is a volatile memory in which the stored data are not retained when its power supply is removed or shut down. The volatile memories include Static Random Access Memory (SRAM) and Dynamic Random Access Memory (DRAM). The other type is a non-volatile memory (NVM) in which the stored data can normally be retained for more than 20 years even after the power supply voltage source is completely disconnected. 
     Today, there are many different kinds of NVM memories aimed for different applications. For example, the most popular NVM today is NAND flash with a very small cell size of about 4λ 2  (λ 2  being the smallest area in the design rule for a given semiconductor process) and is generally used to store huge blocks of data necessary for audio and video serial applications. The second popular NVM is NOR flash with one-transistor cell of about 10λ 2  and is used to store program codes. The third type of NVM is 2-transistor floating gate tunneling oxide (FLOTOX) EEPROM with a cell size of about 80λ 2 . Unlike NAND and NOR Flash RAM that only allow big-block data alterability, EEPROM can achieve the largest number of program/erase (P/E) cycles. In the current design, the EEPROM is capable of 1M P/E cycles when it is operated in units of bytes for small data change applications. 
     There are several disadvantages for NVM. The on-chip high-voltage devices, charge-pump circuits, and complicated double-polycrystalline silicon cell structure are required for basic erase and program operations. Currently the above NVM cell devices are made of a complicated double-polycrystalline silicon high-voltage process. There are several disadvantages for the double-polycrystalline NVM cells. The required voltages for performing program and erase operations are too high for devices that are fabricated using a standard CMOS logic process. For example, the current 0.5 transistor per NAND cell structure requires +20V for Fowler-Nordheim tunneling program or erase operations. For a single transistor NOR flash cell, the channel-hot-electron program operation needs about +10V. However, the Fowler-Nordheim tunneling erase operation requires both +10V and −10V. A current two-transistor EEPROM memory cell structure requires +15V for both Fowler-Nordheim tunneling program erase. As a consequence, the program and erase operations for the above described three NVM cells require an on-chip charge-pump circuit that provides the high-voltage levels in the range from approximately 10V to approximately 20V. The peripheral devices of the NVM array thus require a high voltage breakdown for the operation. The high-voltage breakdown voltages are not compatible with the current process technology for the peripheral single-poly low-voltage logic devices. Having to implement the necessary process modifications to accomplish this high-voltage breakdown device result in increased manufacturing cost. 
     “A New Single-Poly Flash Memory Cell with Low-Voltage and Low-Power Operations for Embedded Applications”, Chi, et al., The 5th Annual IEEE Device Research Conference Digest, June 1997, pp: 126-127, discusses a single-poly flash memory cell structure using triple-well CMOS technology and new program/erase schemes with operating voltage not exceeding the power voltage sources +/−Vcc. Conventional single-poly EPROM, although fully compatible with standard CMOS fabrication, has the disadvantages of high-voltage operations, slow programming, and not electrically erasable. The flash cell with the program/erase schemes permits low-voltage and low-power nonvolatile memory applications in CMOS mixed-signal circuits of system-on-a-chip. 
     U.S. Pat. No. 5,929,478 to Parris, et al. describes a single level gate nonvolatile memory device that includes a floating gate FET and a capacitor fabricated in two P-wells formed in an N-epitaxial layer on a P-substrate. P+ sinkers and N-type buried layers provide isolation between the two P-wells. The NVM device is programmed or erased by biasing the FET and the capacitor to move charge carriers onto or away from a conductive layer which serves as a floating gate of the FET. Data are read from the NVM device by sensing a current flowing in the FET while applying a reading voltage to the capacitor. 
     U.S. Pat. Nos. 6,992,927 and 7,164,606 to Poplevine, et al. provides a NVM array that includes four transistor PMOS non-volatile memory (NVM) cells having commonly connected floating gates. Each of the four transistors executes distinct control, erase, write and read operations, thereby allowing each device to be individually selected and optimized for performing its respective operation. 
     U.S. Provisional patent application Ser. No. 12/378,036, filed by the same applicant as the present invention, presented a single polycrystalline silicon floating gate nonvolatile memory cell that has a MOS capacitor and a storage MOS transistor fabricated with dimensions that can be fabricated using current low voltage logic integrated circuit process. The MOS capacitor has a first plate connected to a gate of the storage MOS transistor to form a floating gate node. Although the single polycrystalline silicon floating gate nonvolatile memory cell using a MOS capacitor can be fabricated using current low voltage logic integrated circuit process, the physical size of the MOS capacitor is relatively large in order to establish a large coupling ratio. As a result, the size of the memory cell is also large and makes it difficult to miniaturize the memory device or the integrated circuit using such single polycrystalline silicon floating gate nonvolatile memory cells. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to provide a single-polycrystalline silicon electrically erasable programmable floating gate memory device that comprises either PMOS or NMOS transistors coupled with at least one PIP or MIM capacitor so as to replace the traditional MOS capacitors for reduce the size of the memory cell. The single-polycrystalline integrated circuit memory device of the invention is logic-process-compatible and comprises gate oxide with varied thickness in the range from below 100 Å to more than 400 Å to meet different design specifications and applications. 
     An object of this invention is to provide a single-polycrystalline thick floating gate transistor that is compatible with the high voltage and CMOS mixed signal process but uses at least one Polycrystalline-Insulator-Polycrystalline (PIP) capacitor. The single-polycrystalline high voltage (HV) device is made of either a PMOS or NMOS device with a gate oxide of around 400 Å thick to achieve superior data retention and low threshold voltage (V T ) disturbance during a read operation. 
     Another object of this invention is to provide a single-polycrystalline thick floating gate transistor that is compatible with the high voltage and CMOS mixed signal process but uses at least one Metal-Insulator-Metal (MIM) capacitor. The single-polycrystalline high voltage (HV) device is made of either a PMOS or NMOS device with a gate oxide of around 400 Å thick to achieve superior data retention and low threshold voltage (VT) disturbance during a read operation. 
     A further object of this invention is to provide a preferable bias voltage set which only requires +/−20V for both program and erase operations employing low-current Fowler Nordheim tunneling scheme for the single-polycrystalline thick floating gate transistor having a gate oxide of about 400 Å. 
     Still another object of this invention is to provide a single-polycrystalline thick floating gate transistor that provides high program and erase endurance cycles. 
     It is yet another object of this invention to provide a single-polycrystalline thick floating gate transistor with substantial cell size reduction by using at least one PIP capacitor rather than an NMOS or PMOS capacitor to avoid having a large well spacing in silicon layout. 
     It is yet further object of this invention to provide a single-polycrystalline thick floating gate transistor with substantial cell size reduction by using at least one MIM capacitor rather than an NMOS or PMOS capacitor to avoid having a large well spacing in silicon layout. 
     Still a further object of this invention is to provide a single-polycrystalline thick floating gate transistor with two preferable coupling PIP or MIM capacitors, one large and one small, in contrast to the traditional approach that only uses one large coupling capacitor. The size of the small capacitor is preferably made about the same as the single-polycrystalline thick floating gate transistor of this invention. The size of the large capacitor is preferably made more than 10 times of the total area of the small capacitor and the gate of the single-polycrystalline thick floating gate transistor. 
     Another object of this invention is to provide program and erase operations for the single-polycrystalline thick floating gate transistor with two coupling PIP or MIM capacitors. During the program and erase operations, the large and small capacitors are coupled with the preferable HV of opposite polarities denoted as VPP and VNN for reducing the required high-voltage. The voltages VPP and VNN applied to the two capacitors have to be reversed between program and erase operations. Accordingly, a HV NMOS or PMOS device with approximately 20V breakdown voltage (BVDS) can be used for proper program and erase operations of the single-polycrystalline thick floating gate transistor of the present invention. 
     A further object of this invention is to provide a single-polycrystalline floating gate transistor with gate oxide of thin or medium thickness. The thickness of the thin oxide is below 100 Å and the thickness of the medium oxide is in the range between 100 Å to 400 Å. The single-polycrystalline floating gate transistor with gate oxide of thin or medium thickness is preferably made compatible with the high voltage and CMOS mixed signal process but using a PIP capacitor. Both thin and medium oxide floating-gate single-polycrystalline devices are preferably made of either PMOS or NMOS device for cell size reduction purpose. It is understandable that the performance of data retention in memory cells with thin or medium gate oxide would not be as good as those with a thicker gate oxide of 400 Å. 
     In accordance with the present invention, there are three approaches to designing the single-polycrystalline silicon electrically erasable programmable floating gate memory device. The first approach is to have a single-polycrystalline silicon floating gate HV MOS transistor and two PIP or MIM capacitors fabricated with dimensions that can be manufactured using current high voltage and mixed signal integrated circuit process. Both two PIP or MIM capacitors have a first plate connected to the gate of the HV MOS transistor so that the gate of the HV MOS transistor is floating and forms a floating gate node of the floating gate HV MOS. The second plate of the PIP or MIM capacitor is formed by another layer of polycrystalline silicon. 
     One of the two PIP or MIM capacitors is made with the smallest size allowed in the design rule of the layout. It is used as the tunneling capacitor. The other PIP or MIM capacitor is made with a large physical size and used as a coupling capacitor. The drain of the HV MOS transistor is connected to the source side of a one pass HV MOS transistor and the source of the HV MOS transistor is connected to a source line. The physical size of the coupling PIP or MIM capacitor in combination with the HV MOS transistor is relatively large in comparison to the physical size of the tunneling PIP or MIM capacitor with a ratio of 10 or greater for the embodiments of this invention. 
     The large ratio between the physical sizes provides a large coupling ratio of approximately greater than 90%. When a voltage is applied to the second plate of the coupling PIP or MIM capacitor and drain (or source or bulk) of the HV MOS transistor, the large coupling ratio enables the coupling of a large fraction of the voltage applied to the second plate of the coupling PIP or MIM capacitor and drain (or source or bulk) of the HV MOS transistor to the floating gate node. A voltage applied to the tunneling PIP or MIM capacitor establishes a voltage field that initiates Fowler-Nordheim tunneling effect. When the voltage at the second plate of the coupling PIP or MIM capacitor and drain (or source or bulk) of the HV MOS transistor is negative and the voltage applied to the tunneling PIP or MIM capacitor is positive, charges present on the floating gate are extracted out of the floating gate. To the contrary, when the voltage at the second plate and drain (or source or bulk) of the HV MOS transistor is positive and the voltage applied to the tunneling PIP or MIM capacitor is negative, charges present underneath the insulator of the tunneling PIP or MIM capacitor are injected into the floating gate. 
     The second approach is to have a single-polycrystalline silicon floating gate medium voltage (MV) MOS transistor and two PIP or MIM capacitors fabricated with dimensions that can be manufactured using current high voltage and mixed signal integrated circuit process. In this embodiment, the MV MOS transistor is defined as a single-polycrystalline silicon floating gate MOS transistor having a thick gate oxide but with a lower voltage level denoted as VDD applied to the source/drain. Both two PIP or MIM capacitors have first plate connected to the gate of the MV MOS transistor so that the gate of the MV MOS transistor is floating and forms a floating gate node of the floating gate MV MOS. The second plate of the PIP or MIM capacitor is formed by another layer of polycrystalline silicon. 
     One of the two PIP or MIM capacitors is made with the smallest size allowed in the design rule of the layout. It is used as the tunneling capacitor. The other PIP or MIM capacitor is made with a large physical size and used a coupling capacitor. The drain of the MV MOS transistor is connected to the source side of a one pass MV MOS transistor and the source of the MV MOS transistor is connected to a source line. The physical size of the coupling PIP or MIM capacitor is relatively large in comparison to the physical size of the tunneling PIP or MIM capacitor with a ratio of 10 or greater for the embodiments of this invention. 
     The large ratio between the physical sizes provides a large coupling ratio of approximately greater than 90%. When a voltage is applied to the second plate of the coupling PIP or MIM capacitor, the large coupling ratio enables the coupling of a large fraction of the voltage applied to the second plate of the coupling PIP or MIM capacitor to the floating gate node. A voltage applied to the tunneling PIP or MIM capacitor establishes a voltage field that initiates Fowler-Nordheim tunnel effect. When the voltage at the second plate of the coupling PIP or MIM capacitor is negative and the voltage applied to the tunneling PIP or MIM capacitor is positive, charges present on the floating gate are extracted out of the floating gate. To the contrary, when the voltage at the second plate of the coupling PIP or MIM capacitor is positive and the voltage applied to the tunneling PIP or MIM capacitor is negative, charges present underneath the insulator of the tunneling PIP or MIM capacitor are injected into the floating gate. 
     The third approach is to have a single-polycrystalline silicon floating gate HV MOS transistor and a coupling PIP or MIM capacitor fabricated with dimensions that can be manufactured using current high voltage and mixed signal integrated circuit process. The coupling PIP or MIM capacitor has a first plate connected to the gate of the HV MOS transistor so that the gate of the HV MOS transistor is floating and forms a floating gate node of the floating gate HV MOS. The second plate of the PIP or MIM capacitor is formed by another layer of polycrystalline silicon. The drain of the HV MOS transistor is connected to the source side of a one pass HV MOS transistor and the source of the HV MOS transistor is connected to a source line. 
     The physical size of the coupling PIP or MIM capacitor is relatively large in comparison to the physical size of the HV MOS transistor with a ratio of 10 or greater for the embodiments of this invention. The large ratio between the physical sizes provides a large coupling ratio of approximately greater than 90%. When a voltage is applied to the second plate of the coupling PIP or MIM capacitor, the large coupling ratio enables the coupling of a large fraction of the voltage applied to the second plate of the coupling PIP or MIM capacitor to the floating gate node. A voltage applied to the drain (or source or bulk) of the HV MOS transistor establishes a voltage field within the gate oxide of the HV MOS transistor so that Fowler-Nordheim tunneling is initiated. When the voltage at the second plate of the coupling capacitor is negative and the voltage applied to the drain (or source) of the HV MOS transistor is positive, charges present on the floating gate are extracted out of the floating gate. To the contrary, when the voltage at the second plate of the coupling PIP or MIM capacitor is positive and the voltage applied to the drain (or source or bulk) of the HV MOS transistor is negative, charges present underneath the HV MOS gate oxide are injected into the floating gate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a schematic diagram of a floating gate HV PMOS transistor with two PIP or MIM capacitors according to the first embodiment of the present invention. 
         FIG. 1   b  is a table illustrating the voltage levels of voltage sources applied to the floating gate HV PMOS transistor with two PIP or MIM capacitors of the first embodiment. 
         FIG. 2   a  is a schematic diagram of a floating gate HV NMOS transistor with two PIP or MIM capacitors according to the second embodiment of the present invention. 
         FIG. 2   b  is a table illustrating the voltage levels of voltage sources applied to the floating gate HV NMOS transistor with two PIP or MIM capacitors of the second embodiment. 
         FIG. 3   a  is a schematic diagram of a floating gate MV NMOS transistor with two PIP or MIM capacitors according to the third embodiment of the present invention. 
         FIG. 3   b  is a table illustrating the voltage levels of voltage sources applied to the floating gate MV NMOS transistor with two PIP or MIM capacitors of the third embodiment. 
         FIG. 4   a  is a schematic diagram of a floating gate MV PMOS transistor with two PIP or MIM capacitors according to the fourth embodiment of the present invention. 
         FIG. 4   b  is a table illustrating the voltage levels of voltage sources applied to the floating gate MV PMOS transistor with two PIP or MIM capacitors of the fourth embodiment. 
         FIG. 5   a  is a schematic diagram of a floating gate HV PMOS transistor with one PIP or MIM capacitor according to the fifth embodiment of the present invention. 
         FIG. 5   b  is a table illustrating the voltage levels of voltage sources applied to the floating gate HV PMOS transistor with one PIP or MIM capacitor of the fifth embodiment. 
         FIG. 6   a  is a schematic diagram of a floating gate HV NMOS transistor with one PIP or MIM capacitor according to the sixth embodiment of the present invention. 
         FIG. 6   b  is a table illustrating the voltage levels of voltage sources applied to the floating gate HV NMOS transistor with one PIP or MIM capacitor of the sixth embodiment. 
         FIG. 7  is a table showing three major categories of technology to form a PIP or MIM capacitor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1   a  shows a schematic diagram of the first embodiment of the single-polycrystalline silicon electrically erasable programmable floating gate memory device of the present invention in which two HV transistors are single-poly PMOS transistors formed on an N-well (NW). The memory cell comprises a one pass HV PMOS transistor  270  and a floating gate HV PMOS storage transistor  280  with two PIP or MIM capacitors  250  and  260 . The floating gate storage transistor HV PMOS  280  and the one pass transistor HV PMOS  270  are formed on NW  220 . Those two HV PMOS transistors  270  and  280  are connected in series. The bit line BL  215  is connected to the drain side of HV PMOS  270  and the source line SL  225  is connected to the source side of HV PMOS  280 . 
     According to the present invention, the single-polycrystalline floating gate HV PMOS storage transistor  280  of the first embodiment has a thick gate oxide with thickness approximately 400 Å, preferably in the range between 360 Å to 440 Å. The memory device requires only high voltages approximately +/−20V for both program and erase operations employing low-current Fowler Nordheim tunneling scheme. Preferably, the high voltage level is in the range between 18V to 22V. 
     With reference to the table in  FIG. 1   b , the program and erase operations for the storage HV PMOS transistor  280  refer to the Fowler-Nordheim tunneling program and erase operations occurring between the floating node FG at the first plate and the second plate of the tunneling PIP or MIM capacitor  260 . During the program operation, VCG  295 , WL  285 , SL  225  and BL  215  are applied with a negative voltage VNN. NW  220  is connected with 0V. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the negative voltage is coupled to the floating node FG, where Ccell is the effective capacitance of the HV PMOS transistor  280 , and CA and CB are the capacitances of the two PIP or MIM capacitors  250  and  260  respectively. In the present invention, the value of CA is preferably greater than Ccell, and the ratio (CA+Ccell)/CB is preferably greater than 10. 
     As for the node VEG  290 , it is applied with a positive voltage VPP and it only contributes less coupling voltage to FG because of the smaller capacitance CB of the PIP or MIM capacitor  260 . With the bias voltages described above, a small electric field (˜1 MV/cm) occurs at the coupling capacitor  250  and the HV PMOS transistor  280 , and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  260 . As a result, electrons are extracted from the charge storage floating gate FG into the second plate of the tunneling PIP or MIM capacitor  260  through the tunneling oxide and the threshold voltage of the HV PMOS transistor  280  is increased. In the Fowler-Nordheim tunneling programming, it consumes only a current below 10 nA during the program operation of the present invention. 
     In a same manner, during the erase operation, VCG  295 , SL  225  and BL  215  are applied with the positive voltage VPP. NW  220  is also connected to VPP. WL  285  is connected to 0V. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the positive voltage is coupled to the floating node FG. As for the node VEG  290 , it is applied with the negative voltage VNN and it only contributes less coupling voltage to FG because of the small capacitance CB of the PIP or MIM capacitor  260 . With the bias voltages described above, a small electric field (˜1 MV/cm) occurs at the coupling capacitor  250  and the HV PMOS transistor  280  and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  260 . As a result, electrons are injected from the second plate of the tunneling PIP or MIM capacitor  260  into the charge storage floating gate FG through the tunneling oxide and the threshold voltage of HV PMOS transistor  280  is decreased. In the Fowler-Nordheim tunneling erasing, it consumes only a current below 10 nA during the erase operation. 
     In the program inhibition for the storage HV PMOS transistor  280 , VCG  295  and WL  285  are still applied with the negative voltage VNN but SL  225  and BL  215  are applied with 0V respectively. NW  220  is also connected with 0V. Therefore, only a certain portion, i.e., CA/(Ccell+CA+CB), of VNN will be coupled to the FG node. Because the node VEG  290  is applied with 0V accordingly, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  260 . As a result, a smaller electric field (2˜4 MV/cm) occurs at the tunneling capacitor  260  and the HV PMOS transistor  280  and a smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  250 . With the biased conditions, the program inhibition prevents the threshold voltage of the HV PMOS transistor  280  from being changed. 
     For an unselected HV PMOS transistor  280  in a memory array during the program operation, there are two cases of program inhibition as shown in the table of  FIG. 1   b . In the first case, VCG  295  and WL  285  are applied with 0V but SL  225  and BL  215  are applied with the negative voltage VNN respectively. NW  220  is connected with 0V. Thus, HV PMOS transistor  280  stays at a shut-off state. In other words, only a certain portion, i.e. much smaller than Ccell/(Ccell+CA+CB), of VNN will be coupled to the FG node. Because the node VEG  290  is applied with VPP accordingly, there is less coupling voltage to the FG node through the tunneling PIP or MIM capacitor  260  and a smaller electric field (2˜4 MV/cm) occurs at the tunneling PIP or MIM capacitor  260 . As a result, the program inhibition can prevent the threshold voltage of the HV PMOS transistor  280  from being changed. 
     In the second case, VCG  295 , WL  285 , VEG  290 , SL  225 , NW  220  and BL  215  are all applied with 0V. Thus, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  260 , the coupling PIP or MIM capacitor  250  or the HV PMOS transistor  280 . Therefore, the program inhibition can prevent the threshold voltage of the HV PMOS transistor  280  from being changed. 
     In a read operation, the selected VCG  295 , WL  285  and VEG  290  are applied with 0V respectively. SL  225  and NW  220  are applied with VDD. Thus, there is a read current flowing through the HV PMOS transistors  280  and  270  with 1V applied to BL  215  once the HV PMOS transistor  280  stays at the erase status. 
       FIG. 2   a  shows a schematic diagram of the second embodiment of the single-polycrystalline silicon electrically erasable programmable floating gate memory device of the present invention in which two high voltage (HV) transistors are single-poly NMOS transistors formed in a shallow p-type diffusion well, commonly referred to as a triple P-well (TPW), on a deep N-well (DNW). The memory cell comprises a one pass HV NMOS transistor  370  and a floating gate HV NMOS storage transistor  380  with two PIP or MIM capacitors  350  and  360 . The floating gate HV NMOS  380  and one pass transistor HV NMOS  370  are formed on TPW  305  in DNW  320 . Those two HV NMOS transistors  370  and  380  are connected in series. The bit line BL  315  is connected to the drain side of HV NMOS  370  and the source line SL  325  is connected to the source side of HV NMOS  380 . 
     According to the present invention, the single-polycrystalline floating gate HV NMOS storage transistor  380  of the second embodiment has a thick gate oxide with thickness approximately 400 Å, preferably in the range between 360 Å to 440 Å. The memory device requires only high voltages approximately +/−20V for both program and erase operations employing low-current Fowler Nordheim tunneling scheme. Preferably, the high voltage level is in the range between 18V to 22V. 
     With reference to the table in  FIG. 2   b , the program and erase operations for the storage HV NMOS transistor  380  refer to the Fowler-Nordheim tunneling program and erase operations occurring between the floating node FG at the first plate and the second plate of the tunneling PIP or MIM capacitor  360 . During the program operation, VCG  395 , WL  385 , SL  325 , BL  315 , TPW  305  and DNW  320  are applied with a positive voltage VPP. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the positive voltage is coupled to the floating node FG, where Ccell is the effective capacitance of the HV NMOS transistor  380 , and CA and CB are the capacitances of the two PIP or MIM capacitors  350  and  360  respectively. In the present invention, the value of CA is preferably greater than Ccell, and the ratio (CA+Ccell)/CB is preferably greater than 10. 
     As for the node VEG  390 , it is applied with a negative voltage VNN and it only contributes less coupling voltage to FG because of the smaller capacitance CB of the PIP or MIM capacitor  360 . With the bias voltages described above, a small electric field (˜1 MV/cm) occurs at the coupling capacitor  350  and the HV NMOS transistor  380 , and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  360 . As a result, electrons are injected from the second plate of the tunneling PIP or MIM capacitor  360  into the charge storage floating gate FG through the tunneling oxide and the threshold voltage of HV NMOS transistor  380  is increased. In the Fowler-Nordheim tunneling programming, it consumes only a current below 10 nA during the program operation of the present invention. 
     In a same manner, during the erase operation, VCG  395 , SL  325 , BL  315  and TPW  305  are applied with the negative voltage VNN. DNW is tied to 0V. WL  385  is also connected to 0V. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the negative voltage is coupled to the floating node FG. As for the node VEG  390 , it is applied with the positive voltage VPP and it only contributes less coupling voltage to FG because of the small capacitance CB of the PIP or MIM capacitor  360 . With the bias voltages just described, a small electric field (˜1 MV/cm) occurs at the coupling capacitor  350  and the HV NMOS transistor  380  and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  360 . As a result, electrons are extracted from the charge storage floating gate FG into the second plate of the tunneling PIP or MIM capacitor  360  through the tunneling oxide and the threshold voltage of HV NMOS transistor  380  is decreased. In the Fowler-Nordheim tunneling erasing, it consumes only a current below 10 nA during the erase operation. 
     In the program inhibition for the storage HV NMOS transistor  380 , VCG  395 , WL  385 , DNW  320 , TPW  305 , SL  325  and BL  315  are still applied with the voltage VPP respectively. Therefore, only a certain portion, i.e., (CA+Ccell)/(Ccell+CA+CB), of VPP will be coupled to the FG node. Because the node VEG  390  is applied with 0V accordingly, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  360 . As a result, a smaller electric field (4˜5 MV/cm) occurs at the tunneling capacitor  360 , and the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  350  and the HV NMOS transistor  380 . With the biased conditions, the program inhibition prevents the threshold voltage of the HV NMOS transistor  380  form being changed. 
     For an unselected HV NMOS transistor  380  in a memory array during the program operation, there are two cases of program inhibition as shown in the table of  FIG. 2   b . For the first case, DNW  320 , TPW  305 , SL  325  and BL  315  are still applied with the positive VPP respectively. VCG  395  and WL  385  are applied with 0V. Thus, only a certain portion, i.e., Ccell/(Ccell+CA+CB), of VPP will be coupled to the FG node. Because the node VEG  390  is applied with VNN accordingly, there is less coupling voltage to the FG node through the tunneling PIP or MIM capacitor  360  and a smaller electric field (4˜5 MV/cm) occurs at the tunneling capacitor  360  and the HV NMOS transistor  380  and the smallest electric field (˜1 MV/cm) occurs at the coupling PIP or MIM capacitor  350 . As a result, the program inhibition can prevent the threshold voltage of the HV NMOS transistor  380  from being changed. 
     In the second case, DNW  320 , TPW  305 , SL  325  and BL  315  are still applied with VPP respectively. VCG  395  and WL  385  are applied with 0V. Thus, only a certain portion, i.e., Ccell/(Ccell+CA+CB), of VPP will be coupled to the FG node. Because the node VEG  390  is applied with 0V accordingly, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  360 . Therefore, the smallest electric field (˜1 MV/cm) occurs at the tunneling capacitor  360  and the coupling PIP or MIM capacitor  350  and the smaller electric field (˜4-5 MV/cm) occurs at the HV NMOS transistor  380 . As a result, the program inhibition can prevent the threshold voltage of the HV NMOS transistor  380  from being changed. 
     In a read operation, the selected VCG  395 , WL  385  and DNW  320  are applied with VDD respectively. SL  325 , VEG  390  and TPW  305  are applied with 0V. Thus, there is a read current flowing through the HV NMOS transistors  380  and  370  with 1V applied to BL  315  once the HV NMOS transistor  380  stays at the erase status. 
       FIG. 3   a  shows a schematic diagram of the third embodiment of the single-polycrystalline silicon electrically erasable programmable floating gate memory device of the present invention in which two MV (medium voltage) transistors are single-poly NMOS transistors formed on a P-substrate. The memory cell comprises a one pass MV NMOS transistor  470  and a floating gate MV NMOS transistor  480  with two PIP or MIM capacitors  450  and  460 . The floating gate MV NMOS  480  and one pass transistor MV NMOS  470  are formed on the P-substrate. Those two MV NMOS transistors  470  and  480  are connected in series. BL  415  is connected to the drain side of MV NMOS  470  and SL  425  is connected to the source side of MV NMOS  480 . In this embodiment, 0V and VDD voltage levels are used to bias the source/drain side of the MV NMOS transistor. However, the coupled high voltage will be established at the tunneling PIP or MIM capacitor  460  while performing erase and program operations. 
     According to the present invention, the single-polycrystalline floating gate MV NMOS storage transistor  480  of the third embodiment has a gate oxide with thickness from below 100 Å to 400 Å. In one configuration, the thickness of the gate oxide is approximately 200 Å, preferably in the range between 180 Å to 220 Å. In this configuration, the memory device requires only medium voltages approximately +/−10V for both program and erase operations employing low-current Fowler Nordheim tunneling scheme. Preferably, the medium voltage level is in the range between 9V to 11V. In another configuration, the thickness of the gate oxide is approximately 100 Å, preferably in the range between 90 Å to 110 Å. In this configuration, the memory device requires only medium voltages approximately +/−5V for both program and erase operations employing low-current Fowler Nordheim tunneling scheme. Preferably, the medium voltage level is in the range between 4.5V to 5.5V. 
     With reference to the table in  FIG. 3   b , the program and erase operations for the storage MV NMOS transistor  480  refer to the Fowler-Nordheim tunneling program and erase operations occurring between the floating node FG at the first plate and the second plate of the tunneling PIP or MIM capacitor  460 . There are two options shown in the table. For the first option, during the program operation, VCG  495  is applied with a positive voltage VPP. However, WL  485 , SL  425  and BL  415  are applied with VDD. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the positive voltage is coupled to the floating node FG, where Ccell is the effective capacitance of the MV NMOS transistor  480 , and CA and CB are the capacitances of the two PIP or MIM capacitors  450  and  460  respectively. In the present invention, the value of CA is preferably much greater than Ccell, and the ratio (CA+Ccell)/CB is preferably greater than 10. 
     As for the node VEG  490 , it is applied with a negative voltage VNN and it only contributes less coupling voltage to FG because of the smaller capacitance CB of the PIP or MIM capacitor  460 . With the bias voltages described above, a smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  450 , a smaller electric field (4˜5 MV/cm) occurs at the MV NMOS transistor  480  and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  460 . As a result, electrons are injected from the second plate of the tunneling PIP or MIM capacitor  460  into the charge storage floating gate FG through the tunneling oxide and the threshold voltage of the MV NMOS transistor  480  is increased. In the Fowler-Nordheim tunneling programming, it consumes only a current below 10 nA during program operation of the present invention. 
     In a same manner, during the erase operation, VCG  495  is applied with the negative voltage VNN. WL  485  is applied with VDD and both SL  425  and BL  415  are applied with 0V. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the negative voltage is coupled to the floating node FG. As for the node VEG  490 , it is applied with the positive voltage VPP and it only contributes less coupling voltage to FG because of the small capacitance CB of the PIP or MIM capacitor  460 . With the bias voltages just described, a smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  450 , a smaller electric field (4˜5 MV/cm) occurs at the MV NMOS transistor  480  and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  460 . As a result, electrons are extracted from the charge storage floating gate FG into the second plate of the tunneling PIP or MIM capacitor  460  through the tunneling oxide and the threshold voltage of the MV NMOS transistor  480  is decreased. In the Fowler-Nordheim tunneling erasing, it consumes only a current below 10 nA during the erase operation. 
     In the program inhibition for the storage MV NMOS transistor  480 , VCG  495  is still applied with the positive voltage VPP. However, WL  485 , SL  425  and BL  415  are just applied with VDD. Therefore, only a certain portion, i.e., (Ccell/(Ccell+CA+CB) of VDD and (CA/(Ccell+CA+CB) of VPP, will be coupled to the FG node. Because the node VEG  490  is applied with 0V accordingly, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  460 . As a result, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  450  and a smaller electric field (4˜5 MV/cm) occurs at the MV NMOS transistor  480  and the tunneling PIP or MIM capacitor  460 . With the biased conditions, the program inhibition prevents the threshold voltage of the MV NMOS transistor  480  form being changed. 
     For an unselected MV NMOS transistor  480  in a memory array during the program operation, there are two cases of program inhibition as shown in the table of  FIG. 3   b . For the first case, SL  425  and BL  415  are still applied with VDD respectively. VCG  495  and WL  485  are applied with 0V. Thus, only a certain portion, i.e., Ccell/(Ccell+CA+CB), of VDD will be coupled to the FG node. Because the node VEG  490  is applied with VNN accordingly, there is less coupling voltage to the FG node through the tunneling PIP or MIM capacitor  460 , a smaller electric field (4˜5 MV/cm) occurs at the tunneling capacitor  460  and the smallest electric field (˜1 MV/cm) occurs at the coupling PIP or MIM capacitor  450  and the MV NMOS transistor  480 . As a result, the program inhibition can prevent the threshold voltage of the MV NMOS transistor  480  from being changed. 
     In the second case, SL  425  and BL  415  are still applied with VDD respectively. VCG  495  and WL  485  are applied with 0V. Thus, only a certain portion, i.e., Ccell/(Ccell+CA+CB), of VDD will be coupled to the FG node. Because the node VEG  490  is applied with 0V accordingly, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  460 . Therefore, the smallest electric field (˜1 MV/cm) occurs at the tunneling capacitor  460 , the coupling PIP or MIM capacitor  450  and the MV NMOS transistor  480 . As a result, the program inhibition can prevent the threshold voltage of the MV NMOS transistor  480  from being changed. 
     For the second option in the table of  FIG. 3   b , during the program operation, VCG  495  is applied with a negative voltage VNN. However, WL  485  is applied with VDD and SL  425  and BL  415  are applied with 0V. Because of the large coupling ratio, i.e., CA/(CA+Ccell+CB)&gt;90%, most of the negative voltage is coupled to the floating node FG. As for the node VEG  490 , it is applied with a positive voltage VPP and it only contributes less coupling voltage to FG because of the smaller capacitance CB of the PIP or MIM capacitor  460 . With the bias voltages described above, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  450 , a smaller electric field (4˜5 MV/cm) occurs at the MV NMOS transistor  480 , and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  460 . As a result, electrons are extracted from the charge storage floating gate FG into the second plate of the tunneling PIP or MIM capacitor  460  through the tunneling oxide and the threshold voltage of MV NMOS transistor  480  is decreased. In the Fowler-Nordheim tunneling programming, it consumes only a current below 10 nA during the program operation. 
     In a same manner, during the erase operation, VCG  495  is applied with VPP. WL  485 , SL  425  and BL  415  are applied with VDD. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the positive voltage is coupled to the floating node FG. As for the node VEG  490 , it is applied with VNN and it only contributes less coupling voltage to FG because of the small capacitance CB of the PIP or MIM capacitor  460 . With the bias voltages just described, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  450 , a smaller electric field (4˜5 MV/cm) occurs at the MV NMOS transistor  480  and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  460 . As a result, electrons are injected from the second plate of the tunneling PIP or MIM capacitor  460  into the charge storage floating gate FG through the tunneling oxide and the threshold voltage of MV NMOS transistor  480  is increased. In the Fowler-Nordheim tunneling erasing, it consumes only a current below 10 nA during program operation. 
     In the programming inhibition for the storage MV NMOS transistor  480 , VCG  495  is still applied with VNN. However, WL  485  is applied with VDD and both SL  425  and BL  415  are just applied with 0V. Therefore, only a certain portion, i.e., (CA/(Ccell+CA+CB), of VNN will be coupled to the FG node. Because the node VEG  490  is applied with 0V accordingly, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  460 . As a result, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  450  and a smaller electric field (4˜5 MV/cm) occurs at the MV NMOS transistor  480  and the tunneling PIP or MIM capacitor  460 . With the biased conditions, the program inhibition prevents the threshold voltage of the MV NMOS transistor  480  from being changed. 
     For an unselected MV NMOS transistor  480  in a memory during the program operation, there are two cases of program inhibition as shown in the table of  FIG. 3   b . For the first case, SL  425  and BL  415  are still applied with 0V respectively. VCG  495  and WL  485  are also applied with 0V. Thus, there is no voltage coupled to the FG node. Because the node VEG  490  is applied with VPP accordingly, there is less coupling voltage to the FG node through the tunneling PIP or MIM capacitor  460 , a smaller electric field (4˜5 MV/cm) occurs at the tunneling capacitor  460  and the smallest electric field (˜1 MV/cm) occurs at the coupling PIP or MIM capacitor  450  and the MV NMOS transistor  480 . As a result, the program inhibition can prevent the threshold voltage of the MV NMOS transistor  480  from being changed. 
     In the second case, SL  425  and BL  415 , VCG  495 , WL  485  and VEG  490  are applied with 0V respectively. Therefore, no electric field occurs at the tunneling capacitor  460 , the coupling PIP or MIM capacitor  450  and the MV NMOS transistor  480 . As a result, the program inhibition can prevent the threshold voltage of the MV NMOS transistor  480  from being changed. 
     In a read operation, the selected VCG  495  and WL  485  are applied with VDD respectively. SL  425  and VEG  490  are applied with 0V. Thus, there is a read current flowing through the MV NMOS transistors  480  and  470  with 1V applied to BL  415  once MV NMOS transistor  480  stays at the erase status in the first option and stays at the program status in the second option. 
       FIG. 4   a  shows a schematic diagram of the fourth embodiment of the single-polycrystalline silicon electrically erasable programmable floating gate memory device of the present invention in which two MV transistors are single-poly PMOS transistors formed in an N-well (NW). The memory cell comprises a one pass MV PMOS transistor  570  and a floating gate MV PMOS storage transistor  580  with two PIP or MIM capacitors  550  and  560 . The floating gate MV PMOS  580  and one pass transistor MV PMOS  570  are formed on NW  505 . Those two MV PMOS transistors  570  and  580  are connected in series. BL  515  is connected to the drain side of MV PMOS  570  and SL  525  is connected to the source side of MV PMOS  580 . In this embodiment, 0V and VDD voltage levels are used to bias the source/drain side of the MV PMOS transistor. However, the coupled high voltage will be established at the tunneling PIP or MIM capacitor  560  while performing erase and program operations. 
     According to the present invention, the single-polycrystalline floating gate MV PMOS storage transistor  580  of the fourth embodiment has a gate oxide with thickness from below 100 Å to 400 Å. In one configuration, the thickness of the gate oxide is approximately 200 Å, preferably in the range between 180 Å to 220 Å. In this configuration, the memory device requires only medium voltages approximately +/−10V for both program and erase operations employing low-current Fowler Nordheim tunneling scheme. Preferably, the medium voltage level is in the range between 9V to 11V. In another configuration, the thickness of the gate oxide is approximately 100 Å, preferably in the range between 90 Å to 110 Å. In this configuration, the memory device requires only medium voltages approximately +/−5V for both program and erase operations employing low-current Fowler Nordheim tunneling scheme. Preferably, the medium voltage level is in the range between 4.5V to 5.5V. 
     With reference to the table in  FIG. 4   b , the program and erase operations for the storage MV PMOS transistor  580  refer to the Fowler-Nordheim tunneling program and erase operations occurring between the floating node FG at the first plate and the second plate of the tunneling PIP or MIM capacitor  560 . There are two options shown in the table. For the first option, during the program operation, VCG  595  is applied with a positive voltage VPP. However, WL  585  is applied with 0V and both SL  525  and BL  515  are just applied with VDD. NW  505  is also connected to VDD. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the positive voltage is coupled to the floating node FG, where Ccell is the effective capacitance of the MV PMOS transistor  580 , and CA and CB are the capacitances of the two PIP or MIM capacitors  550  and  560  respectively. In the present invention, the value of CA is preferably much greater than Ccell, and the ratio (CA+Ccell)/CB is preferably greater than 10. 
     As for the node VEG  590 , it is applied with a negative voltage VNN and it only contributes less coupling voltage to FG because of the smaller capacitance CB of the PIP or MIM capacitor  560 . With the bias voltages described above, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  550 , a smaller electric field (4˜5 MV/cm) occurs at the MV PMOS transistor  580  and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  560 . As a result, electrons are injected from the second plate of the tunneling PIP or MIM capacitor  560  into the charge storage floating gate FG through the tunneling oxide and the threshold voltage of the MV PMOS transistor  580  is decreased. In the Fowler-Nordheim tunneling programming, it consumes only a current below 10 nA during program operation of the present invention. 
     In a same manner, during the erase operation, VCG  595  is applied with the negative voltage VNN. WL  585 , SL  525  and BL  515  are applied with 0V. NW  505  is also connected to 0V. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the negative voltage is coupled to the floating node FG. As for the node VEG  590 , it is applied with the positive voltage VPP and it only contributes less coupling voltage to FG because of the small capacitance CB of the PIP or MIM capacitor  560 . With the bias voltages just described, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  550 , a smaller electric field (4˜5 MV/cm) occurs at the MV PMOS transistor  580  and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  560 . As a result, electrons are extracted from the charge storage floating gate FG into the second plate of the tunneling PIP or MIM capacitor  560  through the tunneling oxide and the threshold voltage of the MV PMOS transistor  580  is increased. In the Fowler-Nordheim tunneling erasing, it consumes only a current below 10 nA during erase operation. 
     In the program inhibition for the storage MV PMOS transistor  580 , VCG  595  is still applied with the positive voltage VPP. However, WL  585  is applied with 0V and both SL  525  and BL  515  are just applied with VDD. NW  505  is also connected to VDD. Therefore, only a certain portion, i.e., (Ccell/(Ccell+CA+CB) of VDD and (CA/(Ccell+CA+CB) of VPP, will be coupled to the FG node. Because the node VEG  590  is applied with 0V accordingly, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  560 . As a result, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  550  and a smaller electric field (4˜5 MV/cm) occurs at the MV PMOS transistor  580  and the tunneling PIP or MIM capacitor  560 . With the biased conditions, the program inhibition prevents the threshold voltage of the MV PMOS transistor  580  form being changed. 
     For an unselected MV PMOS transistor  580  in a memory array during the program operation, there are two cases of program inhibition as shown in the table of  FIG. 4   b . For the first case, SL  525  and BL  515  are still applied with VDD respectively. VCG  595  is applied with 0V and WL  585  is applied with VDD. NW  505  is also connected to VDD. Thus, only a certain portion, i.e., Ccell/(Ccell+CA+CB) of VDD will be coupled to the FG node. Because the node VEG  590  is applied with VNN accordingly, there is less coupling voltage to the FG node through tunneling PIP or MIM capacitor  560 , a smaller electric field (4˜5 MV/cm) occurs at the tunneling capacitor  560  and the smallest electric field (˜1 MV/cm) occurs at the coupling PIP or MIM capacitor  550  and the MV PMOS transistor  580 . As a result, the program inhibition can prevent the threshold voltage of the MV PMOS transistor  580  from being changed. 
     In the second case, SL  525  and BL  515  are still applied with VDD respectively. VCG  595  is applied with 0V and WL  585  is applied with VDD. NW  505  is also connected to VDD. Thus, a certain portion, i.e., Ccell/(Ccell+CA+CB), of VDD will be coupled to the FG node. Because the node VEG  590  is applied with 0V accordingly, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  560 . Therefore, the smallest electric field (˜1 MV/cm) occurs at the tunneling capacitor  560 , the coupling PIP or MIM capacitor  550  and the MV PMOS transistor  580 . As a result, the program inhibition can prevent the threshold voltage of the MV PMOS transistor  580  from being changed. 
     For the second option in the table of  FIG. 4   b , during the program operation, VCG  595  is applied with a negative voltage VNN. However, WL  585 , SL  525  and BL  515  are applied with 0V. NW  505  is also connected to 0V. Because of the large coupling ratio, i.e., CA/(CA+Ccell+CB)&gt;90%, most of the negative voltage is coupled to the floating node FG. As for the node VEG  590 , it is applied with VPP and it only contributes less coupling voltage to FG because of the smaller capacitance CB of the PIP or MIM capacitor  560 . With the bias voltages described above, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  550 , a smaller electric field (4˜5 MV/cm) occurs at the MV PMOS transistor  580  and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  560 . As a result, electrons are extracted from the charge storage floating gate FG into the second plate of the tunneling PIP or MIM capacitor  560  through the tunneling oxide and the threshold voltage of MV PMOS transistor  580  is increased. In the Fowler-Nordheim tunneling programming, it consumes only a current below 10 nA during the program operation. 
     In a same manner, during the erase operation, VCG  595  is applied with VPP. WL  585  is applied with 0V and both SL  525  and BL  515  are applied with VDD. NW  505  is also connected to VDD. Because of the large coupling ratio, i.e., (CA+Ccell)/(CA+Ccell+CB)&gt;90%, most of the positive voltage is coupled to the floating node FG. As for the node VEG  590 , it is applied with VNN and it only contributes less coupling voltage to FG because of the small capacitance CB on the PIP or MIM capacitor  560 . With the bias voltages just described, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  550 , a smaller electric field (4˜5 MV/cm) occurs at the MV PMOS transistor  580  and a strong electric field (&gt;10 MV/cm) occurs at the tunneling PIP or MIM capacitor  560 . As a result, electrons are injected from the second plate of the tunneling PIP or MIM capacitor  560  into the charge storage floating gate FG through the tunneling oxide and the threshold voltage of the MV PMOS transistor  580  is decreased. In the Fowler-Nordheim tunneling programming, it consumes only a current below 10 nA during program operation. 
     In the programming inhibit for the storage MV PMOS transistor  580 , VCG  595  is still applied with VNN. However, WL  585 , SL  525  and BL  515  are just applied with 0V. NW  505  is also connected to 0V. Therefore, only a certain portion, i.e., (CA/(Ccell+CA+CB), of VNN will be coupled to the FG node. Because the node VEG  590  is applied with 0V accordingly, there is no coupling voltage to the FG node through the tunneling PIP or MIM capacitor  560 . As a result, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  550  and a smaller electric field (4˜5 MV/cm) occurs at the MV PMOS transistor  580  and the tunneling PIP or MIM capacitor  560 . With the biased conditions, the program inhibition prevents the threshold voltage of the MV PMOS transistor  580  from being changed. 
     For an unselected MV PMOS transistor  580  in a memory array during the program operation, there are two cases of program inhibition shown in the table of  FIG. 4   b . For the first case, VCG  595 , SL  525  and BL  515  are still applied with 0V respectively. WL  585  is applied with VDD. NW  505  is still connected to 0V. Thus, there is no voltage coupled to the FG node. Because the node VEG  590  is applied with VPP accordingly, there is less coupling voltage to the FG node through the tunneling PIP or MIM capacitor  560 , a smaller electric field (4˜5 MV/cm) occurs at the tunneling capacitor  560  and the smallest electric field (˜1 MV/cm) occurs at the coupling PIP or MIM capacitor  550  and the MV PMOS transistor  580 . As a result, the program inhibition can prevent the threshold voltage of the MV PMOS transistor  580  from being changed. 
     In the second case, SL  525  and BL  515 , VCG  595 , and VEG  590  are applied with 0V respectively. WL  585  is applied with VDD. NW  505  is still connected to 0V. Therefore, no electric field occurs at the tunneling capacitor  560 , the coupling PIP or MIM capacitor  550  and the MV PMOS transistor  580 . As a result, the program inhibition can prevent the threshold voltage of the MV PMOS transistor  580  from being changed. 
     In a read operation, the selected VCG  595  and WL  585  are applied with 0V respectively. SL  525  and NW  505  are applied with VDD and VEG  590  is applied with 0V. Thus, there is a read current flowing through the MV PMOS transistors  580  and  570  with 1V applied to BL  515  once the MV PMOS transistor  580  stays at the program status in the first option and stays at the erase status in second option. 
       FIG. 5   a  shows a schematic diagram of the fifth embodiment of the single-polycrystalline silicon electrically erasable programmable floating gate memory device of the present invention in which two HV transistors are single-poly PMOS transistors formed in an N-well (NW). The memory cell comprises a one pass HV PMOS transistor  670  and a floating gate HV PMOS storage transistor  680  with one PIP or MIM capacitor  650 . The floating gate HV PMOS  680  and one pass transistor HV PMOS  670  are formed on NW  605 . Those two HV PMOS transistors  670  and  680  are connected in series. BL  615  is connected to the drain side of HV PMOS  670  and SL  625  is connected to the source side of HV PMOS  680 . In this embodiment, VNN and VPP voltage levels are used to bias the source/drain side of the HV PMOS transistor. However, the coupled high voltage will be established at the HV PMOS transistor  670  while performing erase and program operations. 
     According to the present invention, the single-polycrystalline floating gate HV PMOS storage transistor  680  of the fifth embodiment has a thick gate oxide with thickness approximately 400 Å, preferably in the range between 360 Å to 440 Å. The memory device requires only high voltages approximately +/−20V for both program and erase operations employing low-current Fowler Nordheim tunneling scheme. Preferably, the high voltage level is in the range between 18V to 22V. 
     With reference to the table in  FIG. 5   b , the program and erase operations for the storage HV PMOS transistor  680  refer to the Fowler-Nordheim tunneling program and erase operations occurring at the insulator oxide of HV PMOS transistor  680 . During the program operation, VCG  695  is applied with VPP. However, WL  685 , SL  625  and BL  615  are applied with VNN. NW  605  is connected to 0V. Because of the large coupling ratio, i.e., CA/(CA+Ccell)&gt;90%, most of the positive voltage is coupled to the floating node FG, where Ccell is the effective capacitance of the HV PMOS transistor  680  and CA is the capacitance of the PIP or MIM capacitor  650 . In the present invention, the ratio CA/Ccell is preferably greater than 10. Therefore, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  650  and a strong electric field (&gt;10 MV/cm) occurs at the HV PMOS transistor  680 . As a result, electrons are injected from the source/drain side of the HV PMOS transistor  680  into the charge storage floating gate FG through the insulator oxide and the threshold voltage of the HV PMOS transistor  680  is decreased. In the Fowler-Nordheim tunneling programming, it consumes only a current below 10 nA during program operation. 
     In a same manner, during the erase operation, VCG  695  is applied with VNN. WL  685  is applied with 0V and SL  625 , BL  615  and NW  605  are applied with VPP. Because of the large coupling ratio, i.e., CN (CA+Ccell)&gt;90%, most of the negative voltage is coupled to the floating node FG. Therefore, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  650  and a strong electric field (&gt;10 MV/cm) occurs at the HV PMOS transistor  680 . As a result, electrons are extracted from the charge storage floating gate FG into the channel underneath the insulator oxide of the HV PMOS transistor  680  and the threshold voltage of the HV PMOS transistor  680  is increased. In the Fowler-Nordheim tunneling erasing, it consumes only a current below 10 nA during erase operation. 
     In the programming inhibition for the storage HV PMOS transistor  680 , VCG  695  is still applied with VPP. However, WL  685  is applied with VNN and both SL  625  and BL  615  are just applied with 0V. NW  605  is also connected to 0V. Thus, a certain portion, i.e., CA/(Ccell+CA), of VPP will be coupled to the FG node. As a result, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  650  and a smaller electric field (4˜5 MV/cm) occurs at the HV PMOS transistor  680 . With the biased conditions, the program inhibition prevents the threshold voltage of the HV PMOS transistor  680  from being changed. 
     For an unselected HV PMOS transistor  680  in a memory array during the program operation, there are two cases of program inhibition shown in the table of  FIG. 5   b . For the first case, SL  625  and BL  615  are still applied with VNN respectively. VCG  695  and WL  685  are applied with 0V. NW  605  is also connected to 0V. Thus, only a certain portion, i.e., Ccell/(Ccell+CA), of VNN will be coupled to the FG node. The smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  650  and the HV PMOS transistor  680 . As a result, the program inhibition can prevent the threshold voltage of the HV PMOS transistor  680  from being changed. 
     In the second case, SL  625  and BL  615 , WL  685 , VCG  695  and NW  605  are applied with 0V respectively. Therefore, no electric field occurs at the coupling PIP or MIM capacitor  650  and the HV PMOS transistor  680 . As a result, the program inhibition can prevent the threshold voltage of the HV PMOS transistor  680  from being changed. 
     In a read operation, the selected VCG  695  and WL  685  are applied with 0V respectively. SL  625  and NW  605  are applied with VDD. Thus, there is a read current flowing through the HV PMOS transistors  680  and  670  with 1V applied to BL  615  once the HV PMOS transistor  680  stays at the program status. 
       FIG. 6   a  shows a schematic diagram of the sixth embodiment of the single-polycrystalline silicon electrically erasable programmable floating gate memory device of the present invention in which two HV transistors are single-poly NMOS transistors formed in a triple P-well (TPW) in a deep N-well (DNW). The memory cell comprises a one pass HV NMOS transistor  770  and a floating gate HV NMOS transistor  780  with one PIP or MIM capacitor  750 . The floating gate HV NMOS  780  and one pass transistor HV NMOS  770  are formed on TPW  705  in DNW  720 . Those two HV NMOS transistors  770  and  780  are connected in series. BL  715  is connected to the drain side of HV NMOS  770  and SL  725  is connected to the source side of HV NMOS  780 . In this embodiment, VNN and VPP voltage levels are used to bias the source/drain side of the HV NMOS. However, the coupled high voltage will be established at the HV NMOS transistor  770  while performing erase and program operations. 
     According to the present invention, the single-polycrystalline floating gate HV NMOS storage transistor  780  of the sixth embodiment has a thick gate oxide with thickness approximately 400 Å, preferably in the range between 360 Å to 440 Å. The memory device requires only high voltages approximately +/−20V for both program and erase operations employing low-current Fowler Nordheim tunneling scheme. Preferably, the high voltage level is in the range between 18V to 22V. 
     With reference to the table in  FIG. 6   b , the program and erase operations for the storage HV NMOS transistor  780  refer to the Fowler-Nordheim tunneling program and erase operations occurring at the insulator oxide of HV NMOS transistor  780 . During the program operation, VCG  795  is applied with VPP. However, WL  785  is applied with 0V and both SL  725  and BL  715  are applied with VNN. TPW  705  is applied with VNN and DNW  720  is applied with 0V. Because of the large coupling ratio, i.e., CA/(CA+Ccell)&gt;90%, most of the positive voltage is coupled to the floating node FG, where Ccell is the effective capacitance of the HV NMOS transistor  780  and CA is the capacitance of the PIP or MIM capacitor  750 . In the present invention, the ratio CA/Ccell is preferably greater than 10. Therefore, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  750  and a strong electric field (&gt;10 MV/cm) occurs at the HV NMOS transistor  780 . As a result, electrons are injected from the channel underneath HV NMOS transistor  780  into the charge storage floating gate FG through the insulator oxide and the threshold voltage of HV NMOS transistor  780  is increased. In the Fowler-Nordheim tunneling programming, it consumes only a current below 10 nA during program operation. 
     In a same manner, during the erase operation, VCG  795  is applied with VNN. WL  785 , SL  725 , BL  715 , DNW  720  and TPW  705  are applied with VPP. Because of the large coupling ratio, i.e., CA/(CA+Ccell)&gt;90%, most of the negative voltage is coupled to the floating node FG. With the bias voltages just described, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  750  and a strong electric field (&gt;10 MV/cm) occurs at the HV NMOS transistor  780 . As a result, electrons are extracted from the charge storage floating gate FG into the channel underneath the insulator oxide of HV NMOS transistor  780  and the threshold voltage of HV NMOS transistor  780  is decreased. In the Fowler-Nordheim tunneling erasing, it consumes only a current below 10 nA during erase operation. 
     In the program inhibition for the storage HV NMOS transistor  780 , VCG  795  is still applied with VPP. However, WL  785 , SL  725 , DNW  720  and BL  715  are just applied with 0V. TPW  705  is applied with VNN. Thus, a certain portion, i.e., CA/(Ccell+CA), of VPP will be coupled to the FG node. Therefore, the smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  750  and a smaller electric field (4˜5 MV/cm) occurs at the HV NMOS transistor  780 . With the biased conditions, the program inhibition prevents the threshold voltage of the HV NMOS transistor  780  form being changed. 
     For an unselected HV NMOS transistor  780  in a memory array during the program operation, there are two cases of program inhibition shown in the table of  FIG. 6   b . For the first case, SL  725  and BL  715  are still applied with VNN respectively. VCG  795  and DNW  720  are applied with 0V, and WL  785  and TPW  705  are applied with VNN. Thus, only a certain portion, i.e., Ccell/(Ccell+CA), of VNN will be coupled to the FG node. The smallest electric field (˜1 MV/cm) occurs at the coupling capacitor  750  and the HV NMOS transistor  780 . As a result, the program inhibition can prevent the threshold voltage of the HV NMOS transistor  780  from being changed. 
     In the second case, SL  725 , DNW  720 , BL  715  and VCG  795  are applied with 0V respectively. WL  785  and TPW  705  are applied with VNN. Therefore, no electric field occurs at the coupling PIP or MIM capacitor  750  and the HV NMOS transistor  780 . As a result, the program inhibition can prevent the threshold voltage of the HV NMOS transistor  780  from being changed. 
     In a read operation, the selected VCG  795  and WL  785  are applied with VDD respectively. SL  725  and TPW  705  are applied with 0V and DNW  720  is applied with VDD. Thus, there is a read current flowing through the HV NMOS transistors  780  and  770  with 1V applied to BL  715  once the HV NMOS transistor  780  stays at the erase status. 
     With reference to the table in  FIG. 7 , a PIP or MIM capacitor of the present invention can be manufactured with three categories of technology. According to the necessary requirement in the electric field of the Fowler-Nordheim tunneling scheme, the oxide thickness and voltage range of VPP/VNN are shown respectively. For category  1 , the thickness of the thick gate-oxide is about 400 Å+/−10%. The corresponding VPP is in the range from 18V to 22V and VNN is in the range between −18V and −22V. For category  2 , the thickness of the thick gate-oxide is about 200 Å+/−10%. The corresponding VPP is in the range from 9V to 11V and VNN is in the range between −9V to −11V. For category  3 , the thickness of the thick gate-oxide is about 100 Å+/−10%. The corresponding VPP is in the range from 4.5V to 5.5V and VNN is in the range between −4.5V and −5.5V. 
     In accordance with the present invention, the two capacitors made of either PIP or MIM are employed to eliminate the large well spacing required in the silicon layout of the memory device for effective cell size reduction and reducing of the required number of high voltages. In addition, with a thick floating-gate in the memory cell, superior data retention and least cell VT disturbance can be achieved in wide temperature and VDD operating ranges. 
     Although the present invention has been described with reference to the exemplary embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.

Technology Category: g