Abstract:
A memory cell array forms a plurality of control areas in a direction orthogonal to the direction of extension of a bit line. A sense amplifier initially charges a bit line in each control area in the memory cell array with a charging voltage controlled by a respective individual bit-line control signal. Bit-line control signal generator circuits are provided plural in accordance with the control areas in the memory cell array. Each bit-line control signal generator circuit receives the potential on a cell source line in a corresponding control area, individually generates and provides the bit-line control signal in the each control area in accordance with the received voltage on the cell source line in each control area.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-208720, filed on Aug. 10, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a semiconductor memory device such as EEPROMs of the NAND-cell, NOR-cell, DINOR (Divided bit line NOR)-cell and AND-cell types, and more particularly to a semiconductor memory device having an improved sense amplifier of the current sense type. 
         [0004]    2. Description of the Related Art 
         [0005]    A sense amplifier in a semiconductor memory device such as a flash memory basically senses the presence/absence or the level of cell current flowing in accordance with data in a memory cell, thereby deciding the value of data. The sense amplifier is usually connected to a bit line (data line) to which a number of memory cells are connected. The sensing scheme is roughly divided into the voltage sense type and the current sense type. 
         [0006]    A sense amplifier of the current sense type precharges a bit line isolated from the memory cells to a certain voltage, discharges the bit line through the selected memory cell, and detects the discharged state of the bit line at a sense node connected to the bit line. At the time of data sensing, the bit line is isolated from the current source load to detect the bit line voltage determined from cell data. 
         [0007]    A sense amplifier of the voltage sense type, on the other hand, supplies read current flowing in a memory cell via the bit line, thereby sensing data. Also in this case, cell data determines the bit line voltage, and eventually data determination at the sense node connected to the bit line detects a difference in voltage at the sense node based on the difference in cell current. 
         [0008]    The sense amplifier of the current sense type and the sense amplifier of the voltage sense type have the following advantages and disadvantages in general. The voltage sense type utilizes charging and discharging bit lines and accordingly has less power consumption. In a mass storage memory with a large bit line capacity, though, charging/discharging is time-consuming and accordingly fast sensing becomes difficult. In addition, the amplitude of the bit line voltage is made relatively large in accordance with cell data and accordingly a noise between adjacent bit lines causes a problem. 
         [0009]    In contrast, the sense amplifier of the current sense type senses data while supplying read current flowing in the memory cell via the bit line, thereby enabling fast sensing. In addition, a clamp transistor (presense amplifier) arranged between the bit line and the sense node is used to reduce the amplitude of the bit line voltage in accordance with cell data and accordingly the noise between bit lines hardly causes a problem. Also in this case, however, reading is executed on alternate bit lines, and other bit lines not subjected to reading are grounded and used as shields to exclude influences between bit lines on reading. 
         [0010]    To the contrary, the bit line potential may be controlled such that it is always fixed at a constant voltage to exclude influences between bit lines and allow all bit lines to be sensed in parallel on sensing. Such a sense amplifier of the ABL (All Bit Line) type has been proposed (JP 2006-500729T, paragraphs 0062-0068, 0076-0079,  FIGS. 7A ,  7 B,  8  and  13 ). 
         [0011]    In such the sense amplifier of the current sense type, however, advanced fine patterning of devices increases the value of current flowing in the cell source line and elevates the potential on the cell source line as a problem. The elevation of the potential on the cell source line decreases the potential difference between the bit line controlled at a certain potential by a clamp transistor and the cell source line. Accordingly, the drain-source voltage Vds in the selected cell lowers and the gate-source voltage Vds in the selected cell also lowers. As a result, the cell current decreases and causes a failure in reading data out of the selected cell. 
         [0012]    To prevent such the read failure, there has been proposed a method called multipath sense, which comprises multiple times of sensing (Patent Document 1). The multipath sense is a method comprising turning off the selected cell after once sense current flows therein at the first sensing, followed by sensing again. Thus, the value of current flowing in the cell source line is suppressed lower than the first sensing and the subsequent sensing can correctly detect the sense current flowing in the selected cell that could not be detected in the previous sensing. 
         [0013]    The multipath sense, however, requires multiple times of sensing and accordingly consumes time in sensing as a problem. In particular, storing multivalue data such as 8-value or 16-value data in a memory cell increases times of sensing to 7 or 15. Accordingly, the requirement of multiple times of sensing in a single threshold decision causes a fatal drawback with respect to the reading time. 
         [0014]    On the other hand, a control may be considered to elevate the gate voltage on the bit-line clamp transistor in accordance with the elevation of the potential on the cell source line. In this case, however, the potential on the cell source line differs from part to part in the memory cell array. Accordingly, the voltage control of bit lines together causes an excessively controlled bit line and a less controlled bit line as a problem. 
       SUMMARY OF THE INVENTION 
       [0015]    In one aspect the present invention provides a semiconductor memory device, comprising: a memory cell array having plural memory cells connected between a bit line and a cell source line; a sense amplifier of the current sense type operative to initially charge the bit line with a charging voltage controlled by a bit-line control signal and detect the value of current flowing in the bit line when a certain gate voltage is given to a data read-targeted memory cell to decide data read out of the memory cell; and a bit-line control signal generator circuit operative to receive the voltage on the cell source line, generate the bit-line control signal in accordance with the received voltage on the cell source line and provide it to the sense amplifier, wherein the memory cell array forms a plurality of control areas in a direction orthogonal to the direction of extension of the bit line, wherein the sense amplifier initially charges a bit line in each control area in the memory cell array with a charging voltage controlled by a respective individual bit-line control signal, wherein the bit-line control signal generator circuit is one of plural bit-line control signal generator circuits provided in accordance with the control areas in the memory cell array, wherein each bit-line control signal generator circuit receives the potential on the cell source line in a corresponding control area, individually generates and provides the bit-line control signal in the each control area in accordance with the received voltage on the cell source line in each control area. 
         [0016]    In one aspect the present invention provides a semiconductor memory device, comprising: a memory cell array having plural memory cells connected between a bit line and a cell source line; a sense amplifier of the current sense type operative to initially charge the bit line with a charging voltage controlled by a bit-line control signal and detect the value of current flowing in the bit line when a certain gate voltage is given to a data read-targeted memory cell to decide data read out of the memory cell; and a bit-line control signal generator circuit operative to receive the voltage on the cell source line, generate the bit-line control signal in accordance with the received voltage on the cell source line and provide it to the sense amplifier, wherein the memory cell array and the sense amplifier are divided into M areas (M is an integer of 3 or more), wherein the bit-line control signal generator circuit receives the voltage on the cell source line in each of the M areas of the memory cells, generates the bit-line control signal in accordance with the received voltage on the cell source line in each area, and supplies the generated bit-line control signal to the sense amplifier in each area. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a block diagram of the major part of a NAND-type flash memory according to one embodiment of the present invention. 
           [0018]      FIG. 2  is a circuit diagram of a memory cell array in the same memory. 
           [0019]      FIG. 3  is a circuit diagram of a sense amplifier in the same memory. 
           [0020]      FIG. 4  is a waveform diagram showing operation of the sense amplifier in the same memory. 
           [0021]      FIG. 5  is a circuit diagram of a BLC generator circuit in the same memory. 
           [0022]      FIG. 6  is a circuit diagram of a PG generator circuit in the same memory. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0023]    The embodiments of the invention will now be described with reference to the drawings. 
         [0024]      FIG. 1  is a block diagram showing the major part of a semiconductor memory device according to one embodiment of the present invention. This semiconductor memory device is a NAND-type flash memory, which comprises: a memory cell array  1 ; a row decoder  2  operative to select a word line and a selection gate line in the memory cell array  1 ; a sense amplifier  3  provided atone end or both ends in a bit line BL (described later) direction in the memory cell array  1  and operative to read data via the bit line BL; a plurality of BLC (bit-line control signal) generator circuits  4  provided in parallel with the sense amplifier  3 ; and a PG generator circuit  5  operative to supply a control signal PG to the BLC generator circuits  4 . 
         [0025]    The memory cell array  1  comprises a plurality of NAND cell units NU arrayed in matrix as shown in  FIG. 2 . A NAND cell unit NU includes: a memory cell string of plural memory cells M 1 -Mn serially connected in such a manner that adjacent ones share a source/drain diffused layer; a selection gate transistor S 1  connected between one end of the memory cell string and the bit line BL; and a selection gate transistor S 2  connected between the other end of the memory cell string and a cell source line CELSRC. Control gates of the memory cells M 0  to Mm- 1  are connected in such a manner that control gates of memory cells arrayed in the lateral direction are connected in common to form word lines WL 0 -WLn. Control gates of the selection gate transistors S 1 , S 2  are connected in such a manner that control gates of selection gate transistors arrayed in the lateral direction are connected in common to form selection gate lines SGD, SGS. 
         [0026]    A set of NAND cell units NU arrayed in the word line WL direction configures a block or the minimum unit of data erase and plural such blocks BLK 0 -BLKm- 1  are arranged in the bit line direction. The memory cell array  1  is divided into plural areas in the direction of extension of the word line WL to form plural control areas CA. 
         [0027]    The sense amplifier  3  may be configured as shown in  FIG. 3 . In this case, a sense amplifier of the ABL type is described below by way of example though the present invention is not particularly limited to the sense amplifier of this type. 
         [0028]    The sense amplifier  3  mainly includes: an initial charging circuit  31  capable of initially charging the bit line BL and a sense node SEN: a sensing capacitor  32  connected to the sense node SEN; a current discriminating circuit  33  operative to detect the value of current flowing in the bit line BL based on the potential on the sense node SEN; a latch  34  operative to hold an output from the current discriminating circuit  33  as read data; a discharging circuit  35  operative to discharge the charge stored on the bit line BL and the sense node SEN; and a bit line selection transistor  36  operative to selectively connect the sense amplifier  3  with the bit line BL. 
         [0029]    The initial charging circuit  31  includes a charge switch PMOS transistor  312  connected to the power source VDD to switch on/off the charging current. The source of the PMOS transistor  312  is directly connected to the power source VDD. A PMOS transistor  314  and an NMOS transistor  315  are serially connected between the drain of the PMOS transistor  312  and the sense node SEN. A serial circuit of an NMOS transistor  316  and an NMOS transistor  317  for voltage clamp is interposed between the sense node SEN and the bit line selection transistor  36 . An NMOS transistor  318  is connected in parallel with the serial circuit of the NMOS transistors  315 ,  316 . Namely, the NMOS transistor  315  supplies initial charging current to the sense node SEN. The NMOS transistor  316  supplies current from the sense node SEN to the bit line BL. The NMOS transistor  318  continuously supplies current to the bit line BL not via the sense node SEN. The NMOS transistor  317  is connected between the NMOS transistors  316 ,  318  and the bit line BL and used in voltage clamping. The NMOS transistors  315 ,  316 ,  318  switch the charging/discharging paths to the bit line BL and the sense node SEN. When the current discriminating circuit  33  detects the potential on the sense node SEN, the NMOS transistor  315  is turned “off” and the NMOS transistor  318  is turned “on”. 
         [0030]    The current discriminating circuit  33  includes a PMOS transistor  331  for detecting the sense node SEN, and a PMOS transistor  332  connected between the source of the transistor  331  and the power source VDD and operative at latch timing. Connected to the drain of the PMOS transistor  331  is the latch  34 , which includes CMOS inverters  341 ,  342  connected in antiparallel. The latch  34  has an output linked to a read bus, not shown. The discharging circuit  35  includes a serial circuit of NMOS transistors  351 ,  352 . 
         [0031]    The sense amplifier  3  has operation periods of precharge, sense, data latch and discharge.  FIG. 4  is a timing chart during a precharge period. To start precharging, first, a control signal INV supplied to the gate of the PMOS transistor  314  is at the low level, and control signals H 00 , XX 0  supplied to the gates of the NMOS transistors  315 ,  316  are at the high level (not shown). In this situation, control signals BLC, BLX supplied to the gates of the NMOS transistors  317 ,  318  rise to a certain voltage that can transfer the power source VDD to the bit line BL. Subsequently, a control signal BLS supplied to the gate of the bit line selection transistor  36  rises and a control signal BLT fed to the gate of the PMOS transistor  312  falls. As a result, the transistors  312 ,  314 ,  315 - 318  and  36  turn on and allow charging current to flow in the bit line BL and the sense node SEN via two paths of the transistors  315 ,  316  and the transistor  318 . 
         [0032]    If the selected cell stores “0” data in the NAND cell unit NU connected to the bit line BL, no on-current flows in the NAND cell unit NU. Therefore, the potential on the bit line BL becomes V BLC −Vth, which is equal to the voltage V BLC  of the bit-line control signal BLC supplied to the gate of the voltage clamp transistor  317  minus the threshold Vth of the transistor  317 . If the selected cell stores “1” data (erased), a certain current flows in the selected cell and the potential on the bit line BL becomes lower than that when the selected cell stores “0” data. 
         [0033]    After completion of the precharge period, the transistor  315  is turned off and the charge stored on the sense node SEN is discharged via the bit line BL and the selected cell if the selected cell holds data “1”. Subsequently, the value of current flowing in the bit line BL via the transistor  318  is controlled. As a result, the bit line BL is kept always at a constant potential to exclude influences to adjacent bit lines. Thereafter, the potential on the sense node SEN is sensed to decide data stored in the selected cell. The decided data is latched in the latch  34  and provided to external via the data line. Subsequently, the charge on the bit line BL and the sense node SEN is discharged via the discharging circuit  35 . 
         [0034]    During the precharge period and on sensing, the current flowing in the bit line BL flows into the cell source line CELSRC in a stroke, thereby elevating the potential on the cell source line CELSRC. Therefore, the BLC generator circuit  4  cooperates with the PG generator circuit  5  to generate the bit-line control signal BLC of the voltage in accordance with the voltage on the cell source line CELSRC and supplies it to the gate of the clamp transistor  317  in the sense amplifier  3 . Namely, in accordance with the elevation of the voltage on the cell source line CELSRC, the voltage of the control signal BLC is also allowed to rise. 
         [0035]    In this embodiment, monitor positions on the cell source line CELSRC are arranged one by one in the control areas CA of the memory cell array  1  and each BLC generator circuit  4  individually controls the bit-line control signal BLC in each control area CA. Namely, the current flowing in the bit line BL elevates the voltage on the cell source line CELSRC as shown in  FIG. 4 . Accordingly, the cell source voltage is observed individually in each control area CA and, based on this result, the bit-line control signals BLC 1 , BLC 2 , . . . are controlled individually. 
         [0036]      FIG. 5  shows a specific configuration example of the BLC generator circuit  4 , and  FIG. 6  shows a specific configuration example of the PG generator circuit  5 . The PG generator circuit  5  comprises a constant current circuit, which includes a serial circuit of a PMOS transistor  51  and resistors  52 ,  53 , and an operational amplifier  54  operative to supply the control signal PG to the gate of the PMOS transistor  51  to control the transistor  51  such that the voltage drop across the resistor  53  caused by the current flowing in the serial circuit meets the reference voltage VREF. The BLC generator circuit  4  comprises a serial circuit of a PMOS transistor  41 , a diode-connected NMOS transistor  42 , and a resistor  43 . The PMOS transistor  41  has a gate given the control signal PG to supply a constant current flowing in the resistor  43  and the resistor  43  has one end connected to the cell source line CELSRC. It is configured such that as the voltage on the cell source line CELSRC rises or falls, the potential on the point of connection between the PMOS transistor  41  and the NMOS transistor  42  also rises or falls. The point of connection between the transistors  41 ,  42  is used as the output of the bit-line control signal BLC. 
         [0037]    The BLC generator circuit  4  is configured in other words as follows. Namely, the NMOS transistor  42  and the resistor  43  serve as a resistor element, which has one end supplied with the voltage on the cell source line CELSRC and the other end used as the output terminal of the bit-line control signal BLC. The PMOS transistor  41  supplies a constant current in the NMOS transistor  42  and the resistor  43  (resistor element). 
         [0038]    The PG generator circuit  5  is configured in other words as follows. Namely, the PG generator circuit  5  supplies the control signal PG for constant current to the gate of the PMOS transistor  41 . The PG generator circuit  5  supplies the control signal PG to the BLC generator circuits  4  in common. In the PG generator circuit  5 , the PMOS transistor  51 , the resistors  52 ,  53 , and the operational amplifier  54  configure a current mirror paired with the PMOS transistor  41  in the BLC generator circuit  4 . 
         [0039]    In this embodiment, the BLC generator circuit  4  is provided in each control area CA in the memory cell array  1 , and the bit-line control signal BLC is generated individually for each control area CA to control the voltage on the bit line BL generated by the sense amplifier  3 . Accordingly, it is possible to prevent the fluctuation of the cell source line CELSRC from causing a read error in each control area CA. In addition, this embodiment makes it possible to read all data by single sensing, which can reduce the read time greatly over the multipath method. 
         [0040]    Desirably, the number of the control areas CA formed through division of the memory cell array  1  is equal to 3 or more. The number of the BLC generator circuits  4  is preferably equal to 3 or more though it is not required to meet the number of the control areas CA. The BLC generator circuit  4  may be one that can control one or more control areas CA. 
         [0041]    The BLC generator circuits  4  are arranged at one end or both ends of the memory cell array  1  in the direction of extension of the bit line BL as is preferable from the viewpoint of layout. 
         [0042]    The above embodiments describe the NAND-type flash memory by way of example. The present invention is though not limited to the NAND-type flash memory but rather can be applied to semiconductor memory devices such as EEPROMs of the NOR type, the DINOR (Divided bit line NOR) type and the AND type as well.