Semiconductor memory device generating accurate internal reference voltage

Abstract: A semiconductor memory device has a memory cell array that outputs cell current to data sensing amplifiers and reference current to reference amplifiers. The reference amplifiers convert the reference current to a reference voltage. The data sensing amplifiers use the reference voltage to convert the cell current to a data voltage signal. According to a first aspect of the invention, reference current is supplied to a reference amplifier from two parallel data paths, each having approximately equal numbers of transistors of two types, one type of which is always switched on, the other type switching on and off. According to a second aspect of the invention, each data sensing amplifier, and the reference amplifier to which it is connected, receive currents from parts of the memory cell array having identical layouts.

Description: The present invention relates to a semiconductor memory device in which data values are sensed by current-to-voltage conversion, more particularly to the provision of a suitable reference current and reference voltage in this type of memory device. Current signals are used for data transport in, for example, non-volatile memory devices with single-transistor memory cells. When selected, the memory cells either conduct or block current, depending on the stored data value. Sensing circuits in the memory device convert the current flow to a data voltage signal Vs that has a high level or a low level, depending an the stored data value. Such a memory device also has reference memory cells that always conduct current when selected. The sensing circuits convert a reference current received from the reference memory cells to a reference voltage signal Vref having a level intermediate between the high and low Vs levels. The difference between Vs and Vref is amplified to produce a '1' or '0' data output signal. For quick and reliable amplification of the Vs-Vref difference, the reference voltage level Vref is preferably disposed halfway between the two Vs levels. In conventional memory devices, however, depending on the location of the memory cell being read, or on the data stored in other memory cells, the Vref level may be quite close to one of the two Vs levels. A resulting problem is that extra time must be allowed for amplification of the Vs-Vref difference, so the stored data cannot be accessed quickly. In the worst case, the wrong data value may be read. Further explanation of this problem will be given in the detailed description of the invention. US 4,648,074, on which the preamble of claim 1 is based, discloses a reference circuit which is particularly useful in combination with a memory array in which data transistors are arranged in rows and columns and in which the conduction paths of the data transistors along each column are connected in series between a common ground line and a bit line forming a stack. A problem exists in that the signal current flowing in a stack varies as a function of the position of a selected data transistor along the stack and of the information stored in the other data transistors of the stack. The reference circuit includes means for generating a different reference current corresponding to each row of data transistors along the stacks, where each reference current mirrors and tracks a particular signal condition of die data transistors being sensed in its corresponding row. US 5,680,357 discloses an electronic memory. The electronic memory includes memory cells and corresponding reference memory cells. Wordlines are connected to the memory cells and the corresponding reference memory cells. If an accessed memory cell has previously been programmed to a first state, an output from the memory cell is connected to a bitline. An output from a selected reference memory cell is connected to a reference bitline. The bitline and the reference bitline are connected to a differential input sense amplifier. The differential input sense amplifier detects the voltage differential between the bitline and the reference bitline. If the memory cell is accessed and the memory cell is in die first state, the memory cell drives the voltage potential on the bitline from a precharge voltage to a discharge voltage at a first rate. If the memory cell is selected and the memory cell is in a second state, the memory cell does not effect the voltage potential on the bitline and the voltage potential on the bitline remains at the precharge voltage. If die reference memory cell is selected, the reference memory cell drive the voltage potential on the reference bitline from the precharge voltage to the discharge voltage at a second rate. The first rate is faster than the second rate. Therefore, if the selected memory cell is in die first state, the differential sense amplifier will output a first voltage level. If the selected memory cell is in the second state, the differential amplifier will output a second voltage level. A general object of the present invention is to shorten the read access time of a semiconductor memory device. A more specific object is to provide a suitable reference current and reference voltage for use in reading data in a semiconductor memory device. According to a first aspect of the invention, a semiconductor memory device has two types of transistors with different on-resistances. The transistors have control terminals coupled to a plurality of word lines. Transistors of the type with the higher on-resistance are switched on and off according to the word-line potential. Transistors of the other type remain switched on regardless of the word-line potential. The semiconductor memory device has at least one column of transistors of these types coupled in series, conducting a cell current when all switched on, each of the transistors constituting a memory cell. The semiconductor memory device also has a reference amplifier receiving a reference current and generating a reference voltage, and a data sensing amplifier using the reference voltage to convert the cell current to a data voltage signal. The reference current is supplied by a reference column having two parallel current paths joining at a common node, which is coupled to the reference amplifier. Each current path has substantially equal numbers of transistors but at least three of the above two types, coupled in series. Each word line is coupled to the control terminals of one transistor on each current path, these two transistors being of opposite type. Reference current flows on one of the two parallel current paths at a time. A suitable reference current is obtained, because substantially equal numbers of transistors of the two types are present on the current path. A suitable reference voltage is therefore obtained. In the attached drawings: FIG. 1 is a circuit diagram illustrating a conventional semiconductor memory device with a NAND architecture; FIG. 2 is a waveform diagram illustrating the reading of data from the memory device in FIG. 1; FIG. 3 is a circuit diagram of another semiconductor memory device with a NAND architecture, embodying the present invention; FIG. 4 is a waveform diagram illustrating the reading of data from the memory device in FIG. 3; FIG. 5 is a circuit diagram illustrating a variation of the memory device in FIG. 3; Embodiments of the invention will be described with reference to the attached drawings, in which like parts are indicated by like reference characters. All of the embodiments are non-volatile semiconductor memory devices. Two conventional non-volatile semiconductor memory devices closely related to the embodiments will also be described. Referring to FIG. 1, the first related conventional non-volatile semiconductor memory device comprises an array of field-effect transistors, some of which are enhancement-mode n-channel metal-oxide-semiconductor (NMOS) transistors 1, the remainder being depletion-mode n-channel metal-oxide-semiconductor (DMOS) transistors 2. The gate terminal (the control terminal) of each of these transistors 1, 2 is coupled to one of a plurality of word lines, designated WL0 to WLn. An NMOS transistor 1 conducts current when its gate terminal is at a positive voltage level referred to below as the high logic level or Vcc level, and blocks current when its gate terminal is at the low logic level or ground level. An NMOS transistor thus has a positive threshold voltage intermediate between the high and low logic levels. A DMOS transistor 2 has a negative threshold level, and conducts current both when its gate terminal is at the high logic level and when its gate terminal is at the low logic level. The memory cell array includes a data-cell column 10A comprising a plurality of NMOS transistors 1 and/or DMOS transistors 2, which are coupled in series on a bit line B. Each transistor 1, 2 in this column 10A constitutes a memory cell. The value of the data stored in the cell depends on whether the transistor is a DMOS or an NMOS transistor. The array also includes an associated reference-cell column 10B comprising only DMOS transistors 2, which are coupled in series to a reference bit line RB. The bit line B and reference bit line RB both receive power at the Vcc voltage level from a power source such as an external terminal (not visible) of the semiconductor memory device. The bit line B is coupled through further transistors 11a, 11b, 12a, 12b to a data sensing amplifier 13. The data sensing amplifier 13 comprises a p-channel metal-oxide-semiconductor (PMOS) transistor 13a and two NMOS transistors 13b, 13c. The source terminal of PMOS transistor 13a receives Vcc from the power source (indicated by an arrow). The drain terminals of PMOS transistor 13a and NMOS transistor 13b are mutually coupled to the gate terminal of NMOS transistor 13c, and to an output terminal for a data voltage signal Vs. The source terminal of NMOS transistor 13b and drain terminal of NMOS transistor 13c are mutually coupled to the bit line B. The source terminal of NMOS transistor 13c is coupled to ground. Transistors 13a, 13b, 13c are thus coupled in series between the power source and ground. The reference bit line RB is coupled through transistors 14a, 14b, 15a, 15b to a reference amplifier 16. The reference amplifier 16 comprises a PMOS transistor 16a coupled in series with two NMOS transistors 16b, 16c between the power source and ground, and another PMOS transistor 16d coupled in series with two more NMOS transistor 16e, 16f between the power source and ground. The drain terminals of transistors 16a, 16b, 16d, 16e are mutually coupled to the gate terminals of transistors 16b, 16c, 16e, 16f, to the gate terminal of transistor 13b in the data sensing amplifier 13, and to an output terminal for a reference voltage Vref. The source terminals of NMOS transistors 16b, 16e and the drain terminals of NMOS transistors 16c, 16f are mutually coupled to the reference bit line RB. The gate terminals of PMOS transistors 13a, 16a, 16d are grounded, so that these transistors are permanently switched on. The gate terminals of NMOS transistors 11a, 14a and DMOS transistors 11b, 14b receive respective gate control signals CG0, CG1 from a control circuit (not visible). The gate terminals of NMOS transistors 12a, 12b, 15a, 15b receive identical column selection signals from a column decoder (not visible). This memory device is said to have a NAND architecture, because of the general resemblance of the data-cell column 10A to an NMOS NAND logic gate. For simplicity, FIG. 1 shows only a single data-cell column, but the memory device may also include other data-cell columns 10A and reference-cell columns 10B coupled to word lines WL0 to WLn, having respective data sensing amplifiers 13 and reference amplifiers 16. Next, the reading of data from one of the memory cells in FIG. 1 will be described. The word lines WL0 to WLn are normally held at the high logic level, so that all transistors 1, 2 in the data-cell column 10A and reference-cell column 10B are switched on. Read access starts when one of the word lines is driven to the low logic level, selecting the row of memory cells to which the word line is coupled. In addition, transistors 11a, 11b, 12a, 12b, 14a, 14b, 15a, 15b are switched on by the gate control signals CG0 and CG1 and column decoder. Since DMOS transistors 2 conduct current regardless of the logic levels of the word lines, when transistors 14a, 14b, 15a, 15b are switched on, a reference current Iref flows from the all-DMOS reference-cell column 10B through the reference amplifier 16 to ground. If the transistor in the selected row of the data-cell column 10A is a DMOS transistor 2, a similar data-cell current Icell flows to the data sensing amplifier 13, reaching ground through NMOS transistor 13c. If the selected transistor in the data-cell column 10A is an NMOS transistor 1, however, this transistor 1 switches off and the flow of data-cell current Icell is blocked. When the data-cell current Icell is blocked, the data sensing amplifier 13 holds the data voltage signal Vs at a comparatively low level, near the threshold level of NMOS transistor 13c. When Icell current flows, the drain potential of NMOS transistor 13c and the source potential of NMOS transistor 13b rise, reducing the gate-source voltage and hence the conductivity of NMOS transistor 13b, thereby raising the data voltage signal Vs. As Icell increases, Vs continues to rise. The data sensing amplifier 13 thus functions as a transresistance amplifier, converting the received cell current Icell to a voltage Vs that increases as Icell increases. The reference amplifier 16 operates in substantially the same way, generating a reference voltage Vref in response to the reference current Iref. Because the reference current is branched through two NMOS transistors 16c, 16f, however, for a given amount of reference current Iref, the reference voltage Vref is only about half as high as the data voltage level Vs produced by the same amount of data-cell current Icell. Because an NMOS transistor 1 has a higher on-resistance than a DMOS transistor 2, the data-cell current Icell varies, depending on the data stored in all the memory cells in the data-cell column 10A. FIG. 2 illustrates this effect when word line WL2, which is coupled to a DMOS transistor in data-cell column 10A, is selected after word line WL3, which is coupled to an NMOS transistor in data-cell column 10A. The vertical axis indicates voltage levels in volts, the high logic level corresponding to Vcc. The horizontal axis indicates time (t) in nanoseconds (ns). If the data-cell column 10A has a maximum number of DMOS transistors (if only the transistor coupled to word line WL3 is an NMOS transistor, for example), the data voltage signal Vs rises to a level Vsmax well above the reference level Vref. If the data-cell column 10A has the minimum number of DMOS transistors (if only the transistor coupled to word line WL2 is a DMOS transistor, for example), the increased on-resistance of the other (NMOS) transistors reduces the data-cell current, and the data voltage signal Vs rises to a level Vsmin only slightly above the reference level Vref. To obtain output data, the difference between Vs and Vref is amplified to the Vcc level, by circuits not shown in the drawings. The small difference ΔVmin between Vsmin and Vref takes time to amplify, leading to the problem of slow data readout, and potentially leading to erroneous data. As a first embodiment of the invention, FIG. 3 illustrates a memory device with a NAND architecture similar to that described above, except that the reference-cell column 17 has two parallel current paths 18, 19, both receiving Vcc from the power source (not visible). On the first current path 18, NMOS transistors 1 alternate with DMOS transistors 2 in a series starting with an NMOS transistor having its gate terminal coupled to word line WL1 and ending with a DMOS transistor having its gate terminal coupled to word line WLn. On the second current path 19, NMOS transistors 1 alternate with DMOS transistors 2 in a similar series, starting with a DMOS transistor having its gate terminal coupled to word line WL1 and ending with an NMOS transistor having its gate terminal coupled to word line WLn. Accordingly, half of the transistors on each current path 18, 19 are NMOS transistors, the other half are DMOS transistors, and each word line is coupled to the gate terminals of one NMOS transistor and one DMOS transistor in the reference-cell column 17. The two current paths 18, 19 pass through separate pairs of gate-control transistors 14a, 14b, 14c, 14d. The two paths then merge at a node N1 and are coupled through a single pair of column selection transistors 15a, 15b to a reference amplifier 16, which has the same structure as in the conventional memory device described above. The data-cell column 10A, transistors 11a, 11b, 12a, 12b, and data sensing amplifier 13 are also the same as in the conventional memory device. Repeated descriptions of these circuit elements will be omitted. Next, the reading of data from the memory device in FIG. 3 will be described. All word lines WL0 to WLn are normally held at the high logic level, so that all transistors in the reference-cell column 17 are switched on. When read access begins, one of the word lines is driven to the low level, interrupting one of the two current paths 18, 19 in the reference-cell column 17. For example, if word line WL3 is driven low, the second current path 19 is interrupted, because the NMOS transistor coupled to word line WL3 on this current path is switched off. The other current path 18 is not interrupted, because the DMOS transistor on word line WL3 on this path 18 remains in the on-state. Reference current Iref therefore flows on just one of the two current paths 18, 19 to the reference amplifier 16. Less reference current Iref flows than in the conventional memory device described above, because half of the transistors on the current path are NMOS transistors 1, which have a higher on-resistance than DMOS transistors 2. The resulting reference voltage Vref is accordingly lower than in the conventional memory device. As in the conventional memory device, if the transistor on the selected word line in the data-cell column 10A is a DMOS transistor 2, data-cell current Icell flows into the data sensing amplifier 13, the size of the current depending on the total number of DMOS transistors 2 in the data-cell column 10A. FIG. 4 shows the resulting data voltage signal Vs waveforms, the horizontal and vertical axes representing time and voltage as in FIG. 2. The Vs waveforms in FIG. 4 are generally similar to the Vs waveforms in FIG. 2, rising to a comparatively high level Vsmax when the number of DMOS transistors 2 in the data-cell column 10A is maximal, and to a somewhat lower level Vsmin when the number of DMOS transistors is minimal. The reference voltage level Vref, however, is now considerably lower than both of these levels Vsmax and Vsmin. The Vref level is approximately one half of a level halfway between Vsmax and Vsmin. Even in the minimal case, the difference ΔVmin between Vsmin and Vref is large enough to be amplified quickly and reliably, providing valid output data in a short time. The problems of slow read access and, in the worst case, unreliable data, which were present in the conventional memory device in FIG. 1, are accordingly solved. A further advantage of the first embodiment is that the alternating layout of DMOS transistors and NMOS transistors in the reference-cell column 17 reduces the effect of mask alignment error and other factors that impair the uniformity of the semiconductor fabrication process, thereby improving the accuracy of the reference voltage and raising the yield of the fabrication process. FIG. 5 shows a variation of the first embodiment in which all of the transistors in the data-cell column 10A and reference-cell column 17 are NMOS transistors 1. In the data-cell column 10A, the source and drain terminals of an NMOS transistor 1 are mutually short-circuited to store one data value, and are left in the normal condition (not short-circuited) to store the opposite data value. In the reference-cell column 17, NMOS transistors 1 having short-circuited source and drain terminals alternate with NMOS transistors that are not short-circuited in this way. The short-circuited NMOS transistors 1 are equivalent in function to the DMOS transistors in FIG. 3, conducting current regardless of the word-line logic level. The on-resistance of the short-circuited NMOS transistors 1 is also lower than the on-resistance of the NMOS transistors that are not short-circuited. The short circuits can be formed by metal interconnections or fuses, or by diffusion of impurities into the semiconductor substrate. As described above, the present invention provides improved access speed and data reliability in semiconductor memory devices with both NAND and NOR architectures. Memory devices to which the invention can be applied include, for example, various types of read-only memories and flash memories. In a flash memory, NMOS, DMOS, and EMOS transistors are created by storing different amounts of positive or negative charge in a floating gate. In the first embodiment, the NMOS transistors 1 and DMOS transistors 2 in do not necessarily have to be coupled in an alternating sequence on each current path 18, 19 in the reference-cell column 17. It is only necessary for each current path to have substantially equal numbers of NMOS transistors and DMOS transistors, and for each word line to be coupled to an NMOS transistor on one current path and a DMOS transistor on the other current path. The invention is not restricted to the NAND and NOR architectures. It can also be practiced in a memory device having an AND architecture, for example.

Claims: A semiconductor memory device having transistors with respective control terminals, the transistors being of a first type and a second type, each transistor (1) of the first type being switched on when its control terminal is at a first potential and off when its control terminal is at a second potential, each transistor (2) of the second type being switched on when its control terminal is at the first potential and also when its control terminal is at the second potential, the transistors of the first type having a higher on-resistance than the transistors of the second type, comprising: a plurality of word lines drivable to said first potential and said second potential; at least one column (10A) of memory cells, each memory cell among said memory cells having a transistor of a type selected from among said first type and said second type, the control terminal of the transistor in the memory cell being coupled to one of said word lines, the transistors in said column of memory cells being mutually coupled in series and conducting a cell current when all switched on; a reference amplifier (16) receiving a reference current and converting the reference current to a reference voltage; a data sensing amplifier (13) coupled to said column of memory cells and said reference amplifier (16), using said reference voltage to convert said cell current to a data voltage signal; and a reference column (17) having two parallel current paths (18, 19) joining at a common node (N1) from which said reference current is supplied to said reference amplifier (16), each of said two parallel current paths having a number of transistors (1) of said first type coupled in series with a substantially equal number of transistors (2) of said second type, each word line among said word lines being coupled to the control terminals of one transistor of said first type on one of said two parallel current paths and one transistor of the second type on another one of said two parallel current paths, characterised in that said number of transistors (1) is at least three. The semiconductor memory device of claim 1, wherein on each of said two parallel current paths (18, 19), the transistors (1) of said first type are coupled in an alternating sequence with the transistors (2) of said second type. The semiconductor memory device of claim 1, wherein the transistors (1) of said first type are n-channel metal-oxide-semiconductor field-effect transistors having a positive threshold voltage, and the transistors (2) of the second type are n-channel metal-oxide-semiconductor field-effect transistors having a negative threshold voltage. The semiconductor memory device of claim 1, wherein the transistors of said second type have short-circuited source and drain terminals. The semiconductor memory device of claim 1, comprising more than one said reference column (10B), and more than one said reference amplifier (16) coupled to said data sensing amplifier (13), said data sensing amplifier (13) receiving an average of the reference voltages supplied by all coupled reference amplifiers.

Inventors: MIYAMOTO KENICHI, MIYAMOTO, KENICHI, Miyamoto, Kenichi, c/o OKI Micro Design Co., Ltd.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 2000
Main classification: G11C17
IPC codes: G11C7, G11C16, G11C17