Patent Abstract:
A programmable read-only memory supplied with power at a specified voltage has word-line drivers that drive the word lines of the memory-cell array to the same potential, regardless of whether the specified voltage has a first value or a second value. This effect is achieved by using different types of transistors to drive the word lines, depending on the specified voltage. As a result, the same memory-cell array, the same programming voltages, and the same wafer process can be used for memories operating at either of the two specified voltage values. Consequently, less time and effort are needed to design memories for different power-supply voltages.

Full Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an electrically programmable read-only memory.  
           [0002]    An electrically programmable read-only memory is commonly referred to as an EPROM or PROM, the latter term being used below. PROMs are fabricated in large numbers on semiconductor wafers, and are widely used in electronic computing devices for the storage of fixed data and programs.  
           [0003]    There is no single standard power-supply voltage for electronic computing devices. Many operate on a five-volt (5-V) supply, but others operate on a lower-voltage supply such as a three-volt (3-V) supply. A PROM manufacturer normally provides different PROM versions specified for operation at different power-supply voltages. The different versions conventionally have the same circuit design, but differ in their wafer process parameters and programming parameters. For example, to ensure reliable operation, the thicknesses of oxide films deposited during the wafer process must be adjusted according to the power-supply voltage, and the programming voltage and the internal cell voltage used during programming must be optimized for each power-supply voltage. Optimization of these programming parameters is particularly difficult, requiring much time and labor in the design and development stage.  
         SUMMARY OF THE INVENTION  
         [0004]    An object of the present invention is to provide a PROM having two different versions, specified for operation at two different power-supply voltages, both versions being manufactured with the same wafer process parameters and programmed with the same programming parameters.  
           [0005]    An attendant object of the invention is to reduce PROM design and development costs.  
           [0006]    Another object is to shorten PROM design and development time.  
           [0007]    The invented PROM operates in a program mode and a read mode on a power supply. Either a first power-supply voltage or a second power-supply voltage is specified when the PROM is manufactured. The PROM has word lines, bit lines, memory cells, an address decoder, and word-line drivers. When selected by the address decoder, a word-line driver drives a word line to a potential that selects the memory cells disposed on the word line. The selected memory cells are programmed with data from the bit lines in the program mode, and supply the programmed data to the bit lines in the read mode.  
           [0008]    The invention employs a method of driving the word lines in which a field-effect transistor of one type is used if the first power-supply voltage is specified, and a field-effect transistor of another type is used if the second power-supply voltage is specified. Both methods drive the word lines to the first power-supply voltage. The two transistors are, for example, a p-channel transistor and an n-channel transistor, or a depletion-mode transistor and an enhancement-mode transistor.  
           [0009]    In a first aspect of the invention, each word-line driver includes a first node, a second node, four transistors, and a wiring pattern that can be configured in different ways by a fabrication mask option. The first node is coupled through the first transistor to the power supply, through the second transistor to ground, and by the wiring pattern to the second node if the first power-supply voltage is specified. The first and second nodes are mutually disconnected if the second power-supply voltage is specified. The second node is coupled through the third transistor to the power supply. The first node is coupled through the fourth transistor to a word line. The substrate of the first transistor is grounded.  
           [0010]    In the read mode, a word line is driven through the second and fourth transistors or the first and fourth transistors, depending on the power-supply voltage specification. The grounded substrate of the first transistor provides a body effect that results in the word line being driven to the same potential in both cases.  
           [0011]    In a second aspect of the invention, each word-line driver includes a transistor that supplies a decoded address signal to a word line, and a logic circuit that turns the transistor on and off according to the decoded address signal and the operating mode. The transistor is a depletion-mode transistor if the first power-supply voltage is specified, and an enhancement-mode transistor if the second power-supply voltage is specified.  
           [0012]    In a third aspect of the invention, each word-line driver includes a transistor that supplies a decoded address signal to a word line, and a wiring circuit that supplies either a control signal or a predetermined potential to the gate electrode of the transistor, depending on the power-supply specification. The transistor is a depletion-mode transistor if the first power-supply voltage is specified, and an enhancement-mode transistor if the second power-supply voltage is specified.  
           [0013]    In the second and third aspects of the invention, in the read mode, the enhancement-mode transistor used with the second power-supply voltage turns off when the word line reaches substantially the first power-supply voltage, so the selected word line is driven to substantially the first power-supply voltage regardless of which power-supply voltage is used.  
           [0014]    In all aspects of the invention, since the word lines are driven to substantially the same potential regardless of which power-supply voltage is used, the PROM can be programmed with the same programming voltage for both power-supply voltages, and the same wafer process can be used for both power-supply voltages. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    In the attached drawings:  
         [0016]    [0016]FIG. 1 is a circuit diagram illustrating a first embodiment of the invention;  
         [0017]    [0017]FIG. 2 is a circuit diagram illustrating a conventional PROM;  
         [0018]    [0018]FIG. 3 is a circuit diagram of a word-line driver in a second embodiment of the invention;  
         [0019]    [0019]FIG. 4 is a circuit diagram of a word-line driver in a third embodiment of the invention; and  
         [0020]    [0020]FIG. 5 is a circuit diagram of a word-line driver in a fourth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    PROMs embodying the invention will be described with reference to the attached drawings, in which like parts are indicated by like reference characters.  
         [0022]    In the descriptions, a metal-oxide-semiconductor field-effect transistor will be referred to as a MOS transistor. Among the various MOS transistor types, an n-channel enhancement-mode MOS transistor will be referred to as an NMOS transistor, a p-channel enhancement-mode MOS transistor will be referred to as a PMOS transistor, and an n-channel depletion-mode MOS transistor will be referred to as a DMOS transistor. Each of these types of transistor has a source electrode, a gate electrode, a drain electrode, and a substrate electrode. The substrate electrode is normally biased at a fixed potential. The source and drain designations are interchangeable. In an n-channel transistor, for example, if the source and drain electrodes are at different potentials, the electrode at the lower potential functions as the source electrode.  
         [0023]    The relevant properties of these transistors are that an NMOS transistor turns on when its gate-source voltage rises above a certain positive threshold value, a PMOS transistor turns on when its gate-source voltage falls below a certain negative threshold value, and a DMOS transistor turns off when its gate-source voltage falls below a certain negative threshold value. There is also a body effect that causes the threshold voltage of an NMOS transistor to rise as its source-substrate voltage rises.  
         [0024]    Referring to FIG. 1, the first embodiment of the invention is a PROM having a memory-cell array  10  with m word lines  11   i  (i=1 to m) and n bit lines  12   j (j=1 to n), where m and n are arbitrary positive integers. Memory cells  13   i,j  are disposed at the intersections of the bit lines and word lines. Each memory cell  13   i,j  comprises a MOS transistor having a floating gate and a control gate. The control gate is coupled to the corresponding word line  11   i , the source electrode of the transistor is coupled to the corresponding bit line  12   j , and the drain electrode of the transistor is coupled to a node that supplies a cell voltage VD.  
         [0025]    The PROM also has an address decoder  20  that receives and decodes an address signal AD to generate decoded address signals that select the word lines  11   i  individually. These decoded address signals are supplied to respective word-line drivers  30 A i  (i=1 to m), and can be regarded as selecting the word-line drivers.  
         [0026]    Each word-line driver  30 A i  comprises DMOS transistors  31 ,  32 ,  33 , an inverter  34 , NMOS transistors  35 ,  36 , a PMOS transistor  37 , and a wiring pattern  38 . The decoded address signal is supplied to the inverter  34  and the gate electrode of NMOS transistor  35 . The inverted signal produced by the inverter  34  is supplied to the gate electrodes of NMOS transistor  36  and PMOS transistor  37 . NMOS transistor  35  has its source electrode coupled to a first internal node N 1 , its drain electrode coupled to a power supply node, generically denoted VDD, and its substrate electrode coupled to ground. NMOS transistor  36  has its source electrode coupled to ground and its drain electrode coupled to node N 1 . PMOS transistor  37  has its source electrode coupled to the power supply VDD and its drain electrode coupled to a second internal node N 2 . The wiring pattern  38  is disposed between nodes N 1  and N 2 .  
         [0027]    The source and drain electrodes of DMOS transistor  31  are coupled in series between node N 1  and word line  11   i . A program mode control signal PGM (active low) is supplied to the gate electrode of this DMOS transistor  31 .  
         [0028]    DMOS transistors  32 ,  33  are coupled in series between a programming power supply, generically denoted VPP, and word line  11   i . The source electrode of DMOS transistor  32  is coupled to word line  11   i , the drain electrode of DMOS transistor  32  is coupled to the source electrode of DMOS transistor  33 , and the drain electrode of DMOS transistor is coupled to VPP. The gate electrodes of DMOS transistors  32 ,  33  are both coupled to the source electrode of DMOS transistor  33  and the drain electrode of DMOS transistor  32 . DMOS transistors  32 ,  33  have a comparatively high series resistance and function as pull-up transistors.  
         [0029]    The PROM also has n sense amplifiers  40   j  (j =1 to n) which are coupled to respective bit lines  12   j . The sense amplifiers detect data on the bit lines in the read mode, and supply data to the bit lines in the program mode.  
         [0030]    The configuration of the wiring pattern  38  is determined by a mask used in the PROM fabrication process. There are two mask options, one causing the wiring pattern  38  to interconnect nodes N 1  and N 2 , the other leaving nodes N 1  and N 2  mutually disconnected. The mask option that interconnects nodes N 1  and N 2  is selected if the PROM is specified for use at a first power-supply voltage VDD of three volts (3 V). The mask option that does not interconnect nodes N 1  and N 2  is selected if the PROM is specified for use at a second power-supply voltage VDD of five volts (5 V). Thus there are two versions of the first embodiment, referred to below as the three-volt version and the five-volt version.  
         [0031]    The operation of these two versions in the read mode and the program mode will be described below.  
         [0032]    First, the programming of the three-volt version will be described. Nodes N 1  and N 2  are interconnected through the wiring pattern  38 , as explained above. The power-supply voltage VDD is 3 V, the programming voltage VPP is 9.75 V, the cell voltage VD is 6 V, and the program mode control signal PGM is low (0 V). All memory cells  13   i,j  originally hold ‘1’data.  
         [0033]    To program the memory cells on a particular word line  11   i , the data to be programmed are set in the sense amplifiers  40   j  (j=1 to n), and an address signal AD is supplied to the address decoder  20 , causing the address decoder  20  to send a high (VDD) decoded address signal to the selected word-line driver  30 A i  and low (0-V) decoded address signals to the other word-line drivers. In the selected word-line driver  30 A i , NMOS transistor  35  and PMOS transistor  37  are placed in the on-state, while NMOS transistor  36  is in the off-state. Since nodes N 1  and N 2  are interconnected, node N 1  receives VDD through PMOS transistor  37 , as well as through NMOS transistor  36 . The potential of node N 1  rises from the ground level toward VDD. As the potential of node N 1  approaches VDD, the gate-source voltage of NMOS transistor  35  approaches zero and thus falls below the threshold level, turning NMOS transistor  35  off, but the gate-source voltage of PMOS transistor  37  remains fixed (at-VDD), so node N 1  continues to receive VDD through PMOS transistor  37 . Thus the potential of node N 1  is brought to substantially the VDD level.  
         [0034]    DMOS transistor  31  is initially in the on-state, so as the potential of node N 1  rises, the potential of word line  11   i  also rises. As these potentials rise, however, DMOS transistor  31  acquires a sufficiently negative gate-source voltage to turn off, isolating word line  11   i  from node N 1 . That allows the potential of word line  11   i  to be pulled up further, to VPP.  
         [0035]    The pull-up function is performed by DMOS transistors  32  and  33 . These transistors are initially in the on-state and stay in the on-state, their gate-source voltages remaining positive or zero, as the potential of word line  11   i  rises to VPP. The control gates of the memory cells  13   i,j  connected to word line  11   i  are thus brought to substantially the VPP level (j =1 to n).  
         [0036]    The bit lines  12   j  coupled to sense amplifiers  40   j  holding ‘0’data are at ground level. The potential difference between VPP and ground is large enough to inject electrons into the floating gates of the corresponding memory cells  13   i.j,  raising the threshold voltages of these memory cells. The bit lines  12   j  coupled to sense amplifiers  40   j  holding ‘1’data are at the VD potential. The potential difference between VPP and VD is not large enough for electron injection to take place.  
         [0037]    In the non-selected word line drivers  30 A k  (k≠i), NMOS transistor  36  is in the on-state while NMOS transistor  35  and PMOS transistor  37  are in the off-state. Node N 1  is therefore at the ground potential. Since the program mode control signal PGM is low, both the source and gate electrodes of DMOS transistor  31  are at the ground potential, so DMOS transistor  31  is in the on-state, and holds word line  11   k  at substantially the ground level. The actual word-line level is slightly higher than ground, because DMOS transistors  32  and  33  are also in the on-state, but the comparatively high series resistance of these DMOS transistors prevents the word line  11   k  from being pulled up high enough to cause electron injection in the connected memory cells  11   k,j .  
         [0038]    Next, the reading of the three-volt version of the PROM will be described. In the read mode the program mode control signal PGM is high (3 V), the cell voltage VD is 1.2 V, and the programming voltage VPP is not supplied.  
         [0039]    When the memory cells coupled to word line  11   i  are read, an address signal AD is supplied to the address decoder  20 , causing the address decoder  20  to send a high (VDD) decoded address signal to word-line driver  30 A i  and low (0-V) decoded address signals to the other word-line drivers. In the selected word-line driver  30 A i , node N 1  is brought to substantially the VDD level as described above. The gate potential of DMOS transistor  31  is also VDD, so DMOS transistor  31  is in the on-state, and the potential of word line  11   i  rises to substantially VDD. Since the programming power supply VPP is off, the word-line potential may be pulled down slightly through DMOS transistors  32  and  33 , but the series resistance of these transistors is high enough that the resulting potential drop can be ignored.  
         [0040]    The control gates of the memory cells  13   i,j  connected to word line  11   i  are thus driven to substantially VDD (j=1 to n). The transistors in the memory cells  13   i,j  that have been programmed with ‘0’data have threshold voltages higher than VDD (3 V) and remain off; the transistors in the memory cells  13   i,j  that have not been programmed with ‘0’data have threshold voltages lower than VDD and turn on, conducting current from VD to the corresponding sense amplifiers  40   j . The sense amplifiers sense the presence or absence of current, thereby sensing the programmed data.  
         [0041]    Next, the programming of the five-volt version will be described. The power-supply voltage VDD is 5 V, the programming voltage VPP is again 9.75 V, and the cell voltage VD is again 6 V, but nodes N 1  and N 2  are not interconnected. PMOS transistor  37  therefore plays no part in the programming operation.  
         [0042]    To program the memory cells on word line  11   i , the program mode control signal PGM is driven low, the data to be programmed are set in the sense amplifiers  40   j  (j=1 to n), and an address signal AD is supplied to the address decoder  20 , causing the address decoder  20  to send a high (5-V) decoded address signal to word-line driver  30 A i  and low (0-V) decoded address signals to the other word-line drivers. In the selected word-line driver  30 A i , NMOS transistor  35  is thereby placed in the on-state and NMOS transistor  36  is placed in the off-state. As the potential of node N 1  rises. the gate-source voltage of NMOS transistor  35  decreases. The threshold voltage of NMOS transistor  35  also increases, due to a considerable body effect, since the substrate of this transistor is at the ground potential. The body effect is such that NMOS transistor  35  stops conducting when node N 1  reaches a potential of substantially three volts (3 V).  
         [0043]    Programming then proceeds as in the three-volt version, with DMOS transistor  31  in the off-state because its gate-source voltage is substantially minus three volts (−3 V). Word line  11   i  is pulled up to VPP.  
         [0044]    Next, the reading of the five-volt version will be described. The program mode control signal PGM is high (5 V), the cell voltage VD is 1.2 V, and the programming voltage VPP is not supplied.  
         [0045]    When the memory cells coupled to word line  11   i  are read, an address signal AD is supplied to the address decoder  20 , causing the address decoder  20  to send a high (5-V) decoded address signal to word-line driver  30 A i  and low (0-V) decoded address signals to the other word-line drivers. In the selected word-line driver  30 A i , node N 1  is brought to a potential of substantially three volts (3 V) as described in the five-volt programming operation. DMOS transistor  31  is in the on-state because its gate potential is high (5 V), and the potential of word line  11   i  rises to the potential of node N 1  (substantially 3 V). The series resistance of DMOS transistors  32  and  33  is high enough that the pull-down effect of these transistors can be ignored. The control gates of the memory cells  13   i,j  connected to word line  11   i  are thus driven to a potential of substantially three volts (j=1 to n). The data stored in the memory cells  13   i,j  are sensed by the sense amplifiers  40   j  as described above.  
         [0046]    The first embodiment functions equally well at power-supply voltages of three and five volts, because the voltages supplied to the memory-cell array  10  are the same in both cases. The same wafer process, the same cell voltage VD, and the same programming voltage VPP can accordingly be used for both the three-volt and the five-volt versions of the first embodiment. The two versions are therefore obtained at substantially the design and development cost of only one version. The design and development of both versions can be completed in about the same length of time as required for just one version.  
         [0047]    For comparison, FIG. 2 shows a conventional PROM in which each word line driver  30   i  includes DMOS transistors  31 ,  32 ,  33  as described above, but lacks the PMOS and NMOS transistors of the first embodiment. When this PROM is read, in the selected word-line driver  30   i , DMOS transistor  31  turns on and conducts the voltage (VDD) output by the address decoder  20  to the selected word line  11   i . The control gates of the memory cells on this word line  11   i  are thus placed at different potentials, depending on whether VDD is three or five volts. For reliable operation at the higher (5-V) power-supply voltage, more electrons must be injected into the floating gates during programming than for the lower (3-V) power-supply voltage. Different programming voltages (VPP) and different cell voltages (VD) must therefore be used, depending on the power-supply voltage, and fabrication parameters such as gate oxide thicknesses of the memory-cell transistors must also be adjusted to allow for the different voltages.  
         [0048]    The three-volt and five-volt versions of the conventional PROM must therefore be designed separately, and a separate wafer process must be developed for each. The concomitant design and development costs are considerably higher than for the first embodiment, and additional design and development time is required.  
         [0049]    A second embodiment of the invention comprises the memory-cell array  10 , address decoder  20 , and sense amplifiers  40   j  described in the first embodiment, and the word-line drivers  30 B i  shown in FIG. 3, which replace the word-line drivers  30 A i  of the first embodiment.  
         [0050]    Each word-line driver  30 B i  has a transistor  31 A of a selectable type coupled in series between the address decoder (not visible) and word line  11   i . When turned on, transistor  31 A conducts a decoded address signal to word line  11   i . Transistor  31 A is a DMOS transistor in the three-volt version of the second embodiment, and an NMOS transistor in the five-volt version. The word-line driver  30 B i  also comprises pull-up DMOS transistors  32 ,  33  as described in the first embodiment, a NAND gate  38 , and an inverter  39 . The program mode control signal PGM is supplied to the inverter  39 . The NAND gate  38  receives the inverted control signal output by the inverter  39 , and the decoded address signal output by the address decoder. The output terminal of the NAND gate  38  is coupled to the gate electrode of transistor  31 A.  
         [0051]    In the program mode (PGM low), in the selected word-line driver  30 B i , both inputs to the NAND gate  38  are high, so the gate electrode of transistor  31 A is at the low (ground) level. As the potential of word line  11   i  rises, the gate-source voltage of transistor  31 A becomes negative, causing transistor  31 A to turn off, regardless of whether transistor  31 A is of the DMOS or NMOS type. Word line  11   i  is then pulled up to the VPP level through DMOS transistors  32  and  33 . Conversely, in the non-selected word-line drivers, since the decoded address signal supplied to the NAND gate  38  is low, the gate electrode of transistor  31 A is at the high level, transistor  31 A is in the on-state, regardless of whether it is of the DMOS or NMOS type, and word line  11   i  is held at the low (ground) level.  
         [0052]    In both versions of the second embodiment, accordingly, the selected word line is raised to the VPP level while other word lines are held at ground level, and programming is carried out as described in the first embodiment.  
         [0053]    In the read mode (PGM high, VPP not supplied), since the output of inverter  39  is low, the logic output of NAND gate  38  is high (VDD) and the gate electrode of transistor  31 A is held at the VDD level. The three-volt and five-volt versions are read as follows.  
         [0054]    In the three-volt PROM version, since transistor  31 A is a DMOS transistor, it is in the on-state regardless of whether the decoded address signal is high (3 V) or low (0 V). The decoded address signal is conducted to word line  11   1  with substantially no voltage drop. The selected word line is thus brought to a 3-V potential, while the non-selected word lines are held at the ground potential.  
         [0055]    In the five-volt version, transistor  31 A is an NMOS transistor and remains in the on-state only as long as its gate potential exceeds its source potential by at least the necessary threshold voltage. In the selected word-line driver  30 B i , the electrode coupled to the word line  11   i  functions as the source electrode while the electrode receiving the high (5-V) decoded address signal functions as the drain electrode. NMOS transistor  31 A is initially in the on-state, but turns off as the potential of the word line  11   i  rises, the turn-off being completed when the word-line potential is substantially three volts. In non-selected word-line drivers, the electrode receiving the low (0 V) decoded address signal functions as the source electrode, so NMOS transistor  31 A remains in the on-state and the word-line potential is held at ground level.  
         [0056]    In the read mode, accordingly, in both the three-volt and five-volt versions, the selected word line is driven to substantially three volts (3 V) while non-selected word lines are held at ground level. The same memory-cell array and the same programming voltages can therefore be used for both the three-volt and five-volt versions, saving design and development time and cost. The wafer process is also the same for both versions, although different masks are used to produce DMOS transistors  31 A in the three-volt version and NMOS transistors  31 A in the five-volt version.  
         [0057]    A third embodiment of the invention comprises the memory-cell array  10 , address decoder  20 , and sense amplifiers  40   j  described in the first embodiment, and the word-line drivers  30 C i  shown in FIG. 4, which replace the word-line drivers  30 A i  of the first embodiment.  
         [0058]    Each word-line driver  30 C i  comprises the DMOS or NMOS transistor  31 A described in the second embodiment, the pull-up DMOS transistors  32 ,  33  described in the first embodiment, a wiring pattern  41  through which the program mode control signal PGM is supplied to the gate electrode of transistor  31 A, and another wiring pattern  42  that couples the gate electrode of transistor  31 A to the power supply VDD. The wiring patterns  41 ,  42  are continuous or open as selected by mask options in the fabrication process. In the three-volt version of the third embodiment, wiring pattern  41  is continuous and wiring pattern  42  is open. In the five-volt version, wiring pattern  41  is open and wiring pattern  42  is continuous.  
         [0059]    In the three-volt version of the PROM, transistor  31 A is a DMOS transistor, and its gate electrode receives the program mode control signal PGM through wiring pattern  41 .  
         [0060]    In the three-volt program mode (PGM low), the DMOS transistor  31 A in the selected word-line driver  30 C i  receives a 3-V decoded address signal from the address decoder. As the potential of word line  11   i  rises, DMOS transistor  31 A turns off because its gate electrode is at the ground potential. Word line  11   i  is then pulled up to the VPP potential through DMOS transistors  32  and  33 . In the non-selected word-line drivers, the decoded address signal is low (0 V), DMOS transistor  31 A is held in the on-state because its source and gate electrodes are both at ground level, and the word line is held at ground level.  
         [0061]    In the three-volt read mode (PGM high, VPP not supplied), DMOS transistor  31 A is always in the on-state because its gate potential (PGM=VDD) is equal to or greater than its source potential (VDD or ground). The word lines are therefore brought to the potentials of the corresponding decoded address signals. The selected word line  11   i  is driven to the VDD level (3 V), while other word lines are held at ground level.  
         [0062]    In the five-volt version of the PROM, transistor  31 A is an NMOS transistor, and its gate electrode is tied to VDD (5 V).  
         [0063]    In the five-volt program mode, NMOS transistor  31 A in the selected word-line driver  30 C i  receives a 5-V (VDD) decoded address signal from the address decoder. As the potential of word line  11   i  rises toward 5 V, the gate-source voltage of NMOS transistor  31 A approaches zero, NMOS transistor  31 A turns off, and word line  11   i  is pulled up to the VPP level through DMOS transistors  32 ,  33 . In the non-selected word-line drivers, the source electrode of NMOS transistor  31 A receives a low (0-V) decoded address signal from the address decoder while its gate electrode is at VDD, so NMOS transistor  31 A is in the on-state and the word line is held at ground level.  
         [0064]    In the five-volt read mode (VPP not supplied), the source electrode of NMOS transistor  31 A in the selected word-line driver  30 C i  is the electrode coupled to word line  11   i . As explained in the second embodiment, NMOS transistor  31 A turns off when word line  11   i  reaches a potential of substantially three volts (3 V). In the non-selected word-line drivers, the source electrode of NMOS transistor  31 A is the electrode receiving zero volts from the address decoder (not visible), so NMOS transistor  31 A is in the on-state and the word line is held at ground level (0 V).  
         [0065]    In both the three-volt and five-volt versions, the selected word line is driven to the VPP potential in the program mode and to a potential of substantially three volts (3 V) in the read mode, while non-selected word lines are held at ground level in both modes. The same memory-cell array and the same programming voltages can accordingly be used for both the three-volt and five-volt versions, leading to savings in design and development time and cost. The same wafer process is also used for both versions, although with different masks as noted in the second embodiment.  
         [0066]    A fourth embodiment of the invention comprises the memory-cell array  10 , address decoder  20 , and sense amplifiers  40 ; described in the first embodiment, and the word-line drivers  30 D i  shown in FIG. 5, which replace the word-line drivers  30 A i  of the first embodiment. These word-line drivers  30 D i  are identical to the word-line drivers  30 C i  in the third embodiment, except that wiring pattern  42  couples the gate electrode of transistor  31 A to a constant-voltage source  43 . In the read mode, the constant-voltage source  43  divides the power-supply voltage VDD to generate a predetermined potential MV intermediate between VDD and ground. In the program mode, the constant-voltage source  43  outputs the power-supply voltage VDD.  
         [0067]    The same constant-voltage source  43  may be used for all of the word-line drivers  30 D i . The constant-voltage source  43  includes well-known means such as resistors (not visible) for dividing VDD in the read mode.  
         [0068]    The fourth embodiment operates in the same way as the third embodiment, except that in the five-volt version of the PROM, in the read mode, the gate potential of NMOS transistor  31 A is at the MV potential instead of the VDD potential. This MV potential is set at a level that makes ;NMOS transistor  31 A turn off when word line  11   i  reaches a potential of exactly three volts (3 V). In the read mode, accordingly, the selected word line is driven to a potential of three volts (3 V) with high accuracy in both the three-volt and five-volt versions of the PROM.  
         [0069]    It is comparatively easy to determine the MV potential level that will make NMOS transistor  31 A turn off when the selected word line reaches three volts, and to design a constant-voltage source  43  to generate this MV potential. Compared with the third embodiment, the fourth embodiment thus provides an improved five-volt PROM version at only a small additional design and development cost. As in the third embodiment, the same wafer process can be used for both versions.  
         [0070]    The invention has been described in relation to three-volt and five-volt power supplies, but these voltages are of course only examples. The invention can be practiced in a PROM with versions suitable for any two power-supply voltages.  
         [0071]    The memory-cell array is not limited to the circuit configuration shown in FIG. 1.  
         [0072]    Those skilled in the art will recognize that further variations are possible within the scope claimed below.

Technology Classification (CPC): 6