Method and circuit for tunnel-effect programming of floating-gate MOSFETS

Abstract: The present programming method exploits the close dependence of tunneling current on the voltage drop (V ox ) across the tunnel oxide layer, for which purpose, a bootstrapped capacitor (3) connected to the drain terminal (D) of the transistor (1) is employed, the charge state of which determines the bias of the tunnel oxide and is in turn determined by the charge state in the floating gate (FG). Biasing by the bootstrapped capacitor is such as to permit passage of the tunneling current (I TUN ) and, hence, a change in the threshold voltage of the transistor until the floating gate reaches the desired charge level, and to prevent passage of the tunneling current upon the transistor reaching the desired threshold. Programming is thus performed automatically and to a predetermined degree of accuracy, with no need for a special circuit for arresting the programming operation when the desired threshold is reached.

Description: The present invention relates to a method and circuit for tunnel-effect programming floating-gate MOSFET transistors, and is especially advantageous for programming nonvolatile EEPROM and flash-EEPROM memories to which the following description refers purely by way of example. As we know, of modern solid-state electronic components, nonvolatile semiconductor memories (in particular, electrically erasable programmable types) now rank uppermost by virtue of being able to store and preserve information for prolonged periods (decades, under normal operating conditions) even in the absence of supply, thus providing a solution to all those applications in which information must be preserved when the equipment or device to which the memories are connected is turned off. Nonvolatile memories may be digital or analog (the latter of more recent and less widespread application), and generally comprise numerous elementary cells for storing an elementary item (a bit in the case of digital memories, a specific electric level in the case of analog types). Regardless, however, of the type of fabrication technology employed, the information stored in any commercial memory cell is represented by a given charge (electrons) inside the cell, and more specifically in a region (floating gate) insulated electrically from the rest of the cell. In digital applications, the stored charge assumes two values corresponding respectively to storage of a logic "0" or "1"; whereas, in the case of analog memories, the charge varies within certain limits, and is correlated to the electric level of the signals to be stored via appropriate read and write circuits. The main problem in programming nonvolatile memory cells lies in controlling the stored charge. This is particularly important in the case of analog applications, wherein charge control determines the resolution and accuracy with which the electric signal is stored, but is nonetheless important also in the case of digital memories for minimizing as far as possible the difference between the charges corresponding to the two logic states (also referred to as the write and erase states of the cell) and so minimizing wear and extending the working life of the cell. Moreover, in the case of flash-EEPROM memories, charge control is essential for minimizing read errors due, for example, to the presence of overerased cells. As we know, the charge of the memory cells in question is altered by Fowler-Nordheim-effect tunneling of a thin dielectric layer (EEPROM and flash-EEPROM cells); and since tunneling depends exponentially on the electric field involved, and since the programming current is extremely low (a few nA), it is difficult to design a circuit capable of automatically limiting the injected charge on the basis of quantities (voltage or current) set during programming, and so accurately controlling the stored charge. One proposal for solving this problem is to sample the state of the cell during programming, and to interrupt programming upon the stored charge (or a quantity which is a direct function of it) assuming a predetermined value. For example, it has already been proposed (see USA Patents US-A-4,357,685 and US-A-4,890,259) to divide the programming cycle into a given number of short partial subcycles during which only a small fraction of the charge required for the cell to assume the desired final state is injected. At the end of each partial cycle, the state of the cell is tested to determine whether to continue with further partial cycles, or to terminate programming by virtue of the required charge being achieved. The state of the cell is tested on the basis of various electric quantities related to the charge stored in the floating gate (cell threshold, current flow under given bias conditions, etc.). This solution presents several major drawbacks, mainly due to the discrete nature of the charge state achievable, and to resolution of the injected charge depending on the number and duration of the partial cycles. Consequently, accuracy is improved only alongside an increase in the number of cycles and, consequently, in programming time, so that, in practice, a trade-off is inevitable between programming time and accuracy, the results of which are not always satisfactory. As a result, the above known method is limited solely to applications wherein no more than rough control of the stored charge is required. EP-A-0 541 221, which is comprised in the state of art under Art. 54(3) EPC, discloses in fig. 2A a method and apparatus for erasing flash cells, including a bootstrap capacitor one terminal whereof is connectable with a floating gate cell and the other terminal whereof receives a cyclical clock signal alternating between a low state, wherein the capacitor charges, and a high state, wherein the capacitor is connected to the floating gate cell. It is an object of the present invention to provide a programming method designed to overcome the drawbacks typically associated with known solutions, and which in particular provides for straightforward, high-precision programming of MOSFET transistors. According to the present invention, there is provided a method of tunnel-effect programming floating-gate MOSFET transistors, as claimed in Claims 1 and 4. The present invention also relates to a circuit for tunnel-effect programming floating-gate MOSFET transistors, as claimed in Claims 7 and 8. In practice, the present invention exploits the close dependance of the tunneling current on the voltage drop across the oxide layer. Therefore, as opposed to tunneling by biasing the tunnel oxide layer by means of a low-impedance voltage source or a capacitor recharged periodically to simulate a voltage source, a bootstrapped capacitor is employed, possibly in series with a voltage source. The charge of the capacitor determines the tunnel oxide bias, and in turn is critically dependent on the state of the MOSFET transistor. The bias produced by the capacitor is such as to allow passage of the tunneling current until the floating gate reaches the required charge, and to cut off the tunneling current upon the transistor reaching the required threshold, thus terminating programming of the cell. As such, programming is terminated automatically and to a predetermined degree of accuracy, with no need for a special circuit for terminating the programming operation upon the required threshold being reached. In particular, when erasing a transistor presenting an excessive charge at the floating gate, and hence a higher threshold voltage than desired, the transistor is biased at the control gate to a value correlated to the desired threshold level, and connected to the bootstrapped capacitor, charged beforehand, so that the resulting voltage drop across the tunnel oxide is such as to extract electrons from the floating gate through the tunnel oxide, and so reduce the threshold voltage. In view of the low tunneling current involved, substantially no discharge occurs of the capacitor, and charge extraction continues until the threshold voltage reaches the desired value. At this point, a high current is conducted by the transistor, thus rapidly discharging the bootstrapped capacitor, which discharge in turn reduces the voltage drop across the tunnel oxide, and arrests charge extraction from the floating gate, thus automatically terminating the programming step at the required value. Conversely, when writing a transistor having a lower threshold voltage than desired, the transistor is biased accordingly to the desired threshold value, and connected to the capacitor, charged beforehand, so as to turn on the transistor and rapidly discharge the capacitor; the control gate of the transistor is then supplied with a write potential of such a value that, with the capacitor discharged, the voltage drop across the tunnel oxide is sufficient to tunnel charges towards the floating gate, thus increasing the threshold voltage. The capacitor is then recharged and connected to the transistor, and the above cycle is repeated several times. Consequently, upon charge injection bringing the threshold of the transistor to the desired value, and the (charged) capacitor being connected to the transistor, the transistor is not turned on, and the capacitor is only partially discharged. When the write potential is applied to the control gate of the transistor, therefore, the voltage drop across the gate oxide of the transistor is reduced so that no further charges can be injected, the write step is arrested, and, in the following cycles, the transistor remains at the desired threshold value. Two preferred, non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which: Fig.1 shows the bias circuit of a floating-gate MOSFET transistor for effecting controlled erase, according to a first embodiment of the present invention; Fig.2 shows the bias circuit of a floating-gate MOSFET transistor for effecting controlled write, according to a second embodiment of the present invention; Fig.3 shows graphs of a number of significant electric quantities during the controlled erase step; Fig.4 shows a graph of the threshold voltage of the cell as a function of control gate voltage, following controlled erase according to the present invention; Fig.5 shows graphs of a number of control signals in the Fig.2 circuit; Fig.6 shows graphs of a number of significant electric quantities during the controlled write step according to the present invention. To begin with, a preferred embodiment will be described for erasing a floating-gate MOSFET transistor defining, for example, a nonvolatile memory array cell. As we know, the erase step involves removing electric charges (electrons) from the floating gate of the cell to reduce its threshold voltage, i.e. the value of the voltage to be applied between the control gate region and the source region of the cell for it to be turned on. Fig.1 shows a transistor 1 and a drive circuit 2 substantially comprising a bootstrapped capacitor 3 and a number of switches. More specifically, capacitor 3 presents a first terminal 4 connected to the drain terminal D of transistor 1 and to supply voltage V 1 via switch 5 and a second terminal 6 at voltage V p , preferably an increasing ramp voltage at a first step, and then of constant value up to the end of the erase step. The source terminal S of transistor 1 is grounded via switch 7 and the control gate terminal CG is biased to a constant voltage V CGE via switch 8. Capacitor 3 must present a capacitance of over, for example, roughly ten times the parasitic capacitance between the drain region and substrate of transistor 1, e.g. 100 fF. Assuming transistor 1 presents a higher threshold voltage than desired, due to an excess of electrons at the floating gate, and must therefore be erased, according to a preferred embodiment of the present method, capacitor 3 is precharged to voltage V 1 by closing switch 5 and maintaining terminal 6 grounded. During this step, switches 7 and 8 are open. Switch 5 is then opened and switches 7 and 8 closed so as to ground source terminal S of transistor 1 and connect control gate terminal CG to voltage V CGE correlated to the desired threshold voltage, as shown below. Drain terminal D of transistor 1 is thus at voltage V D = V 1 ; and V p is increased linearly to bring about a similar increase in voltage V D at the drain terminal, by virtue of the latter being related to V p by the equation: V D = V p C/(C+C D ) where C is the capacitance of capacitor 3, and C D the capacitance between the drain region and ground (substrate of the transistor). When V D becomes high enough, tunneling current flows through capacitor 3 to floating gate FG of transistor 1, thus reducing the charge stored in the floating gate and so gradually reducing the threshold voltage of the transistor. As tunneling current I TUN at this step is very low (in the order of tenths of a nA), substantially no change occurs in the charge of bootstrapped capacitor 3 or its voltage V C . As the charge stored in floating gate FG is reduced gradually, the threshold voltage of transistor 1 eventually reaches the desired value set by V CGE , at which point, transistor 1 switches on and conducts rapidly increasing source current I S , the value of which is such as to rapidly discharge the capacitor and so rapidly reduce voltage V C . As a result, drain voltage V D not only stops increasing linearly with V p but is actually decreased slightly, which decrease, however, is sufficient to reduce and cut off tunneling current I TUN , thus automatically arresting erasure of the transistor and freezing the value of the stored charge. The final charge stored in the floating gate thus depends on voltage V CGE , by virtue of this determining the charge needed to turn on the transistor and, hence, for arresting the programming operation. This voltage, being fixed during programming and controllable continuously, provides for achieving a high degree of resolution as regards control of the final charge in the floating gate and, hence, the final threshold. Programming is also largely independent of, and so presents a good degree of immunity to, the characteristics of the programming ramp, wear of the tunnel oxide, and the initial charge in the floating gate. Fig.3 shows graphs of a number of electric quantities relative to a transistor 1 connected to bias circuit 2 in Fig.1 during erasure, and for two different desired threshold voltage values. In both cases, transistor 1 presented an initial threshold voltage of 6.95 V, corresponding to a floating gate voltage with respect to ground V FG = -4 V with all the terminals of the cell grounded. In both cases, V 1 = 5 V, and V p was increased linearly from 0 to 15 V between instants t 0 = 0 and t 3 = 1 ms, and held at the maximum value up to instant t 4 . Before instant t 0 , the capacitor was charged to voltage V 1 , bringing the drain terminal V D to 5 V. At instant t 0 , voltage V CGE = 1 V is applied to the control gate terminal. As a result (solid line curves) voltage V D begins increasing, following voltage V p . At this step, floating gate voltage V FG also increases, but more slowly, due to capacitive coupling of the floating gate to the control gate, so that voltage drop V ox between the floating gate region and the drain terminal also increases. At instant t 1 , V ox reaches such a value as to produce a significant tunneling current I TUN , so that, at this step, the charge in the floating gate region decreases, but with no substantial change in the charge of the capacitor, as shown by the voltage V D curve. At instant t 2 , the remaining charge in the floating gate is such that transistor 1 switches on, thus discharging capacitor 3; current I S presents a peak; voltage V D decreases; and current I TUN is reduced. At instant t 4 , all the transistor terminals are grounded for evaluation. If, on the other hand, voltage V CGE is set to 3 V, this gives the pattern shown by the dotted line curves, which is more or less similar to that described above, except that erasure of transistor 1 commences later due to the higher voltage of floating gate FG. Also, due to transistor 1 being turned on earlier, a much smaller total charge is extracted, so that the final floating gate voltage V FG is lower. Fig.4 shows the relationship between threshold voltage V TH , following erasure according to the present method, and applied control gate voltage V CGE , as determined in experimental tests performed using various V CGE values. As can be seen, the relationship is linear with a high degree of precision over a wide range of values, as required for precision analog applications. It should be pointed out that testing was conducted using a transistor with a non-negligible capacitive coupling between the floating gate and drain, which makes threshold voltage V TH also dependent on V D . The coupling may, however, be reduced in known manner, and in any case the dependence of voltage V CGE on the desired V TH value may be calculated. If necessary, special circuits may be provided for automatically generating voltage V CGE on the basis of the desired threshold. The controlled erase procedure described above may be employed in erase mode, for reducing the cell and transistor threshold value, as well as in write mode, for increasing the cell and transistor threshold by initially overwriting the transistor to above the desired final value and then erasing it in controlled manner (as described above). Particularly in the case of nonvolatile memories, however, it is frequently desirable not to overwrite the cells, to prevent impairing or at any rate reducing reliability of the tunnel oxide layer, for which purpose a second controlled write procedure may be adopted as described below. Fig.2 shows the drive circuit 2' for performing the controlled write procedure. In view of the similarity between this and circuit 2 in Fig.1, which may be implemented by a single circuit with only minor alterations, any parts common to both are indicated using the same numbering system. In the Fig.2 circuit, capacitor 3 presents terminal 6 grounded, and terminal 4 connected via switch 5 to voltage V 2 ; and control gate terminal CG is connected to a switch 10 for selectively connecting the control gate to voltage V R or V CGW , where V R is a rising or continuous voltage for enabling passage of the tunneling current and, hence, electron injection into the floating gate, and V CGW is correlated to the desired threshold voltage. A preferred embodiment will now be described with reference also to Fig.5, which shows graphs of the control signals Φ 1 , Φ 2 , Φ 3 of respective switches 10, 7, 5, and wherein a high level of signals Φ 3 , Φ 2 of switches 5, 7 corresponds to a closed switch, and a low level to an open switch, and a high level of signal Φ 1 of switch 10 corresponds to connection of the control gate to V CGW , and a low level to connection to V R . To begin with, the capacitor is charged to a medium/high voltage V 2 , such as to prevent the passage of tunneling current when the control gate is at voltage V R and capacitor 3 is charged. At this step (indicated by 1 in Fig.5), switch 7 is open, and switch 10 connects the control gate to voltage V CGW . At the next step (2 in Fig.5), switch 5 is opened, switch 10 remains in the same position as before (supplying voltage V CGW to control gate terminal CG), and switch 7 is closed. Since transistor 1 presents a threshold below the desired value, this comes on, thus resulting in rapid discharge of capacitor 3 and the passage of a high current I S . At the next step (3 in Fig.5), switch 7 is opened; switch 10 switches to supply voltage V R to the control gate; switch 5 remains open; and, since the capacitor is fully or mostly discharged, voltage V D at the drain terminal is low, and voltage V ox across the tunnel oxide is such as to generate tunneling current I TUN towards the drain region, thus injecting electrons into floating gate FG and so increasing the threshold voltage. Capacitor 3 is then recharged by closing switch 5 and switching switch 10 (switch 7 remains open), and the above three-step cycle is repeated, the threshold voltage increasing at each cycle until the desired value is reached. When, in step 2 of the cycle following the one in which the desired threshold is reached, the capacitor is charged to V 2 , voltage V CGW is applied to the control gate of transistor 1, and switch 7 is closed, transistor 1 therefore remains off, or is turned on only partially, thus preventing discharge of capacitor 3 or enabling only a small amount of discharge as compared with the previous cycles, so that drain terminal D remains at a high voltage. In step 3 of the same cycle, therefore, when control gate CG of transistor 1 is connected to high voltage V R , voltage drop V ox across the oxide layer is low, thus preventing further electron injection. Even in the event the above cycle is repeated, this has no effect on the stored charge or threshold voltage, the value of which thus remains constant. Fig.6 shows two embodiments of the above write method, wherein the write operation is divided into 20 cycles, each lasting 110 µs. In both cases, a voltage V 2 of 8 V was selected; and voltage V R presents the pattern shown, rising initially in a ramp from 0 V to maximum (e.g. 15 V) at t = 1 ms, and then constant up to t = 2.2 ms. It should be pointed out, however, that voltage V R , even over the ramp portion, does not increase continuously as shown, but only in step 3 of Fig.5, and remains constant in steps 1 and 2. In view of the brevity of steps 1 and 2 as compared with step 3, however, the V R curve has been simplified. In the first embodiment, voltage V CGW equals 4 V, in which case, as shown by the solid line curves, drain terminal voltage V D initially continues switching between 8 V (after charging the capacitor) and 0 V (due to discharging of the capacitor when switch 7 is closed and voltage V CGW is applied to control gate terminal CG). At the same time, the floating gate voltage continues rising, following the increase in V R , with the exception of the peaks, when the capacitor is charged, due to capacitive coupling with the drain region. As voltage V R increases, voltage drop V ox across the oxide layer is eventually sufficient to cause electrons to be injected into the floating gate. This step corresponds to cycle a in Fig.6, in which current I TUN starts flowing when voltage V R is applied to the control gate. Injection continues into the next cycle b , at the end of which the charge stored in the floating gate has brought about the desired threshold value. When the capacitor is charged, therefore, and voltage V CGW is applied to control gate CG, the capacitor discharges only partially; and, when voltage V R is again applied, no tunneling current is supplied, thus freezing the stored charge at the value reached in cycle b . In the second embodiment, voltage V CGW equals 5 V, in which case, electron injection also continues into cycle c , as shown by the dotted line curves (in this case in fact, voltage V D is still low in cycle c ) thus enabling greater electron storage and, hence, a higher threshold. The Applicant's calculations give a final threshold voltage value of 4.49 V with V CGW = 4 V, and 5.47 V with V CGW = 5 V. Whichever the case, in write mode also, a good degree of linearity exists between voltage V CGW applied to the control gate and the threshold voltage achievable. Though more complex than the controlled erase procedure, and despite involving the repetition of elementary cycles and, hence, intrinsic quantization errors, by appropriately selecting the times and voltage values employed, the controlled write procedure provides for a high degree of precision and reliability, with no need for special measuring operations, by virtue, as explained, of the write operation terminating automatically when the desired threshold value is reached. The advantages of the programming method and circuit according to the present invention will be clear from the foregoing description. In particular, with a solution of straightforward design and circuitry, they provide for automatically arresting the programming operation upon the desired threshold being reached. In the case of controlled erase, programming is performed in one cycle, with no charge discretizing problems, and with an excellent trade-off in terms of resolution and programming time. In the case of controlled write, the discretizing error is containable, and the overall performance time of the procedure is at any rate less than that required for cyclically measuring the charge, as with traditional methods. To those skilled in the art it will be clear that changes may be made to the method and circuit described and illustrated herein purely by way of example, without, however, departing from the scope of the present invention. For example, as opposed to a ramp pattern, voltage V p applied to the bootstrapped capacitor in erase mode, and voltage V R applied to the control gate in write mode, may present an appropriate constant value, though a rising voltage, at least initially, is preferable to prevent sharply stressing and damaging the tunnel oxide. Other voltage patterns may also be employed, and the voltages may be applied differently than as described herein. Changes may also be made to the programming circuit. In particular, one circuit may be used for both write and read, by providing a two-position switch connected to terminal 6 of the capacitor, and by so controlling the switch that terminal 6 is connected to voltage V p for erasing, and is grounded for writing. The switches themselves may be formed in any manner, e.g. by means of MOS transistors; and the circuit may be formed separately or integrated with MOSFET transistor 1 and the voltage source section.

Claims: A method for tunnel-effect erasing a MOSFET transistor (1) comprising a control gate terminal (CG), a pair of source and drain terminals (S, D), and a floating gate region (FG) insulated with respect to said pair of terminals by a dielectric layer, the method comprising the steps of: a) connecting one of said source and drain terminals (S, D) to a first terminal (4) of a bootstrap capacitor (3); b) connecting the other one of said source and drain terminals (S, D) to a reference potential source; c) connecting the control gate terminal (CG) to a control gate voltage (V CGE ) corresponding to a desired threshold to which the MOS transistor is to be erased; d) increasing only once for an entire erase operation a voltage (V p ) applied to a second terminal (6) of said bootstrap capacitor (3); e) whereby a tunneling current flows from the bootstrap capacitor (3) towards the floating gate through said dielectric layer and the threshold voltage of the MOS transistor is reduced, until the MOS transistor becomes conductive and pulls down the first terminal (4) of the bootstrap capacitor (3) towards the reference potential so that the tunneling current is automatically turned off. A method as claimed in Claim 1, characterized in that, before said step d), it comprises the step of: initially charging said bootstrapp capacitor (3) to a predetermined charge voltage (V 1 ). A method as claimed in Claim 1 or 2, characterized by the fact that said erase voltage (V p ) increases from a minimum to a maximum value. A method of tunnel-effect writing a MOSFET transistor (1) comprising a control gate terminal (CG), a pair of source and drain terminals (S,D) and a floating gate region (FD) insulated with respect to said pair of terminals by a dielectric layer, the method comprising the steps of: a) connecting one of said source and drain terminals (S, D) to a first terminal (4) of a capacitor (3); b) connecting the other one of said source and drain terminals (S, D) to a reference potential source; c) precharging said capacitor (3) to a first voltage (V2); d) connecting the control gate terminal (CG) to a control gate voltage (V CGW ) corresponding to a desired threshold voltage to which the MOS transistor is to be written, whereby said capacitor is conditionally discharged through the MOS transistor (1); e) connecting the control gate terminal (CG) to a second voltage (V R ), whereby tunnel current conditionally flows through the dielectric layer into the capacitor (e); f) repeating steps c) through e). A method as claimed in Claim 4, characterized in that said second voltage (V R ) increases from a minimum to a maximum value. A method as claimed in Claim 1 or 4, characterized in that said first terminal is a drain terminal (D). A device comprising a MOSFET transistor (1) and a circuit for tunnel-effect erasing said MOSFET transistor (1), said MOSFET transistor (1) comprising a control gate terminal (CG), a pair of source and drain terminals (S, D), and a floating gate region (FG) insulated with respect to said pair of terminals by a dielectric layer, the circuit comprising: a) a bootstrap capacitor (3); b) first connection means (4) for connecting one of said source and drain terminals (S, D) to a first terminal (4) of said bootstrap capacitor (3); c) second connection means (7) for connecting the other one of said source and drain terminals (S, D) to a reference potential source; d) third connection means (8) for connecting the control gate terminal (CG) to a control gate voltage (V CGE ) corresponding to a desired threshold to which the MOS transistor (1) is to be erased; e) single cycle increasing means for increasing only once for an entire erase operation a voltage (V p ) applied to a second terminal (6) of a said bootstrap capacitor (3); whereby, in operation a tunneling current flows from the bootstrap capacitor (3) towards the floating gate through said dielectric layer and the threshold voltage of the MOS transistor (1) is reduced, until the MOS transistor becomes conductive and pulls down the first terminal (4) of the bootstrap capacitor (3) towards the reference potential so that the tunneling current is automatically turned off. A device comprising a MOSFET transistor (1) and a circuit for tunnel-effect writing said MOSFET transistor (1), said MOSFET transistor (1) comprising a control gate terminal (CG), a pair of source and drain terminals (S, D) and a floating gate region (FD) insulated with respect to said pair of terminals by a dielectric layer, the circuit comprising: a) a capacitor (3) ; b) first connection means (4) connecting one of said source and drain terminals (S, D) to a first terminal (4) of said capacitor (3); c) second connection means (7) connecting the other one of said source and drain terminals (S, D) to a reference potential source; d) precharge means (5) for precharging said capacitor (3) to a first voltage (V2) ; e) first switch means (10) connecting the control gate terminal (CG) to a control gate voltage (V CGW ) corresponding to a desired threshold voltage to which the MOS transistor (1) is to be written, whereby said capacitor (3) is conditionally discharged through the MOS transistor (1); f) second switch means (10) connecting the control gate terminal (CG) to a second voltage (V R ), whereby tunnel current conditionally flows through the dielectric layer into the capacitor (3); g) activation means repeatedly activating said precharge means (5), said first and said second switching means (10). A circuit according to claim 7, characterized in that it comprises a first switch (5) interposed between a reference potential line (V 1 ) and said first terminal (4) of said bootstrap capacitor (3) and in that said third connecting means comprises a second switch (8). A circuit according to claim 8, characterized in that said precharge means (5) comprises a switch interposed between a reference potential line (V 2 ) and said first terminal (4) of said capacitor (3). A circuit as claimed in Claim 8, characterized in that said first and second switch means comprise a switch (10) switchable between a position connecting said control gate terminal (CG) to said control gate voltage (V CGW ), and a position connecting said control gate terminal to said second voltage (V R ). A circuit as claimed in one of Claims 7 to 11, characterized in that said second connection means comprises a third switch (7).

Inventors: BRIOZZO LUCIANO, LANZONI MASSIMO, RICCO BRUNO, BRIOZZO, LUCIANO, LANZONI, MASSIMO, RICCO, BRUNO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: G11C16
IPC codes: G11C17, G11C16