Abstract:
An electrically erasable programmable read-only memory, comprising a plurality of floating gate tunneling metal oxide semiconductor field effect transistors, does not require an addressing transistor in each cell. Instead, the gate decoder applies a sufficiently negative gate voltage to unselected ones of the transistors so that they are turned off regardless of the amount of charge on their floating polysilicon gates. Writing and erasure of data is performed without disturbing data in memory cells not selected for writing or erasure despite the absence of a series connected addressing transistor.

Description:
TECHNICAL FIELD 
     This invention is related to electrically erasable programmable read-only memories (EEPROM&#39;s) comprising a variable threshold voltage metal oxide semiconductor (VTVMOS) field effect transistor (FET) in each memory cell. 
     BACKGROUND ART 
     Electrically erasable programmable read-only memories (EEPROM&#39;s) became practical with the advent of the variable threshold voltage metal oxide semiconductor (VTVMOS) field effect transistor (FET), the VTVMOS FET being described in U.S. Pat. No. 4,115,914 by Eliyahou Harari, entitled &#34;Electrically Erasable Non-Volatile Semiconductor Memory&#34; and assigned to the assignee of the present application. Typically, the VTVMOS FET includes a source, a drain, an overlying control gate and a polysilicon floating gate formed beneath the control gate and overlying the source and at least a portion of the source-to-drain channel. The dielectric insulation under the polysilicon floating gate has a thin &#34;tunneling&#34; spot on the order of 100 angstroms thickness overlying the source. As is well known in the art, data may be stored permanently in a memory comprising an array of such VTVMOS FETs by simply causing electrons to tunnel through the thin spot in the dielectric insulation between the source and the floating polysilicon gate. Such tunneling of electrons between the source and the floating gate changes the charge on the floating gate and thus the threshold voltage of the VTVMOS FET. 
     One problem in the prior art is that if an excess of electrons tunnels between the floating gate and the source during a non-volatile storage or &#34;write&#34; operation, the threshold voltage of the VTVMOS FET may be decreased so much that the VTVMOS FET cannot be turned off during normal operation of the memory. For example, in an n-channel VTVMOS FET, electrons may tunnel from the floating polysilicon gate to the source in such a great number that the polysilicon gate acquires a large positive charge. If the positive charge is sufficient, the threshold voltage of the n-channel VTVMOS FET (as measured by varying the voltage of the control gate) may be decreased from the usual threshold of +1 volt to a negative voltage on the order of -1.5 volts. The operating voltage of such memory arrays typically varies between +5 volts (for &#34;on&#34;) and 0 volts (for &#34;off&#34;), the latter being insufficient to turn off such an excessively charged VTVMOS FET. Accordingly, the VTVMOS transistor cannot be &#34;unselected&#34; and therefore will distort data read out of the memory from a selected memory cell sharing the same output node. 
     One possible solution is to control the writing voltages applied to the gate and the source in such a manner as to prevent excessive charging of the floating polysilicon gate. This solution has the disadvantage that it is difficult to control the electron tunneling current between the floating gate and the source. Accordingly, attempting to prevent excessive charging of the floating gate during a writing step is impractical. 
     A more practical solution is the addition in each memory cell of a series connected addressing field effect transistor having a single control gate which controls the selection of the VTVMOS FET in the cell. The addition of an extra transistor in each memory cell decreases the the density of the memory array and increases access time, both significant disadvantages. 
     SUMMARY OF THE INVENTION 
     The present invention is an EEPROM comprising an array of VTVMOS FETs of the type disclosed in the Harari patent referred to above, each memory cell comprising a single one of the VTVMOS transistors and not requiring the presence of an extra addressing transistor in the memory cell. The EEPROM of the present invention prevents a false reading of data from unselected VTVMOS FETs even if the floating gate of one of them has been depleted or charged excessively. The excessively depleted VTVMOS FET is prevented from distorting the data read from the memory by forcing a large negative voltage (on the order of -5 volts) onto the overlying control gates of unselected VTVMOS FETs. This large negative voltage is sufficient to turn off source-to-drain channel current even if the floating gate has been excessively charged. Accordingly, there is no necessity for an extra addressing transistor to prevent a false readout of data. None of the source or drain pn junctions in the array may be forward biased during writing and erasure operations. This situation is avoided by the complementary structure of the invention wherein the VTVMOS FETs are of a first conductivity-type and are formed in a well region of a second conductivity-type on a substrate of the first conductivity type. This structure includes means for varying the potential of the well region in such a manner as to prevent forward biasing any of the source or drain pn junctions during reading, writing or erasing. 
     Thus, the invention includes a method of storing information, comprising fabricating the foregoing complementary micro-electronic structure and then operating it in the manner described herein. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention is best understood by reference to the accompanying drawings, of which: 
     FIG. 1 is a simplified schematic diagram of the EEPROM array of this invention; 
     FIG. 2 is a simplified plan view of the VTVMOS FET comprising a single memory cell in the array of FIG. 1, and is of the type well known in the prior art; 
     FIG. 3 is a simplified schematic diagram of the memory array of FIG. 1 illustrating the erase step; and 
     FIG. 4 is a simplified schematic diagram of the memory array of FIG. 1 illustrating the writing step. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the novel EEPROM of this invention is formed on a semiconductor substrate 5 and comprises an array of n-channel VTVMOS FETs 10 organized by row and column and formed in a p-type well 11 on the substrate 5. Each of the VTVMOS FETs 10 is controlled by a gate decoder 12, a source decoder 14, and a drain decoder 16, each decoder being formed in the substrate 5. Each VTVMOS FET 10 is of the type illustrated in FIG. 2 and was previously described in U.S. Pat. No. 4,115,914 filed by Eliyahou Harari and assigned to the assignee of the present application. The VTVMOS transistor 10 of FIG. 2 includes source and drain diffusions 10a, 10b of n-type conductivity which are shared with other VTVMOS transistors in the same column. A floating polysilicon gate 10c is insulated from the substrate 5 by a dielectric layer having a thin (&#34;gate oxide&#34;) region 10d overlying the source-to-drain channel and an extremely thin spot 10e overlying source diffusion 10a through which electrons may tunnel between the source diffusion 10a and the floating polysilicon gate 10c. A metal control gate 10g overlies the floating gate 10c and is insulated therefrom by another thin layer. 
     The gate decoder 12 is connected to each control gate 10g shared by VTVMOS transistors in the same row. The source decoder 14 is connected to source diffusions 10a shared by VTVMOS transistors in the same column while the drain decoder 16 is connected to drain diffusions 10b shared by VTVMOS transistors in the same column. 
     A well control circuit 24 controls the potential of the well 11 during reading, writing and erasing so that none of the sources 10a or drains 10b becomes forward-biased with respect to the well 11. 
     READING 
     A method for reading data from the memory of this invention is illustrated in FIG. 1. A VTVMOS FET 10 in a particular row and column may be selected for reading to the exclusion of all other VTVMOS FETs in the array as follows: the gate decoder 12 applies +5 volts to the control gate 10g of the selected row wherein the selected VTVMOS FET 10 resides. Simultaneously, the gate decoder 12 continuously applies -5 volts to the control gates of all other rows of VTVMOS FETs. The source decoder 14 maintains 0 volts on all the source diffusions 10a. The drain decoder connects the drain diffusions 10b of the selected column (wherein the selected VTVMOS FET 10 resides) to a sense amplifier 20. All of these operations are performed in two steps tabulated in the following table. 
     
                       TABLE I______________________________________                Precharging     Output     Step 1        Step 2Subcircuit     Terminal   (Before Reading)                              Reading______________________________________gate      selected   -5 volts      +5 voltsdecoder   gate     unselected -5 volts      -5 volts     gatesdrain     selected   +5 volts      to sensedecoder   drain                    amplifier     unselected +5 volts      not used     drainsource    all         0 volts       0 voltsdecoder   sourceswell control     well        0 volts       0 voltscircuit     substrate  +5 volts      +5 volts______________________________________ 
    
     As shown in Table I above, the reading operation includes a precharging step in which all of the drain diffusions 10b are first precharged to +5 volts while all of the transistors in the array are turned off by keeping all gate voltages below the source voltage. Accordingly, if the floating gate 10c of the selected VTVMOS FET 10 has been depleted of electrons, the source-to-drain impedance of the VTVMOS FET 10 is lower than it would be if the floating gate were not charged. Thus, at the instant the gate decoder 12 applies +5 volts to the control gate 10g, the sense amplifier 20 detects a relatively faster movement in the potential of the drain diffusion 10b downward from +5 volts toward 0 volts. Conversely, if the floating gate 10c has an excess of electrons stored on it, the source-to-drain impedance of the VTVMOS FET 10 is higher than it would be if the floating gate were not charged. Thus, the sense amplifier 20 detects a relatively slower movement in the potential of the drain diffusion 10b. Accordingly, the sense amplifier 20 either senses a faster change in the drain potential (a logic &#34;one&#34;) or senses slower change in the drain potential (a logic &#34;zero&#34;). 
     One problem solved in this invention is that an unselected VTVMOS FET in the same column with the selected VTVMOS FET 10 may have had its floating gate excessively depleted of electrons during a previous writing operation so that it has become a depletion mode field effect transistor (that is to say, a field effect transistor which is normally &#34;on&#34;). In the prior art, the gate decoder 12 would have merely applied 0 volts to the control gates of all of the unselected rows of VTVMOS FETs, which is insufficient to turn off an unselected VTVMOS FET having an excessively charged floating gate. The unselected FET would therefore distort the drain potential sensed by the sense amplifier 20, giving rise to a false reading. However, in the present invention the gate decoder 12 applies -5 volts to the control gates all of the unselected rows of VTVMOS FETs in the selected column, which causes all the unselected VTVMOS FETs in that column to be turned off, including those having excessively charged floating gates. 
     WRITING AND ERASING 
     The erasing and writing operations are summarized in Table II below and are illustrated in FIGS. 3 and 4 respectively. 
     
                       TABLE II______________________________________   Output     OperationSubcircuit     Terminal     Erase      Write______________________________________source decoder     selected source                  +5 volts   -12 volts     unselected source                  -5 volts    0 voltsgate decoder     selected gate                  -12 volts  +5 volts     unselected gate                   0 volts   -5 voltsdrain decoder     all drains   floating   floatingwell control     well         -5 volts   -12 voltscircuit     substrate    +5 volts   +5 volts______________________________________ 
    
     Data is written or erased in selected memory cells without affecting data previously stored in the unselected memory cells of the array. The purpose of the erase operation is to withdraw electrons from the floating gate 10c to the source 10a through the tunneling spot 10e; the purpose of the write operation is to store electrons on the polysilicon floating gate 10c by supplying them from the source 10a through the tunneling spot 10e to the gate 10c. During writing, tunneling of electrons from the source 10a to the floating gate 10c requires that the floating gate 10c be at a positive potential (typically on the order of 17 volts) with respect to the source 10a; during erasing, tunneling of electrons from the floating gate 10c to the source 10a requires that the floating gate be at a negative potential (typically on the order of -17 volts) with respect to the source 10a. 
     FIG. 3 denotes the voltages applied during the erase operation by the gate decoder 12 and by the source decoder 14. During erasure, the gate decoder 12 applies -12 volts to the selected control gate 10g and 0 volts to the unselected control gates of the array, while the source decoder 14 applies +5 volts to the selected source 10a and -5 volts to the unselected sources in the remainder of the array. Simultaneously, the well control circuit 24 applies -5 volts to the well 11 to prevent forward biasing the pn junctions between the unselected sources and the well. 
     FIG. 3 also denotes the resulting control gate-to-source potentials during erasure in each of the VTVMOS FETs in the array. The selected VTVMOS FET 10 has a gate-to-source potential of -17 volts, which is sufficient to cause electron tunneling from the floating gate 10c. As illustrated in FIG. 3, the other unselected VTVMOS FETs have control gate-to-source voltages of either -7, -5 or +5 volts depending upon their location, which voltages are insufficient to cause electrons to tunnel between the floating gate and the source. 
     During the write operation illustrated in FIG. 4, the gate decoder 12 applies +5 volts to the control gate 10g of the selected row of VTVMOS FETs and -5 volts to the control gates of the unselected rows of VTVMOS FETs. The source decoder 14 applies -12 volts to the source diffusion of the selected column of VTVMOS FETs and 0 volts to the source diffusions of the remaining unselected columns of VTVMOS FETs. Simultaneously, the well control circuit 24 applies -12 volts to the well 11 to prevent forward biasing the pn junction between the selected source 10a and the well 11. The resulting control gate-to-source potential of the selected VTVMOS FET 10 is +17 volts. This is sufficient to cause electrons to tunnel from the source 10a to the floating gate 10c so that the floating gate 10c acquires a negative charge. As illustrated in FIG. 4, the unselected VTVMOS FETs in the array have control gate-to-source voltages of either +7, +5 or -5 volts depending upon their location, which voltages are insufficient to cause tunneling of electrons between the source and the floating gate. 
     In summary, the memory array of this invention is more dense than prior art memory arrays which used a VTVMOS FET in combination with a series connected addressing FET in each memory cell. At the same time, the memory array of this invention is bit-addressable in the writing and erasure operations. A complementary metal oxide semiconductor (CMOS) structure, wherein each n-channel VTVMOS FET is formed in a common p-type well on an n-type substrate, prevents the source and drain junctions from forward biasing. More importantly, it permits operating the VTVMOS FETs in a well, the voltage of which is less negative than the supply voltage of the gate decoder (-5 volts). This is necessary (but not mandatory) to reduce efects of well voltage on threshold voltage, sometimes called &#34;back gate bias&#34; in the art. This feature makes it easier to physically implement the novel reading, writing and erasing means of this invention without requiring of an extra addressing transistor in each memory cell. It is possible, of course, to implement the present invention without the complementary well by selecting a large substrate voltage, which is not as desirable as the preferred embodiment disclosed herein.