Bit-line voltage generation circuit for a non-volatile memory device and corresponding method

Abstract

An embodiment voltage generation circuit, for a memory having a memory array with a plurality of memory cells coupled to respective wordlines and local bit-lines, each having a storage element and selector element, a bipolar transistor being coupled to the storage element for selective flow of a cell current during reading or verifying operations, and a base terminal of the selector element being coupled to a respective wordline; associated to each bit-line is a biasing transistor having a control terminal, and the circuit generates a cascode voltage for this control terminal; a driver stage is coupled to one end of each wordline. The circuit generates the cascode voltage based on a reference voltage, which is a function of the emulation of a voltage drop on the driver stage, on the wordline, and on the memory cell as a result of a current associated to the corresponding selector element.

Claims

1. A voltage generation circuit for a non-volatile memory device comprising a memory array having a plurality of memory cells coupled to respective wordlines and local bit-lines, each memory cell comprising a storage element and a selector element including a bipolar transistor coupled to the storage element and configured to selectively enable flow of a cell current during reading or verifying operations, wherein a base terminal of the selector element is coupled to a respective wordline, wherein a sense amplifier stage is associated with the local bit-lines via a column decoder, wherein the sense amplifier stage comprises at its input respective biasing transistors each having a control terminal, and wherein a driver stage is coupled to each wordline for biasing thereof during the reading or verifying operations, wherein the voltage generation circuit is configured to: generate a cascode voltage for the control terminals of the biasing transistors based on a reference voltage, wherein the reference voltage is a function of an emulation of a respective voltage drop on the driver stage, on the wordline, and on the memory cell, as a result of a current associated with the corresponding selector element.

2. The circuit according to claim 1, comprising: a reference-generation stage configured to generate, on an output, the reference voltage starting from a voltage reference received at input; and an output stage, configured to generate the cascode voltage based on the reference voltage; wherein the output stage comprises: an operational amplifier having a non-inverting terminal that receives the reference voltage from the reference-generation stage, an inverting terminal connected to a first internal node, and an output terminal that supplies the cascode voltage; an output transistor having its control terminal connected to the output terminal of the operational amplifier receiving the cascode voltage and coupled between a supply line and the first internal node; and a current generator coupled to the first internal node and configured to generate a column current having a value representing an intermediate value of a current circulating in the memory cells.

3. The circuit according to claim 2, wherein the reference-generation stage comprises: a first emulation block coupling the output transistor to the first internal node, and configured to emulate the column decoder with respect to a first voltage drop due to the column current.

4. The circuit according to claim 3, wherein the first emulation block comprises a number of PMOS-type emulation transistors that are cascaded between the output transistor and the first internal node, and have a corresponding control terminal connected to a ground terminal to be in a conducting state.

5. The circuit according to claim 3, wherein the reference-generation stage comprises: a second emulation block, configured to emulate a phase-change element of a memory cell with respect to a corresponding second voltage drop; a third emulation block, configured to emulate the wordline of the memory array and memory cells associated therewith with respect to a corresponding third voltage drop; and a fourth emulation block, configured to emulate the driver stage, with respect to a corresponding fourth voltage drop; wherein the reference-generation stage is configured so that a contribution, for creation of the reference voltage, of the second voltage drop is controllable separately and individually with respect to contributions of the third voltage drop and fourth voltage drop.

6. The circuit according to claim 5, wherein the second emulation block comprises an emulation resistor, which is coupled between the output and an internal node, connected to the third emulation block, and is designed to receive a mirrored current from a first current mirror; the first current mirror having a mirror branch, which generates a mirror current based on the voltage reference, a first mirrored branch, which is connected to the output and to the emulation resistor and supplying the mirrored current based on a mirror ratio with respect to the mirror current, and a second mirrored branch, which is connected to a further internal node; and wherein the reference-generation stage further comprises a second current mirror having a unit mirror ratio, coupled between the internal node and the further internal node and configured to extract from the internal node a current equal to the mirrored current flowing through the emulation resistor.

7. The circuit according to claim 6, wherein the reference-generation stage further comprises: a resistive block, formed by a number of resistors having a same resistance and on which the voltage reference is present; wherein the mirror current is the current circulating in the resistive block, and wherein the emulation resistor is matched to the resistors, having substantially the same resistance.

8. The circuit according to claim 5, wherein the third emulation block comprises: a metallization line, configured to emulate the wordline, with respect to a relative voltage drop; a first, a second and a third cell-current generator configured to emulate injection of a cell current on the metallization line and coupled to the metallization line at an initial end, a central intermediate point, and a final end, respectively; and a first selector transistor, a second selector transistor, and a third selector transistor, of a bipolar type, which are configured to emulate selector elements of respective memory cells, are connected between the first, second, and third cell-current generator, respectively, and a ground terminal and have their base terminals connected to the initial end, to the central intermediate point, and to the final end, respectively, of the metallization line, wherein an emitter terminal of the second selector transistor is connected to the second emulation block.

9. The circuit according to claim 8, wherein the cell current has a value representing the current of a memory cell in a programmed state and is greater than the column current.

10. The circuit according to claim 8, wherein the metallization line has an overall length that is approximately half a length of the wordline and is folded a number of times in a serpentine fashion.

11. The circuit according to claim 8, wherein the fourth emulation block comprises a number of NMOS-type pull-down transistors that are connected in series between the initial end of the metallization line and a ground terminal, and have respective control terminals coupled to the supply line to be in a conducting state.

12. The circuit according to claim 8, wherein the reference voltage is given by a sum of the second voltage drop on the second emulation block, of the third voltage drop on the third emulation block, and of the fourth voltage drop on the fourth emulation block; and the third voltage drop being given by a sum of a voltage drop on the metallization line and of a voltage drop between the base terminal and the emitter terminal of the second selector transistor.

13. The circuit according to claim 1, wherein the memory cells are phase-change memory cells.

14. An electronic apparatus comprising: a non-volatile memory device comprising: a memory array having a plurality of memory cells coupled to respective wordlines and local bit-lines, each memory cell comprising a storage element and a selector element, each selector element comprising a bipolar transistor coupled to the storage element and configured to selectively enable flow of a cell current during reading or verifying operations, wherein a base terminal of the selector element is coupled to a respective wordline, wherein a biasing transistor has a control terminal associated with each local bit-line, via a column decoder, and wherein a driver stage is coupled to one end of each wordline for biasing thereof during the reading or verification operations; and a voltage generation circuit configured to generate a cascode voltage for control terminals of the biasing transistors based on a reference voltage, wherein the reference voltage is a function of an emulation of a respective voltage drop on the driver stage, on the wordline, and on the memory cell, as a result of a current associated with the corresponding selector element.

15. The electronic apparatus according to claim 14, further comprising: a controller coupled to the non-volatile memory device.

16. The electronic apparatus according to claim 14, wherein the memory cells are phase-change memory cells.

17. A voltage generation method, for a non-volatile memory device comprising a memory array having a plurality of memory cells coupled to respective wordlines and local bit-lines, each memory cell comprising a storage element and a selector element including a bipolar transistor coupled to the storage element and configured to selectively enable flow of a cell current during reading or verifying operations, a base terminal of the selector element being coupled to a respective wordline, a biasing transistor having a control terminal being associated with each local bit-line, via a column decoder, and a driver stage being coupled to one end of each wordline for biasing thereof during the reading or verification operations, the method comprising: generating a cascode voltage for the control terminal based on a reference voltage, the reference voltage being a function of an emulation of a voltage drop on the driver stage, on the wordline, and on the memory cell, as a result of a current associated with the corresponding selector element.

18. The method according to claim 17, wherein generating the cascode voltage comprises controlling separately and individually a contribution for the generation of the reference voltage of a voltage drop on an emulation block, the emulation block configured to emulate a phase-change element of the memory cell, with respect to contributions of further emulation blocks, the further emulation blocks configured to emulate the wordline of the memory array and memory cells associated therewith, and, respectively, the driver stage.

19. The method according to claim 18, wherein generating the cascode voltage further comprises emulating the column decoder with respect to a corresponding voltage drop due to a column current having a value representing an intermediate value of a current circulating in the memory cells.

20. The method according to claim 17, wherein the memory cells are phase-change memory cells.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the present invention, preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:

(2) FIG. 1 is a schematic illustration of a portion of a non-volatile memory device, of a known type, during a differential reading operation;

(3) FIG. 2 shows the circuit diagram of a voltage generation unit in the memory device of FIG. 1;

(4) FIG. 3 is a schematic illustration of a memory cell and associated wordlines and bit-lines in the memory device of FIG. 1;

(5) FIG. 4 is a schematic block diagram of a memory device according to one aspect of the present solution;

(6) FIG. 5 shows the circuit diagram of a bit-line voltage generation circuit in the device of FIG. 4, according to the present solution; and

(7) FIG. 6 is a simplified block diagram of an electronic system incorporating the memory device of FIG. 4.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(8) FIG. 4 is a schematic depiction of a non-volatile memory device, in particular of the phase-change type, designated as a whole by 20, comprising a memory array 21, the memory cells of which (here not illustrated) are coupled to respective wordlines WL (aligned in rows) and respective bit-lines (aligned in columns); in particular, the bit-lines are divided into main bit-lines and local bit-lines, in a way substantially similar to what has been illustrated previously with reference to FIG. 1.

(9) The memory device 20 comprises, as illustrated schematically, a row decoder 24 and a column decoder 25, configured to appropriately address and bias, respectively, the wordlines WL and the local bit-lines BL, each time selected for the memory operations (in particular, reading and verifying operations), based on input addresses Add.

(10) As discussed previously with reference to FIG. 1 (to which reference is made), the column decoder 25 comprises (for each local bit-line BL) a certain number of selection transistors 6 (not illustrated herein), for example PMOS transistors, which are cascaded between the respective (local) bit-line BL and a respective sensing main bit-line MBLs and receive, on a corresponding control terminal, a respective selection signal (Y.sub.o, Y.sub.n).

(11) The memory device 20 further comprises the sense amplifier (SA) stage, here designated by 29, selectively coupled to the sensing main bit-lines MBLs associated to the memory cells 2, by way of the column decoder 25, and configured to compare the corresponding cell currents I.sub.cell in order to enable differential reading (or verifying) of the datum stored.

(12) In a way here not illustrated (but described previously with respect to FIG. 1), the sense amplifier stage 29 comprises the biasing transistors 7, in particular NMOS transistors, connected between each sensing main bit-line MBLs and an input block 9a of the same sense amplifier stage 29.

(13) A voltage generation circuit 30 is operatively coupled to the sense amplifier stage 29 and is configured to generate an appropriate value for the cascode voltage V.sub.case (in particular, for the control terminal of the aforesaid biasing transistors 7; see once again also the foregoing discussion and the aforesaid FIG. 1) in order to obtain a desired value for the voltage present on the bit-lines selected for the reading (or verifying) operations.

(14) As will be described in detail hereinafter, according to an aspect of the present solution, the voltage generation circuit 30 is configured so as to generate the aforesaid cascode voltage V.sub.casc, taking into account the factors linked to the nature of the selector elements 2b of a BJT type of the memory cells 2, thus enabling correct biasing of the bit-lines and preventing errors in the memory operations, in particular the reading (or verifying) operations.

(15) In detail, and with reference now to FIG. 5, the voltage generation circuit 30 comprises a reference-generation stage 32, configured to generate a reference voltage V.sub.ref, having an appropriate value (which, as will be described in detail, is a function of the aforesaid factors linked to the nature of the selector elements 2b of a BJT type), starting from a voltage reference V.sub.r, which is generated by a voltage generator, for example of a band-gap type (here not illustrated), and hence has a stable and precise value irrespective of variations in the supply voltage and temperature and irrespective of process spread.

(16) The voltage generation circuit 30 is provided with an output stage 34, comprising an operational amplifier 40, having a non-inverting terminal that receives the aforesaid reference voltage V.sub.ref from the reference-generation stage 32, an inverting terminal connected to a first internal node 41, on which a feedback voltage V.sub.fb is present, and an output terminal that supplies the cascode voltage V.sub.casc to be sent to the sense amplifier stage 29 of the memory device 20, in particular to the control terminal of a biasing transistor 7, of an NMOS type, connected to a respective sensing main bit-line MBLs.

(17) The output stage 34 further comprises an output transistor 42, of an NMOS type, having characteristics corresponding to those of the biasing transistor 7, with its control terminal connected to the output terminal of the operational amplifier 40 that receives the aforesaid cascode voltage V.sub.casc and coupled between a supply line that receives the power-supply voltage V.sub.dd and a second internal node 43.

(18) The second internal node 43 is coupled to the first internal node 41 through a first emulation block 45, configured to emulate the column decoder 25. This first emulation block 45 thus comprises a certain number of emulation transistors 46, in particular PMOS transistors, which are cascaded between the second internal node 43 and the first internal node 41 and in this case have their corresponding control terminal connected to the ground terminal GND (so as to always be in a conducting state).

(19) Two emulation transistors 46a, 46b are illustrated by way of example in FIG. 5, which are connected in series between the aforesaid second internal node 43 and the aforesaid first internal node 41 (and are hence designed to emulate the selection transistors 6a, 6b that receive the first and second selection signals Y.sub.o, Y.sub.n of the column decoder 25).

(20) The output stage 34 further comprises a current generator 47, coupled between the aforesaid first internal node 41 and a reference terminal (in particular, the ground terminal GND). The current generator 47 is configured to generate a column current I.sub.col, the value of which represents an intermediate value between the distribution of the current (circulating in the memory cells 2) associated to the SET state and that associated to the RESET state (in other words, the value of the column current I.sub.col is intermediate between the lower-tail value of the distribution of the SET current and the upper-tail value of the distribution of the RESET current and may, for example, be equal to 8 μA).

(21) The first internal node 41, connected to the inverting terminal of the operational amplifier 40, thus has, as a result of the unit feedback gain of the operational amplifier 40, a voltage value equal to the reference voltage V.sub.ref, while the second internal node 43 has a voltage V.sub.MBL equal to the sum of the aforesaid reference voltage V.sub.ref and a voltage drop V.sub.drop on the first emulation block 45 (and on the corresponding emulation transistors 46) as a result of the column current I.sub.col:V.sub.MBL=V.sub.ref+V.sub.drop.

(22) As it is evident from an examination of the aforesaid FIG. 5, this voltage V.sub.MBL has a value corresponding to the desired voltage to set on the sensing main bit-line MBLs selected for the memory operations, in particular the reading (or verifying) operations, while the aforesaid reference voltage V.sub.ref corresponds to the voltage on the corresponding local bit-line BL.

(23) In greater detail, the reference-generation stage 32 has an input IN designed to receive the voltage reference V.sub.r, generated by the voltage generator, for example of a band-gap type, and an output OUT designed to supply the reference voltage V.sub.ref.

(24) The reference-generation stage 32 comprises an input operational amplifier 50, having a non-inverting terminal that is connected to the aforesaid input IN and receives the voltage reference V.sub.r, a non-inverting terminal connected to a third internal node 51, and an output terminal connected to a first current mirror 52.

(25) A resistive block 53 is connected between the third internal node 51 and the ground terminal GND. In particular, the resistive block 53 is constituted by the series of a certain number of resistors 53a (in the example illustrated, ten) having a same resistance value R.sub.x, for example 20 kΩ. A first current I.sub.1 flows in the resistive block 53 (i.e., in the aforesaid series of resistors) and in the example has a value I.sub.1=V.sub.r/(10.Math.R.sub.x).

(26) The first current mirror 52 comprises: a first mirror transistor 52a (of a PMOS type), which is connected between the third internal node 51 and the line set at the supply voltage V.sub.dd and has its control terminal connected to the output terminal of the input operational amplifier 50; a second mirror transistor 52b (also of a PMOS type), which is coupled, with a mirror ratio N/2 (where N has a value that can be configured, for example, between 6 and 14 in unit steps), to the first mirror transistor sea, is connected between the output OUT and the line set at the supply voltage V.sub.dd, and has its control terminal connected to the same output terminal of the input operational amplifier 50; and a third mirror transistor 52c (also of a PMOS type), which is coupled, with the same mirror ratio N/2, to the first mirror transistor sea, is connected between a fourth internal node 54 and the terminal set at the supply voltage V.sub.dd, and has its control terminal connected to the same output terminal of the input operational amplifier 50.

(27) By virtue of the current mirroring by the first current mirror 52, the second and third mirror transistors 52b, 52c supply on the output OUT and on the fourth internal node 54, respectively, a second current I.sub.2, mirrored with respect to the first current I.sub.1 and having a value equal to I.sub.1.Math.N/2.

(28) The reference-generation stage 32 further comprises a second emulation block 55, configured to emulate the phase-change element 2a of the memory cell 2. This second emulation block 55 is provided by an emulation resistor 56, which is made so as to be matched with the resistors 53a of the resistive block 53 and has, in particular, the same value of resistance R.sub.x (in a way that will be evident, this emulation resistor 56 may be physically made at the resistive block 53 and with the same manufacturing techniques so as to achieve a minimal spread between the respective resistance values).

(29) In particular, the aforesaid emulation resistor 56 is connected between the output OUT and a fifth internal node 58, and is traversed by the second current I.sub.2 so as to determine a voltage drop V.sub.RES equal to R.sub.x.Math.I.sub.1.Math.N/2.

(30) The reference-generation stage 32 further comprises a second current mirror 62, with a unit mirror ratio, coupled to the aforesaid fifth internal node 58 and to the aforesaid fourth internal node 54 and comprising: a respective first mirror transistor 62a (of an NMOS type), in diode configuration, connected between the fourth internal node 54 and the ground terminal GND; and a respective second mirror transistor 62b (also of an NMOS type), which is connected between the fifth internal node 58 and the ground terminal GND and has its control terminal connected to the control terminal of the respective first mirror transistor 62a.

(31) In particular, as a result of current mirroring by the second current mirror 62, the respective second mirror transistor 62b extracts from the fifth internal node 58 the entire second current I.sub.2 that flows through the second emulation block 55 (i.e., through the emulation resistor 56).

(32) The reference-generation stage 32 further comprises a third emulation block 65, coupled to the aforesaid fifth internal node 58.

(33) The third emulation block 65 is configured to emulate a wordline WL of the memory array 21 and the corresponding memory cells 2 associated thereto, in particular as regards the voltage drop due to the current circulating (as discussed previously) as a result of the base current I.sub.B of the selector element 2b, of a BJT type, associated to the same memory cell 2.

(34) The third emulation block 65 comprises: a metallization line 66, referred to as “dummy”, configured to emulate the resistive contribution of the wordlines WL; and a first, a second, and a third cell-current generator 68a, 68b, 68c which are configured to emulate the injection of a cell current I.sub.cell by a respective memory cell and are coupled to the metallization line 66 at an initial end, at a central intermediate point, and at a final end, respectively.

(35) In particular, the cell current I.sub.cell has a value that represents the current of a memory cell that is in the SET state (programmed state) and is hence higher than the aforesaid column current I.sub.col, being, for example, equal to 22 μA.

(36) The third emulation block 65 further comprises a first selector transistor 69a, a second selector transistor 69b, and a third selector transistor 69c, in particular, bipolar transistors, which are configured to emulate the selector elements 2b of the memory cells 2, are connected between the first, second, and third cell-current generator 68a, 68b, 68c, respectively, and the ground terminal GND, and have their base terminal connected to the initial end, to the central intermediate point, and to the final end, respectively, of the metallization line 66.

(37) In particular, the emitter of the second selector transistor 69b (having its base terminal connected to the central intermediate point of the metallization line 66) is connected to the aforesaid fifth internal node 58.

(38) It is to be noted that on a real wordline WL of the memory array 21 a larger number of memory cells 2 is present (which are associated to the selected local bit-lines BL), for example thirty-seven in number, whereas only three cell-current generators 68a, 68b, 68c (which each emulate the cell current I.sub.cell of a single memory cell 2) are coupled to the metallization line 66.

(39) For this reason, the metallization line 66 is chosen of an appropriate length so as to have a resistance such as to cause a voltage drop that will appropriately emulate the real voltage drop on the wordline WL, and in particular represents the mean value of the wordline voltage distribution during the reading (or verifying) operations.

(40) Advantageously (also in order to reduce the occupation of area), the metallization line 66 has an overall length that is approximately half the length of the wordline WL and is moreover folded a number of times in serpentine fashion.

(41) It is also noted that the point of acquisition of the voltage drop, i.e., at the emitter of the second selector transistor 69b connected to the central intermediate point of the metallization line 66, effectively enables acquisition of a voltage drop of an average value from among the ones available on the same metallization line 66.

(42) The reference-generation stage 32 further comprises a fourth emulation block 70, coupled to the aforesaid initial end of the metallization line 66.

(43) The fourth emulation block 70 is configured to emulate the pull-down portion of the driver stage 15 coupled to the wordline WL, in particular as regards the voltage drop due to the current circulating in the same wordline WL (as discussed previously).

(44) The fourth emulation block 70, as illustrated schematically once again in FIG. 5, hence comprises a certain number of pull-down transistors 71 (in the example represented, four), of an NMOS type, which are connected in series between the aforesaid initial end of the metallization line 66 and the ground terminal GND and have respective control terminals coupled to the line set at the supply voltage V.sub.dd, so that the NMOS pull-down transistors 71 will always be in the ON (active or conductive) state.

(45) In particular, the fourth emulation block 70 is provided so as to determine a voltage drop that will appropriately emulate the real drop on the pull-down portion of the driver stage 15.

(46) For this reason, in order to take into account the lower current on the metallization line 66 as compared to that of the wordline WL of the memory array 21, the NMOS pull-down transistors 71 are made so as to have a higher resistivity than the corresponding pull-down transistors of the driver stage 15, for example a resistivity that is four times higher.

(47) For this purpose, the NMOS pull-down transistors 71 may be provided with a smaller channel width W, for example equal to one quarter, as compared to the corresponding channel width of the pull-down transistors of the driver stage 15 (advantageously also obtaining a reduction in dimensions for the fourth emulation block 70).

(48) Based on what has been described previously, it will be evident that the reference voltage V.sub.ref on the output OUT of the reference-generation stage 32 may be expressed as the sum of the following contributions: the voltage drop on the fourth emulation block 70 (i.e., on the cascade of the NMOS pull-down transistors 71), designated by V.sub.pd; the voltage drop on the metallization line 66, designated by V.sub.WL; the voltage drop between the base and the emitter of the second selector transistor 69b, designated by V.sub.BE; and the voltage drop on the second emulation block 55 (i.e., on the emulation resistor 56), designated, as pointed out previously, by V.sub.RES.

(49) The reference voltage V.sub.ref can hence be expressed as: V.sub.ref=V.sub.pd+V.sub.WL+V.sub.BE+V.sub.RES.

(50) In particular, the sum of the aforesaid contributions (V.sub.pd+V.sub.WL+V.sub.BE) represents the voltage on the emitter of the second selector transistor 69b, designated by V.sub.E, which hence depends only on the cell current I.sub.cell, whereas the contribution V.sub.RES depends only on the second current I.sub.2 that flows through the emulation resistor 56, this contribution V.sub.RES being equal to (V.sub.r/10).Math.N/2 as a result, as discussed previously, of the current-mirroring operation carried out by the first and second current mirrors 52, 62 (it is to be noted that this voltage does not depend on the resistance value R.sub.x).

(51) In other words, the aforesaid reference voltage V.sub.ref is the sum of two macro-contributions, the first being the aforesaid emitter voltage V.sub.E and the second being the aforesaid voltage V.sub.RES on the emulation resistor 56, which are altogether independent of one another and do not have any effect on each other, it thus being possible to control them separately and individually for generation of the aforesaid reference voltage V.sub.ref (in particular, as a function of the cell current I.sub.cell for the first contribution, and of the second current I.sub.2, i.e., of the voltage reference V.sub.r and of the mirroring factor N/2, for the second contribution).

(52) As mentioned previously, to the aforesaid contributions defining the reference voltage V.sub.ref, a further contribution is to be added (externally to the reference-generation stage 32) for definition of the bit-line voltage as a result of the cascode voltage V.sub.casc, i.e. the aforesaid voltage drop V.sub.drop on the emulation transistors 46 of the first emulation block 45.

(53) It is to be noted that also this further contribution (represented by the voltage drop V.sub.drop), which depends only on the column current I.sub.col, is altogether independent of, and not affected by, the other contributions (the aforesaid emitter voltage V.sub.E and the aforesaid voltage V.sub.RES on the emulation resistor 56), thus being possible to control it separately and individually (in particular, via definition of the column current I.sub.col).

(54) The advantages of the present solution emerge clearly from the foregoing description.

(55) In any case, it is once again underlined that the voltage generation circuit 30 generates the cascode voltage V.sub.casc for definition of the bit-line voltage during the memory operations, in particular the reading (or verifying) operations, taking into account all the significant factors linked to the nature of the BJT selector elements 2b of the memory cells 2, in particular linked to the corresponding base currents I.sub.B, thus enabling correct biasing of the bit-lines and preventing any degradation of performance or errors in the memory operations, in particular in the reading (or verifying) operations.

(56) The aforesaid voltage generation circuit 30 in particular allows to consider the voltage drop on the driver stage 15 of the wordline WL, the voltage drop on the same wordline WL, the voltage drop on the selector element 2b of the memory cell 2, and moreover the voltage drop on the phase-change element 2a of the same memory cell 2.

(57) Advantageously, the contributions that determine the bit-line voltage can be controlled and configured individually and separately (since they do not have any effect on each other).

(58) The aforesaid characteristics hence render particularly advantageous use of the memory device 1 in an electronic system 80, for example as illustrated schematically in FIG. 6.

(59) The electronic system 80 may be used in electronic devices, such as: a PDA (Personal Digital Assistant); a portable or fixed computer, possibly with wireless data-transfer capability; a mobile phone; a digital audio player; a photographic or video camera; or further portable devices capable of processing, storing, transmitting, and receiving information.

(60) The electronic system 80 comprises the memory device 20, provided with the memory array 21 (not illustrated herein) of memory cells 2 of the phase-change type, described previously, and a controller 81 (for example, provided with a microprocessor, a DSP, or a microcontroller), both coupled to a bus 86 designed to route signals (for example, for address selection) towards the memory device 20.

(61) Furthermore, the electronic system 80 may optionally comprise, coupled to the bus 86, one or more of the following: an input/output device 82 (for example, provided with a keypad and a display), for input and display of data; a wireless interface 84, for example an antenna, for transmitting and receiving data through a radio-frequency wireless communication network; a RAM 85; a battery 87, which can be used as electric-power supply source in the electronic system 80; and a photographic and/or video camera 88.

(62) According to a different embodiment, the controller 81 may be coupled to the memory device 20 by means of a dedicated connection different from, and possibly additional to, the bus 86 (the latter may be present or absent).

(63) Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims.

(64) In particular, it is underlined that, even though what has been described advantageously applies to non-volatile memory devices of a PCM type, the same solution may be applied to further and different non-volatile memory devices, in which the state of the memory cells is associated to a current circulating therein.

(65) Furthermore, the phase-change element of the memory cells may in general be replaced by a generic variable-resistance element, provided via any available technology (even not of the phase-change type).