Semiconductor memory in which data readout operation is carried out over wide power voltage range

Abstract: A programmable read-only memory is disclosed in which each of memory cells is composed of a field effect transistor having a floating gate. This memory is equipped with a word line pull-up circuit which includes a pulse generator generating a pulse signal in response to a change in address signals and a capacitor coupled between the output terminal of the pulse generator and a selected word line, thereby the selected word line taking a level larger than a power supply voltage.

Description: BACKGROUND OF THE INVENTION The present invention relates to a semiconductor memory and, more particularly, to a driving of word lines in a programmable read-only memory (referred hereinafter to as a "PROM") having memory cells each constituted with a field effect transistor having a floating gate. In such a PROM, a data readout operation is performed by using a difference in threshold voltage between a programmed memory cell and a non-programmed memory cell. The floating gate of the programmed memory cell is charged negative or positive due to electrons or holes injected therein, and therefore, the threshold value thereof takes a high value V TMW . On the other hand, since the floating gate of the non-programmed memory cell is not charged, its threshold value takes a low value V TMO . Each memory cell is applied with a voltage between V TMW and V TMO as a readout voltage. Therefore, the non-programmed memory cell is turned on while the programmed memory cell is kept non-conductive. Thus, an output data is obtained as "1" or "0" dependent upon whether or not the selected memory cell is programmed. The readout voltage is reduced with a drop of a power supply voltage. When the readout voltage becomes smaller than the low threshold voltage V TMO due to the drop of the power voltage, all of the memory cells are kept non-conductive regardless of whether or not they are programmed. The low threshold value V TMO of an N channel non-programmed memory cell is about 2.5 V, and a readout voltage of at least 3 V is required in order to make the memory cell conductive realiably. Therefore, when the readout voltage is lowered below 3 V due to the power voltage drop, the data readout from the EPROM becomes impossible. A semiconductor memory according to the preamble of claim 1 is disclosed in GB-A-2 144 006. In this memory distributors and a voltage step-up circuit, which operate only a data write mode, are provided to supply a selected word line with a programming voltage higher than a predetermined programming voltage to shorten the data write operation, i.e. the programming operation. However, for reading out data from the memory a fixed read-out voltage is supplied to the selected word line, so that the problems as indicated above are also present in this memory. EP-A-106 222 is related to the problem of providing a static type memory free from soft errors caused by alpha particles in a battery back-up mode. For this purpose the power supply voltage is boosted and supplied to power terminals of the memory cells. It is an object of the present invention to provide a semiconductor memory in which data read-out operations are performed over an expanded power voltage range. This object is achieved by a semiconductor memory having the features of the claim. According to the invention the voltage of the selected word line is raised in the reading operation relative to a power supply voltage. This can be achieved by utilizing a capacitive element which is connected to the selected word line and which is first charged up to the selection level of the word line. Thereafter, a voltage higher than the non-selection level is applied to the other end of the capacitive element. As a result, the potential on the selected word line is raised by the bootstrap effect of the capacitive element. In other words, a voltage which is higher than the prior art readout voltage is applied to the selected memory cell as an actual readout voltage. Thus, the readout operation is performed even when the source voltage drops. BRIEF DESCRIPTION OF THE DRAWINGS The above objects, advantages and features of the present invention are more apparent from the flowing description taken in conjunction with the accompanying drawings, in which Fig. 1 is a block diagram of a semiconductor memory according to an embodiment of the present invention; Fig. 2 is a circuit diagram showing one portion of the memory shown in Fig. 1; Fig. 3 is a timing chart of an operation of the circuit shown in Fig. 2; Fig. 4 is a circuit diagram showing one portion of another embodiment of the present invention; Figs. 5A and 5B are timing charts representing a circuit operation of Fig. 4 when a power voltage is above a predetermined level and when it is below the predetermined level, respectively; and Fig. 6 shows control gate voltage V CG versus drain current I DS characteristics curves of a non-programmed memory cell and a programmed memory cell, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a block diagram of a PROM according to an embodiment of the present invention. In Fig. 1, a plurality of memory cells MC₁₁ to MC mn are arranged in n lines and m rows to constitute a memory cell array 1. Each memory cell is constituted with an N channel field effect transistor having a control gate CG, a floating gate FG and a source-drain current path. The control gate CG of each memory cell is connected to one of word lines Wo to Wm and the source-drain path thereof is connected between one of digit lines D1 to Dn and a ground point. Between the digit lines D1 to Dn and a sense amplifier 7, N channel gate transistors MG1 to MGn are connected, respectively. Row address signals RAo to RAi are supplied through row address terminals 11-0 to 11-i to a row address buffer 3. Signals RAoo to RAii from the buffer 3 are supplied to a row address decoder 2. The decoder 2 thereby changes one of selection signals Xo to Xm to a selection level and energizes an associated word line W. Column address signals CAo to CAj are supplied via column address terminals 12-o to 12-i to a column address buffer 5. A column address decoder 4 responds to the outputs CAoo to CAjj of the buffer 5 and raises one of decoded signals Yo to Yn to the high level, so that an associated gate transistor MG is turned on. As a result, one memory cell MC is selected. A signal WE supplied to a terminal 13 is used to bring the PROM into a data readout operation or a data programming operation. When the signal WE takes the high level, a read/write control circuit 6 changes a read-enable signal RE to the high level and a write-enable signal WE to the low level. Therefore, the sense amplifier 7 is activated to amplify a data in the selected memory cell and provides the amplified data at a terminal 14 as an output data Dout. When the signal WE is in the low level, the write-enable signal WE becomes the high level and a data programming operation is performed. At this time, a programming voltage Vpp is supplied to a terminal 15. A programming circuit 9 responds to an output produced by a data input circuit 8 according to an input data D IN supplied to the terminal 14 and controls whether or not the programming voltage Vpp is applied to the selected word line W and the selected digit line D. When the selected memory cell is supplied with the voltage Vpp at its control gate CG and across its the source-drain current, electrons are injected into its floating gate FG. The gate FG is thereby charged negative. As a result, the threshold value thereof becomes high, and a programmed memory cell is thus formed. Fig. 6 shows control gate voltage V CG versus drain-source current I DS characteristics curves 60 and 50 of a programmed memory cell and a non-programmed memory cell, respectively. In the non-programmed memory cell, the floating gate thereof is not charged negative. Therefore, its threshold value V TMO is about 2.5 V and it is turned on by a control gate voltage V CG exceeding the threshold value, so that a drain current I DS increases. On the other hand, the programmed memory cell has a threshold value T TMW of about 13 V since its floating gate is charged negative. Therefore, the programmed memory cell is not turned on so long as its control gate voltage V CG does not exceed the threshold value V TMW . In the data readout operation, an intermediate voltage between the threshold values V TMO and V TMW is thus applied to the control gate CG of the selected memory cell MC as a readout voltage. In other words, the row address decoder 2 responds to the row address signals RAo to RAi and changes one of the selection signals Xo to Xm to this intermediate voltage to energize the associated word line W. The power voltage Vcc applied to a power supply terminal 16 is 5 V, and thus used as that intermediate voltage, advantageously. PROMs have been employed in various equipments powered by batteries. Electromotive force of a battery reduces with its use. This means that the readout voltage to be supplied to the memory cell drops gradually. The threshold value V TMO of the non-programmed memory cell is about 2.5 V, but a control gate voltage V CG of at least 3 V is required in order that the non-programmed memory cell produces a readout current I D . When the readout voltage (i.e., control gate voltage V CG ) becomes lower than 3 V due to the drop in the power voltage V CC , the sense amplifier will judge the non-programmed memory cell as a programmed memory cell. That is, the data readout operation becomes impossible. In order to broaden the range of the power voltage V CC within which the data readout operation can be performed, the PROM shown in Fig. 1 further includes a signal generator 10 generating word line pull-up signals RSo to RSm in response to a potential change in the outputs RAoo to RAii of the row address buffer 3 (i.e., potential change in the row address signals RAo to RAi), and capacitors Co to Cm connected, respectively, between output out terminals of the signal generator 10 and the word lines Wo to Wm. The pull-up signals RSo to RSm are generated after the row address decoder 2 raises the selected signal X to the selection level. Therefore, the capacitor C connected to the energized word line is first charged to the selection level and thereafter supplied with the pull-up signal RS. Accordingly, the word line W is further charged up to a level of a sum of the selection level of the signal X and the signal level of the signal RS. As a result, the non-programmed memory cell receives at its control gate a voltage V CG that is enough to produce the readout current I R even when the power voltage V CC drops below 3 V (Fig. 6). Fig. 2 is a circuit diagram of a row address decoder 2-0 and a word line pull-up signal generator 10-0 both associated with the word line Wo. The decoder 2-0 includes P-channel transistors M₂₀ and M₂₁ and N-channel transistors M₂₂, and M₂₃, these transistors constituting a NOR circuit having 2 inputs to which the signals RA oo and RA₁₁ are supplied from the address buffer 3. When both of the signals RA₀₀ and RA₁₁ are in the low level, an output of the NOR circuit becomes the high level. The output of the NOR circuit is supplied through an N-channel depletion type transistor M₂₄ to the word line Wo as a selection signal Xo. The read-enable signal RE is supplied to the gate of the transistor M₂₄. The signal generator 10-0 generating the word line pull-up signal RSo includes an RA₀₀ responsive circuit 10-0-1 and an RA₁₁ responsive circuit 10-0-2. Since these responsive circuits have the same construction, only the circuit 10-0-1 is shown in detail in Fig. 2. In the circuit 10-0-1, the signal RA₀₀ is delayed by a delay circuit 20 to provide a delayed signal V₂₀. The delay circuit 20 may be constructed with a plurality of inverters connected in series. The signal V₂₀ is supplied through an N-channel transistor M₂₅ having a gate supplied with the power voltage V CC to an inverter 21 whose output is in turn supplied to an inverter 22. An inverted signal V₂₂ from the inverter 22 is supplied through an N-channel transistor M₂₆ having a gate supplied with the power voltage V CC to an inverter 23. The outputs V₂₁ and V₂₃ of the inverters 21 and 23 are supplied to gates of N-channel transistors M₂₇ and M₂₈, respectively. Source-drain current paths of the transistors M₂₇ and M₂₈ are connected between the output terminal of the delay circuit 20 and a node N₁ and between the output terminal of the inverter 22 and the node N₁, respectively. A signal appearing at the node N₁ is derived as an output of the circuit 10-0-1 and, together with an output of the RA₁₁ responsive circuit 10-0-2, supplied to an NOR circuit 24. An output of the NOR circuit 24 is supplied to an inverter 25 whose output is derived as the pull-up signal RSo of the generator 10-0. A capacitor Co is connected between the output terminal of the signal generator 10-0 and the word line Wo. Fig. 3 shows a timing chart representative of the circuit operation. Before a time point T₁, the row address decoder 2 does not energize the word line Wo and does another word line. Therefore, the signals, RA₀₀ and RA₁₁ takes the high level and the low level, respectively, for example. At the time point T₁, at least one of the row address signals RAo to RAi changes it potential, resulting in that the signal RA oo from the address buffer 3 changes from the high level to the low level. On the other hand, the signal RA₁₁ is kept the low level according to the previous address information. Consequently, the transistors M₂₀ and M₂₁ are turned on and the transistors M₂₂ and M₂₃ are turned off. The potential at a node N2 is thus raised the power voltage V cc (high level). The selection signal Xo takes the selection level and the word line Wo is energized to be charged. The potential Vr of the word line Wo takes a V xo level. The level V xo is slightly lower than the power voltage V CC as shown in Fig. 3 due to the existence of the transistor M₂₄. The level change of the signal RA₀₀ is supplied to the circuit 10-0-1 and delayed by the delay circuit 20. The delayed output V₂₀ of the delay circuit 20 is changed to the low level at a time point T2. The delay time of the circuit 20 is selected to be longer than a time required for charging the word line Wo to the level V xo . At the time point T2, the output signal V₂₂ of the inverter 22 is changed to the high level. The transistor 26 acts as a delay element. Therefore, the gate of the transistor M₂₈ is in the high level at the time point T₂. The node N₁ is thereby changed to the high level. The output of the inverter 25 (therefore, the signal RSo) takes the power voltage V CC (high level). Since the capacitor Co is charged to the level V xo for a time from the time instance T₁ to the time instance T₂, the potential Vr of the word line Wo is pulled up by the high level signal RSo supplied to the capacitor Co and thus takes a (V xo + V CC ) level as shown in Fig. 3. This potential level is supplied to the control gates CG of the memory cells MC11 to MCln (Fig. 1) as a readout voltage. Assuming that the column address decoder 4 turns the gate transistor MG1 on in response to the column address signals CAo to CAj, a memory cell MC11 is selected. Further assuming that the memory cell MC11 is a non-programmed cell, this cell is turned on to produce the readout current I D . As as result, the sense amplifier 7 produces an output data Dout of "0", for example. If the cell MC₁₁ is programmed, its threshold value V TMW is about 13 V. On the other hand, the readout voltage level (V xo + V CC ) is lower than 10 V. Therefore, the cell MC₁₁ is maintained in the non-conductive state and the sense amplifier 7 produces an output data Dout of "1". The N-channel depletion transistor M₂₄ has a negative threshold value. The node N₂ to which one of the drain and source of the transistor M₂₄ is connected takes the power voltage V CC , and the word line Wo to which the other of them is connected takes the (V xo + V CC ) level. As a result, the back gate biasing-effect occurs to shift the threshold value of the transistor M₂₄ to a positive value. In the data readout operation, the signal RE, i.e. the gate of the transistor M₂₄, takes the power voltage V CC (high level), and the source and drain thereof take a potential that is equal to or higher than the gate potential. Therefore, the transistor M₂₄ is turned off when the potential of the word line Wo exceeds the power voltage V CC . The word line Wo and the decoder circuit 2-0 are thereby separated from each other. Due to the delay effects of the transistors M₂₅ and M₂₆, the outputs V₂₁ and V₂₃ of the inverters 21 and 23 are inverted respectively to the high level and the low level at a time point 3. Transistors M₂₇ and M₂₈ are thereby turned on and off, respectively. The signal V₂₀ is in the low level at the time point T3. Therefore, the node N₁ is discharged to the low level and the signal RSo is inverted to the low level. The potential Vr of the word line Wo is thereby discharged to the V xo level. As a result, the data readout operation responsive to the change in the address signals is completed. The readout voltage which is actually supplied to the selected memory cell MC takes the level which is approximately as high as twice the power voltage V CC . Therefore, the non-programmed cell receives the control gate voltage V CG which is required for obtaining the readout current I D even when the power voltage V CC is lowered to about 1.5 V. Thus, the range of the power voltage within which the data readout operation is carried out is expanded in the PROM shown in Figs. 1 and 2. When the signal RAoo is changed to the high level in response to a change in the row address signals to select another memory cell, the transistors M₂₀ and M₂₃ are turned off and on, respectively, so that the word line Wo is discharged to the ground level (low level). The signal generator 10-0 responds to the change in the address signals and produces again the high level pull-up signal RSo at a time point T5, but at this time, the transistor M₂₃ clamps the word line Wo at the ground level. Therefore, a substantial level increase does not occure on the word line Wo. The readout voltage taking the (V xo + V CC ) level is supplied to a newly selected memory cell by the operations of the signal generator 10 and other capacitors C shown in Fig. 1. Fig. 4 shows another embodiment of the present invention, in which only portions corresponding to those shown in Fig. 2 are shown. The same constituents as those shown in Fig. 2 are depicted by the same reference numerals and symbols to omit their further description. Features of this embodiment are that, a readout voltage V R higher than V CC is applied to a selected memory cell only when the source voltage V CC is lower than a predetermined value, and that means is provided for reliably separating the row decoder circuit from the boosted word line. A comparator 32 is used to detect the power voltage V CC . The non-inverted input terminal(+) of the comparator 32 is supplied with the power voltage V CC and the inverted input terminal(-) thereof is supplied with a reference voltage Vref from a reference voltage source 33. The reference voltage Vref is selected to be a voltage value by which the non-programmed memory cell produces the readout current I R , i.e., 3 V (refer to Fig. 6). The output V D of the comparator 32 is supplied to one input terminal of an NOR circuit 30 which has the other input terminal supplied with an output RSo' of an NOR circuit 24. The output of the NOR circuit 30 is supplied to the capacitor Co as the pull-up signal RSo and further to an inverter 31. The output of the inverter 31 is supplied to the gate of a transistor M₂₄ as a gate control signal GC. When the power voltage V CC is higher than the reference voltage Vref, that is, when the readout current I D is obtained by applying a readout voltage taking the power voltage V CC level to a non-programmed memory cell, the output V D of the comparator 32 takes the high level, as shown in Fig. 5A. Therefore, the signal RSo is clamped to the low level regardless of the level change in the signal RAoo or RA11 caused by the change in the address signals. The signal GC takes therefore the high level. Consequently, the voltage V R of the word line Wo takes the Vxo level and is supplied to the memory cell MC₁₁ as a readout voltage. When the source voltage V CC is reduced to a level V CC1 that is lower than the reference voltage Vref, the comparator 32 produces a low level output V D as shown in Fig. 5B. When the signal RA11 is changed to the low level due to the change in the address signal (the signal RAoo is in low level), the transistors M₂₀ and M₂₁ are turned on to charge the word line Wo to the Vxo level. As described with reference to Fig. 2, the output RSo' of the NOR circuit 24 is inverted to the low level and the signal RSo is inverted to the high level after the capacitor Co is charged to the Vox level. As a result, the word line is further charged, so that its potential V R takes the (Vxo + V CC1 ) level. Since the signal RSo is in high level, the signal CG, i.e. the gate of the transistor M₂₄, takes the low level. Thus, the gate of the transistor M₂₄ takes the low level while both of the source and drain thereof are takes the level higher than the level at the gate. Therefore, the transistor M₂₄ is ensured to be the non-conductive state even if the threshold value thereof has deviation from a designed value. The decoder circuit 2-0 is thereby separated completely from the word line Wo. When the signal RSo is inverted to the low level, the potential V R of the word line is decreased to the Vxo level. The range of the source voltage within which the data readout operation is carried out is also expanded in this embodiment. Furthermore, since the bootstrap effect of the word line potential is performed only when the power voltage is lowered, the problem that the programmed memory cell may be turned on when the power voltage V CC is increased, is resolved, which may otherwise occur in the memory shown in Figs. 1 and 2. The transistor M₂₄ shows in Fig. 2 can be supplied at its gate with the low level at the boosting time of the word line in accordance with the teaching of Fig. 4. It should be apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope of the invention. For instance, the signal generators 10-0-1 and 10-0-2 shown in Figs. 2 and 4 may respond only to a change in the signals RAoo and RA11 from the high level to the low level and produce the high level signal. Although the respective embodiments described hereinbefore are constituted with complementary insulated-gate field effect transistors, they may be constituted only with N channel transistors or P channel transistors.

Claims: A semiconductor memory comprising a memory cell array (1) including a plurality of memory cells (MC), a plurality of word lines (WO-Wm) and a plurality of digit lines (D1-Dn), each of said memory cells including a field effect transistor having a floating gate (FG), a control gate (CG) connected to one of said word lines (W) and a source-drain path connected to one of said digit lines (D), said field effect transistor having a first threshold voltage (VTMO) lower than a power supply voltage (Vcc) in a non-programmed state and a second threshold voltage (VTMW) higher than said power supply voltage (Vcc) in a programmed state, and means (2-5, MG) responsive to a set of address signals (RA, CA) for selecting at least one word line (W) and at least one digit line (D) to designate at least one of said memory cells (MC), said selecting means including means (2, 3) for supplying the selected word line with a read-out voltage (Vxo) between said first threshold value (VMTO) and said power supply voltage (Vcc) to read out data from the designated memory cell, characterized by further comprising means (10, CO-Cm) for supplying, after said selected word line is charged with said read-out voltage (Vxo), said selected word line with such an additional voltage (Vcc) that raises the voltage of said selected word line up to a level (Vxo+Vcc) above said power supply voltage (Vcc) but below said second threshold voltage (VTMW), said additional voltage being equal to said power supply voltage.

Inventors: WATANABE TAKESHI C O NEC CORPO, WATANABE, TAKESHI C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1986
Main classification: G11C16
IPC codes: G11C8, G11C16, G11C17