Improved bit line discharge method and circuit for a semiconductor memory

The present invention generally relates to the field of semiconductor memories, particularly to non-volatile memories and, even more particularly, to memories based on phase-change materials, also known in the art as ovonic unified memories. More specifically, the invention relates to an improved bit line discharge control method, and a related circuit, for controlling the discharge of bit lines in a matrix of memory cells, particularly phase-change memory cells.

Ovonic unified or phase-change memories are an emerging type of electrically-alterable non-volatile semiconductor memories. These memories exploit the properties of materials (phase-change materials) that can be reversibly switched between an amorphous phase and a crystalline phase. A phase-change material exhibits different electrical characteristics, particularly a different resistivity, peculiar of each one of the two phases; thus, each material phase can be conventionally associated with a corresponding one of the two logic values, "1" and "0". An example of a phase-change semiconductor memory is described in US-A-5,166,758.

Typically, the memory includes a matrix of phase-change memory cells, arranged in rows and columns with associated word lines and bit lines, respectively. Each memory cell consists of a storage element connected in series to an access element; as the access element, the base-emitter junction of a Bipolar Junction Transistor, particularly a PNP BJT, associated with the storage element can be used.

In a stand-by condition, or before any read or write access to the memory, the word lines are kept at a relatively high voltage (word line deselection voltage), so that all the access elements are reverse biased.

During a write or read operation, the voltage of a selected word line is lowered, and one or more selected bit lines are brought to a voltage corresponding to the operation to be performed, while the remaining, non-selected bit lines are left floating. In this condition, the access elements associated with the memory cells of the selected word line and of the selected bit lines are forward biased, and the access to the corresponding storage elements is enabled.

However, leakage currents inevitably flow through the access elements even when they are reverse biased; the leakage currents, delivered to the access elements by the word line drivers, cause the charging of stray capacitances intrinsically associated with the bit lines, and the voltage of the bit lines raises therefore towards the word line deselection voltage.

If, as a consequence of the leakage currents, the voltage of a generic bit line reaches and, possibly, exceeds the turn-on voltage of the access elements (the threshold voltage of the base-emitter junction diodes, typically 0.6 V), an undesired access to the storage element that belongs to such a bit line and to a selected word line takes place, even if the bit line is actually kept unselected.

In this condition, the current flowing through the spuriously selected storage element may cause the voltage of the selected word line to rise, disturbing the operation of the memory device. Even worse, when the current flowing through the spuriously selected storage element reaches a value sufficient to cause a switch of phase in the phase-change material, the phase switch can cause an undesired change of the logic value stored in that memory cell. Even when the phase switch is only temporary and not permanent, a wrong value can be read if the memory cell is selected before a corresponding recovery time.

These problems are exacerbated as the temperature increases, since the leakage currents typically depend on the temperature according to an exponential law. In addition, the total leakage current injected into each storage element in the selected word line is directly proportional to the number of access elements in the corresponding deselected bit line.

For this reasons, before any operation involving the selection of a word line, the bit line voltage has to be brought to a properly low value, typically the ground, and thus the bit line stray capacitances need to be discharged. To this purpose, controlled switching elements, typically pass transistors, are associated with the bit lines, which are activated during a bit line discharge phase preceding any operation that involves the selection of a word line, so as to electrically connect the bit lines to a reference voltage line, for example, the ground line, and to discharge the associated stray capacitances.

A problem arises when, due to a manufacturing defect, a short-circuit (or, generally, a low-resistance path) exists between a generic bit line and a generic word line of the matrix. Especially in very dense memory matrices, this type of defect may be relatively highly probable. Known redundancy techniques allow repairing such a defect, by functionally replacing the defective bit line and word line with redundant bit line and word line; any attempt of selecting the defective bit line and word line would automatically result in a selection of the redundant bit line and word line.

Regretfully, according to the known approach, during the bit line discharge phase, before the desired word line is selected (thus, when all the word lines are still kept at the word line deselection voltage), the short-circuit between the defective word line and bit line causes a short-circuit between the word line driver supply voltage line (the line supplying the word line deselection voltage to the word line driver) and the reference voltage line; this short-circuit causes the flow of a significant crowbar current, for a time corresponding to the activation time of the bit line discharge pass transistors (typically, some tens of nanoseconds). This may easily cause the memory to exhibit an unacceptably high power consumption. Additionally, when the word line deselection voltage is generated on-chip by means of charge-pump voltage generators, the excessive current absorption due to the short circuit may easily cause the charge-pump output voltage to fall below a safety level, with the consequence that the correct operation of the whole memory might be prejudiced (for example, a too low word line deselection voltage may prevent the proper deactivation of the memory cell access elements, with the possible consequence that the data stored in the memory cells of the selected bit line are corrupted).

In view of the state of the art outlined in the foregoing, it has been an object of the present invention to solve the above-mentioned problems.

In particular, it has been an object of the present invention to avoid the flow of any significant crowbar current from the word line driver supply voltage line to the reference voltage line in presence of manufacturing defects consisting in a short-circuit between a bit line and a word line.

Generally stated, the Applicant has found that in order to avoid undesired and, possibly, even dangerous crowbar currents, it is necessary to condition the way in which the bit line discharge operation is accomplished to the fact that a bit line may be defective.

According to a first aspect of the present invention, the above-stated and other objects have been attained by means of a bit line discharge control method as set forth in appended claim 1.

Briefly stated, the method comprises conditioning a value of a current flowing through a bit line during a bit line discharge phase to an absence of an indication of defectiveness of the bit line.

According to a second aspect of the present invention, there is provided a bit line discharge control circuit as set forth in claim 9.

The circuit comprises bit line discharge means, associated with the bit lines, activatable in a bit line discharge operating phase of the memory for enabling a discharge of bit line capacitances associated with the bit lines.

Bit line current control means are additionally provided, controllable for at least limiting a current of an associated bit line in the bit line discharge operating phase.

These and other features, and the advantages of the present invention will be made apparent by the following detailed description of some embodiments thereof, provided merely by way of non-limitative examples, description that will be conducted making reference to the attached drawings, wherein: <ul id="ul0001" list-style="none" compact="compact"><li>Figure 1 schematically shows the relevant circuit blocks of a memory featuring a bit line discharge control, according to an embodiment of the present invention;</li><li><b>Figure 2</b> schematically shows current-voltage (I-V) characteristic curves of a variable-resistance, phase-change programmable non-volatile storage element used in memory cells of the memory of <b>Figure 1;</b></li><li><b>Figure 3</b> schematically shows an exemplary practical implementation of the bit line discharge control schematically depicted in <b>Figure 1</b>, according to an embodiment of the present invention;</li><li><b>Figure 4</b> schematically shows another exemplary practical implementation of the bit line discharge control schematically depicted in Figure 1, according to another embodiment of the present invention;</li><li><b>Figure 5</b> schematically shows the structure of a bit line group discharge control circuit depicted as a block in <b>Figure 4,</b> in an embodiment of the present invention; and</li><li><b>Figure 6</b> schematically shows the structure of a word line driver supply control according to another embodiment of the present invention, adapted to be used either in substitution of, or in combination with the bit line discharge control of the previous embodiments.</li></ul>

With reference to the drawings, in <b>Figure 1</b> the functional blocks of a semiconductor memory <b>100</b> relevant to the present invention are schematically shown. Specifically, the semiconductor memory <b>100</b> is an ovonic unified electrically-alterable non-volatile memory device.

The memory <b>100</b> includes a two-dimensional arrangement or matrix <b>105</b> of a plurality of memory cells <b>MC;</b> the memory cells <b>MC</b> in the matrix <b>105</b> are arranged by, <i>e.g.</i>, <i>m</i> rows and <i>n</i> columns; a respective word line <b>WL1 - WLm</b> is associated with each matrix row, and a respective bit line <b>BL1 - BLn</b> is associated with each matrix column.

Each memory cell MC in the matrix <b>105</b> includes a programmable storage element <b>P</b> having a programmable resistivity; the programmable storage element <b>P</b> is connected in series to an associated access element, in the shown example consisting of the emitter-base junction of a PNP Bipolar Junction Transistor (BJT) <b>T</b> which is associated with the programmable element <b>P</b>.

Each programmable storage element <b>P</b> is made of a phase-change material; typically, the phase-change material consists of a calcogenide (<i>e</i>.<i>g</i>., a Ge₂Sb₂Te₅ alloy). The phase-change material can be reversibly switched between a generally amorphous, disordered phase and a generally crystalline, highly ordered phase. The two phases of the material exhibit different electrical characteristics, as depicted schematically in <b>Figure 2,</b> wherein <b>Vp</b> and <b>Ip</b> denote the voltage across and the current flowing through the generic programmable storage element <b>P</b>, respectively; particularly, when the material is in the amorphous phase, it exhibits a relatively high resistivity, whereas when the material is in the crystalline phase, it has a low resistivity. It is the material resistivity difference in the two phases that is exploited to store a binary value; if the programmable storage element <b>P</b> is in the amorphous phase, characterized by a high resistivity, the memory cell <b>MC</b> is conventionally considered in a reset state, and this state is associated with a binary value, for example a "0"; if the programmable storage element <b>P</b> is in the crystalline phase, characterized by a low resistivity, the memory cell <b>MC</b> is conventionally considered in a set state, which is associated with the opposite binary value, in the example herein considered a "1".

Without entering into particulars, which are <i>per-se</i> well known in the art, the phase of the phase-change material is stable below a predefined temperature (such as 150°C). The material phase can be changed by heating the material over such a temperature; for this purpose, a voltage <b>Vp</b> higher than a corresponding phase-change value Vpc (for example, 0.6V) is applied to the programmable storage element <b>P</b>; the applied voltage causes the flow of a current <b>Ip</b> through a resistive element placed in contact to the phase-change material of the programmable storage element <b>P</b>, which resistive element acts as a local Joule-effect heater, and accordingly raises the temperature of the phase-change material. Depending on the voltage applied thereacross, and thus on the current flowing therethrough, if the programmable storage element <b>P</b> is heated over a nucleation temperature (typically 200°C) and then cooled slowly, the phase-change material becomes crystalline; conversely, if the programmable storage element <b>P</b> is heated over a higher, melting temperature (such as 600°C) and then cooled rapidly, the phase-change material becomes amorphous.

The state of the programmable storage element <b>P</b> can be detected by applying thereacross a read voltage (suitably lower than the phase-change value Vpc, not to cause an undesired change of phase of the material). The resulting current that flows through the programmable storage element <b>P</b> has a value that depends on the resistivity of the phase-change material, and provides therefore an indication of the material phase.

Still referring to <b>Figure 1,</b> each memory cell <b>MC</b> in the matrix <b>105</b> is connected to a respective one of the word lines <b>WL1 - WLm</b> and a respective one of the bit lines <b>BL1 - BLn</b>. In particular, within each memory cell <b>MC</b> the programmable storage element <b>P</b> has a first terminal connected to the corresponding bit line <b>BL1 - BLn</b> and a second terminal connected to the emitter terminal of the associated BJT <b>T;</b> the BJT <b>T</b> has the base terminal connected to the corresponding word line <b>WL1 - WLm</b> and the collector terminal connected to a reference voltage line, typically the ground.

A generic memory cell <b>MC</b> within the matrix <b>105</b> is accessed by selecting the corresponding row and column pair, <i>i.e.</i> by selecting the corresponding word line and bit line pair. To this purpose, word line selector circuits <b>110</b> and bit line selector circuits <b>115</b> are provided; these circuits, known <i>per-se</i> and therefore merely schematized as blocks in the drawings, perform the selection of the word lines and of the bit lines on the basis of a row address binary code <b>RADD</b> and a column address binary code <b>CADD</b>, respectively, part of a memory address binary code <b>ADD,</b> for example received by the memory device <b>100</b> from a device external to the memory <i>(e.g.,</i> a microprocessor). The word line selector circuits <b>110</b> decode the row address code <b>RADD</b> and select a corresponding one of the word lines <b>WL1 - WLm,</b> identified by the specific row address code <b>RADD</b> received; the bit line selector circuits <b>115</b> decode the column address code <b>CADD</b> and select a corresponding one or, more generally, a corresponding bit line packet of the bit lines <b>BL1 - BLn,</b> depending on the degree of parallelism of the memory device <b>100</b>, <i>i.e.</i> on the size of the memory locations (typically, eight or sixteen bits), identified by the specific column address code <b>CADD</b> received.

The bit line selector circuits <b>115</b> interface with read/write circuits, also known per-se and therefore schematized as a block 120. The read/write circuits <b>120</b> include all the components (<i>e.g.,</i> sense amplifiers, comparators, reference current/voltage generators, pulse generators, program load circuits and the like) which are normally required for writing the desired logic values into the selected memory cells <b>MC,</b> and for reading the logic values currently stored therein.

As discussed in the introductory part of the present description, in a stand-by operating condition, as well as before any read or write access to the memory device <b>100,</b> the word line selection circuits <b>110</b> keep all the word lines <b>Wh1</b> - <b>WLm</b> at a relatively high voltage Vh, also referred to as the word line deselection voltage (a typical value of this voltage may for example be 4.5V; a similar voltage is used for keeping the word lines deselected during a memory write operation); the word line deselection voltage Vh is such that the emitter-base junction of the BJT <b>T</b> in every memory cell <b>MC</b> is kept reverse biased; in this way, none of the memory cells <b>MC</b> is accessed, because all the access elements are disabled. At the same time, the bit line selection circuits <b>115</b> keep all the bit lines <b>BL1 - BLn</b> disconnected, and thus isolated, from the read/write circuits <b>120.</b>

During a read or a write operation, the word line selection circuits <b>110</b> lower the voltage of a selected one of the word lines <b>WL1 - WLm</b> to a word line selection voltage V1 (having for example value equal to 0V - the ground), while the remaining word lines are kept at the word line deselection voltage Vh; similarly, the bit line selection circuits <b>115</b> couple a selected one of the bit lines <b>BL1 - BLn</b> (more typically, a selected bit line packet of, <i>e.g.</i>, eight or sixteen bit lines, depending on the degree of parallelism of the memory 100) to the read circuits or to the write circuits, depending on the operation to be performed; the selected bit lines are brought to a voltage corresponding to the operation to be performed, while the remaining, non-selected bit lines are left floating. In this condition, the access elements associated with the memory cells <b>MC</b> belonging to the selected word line and the selected bit lines are forward biased, and enable the access to the corresponding storage elements <b>P</b>.

In greater detail, the voltage applied to the selected bit lines is always comprised between a base-emitter threshold voltage Vbe of the access transistors <b>T</b> (typically, 0.6V) and the voltage difference (Vh - Vbe) (typically, 4.5 - 0.6 = 3.9V). Therefore, only the base-emitter junctions of the access transistors <b>T</b> that are associated with the selected word line and the selected bit lines are forward biased, while the base-emitter junctions of the other access transistors <b>T</b> should remain reverse biased at a voltage at least equal to -Vbe.

In particular, during a write operation a relatively high voltage pulse is applied to each selected bit line; the voltage pulse has an amplitude and a time duration depending on the desired final state of the correspondingly selected programmable element <b>P</b> (crystalline or amorphous). On the other hand, during a read operation a relatively low voltage (for example, 1V) is applied to the selected bit lines. In the read/write circuits <b>120,</b> the current flowing through each corresponding programmable element <b>P</b> is compared to a reference value (typically provided by a reference memory cell); when the programmable element <b>P</b> is in the crystalline state, a current higher than the reference value is detected, and this situation is conventionally decreed to correspond to the logic value "1", whereas when the programmable element <b>P</b> is in the amorphous state, a current lower than the reference value is detected, and this situation is conventionally decreed to correspond to the logic value "0".

As mentioned in the introductory part of the present description, malfunctionings of the memory device may be induced by leakage currents that flow through the base-emitter junctions of the access BJTs <b>T</b> even when the base-emitter junctions thereof are reverse-biased; during the memory device stand-by or, generally, before a read or write operation, these leakage currents, delivered by word line drivers in the word line selection circuits 110 (such as the word line driver <b>130-h</b>, schematically represented in <b>Figure 1</b> and associated with the word line <b>WLh),</b> cause the charging of stray capacitances <b>C</b>_<b>BL1</b><b> - C</b>_<b>BLn</b> intrinsically associated with the bit lines <b>BL1</b> - <b>BLn;</b> the voltage of the bit lines raises therefore towards the word line deselection voltage Vh.

In order to avoid the already cited problems of spurious access to undesired memory cells, caused by this rise of the bit line voltage, the bit line stray capacitances <b>C</b>_<b>BL1</b> - <b>C</b>_<b>BLn</b> need to be discharged before performing any operation that involves the selection of a word line <b>WL1 - WLm,</b> such as a memory read or a write operation.

To this purpose, bit line discharge circuits 125-1 <b>- 125-n</b> are provided, associated with each bit line <b>BL1 - BLn;</b> the bit line discharge circuits <b>125-1 - 125-n</b> are enabled in a bit line discharge phase of the memory device operation, preceding any operation that involves the selection of a word line <b>WL1 - WLm,</b> for discharging the bit line stray capacitances <b>C</b>_<b>BL1</b><b> - C</b>_<b>BLn</b>.

As known in the art, the generic bit line discharge circuit <b>125-1 - 125-n</b> is simply implemented by means of a transistor, particularly an N-channel MOSFET having the drain terminal connected to the corresponding bit line, the source terminal connected to the reference voltage line (<i>e.g.,</i> the ground <b>GND</b>) and the gate terminal controlled by a discharge enable signal. Before a write or a read operation, the discharge enable signal is temporarily asserted to a sufficiently high positive voltage, so that all the discharge MOSFETs turn on and connect the bit lines to the reference voltage line. The discharge currents that flow through the discharge transistors cause the discharge of the bit line stray capacitances and the fall of the bit line voltage to the reference voltage, <i>e</i>.<i>g</i>., the ground. Then, before selecting the desired word line, the discharge enable signal is deasserted and the discharge MOSFETs turned off.

As discussed in the introduction to the present description, this known solution is not particularly satisfactory. Manufacturing processes of integrated circuits are inherently affected by defectiveness, especially in the early stages of a newly-developed manufacturing technology. Semiconductor memories, being characterized by a very large integration scale, are greatly affected by manufacturing defects. Within a semiconductor memory, defects in the memory cell matrix <b>105</b> are highly probable; unfortunately, a defect that impairs the functionality of even a single memory cell <b>MC</b> may cause the whole memory device to be discarded, with a consequent significant reduction of the process yield, and an increase of costs. For this reason, redundant memory cells are normally provided in the memory, organized for example by redundant word lines and/or redundant bit lines that are used for functionally replacing a defective word line or bit line. This is shown in an extremely schematic and simplified way in <b>Figure 1,</b> wherein reference numerals <b>RWL</b> and <b>RBL</b> identify a redundant word line and, respectively, a redundant bit line, i.e., a word line and a bit line of redundant memory cells <b>RMC,</b> structurally identical to the memory cells <b>MC</b> and provided in the memory matrix <b>105</b> in addition to the memory cells <b>MC</b> that would be strictly necessary for achieving the desired storage capacity. A redundancy control circuitry is provided, known per-se and therefore not shown in detail (in the drawing, a block 135 is intended to include the redundancy control circuitry as well as the memory operation control circuitry). Under the control of the redundancy control circuitry <b>135</b>, the redundant bit line <b>RBL</b> is exploited to functionally replace a defective bit line among the bit lines <b>BL1 - BLn</b>, and the redundant word line <b>RWL</b> is similarly exploited to functionally replace a defective word line among the word lines <b>WL1 - WLm.</b> For example, if it is detected that the bit line <b>BLk</b> contains a defective memory cell <b>MC,</b> an address code of the defective bit line is stored in the redundancy control circuitry <b>135</b> (for example, in a non-volatile Content-Addressable Memory or CAM <b>155</b>); the redundancy control circuitry <b>135</b> constantly compares the currently supplied address code to the stored address code; in case coincidence is detected, the access to the defective bit line <b>BLk</b> is prevented and, in turn, the redundancy bit line <b>RBL</b> is accessed and coupled to the read/write circuits 120 (in <b>Figure 1</b> this operation is schematically represented by a switch <b>140,</b> controlled by the redundancy control circuitry through a redundant bit line selection signal <b>RBLen</b>, and interposed between the bit line selection circuits and the redundancy bit line, on one side, and the read/write circuits, on the other side). Similarly, if the redundant word line <b>RWL</b> is exploited for functionally replacing the defective word line <b>WLh</b>, an address code thereof is stored in the (CAM of the) redundancy control circuitry and, in case of coincidence of the stored defective word line address code with the currently supplied address code, the redundant word line <b>RWL</b> is selected in place of the defective word line <b>WLh</b> (in <b>Figure 1</b> this operation is schematically represented by the provision of a redundant word line selector block <b>145,</b> driven by a redundant word line selection signal <b>RWLen</b> which is also used to disable the word line selection circuits <b>110).</b>

A defect that is rather frequently encountered in a matrix of memory cells is the existence of a low-resistance path, or even a short-circuit <b>150</b> between one <b>BLk</b> of the matrix bit lines <b>BL1 - BLn</b> and one <b>WLh</b> of the matrix word lines <b>WL1 - WLm.</b> Albeit this type of defect can be repaired in the way described in the foregoing, by functionally replacing the defective bit line <b>BLk</b> and the defective word line <b>WLh</b> short-circuited thereto with a redundant pair of bit line <b>RBL</b> and word line <b>RWL,</b> the implementation of the known solution adopted for discharging the bit line stray capacitances would result in a significant crowbar current flowing, from the word line deselection voltage distribution line <b>Vh</b> to the reference voltage distribution line, through the word line driver <b>135-h</b>, the short-circuit <b>150</b> and the discharge MOSFET associated with the defective bit line <b>BLk</b>. This may easily cause the memory to exhibit an unacceptably high power consumption and, when the word line deselection voltage is generated on-chip by means of charge-pump voltage generators, the excessive current absorption may easily cause the charge-pump output voltage to fall below a safety level, with the consequence that the operation of the entire memory might be prejudiced.

In order to solve this problem, according to an embodiment of the present invention, the bit line discharge circuits <b>125-1 - 125-n</b> are controllable so as to exhibit at least three different operating modes: a first operating mode, in which the bit line discharge is inhibited, and no discharge current is allowed to flow; a second operating mode, in which the bit line discharge is enabled, and a relatively large bit line discharge current is allowed to flow; and a third operating mode, in which, although the bit line discharge is enabled, the bit line discharge current is at least substantially limited in value, possibly substantially reduced to zero.

As schematically depicted in <b>Figure 1,</b> each bit line discharge circuit <b>125-1 - 125-n</b> receives from the control circuitry <b>135</b> a common discharge enable signal <b>DSCen</b>, asserted by the control circuitry <b>135</b> in the bit line discharge phase that precedes any operation involving the selection of a generic word line, so as to enable the discharge of the bit line stray capacitances. In addition, each bit line discharge circuit <b>125-1 - 125-n</b> receives from the control circuitry <b>135</b> a respective control signal <b>CNT1 - CNTn,</b> for selecting the bit line discharge circuit <b>125-1 - 125-n</b> operating mode among the second and third operating modes cited above. For example, when the generic control signal <b>CNTk</b> is kept deasserted, upon assertion of the discharge enable signal <b>DSCen</b> the corresponding bit line discharge circuit <b>125-k</b> allows a relatively high discharge current to flow, thereby allowing a relatively fast discharge of the stray capacitance <b>C</b>_<b>BLk</b> associated with the bit line <b>BLk.</b> On the contrary, if the generic control signal <b>CNTk</b> is asserted, upon assertion of the discharge enable signal <b>DSCen</b> the bit line discharge circuit <b>125-k</b> does not allow the flow of a relatively high discharge current, and instead significantly limits, or even reduces substantially to zero, the current that can flow through the bit line <b>BLk.</b> Exploiting this peculiar operation mode of the bit line discharge circuits <b>125-1 - 125-n,</b> it is possible to avoid the substantial crowbar current that, in the solution known in the art, would flow when a defective bit line (the bit line <b>BLk</b> in the shown example) has been found to be short-circuited to a word line <b>WLh:</b> it is in this case sufficient that the bit line discharge circuit <b>125-k,</b> associated with the defective bit line, is individually controlled so as to at least substantially limit the discharge current that can flow therethrough.

It is observed that the generic control signal <b>CNT-k</b> may be used either as a control signal, controlling the operating mode and particularly the activation of the respective bit line discharge circuit <b>125-k</b> each time a bit line discharge phase is entered (<i>i.e.</i>, when the signal discharge enable signal <b>DSCen</b> is asserted), or, alternatively, as a set signal, used in a setting phase of the bit line discharge circuit <b>125-k</b> for setting, substantially once and for all, the bit line discharge circuit <b>125-k</b> in the desired, discharge current-limiting operating mode, for example at the time the defect in the bit line <b>BLk</b> is discovered and the defective bit line is repaired by means of a redundant bit line <b>RBL,</b> so that each time the discharge enable signal <b>DSCen</b> is asserted, the discharge circuit <b>125-k</b> automatically enters the current-limiting operating mode.

It is also observed that a bit line discharge circuit <b>R125,</b> controlled by a control signal <b>RCNT</b> and similar to those <b>125-1 - 125-n</b> associated with the bit lines <b>BL1</b> - <b>BLn</b> is also associated with the redundancy bit line <b>RBL,</b> for allowing the discharge of a stray capacitance <b>C</b>_<b>RaL</b> associated therewith.

In the following, some practical embodiments of the present invention will be described, being intended that none of the described embodiments is to be construed as limitative, but merely as exemplary.

Referring to <b>Figure 3,</b> associated with each bit line <b>BL1 - BLn</b> a respective bit line discharge circuit is provided, consisting of a discharge-enabling transistor <b>DT1 - DTn,</b> particularly but not limitatively an N-channel MOSFET, with a drain electrode connected to the respective bit line and the source electrode connected to a first terminal of a variable impedance element <b>300-1 - 300-n,</b> particularly a variable resistance element, having a second terminal connected to the reference voltage line. The variable resistance elements <b>300-1 - 300-n</b> have a resistance variable in a controlled way between at least a first and a second values, the first resistance value being relatively low, whereas the second resistance value is substantially higher. Expediently, especially from a manufacturing process viewpoint, the variable resistance elements <b>300-1 - 300-n</b> are formed by phase-change material elements similar to those making up the storage elements <b>P</b> of the memory cells <b>MC.</b>

The control electrodes of all the discharge-enabling transistor <b>DT1 - DTn</b> are driven by a common control signal <b>DT-CNT,</b> generated by a control circuit <b>305.</b> The operation of the control circuit <b>305</b> is controlled by the memory control circuit <b>135</b> through the bit line discharge enable signal <b>DSCen</b> and a bit line discharge circuit program enable signal <b>DSCpg</b>.

The provision of the variable resistance element <b>300-1 - 300-n</b> in series to the discharge-enabling transistor <b>DT1 - DTn</b> allows controlling the bit line discharge current: the discharge current flowing through the generic variable resistance element <b>300-1 - 300-n</b> (<i>e.g.</i>, the current Id flowing the element <b>300-k</b>) produces a voltage drop (<b>Vdrop</b>) thereacross that depends on the resistance of the variable resistance element; the amount of the voltage drop affects the gate-source drive voltage of the transistor <b>DT1 - DTn</b>. Thus, in case the variable resistance element <b>300-1 - 300-n</b> is in the low resistance state, the voltage drop thereacross is substantially zero, the respective transistor <b>DT1</b> - <b>DTn</b> will experience a full gate-source drive voltage, and the flow of a relatively high discharge current will be sustained. On the contrary, if the variable resistance element is in to the high resistance state, a negative feedback control loop is automatically created, thereby the discharge current is automatically limited to a desired value: an increase in the discharge current would produce an increased voltage drop across the variable resistance element <b>300-1 - 300-n,</b> and thus a reduction in the gate-source drive voltage of the transistor <b>DT1 - DTn</b>, and a consequent reduction in the discharge current; for a sufficiently high resistance value, the discharge current is substantially reduced to zero.

The specific first and second resistance values are not critical per-se; it is sufficient that the first resistance value is low enough not to cause a significant reduction of the drive of the corresponding transistor <b>DT1 - DTn,</b> while the opposite holds true for the second resistance value, depending on the discharge current, the transistor dimensions and the like.

Essentially, the control circuit <b>305</b> is a controlled variable voltage generator, adapted to generate variable voltage levels (either discrete or continuous, for example voltage ramps) depending on the operations to be performed, namely programming of the variable resistance elements 300-1 - 300-n or enabling of the bit line discharge, as determined by the state of the control signals <b>DSCen</b> and <b>DSCpg</b>.

In particular, and by way of example, during a bit line discharge phase, the assertion of the bit line discharge enable signal <b>DSCen</b> by the memory control circuit <b>135</b> causes the control circuit <b>305</b> to drive the control signal line <b>DT-CNT</b> (normally kept at, <i>e</i>.<i>g</i>., the reference voltage) to a first voltage, sufficient to cause the transistors <b>DT1 - DTk</b> to turn on, so as to enable the flow of a discharge current. It is observed that the value of this first voltage is determined on the basis of the need of preventing that, during the bit line discharge phase, the voltage across the phase-change elements <b>300-1 - 300-k</b> exceeds the phase-change material switching value, not to alter their state. In particular, in order to ensure that the state of the phase-change elements <b>300-1 - 300-k</b> is not changed during a bit line discharge, the following relations need to be satisfied:<maths id="math0001" num=""><math display="block"><mrow><msup><mrow><mtext>Id = K * (Vd - Ri*Id - Vth)</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>, with Ri*Id &lt; Vtr</mtext></mrow></math><img file="EP1526548A1_D0001.tif" /></maths> where Id the discharge current, flowing through the generic transistor <b>DTk</b> and the associated variable resistance element <b>300-k,</b> Vd is the voltage at the gate of the transistor <b>DTk,</b> Ri is the resistance of the variable resistance element <b>300-k,</b> Vth is the threshold voltage of the transistor <b>DTk,</b> K is the proportionality constant of the transistor <b>DTk</b> (assumed to operate in saturation when activated), and Vtr is the threshold voltage of the calcogenide material. A suitable voltage may for example be 1.5V.

It is observed that instead of a digital signal, taking only two discrete voltage values (a lower, transistor turn-off voltage for disabling the transistors <b>DT1 - DTn</b>, and a higher, transistor turn-on voltage for enabling the transistors), the control circuit 305 may generate an analog signal, for example a voltage ramp (or a staircase signal). The adoption of a voltage ramp is considered preferable, because in this way the discharge of the bit line stray capacitances is better controlled, and in particular the ramp-up current is kept limited.

In order to set the variable resistance elements <b>300-1 - 300-n</b> in either one of the two (low resistance or high resistance) states, the memory control circuit <b>135</b> asserts the signal <b>DSCpg.</b> A procedure totally similar to that adopted for programming a generic memory cell <b>MC</b> in the array <b>105</b> may be expediently adopted (with the exception that the word line selector <b>110</b> keeps all the word lines <b>WL1 - WLm</b> deselected, so as to prevent that the memory cells <b>MC</b> belonging to the selected bit line are programmed). Specifically, in order to program the variable resistance element <b>300-k</b> associated with the generic bit line <b>BLk,</b> such a bit line is caused to be selected by the bit line selection circuits <b>115</b> and coupled to the write circuits <b>120</b> (to this purpose, the memory control circuitry <b>135</b> may internally furnish to the bit line selection circuits <b>115</b> the address code of the bit line <b>BLk).</b> The write circuits bias the bit line <b>BLk</b> at the proper voltage, depending on the state desired for the variable resistance element <b>300-1 - 300-n</b> (crystalline - low resistance, or amorphous - high resistance). The assertion of the control signal <b>DSCpg</b> causes the control circuit <b>305</b> to place on the control signal line <b>DT-CNT</b> a voltage suitable to cause the flow through the phase-change material element forming the variable-resistance element <b>300-k</b> of a current adapted to cause the element to be programmed in the desired state; for example, a first, lower programming voltage of, <i>e.g.</i>, 1.5V is placed on the control signal line <b>DT-CNT</b> to cause the selected variable resistance element <b>300-1 - 300-n</b> to be set, while a second, higher programming voltage of, <i>e.g.,</i> 3V is used to cause the variable resistance element to be reset. The signal <b>DSCpg</b> can also be exploited to keep the word line selector <b>110</b> disabled, thereby the word lines <b>WL1 - WLm</b> are kept deselected.

Should the variable resistance elements <b>300-1 - 300-n</b> need a periodical refresh of their programming state, this operation might be expediently performed during a bit line discharge operation (provided that the duration of the bit line discharge operation is sufficient for this purpose). In this way, the refresh operation is made completely transparent to an external user of the memory 100.

It is also observed that a measure for reducing the dimensions of the transistors <b>DT1 - DTk</b> (and thus the semiconductor area) may call for providing, in the read/write circuits 120, an additional transistor, for example an N-channel MOSFET shown in phantom in <b>Figure 3</b> and identified therein by <b>N1,</b> arranged for contributing to the discharge of the selected bit line.

It is observed that although the exploitation of the write circuits already provided in the memory for programming the memory cells <b>MC</b> of the matrix <b>105</b> is clearly advantageous, nothing prevents the designer from providing dedicated write circuits for writing the variable resistance elements <b>300-1 - 300-n.</b> Also, it is observed that the exploitation of phase-change calcogenides for implementing the variable resistance elements is not limitative to the present invention: any other component adapted to exhibit a selectively programmable, variable resistance can be used, for example a variable threshold voltage MOS transistor of the type used in electrically alterable semiconductor memories such as Flash memories.

Another possible practical implementation of the present invention is schematically depicted in <b>Figure 4.</b> In this case, instead of providing programmable variable resistance elements in series to the discharge enabling transistors <b>DT1 - DTn,</b> a programmable coding is provided for selectively controlling the activation of the discharge of the different bit lines. Specifically, each bit line <b>BL1</b> - <b>BLn</b> has associated therewith a controlled discharge switch <b>DS1</b> - <b>DSn,</b> for example a discharge transistor, particularly an N-channel MOSFET similar to the discharge transistors <b>DT1 - DTn</b> of the previous embodiment; the controlled discharge switch <b>DS1 - DSn</b> is selectively activatable for connecting the respective bit line to the reference voltage distribution line, so as to discharge the bit line stray capacitance (not shown for simplicity in <b>Figure 4</b>).

In the example shown in the drawing, it is assumed that the bit lines <b>BL1</b> - <b>BLn</b> of the matrix <b>105</b> are ideally gathered in groups <b>BL1 - BLj,..., BLk - BLn</b> of bit lines, each bit line group including a same number of bit lines (in the drawing, the bit lines of a same group are depicted as physically adjacent in the matrix 105, but this is not to be construed as limitative). At least one similar group of redundant bit lines <b>RBL1</b> - <b>RBLj</b> is also provided, made up of redundant memory cells <b>RMC.</b> The redundant bit line group <b>RBL1 - RBLj</b> include a number of redundant bit lines equal to the number of bit lines in each bit line <b>BL1 - BLj,..., BLk - BLn,</b> and is exploited for functionally replacing any one of the bit line groups <b>BL1 - BLj,..., BLk - BLn</b> in case of one or more defects are discovered therein.

The choice of the actual number of bit lines to be included in each bit line group depends on design, architectural and semiconductor chip area considerations, because a bit line group will be in this case the elemental unit that can be functionally repaired in block.

All the discharge switches <b>DS1 - DSj, ..., DSk - DSn</b> associated with the bit lines of a same bit line group <b>BL1</b> - <b>BLj,..., BLk - BLn</b> are controlled by a same bit line group discharge enabling signal <b>DSC1 - DSCq</b> that, in the shown example, drives the gates of the discharge MOSFETs. The discharge enabling signals <b>DSC1 - DSCq</b> are generated by a bit line group discharge control circuit 400. The activation of the discharge enabling signals DSC1 - DSCq (to a variable activation voltage value, as described in detail in the following) by the bit line group discharge control circuit 400 is conditioned to the assertion of the bit line discharge enable signal DCSen, received from the memory control circuitry 135. Additionally, the bit line group discharge control circuit 400 implements a programmable, selective activation scheme of the different bit line group discharge enabling signals <b>DSC1 - DSCq,</b> thereby in case one of the bit line groups <b>BL1 - BLj,..., BLk - BLn</b> has been functionally replaced (in jargon, repaired) by the redundant bit line group <b>RBL1 - RBLj,</b> for example because one of the bit lines in the repaired bit line group contained a short-circuited bit line/word line pair, the bit line group discharge enabling signal <b>DSC1 - DSCq</b> associated with the repaired bit line group is not activated, or it is activated at a voltage level such as to significantly limit the discharge current sustained by the corresponding discharge transistors DS1 - DSj, ..., DSk - DSn, thereby the possible flow of crowbar currents is avoided.

A possible implementation of the bit line group discharge control circuit <b>400</b> is schematically shown in <b>Figure 5.</b> The generic bit line group discharge enabling signal <b>DSC1 - DSCq</b> is tapped off from an intermediate node of a respective programmable voltage partition network <b>500-11, 500-12 - 500-q1, 500-q2,</b> made up of programmable, variable impedance elements, particularly programmable variable resistance elements, for example based on a phase-change material similar to the storage elements <b>P</b> of the memory cells <b>MC.</b>

A respective programming circuit <b>505-1 - 505-q</b> is associated with each programmable voltage partition network <b>500-11, 500-12</b> - <b>500-q1, 500-q2.</b> The operation of the programming circuits <b>505-1 - 505-q</b> is controlled by a common program enable signal DSCpg, generated by the memory control circuit <b>135,</b> and by individual program enabling signals <b>EN1 - ENq,</b> generated by a decoder <b>510</b> that receives (from the redundancy control circuitry included in the memory control circuit <b>135)</b> and decodes a bit line group address code <b>BLADD</b> identifying the bit line group <b>BL1</b> - <b>BLj,..., BLk - BLn</b> that has been repaired. The programming circuits <b>505-1 - 505-q</b> are adapted to generate voltages suitable to program the phase-change material elements, i.e. suitable to put the phase-change material elements either in the set or in the reset state; to this purpose, the programming circuits <b>505-1 - 505-q</b> may include a circuitry similar to that used in the memory write circuits <b>120</b> for programming the memory cells <b>MC,</b><i>i.e.</i> circuits adapted to apply voltage pulses of different amplitude to the phase-change material elements, for rising the temperature thereof over a nucleation or a melting temperature.

In particular, when it is intended to limit or even cut off the discharge current of a given bit line group, because one or more bit lines in such a group are defective, the program enable signal <b>DSCpg</b> is asserted by the memory control circuit <b>135,</b> and the address code <b>BLADD</b> of that bit line group (stored in the CAM <b>155</b> of the redundancy circuitry) is supplied to the decoder <b>510,</b> which asserts a specified one of the enable signals <b>EN1 - ENq,</b> corresponding to that bit line group address code. The respective programming circuit <b>505-1 - 505-q</b> thus causes the variable resistance elements <b>500-11, 500-12 - 500-q1, 500-q2</b> in the associated voltage partition network to be put in the reset state. Then, during a normal operation of the memory, when a bit line discharge phase is entered and the discharge enable signal <b>DSCen</b> is asserted, a full voltage is placed by each circuit <b>505-1 - 505-q</b> across the respective voltage partition network, on a line <b>DSC1' - DSCq';</b> if the two variable resistance elements <b>500-11, 500-12 - 500-q1, 500-q2</b> of a given voltage partition network are respectively in the set state and in the reset state (low resistance and high resistance, respectively), the corresponding bit line group discharge enabling signal <b>DSC1 - DSCq</b> will take the full voltage present on the line <b>DSC1' - DSCq';</b> the respective discharge transistor <b>DS1 - DSn</b> will thus be strongly turned on, and a relatively large discharge current will flow therethrough. If instead the two variable resistance elements <b>500-11, 500-12 - 500-q1, 500-q2</b> of a given voltage partition network are respectively in the reset state and in the set state (high resistance and low resistance, respectively), the corresponding bit line group discharge enabling signal <b>DSC1 - DSCq</b> will be relatively low in voltage compared to the voltage level present on the corresponding line <b>DSC1'</b> - <b>DSCq';</b> the associated discharge transistor <b>DS1 - DSn</b> will thus be only weakly turned on (or, possibly, kept off), thereby the discharge current will be limited.

Each bit line group discharge enabling signal <b>DSC1</b> - <b>DSCq</b> can thus take three different voltage values: a first voltage value, equal to the reference voltage, when the bit line discharge phase is not entered (signal DSCen deasserted); during the bit line discharge phase, until the signal <b>DSCen</b> is asserted, the generic bit line group discharge enabling signal <b>DSC1 - DSCq</b> may take a second, higher voltage value (<i>e.g.</i>, around 1.5V), adapted to allow the discharge transistor <b>DS1 - DSn</b> sustaining a relatively large bit line discharge current, or a third, lower voltage value (possibly proximate to the reference voltage, for example around 800 mV), adapted to cause the discharge transistor <b>DS1 - DSn</b> to at least limit the bit line discharge current, the second and third voltage values depending on the programming of the respective voltage partition network <b>500-11, 500-12 - 500-q1, 500-q2.</b>

It is pointed out that although in the foregoing a discharge switch control decoding scheme by groups of bit lines has been described, nothing prevents in principle from implementing a finer discharge switch control decoding, down to the level of the single bit line, although this solution may result heavy from the area occupation point of view).

It is also observed that although in the foregoing the memory device has been assumed to include one memory matrix <b>105,</b> the memory cells <b>MC</b> may be arranged in two or more matrixes, also referred to in jargon as "tiles". With reference to the embodiment described in connection with <b>Figure 4,</b> in this case several bit line group discharge control circuits 400 might be provided, one for each tile, selected via a tile address code stored in the CAM <b>155</b> of redundancy circuitry.

Thanks to the bit line discharge control according to the previously described embodiments of the present invention, it is possible to control the bit line discharge currents; thus, it is possible to prevent the flow of relatively large crowbar currents arising when in the memory matrix one or more defective bit lines exist that exhibit a low-resistance path, or a short-circuit, to a matrix word line. Thanks to this, not only the overall current consumption of the memory device can be kept low, but also the operation of the memory device as a whole is not compromised, especially in case the voltages necessary for accessing the memory cells are generated on-chip by charge pump voltage generators.

It is however observed that in case of a short-circuit between a bit line and a word line, a slight current may still flow from the word line deselection voltage distribution line <b>Vh</b> that supplies the word line drivers, to the reference voltage distribution line (<i>e.g.,</i> the ground), through a circuit path including the defective word line, the short-circuit to the defective bit line and the memory cell <b>MC</b> that belongs to the defective bit line and to another, non-defective word line being selected. For better clarity, referring to <b>Figure 6,</b> assuming that a short circuit exists between the generic bit line <b>BLk</b> and the generic word line <b>WLh,</b> and that another generic word line <b>WLj</b> is selected by the word line selector circuits <b>110</b> (bringing the potential thereof to ground), a slight current flows from the voltage line <b>Vh</b> through the word line driver <b>130-h</b> of the defective word line <b>WLh,</b> the short circuit <b>150,</b> the defective bit line <b>BLk,</b> the storage element <b>P</b> and the emitter-base junction of the respective selection BJT T of the memory cell <b>MC</b> belonging to the defective bit line <b>BLk</b> and the non-defective, selected word line <b>WLj</b> and, finally, the word line driver <b>130-j</b> thereof. It is pointed out that such a current is normally small, being limited to the current carried by the selection BJT T; in particular, this current depends on the emitter area of the BJT <b>T,</b> and thus on the dimensions of the memory cell <b>MC,</b> and in current technologies is not higher than 700 - 800 µA. However, it may happen that the charge-pump voltage generator generating the word line deselection voltage Vh is not capable of delivering this extra current, and/or the spurious increase in the voltage of the selected word line <b>WLj</b> (due to the slight voltage drop on the word line driver <b>130-j</b> caused by the base current of the selection BJT <b>T</b>, equal to the emitter current divided by a factor equal to the transistor beta plus 1) is considered unacceptable.

<b>Figure 6</b> schematically shows an embodiment of the invention adapted to avoid the flow of even this slight current. The word lines <b>WL1 - WLm</b> are gathered in groups <b>WL1 - WLh,..., WLj - WLm</b> of word lines, each word line group including a same number of word lines (in the drawing, the word lines of a same group are depicted as physically adjacent in the matrix <b>105,</b> but this is not to be construed as a limitation). At least one similar group of redundant word lines <b>RWL1 - RWLh</b> is also provided, made up of redundant memory cells <b>RMC;</b> the redundant word line group <b>RWL1 - RWLh</b> includes a number of redundant word lines equal to the number of word lines in each word line group <b>Wh1</b> - <b>WLh,..., WLj - WLm,</b> and is exploited for functionally replacing any one of the word line groups <b>WL1 - WLh,..., WLj - WLm</b> in case one or more defects are discovered therein. As far as the number of word lines to be included in each word line group is concerned, considerations similar to those made in connection with the bit line groups of <b>Figure 4</b> can be made.

Also schematically shown in <b>Figure 6</b> is a two-level row address code decoding scheme, with a first-level decoder, or predecoder <b>605,</b> for the selection of one word line group among the different word line groups <b>WL1 - WLh,..., WLj - WLm,</b> and a number of second-level decoders <b>610-1 - 610-p,</b> each one associated with a respective word line group, for selecting one word line within the selected word line group.

A word line driver supply control scheme is implemented, according to which all the word line drivers <b>130-1 - 130-h,..., 130-j - 130-m</b> driving the word lines of a same word line group receive the supply voltage from the supply voltage distribution line <b>Vh</b> through a respective switchable path arrangement <b>615-1 - 615-p,</b> including a switch <b>SW</b> adapted to switching a connection to the voltage supply line <b>Vh</b> of a voltage supply terminal of the respective word line driver <b>130-1 - 130-h,..., 130-j - 130-m</b> between a first path <b>620,</b> having a negligible resistance, and a second path 625, having a high (possibly, substantially infinite) resistance. The second path <b>625</b> may for example include a resistive element having a sufficiently high resistance, for example a phase-change material based element similar to the storage elements P of the memory cells, or a current generator adapted to limiting the current flow to a prescribed value, for example approximately 10 µA, or a transistor (<i>e.g.</i>, a P-channel MOSFET) with a suitably controlled drive, so as to limit the current flowing therethrough. Any current-limiting element, device or circuit arrangement can in general be used, even a simple open circuit.

Redundancy word line drivers <b>R130-1 - R130-h</b> are also schematically shown, driving the redundancy word lines <b>RWL1 - RWLh</b> in response to a redundancy word line address decoder <b>R610,</b> performing a function similar to the decoders <b>610-1 - 610-p</b> on a redundancy word line address code provided by the redundancy control circuitry <b>135.</b> A switchable path arrangement <b>R615-1</b> similar to those provided for the word line drivers <b>130-1 - 130-h,..., 130-j - 130-m</b> is preferably provided for the redundancy word line drivers <b>R130-1 - R130-h.</b>

The defect represented by the short-circuit <b>150</b> between the word line <b>WLh</b> and the bit line <b>BLk</b> is repaired by functionally replacing the word line group <b>WL1 - WLh</b> to which the defective word line belong with the redundant word line group <b>RWL1 - RWLh,</b> and functionally replacing the defective bit line <b>BLk</b> with a redundancy bit line, for simplicity not depicted in <b>Figure 6</b> (or, as shown in <b>Figure 4,</b> replacing the bit line group to which the defective bit line <b>BLk</b> belongs with the redundant bit line group <b>RBL1</b> - <b>RBLj).</b> In addition, the switchable path arrangement <b>615-1</b> of the defective word line group <b>WL1 - WLh</b> is set to the second, high resistance path, thereby the current that can be delivered by the voltage distribution line <b>Vh</b> to the word line drivers <b>1301 - 130-h</b> is significantly limited, possibly reduced to zero. For example, the setting of the switchable path arrangement may be accomplished at the same time as the address of the defective word line group is written into the redundancy control circuitry <b>135</b>.

In other words, in this embodiment of the invention a controlled current limiting arrangement is provided in a circuit path associated with a word line and bit line pair and including one of the word line driver circuits, the associated word line, one of the bit lines and the associated bit line discharge circuit, and the current limiting arrangement is capable of at least limiting a current flowing through the circuit path in response to an indication of defectiveness of at least one among the word line and the bit line.

In this way, not only is the defect <b>150</b> repaired, and the crowbar current flowing through the short-circuit <b>150</b> in the bit line discharge phase significantly limited or, possibly, reduced to zero, but the problems of spurious currents flowing through the defective word lines and bit lines are also solved.

It is pointed out that, as discussed in connection with the invention embodiment depicted in <b>Figure 4,</b> also in the present case nothing prevents in principle from implementing a finer word line driver controlled supply scheme, down to the level of the single word line, although this solution may result heavy from the area occupation point of view.

It is observed that the word line driver controlled supply according to the embodiment depicted in <b>Figure 6</b> may be used in substitution of, or in conjunction with the previously described embodiments of the present invention.

In other words, according to the present invention, the undesired crowbar current that would flow during the bit line discharge phase due to the presence of a low-resistance path or short-circuit between a word line and a bit line is substantially reduced or even avoided by providing a control of the current allowed to flow through the bit line in the discharge phase, and such a current control can for example be actuated either at the level of the bit line discharge circuit, or at the level of the word line driver associated with the word line short-circuited to the bit line.

Thanks to the present invention, the existence of defects such as a low-resistance path, or a short-circuit, between a matrix bit line and a matrix word line can be remedied by simply functionally replacing the defective bit line and word line pair with a redundant pair of bit line and word line, without the need of discarding the memory device or functionally replacing the whole memory matrix ("tile").

The present invention also makes it feasible to adopt a different bit line discharge policy compared to the one conventionally followed. The Applicant has in fact observed that the conventional bit line discharge policy, providing for carrying out a bit line discharge operation before any memory operation involving the selection of a word line, such as upon leaving a stand-by condition, has the disadvantage that the bit line discharge time adds up to the memory access time. Such a discharge time is not negligible, because during stand-by the bit line voltage normally reaches the word line deselection voltage, typically 4.5 V; if the bit line stray capacitance is of the order of some nF, the time necessary for discharging such a capacitance to ground may easily be of some µs. On the other hand, having highly conductive bit line discharge transistors for speeding up the bit line discharge is not practical, because of reasons of semiconductor chip area, and of noise induced on the memory reference voltage (similarly to what happen in fast output buffers). This problems may be solved if the bit line discharge circuits <b>125-1 - 125-n,</b> instead of being temporarily activated before any access to the memory, are always kept activated, except during the access (read or write) to the memory, and the word line deselection voltage during stand-by is reduced from 4.5 V to, <i>e.g.</i>, 2 V (the word line deselection value during a read access to the memory); in other words, the bit line discharge phase is not limited to a relatively short time interval before any access operation, but lasts for all time preceding and following a memory access. In this way, not only is the amount of the leakage currents greatly reduced, but the bit lines are always kept at the reference voltage, and there is no need to preliminary discharge the bit line stray capacitances upon leaving stand-by. Thanks to the presence invention, the presence of a defective pair of short-circuited bit line and word line does not cause problems of crowbar current, because the associated bit line discharge circuit will be disabled.

Although the present invention has been disclosed and described by way of some embodiments, it is apparent to those skilled in the art that several modifications to the described embodiments, as well as other embodiments of the present invention are possible without departing from the scope thereof as defined in the appended claims.