Method and circuit for reading low-supply-voltage nonvolatile memory cells

Abstract: The read circuit (30) has an array branch (31) connected to an array cell (33), and a reference branch (32) connected to a reference cell (37); the array branch (31) presents an array load transistor (42) interposed between a supply line (36) and the array cell (33), and the reference branch (32) presents a reference load transistor (43) interposed between the supply line (36) and the reference cell (37); and the array and reference load transistors (42, 43) form a current mirror wherein the array load transistor (42) is diode-connected and presents a first predetermined channel width/length ratio (W 1 /L 1 ), and the reference load transistor (43) presents a second predetermined channel width/length ratio (W 2 /L 2 ) N times greater than the first ratio (W 1 /L 1 ), so that the current (I' M ) flowing in the array cell (33) is supplied, amplified, to the reference branch.

Description: The present invention relates to a method and circuit for reading low-supply-voltage nonvolatile memory cells. As is known, the most commonly used method of reading memory cells is to compare a quantity related to the current flowing through the memory cell to be read with a similar quantity related to the current flowing through a reference cell of known content, and using a circuit arrangement of the type shown in Figure 1. Number 1 in Figure 1 indicates a circuit for reading the cells of a memory array, comprising an array branch 2 connected by an array bit line 5 to an array cell 4 to be read; and a reference branch 3 connected by a reference bit line 9 to a reference cell 8 of known content. Array branch 2 comprises a current/voltage converter 6 - hereinafter referred to as an array converter - connected on one side to array bit line 5 by a decoding circuit (not shown), and on the other side to a supply line 7 at voltage V CC . Reference branch 3 comprises a current/voltage converter 10 - hereinafter referred to as a reference converter - connected on one side to reference bit line 9 by a decoding circuit (not shown), and on the other side to supply line 7. Each current/voltage converter 6, 10 comprises a precharge and biasing circuit 11, 12 (for appropriately biasing the respective bit line 5, 9 to prevent spurious write phenomena, such as soft-writing, and possibly also for equalizing lines 5 and 9) and a load 13, 14 connected in series with each other; the nodes 15, 16 between respective precharge and biasing circuits 11, 12 and respective loads 13, 14 form the outputs of respective current/voltage converters 6, 10 and are connected to the input of a sense amplifier 17 for comparing the voltages of nodes 15 and 16 to determine the content of array cell 4. Load 13 comprises a P-channel MOS transistor with a predetermined channel width to length ratio W 1 /L 1 ; load 14 comprises a diode-connected P-channel MOS transistor with a channel width to length ratio W 2 /L 2 N times greater than ratio W 1 /L 1 ; and MOS transistors 13, 14 present the control terminals connected to each other, the source terminals connected to supply line 7, and the drain terminals connected respectively to nodes 15 and 16 to form a current mirror in which the current flowing in transistor 13 is N times less than that flowing in transistor 14. Array bit line 5 is connected to a number of array cells 4 located in the same array column, and the capacitances of which are shown schematically in Figure 1 by an equivalent array capacitor 18, which, for the sake of simplicity, is shown connected directly to node 15. Similarly, reference bit line 9 is connected to a number of reference cells located in the same reference column, and the capacitances of which are shown schematically in Figure 1 by an equivalent reference capacitor 19 connected to node 16. To safely determine the content of array cell 4, i.e. whether the cell is erased or written, reference cell 8, when biased at a given read voltage, generates a reference current I R , the value of which is mirrored in array branch 6 and is between the value of the current generated by an erased array cell 4 and the value of the current generated by a written array cell 4. In the Figure 1 circuit, therefore, sense amplifier 17 compares the value of the voltage V R present at reference node 16 and related to current I R generated by reference cell 8, with the value of the voltage V M present at array node 15 and related to the difference between the current I M generated by array cell 4 and the current I O supplied by load 13 (and in turn equal to the current I R mirrored - multiplied by factor 1/N - in array branch 2). To give a clearer idea of the concept involved, the Figure 2 diagram shows the characteristics I DS =f(V GS ) of array and reference cells 4 and 8. More specifically, A indicates the characteristic of array cell 4 when erased (distributed about that of reference cell 8); B indicates the characteristic of array cell 4 when written; and C indicates the characteristic of reference cell 8 as mirrored by mirror circuit 13, 14 in array branch 2. As can be seen, in the Figure 1 circuit solution, the slope of mirrored characteristic C of reference cell 8 is less by a factor N than that of characteristics A, B of erased and written array cells 4, due to the current I O flowing in transistor 13 being N times less than that imposed by reference cell 8 in transistor 14; and the characteristic of reference cell 8 presents a reference threshold value equal to the threshold value V th1 of erased array cell 4. As shown in Figure 2, in the Figure 1 circuit solution, the maximum read voltage of array and reference cells 4 and 8 is substantially equal to the value at which characteristic B of written array cell 4 intersects characteristic C of reference cell 8, and, since the read voltage also represents the supply voltage of read circuit 1, there is therefore a limit to the maximum permissible supply voltage value V CC . To overcome the problem, according to a (not shown) solution described in EP-A-O 676 768 published on 11.10.95, the characteristic of reference cell 8 is approximated, not by a single straight line, but by a broken line comprising two segments: a first segment presenting the same slope as curve C in Figure 2 and comprised between the threshold voltage of an erased cell and a predetermined intermediate voltage; and a second segment presenting the same slope as curves A and B. By being divided into two segments, the characteristic of reference cell 8 (shown by the dotted-line curve D in Figure 2) lies between characteristics A and B and no longer intersects characteristic B of written array cell 4, even in the event of a high read, and hence supply, voltage. In the known solutions described, the ratio between the current flowing in loads 13 and 14 is essential, as stated, for distinguishing the written from the erased cells. In fact, as is known, to reduce the read time of array cells 4 when erased, the current flowing in array cell 4 must be much greater than that flowing in transistor 13 so as to rapidly discharge capacitor 18 and hence rapidly reduce the voltage at node 15; whereas, to reduce the read time of array cells 4 when written, the current generated by reference cell 8 must be high, so that the corresponding mirrored current I O in transistor 13 rapidly charges capacitor 18 and so brings the voltage at node 15 rapidly to a value close to that of supply voltage V CC . The present tendency, however, is to reduce supply voltage as far as possible to permit wider use of read circuits 1, even in portable equipment and low-supply applications. Reducing the supply voltage, however, results in a considerable reduction in the currents I R , I M generated by array and reference cells 4, 8 and hence an increase in the read time of circuit 1. A memory for low supply voltage applications in accordance with the preamble of claim 1 is disclosed in US 4,725,984. US 5,396,467 discloses a non-volatile memory having a differential sense amplifier. It is an object of the present invention to provide a read method and circuit permitting read times compatible with demand, even in the event of low supply voltage. According to the present invention, there are provided a method and circuit for reading low-supply-voltage nonvolatile memory cells, as claimed respectively in Claims 1 and 3. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows the electric diagram of a known read circuit; Figure 2 shows the characteristics for evaluating the memory cells according to the Figure 1 solution; Figure 3 shows the electric diagram of a read circuit in accordance with the present invention; Figure 4 shows the characteristics obtainable using the Figure 3 circuit; Figures 5 and 6 show plots of the output voltages of the Figure 3 read circuit. Number 50 in Figure 3 indicates a nonvolatile memory, e.g. a flash-EEPROM memory, comprising a memory array 51 including a plurality of array cells 33 connected to a read circuit 30 in accordance with the present invention. Like circuit 1 in Figure 1, read circuit 30 comprises an array branch 31 connected to an array cell 33 by an array bit line 34; and a reference branch 32 connected to a reference cell 37 by a reference bit line 38. Array branch 31 comprises an array converter 35 connected on one side to array bit line 34 and on the other to a supply line 36 at voltage V CC ; and reference branch 32 comprises a reference converter 39 connected on one side to reference bit line 38 and on the other to supply line 36. Like circuit 1 in Figure 1, each converter 35, 39 comprises a precharge and biasing circuit 40, 41 and a load 42, 43 connected in series with each other; the nodes 44, 45 between respective precharge and biasing circuits 40, 41 and respective loads 42, 43 are connected to the input of a sense amplifier 46. Unlike known read circuit 1, load 42 comprises a P-channel MOS transistor with a predetermined ratio W 1 /L 1 ; and load 43 comprises a diode-connected P-channel MOS transistor with a ratio W 2 /L 2 N times greater than ratio W 1 /L 1 . MOS transistors 42, 43 are connected to form a current mirror, so that the current flowing in transistor 43 is N times greater than that flowing in transistor 42. Like circuit 1 in Figure 1, each node 44, 45 is connected to an equivalent capacitor 47, 48 representing the capacitances of the cells connected to node 44, 45. Read circuit 30 operates in the same way as circuit 1, except that, in this case, sense amplifier 46 compares the value of the voltage V' M present at node 44 and related to the current I' M generated by array cell 33, with the value of the voltage V' R present at node 45 and related to the difference between the current I' R generated by reference cell 37 and the current I' O supplied by load 43 (and in turn equal to the current I' M mirrored - multiplied by factor N - in reference branch 32). Unlike read circuit 1 in Figure 1, therefore, it is current I' M generated by array cell 33 that is mirrored, instead of current I' R generated by reference cell 37. Moreover, using a single reference cell 37 outside memory array 51, its threshold voltage may be adjusted to a low value in the course of EWS (Electrical Wafer Sort) testing, so that reference cell 37 conducts a high current even in the presence of a low supply voltage V CC . On the other hand, a similar reduction in the threshold voltage of array cell 33 is not possible, as this would increase the number of depleted cells in memory array 51. The solution according to the present invention, however, provides for a high read current of array cells 33, and hence for reducing read time, by multiplying by N the current flowing in cells 33. Circuit 30 therefore provides for achieving the characteristics shown in Figure 4, in which E indicates the characteristic of array cell 33 when erased (coinciding with that of reference cell 37); F indicates the characteristic of array cell 33 when written; and G and H respectively indicate the erased and written characteristics of array cell 33 mirrored in reference branch 32. In Figure 4, V th1 and V th2 have the same meaning as in Figure 2. As can be seen, in this case, the slope of characteristics G, H of mirrored current I' O is greater by a factor N than the slope of characteristic E of reference current I' R with which current I' O is compared, so that the comparison is made between currents meeting both the switching requirements of circuit 30 when reading erased and written array cells 3, and the requirements governing the maximum read voltage that may be applied. In fact, when reading an erased array cell 33, on the one hand, cell 33 draws a current I' M supplied entirely by load 42, i.e. without discharging capacitor 47 connected to node 44; and, on the other hand, the current flowing in load 42, when amplified by mirror circuit 42, 43 in reference branch 32, rapidly charges capacitor 48 connected to node 45, by virtue of being much greater than the current I' R required by reference cell 37, thus rapidly pulling up the voltage at node 45. Similarly, when reading a written array cell 33, this does not draw current at the read voltage considered, so that, current I' O being substantially zero, the current I' R drawn by reference cell 37 rapidly discharges capacitor 48 connected to node 45, thus rapidly lowering the voltage V' R at node 45. Figures 5 and 6 show plots of voltages V' M , V' R at respective output nodes 44, 45 of array and reference converters 35, 39 as a function of supply voltage V CC (equal to read voltage V gs applied to the control terminals of array and reference cells 33, 37) and with a difference of 3 V (Figure 5) and 1 V (Figure 6) between the threshold voltage V th1 of a written array cell 33 and the threshold voltage V th2 of (virgin) reference cell 37. As shown in Figure 5, for supply voltages V CC , and hence read voltages V gs , below the threshold voltage V th1 of array cell 33, voltages V' M and V' R are equal and roughly follow supply voltage V CC , in that no current is drawn by either of cells 33, 37. For supply voltages between V th1 and V th2 , only reference cell 37 draws current, so that voltage V' R at node 45 falls rapidly and remains close to zero, while voltage V' M at node 44 continues to substantially follow supply voltage V CC (array cell 33 is still turned off). Finally, upon the supply voltage exceeding threshold V th2 , array cell 33 also starts conducting and drawing current, and voltage V' R at node 45 also follows supply voltage V CC . In Figure 6, the behaviour of cells 33, 37 as a function of supply voltage V CC is the same as in Figure 5, and the Figure 6 plot also shows output voltages V' M and V' R for supply voltages V CC well above thresholds V th1 , V th2 . More specifically, when supply voltage V CC reaches a predetermined value (about 7 V in the example shown), voltage V' R at node 45 exceeds voltage V' M at node 44, so that said value represents the maximum read voltage permissible for read circuit 30 to function properly. The advantages of read circuit 30 according to the present invention are as follows. In particular, it provides for reading cells 33 of memory array 51 even in the presence of low supply voltage, and with no increase in read time as compared with high-supply-voltage operation, by amplifying the current I' M generated by array cell 33, and, what is more, is both simple and reliable. Finally, the circuit solution adopted involves no increase in the fabrication cost or layout requirements of read circuit 30, by virtue of featuring no additional components and simply shifting a diode connection of a transistor. Clearly, changes may be made to read circuit 30 as described and illustrated herein without, however, departing from the scope of the present invention, as defined in the appended claims.

Claims: A low supply voltage nonvolatile memory (50) including a memory array (51); a read circuit (30) comprising an array branch (31) connected to an array cell (33) to be read; a reference branch (32) connected to a reference cell (37) of known content; amplifying means (42, 43) for amplifying an array current (I' M ) flowing in said array cell (33); supply means (42, 43) for supplying said amplified array current (I' O ) to said reference branch (32); characterized by comparing means (46) for comparing a quantity (V' M ) related to said array current (I' M ) with a quantity (V' R ) related to the difference between a reference current (I' R ) flowing in said reference cell (37) and said amplified array current (I' O ); said reference cell (37) being arranged outside said memory array (51). A memory as claimed in claim 1, characterized in that said amplifying means (42, 43) comprise a current mirror (42, 43) including a diode-connected array load transistor (42) arranged between a reference potential line (36) and said array cell (33), and having a first predetermined channel width/length ratio (W1/L1), and a reference load transistor (43) arranged between said reference potential line (36) and said reference cell (37), and having a second predetermined channel width/length ratio (W2/L2) N times greater than said first ratio (W1/L1). A read method for a low supply voltage nonvolatile memory (50) comprising a memory array (51) and a read circuit including an array branch (31) connected to an array cell (33), and a reference branch (32) connected to a reference cell (37); said read method comprising the steps of generating an array current (I' M ) flowing in said array cell (33); amplifying said array current (I' M ); supplying said amplified array current (I' o ) to said reference branch (32); and generating a reference current (I' R ) flowing in said reference cell (37); characterized by comparing a quantity (V' M ) related to said array current (I' M ) with a quantity (V' R ) related to the difference between said reference current (I' R ) and said amplified array current (I' O ); wherein said step of generating a reference current (I' R ) comprises the step of arranging said reference cell (37) outside said memory array (51). A read method as claimed in claim 3, characterized in that said steps of amplifying said array current (I' M ) and supplying said amplified array current (I' O ) to said reference branch (32) comprise the step of providing a current mirror (42, 43) including a diode-connected array load transistor (42) arranged between a reference potential line (36) and said array cell (33), and having a first predetermined channel width/length ratio (W 1 /L 1 ), and a reference load transistor (43) arranged between said reference potential line (36) and said reference cell (37), and having a second predetermined channel width/length ratio (W 2 /L 2 ) N times greater than said first ratio (W 1 /L 1 ).

Inventors: CAMPARDO GIOVANNI, COMMODARO STEFANO, RINO MICHELONI, CAMPARDO, GIOVANNI, COMMODARO, STEFANO, RINO, MICHELONI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G11C16
IPC codes: G11C16