Sense circuit for reading data stored in nonvolatile memory cells

The present invention relates to a circuit for reading the information stored in ROM and EPROM type memories according to a differential sensing mode and, in particular to an improved circuit for generating an offsetting current for discriminating between the currents which flow through a certain cell of the memory array which has been selectively addressed for reading and a virgin reference cell, according to a current offset sensing mode.

Among semiconductor nonvolatile memories, EPROM memories represent one of the most advanced field of integration in silicon. Starting from nowadays common 1 megabit devices, new devices with a capacity of up to 4 megabit have been presented lately and new ambitious goals are announced.

Together with an ever increasing packing density, the memory market requires improved performances in terms of access time, write time and power consumption. The reduction of the size of the devices poses serious problems to the achievement of these aims. In particular, the access time during a reading phase is penalized by a consequent reduction of the actual current through the memory cell and an increased influence of parasitic electric factors of the integrated structure of the cells. For these reasons, the circuits used for reading the information stored in the cells must possess an enhanced precision and reliability.

The article entitled "L'amplilicatore di Lettura nei Dispositivi di Memoria EPROM" by G. Compardo, M. Dallabora and D. Novosel, published on the journal "Alta Frequenza" Vol. LVII--No. 6--July-August 1988, contains a comprehensive review of the different sense circuits which are commonly used. The relevant content of this article is intended to be incorporated herein by express reference thereto.

Basically, the architecture of a differential type sense circuit are by far more precise and less sensitive to the effects of "process spread", temperature and supply voltage variations, by treating them essentially as common mode contributions. On the other hand, there are .[.two.]. .Iadd.three .Iaddend.differential sense systems: a load-offset type.Iadd., a current unbalance type, .Iaddend.and a current-offset type. A load-offset (or load-unbalance) type sense circuit .[.and the relative operating characteristics are.]. .Iadd.is .Iaddend.reproduced in .[.FIGS. 1 and 2, respectively.]. .Iadd.FIG. 1.Iaddend.. The reading system is based, as it is well known, upon the unbalancing of the load transistors, respectively of the "array cell" side and of the "reference cell" side (where the reference cell is commonly constituted by a virgin memory cell), by making larger than unity the dimensional ratio between the two load transistors.

The unbalancing of the loads permits the reading of a virgin cell by introducing an element of asymmetry between the two branches of the differential sense circuit. .Iadd.The minimum working voltage in this case is limited by the threshold of a virgin cell V th , while the maximum operating voltage VCC max is given by the equality of V ref and V mat . .Iaddend.

The characteristics of a current-unbalance type circuit are shown in FIG. 2. In the diagram of FIG. 2, the respective characteristics of a virgin cell (I on ) and of a written cell (I off ) and of the reference current (I ref ) are all shown on a single diagram purposely. .Iadd.Here, the reference cell has the same threshold but different current characteristics than the matrix cells, and the loads are equal. .Iaddend.

In this type of sense circuit, the reference current I ref varies with the varying of the supply voltage VCC which is applied to the control gate of the reference cell. The minimum working voltage VCC min is in this case limited by the threshold of a virgin cell V th , while the maximum operating voltage VCC max is given by the equality between I ref and I off .

Clearly the .Iadd.current-unbalance .Iaddend.system .Iadd.of FIG. 2 .Iaddend.has limitations, in static terms, toward the value of VCC max and this constitutes a drawback. A .[.further.]. drawback .Iadd.of the load-unbalance system of FIG. 1 .Iaddend.may be identified by the necessity of forming transistors having different geometrical dimensions among each other and by the necessity of employing a relatively complex sense circuit.

A typical circuit diagram and relative characteristics of a sensing system operating in a current-offset mode, are shown in FIGS. 3 and 4, respectively. According to this known alternative approach, the operating range in terms of voltage VCC is widened by modifying the reference current I ref characteristic. In the relative circuit diagram, as it may be observed in FIG. 3, the load transistors have similar dimensions. The element of asymmetry which is necessary for discriminating the sensing of a virgin cell, is provided by means of a constant current I offset which is purposely added in parallel to the cell current .Iadd.I cell .Iaddend.of the array side.

In this way, the electrical characteristics of the cells are caused to be parallel to the characteristic of the reference current. The remarkable result is that the VCC max value tends theorethically to infinity for all the cells which have undergone a threshold shift (δV th ) larger than the value of said additional constant current (I offset ) divided by the current gain of the cell: i.e. ##EQU1##

As described in the above mentioned article, in order to improve discrimination at the limit VCC min , the contribution of the offset current is added singly on the reference side and doubled on the array side. This is schematically shown in FIG. 3 by the "two generators": I offset and I offset /2, which determine the respective offset currents through the two branches of the input circuit of the differential sense amplifier A. In this way, the three characteristics: .[.I on , I ref and I off .]. .Iadd.I maton , I ref , and I matoff .Iaddend., of the operating diagram of FIG. 4, are always kept separated for:

VCC>V.sub.th =VCC.sub.min

There is however another order of considerations on sense circuits concerning their dynamic characteristics. In other words, the static characteristics discussed above, define the correct margin of operation, that is of correct interpretation of the data stored in an array cell, in function of the supply voltage and therefore they offer a measure of the width of the operating range of the sense circuits toward VCC, in the hypotesis that all transients, such as the precharging of the column lines (BITLINES), the signal propagation along the relative row of the array and so forth have terminated.

Vicerversa, the dynamic characteristics have a relevance on the currents passing in the two branches of the input circuit of the differential sense amplifier during a typical reading phase, during which the actual supply voltage of the device is set, thus providing for an evaluation of the correct operation margin during a transient. These dynamic characteristics are indicative of how small will be the access time of the memory, because the response of the sense circuits will be correct only from the moment in which the current of the reference side is smaller than the current of the array side when reading a conducting cell (ON-programmed cell), and viceversa, when reading a nonconducting cell (OFFprogrammed cell).

Moreover, this latter consideration is fundamental for recognizing an intrinsic limit of a current-offset type sense circuit.

In fact, while for the case of a load-unbalance system, the current in the two branches (reference side and array side) of the sense circuit are intrinsically different for any value of the gate voltage of the cells (and therefore the circuit provides a correct sensing during a transient), in the case of a current-offset sense circuit, because the circuit is powered at VCC, there is not a transient phase for the value of the current set by the offsetting circuit and therefore, when an OFF-programmed cell must be read, the sense circuit initially provides an erroneous response because the reference side current, for a low value of the voltage applied to the gate of the array cell and of the reference cell, is lower than the current of the array side, which current represents the sum of the offsetting current, which is intrinsically constant, and of the current of the OFF-programmed cell, which is practically nil.

Only when the voltage on the gate of the reference cell whose value depends from the propagation of the signal through the array's row, becomes sufficient to generate a current through the cell greater than the offsetting current, a correct response is obtained from the circuit. This limitation at transients negatively affects the access time in comparison to the case where a load-unbalance sense circuit is used, which, on the other hand, has the disadvantage of providing a reduced operating voltage range, as seen before.

There is a need for a sense circuit which although possessing static operating characteristics comparable to the static operating characteristics of a current-offset type circuit or anyway having a wide operating range toward the maximum supply voltage (VCC max ) also possess improved dynamic characteristics as compared to a current-offset circuit of the known type.

This objective is reached by means of the current-offset sense circuit which forms the object of the present invention.

Basically, the device for generating an unbalance between a reference cell branch of the sense circuit and another branch of the circuit comprising the selectively addressed array cell to be read, which is employed is basically a current generator and therefore, as in the case of a currentoffset sense circuit of the prior art, the sizes of the load transistors of the array side and of the reference side of the differential sense circuit are advantageously identical, thus avoiding the above mentioned drawbacks typical of a load-unbalance sense circuit. The circuit is driven essentially by the same gate voltage which is present on the array cells and for this purpose it is necessary to introduce in the array of cells a supplementary row which is decoded at every reading, and which replicates during transients the behaviour of the array's row which is selected for reading.

In the drawings attached to the present specification:

FIG. 1 is a basic circuit diagram of a load-unbalance type, sense circuit;

FIG. 2 is a diagram depicting the operation of .[.the.]. .Iadd.a current-unbalance type .Iaddend.sense circuit .[.of FIG. 1.].;

FIG. 3 is a basic circuit diagram of a current-offset type, sense circuit;

FIG. 4 is a diagram depicting the operation of the sense circuit of FIG. 3;

FIG. 5 is a circuit diagram of an offsetting current generating circuit made in accordance with a first embodiment of the present invention;

FIG. 6 depicts the dynamic characteristics of operation of the circuit of FIG. 5;

FIG. 7 depicts the static characteristics of operation of the circuit of FIG. 5;

FIG. 8 is a circuit diagram of the offsetting current generating circuit of the invention according to a different embodiment;

FIG. 9 depicts the dynamic characteristics of operation of the circuit of FIG. 8; and

FIG. 10 depicts the static characteristics of operation of the circuit of FIG. 8.

For simplicity's sake, in the circuit diagram of FIGS. 5 and 8, only the offsetting current generating circuit of the offset-current-type sense circuit as shown in FIG. 3 is depicted; the circuit nodes M and N of the circuit of FIG. 3 being indicated in the so-partialized circuit diagrams of FIGS. 5 and 8.

Moreover the label DUMMY ROW used in FIGS. 5 and 8, indicates a drive voltage line derived from a supplementary row of cells of the array of cells which is decoded at every reading and which therefore replicates perfectly the behaviour of .[.anyone.]. .Iadd.current unbalance .Iaddend.row of memory cells of the array which is selected for reading, during a transient.

Normally, the reference column line, REFERENCE BITLINE, serves a certain number of column lines, MATRIX BITLINE, of the array and to these is adjacently formed on the silicon chip so as to make as similar as possible the voltages present on the gate of the reference cell and on the gate of a selected cell of the array also during transients, the latter having a nonnegligeable duration because of the RC value of a row of the array. This topographic closeness between these lines further enhances a maximum geometrical identity of the relative reference cells and array cells.

With reference to the circuit shown in FIG. 5, n1, n3, n4 and n5 are cells (transistors) electrically equivalent to anyone memory cell of the memory array programmed in an ON-condition. According to this first preferred embodiment, the circuit is provided also with two compensation transistors n2 and n6, which are functionally unnecessary, but serve advantageously as compensating elements when the array cells, programmed in an-OFF condition (written cells) are not in a perfectly cut-off condition, but which in practice begin to conduct a certain current, in the order microampers, beyond a certain level of the driving voltage applied thereto. This condition is "replicated", by analogy, also by said transistors (cells) n2 and n6, which are electrically equivalent to any other memory cell of the array programmed in an OFF-condition, thus obtaining as a result a perfect compensation of the current which may be conducted by written memory cells and which would disturb the process of unbalancing the currents which is performed by the circuit of the invention, thus introducing a certain limitation toward the maximum value of the driving voltage (VCC max ) by "bending" the current characteristic of the reference branch of the circuit toward the current characteristic of a written (OFF) cell until crossing it. As shown, n1, n2 and n6 are driven by said supplementary array row, DUMMY ROW, and are topologically near, on the silicon chip, to the reference column line: REFERENCE BITLINE and to the respective array column lines: MATRIX BITLINE. This means that during a transient the voltages on the gates of the reference cell and of the array cell (not shown in FIG. 5), as well as on the gates of the transistors n1, n2 and n6, are very similar and may be considered equal to each other.

By supposing initially to neglect the contribution given by the optional compensation transistors n2 and n6 and by observing that, by assuming that all the cells (array, reference and n1) are in a saturation condition, the current of the cell n1 is equal in any instant to the current flowing through any selected ON-programmed cell of the array, because these currents depend exclusively on the respective gate voltages, which are equal in both cells also during a transient. The current I on generated by the cell n1, which is identical to the current flowing through the selected cell of the array which must be read, is mirrored by the current mirror formed by the transistors p1 and p2 into the right hand branch of the mirror and it is divided into two identical semi-currents by means of the n-channel current mirror formed by the transistors n3, n4 and n5, all having the same size, and it is further delivered to the column line of the array side (MATRIX BITLINE). By defining the so-obtained current: I offset ; and having neglected for the moment the contribution of the cell n2, the following relation holds:

I.sub.on =2*I.sub.offset.

The effect of unbalancing the currents of the two input branches (MATRIX BITLINE and REFERENCE BITLINE, respectively) of the differential sense circuit so obtained, is from both a static and a dynamic point of view, substantially similar to the effect obtained in a .[.load-unbalance.]. .Iadd.current-unbalance .Iaddend.type, sense circuit, .[.where the size of the load of the reference side is twice the size of the load of the array side,.]. thus producing an operating characteristic similar to the one depicted in FIG. 2. The unbalancing circuit is, on the other hand, exempt of the drawbacks of a .[.load-unbalance.]. .Iadd.conventional current-unbalance .Iaddend.circuit. Any limitation toward the maximum value of the driving voltage (VCC max ) is effectively eliminated by means of the optional compensation transistors n2 and n6, which are, as already said, equivalent to a programmed array cell, i.e. have a high threshold, as a cell in an OFF-condition, and they are driven by the supplementary row DUMMY ROW, of the array as the cell n1. The drain of the transistor n6 is connected to the reference column line: REFERENCE BITLINE, while the eventual current contribution of the transistor (cell) n2, which is substantially connected in common with the first transistor n1, and which will essentially be identical to the current eventually carried by a written memory cell (OFF-programmed cell) of the array, will be summed with the current generated by n1, thus producing a sum current (I on +I off ) of the currents generated by n1 and n2, respectively.

The result is easily analyzed as follows: ##EQU2##

The dynamic characteristics of operation of the current generating circuit of FIG. 5 are shown in the diagram of FIG. 6 and the static characteristics of operation of the circuit shown in the diagram of FIG. 7 make evident the advantages which are achieved by the circuit of the invention, in comparison either with a .[.load-unbalance-type.]. .Iadd.current-unbalance type .Iaddend.architecture of the sense circuit, represented by the wide range of the operating voltage, substantially unlimited toward the VCC max , or the current-offset-type circuits of prior art architecture, represented by the freedom from error in the transient response, which entails a smaller access time of the memory array.

In accordance with an alternative embodiment of the invention, the current generating circuit for the sense circuit of the invention may also be realized in the form shown in the diagram of FIG. 8. The diagram is similar to that of FIG. 5, and therein the current contribution given by the cell n2 at the moment of generating the unbalancing current lacks completely. As already seen before, the effect which is obtained in terms of offsetting the currents in the two branches: MATRIX BITLINE and REFERENCE BITLINE, may be considered similar to the effect occurring in a .[.load.]..Iadd.current.Iaddend.-unbalancetype sense circuit. Although, if the effect of the cell n6', connected as shown in the figure to the REFERENCE BITLINE, is considered too, and assuming the transistors n3, n4 and n5 also electrically equivalent to array cells, programmed in an ON-condition, it is evident that upon an increase of the VCC, the gate voltage of the transistor n6' will assume a value sufficiently high to drive a current on the reference column line, REFERENCE BITLINE, having an intensity, which though lower than that of an eventual current carried by an array cell, programmed in an OFF-condition (written cell) during reading, is sufficient to shift toward a higher voltage the point wherein the reference current I ref characteristic crosses the current I off of an array cell, programmed in an OFFcondition (written cell), thus increasing the VCC max limit of operation of the circuit.

In the diagram of FIG. 9 the dynamic characteristics of the circuit of FIG. 8 are depicted, which clearly appear even further improved in comparison with the dynamic characteristics of the circuit of FIG. 5, as will be remarked later.

In the diagram of FIG. 10, the static operation characteristics of a reading circuit employing the current generating circuit of FIG. 8 are depicted. The mentioned increased limit of the operation range toward VCC max may be recognized by the fact that the reference current I ref curve no longer crosses the .[.I off .]. .Iadd.I matoff .Iaddend.current curve of an OFF-programmed cell, notwithstanding that the voltage VCC become larger than 10 V.

The current generating circuit according to the FIG. 8 embodiment, does not provide a static operation performance comparable to that of the circuit made in accordance with the FIG. 5 embodiment which, as shown before, has no limitation toward VCC by virtue of the contribution of transistors n2 and n6. The circuit of FIG. 8 is nevertheless simpler to implement because the required supplementary row, DUMMY ROW, drives a single transistor instead of three transistors and therefore has a reduced capacitive load, positively reflecting upon the dynamic response of the current generating circuit. In fact, by comparing the dynamic characteristics of FIG. 6 and 9, it may be observed that, for the circuit of FIG. 8, the characteristic curves of the reference current I ref and of the current I on of an ON-programmed cell, diverge more rapidly during a transient than those of the circuit of FIG. 5.