Abstract:
A nonvolatile memory cell includes first and second MOS transistors, such as a PMOS transistor and NMOS transistor in a CMOS cell. One of the two transistors provides a floating gate for storing data while the other transistor is provided with a control gate for selecting the memory cell, and is connected with a bit line for reading data stored in the cell. The nonvolatile memory cell may be integrated into a logic device, such as a CMOS gate array, using PMOS and NMOS transistor cells formed in the gate array. In that case, the nonvolatile memory cell may be fabricated in a logic device with the standard processes used to produce the logic device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of application Ser. No. 09/218,026 filed Dec. 22, 1998, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     This invention pertains to the field of semiconductor memory devices, and more particularly, to a nonvolatile memory cell produced of semiconductor transistor structures common to semiconductor logic devices. 
     2) Background of the Related Art 
     Semiconductor memory devices may be largely divided into Random Access Memories (RAMs) and Read Only Memories (ROMs). RAMs, also referred to as volatile memories because the stored data is destroyed with the passage of time upon removal of the power supply, allow rapid data storage and data retrieval. ROMS, also referred to as nonvolatile memories because they retain data once it is entered, typically have slower data storage and data retrieval times. 
     One popular type of ROM is the Electrically Erasable Programmable Read Only Memory (EEPROM) in which data is electrically programmed and erased. A flash EEPROM, which is electrically erasable at high speed without being removed from a circuit board, offers the advantages of a simple memory cell structure, cheap cost, and no need for a refresh signal to retain the data. 
     EEPROM cells are largely divided into two types: a NOR type EEPROM and a NAND type EEPROM. 
     FIG. 1 shows a circuit diagram for a NOR-type flash EEPROM cell  100 . A metal oxide semiconductor (MOS) transistor  120  is provided with a floating gate  110  and a control gate  160 . The floating gate  110  may be programmed with a charge according to the data to be stored in the memory cell  100 . Data is read by selecting the transistor on a word line connected to the control gate  160  and detecting the presence or absence of a current through the transistor on a bit line  170  connected to one terminal of the device. 
     A number of different memory cell structures have been used for a nonvolatile memory device. FIG. 2 shows one configuration of a NOR-type Flash EEPROM cell  200  according to the prior art. The memory cell  200  comprises a semiconductor substrate  205  having first and second impurity regions  225  and  235  formed in a top surface of a well  202 . The first impurity region  225  is connected with a bit line  270  of the memory device  200 , while the second impurity region  235  is connected with ground potential. A first oxide later  230  is deposited on the top surface of the semiconductor substrate where the first and second impurity regions  225  and  235  are formed. 
     A floating gate  210  is formed on the first oxide layer  230  above and between the first and second impurity regions  225  and  235 . A control gate  260  is also formed above and between the first and second impurity regions  225  and  235 . A portion of the control gate  260  is formed above the floating gate  210 , separated by a second oxide layer  250 . The floating gate  210  and the control gate  260  may each be formed of conductive polysilicon layers. 
     To program the EEPROM device  200  with a potential V p , the word line connected with the control gate  260  is supplied with a large positive potential V PGM  (e.g., V PGM =12.5 Volts). This causes an injection of electrons onto the floating gate  210 . To read the data from the cell  200 , a lower positive voltage V cc , (e.g., 5 volts) is applied to the control gate while the bit line is supplied with a smaller positive voltage (e.g., 1-2 volts). Data  0  or  1  is read from the cell  200  according to the presence or absence of a current path through the cell, relying on the principle that the threshold voltage V th  of the cell is changed to a voltage greater than +5V when electrons are stored in the cell, while the threshold voltage V th  is about 1.5V when electrons are not stored on the floating gate. Data may be erased though exposure to ultraviolet light radiation, or through a separate erase gate (not shown). 
     Disadvantageously, these prior art nonvolatile memory cell structures are not readily adaptable to integration in a logic device such as a gate array. For example, the EEPROM cell  200  requires two polysilicon layers or more, whereas the typical gate array process uses a single polysilicon process. Yet, it is desirable to provide nonvolatile memory cells in a gate array device. 
     Accordingly, it would be advantageous to provide a nonvolatile memory cell which may be easily integrated into a semiconductor logic device. It would also be advantageous to provide a nonvolatile memory cell which may be easily integrated into a gate array logic device. It would be further advantageous to provide a nonvolatile memory cell which can use the same process technologies and array structures which are used to manufacture gate array logic circuitry. Other and further objects and advantages will appear hereinafter. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a nonvolatile memory cell constructed from MOS transistor structures. 
     In one aspect of the invention, a nonvolatile memory call includes first and second MOS transistors. A gate of the first transistor is a control gate while a gate of the other transistor is a floating gate. In a preferred embodiment, the nonvolatile memory cell includes a PMOS transistor and an NMOS transistor in a CMOS cell. 
     In another aspect of the invention, a nonvolatile memory cell may be integrated into a logic device, such as a CMOS gate array, using PMOS and NMOS transistor cells formed in the gate array. 
     In another aspect of the present invention, a nonvolatile memory cell may be fabricated in a logic device with the standard processes normally used to produce such a logic device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a nonvolatile memory cell device. 
     FIG. 2 shows a NOR-type flash EEPROM memory cell. 
     FIG. 3 shows a nonvolatile memory cell structure according to a first preferred embodiment of the present invention. 
     FIG. 4 shows a circuit diagram of a nonvolatile memory cell according to a first preferred embodiment of the present invention. 
     FIG. 5 shows a nonvolatile memory cell structure according to a second preferred embodiment of the present invention. 
     FIG. 6 shows a circuit diagram of a nonvolatile memory cell according to a second preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a first preferred embodiment of a nonvolatile memory cell structure according to one or more aspects of the present invention. In the preferred embodiment of FIG. 3, a nonvolatile memory cell  300  comprises an N-well  302  and a P-well  304  formed in a top surface of a semiconductor substrate  305  and separated by a field oxide  308 . A first impurity region  325  and a second impurity region  335 , each doped with a P-type impurity, are formed in a top surface of the N-well  302 . A V cc  supply line  330  is formed and connected to the first impurity region  325 . A gate electrode for a floating gate  310  is formed on an oxide film  317  above and between the impurity regions, producing a PMOS transistor  320 . 
     The P-well  304  also has formed in a top surface a first impurity region  355  and a second impurity region  365 , each doped with an N-type impurity. A gate electrode  345  is formed on an oxide layer  344  above and between the impurity regions  355  and  365 , producing a NMOS transistor  350 . The gate electrode  345  is connected with a control line  360  of the nonvolatile memory cell  300 . 
     A metalization layer  346  is formed above the field oxide region  308 . The metalization layer  346  connects the second impurity region  335  of the PMOS transistor  320  with the second impurity region  365  of the NMOS transistor  350 . In a preferred embodiment, a dielectric layer  347  is formed between the metalization layer  346  and the field oxide region  308 . 
     Also, the first impurity region  355  of the NMOS transistor  350  is connected with a bit line  370 . 
     FIG. 4 is a circuit diagram for a first preferred embodiment of a nonvolatile memory cell  400  according to one or more aspects of the present invention. In the memory cell  400 , a first MOS transistor  450  has a gate connected to a control gate  460  of the nonvolatile memory cell, a source connected to a Bit-line  470  and a drain connected to a drain of a second MOS transistor  420 . The second MOS transistor  420  has a source connected with a supply V cc    420  and a gate which is the floating gate  410  of the non-volatile memory cell  400 . In a preferred embodiment, the first MOS transistor  450  is an NMOS transistor and the second MOS transistor  420  is a PMOS transistor. 
     Operation of the nonvolatile memory cell  300  will now be described. When it is desired to program the memory cell  300 , electrons are injected onto the floating gate  310  of the PMOS transistor  320 . To accomplish this, for example, a negative voltage (e.g., −3 to −18 volts) may be applied on the V cc  supply line  330  while the N-well  302  is held at ground potential. Electrons induced by the breakdown of the PN junction at the source of the PMOS transistor  320  are injected thereby onto the floating gate  310 . 
     To read the data stored in the memory cell  300 , it is determined whether or not a current will pass through the PMOS transistor  320  with the gate floating. To accomplish this, for example, a positive voltage (e.g., +1 to +10 volts) may be applied on the V cc  supply line  330  which in turn applies the same potential to the N-well  302 . Alternatively, the N-well  302  maye be held at a higher potential. The control gate  360  is supplied with a voltage to turn on the NMOS transistor  350  and the current through the transistor is sensed via the bit line  370 . If a current is conducted, then the memory cell  300  is considered to store a first data value (e.g. “1”) and if no current conducts, then the memory cell  300  is considered to store a second data value (e.g., “0”). 
     To erase the memory cell  300 , electrons are emitted from the floating gate  310  of the PMOS transistor  320  through the gate oxide  317  into the source or drain of the PMOS transistor  320  or into the substrate  305 . To accomplish this, for example, a large positive voltage (e.g., +15 to +18 volts) may be applied on the V cc  supply line  330  and/or the N-well  302 . This will cause Fowler-Nordheim emission of electrons from the floating gate  310 . Alternatively, the nonvolatile memory cell  300  may be erased by exposure to intense ultraviolet (UV) light in which case the memory cell  300  is a UVPROM. 
     Thus, a nonvolatile memory cell is formed from a CMOS structure comprising a PMOS and NMOS transistor appropriately fabricated and connected together. 
     In a preferred embodiment, the nonvolatile memory cell may be formed in a gate array device comprising a plurality of PMOS and NMOS transistors formed in N-wells and P-wells in a top surface of a semiconductor substrate. 
     FIG. 5 shows a first preferred embodiment of a nonvolatile memory cell structure according to one or more aspects of the present invention. In the preferred embodiment of FIG. 5, a nonvolatile memory cell  500  comprises an N-well  502  and a P-well  504  formed in a top surface of a semiconductor substrate  505  and separated by a field oxide  508 . A first impurity region  525  and a second impurity region  535 , each doped with a P-type impurity, are formed in a top surface of the N-well  502 . A gate electrode  515  is formed on an oxide film  517  above and between the impurity regions, producing a PMOS transistor  520 . 
     A bit line  570  is formed and connected to the first impurity region  525 . The gate electrode  515  is connected with a control line  560  of the nonvolatile memory cell  500 . 
     The P-well  504  also has formed in a top surface a first impurity region  555  and a second impurity region  565 , each doped with an N-type impurity. A gate electrode forming a floating gate  510  of the memory cell  500  is formed on an oxide layer  544  above and between the impurity regions  555  and  565 , producing a NMOS transistor  550 . 
     A metalization layer  546  is formed above the field oxide region  508 . The metalization layer  546  connects the second impurity region  535  of the PMOS transistor  520  with the second impurity region  565  of the NMOS transistor  550 . In a preferred embodiment, a dielectric layer  547  is formed between the metalization layer  546  and the field oxide region  508 . 
     Also, the first impurity region  555  of the NMOS transistor  550  is connected with a V cc  supply line  530 . 
     FIG. 6 is a circuit diagram for a first preferred embodiment of a nonvolatile memory cell  600  according to one or more aspects of the present invention. In the memory cell  600 , a first MOS transistor  650  has a gate connected to a floating gate  610  of the nonvolatile memory cell, a source connected to a supply V cc    620  and a drain connected to a drain of a second MOS transistor  620 . The second MOS transistor  620  has a source connected with a Bit-line  670  and a gate which is the control gate  660  of the non-volatile memory cell  600 . In a preferred embodiment, the first MOS transistor  650  is an NMOS transistor and the second MOS transistor  620  is a PMOS transistor. 
     Operation of the nonvolatile memory cell  500  will now be described. When it is desired to program the memory cell  500 , electrons are injected onto the floating gate  510  of the NMOS transistor  550 . To accomplish this, for example, a positive voltage (e.g., +3 to +18 volts) may be applied on the V cc  supply line  530  while the P-well  504  is held at ground potential. Electrons induced by the breakdown of the PN junction at the source of the NMOS transistor  550  are injected thereby onto the floating gate  510 . 
     To read the data stored in the memory cell  500 , it is determined whether or not a current will pass through the PMOS transistor  520  with the gate floating. To accomplish this, for example, a negative voltage (e.g., −1 to −10 volts) may be applied on the V cc supply line  530  while the P-well  504  is held at ground potential. The control gate  560  is supplied with a voltage to turn on the PMOS transistor  520  and the current through the transistor is sensed via the bit line  570 . If a current is conducted, then the memory cell  500  is considered to store a first data value (e.g. “1”) and if no current conducts, then the memory cell  500  is considered to store a second data value (e.g., “0”). 
     To erase the memory cell  500 , electrons are emitted from the floating gate  510  of the NMOS transistor  550  through the gate oxide  544  into the source or drain of the NMOS transistor  550  or into the substrate  505 . To accomplish this, for example, a large positive voltage (e.g., +15 to +18 volts) may be applied on the V cc  supply line  530 , and/or the P-well  504 . This will cause Fowler-Nordheim emission of electrons from the floating gate  510 . Alternatively, the nonvolatile memory cell  500  may be erased by exposure to intense ultraviolet (UV) light in which case the memory cell  500  is a UVPROM. 
     While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. For example, although the embodiments shown in FIGS. 3 and 5 have a P-type substrate, the devices could be produced in an N-type semiconductor substrate. Also, although the above-described embodiments use one NMOS transistor and one PMOS transistor, both of the transistors could be NMOS transistors or PMOS transistors. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.