Voltage regulator for semiconductor non-volatile electrically programmable memory devices

Abstract: A voltage (Vpd) regulator (3) for electrically programmable non-volatile memory cells (2) comprises a gain stage (diff1) which is supplied a voltage (hv) from a voltage booster (4) connected to a supply voltage (Vcc) reference, having an input terminal connected to an output (A) of a voltage divider (8) and an output terminal connected to at least one transistor (M2) of a pull-up and pull-down differential pair to output (OUT) the regulated voltage (Vpd) for programming at least one column or bit line of the memory cells (2), and further comprises a second gain stage (diff2) having an input terminal connected to a second output (B) of the divider (8). The second stage (diff2) has an output connected to turn off the second transistor (M3) in the complementary pair upon the regulated voltage (Vpd) exceeding a predetermined value.

Description: Field of the Invention This invention relates to a voltage regulator for non-volatile storage devices of the electrically programmable semiconductor type. In particular, the invention concerns a voltage regulator which comprises a gain stage being supplied a voltage from a voltage booster connected to a supply voltage reference, and has an input terminal connected to an output of a voltage divider and an output terminal connected to at least one transistor of a pull-up and pull-down differential pair, to thereby output the regulated voltage for programming at least one column or bit line of said memory cells. As is known, a semiconductor non-volatile storage structure comprises an array of memory cells, each formed of a MOS transistor whose gate electrode, located over its channel region, is a so-called floating gate. This electrode has high DC impedance toward all the other terminals of the same cell and the circuit in which the cell is connected. The cell further comprises a second electrode, called the control gate electrode, which is driven by means of appropriate control voltages. The other electrodes of the transistor are known as its drain, source and body terminals. A non-volatile memory integrated on a semiconductor comprises a array of cells arranged into a matrix of rows, called the word lines, and columns, called the bit lines. The cells in one word line have in common the electric line that drives their control gates, whereas the cells in one bit line have their drain terminals in common. By the application of appropriate voltage values to the cell terminals, the amount of the charge present on the floating gate can be changed, e.g. utilising phenomena known as Fowler-Nordheim's Tunnelling and/or Channel Hot Electron Injection. This results in the transistor being placed in either of two logic states: a first state of "high" threshold voltage, and a second state of "low" voltage threshold. Since the floating gate has high impedance toward any other terminals of the cell, the stored charge can persist therein for an indefinite length of time, even after the power supply to the circuit which contains it has been cut off. The cell has, therefore, the characteristics of a non-volatile memory. The operation whereby a charge is stored into the floating gate is called "programming". The programming of each individual cell is carried out by hot electron injection and results in electrons being trapped in the floating gate when the control gate is applied approximately 12 volts, with the source terminal being connected to ground and the drain terminal held at a drain voltage of about 5.5 volts. It is well recognised that non-volatile memory cells, particularly of the FLASH type, require accurate control of the drain voltage applied to the bit line during the programming step. This voltage is, in fact, to satisfy a number of conditions, as listed below: it should be high enough to allow of fast cell programming; at the same time, it should be sufficiently low to prevent the inception of a phenomenon known as "soft erasing", which consists of the cell becoming partially erased or its characteristics deteriorating in use; and at all events, it should not trigger on the phenomenon known by the term "parasitic bipolar", for reliability reasons. The optimum range for the drain voltage is usually a fairly narrow one, being typically of 5 to 6 volts. It should be further noted that the above-listed conditions are apt to vary with the fabrication process, specifically with the length of the memory cell. Fabrication processes introduce dimensional variations in the cells, especially in the polysilicon layers, and this affects the value of the drain voltage to be applied. On the whole, the above considerations lead to conclude that the storage circuit should be provided with a voltage regulator of a specially refined and precise character, in order to have the bit line supplied with the appropriate drain voltage during the programming step. The state of the art does provide some solutions directed to fill this demand. Background Art In general, since there basically are two voltages being supplied from external sources to the storage circuit, namely a supply voltage Vcc of 5 volts and a programming voltage Vpp of 12 volts, the drain voltage is regulated by dividing the programming voltage Vpp. In this way, a drain voltage can be provided which is relatively stable to temperature and to the parameters of the circuit fabrication process, although it may vary by as much as ±5% with the programming voltage Vpp. However, since during the programming step, the threshold voltage of the cell tends to become progressively higher, the current draw of the cell is bound to decrease over time. Through the resistive divider, the voltage regulation is effected on the basis of a mean value of the programming current, and a regulation so conceived cannot give good results continually. A second prior technical solution is described in U.S. Patent No. 5,263,000, wherein the drain voltage is arranged to be that supplied from a voltage booster being powered by the supply voltage Vcc at 5 volts. Arranged after the voltage booster is a differential amplifier which drives an output stage comprising a series of two transistors, in the pull-up and pull-down configurations. Not even the last-mentioned solution is devoid of drawbacks. In particular, an output stage so configured exhibits current consumption of a very high order, presumably because of some current going astray between the voltage booster output and ground. This can pose some problems at the programming stage because, if the current flowed through the bit line -- and hence, the output stage of the regulator -- differs from the design current, then the voltage on the bit line will also be inappropriate, or at least other than the desired one. Also EP-A-0 538 121 discloses a programming voltage regulator for programming non-volatile memory cells. It contains a pull-up transistor and a pull-down transistor, and a comparing circuit controlling the pull-up transistor. Overshooting drain voltage can therefore only be compensated in a current consuming manner by means of a resistor. EP-A-0 321 226 discloses a voltage regulator having both pull-up and pull-down regulation. It is, however, not concerned with high voltages programming non-volatile cells. The underlying technical problem of this invention is to provide a voltage regulator which can constantly output to the bit line a suitable voltage value to ensure optimum programming of the memory cells, while affording savings in current draw, and therefore, in dissipated power; all this being effective to overcome the limitations of the solutions currently provided by the state of the art. Summary of the Invention The idea on which this invention stands is that of having the drain voltage regulated by a single pull-up and utilising for down-pulling the output stage of the current regulator of the cells, under either steady-state or slow transient phase conditions. Based on this idea, the technical problem is solved by a voltage regulator as indicated in the preamble and defined in the characterising part of the appended claim 1. The features and advantages of the voltage regulating circuit of this invention will be apparent from the following detailed description of an embodiment thereof, to be taken by way of non-limitative example with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 is a diagrammatic view of a programming circuit which incorporates a voltage regulator according to the invention; Figure 2 shows diagramatically an embodiment of the regulator according to the invention; and Figures 3 and 4 are respective plotting of the characteristic curves of simulations carried out with the regulator according to the invention. Detailed Description With reference to the drawing views, schematically shown at 1 is a general diagram of a program circuit serving electrically programmable, non-volatile memory cells 2, e.g. cells of the FLASH type. The circuit 1 includes a voltage regulator 3 according to the invention. Each cell 2 is formed by a MOS transistor of the floating gate G type which also includes a control gate CG. The cells 2 are arranged into rows or word lines WL, and columns or bit lines 5. The cells of one word line have in common the electric line that drives their respective control gates CG, while the cells of one bit line 5 have their drain terminals D in common. Shown schematically in Figure 1, by way of example only, is a matrix of cells having memory words of eight bits. Connected between the output of the regulator 3 and each bit line 5 is a program-load switch block 7 controlled by a corresponding signal pgml. The blocks 7 receive, as output from the regulator 3, the drain voltage Vpd for application to the bit line 5. The voltage Vpd will be transferred to an output Yms of each block 7, whenever the corresponding bit requires programming. The source terminals S of all the cells 2 are connected to a common voltage reference, e.g. a signal ground GND. Each bit line 5 further comprises a set of MOS transistors, all designated 6, which are connected in series with one another for selection of the bit line to which they belong. These transistors are enabled by column decode signals Yo, Yn and Ym being supplied at 12 volts and applied to respective gate terminals. The program circuit 1 includes a voltage booster 4, i.e. a charge pumping circuit which is supplied 5 volts from a supply voltage Vcc. The voltage booster 4 can be viewed, for example, as a generator of a voltage hv which is twice (2Vcc) as high as the supply voltage Vcc and has a resistance of preferably 500 ohms connected in series therewith. The value of the output voltage hv from the voltage booster is, of course, dependent on the supply voltage Vcc and the current supply; the latter is then dependent on the number (0 to 8) of bits to be programmed. Consequently, the output voltage hv from the voltage booster will vary with the parameters it is tied to. The circuit 1 further includes a voltage regulator 3 intended, in accordance with the invention, for regulating the drain voltage Vpd of the cells during the programming thereof. Actually, this voltage Vpd would be applied to the bit line 5 but for a negative-going shift in level caused by the write selection transistors 6 provided. The regulator 3 is controlled by a part-on control signal which is applied to a resistive divider to be described, the voltage booster being controlled by a pump-on control signal. The construction of the voltage regulator 3 is shown in detail in Figure 2, and comprises a resistive divider 8 formed of three resistors R1, R2, R3 connected in series with one another between a voltage reference held at the output drain voltage Vpd and a ground. Connected between the last series resistor R3 and ground is an enable transistor M1 controlled by the part-on control signal being applied to its gate terminal. In parallel with the first series resistor R1 is a compensation capacitor C1 which introduces a "zero" into the 0 dB frequency to increase the phase margin for the regulator. The interconnect point between the first R1 and the second resistor is a first output of the divider, and is represented by a circuit node A. The node A is connected to a non-inverting input of a first differential amplifier diff1 which is supplied a voltage hv from the voltage booster 4. A second inverting input of the differential diff1 is held at a reference voltage Vrif which is set at 2 volts by a band-gap voltage regulator, not shown because conventional. The output of the differential diff1 is connected to the gate terminal of a MOS transistor M2 of the P-channel type which is connected, in series with further transistors M3 and M4, between the output of the voltage booster 4 and ground. This transistor M2 has its body terminal shorted to its drain terminal, the latter being connected to the voltage booster output. Also provided is a transistor M6 having terminals connected between the gate and drain terminals of the transistor M2. The source terminal of the transistor M2 represents the output OUT of the regulator 3 whence the drain voltage value Vpd to be applied to the bit line 5 is taken. Advantageously, the output OUT of the regulator 3 is latched to the supply voltage Vcc by a transistor M5 in a diode configuration. In addition, the output OUT is connected to ground through a series of a resistor R4 and a capacitor C2. The RC network including the resistor R4 and capacitor C2 is effective to filter the output terminal OUT, which forms a circuit node weakly charged capacitively. The interconnect point between the second R2 and the third R3 resistor is a second output of the divider 8, represented by a circuit node B. The node B is connected to a non-inverting input of a second differential amplifier diff2 which, unlike the first, is supplied the supply voltage Vcc of the circuit 1. A second inverting input of the second differential diff2 is held at the reference voltage Vrif. The output of the second differential diff2 is connected to the gate terminal of the N-channel MOS transistor M3, which is connected in series between the aforementioned transistor M2 and the transistor M4 with its drain terminal run to the source terminal of the transistor M3. In essence, the differential amplifiers diff1 and diff2 respectively drive a pull-up M2 and a pull-down M3. The first, diff1, of these differentials is supplied the output voltage hv from the voltage booster in order to turn off the pull-up M2 upon the drain voltage Vpd exceeding 5.5 volts, as will be explained hereinafter. The operating principle of this invention provides for regulation of the drain voltage Vpd through a single pull-up transistor, substantially utilising for down-pulling either the current of the memory cells in the steady-state condition or the slow transient phases. As to the faster transient phases, such as the turn-on of the voltage booster 4 or the turn-off of the program-load switch blocks 7, the second differential amplifier diff2 is effective to prevent overshooting of the drain voltage Vpd by discharging it to ground. This is what would happen, in particular, if the first differential amplifier diff1 were not fast enough to turn off the transistor M2 upon the drain voltage Vpd exceeding 5.5 volts. For this reason, the resistive divider 8 is dimensioned such that, Va > Vrif when Vpd > 5.5V Vb > Vrif when Vpd > 5.8V. In the former case, the differential diff1 brings its output to the voltage value of hv, thus turning off the transistor M2; in the latter case, on the other hand, the second differential diff2 brings its output to the voltage value of Vcc, thereby turning on the transistor M3 and discharging the drain voltage Vpd to ground. The operation of the voltage regulator 3 of this invention will now be described in detail. An initial state will be assumed wherein the voltage booster 4 and the regulator 3 are in the off state preliminary to a programming step. In this situation, the drain voltage Vpd will be held at a value of about 4 volts by the diode M5 bias. When the programming step is commenced, the resistive divider 8 is activated and the value of the reference voltage Vrif brought to a steady state at 2 volts, already before the voltage booster 4 is fully on. This brings the part-on signal to the same value as the supply Vcc. Meantime, the two differential amplifiers diff1 and diff2 will turn on the transistor M2 and hold the transistor M3 in the off state. In fact, were not the transistor M3 turned off, the drain voltage Vpd would be enabled to discharge toward ground. Subsequently, the concurrent turning on of the voltage booster 4 and the regulator 3, upon reception of the pump-on signal, will allow the value of the voltage hv to be raised quite rapidly. Specifically, within approximately 200 ns, the output of the voltage booster 4 will attain twice the value (2Vcc) of the supply voltage and retain that value for as long as the drain voltage Vpd keeps below the limit value of 5.5 volts. During this high-speed transient phase, if the voltage Vpd exceeds 5.8 volts, both voltage values Va and Vb, as picked up from the outputs A and B of the resistive divider 8, will be higher in value than the reference voltage Vrif. Consequently, the transistor M2 will be controlled to the off state by the differential diff1, whereas the transistor M3 will be turned on by the differential diff2 to allow of the drain voltage Vpd being discharged toward ground. This situation would persist until the voltage Vpd drops below 5.8 volts. During the next phase, wherein the voltage Vpd stays at 5.5 to 5.8 volts, both transistors M2 and M3 will be off, and it will be the current flowing through the memory cells 2 that brings the voltage Vpd back toward 5.5 volts, that is, toward the cut-in point of the differential diff1. During the initial program transient just described, the current draw by the memory cells 2 is at a maximum. On termination of this transient, the current draw by the cells 2 will decrease as the cells are being programmed. This causes the cut-in point of the voltage booster 4 to shift toward higher values of the output voltage throughout the duration of the program pulse. The variations of hv are followed by the first differential diff1, which will drive the gate terminal of the transistor M2 so as to keep the value of Vpd constant. The end of the program pulse is given by the program-load blocks 7 being turned off, which removes the voltage Vpd from the drain terminals of the cells. The break in the connection to the load has no significant effects on the regulation of the voltage Vpd. The cycle is then completed with the turning off of the voltage booster 4 and the regulator 3. Plotted in Figure 3 of the accompanying drawings are the results of a simulation carried out by utilising, instead of the memory cells 2, a current generator Iload which has a characteristic curve as shown in Figure 4. Since the current drawn by the cells 2 is not supplied directly from a power supply, but rather supplied through a voltage booster, the invention has a major advantage in that the pull-down transistor M3 is held normally off. This means that that transistor will not contribute toward charging the voltage booster 4. Another advantage of the invention is that the high-voltage output from the voltage booster 4 is filtered through the P-channel transistor M2, whose gate terminal is driven by the output from the differential diff1, the latter being powered from the voltage booster 4 itself. In this way, the ripple present on the voltage hv is carried over to both the source terminal and gate terminal of the transistor M2, thereby keeping the gate-source voltage drop virtually constant. It matters to observe that the two differential amplifiers are comprised of high-voltage transistors, since they are supplied the high-voltage output directly from the voltage booster 4. However, the second differential which drives the transistor M3 could comprise standard transistors, in view of it being supplied the voltage Vcc directly. The capacitances incorporated to the circuit 1 are implemented in N-wells, as are the resistors R1, R2 and R3 of the resistive divider. The resistor R4 is instead formed by a region N+.

Claims: A voltage (Vpd) regulator (3) for non-volatile memory cells (2) of the electrically programmable type, comprising a gain stage (diffl) which is supplied a voltage (hv) from a voltage booster (4) connected to a supply voltage (Vcc) reference, having an input terminal connected to an output (A) of a voltage divider (8) and an output terminal connected to control at least one transistor (M2) of a pull-up and pull-down complementary pair of serially connected transistors (M2, M3) to output (OUT) the regulated voltage (Vpd) for programming at least one column or bit line of said memory cells (2), characterised in that it further comprises a second gain stage (diff2) having an input terminal connected to a second output (B) of the divider (8) and an output connected to turn on the second transistor (M3) in said complementary pair upon the regulated voltage (Vpd) exceeding a predetermined value. A voltage regulator according to Claim 1, characterised in that said voltage divider (8) is connected between a first voltage reference, being held at the value of the regulated voltage (Vpd), and a second voltage reference (GND). A voltage regulator according to Claim 1, characterised in that the first (diff1) and second (diff2) gain stages have respective second inputs which are held at the same reference voltage (Vrif) value. A voltage regulator according to Claim 1, characterised in that said divider (8) is of the resistive type and comprises a series of resistors (R1,R2,R3), of which the first (R1) is connected in parallel with a capacitor (C1). A voltage regulator according to Claim 1, characterised in that said second gain stage (diff2) is supplied said supply voltage (Vcc). A voltage regulator according to Claim 2, characterised in that it comprises an RC circuit between the output (OUT) and said second voltage reference. A programming circuit for an array of electrically programmable non-volatile memory cells (2), characterised in that it includes at least one voltage regulator as claimed in Claim 1.

Inventors: CANE MARCELLO, SCHIPPERS STEFAN, TASSAN CASER FABIO, CANE, MARCELLO, SCHIPPERS, STEFAN, TASSAN CASER, FABIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: G11C16
IPC codes: G11C5, G11C16