Page-mode memory device with multiple-level memory cells

Abstract: A page-mode semiconductor memory device comprises a matrix (1) of memory cells (MC') arranged in rows (R) and columns (C), each row (R) forming a memory page (MP1-MPn) of the memory device and comprising at least one group (MW1,MW2-MW2m-1,MW2m) of memory cells (MC'), memory page selection means (2) for selecting a row (R) of the matrix (1), and a plurality of sensing circuits (3') each one associated to a respective column (C) of the matrix. The memory cells (MC') are multiple-level memory cells which can be programmed in a plurality of c=2 b (b>1) programming states to store b information bits, and the sensing circuits (3') are serial-dichotomic sensing circuits capable of determining, in a number b of consecutive approximation steps, the b information bits stored in the memory cells (MC'), at each step one of said b information bits being determined, said at least one group (MW1,MW2-MW2m-1,MW2m) of memory cells (MC') of a row (R) forming a number b of memory words of a memory page (MP1-MPn).

Description: The present invention relates to a page-mode memory device with multiple-level memory cells. Semiconductor memory devices are known which are organized in a plurality of so-called "memory pages". Fig. 1 schematically shows the structure of a conventional page-mode memory device: memory cells MC, for example EPROM or Flash EPROM memory cells, are arranged in rows R and columns C of a memory matrix 1. Each memory cell is for example a floating gate MOS transistor having a drain electrode connected to a respective column C, a source electrode connected to ground and a control gate electrode connected to a respective row R. Each row R of the memory matrix 1 forms a memory page MP1-MPn of the memory device, and contains a number of memory cells MC equal to the number of memory words MW1-MWm of each memory page multiplied by the number of bits per each memory word (word size): for example, in the case of a memory device organized in pages of sixteen memory words of sixteen bits per word, each row R of the memory matrix 1 contains 16*16=256 memory cells. A memory page is selected when the corresponding row R of the memory matrix 1 is selected. A line decoder circuit 2 is supplied with line address signals LADD for decoding a configuration of the line address signals and for accordingly selecting one of the rows R of memory cells of the memory matrix 1. A plurality of sensing circuits 3 is also provided, each one associated to a respective column C of the memory matrix 1. The number of sensing circuits 3 to be provided is strictly equal to the number of memory words per memory page, multiplied by the word size: in the above-mentioned example, 256 sensing circuits 3 are to be provided. A plurality of memory registers MR1-MRm is also provided for storing all the memory words MW1-MWm of a selected memory page MP1-MPn. Each memory register MR1-MRm has the same size as the size of the memory words (in the previous example, each memory register must be capable of storing sixteen bits). The plurality of memory registers MR1-MRm forms a so-called "line buffer". A word decoder circuit 5, supplied with word address signals WADD, allows to select one memory word among the plurality of memory words MW1-MWm of the selected memory page MP1-MPn, and to supply the selected memory word to output buffer circuits 6 of the memory device. A read operation of a page-mode memory device starts when one or more of the line address signals LADD change: the line decoder circuit 2 decodes the current configuration of the line address signals LADD and accordingly to the new configuration selects one of the rows R of memory cells of the memory matrix 1. The sensing circuits 3 read the content of the memory cells MC belonging to the selected row R, and the read data are stored in the memory registers MR1-MRm: in this way, all the memory words of the selected memory page are simultaneously read and the read data are stored in the line buffer, the information content of the selected memory page being loaded in the memory registers MR1-MRm of the line buffer. The word decoder circuit 5 selects, according to the configuration of the word address signals WADD, one of the plurality of memory words of the selected memory page and transfers the content of the selected memory word to the output buffer circuits 6. This operation takes place only when the line address signals LADD change, and takes a time equal to a conventional read operation in conventional, non-paged memory devices. Since however all the memory words of a memory page are simultaneously read and the read data are stored in the line buffer, each successive reading of a memory word of the same memory page is much faster, approximately one third of the read access time. In fact, in this case it is not necessary to read again the memory cells MC of the memory array 1, being sufficient to select, by changing the configuration of the word address signals WADD, another memory register of the plurality MR1-MRm. Changing of one of the line address signals LADD causes instead a new memory page to be loaded in the line buffer. The higher the number of memory words per memory page, the higher the probability that two consecutive accesses to the memory device "hit" the same memory page. This kind of memory devices are particularly suitable in high-speed applications requiring low read times. However, they occupy a significant chip area, due to the great number of sensing circuits which must be provided (as already mentioned, in word page-mode memory device having a word size of sixteen bits, 256 sensing circuits must be provided instead of the 16 required in a non-paged memory device with the same word size). As already mentioned, the number of sensing circuits is directly dependent on the word size, and on the number of memory words per memory page. Also, this kind of memory devices shows a significant power consumption, because all the sensing circuits operate simultanoeusly. In view of the state of the art described, it is an object of the present invention to provide a new page-mode memory device architecture which, given the memory word size and the number of memory words per memory page, allows for a reduced chip area and power consumption with respect to conventional non-paged memory devices. According to the present invention, such object is achieved by means of a page-mode semiconductor memory device comprising a matrix of memory cells arranged in rows and columns, each row forming a memory page of the memory device and comprising at least one group of memory cells, memory page selection means for selecting a row of the matrix, and a plurality of sensing circuits each one associated to a respective column of the matrix, characterized in that said memory cells are multiple-level memory cells which can be programmed in a plurality of c=2 b (b>1) programming states to store b information bits, and in that said sensing circuits are serial-dichotomic sensing circuits capable of determining, in a number b of consecutive approximation steps, the b information bits stored in the memory cells, at each step one of said b information bits being determined, said at least one group of memory cells of a row forming a number b of memory words of a memory page. Serial-dichotomic sensing circuits for sensing, according to a serial-dichotomic method, multiple-level memory cells (i.e., memory cells which can be programmed in a plurality of programming states and which store more than one bit of information) are disclosed in the co-pending European Patent applications No. 95830023.8 and No. 95830110.3, both in the name of the same Applicant. These circuits occupy an area substantially equal to that of conventional sensing circuits for reading two-level memory cells, and have a similar power consumption. Thanks to the present invention, it is possible to increase the number of memory words per memory page without increasing the number of sensing circuits. This allows to increase the memory capacity without significantly increasing the chip area and the power consumption of the memory device. These and other features of the present invention will be made more evident by the following detailed description of two particular embodiments, described as non limiting examples in the annexed drawings, wherein: Fig. 1 schematically shows a page-mode memory device according to the prior art; Fig. 2 schematically shows a page-mode memory device according to a first embodiment of the present invention, with four-levels memory cells; Fig. 3 schematically shows a sensing circuit of the memory device of Fig. 2; Fig. 4 schematically shows a variable reference current generator of the sensing circuit of Fig. 3; Fig. 5 shows a successive approximation register of the sensing circuit of Fig. 4; Fig. 6 diagramatically shows the distribution of currents sunk by a four-level memory cells, and the distribution of reference currents generated by the variable reference current generator of Fig. 4; Figs. 7 and 8 diagramatically shows the steps of the serial-dichotomic sensing method for sensing a four-level memory cell in two different programming states. Fig. 9 schematically shows a page-mode memory device according to a second embodiment of the invention, with generally multiple-level memory cells. Fig. 1 schematically shows a conventional page-mode memory device, and has already been described. Fig. 2 schematically shows the main circuit blocks of a page-mode memory device according to a first embodiment of the present invention. As in conventional page-mode memory devices, a memory matrix 1 of memory cells MC' is provided, the memory cells MC' being arranged in rows R and columns C. The memory cells MC' are for example floating-gate MOS transistors of the type conventionally used in EPROM or Flash EEPROMs devices. A line decoder 2 is supplied by Line address signals LADD which allow to select one row R of the memory matrix at a time. Each row R forms a memory page MP1-MPn of the memory device. To each column C of the memory matrix 1 there is associated a respective sensing circuit 3' for reading the information content of the memory cell MC' belonging to the selected row R. In this embodiment of the invention, it is supposed for the sake of simplicity that each memory cell MC' is a four-levels memory cell, i.e. a memory cell capable of assuming four different programming states; a four-level memory cell, differently from conventional two-levels memory cells, is capable of storing two bits of information, instead of a single information bit. However this is not to be intended as a limitation of the invention, the latter providing more generally for the use of multiple-level memory cells capable of storing b bits of information. The sensing circuits 3', differently from conventional sensing circuits used to read two-levels memory cells, must be capable of discriminating the four different programming states of the memory cells. Each sensing circuit 3' has two respective output signals 01 and 02, corresponding to the two bits of information that are stored in the memory cell MC' which is to be read. A first one 01 of the two output signals of a given sensing circuit 3' supply a respective memory unit of a memory register MR11-MR1m, and the other output signal 02 of the sensing circuit 3' supplies a respective memory unit of another memory register MR21-MR2m. Each memory register MR11-MR1m and MR21-MR2m has a size equal to the size of the memory words of the memory device. A word decoder 5, supplied with word address signals WADD, allows to select one memory word of the memory page, and to supply the selected word to output buffer circuits 6. It can be seen that in the page-mode memory device according to this first embodiment of the invention, using four-levels memory cells MC', the number of memory words MW1-MW2m per memory page MP1-MPn is double with respect to a conventional page-mode memory device having the same number of memory cells and the same number of sensing circuits, with a significant reduction of chip area and power consumption. This is made possible by the use of four-levels memory cells instead of conventional two-levels memory cells, and using sensing circuits 3' capable of discriminating the four different programming states of the four-level memory cells. Advantageously, the sensing circuits 3' are of the type described in the co-pending European Patent Application No. 95830023.8 or in the co-pending European Patent Application No. 95830110.3, both in the name of the same Applicant, the content of which is enclosed herein by reference. The sensing circuits 3' operate according to a serial-dichotomic sensing method which is also described in the first one of the two above-mentioned patent applications. The serial dichotomic sensing method, applied to the sensing of a multiple-level memory cell which can be programmed in a plurality of c=2 b programming states which correspond to a discrete set of cell current values, is a serial sensing method which allows to obtain the digital code stored in the memory cell after a number b of steps independently of the particular programming state of the memory cell itself. In a first step, the cell current is compared with a first reference current having a value comprised between a minimum and a maximum value of said set of cell current values, thus dividing said set of cell current values in two sub-sets to determine to which of the two sub-sets of cell current values the cell current belongs. In a second step, the cell current is compared with a second reference current having a value comprised between a minimum and a maximum value of said sub-set, thus dividing the sub-set of cell current values in two further sub-sets to determine to which of said further sub-sets the cell current belongs, and so on until in the last step only two memory cell currents are left. Fig. 3 schematically shows the structure of the sensing circuits 3'. The circuit substantially comprises a digitally-driven variable reference current generator G for generating a variable reference current IR, a current comparator 7 for comparing the reference current IR generated by the variable reference current generator G with a current IC sunk by a memory cell MC' to be read (which is biased by a control gate voltage VG) and a successive approximation register 8 supplied by an output CMP of comparator 7. The successive approximation register 8 is further supplied by a preset signal PR and a timing signal CK, and generates the two output signals 01, 02 of the sensing circuit and digital control signals Q0, Q1 for the variable reference current generator G. Fig. 4 schematically shows the structure of the variable reference current generator G, which comprises three distinct reference current generators IR0, IR1 and Ioff. Ioff is an offset current generator which generates a constant current and is permanently connected to the non-inverting input of the current comparator 7. IR0 and IR1 can instead be selectively connected to the non-inverting input of current comparator 7 by means of respective switches SW0, SW1 respectively activated by the control signals Q0 and Q1. Fig. 5 is a circuit diagram of the successive approximation register 8. The circuit comprises two delay-type flip-flops FF0, FF1, each one having a clock input CK and a preset input PR. The clock inputs CK and the preset inputs PR of flip-flops FF0, FF1 are commonly connected to the timing signal CK and to the preset signal PR (more precisely, FF0 receives the logic complement of PR, as indicated by the Logic inversion dot at the input PR of FF0). Each flip-flop FF0, FF1 has a data input D0, D1, a "true" data output Q0, Q1 and a "logic complemented" data output Q0N, Q1N which is the logic complement of Q0, Q1, respectively. As known, in a delay-type flip-flop the true data output after a clock pulse takes the Logic value of the data input during said clock pulse. The data input D0 of flip-flop FF0 is supplied with the complemented data output Q0N of FF0 itself. The data input D1 of flip-flop FF1 is supplied with an output of a NOR gate 9 whose inputs are the output signal CMP of the current comparator 7 and and the complemented data output Q0N of flip-flop FF0. The output signal CMP of the current comparator 7, once complemented, is the output signal 01 of the sensing circuit 3', and forms the least significant bit of a two-bit code stored in the memory cell MC' to be sensed; the true data output Q1 of flip-flop FF1 is the output signal 02 of the sensing circuit 3' and forms the most significant bit of said two-bit code. The true data outputs Q0, Q1 of flip-flops FF0, FF1 are the control signals for the switches SW0, SW1 of the variable reference current generator G; switch SW0,SW1 closes when the respective control signal Q0, Q1 is a logic "1", otherwise switch SW0, SW1 is open. The preset signal is used at the circuit power-up to assure that the starting condition of flip-flops FF0, FF1 is that corresponding to Q0="1" and Q1="0", i.e. to a reference current IR=Ioff+IR0. The operation of the sensing circuit 3' will be now described making reference to the diagrams of Figs. 6 to 8. Fig. 6 is a diagram showing, in the ordinate axis, the distribution of current values IC for a four-levels memory cell to be sensed. When the memory cell is biased with a control gate voltage VG, the four different programming states correspond to four different values IC0-IC3 of the current IC sunk by the memory cell. Fig. 6 also shows, on branches of a decision tree, the three different values I0-I2 that can be taken by the reference current IR; it is to be noted that the reference current values I0-I2 are intermediate between successive values of the memory cell current IC. I0 is equal to Ioff, I1 is equal to Ioff+IR0 and I2 is equal to Ioff+IR1. As a first example of operation of the serial dichotomic sensing method, it is first assumed that the programming state of the memory cell to be read corresponds, once biased, to a current IC=IC2 (Fig. 7). As previously mentioned, flip-flops FF0, FF1 are preset in a condition such that Q0="1" and Q1="0", corresponding to IR=Ioff+IR0=I1. In the first step of the sensing process, starting at the rising edge of timing signal CK current comparator 7 compares the cell current IC with the reference current I1; since IC=IC2 is higher than I1, signal CMP="0", and on the falling edge of timing signal CK the logic state of the true data outputs Q0, Q1 changes to Q0="0" and Q1="1". This causes switch SW0 to open and switch SW1 to close, so that in the next step of the sensing process the cell current IC is compared with a reference current IR=Ioff+IR1=I2. It is to be noted that at the end of the first step of the sensing process the output signal 02 of the sensing circuit 3' already provides the correct value of the most significant bit of the two-bit code stored in the memory cell to be read. Since IC is now Lower than IR, at the rising edge of signal CK signal CMP="1" and the output signal 01 of the sensing circuit, corresponding to the least significant bit of the two-bit code stored in the memory cell, is equal to "0". At the next falling edge of the timing signal CK flip-flops FF0, FF1 are automatically preset to the initial state Q0="1", Q1="0", and the sensing circuit is ready to start another sensing process. As a second example, it is assumed that the programming state of the memory cell to be read corresponds to a current IC=IC0 (Fig. 8). Again, the starting condition of the successive approximation register 8 corresponds to Q0="1" and Q1="0", i.e. switch SW0 closed and switch SW1 open; the reference current IR is equal to Ioff+IR1, i.e. to I1. In the first step of the sensing process, starting at the rising edge of timing signal CK current comparator 7 compares the cell current IC with the reference current I1; since IC=ICO is Lower than I1, signal CMP="1", and on the falling edge of timing signal CK the logic state of the true data outputs Q0, Q1 changes to Q0="0" and Q1="0". This causes switch SW0 to open, so that in the next step of the sensing process the cell current IC is compared with a reference current IR=Ioff=I0. It is to be noted that at the end of the first step of the sensing process the output signal O2 of the sensing circuit 3' already provides the correct value of the most significant bit of the two-bit code stored in the memory cell to be read. Since IC is again lower than IR, at the rising edge of signal CK signal CMP="0" and the output signal O1 of the sensing circuit, corresponding to the Least significant bit of the two-bit code stored in the memory cell, is equal to "0". At the next falling edge of the timing signal CK flip-flops FF0, FF1 are automatically preset to the initial state Q0="1", Q1="0", and the sensing circuit is ready to start another sensing process. In both cases, the most significant bit 02 of the two-bit code stored in the memory cell to be read is available after the first step of the sensing process, while the least significant bit 01 of said two-bit code is available after (3/2)T from the beginning of the sensing process, where T is the period of the timing signal CK, i.e. before the end of the second step of the sensing process. Also, at the end of the second step of the sensing process the successive approximation register 8 automatically presets in the correct initial condition. With reference to Fig. 2, when a row R of the memory matrix 1 is selected, after the first step of the sensing process the memory registers MR21-MR2m store the first m memory words of the selected memory page, and after the second step also the memory registers MR11-MR1m store the remaining m memory words of the memory page. Obviously, the time needed for reading the content of the 2m memory words of a memory page is higher than that necessary to read m memory words in a conventional page-mode memory device with two-level memory cells. However, twice the number of memory words are now stored in the line buffer, so that the higher time needed to load the content of a memory page in the line buffer is compensated by the fact that there is a greater probability that a subsequent access to the memory device "hits" a memory word belonging to the same memory page. The higher time necessary to load a memory page in the line buffer is thus compensated by a higher probability of high-speed accesses to the line buffer. Fig. 9 is a generalization of the structure shown in Fig. 2 to the case of memory cells MC' capable of being programmed in c=2 b distinct states, where b is the number of information bits that can be stored in the memory cell (multiple-level memory cells). Obviously, the sensing circuits 3' must be able of discriminating the c distinct programming states of the memory cells, and have b output signals 01-0b. Output signal 01 supplies a respective memory unit of a first memory register MR11, output signal 02 supplies a respective memory unit of a memory register MR21, and so on, output signal 0b supplying a respective memory unit of a memory register MRb1. It is possible to see that in this case the number of memory words per memory page is b times the number of memory words of a conventional page-mode memory device having the same number of memory cells and the same number of sensing circuits, with a great increase in the memory capacity without a corresponding increase of the chip area and of the power consumption. As in the previous particular example relating to four-levels memory cells, the sensing circuits can be advantageously of the kind suitable to implement the serial-dichotomic sensing method. The structure of the sensing circuits 3' can be easily derived by generalization from the structure shown in Figs. 3 to 5. The number of steps required to complete a sensing process of a memory cell with c=2 b different programming states is equal to b. It should be underlined that no limitations exist on the number b of bits that can be stored in the memory cells: thanks to the use of a page-mode architecture, the number of bits per memory cell is not necessarily a power of 2 (2,4,8,16... bits per memory cell), but can be any integer number. Finally, it should be noted that the line buffer (memory registers MR11-MR1m, MR21-MR2m) and the memory word selection circuit 5 could be eliminated, provided that the possibility of a random access to the memory words of a memory page is abandoned for a sequential reading of the memory words of the memory page. For each group (MW1,MW2)-(MW2m-1,MW2m) of memory cells it is necessary to provide a parallel input-serial output (PISO) memory register for generating a serial stream of bits. At each step of the serial-dichotomic sensing process, the PISO registers are loaded with new memory words of the selected memory page.

Claims: Page-mode semiconductor memory device comprising a matrix (1) of memory cells (MC') arranged in rows (R) and columns (C), each row (R) forming a memory page (MP1-MPn) of the memory device and comprising at least one group (MW1,MW2-My2m-1,MW2m) of memory cells (MC'), memory page selection means (2) for selecting a row (R) of the matrix (1), and a plurality of sensing circuits (3') each one associated to a respective column (C) of the matrix, characterized in that said memory cells (MC') are multiple-level memory cells which can be programmed in a plurality of c=2 b , wherein b>1, programming states to store b information bits, and in that said sensing circuits (3') are serial-dichotomic sensing circuits capable of determining, in a number b of consecutive approximation steps, the b information bits stored in the memory cells (MC'), at each step one of said b information bits being determined, said at least one group (MW1,MW2-MW2m-1,MW2m) of memory cells (MC') of a row (R) forming a number b of memory words of a memory page (MP1-MPn). Memory device according to claim 1, characterized in that each row (R) comprises a plurality of said groups (MW1,MW2-MW2m-1,MW2m) of memory cells (MC'), each group forming a number b of memory words. Memory device according to claim 2, characterized by comprising a plurality of memory registers (MR11-MR1m,MR21-MR2m;MR11-MR1b) equal to the number of memory words (MW1,MW2-MW2m-1,MW2m) contained in each memory page (MP1-MPn), said plurality of memory registers being divided in groups of b memory registers associated to respective groups of b sensing circuits (3'), each sensing circuit (3') having a number b of output signals (01,02;01-0b) each one supplying a respective memory unit of a respective memory register of the associated group of memory registers, each group of memory registers being loaded by the b memory words stored in a respective group (MW1,MW2-MW2m-1,MW2m) of memory cells (MC') of a selected row (R). Memory device according to claim 3, characterized by comprising memory word selection means (5) for selecting one among said plurality of memory registers (MR11-MR1m,MR21-MR2m;MR11-MRb1). Memory device according to claim 1, characterized in that a parallel input-serial output memory register is associated to said at least one group (MW1,MW2-MW2m-1,MW2m) for storing, at each approximation step, one of the b memory words of the memory page, and for serially outputting the bits of said memory word. Memory device according to claim 1, characterized in that each sensing circuit (3') comprises a variable reference current generator (G) generating a reference current (IR) which can take values (I0-I2) intermediate to a plurality of values (IC0-IC3) of a memory cell current (IC) sunk by a memory cell (MC') belonging to the column (C) associated to the sensing circuit and to the row (R) selected by said memory page selection means (2), a current comparator (7) for comparing the memory cell current (IC) with the reference current (IR), and a successive approximation register (8) supplied with an output signal (CMP) of the current comparator (7) and generating control signals (Q0,Q1) for the variable reference current generator (G). Memory device according to claim 6, characterized in that said variable reference current generator (G) comprises an offset current generator (Ioff) permanently coupled to a reference input of the current comparator (7), and a plurality of b-2 distinct current generators (IR0,IR1) independently activatable by the successive approximation register (8). Memory device according to claim 6, characterized in that the successive approximation register comprises a sequential circuit (FF0,FF1,9) which, starting from an initial state causing the variable reference current generator (G) to generate a reference current (IR) with value (I1) intermediate between a lowest (IC0) and a highest (IC3) value of the memory cell current (IC) and dichotomizing the plurality of values of the memory cell current (IC), evolves through a succession of states each one determined by the preceding state and by the output signal (CMP) of the current comparator (7), each state of the sequential circuit causing the variable reference current generator (G) to generate a respective reference current (IR) with value intermediate a minimum value and a maximum value of a sub-plurality of memory cell current values to which the cell current belongs and which has been determined in the preceding step. Memory device according to claim 8, characterized in that said sequential circuit (FF0,FF1,9) automatically presets to said initial state after the sensing of a memory cell (MC') has been completed. Memory device according to claim 9, characterized in that said memory cells (MC') are four-level memory cells being programmable in four different programming states (IC0-IC3) to store two bits of information, said variable reference current generator (G) comprises an offset current generator (Ioff) permanently coupled to a reference input of the current comparator (7) and a first and a second selectively activatable current generators (IR0,IR1), said sequential circuit (FF0,FF1,9) comprising a first delay-type flip-flop (FF0) having a data output controlling the first activatable current generator (IR0) and a data input (D0) connected to a complemented data output (Q0N) of the first flip-flop (FF0), and a second delay-type flip-flop (FF1) having a data output (Q1) controlling the second activatable current generator (IR1) and a data input (D1) connected to an output of NOR logic means (9) supplied with the complemented data output (Q0N) of the first flip-flop (FF0) and with the output signal (CMP) of the current comparator, the first and second flip-flops (FF0,FF1) having timing inputs (CK) supplied by a timing signal (CK).

Inventors: CALLIGARO CRISTIANO, CAPPELLETTI PAOLO, GASTALDI ROBERTO, MANSTRETTA ALESSANDRO, TORELLI GUIDO, CALLIGARO, CRISTIANO, CAPPELLETTI, PAOLO, GASTALDI, ROBERTO, MANSTRETTA, ALESSANDRO, TORELLI, GUIDO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G11C11
IPC codes: G11C11, G11C16