NONVOLATILE SEMICONDUCTOR MEMORY DEVICE

Abstract: In a nonvolatile semiconductor memory device, a wiring layer (R) is connected between a power source (VP) and a memory cell (M11). Resistance of the wiring layer is larger than the on-resistance of a load transistor (104), so that the load transistor substan­ tially determines the load characteristic. Therefore, the load characteristic curve is more gentle in inclina­ tion and more rectilinear in shape. This makes the data writing operation stable against a variance in the channel lengths of manufactured transistors forming the memory cells.

Description: This invention relates to a nonvolatile semiconductor memory device having a data write load circuit and with an improved data write characteristic. A nonvolatile semiconductor memory device, particularly an EPROM whose memory cells are double gate nonvolatile MOS transistors with a floating gate and a control gate, allows data to be rewritten thereinto. Because of this feature, the EPROMs have been used in various types of computer systems. In a negative state of the memory cell, or the cell transistor, in which electrons have been injected into the floating gate, the threshold voltage of the memory cell is high. Therefore, in this state, even if a high level voltage of 5 V, for example, is applied to the control gate of the cell transistor, the transistor will not be conductive. In a neutral state where no electrons are injected into the floating gate, the threshold voltage maintains its initial low voltage. In this state, if the high level voltage is applied to the control gate, the cell transistor is conductive. If logical "1" and "0" are respectively assigned to the conductive and nonconductive states of the cell transistor, the data can be detected. To inject electrons into the floating gate, a write or program voltage much higher than the normal power source voltage (5 V) is applied to the floating gate and the drain. The program voltage is 12.5 to 21 V, for example. Under such a high program voltage, impact ionization occurs in the channel region near the drain, to generate hot electrons. In turn, the hot electrons are injected into the floating gate. The electrons once injected into the floating gate are left in the floating gate unless a data erase operation is applied to the cell transistor. In this way, the cell transistor can store the data in an nonvolatilised manner. Fig. 1 shows a schematic diagram of an EPROM using the nonvolatile semiconductor memory cell as mentioned above. In the figure, WLl to WLm designate row lines to which the decoded row signals are supplied from row decoder 101. COLl to COLn indicate column select lines to which the decoded column signals are supplied from column decoder 102. Column select lines COLl to COLn are coupled with the gates of column select MOS transistors Cl to Cn of enhancement type (E type). These transistors are driven by the decoded column signals coming through "n" column select lines COLl to COLn. The first ends of these column select transistors Cl to Cn are connected together to node 103, while the second ends are connected to "n" column lines BLl to BLn. These column lines BLl to BLn are laid out crossing row lines WLl to WLm. Double gate memory cells Mll to Mmn are placed at the intersections of the column and row lines. The gates of these cell transistors M11 to Mmn are connected to row lines WLl to WLm, respectively. The drains are respectively connected to column lines BLl to BLn. The sources are connected together with ground voltage VS of 0 V, for example. Node 103 is connected to the source of MOS transistor 104. The drain of this transistor 104 is coupled with external program voltage VP. The gate is connected to the output node of data input circuit 105. Data input circuit 105 outputs input data DIN, which is set at the VS voltage or a high level voltage according to the logical state, "1" or "0", of the programmed data. In a read mode, sense amplifier 105 coupled with node 103 senses the data which depends on the potential at node 103. For programming or writing data "0" into a memory cell Mll, for example, the input data signal DIN output from data input circuit 105 is set at a high level voltage, and the decoded output from column decoder 102 sets column select line COLl at a high level voltage. The high level voltage DIN renders transistor 104 conductive. The high level voltage on column select line COLl renders column select transistor Cl conductive. As a result, program voltage VP is applied to column line BLl. At this time, the decoded output signal from row decoder 101 provides a high level voltage on row line WLl, so that a high level voltage is applied to both the control gate and the drain of memory cell Mll. Under this condition, impact ionization occurs in the channel region in the vicinity of the drain, to generate hot electrons. The generated hot electrons are in turn injected into the floating gate of cell transistor Mll. In this way, the data write operation is performed. For programming data "1" into memory cell Mll, the input data signal DIN from data input circuit 105 is set at 0 V, or the VS potential, so that transistor 104 is rendered nonconductive. Under this condition, program voltage VP is not applied to column line BLl. Further, the floating gate of selected memory cell Mll keeps the neutral state. This state of the cell indicates the storage of data "1". Recent semiconductor memory device formed of the nonvolatile memory cells progressively increases its memory capacity. With this, the time for programming the data into the memory cells is longer. The programming time must be reduced. To this end, in programming data, the cell transistor is generally operated in the avalanche region providing a high programming efficiency. Fig. 2A typically shows one of the single memory cells of the EPROM of Fig. 1 and a data write circuit for writing data into the memory cell. In the circuit, the memory cell is denoted as Mll. MOS transistors 104 and Cl make up the data write circuit. Fig. 2B shows data write characteristics of memory cells Mll with different channel lengths, and a load characteristic of the load circuit or the data write circuit. As shown, the data write characteristic is represented by a relationship of drain current ID versus drain voltage VD. Fig. 2B will be used mainly for explaining how the optimum operating point of the cell transistor depends on the channel length of the transistor in a data write mode. For plotting the data write characteristic curves I to V, a high level voltage was applied to the gates of MOS transistors 104 and Cl, and a program high voltage was applied to the control gate of memory cell Mll. Curve I indicates an ID - VD characteristic of memory cell Mll. Rectilinear curve IV indicates a load characteristic of the write circuit formed of MOS transistors 104 and Cl. The memory circuit is operated at the intersection A of curves I and IV. In other words, the circuit is operated with the drain voltage and the drain currents as indicated by point A. V denotes a line extending through point (VP-VTH) on the axis of abscissas and point A. VTH denotes the threshold voltage of MOS transistor 104. In manufacturing memory devices including the MOS transistors, nevertherless, a variance in the channel lengths of the manufactured transistors is inevitable although it is within a limited range. The above optimum operating point of the memory circuit in a data program or write mode, depends greatly on the channel length of the transistor. This will be described referring to Fig. 2B. Curve II shows a data write characteristic of the memory circuit in which the cell transistor has a channel length longer than a specified value. When it is shorter, the data write characteristic varies as indicated by curve III. For writing data, the memory circuit having the cell transistor with the long channel length is operated at the intersection B of curves II and IV. Similarly, the memory circuit having the cell transistor with the short channel length is operated at the intersection C of curves III and IV. Point B resides outside the avalanche region. Therefore, an insufficient number of electrons are injected into the floating gate, leading to error data programming. Point C resides in the avalanche region. At this point, however, drain current ID is very large. This indicates that the circuit operation at this point C consumes a large power. In the MOS transistor 104, which is operated in a saturation region, drain current ID is mathematically expressed ID = β 2 (VG-VTH)² (1) where β is the current amplification factor of transistor 104, VG the gate voltage, and VTH the threshold voltage. The equation (1) implies that drain current ID varies proportional to the square of (VG - VTH) which represents as a difference between gate voltage VG and threshold voltage VTH. Thus, the load characteristic curve is steep, and points B and C are greatly shifted from the optimum operating point A. The problems of the circuit when it is operated at point B or C are as already mentioned. If the load characteristic curve has a more gentle inclination as indicated by rectilinear curve V, the operating points B and C are within the avalanche region and the drain currents required are more near that for the optimum operating point A. Another cause for the steepness of the load characteristic curve is that drain current ID starts to rise at the drain voltage which is lower than program voltage VP by threshold voltage VTH. As described above, the conventional memory devices involve the problems of long data write time and excessive power dissipation. Prior art document EP-A3-0 157 389 discloses a semiconductor integrated memory circuit including a first floating gate type MOS transistor which constitutes a memory cell. One end of a current path of the first MOS transistor is grounded and the other end is connected to a high voltage terminal through second and third MOS transistors in series. Row and column selection signals from row and column decoders are supplied to gates of the first and second MOS transistors, while a data signal is supplied to a gate of the third MOS transistor in the writing mode. It is an object of this invention to provide a nonvolatile semiconductor memory device whose data write characteristic is stable against a variance of the channel lengths of cell transistors, with a lessened inclination of the load characteristic curve. To solve this object the present invention provides a nonvolatile semiconductor memory device as defined in claim 1. The load resistor substantially determines the load characteristic. Therefore, the load characteristic curve is more gentle in inclination and more rectilinear in shape. This makes the data writing operation stable against a variance in the channel lengths of the manufactured cell transistors. Location of the wiring layer on the field region between the adjacent two column select transistors, requires no additional chip area for the wiring layer. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 shows a circuit diagram of a conventional PROM; Fig. 2A shows a circuit diagram of a circuit including a memory cell circuit of one bit and a data write circuit; Fig. 2B shows a graphical representation of the ID - VD curves of cell transistors with different channel lengths, and a load characteristic of a load transistor contained in the data write circuit; Figs. 3A to 3C show circuit diagrams of three types of circuits each including a one-bit memory cell circuit and a data write circuit, which are arranged according to an embodiment of this invention; Fig. 4A shows a plan view showing a PROM, which is patterned according to the schemes of Figs. 3B and 3C; Fig. 4B shows a sectional view taken on line VIIB - VIIB in Fig. 4A; Figs. 5A to 5C show circuit diagrams of three types of circuits each including a one-bit memory cell circuit and a data write circuit, which are arranged according to another embodiment of this invention; and Fig. 5D shows a graphical representation of the ID - VD curves of the cell transistors and a load characteristic curve of the load transistor, those transistors being contained in the Figs. 5A to 5C. Reference is first made to Figs. 3A to 3C illustrating three types of circuit arrangements each including a one-bit memory cell circuit and a data write circuit, which are constructed according to an embodiment of this invention. In those circuits, like reference symbols are used for designating like portions in the figures used for the prior art description. As shown, load transistor 104, column select transistor Cl, and cell transistor Mll, and further resistor R are connected in series between high potential power source VP for data programming and a ground potential. Cell transistor Mll is a double-gate nonvolatile transistor having a control gate and a floating gate. The gate of load transistor 104 is coupled for reception with a programmed data input signal DIN. The gate of column select transistor Cl is coupled with a column select signal COLl output from a column decoder (not shown). The control gate of cell transistor Mll is coupled with row select signal WLl from a row select decoder (not shown). In the Fig. 3A circuit, resistor R is inserted between program power source VP and load transistor 104. In the Fig. 3B circuit, resistor R is inserted between load transistor 104 and column select transistor Cl. In the Fig. 3C circuit, resistor R is connected between column select transistor Cl and cell transistor Mll. Fig. 4A illustrates a pattern of a part of PROM, which is arranged according to the schemes of Figs. 3B and 3C. A sectional view taken on line VIIB - VIIB in Fig. 4A is illustrated in Fig. 4B. In these figures, reference numeral 1 designates a semiconductor substrate, 2 a source or drain of a column select transistor, 3 a gate electrode lead wire, 4 a field insulating film. In this instance, resistor layer R is formed of a polysilicon layer, but it may be substituted by an impurity diffusion layer. Interconnection wires 12 to 14, and 103 are made of aluminum, and in contact with resistor layers R through contact holes 11. Gate electrode 3 is made of polysilicon. It is preferable that resistance of resistor R is equal to or larger than the on-resistance of load transistor 104. If the resistance of resistor R is so selected that the resistor R predominantly determines the load characteristic curve, then the load characteristic curve is more rectilinear and more gentle in inclination and close to curve V of Fig. 2B. The layers of resistors R are formed on the field oxide films between the adjacent column select transistors Cl to Cn, as shown in Figs. 4A and 4B. Therefore, formation of the resistor layers does not require any additional area for the semiconductor chip. Figs. 5A to 5C show circuit diagrams of circuits including a one-bit memory cell and a data write circuit, which are modifications of the Figs. 3A to 3C circuits. In the instances, the potential of data input signal DIN and column select signal COLl are increased to (VP + VTH) or more. In this embodiment, presence of resistor R provides a load characteristic curve which is more rectilinear in shape and more gentle in inclination. Further, the drain current starts to flow at program voltage VP. This makes the load characteristic curve further gentle, as shown by curve V in Fig. 4D. Thus, this embodiment provides a further improvement of the data write characteristic. Curves I, II and III in Fig. 4D corresponds to curves I, II and III in Fig. 2B, respectively. Curves IV denotes a load characteristic in a case where signals DIN and COLl are increased to (VP+VTH) or more and no resistor R is provided. A indicates a cross-point of curves I and IV. B' indicates a cross-point of curves II and IV, and C' denotes a cross-point of curves III and IV. As seen from the foregoing, load resistors are additionally used. The resistance of the load resistor is so selected as to substantially determine the load characteristic of the load circuit, so that the load characteristic curve is more gentle in inclination and more rectilinear in shape. Therefore, the data writing is stable against a variance in the channel lengths of the manufactured cell transistors. Further, use of additional load resistors brings about no increase of chip area used.

Claims: A nonvolatile semiconductor memory device comprising: a power source (VP); a nonvolatile memory cell (Mll) consisting of a floating gate transistor being programmed by hot electron injection onto the floating gate; and a load transistor (104) connected in series with the current path of the floating gate transistor and between said power source (VP) and said nonvolatile memory cell (Mll), the gate of said load transistor (104) being coupled for reception with an input data signal, characterized by further comprising a wiring layer (R) connected in series with said load transistor (104) between said power source (VP) and said memory cell (Mll), a resistance of said wiring layer (R) being larger than a resistance of said load transistor (104) when said load transistor is turned on. The memory device according to claim 1, characterized in that said wiring layer (R) is a polysilicon layer. The memory device according to claim 1, characterized in that said wiring layer (R) is an impurity diffusion layer. The memory device according to claim 1, characterized by further comprising a column select transistor (C1) connected between said load transistor (104) and said memory cell (M11). The memory device according to claim 1, characterized in that said wiring layer (R) is inserted between said power source (VP) and said load transistor (104). The memory device according to claim 4, characterized in that said wiring layer (R) is inserted between said column select transistor (104) and said load transistor (104). The memory device according to claim 4, characterized in that said wiring layer (R) is inserted between said column select transistor (104) and said memory cell (Mll). The memory device according to claim 1, characterized in that a column select transistor (Cl) is connected between said load transistor (104) and said memory cell (Mll), said wiring layer (R) being laid on a field region between adjacent column select transistors.

Inventors: ASANO, MASAMICHI, IMAI, MIZUHO, IWAHASHI, HIROSHI, MINAGAWA, HIDENOBU, NAKAI, HIROTO, TATSUMI, YUUICHI
Applicants: KABUSHIKI KAISHA TOSHIBA, TOSHIBA MICRO-COMPUTER ENGINEERING CORPORATION
Application year: 1988
Main classification: G11C16
IPC codes: G11C16