Abstract:
There is provided a non-volatile memory which enables high accuracy threshold control in a writing operation. In the present invention, a drain voltage and a drain current of a memory transistor are controlled to carry out a writing operation of a hot electron injection system, which is wherein a charge injection speed does not depend on a threshold voltage. FIGS.  1 A and  1 B are views of a circuit structure for controlling the writing. In FIGS.  1 A and  1 B, an output of an operational amplifier 103 is connected to a control gate of a memory transistor 101, a constant current source 102 is connected to a drain electrode, and a source electrode is grounded. The constant current source 102 and a voltage Vpgm are respectively connected to two input terminals of the operational amplifier 103.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application is a continuation of copending U.S. application Ser. No. 12/407,539 filed on Mar. 19, 2009 which is a divisional of U.S. application Ser. No. 11/924,090 filed on Oct. 25, 2007 (now U.S. Pat. No. 7,512,006) which is a divisional of U.S. application Ser. No. 11/109,325 filed on Apr. 19, 2005 (now U.S. Pat. No. 7,298,652 issued Nov. 20, 2007) which is a continuation of U.S. application Ser. No. 10/383,209, filed on Mar. 6, 2003 (now U.S. Pat. No. 6,914,818 issued Jul. 5, 2005) which is a divisional of U.S. application Ser. No. 09/976,770 filed on Oct. 12, 2001 (now U.S. Pat. No. 6,556,475 issued Apr. 22, 2003). 
       1. FIELD OF THE INVENTION 
       [0002]    The present invention relates to a semiconductor non-volatile memory, and particularly to an electrically writable and erasable semiconductor non-volatile memory (also called an EEPROM or Electrically Erasable and Programmable Read Only Memory). The present invention is particularly effective for a semiconductor non-volatile memory using a multi-value technique. Besides, the present invention relates to a semiconductor device including the semiconductor non-volatile memory. 
       2. DESCRIPTION OF THE RELATED ART 
       [0003]    In the present specification, an electrically writable and erasable semiconductor non-volatile memory (EEPROM) indicates all semiconductor non-volatile memories in which electrical writing and electrical erasing are literally enabled, and includes, for example, a full function EEPROM and a flash memory in its category. Unless otherwise specified, the non-volatile memory and the semiconductor non-volatile memory are used to mean the EEPROM. Besides, the semiconductor device indicates all devices functioning by using semiconductor characteristics, and includes, for example, a microprocessor, an electro-optic device typified by a liquid crystal display device and an EL display device, and an electronic equipment incorporating a microprocessor or an electro-optic device in its category. 
         [0004]    In recent years, an electrically writable and erasable semiconductor non-volatile memory (EEPROM), especially a flash memory has attracted attention as a promising candidate of a memory substituting for a magnetic disk or a DRAM. Above all, a so-called multi-value non-volatile memory each memory element of which stores data of three or higher values has attracted attention as a large capacity memory. 
         [0005]    The non-volatile memory is divided into types, such as a NOR type, a NAND type, an AND type or a DINOR type, based on differences in their circuit structures and operation methods. As a memory element constituting the non-volatile memory, there is known a memory transistor including a floating gate, a memory transistor including a cluster layer, a memory transistor having MNOS (Metal-Nitride-Oxide-Semiconductor) structure or MONOS (Metal-Oxide-Nitride-Oxide-Semiconductor) structure, or the like. 
         [0006]    In a conventional non-volatile memory, a hot electron injection method (called an HE injection system) and a charge injection method by FN tunnel current (called an FN current system) can be cited as a typical writing operation. In the HE injection system, a high control gate voltage and a high drain voltage are applied to a memory transistor to cause impact ionization, and a generated hot electron is drawn into a gate electrode side, so that an electric charge is injected into the memory transistor. On the other hand, in the FN current system, a high voltage is applied between a control gate electrode and a substrate to cause an FN tunnel current to flow, so that an electric charge is injected into a memory transistor. 
         [0007]    In either case, in order to confirm that a threshold voltage after writing is within a predetermined range, verify writing is normally carried out. Especially, in a multi-value non-volatile memory, since it is necessary to control the threshold voltage after writing with high accuracy, the verify writing is indispensable. The verify writing is a method in which a small amount of charge injection and readout for confirming the threshold voltage are alternately carried out. This operation is repeated until the threshold voltage after writing falls within a predetermined range. 
         [0008]    In the foregoing method of charge injection, there has been a problem that it is difficult to control the amount of charge injection with accuracy. This is because even in the case where the same operation voltage is applied to the memory transistor, if the charge storage amount of the memory transistor, that is, the threshold voltage is different, the charge injection speed is different. 
         [0009]    Thus, naturally in memory transistors having different threshold voltages, even in the same memory transistor, the charge injection speed is changed as time elapses. For example, in the case where an electron is injected, a threshold voltage is increased, and an effective gate voltage seen from a channel region is decreased, so that the charge injection speed is decreased. 
         [0010]    Further, if the charge injection speed fluctuates by a factor such as fluctuation in film quality of a tunnel oxide film, even if threshold voltages of memory transistors before writing are equal to each other, a difference gradually occurs in the charge storage amount. Then, a vicious circle occurs in which when a difference occurs in the threshold voltage, a charge injection amount further fluctuates. 
         [0011]    The verify writing is one of methods for solving such problems. In the verify writing, since confirmation of the threshold voltage is carried out each time a small amount of charge is injected, it is not necessary to control the charge injection amount with high accuracy. However, in the verify writing, the operation of charge injection is divided into parts, and a readout operation is carried out between them, so that there is a problem that it essentially takes a period of time. 
         [0012]    Incidentally, also in the verify writing, it is important to control the charge injection amount with accuracy. By controlling the charge injection amount with high accuracy, the number of times of division of the charge injection operation can be decreased, and so a verify writing time period can be shortened. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention has been made in view of the above problems. An object of the present invention is to provide a non-volatile memory for carrying out a writing operation in which a charge injection amount can be controlled with high accuracy. Another object of the invention is to provide a non-volatile memory which realizes shortening of a verify writing time, and further, realizes substantial shortening of a writing time by a writing system without using a verify system. A still another object of the invention is to provide a semiconductor device including such a non-volatile memory. 
         [0014]    As already described, in the conventional writing method, the charge injection speed depends on the charge storage amount of the memory transistor, and accordingly, depends on the threshold voltage. As a result, naturally in memory transistors of different states, even in the same memory transistor, the charge injection speed is changed as time elapses, and control of the charge injection amount has been difficult. 
         [0015]    On the contrary, this means that in order to control the charge injection amount with high accuracy, a writing method is effective which has a charge injection speed independent on the charge storage amount of a memory transistor, that is, the threshold voltage. The inventor of the present invention has considered that the charge injection speed depends on the threshold voltage of the memory transistor since a control gate voltage of the memory transistor is directly controlled at the time of writing, and has devised, as a new writing method, a method of controlling a drain voltage and a drain current of the memory transistor. 
         [0016]    That is, a non-volatile memory of the present invention is characterized in that a writing operation of a hot electron injection system is carried out by controlling a drain voltage and a drain current of a memory transistor. This writing method is characterized in that a control gate voltage is not directly controlled, but the control gate voltage is indirectly controlled so that the drain current becomes constant, and is characterized in that a charge injection speed does not depend on a threshold voltage. 
         [0017]    In order to understand a writing operation in a non-volatile memory of the present invention, a simple circuit for carrying out writing into one memory element is cited as an example, and its operation method will be described.  FIGS. 1A and 1B  are used for the description. 
         [0018]    As a memory element, various elements as shown in  FIGS. 26A to 26D  can be used (see embodiment 3 for more details).  FIG. 26A  shows a memory transistor in which a first insulating film  2602 , a cluster layer  2603  constituted by semiconductor or conductor clusters  2604 , a second insulating film  2605 , and a control gate electrode  2606  are successively stacked on an active region  2601 , and which is hereinafter referred to as a memory transistor having a cluster layer.  FIG. 26D  shows a memory transistor in which a first insulating film  2617 , a floating gate electrode  2618  constituted by a semiconductor film or a conductive film, a second insulating film  2619 , and a control gate electrode  2620  are successively stacked on an active region  2616 , and which is hereinafter referred to as a memory transistor having a floating gate.  FIGS. 26B and 26C  show a memory transistor of MNOS structure and a memory transistor of MONOS structure, respectively. 
         [0019]    Here,  FIG. 1A  shows an example of a circuit diagram using, as a memory element, the memory transistor having the cluster layer, the memory transistor of the MNOS structure, or the memory transistor of the MONOS structure, and  FIG. 1B  shows an example of a circuit diagram using the memory transistor having the floating gate. As symbolized by symbols of the respective memory elements, the memory transistor shown in  FIG. 1A  is characterized in that a region for storing an electric charge is spatially discrete, and the memory transistor shown in  FIG. 1B  is characterized in that a region for storing an electric charge is continuous. In both cases, the operation methods are quite identical to each other, and here, the description will be given with reference to  FIG. 1A  as an example. 
         [0020]    The circuit diagram shown in  FIG. 1A  is composed of a memory transistor  101 ( a ), a constant current source  102  for supplying an amount of current I, and an operational amplifier  103 . The output of the operational amplifier  103  is connected to a control gate electrode of the memory transistor  101 ( a ), the constant current source  102  is connected to a drain electrode, and a source electrode is grounded. The constant current source  102  and a voltage Vpgm are respectively connected to two input terminals of the operational amplifier  103 . 
         [0021]    The circuit shown in  FIGS. 1A and 1B  have negative feedback characteristics, and an operation is carried out in a state where two potentials inputted to the operational amplifier  103  are always equal to each other. As a result, in the circuit diagram shown in  FIG. 1A , the memory transistor  101 ( a ) operates in a state where a drain voltage is equal to Vpgm and a drain current is equal to I. 
         [0022]    Actually, in the case where the drain voltage of the memory transistor  101 ( a ) is higher than Vpgm, a positive current is outputted from the operational amplifier  103 , and the control gate voltage of the memory transistor  101 ( a ) is raised. As a result, the conductance of the memory transistor  101 ( a ) is increased, and the drain voltage is lowered. To the contrary, in the case where the drain voltage of the memory transistor  101 ( a ) is lower than Vpgm, a negative current is outputted from the operational amplifier  103 , and the control gate voltage of the memory transistor  101 ( a ) is lowered. As a result, it is understood that the conductance of the memory transistor  101 ( a ) is decreased, and the drain voltage is raised. 
         [0023]    In the present invention, as described above, the drain voltage and the drain current of the memory transistor are controlled, so that the writing operation of the HE injection system is carried out. Incidentally, Vpgm is called a writing voltage, and I is called a writing current. 
         [0024]    When the foregoing writing method is used, and the drain voltage and the drain current are made constant, a channel region of the memory transistor has the same state independently of a charge storage amount of a floating gate. That is, the control gate voltage is shifted by the same amount as a shift of a threshold voltage, and an effective gate voltage seen from the channel region is brought into a state where it is always kept constant. Further, if the drain voltage and the drain current are constant, a certain amount of hot electron is generated, so that a charge injection speed to the floating gate becomes constant. 
         [0025]    This means that in the case where the writing voltage Vpgm and the writing current I are constant, a relation between a shift AVth of the threshold voltage of the memory transistor and a writing time tw is expressed by a straight line as shown by a  201  of  FIG. 2 . As a result, in the non-volatile memory of the present invention, by controlling the writing time, it becomes possible to control the threshold voltage with high accuracy. 
         [0026]    Incidentally,  FIG. 2  also shows a relation between a shift of a threshold voltage in a conventional writing method and a writing time (curve  202  of  FIG. 2 ). In the conventional writing method, in the case where the control gate voltage is made constant, since an effective gate voltage seen from the channel region is decreased as time elapses, a shift amount of threshold voltage is decreased. Thus, it is understood that it is difficult to control the shift of the threshold voltage with high accuracy by controlling the writing time. 
         [0027]    In the present invention, the charge injection speed at the time of writing can be optimized by setting the drain voltage and the drain current of the memory transistor to suitable values. Since the writing method of the present invention is the hot electron injection, the drain voltage is required to have such a magnitude as to generate a hot electron. On the other hand, if plenty of hot electrons are generated, since the controllability of charge injection amount is lowered, it is not preferable that the drain voltage is excessively large. It is preferable that the drain voltage of the memory transistor is 3 V or higher though it depends on the size of the memory transistor, and operates in a saturation region relatively close to a linear region. Further, the writing speed is raised by setting the writing voltage Vpgm and the writing current I to large values in such a region, and on the other hand, it becomes possible to suppress consumed current by setting them to small values. With respect to the writing voltage Vpgm and the writing current I, the optimum operation points may be set according to the use of the non-volatile memory. 
         [0028]    Besides, since the foregoing writing method is the method of hot electron injection, an electron having energy much higher than an energy barrier due to a tunnel oxide film is mainly concerned. Thus, fluctuation in film quality of the tunnel oxide film hardly influences the injection amount of hot electron, and as compared with the method of charge injection by the FN tunnel current, threshold control having little fluctuation becomes possible. 
         [0029]    The non-volatile memory of the present invention uses the foregoing writing operation, so that substantial shortening of a writing time as will be set forth below becomes possible. 
         [0030]    First, reference is made to  FIG. 3 .  FIG. 3  qualitatively shows threshold distributions after the conventional writing and the writing of the present invention are carried out without a verify operation, to a memory transistor with the uniform threshold voltages. A distribution  302  expresses a distribution of threshold voltage after the conventional writing, and a distribution  301  expresses a distribution of threshold voltage after the writing of the present invention. Reference character Vtar designates a set threshold voltage after writing. In  FIG. 3 , when the distribution  302  after the writing of the present invention is compared with the distribution  301  after the conventional writing, both a difference between the center of the distribution and Vtar, and a distribution width are small (ΔV&lt;ΔV′ and δv&lt;δv′). 
         [0031]    In general, in the case where a difference between threshold voltages expressing different states is small as compared with a distribution width after writing and a distance between centers, since it is impossible to bring all memory cells into the predetermined state by one writing operation, verify writing becomes necessary. On the contrary, in the case where a difference between threshold voltages is larger than a distribution width after writing and a difference between centers, it is possible to carry out writing by one writing operation. Incidentally, the verify writing is a method in which writing is not carried out at one time, but a small amount of writing and readout for confirming a threshold are alternately carried out. This operation is repeated until a threshold reaches a predetermined value. 
         [0032]    In the conventional writing method, since the distribution width δv′ after the writing and the difference ΔV&#39; between the centers are large as shown in  FIG. 3 , the method of verify writing is normally used. 
         [0033]    In the case where the writing method of the present invention is used, as shown in  FIG. 3 , since the distribution width δv after the writing and the difference ΔV between the centers are small, even in such a difference between threshold voltages that a verify operation is required in the conventional method, it becomes possible to carry out a writing operation in which a verify operation is not carried out. Specifically, in the case where the difference between threshold voltages of different states is larger than the distribution width δv and the difference ΔV between the centers after the writing of the present invention, the writing operation in which the verify operation is not carried out becomes possible. 
         [0034]    Besides, like a multi-value non-volatile memory, in the case where the difference between threshold voltages expressing different states is small, even in the case where the writing method according to the present invention is used, the verify writing becomes necessary. In this case, it is appropriate that the writing operation is constituted by a first writing operation in which the verify operation is not carried out and a second writing operation in which the verify operation is carried out. Specifically, writing into a state slightly lower (approximately 8v or ΔV) than a final threshold voltage is carried out by the first writing operation, and a verify writing operation is carried out for a remaining shortage (approximately δv or ΔV) of the threshold voltage. Since the amount of writing by the second writing operation is very small, the number of verify times can be greatly decreased as compared with the conventional verify writing. 
         [0035]    Thus, in the case where the writing method of the present invention is used, naturally in the writing operation in which the verify operation is not carried out, also in the writing operation in which the verify operation is carried out, the number of verify times can be decreased, and it becomes possible to greatly shorten the writing time. 
         [0036]    Besides, in the conventional writing method, as shown in  FIG. 2 , when the threshold voltage is increased, the charge injection speed is lowered, and the writing operation becomes slow. On the other hand, in the writing method of the present invention, even in a high threshold voltage, the charge injection speed is constant, and the high speed writing operation becomes possible. 
         [0037]    The structure of the present invention will be set forth below. 
         [0038]    According to the present invention, there is provided an electrically writable and erasable non-volatile memory constituted by a memory element including a charge storage region between an active region and a control gate electrode, characterized in that
       an amount of charge injection into the charge storage region is controlled by controlling an amount of current flowing through the memory element and a drain voltage of the memory element.       
 
         [0040]    Further, according to the present invention, there is provided an electrically writable and erasable non-volatile memory constituted by a memory element including a charge storage region between an active region and a control gate electrode, characterized in that
       charge injection into the charge storage region is carried out by bringing an amount of current flowing through the memory element and a drain voltage of the memory element into a constant state, and   an amount of charge injection into the charge storage region is controlled by controlling a period of time for a current flows to the memory element.       
 
         [0043]    Further, according to the present invention, there is provided an electrically writable and erasable non-volatile memory including at least a memory cell array in which a plurality of memory cells are arranged in a matrix form and a writing circuit, characterized in that
       each of the plurality of memory cells includes a memory element and a selection transistor,   the memory element includes a charge storage region between an active region and a control gate electrode, and   the writing circuit carries out a writing operation by controlling an amount of current flowing through the memory element and a drain voltage of the memory element.       
 
         [0047]    Further, according to the present invention, there is provided an electrically writable and erasable non-volatile memory including at least a memory cell array in which a plurality of memory cells are arranged in a matrix form and a writing circuit, characterized in that
       each of the plurality of memory cells includes a memory element and a selection transistor,   the memory element includes a charge storage region between an active region and a control gate electrode,   the writing circuit has a function to hold an amount of current flowing through the memory element and a drain voltage of the memory element constant, and   the writing circuit carries out a writing operation by controlling a period of time for the amount of the current flowing through the memory element and the drain voltage of the memory element are held constant.       
 
         [0052]    Further, according to the present invention, there is provided an electrically writable and erasable non-volatile memory including at least a memory cell array in which a plurality of memory cells are arranged in a matrix form and a writing circuit, characterized in that
       each of the plurality of memory cells includes a memory element and a selection transistor,   the memory element includes a charge storage region between an active region and a control gate electrode,   the memory element stores states of k values not less than two values by an erase state having a threshold voltage Vth 0 , and states having (k-1) different threshold voltages Vth 1 , Vth 2 , . . . , Vth(k- 1 ) higher than the threshold voltage Vth 0 ,   the writing circuit has a function to hold an amount of current flowing through the memory element and a drain voltage of the memory element constant,   the writing circuit carries out a writing operation by controlling a writing time in which the amount of the current flowing through the memory element and the drain voltage of the memory element are held constant; and   ratios between writing times tw 1 , tw 2 , . . . , tw(k- 1 ) for writing from the erase state to the states having the threshold voltages Vth 1 , Vth 2 , . . . , Vth(k- 1 ) are tw 1  :tw 2 : . . . :tw(k- 1 )=(Vth 1 -Vth 0 ):(Vth 2 -Vth 0 ): . . . :(Vth(k- 1 )−Vth 0 ).       
 
         [0059]    Further, the memory element may store multi-value data. 
         [0060]    Further, the non-volatile memory may carry out a readout operation by reading a gate voltage of the memory element. 
         [0061]    Further, in a writing operation of the non-volatile memory, a verify operation may not be carried out. 
         [0062]    Further, the writing operation of the non-volatile memory includes a first writing operation and a second writing operation, and a verify operation is not carried out in the first writing operation while a verify operation may be carried out in the second writing operation. 
         [0063]    Further, the memory element constituting the non-volatile memory may be a memory transistor in which a first insulating film, a floating gate electrode constituted by a semiconductor film or a conductive film, a second insulating film, and a control gate electrode are successively stacked on the active region. 
         [0064]    The memory element constituting the non-volatile memory may be a memory transistor in which a first insulating film, a cluster layer which includes clusters made of semiconductor or conductor as charge-trapped centers, a second insulating film, and a control gate electrode are successively stacked on the active region. 
         [0065]    The memory element constituting the non-volatile memory may be a memory transistor of MNOS structure or MONOS structure. 
         [0066]    Further, there is provided a semiconductor device characterized in that the non-volatile memory is used as a recording medium. 
         [0067]    Further, there is provided a microprocessor as the semiconductor device. 
         [0068]    Further, there is provided a display, a video camera, a goggle display, a DVD player, a head mount display, a personal computer, a portable telephone, or a car audio, as the semiconductor device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0069]    In the accompanying drawings: 
           [0070]      FIGS. 1A and 1B  are circuit diagrams for explaining a writing operation of a non-volatile memory of the present invention; 
           [0071]      FIG. 2  is a view showing a relation between a threshold voltage of the non-volatile memory of the present invention and a writing time; 
           [0072]      FIG. 3  is a view showing a threshold distribution after the writing operation of the non-volatile memory of the present invention; 
           [0073]      FIG. 4  is a view showing the whole structure of the non-volatile memory of the present invention; 
           [0074]      FIG. 5  is a view showing the partial structure of the non-volatile memory of the present invention; 
           [0075]      FIGS. 6A to 6C  are views showing the partial structure of the non-volatile memory of the present invention; 
           [0076]      FIG. 7  is a view showing the partial structure of the non-volatile memory of the present invention; 
           [0077]      FIG. 8  is a circuit diagram for explaining a writing operation of the non-volatile memory of the present invention; 
           [0078]      FIG. 9  is a view showing an operation point at the time of writing of the non-volatile memory of the present invention; 
           [0079]      FIG. 10  is a circuit diagram for explaining a readout operation of the non-volatile memory of the present invention; 
           [0080]      FIG. 11  is a view showing an operation point at the time of readout of the non-volatile memory of the present invention; 
           [0081]      FIG. 12  is a circuit diagram for explaining an erase operation of the non-volatile memory of the present invention; 
           [0082]      FIG. 13  is a view showing a threshold distribution of the non-volatile memory of the present invention; 
           [0083]      FIG. 14  is a view showing the relation between the threshold voltage of the non-volatile memory of the present invention and the writing time; 
           [0084]      FIG. 15  is a diagram of a writing circuit of the non-volatile memory of the present invention; 
           [0085]      FIG. 16  is a timing chart at the time of writing of the non-volatile memory of the present invention; 
           [0086]      FIG. 17  is a diagram of a writing circuit of the non-volatile memory of the present invention; 
           [0087]      FIG. 18  is a view showing the whole structure of the non-volatile memory of the present invention; 
           [0088]      FIG. 19  is a view showing a partial structure of the non-volatile memory of the present invention; 
           [0089]      FIG. 20  is a circuit diagram for explaining a writing operation of the non-volatile memory of the present invention; 
           [0090]      FIG. 21  is a circuit diagram for explaining a readout operation of the non-volatile memory of the present invention; 
           [0091]      FIG. 22  is a circuit diagram for explaining an erase operation of the non-volatile memory of the present invention; 
           [0092]      FIG. 23  is a view showing a semiconductor circuit using the non-volatile memory of the present invention; 
           [0093]      FIGS. 24A to 24F  are views showing electronic instruments using the non-volatile memory of the present invention; 
           [0094]      FIGS. 25A and 25B  are views showing electronic instruments using the non-volatile memory of the present invention; and 
           [0095]      FIGS. 26A to 26D  are sectional views of memory elements constituting the non-volatile memory of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0096]    The mode for carrying out the invention will next be described. 
         [0097]    In this embodiment mode, a description will be given of a circuit structure of a non-volatile memory for carrying out the foregoing writing operation and an operation method. Incidentally, here, for simplification, although the description is given of the case of a two-value non-volatile memory and a writing method in which a verify operation is not carried out, it is easy to apply this embodiment mode to a multi-value non-volatile memory and a case where the verify writing is carried out. 
         [0098]      FIG. 4  shows an example of a circuit diagram of a non-volatile memory of the present invention including a memory cell array of m rows and n columns (m and n are respectively integers equal to or larger than 1). The non-volatile memory shown in  FIG. 4  is constituted by a memory cell array  401 , an X-address decoder  402 , a Y-address decoder  403 , control circuits  404 ( 1 ) to  404 ( n ), a read/write circuit  405 , and other peripheral circuits (not shown). The other peripheral circuits include an address buffer circuit, a power generation circuit, a booster circuit, a power control circuit, other control circuits and the like, and are provided as the need arises. 
         [0099]    Incidentally, in the present specification, although a writing circuit indicates the whole circuit concerned with a writing operation, there is also a case where only a part thereof that is concerned with the writing operation is picked out and is called the writing circuit. For example, in the circuit diagram of the non-volatile memory shown in  FIG. 4 , although the writing circuit indicates the whole circuit concerned with the writing operation including the write/read circuit  405  and the control circuits  404 , there is also a case where the writing circuit indicates only the write/read circuit  405  as a part thereof. Besides, the same is true of the readout circuit, and although the readout circuit indicates the whole circuit concerned with a readout operation, there is also a case where only a part thereof is picked out and is called the readout circuit. In the case where it is not clear which one the word exactly indicates, it is assumed that the word indicates the whole of the writing circuit and the readout circuit concerned with the whole operation. 
         [0100]    In the non-volatile memory of this embodiment mode, each memory cell is constituted by a memory transistor MTr and a selection transistor STr. The memory transistor MTr is a memory transistor including a floating gate, and the selection transistor STr is a normal transistor. As the memory transistor MTr, it is also possible to use a memory element in which a charge storage region is spatially discrete, such as a memory transistor including a cluster layer, or a memory transistor of MNOS structure or MONOS structure (see embodiment 3). In the case where each memory cell stores one-bit data, the non-volatile memory of this embodiment mode has memory capacity of m×n bits. 
         [0101]    In  FIG. 4 , a memory cell (i,j) (1≦i ≦m, 1≦j≦n) is connected to a selection line SLi, a word line WLj, a bit line BLj, and a common source line SC. Specifically, the word line WLj is connected to a control gate electrode of the memory transistor MTr, and the common source line SC is connected to a source electrode thereof. Besides, the selection line SLi is connected to a gate electrode of the selection transistor STr, and the bit line BLi is connected to a drain electrode thereof. The memory transistor MTr and the selection transistor STr constituting the memory cell (i, j) are connected in series with each other. The selection lines SL 1  to SLm are connected to the Y-address decoder  403 , and the word line WLj and the bit line BLj are connected to the control circuit  404 ( j ) (1≦j≦n). The control circuits  404 ( 1 ) to  404 ( n ) are connected to the X-address decoder  402  and the read/write circuit  405 . 
         [0102]    Next, the control circuits  404 ( 1 ) to  404 ( n ) will be described. Since all the n control circuits are equal to one another, hereinafter, one of them is exemplified as a control circuit  404 .  FIG. 5  shows an example of a circuit diagram of the control circuit  404 , and the control circuit  404  is constituted by an operational amplifier  501 , a constant current source  502 , and a plurality of switch transistors  503  to  510 . One of inputs of the operational amplifier  501  is connected to a voltage Vin, and the other is connected to the bit line BL through the constant current source  502  and the switch transistors  506  and  508 . Besides, the word line WL is connected to the output of the operational amplifier  501  through the switch transistors  505  and  507 , is connected to an erase voltage Verase through the switch transistors  503  and  510 , and is connected to a readout voltage Vout through the switch transistors  504  and  509 . 
         [0103]    The switch transistors  503  and  504  to  506  are respectively connected to output signals Vxdec(b) and Vxdec(a) of the X-address decoder, the switch transistors  507  and  508  are connected to a write/read signal Sw/r, the switch transistor  509  is connected to a readout signal Sread, and the switch transistor  510  is connected to an erase signal Serase. Further, the output signal Vxdec(a) of the X-address decoder and the write/read signal Sw/r are connected to the operational amplifier  501 . 
         [0104]    As the operational amplifier  501  and the constant current source  502 , for example, circuit diagrams shown in  FIGS. 6A to 6C  and  FIG. 7  can be used.  FIG. 6A  shows a symbol of the operational amplifier  501 , and terminals A, B, C, D and E are defined.  FIG. 6B  is a circuit diagram of the operational amplifier  501 . The operational amplifier  501  is a differential amplification circuit constituted by two p-channel transistors  602  and  603 , two n-channel transistors  604  and  605 , and a constant current source  601 . Further, as a switch of the operational amplifier, n-channel switch transistors  606  and  607  are connected in series with the constant current source.  FIG. 6C  shows, as the constant current source  601 , a current mirror type constant current source made of a load resistance  610 , and two n-channel transistors  608  and  609 .  FIG. 7  shows a current mirror type constant current source made of a load resistance  703  and two p-channel transistors  701  and  702 , as a circuit diagram of the constant current source  502  in  FIG. 5 . Of course, other well-known circuits may be used as the operational amplifier  501  and the constant current source  502 . 
         [0105]    Next, an operation method of the non-volatile memory of this embodiment mode will be described. Here, writing and readout to the memory cell (1, 1), and simultaneous erasing of the memory cells (1, 1) to (1, m) of the first column will be described. Of course, a similar operation can be carried out for other memory cells as well. Incidentally, in this embodiment mode, the description will be given of the case where each memory transistor stores two-value data, and a verify operation is not carried out at the time of writing. 
         [0106]    First, before each operation is carried out, the memory cell of the first column is selected by the X-address decoder. Specifically, the switch transistors  503  to  506  constituting the control circuit  404 ( 1 ) are turned on by the output signal of the X-address decoder, and all the switch transistor  503  to  506  constituting the control circuits  404 ( 2 ) to  404 ( n ) are turned off. At the same time with the selection of the control circuit  404 ( 1 ), an operation mode of the control circuit  404 ( 1 ) is selected by the erase signal Serase, the write/read signal Sw/r, and the readout signal Sread. 
         [0107]    All the switch transistors  509  and  510  connected to the erase signal Serase and the readout signal Sread are turned off in the writing operation. Hereinafter, a signal by which all switch transistors connected to a signal line are turned off is called an off signal, and a signal by which all switch transistors connected to a signal line are turned on is called an on signal. That is, the off signal is inputted as the erase signal Serase and the readout signal Sread. On the other hand, the on signal is inputted as the write/read signal Sw/r. As a result, a circuit structure concerned with the writing operation to the memory cell (1, 1) can be expressed as in  FIG. 8 . In  FIG. 8 , reference character VWL designates a potential of the word line; VBL, a potential of the bit line; Vsc, a potential of the common source line; Vsel, a potential of the selection line; Vpgm, a writing potential; and I, a current supplied by the constant current source. The circuit diagram shown in  FIG. 8  is the same as the circuit diagram shown in  FIG. 1  except for the selection transistor STr, and the operation principle thereof is the same as that already described. 
         [0108]    The operation voltage at the time of writing may be made, for example, Vsel=10V, Vsc=GND, Vpgm=6 V, and I=10 mA. An operation point of the memory transistor in this case is shown in  FIG. 9 . Curves  901  and  902  respectively express Vd-Id curves of the memory transistor MTr and the selection transistor STr, and the operation point of the memory transistor MTr is expressed by an intersection P of the two curves. Here, the conductance of the selection transistor STr is set sufficiently larger than the conductance of the memory transistor MTr. It is preferable that the operation voltage at the time of writing is set such that the writing voltage Vpgm is made to have such a magnitude that weak impact ionization occurs, and the operation point of the memory transistor is set to be within a saturation region close to a linear region. 
         [0109]    The writing operation in which a charge injection speed to the memory cell (1, 1) is constant becomes possible by the foregoing writing method. In this embodiment mode, the memory transistor stores one-bit (two values of “0” and “1”) information, nothing is done in the case where “0” is written, and the foregoing writing operation is carried out for the time tw in the case where “1” is written. Incidentally, it is assumed that the writing operation is carried out in the state where threshold voltages Vth are well uniform. The time tw is a writing time necessary for obtaining the state of “1” and is obtained in advance. In this embodiment mode, the verify operation is thus not carried out, but only the writing operation for the time tw is carried out, so that the writing time can be greatly shortened as compared with the conventional verify writing. Of course, the reason why such writing operation is enabled is that the threshold distribution having high accuracy and a small distribution width can be obtained by the writing method of the present invention. 
         [0110]    In the memory cells (2, 1) to (m, 1) in which writing is not carried out, the potential Vsel of the selection line is made 0 V, and the selection transistors are turned off. Further, it is necessary to set the operation voltage at the time of writing so that the potential VWL of the word line at the time of writing has such a degree that erroneous writing by a tunnel current is not carried out in the memory cells (2, 1) to (m, 1). Besides, with respect to the memory cells other than those of the first column, the control circuits  404 ( 2 ) to  404 ( n ) are in non-selection states, and both the bit line and the word line are brought into floating states, so that there is no problem of erroneous operation. 
         [0111]    Of course, in a column in which the control circuit  404  is in the non-selection state, a circuit may be structured so that the bit line and the word line have 0 V. 
         [0112]    Next, a readout operation will be described. In the readout operation, the off signal is inputted as the erase signal Serase, and the on signal is inputted as the write/read signal Sw/r and the readout signal Sread. As a result, a circuit structure concerned with the readout operation to the memory cell (1, 1) can be expressed as in  FIG. 10 . In  FIG. 10 , reference character VWL designates a potential of the word line; VBL, a potential of the bit line; Vsc, a potential of the common source line; Vsel, a potential of the selection line; and Vout, a readout potential to be outputted. A potential Vread is inputted to an operational amplifier  1001 , and a constant current I is supplied from a constant current source  1002 . The circuit diagram shown in  FIG. 10  is identical with the circuit diagram shown in  FIG. 1  except for the selection transistor STr and an output portion of the readout potential Vout, and the operation principle thereof is the same as that already described. 
         [0113]    The operation voltage at the time of readout may be made, for example, Vsel=3 V, Vsc=GND, Vread=1 V, and I=1 mA. An operation point of the memory transistor in this case is shown in  FIG. 11 . Curves  1101  and  1102  respectively express Vd-Id curves of the memory transistor MTr and the selection transistor STr, and the operation point of the memory transistor MTr is expressed by an intersection P of the two curves. Incidentally, the conductance of the selection transistor STr is set sufficiently larger than the conductance of the memory transistor MTr. It is necessary that the operation voltage at the time of readout is made to satisfy conditions under which hot electrons are not generated by impact ionization. For that purpose, as shown in  FIG. 11 , it is appropriate that Vread is set low. 
         [0114]    Then, data stored in the memory cell can be read out by reading the readout potential Vout under this operation voltage. This readout operation uses a phenomenon that when the threshold voltage of the memory transistor MTr is increased by AVth, the control gate voltage of the memory transistor MTr, that is, the readout potential Vout is also increased by ΔVth due to negative feedback characteristics of the control circuit  404 ( 1 ). 
         [0115]    In the memory cells (2, 1) to (m, 1) in which readout is not carried out, the potential Vsel of the selection line is made 0 V, and the selection transistor is turned off. As a result, the readout potential Vout is determined only for the memory cell (1, 1), and there is no fear of erroneous readout. Besides, since the operation voltage is low, there is also no problem of erroneous writing. Also with respect to the memory cells other than those of the first column, the control circuits  404 ( 2 ) to  404 ( n ) are in non-selection states, and both the bit line and the word line are in floating states, so that there is no problem of erroneous operation. 
         [0116]    Of course, in a column in which the control circuit  404  is in the non-selection state, a circuit may be structured such that the bit line and the word line have 0 V. 
         [0117]    Incidentally, in this embodiment mode, although the method of reading the word line potential is adopted as the readout method, a method of reading out a bit line potential can also be used. 
         [0118]    Finally, an erase operation will be described. At the time of the erase operation, the on signal is inputted as the erase signal Serase, and the off signal is inputted as the write/read signal Sw/r and the readout signal Sread. As a result, a circuit structure concerned with the erase operation to the memory cells (1, 1) to (m, 1) is expressed as in  FIG. 12 . In  FIG. 12 , reference character VWL designates a potential of the word line; Vsc, a potential of the common source line; and Vsel(1) to Vsel(m), potentials of the selection lines. As shown in  FIG. 12 , in the erase operation, an operational amplifier and a constant current source do not participate. 
         [0119]    As the operation voltage at the time of erasing, it is appropriate that for example, VWL is made −8 V, potentials Vsel(1) to Vsel(m) of all selection lines are made 0 V, and a substrate potential (or well potential) is made  8  V. As a result, a high potential difference is generated between the control gate and the substrate of the memory transistor constituting the memory cell of the first column, and a charge stored in the floating gate is drawn into the substrate by an FN tunnel current. Incidentally, the selection transistor is turned off, and the drain region of the memory transistor is brought into the floating state. With respect to the memory cells other than those of the first column, the control circuits  404 ( 2 ) to  404 ( n ) are in the non-selection state, and the word line is in the floating state, so that there is no problem of erroneous erasing. Incidentally, instead of making the substrate potential 8 V, it is also possible to provide a partially overlapping region (overlap region) between the source region and the floating gate of the memory transistor, and to make the common source potential Vsc 8 V. 
         [0120]    In order to decrease a distribution width after a writing operation in which a verify operation is not carried out, it is naturally necessary that a distribution width of a threshold voltage before writing is small. However, in the foregoing erase operation, it is generally difficult to realize a narrow distribution width. Then, in this embodiment mode, after the erase operation by the FN tunnel, verify writing into an erase state is newly carried out. It is preferable that the threshold voltage of the erase state is made a value slightly higher than that of the distribution after erasing. Incidentally, a well-known circuit structure and a well-known operation method may be used in the verify writing into the erase state. 
         [0121]    In the manner as described above, the writing operation, the readout operation, and the erase operation of the non-volatile memory of the present invention are carried out. The operation voltages explained in this embodiment mode are summarized as shown in Table 1. Incidentally, in Table 1, an operation voltage in the case where, as an erase method, a charge is drawn into a source region is written. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Writing 
                 Readout 
                 Erase 
               
               
                   
                 time 
                 time 
                 time 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Sw/r 
                 ON 
                 ON 
                 OFF 
               
               
                   
                 Sread 
                 OFF 
                 ON 
                 OFF 
               
               
                   
                 Serase 
                 OFF 
                 OFF 
                 ON 
               
               
                   
                 Vpgm/Vread 
                  6 V 
                 1 V 
                 — 
               
               
                   
                 I 
                 10 mA 
                 1 mA 
                 — 
               
               
                   
                 Vsel 
                 10 V 
                 3 V 
                   0 V 
               
               
                   
                 Vsc 
                  0 V 
                 0 V 
                   8 V 
               
               
                   
                 Verase 
                 — 
                 — 
                 −8 V 
               
               
                   
                   
               
             
          
         
       
     
         [0122]    In this embodiment mode, the two-value non-volatile memory has been described. However, the present invention is not limited to the two-value non-volatile memory. Rather, since the feature of the non-volatile memory of the present invention exists in high accuracy threshold controllability, its effect is exhibited especially in a multi-value non-volatile memory in which a margin of a threshold distribution is small. When the present invention is applied to the multi-value non-volatile memory, as compared with the prior art, the writing time can be shortened, and a high multi-value degree can be realized. With respect to an example in which the present invention is applied to the multi-value memory, reference can be made to embodiment 1. Incidentally, in the present specification, the number of states which one memory cell can store is called a multi-value degree. 
         [0123]    Besides, in this embodiment mode, although the writing method in which the verify operation is not performed has been described, a writing method using a verify operation may be carried out, of course. In that case, it is appropriate that the writing operation is constituted by a first writing operation in which the verify operation is not carried out and a second writing operation in which the verify operation is carried out. Specifically, it is preferable that first, in the first writing operation, high accuracy threshold control characteristics are actively used, and writing into a state slightly lower than a final threshold voltage is carried out, and then, verify writing as to an insufficiency of remaining threshold voltage is carried out. As a result, as compared with the conventional verify writing, the number of times of verify operation can be decreased, and a verify writing time can be shortened. Incidentally, a well-known circuit structure and a well-known operation method may be used for the verify writing. 
         [0124]    In this embodiment mode, although the writing and readout to the memory cell (1, 1) and the simultaneous erasing of the memory cells (1, 1) to (m, 1) of the first column has been described, it is also possible to simultaneously carry out these operations over plural columns. Especially, by selecting all the control circuits  404 ( 1 ) to  404 ( n ), it is possible to carry out the simultaneous writing and readout to the memory cells (1, 1) to (1, n) of the first row, and the simultaneous erasing of all the memory cells. Of course, as the write/read circuit  405 , a circuit capable of processing n-bit data in parallel becomes necessary. Besides, it is necessary that the total of amounts of currents simultaneously flowing in the respective operations does not exceed current driving capacity of the circuit. 
         [0125]    Incidentally, since the writing method of the present invention uses negative feedback, there may be a case where it takes a time until the operation voltage becomes stable. Thus, it is preferable that as the need arises, a discharge circuit or a precharge circuit is provided in the control circuit to shorten a period of time until it takes the operation voltage becomes stable. 
       Embodiment 1 
       [0126]    In this embodiment, a description will be given of an example in which a non-volatile memory of the present invention is applied to a multi-value memory. The non-volatile memory of the present invention is characterized by a writing operation superior in controllability of threshold voltage, and its effect is exhibited especially in a multi-value non-volatile memory in which a margin of a threshold distribution is small. In the multi-value non-volatile memory to which the present invention is applied, it is possible to shorten a writing time as compared with the prior art, and can realize a high multi-value degree. 
         [0127]    The multi-value non-volatile memory is constituted by a memory element capable of storing three or more states. For example, there is a method of dividing a threshold voltage of a memory element into three or more states, or a method of using a memory element capable of storing electric charges in plural places. In this embodiment, a description will be given of a four-value non-volatile memory in which the threshold voltage of the memory element is divided into four states, and four states of “0” (erase state), “1”, “2” and “3” are stored.  FIG. 13  shows distributions of the threshold voltages expressing the four states. In  FIG. 13 , Vth 0 , Vth 0 , Vth 2 , and Vth 3  are respectively center values of the threshold distributions of the states “0”, “1”, “2” and “3”. 
         [0128]      FIG. 14  shows a relation between the threshold voltage of the memory element and a writing time in the case where the writing method of the present invention is used. In  FIG. 14 , times tw 1 , tw 2 , and tw 3  respectively designate writing times from the erase state “0” to the states “1”, “2” and “3”. Since the threshold voltage and the writing time have a linear relation, in the case where intervals between the threshold voltages of the four states are equal to one another, ratios between tw 1 , tw 2  and tw 3  become tw 1 : tw 2 : tw 3 =1:2:3. Like this, in the writing method of the present invention, writing into the respective states can be carried out easily and with high accuracy by controlling the writing time. More generally, in the case where writing is carried out by one writing operation, it is appropriate that the ratios between necessary writing times are made coincident with the ratios between intervals of threshold voltages in the respective states. 
         [0129]    For example, in the case where the memory element stores states of k values not lower than two values by dividing them into the erase state having the threshold voltage Vth 0  and the states having (k- 1 ) different threshold voltages Vth 1 , Vth 2 , Vth(k- 1 ) higher than the threshold voltage Vth 0 , it is appropriate that the ratios between the writing times tw 1 , tw 2 , . . . , tw(k- 1 ) for writing from the erase state to the states having the threshold voltages Vth 1 , Vth 2 , Vth(k- 1 ) are made tw 1 : tw 2 : :tw(k- 1 )=(Vth 1 -Vth 0 ):(Vth 2 -Vth 0 ): . . . :(Vth(k- 1 )−Vth 0 ). 
         [0130]    Hereinafter, a circuit structure and an operation method of the four-value non-volatile memory of the present invention will be described. For simplification, it is assumed that threshold distributions expressing four-value states are as shown in  FIG. 13 , and intervals between threshold voltages of the respective states are equal to one another. 
         [0131]    The circuit structure of the multi-value non-volatile memory may be basically made equal to that of the two-value non-volatile memory, and with respect to a memory cell array, an address decoder, a control circuit, and the like, the same circuits as those of the circuit diagrams shown in  FIGS. 4 to 7  can be used. The multi-value circuit structure is different from the two-value circuit structure mainly in a readout circuit and a writing circuit. In the four-value non-volatile memory, it is necessary that the readout circuit distinguishes between four states and read them out as two-bit data, and the writing circuit carries out writing into the four states on the basis of the two-bit data. Accordingly, the readout circuit and the writing circuit become different for every non-volatile memory (including that of two values) different in the multi-value degree. 
         [0132]    As the writing method, one writing operation in which a verify operation is not carried out is considered. In this case, it is appropriate that on the basis of the inputted two-bit data, the writing operation in which the writing time is 0, tw 1 , tw 2 , or tw 3  is carried out. The writing operation may be made similar to the method explained in this embodiment mode, and the on pulse signal having a width of 0, tw 1 , tw 2  or tw 3  may be inputted as the write/read signal. Of course, it is desirable to use, as the operation voltage, an optimum value determined by the writing time and the like. In the writing method of the present invention, since a charge injection speed is constant, writing can be carried out by simple time control in which the ratios of the writing times to the four states become 0:1:2:3. 
         [0133]    As the readout method, first, similarly to the two-value memory, the readout potential Vout is read out of the control circuit. The readout circuit judges the states “0”, “1”, “2” and “3” according to the value of the read Vout, and conversion into two-bit data is carried out. As the readout circuit, a well-known readout circuit used for a conventional multi-value non-volatile memory can be used. 
         [0134]    As the erase method, since it is appropriate that erasing to “0” (erase state) is carried out irrespective of the state of the memory element, similarly to the two-value memory, the erase operation by the FN tunnel current may be carried out. Incidentally, in the case where the verify operation is not carried out in writing, since it is necessary to make threshold voltages before writing uniform, similarly to the case explained in this embodiment mode, it is preferable that the verify writing into the erase state “0” is carried out after the FN tunnel erasing. A well-known circuit structure and a well-known operation method may be used for verify writing into the erase state “0”. 
         [0135]    The example of the writing operation will be described in more detail.  FIG. 15  shows an example of a writing circuit diagram. A writing circuit  1501  is constituted by a latch  1502 , a pulse generation circuit  1503 , and the like. When two-bit writing data is inputted to the writing circuit  1501 , it is held by the latch  1502 , and a plurality of signals are formed by the pulse generation circuit  1503 . These signals are suitably selected in accordance with the data held in the latch  1502 , and are outputted as a writing pulse signal Sw and a writing finish signal Swf. 
         [0136]      FIG. 16  shows a timing chart of the writing pulse signal Sw and the writing finish signal Swf. As shown in  FIG. 16 , the writing pulse signals Sw have different pulse widths on the basis of the inputted data, and the writing finish signal Swf rises after the writing pulse falls down and notifies the finish of the writing. Specifically, in the case where data corresponding to the states “0”, “1”, “2”, or “3” is inputted, the writing signal Sw has the pulse width of 0, tw 1 , tw 2  or tw 3 , respectively. 
         [0137]    The outputted writing pulse signal Sw is connected to the input terminal of an operational amplifier and a switch transistor for switching the connection with Vpgm and GND. In the case where the writing pulse signal Sw is on, Vpgm is connected to the input terminal of the operational amplifier, and in the case where it is off, GND is connected to the input terminal of the operational amplifier. 
         [0138]    In the foregoing pulse generation circuit  1503 , for example, three rising signals in which a time is shifted by tw 1 , tw 2  and tw 3  are prepared by a shift register, and pulses having different widths can be prepared by taking an exclusive OR (XOR) to the original signal. The writing pulse signal Sw may be selected from the pulse signals having different widths, and the writing finish signal Swf may be selected from the rising signals shifted in time, on the basis of the input data. 
         [0139]    In the manner described above, the writing operation, the readout operation, and the erase operation of the non-volatile memory of the present invention are carried out. 
         [0140]    In this embodiment, although the description has been given of the four-value case in behalf of the multi-value memory, the present invention can be applied to the multi-value memory of 8 values, 16 values, or higher values quite similarly to this embodiment. Besides, in this embodiment, although the description has been given of the case where intervals between the threshold distributions expressing the multi-value states are uniform, even in the case where the intervals between the threshold distributions expressing the multi-value states are not uniform, a completely similar operation can be carried out by merely changing the writing pulse width. 
         [0141]    In the writing operation of the present invention, as compared with the prior art, the distribution width of the threshold voltage after writing is small, and the controllability of the distribution is superior, so that also in the multi-value non-volatile memory, it becomes possible to carry out writing by one writing operation in which the verify operation is not carried out. Also in the case where the verify operation is carried out, first, writing (first writing) into a state in which threshold voltage is slightly lower than a state after writing is carried out, and next, verify writing (second writing) is carried out, so that it becomes possible to greatly shorten the writing time. Incidentally, verify writing may be started after confirmation of the rising of the writing finish signal Swf. Although a verify writing circuit is not shown, a well-known circuit structure and a well-known operation method may be used. 
         [0142]    Besides, the foregoing operation method of the multi-value memory can be carried out simultaneously over plural columns. In that case, a writing circuit diagram as shown in  FIG. 17  can be used. In  FIG. 17 , a latch  1702  is provided for each column, and in accordance with data held in the latch  1702 , a suitable writing pulse signal is selected from a common pulse generation circuit  1701 . As a writing finish signal Swf, it is appropriate that for example, a signal rising after a writing time tw 3  is used. In this way, by making the writing operation, the readout operation, and the erase operation parallel, the speed of the operation can be made high. 
         [0143]    Incidentally, as a method of controlling a threshold voltage with high accuracy, although the control method using time as described above is preferable, more generally, the writing voltage Vpgm and the writing time are freely set and the writing operation to the respective multi-value states may be carried out. 
       Embodiment 2 
       [0144]    In this embodiment, a description will be given of an example which is different from the circuit structure of the non-volatile memory described in this embodiment mode.  FIG. 18  is a circuit diagram of a non-volatile memory of this embodiment including a memory cell array of m rows and n columns (m and n are respectively integers equal to or larger than 1). The non-volatile memory shown in  FIG. 18  is constituted by a memory cell array  1801 , an X-address decoder  1802 , a Y-address decoder  1803 , control circuits  1804 ( 1 ) to  1804 ( n ), a read/write circuit  1805 , and other peripheral circuits (not shown). The other peripheral circuits include an address buffer circuit, a power generation circuit, a boosting circuit, a power control circuit, other control circuits, and the like, and are provided as the need arises. 
         [0145]    Each memory cell is constituted by a memory transistor MTr and a selection transistor STr. The memory transistor MTr is a transistor including a floating gate, and the selection transistor STr is a normal transistor. As the memory transistor MTr, a memory transistor including a cluster layer, or a memory transistor of MNOS structure or MONOS structure may be used (see embodiment 3). 
         [0146]    In  FIG. 18 , a memory cell (i, j) (1≦i≦m, j≦n) is connected to a selection line SLi, a word line WLj, a bit line BLj, and a common source line SC. Specifically, one of a source and a drain electrodes of the selection transistor STr is connected to a control gate electrode of the memory transistor MTr, the common source line SC is connected to a source electrode thereof, and the bit line BLi is connected to a drain electrode thereof. The word line WLj is connected to the remaining one of the source and the drain electrodes of the selection transistor STr, and the selection line SLi is connected to a gate electrode thereof. The selection lines SL 1  to SLm are connected to the Y-address decoder  1803 , and the word line WLj and the bit line BLj are connected to the control circuit  1804 ( j ) ( 1 ≦j≦n). Besides, the control circuits  1804 ( 1 ) to  1804 ( n ) are connected to the X-address decoder  1802  and the read/write circuit  1805 . 
         [0147]    Next, the control circuits  1804 ( 1 ) to  1804 ( n ) will be described. Since all the n control circuits are quite identical to one another, one of them is called a control circuit  1804  in behalf of them.  FIG. 19  shows an example of a circuit diagram of the control circuit  1804 , and the control circuit  1804  is constituted by an operational amplifier  1901 , a constant current source  1902 , and a plurality of switch transistors  1903  to  1911 . Since the operational amplifier  1901 , the constant current source  1902 , and the plurality of switch transistors  1903  to  1910 , which constitute the control circuit  1804 , are quite identical to those of the control circuit shown in  FIG. 5 , their explanation is omitted. As circuit diagrams of the operational amplifier  1901  and the constant current source  1902 , for example, the circuit diagrams shown in  FIGS. 6 and 7  can be used. Of course, another well-known circuit can also be used. The control circuit  1804  is different from the control circuit shown in  FIG. 5  in that the former includes a discharge circuit. That is, the word line WL is connected to GND through the switch transistor  1911 . A gate electrode of the switch transistor  1911  is connected to a discharge signal Sdc. 
         [0148]    Next, an operation method of the non-volatile memory of this embodiment will be described. Here, writing and readout to the memory cell (1, 1) and simultaneous erasing of the first column memory cells (1, 1) to (1, m) will be described. Of course, a similar operation can be carried out for other memory cells. Incidentally, in this embodiment, a description will be given of a case where each memory transistor stores two-value data, and a verify operation is not carried out at the time of writing. 
         [0149]    First, before respective operations are carried out, the first column memory cells are selected by the X-address decoder. Specifically, the switch transistors  1903  to  1906  constituting the control circuit  1804 ( 1 ) are turned on by the output signal of the X-address decoder, and all the switch transistors  1903  to  1906  constituting the control circuits  1804 ( 2 ) to  1804 ( n ) are turned off. At the same time as the selection of the control circuit  1804 ( 1 ), the operation mode of the control circuit  1804 ( 1 ) is selected by the erase signal Serase, the write/read signal Sw/r, and the readout signal Sread. 
         [0150]    The feature of the circuit of the memory cell of this embodiment is that the selection transistor is connected between the control gate electrode of the memory transistor MTr and the word line WL. As described below, in the writing and readout operations, since the selection transistor constituting the row in which an operation is not carried out is turned off, the control gate electrode of the memory transistor MTr is brought into the floating state. In the driving method of this embodiment, in order to prevent an erroneous operation from occurring in these memory cells, before the selection transistor is turned off in the writing and readout operations, the control gate electrodes of all the memory transistors MTr are connected to GND using the discharge circuit. 
         [0151]    First, in the writing operation, the off signal is inputted as the erase signal erase and the readout signal Sread, and the on signal is inputted as the write/read signal Sw/r. As a result, a circuit structure concerned with the writing operation to the memory cell (1, 1) is expressed as in  FIG. 20 . In  FIG. 20 , reference character VWL designates a potential of the word line; VBL, a potential of the bit line; Vsc, a potential of the common source line; Vsel, a potential of the selection line; Vpgm, a writing potential; I, a current supplied by the constant current source; and Sdc, a discharge signal. The circuit diagram shown in  FIG. 20  is coincident with the circuit diagram shown in  FIG. 1  except for the selection transistor STr and the discharge circuit, and the operation principle is not described since its explanation has already been given. 
         [0152]    The operation voltage at the time of writing may be made, for example, Sdc=0 V, Vsel=12 V, Vsc=GND, Vpgm=6 V, and I=10 mA. It may be considered that the operation point of the memory transistor in this case is substantially the same as the example (see  FIG. 9 ) shown in this embodiment mode. With respect to the operation voltage at the time of writing, it is preferable that the writing voltage Vpgm is made to have such a magnitude that weak impact ionization occurs, and the operation point of the memory transistor is set to be within a saturation region close to a linear region. 
         [0153]    In this embodiment, the memory transistor stores one-bit (two values of “0” and “1”) information, and in the case where “0” is written, nothing is done, and in the case where “1” is written, the foregoing writing operation is carried out for a previously examined time tw. Incidentally, it is assumed that the writing operation is carried out under the conditions that the threshold voltage Vth is positive and is well uniform. 
         [0154]    Thus in this embodiment, since the verify operation is not carried out but the writing operation for the time tw is carried out, so that as compared with the conventional verify writing, the writing time can be greatly shortened. Of course, the reason why such writing operation becomes possible is that the threshold distribution having high accuracy and small threshold distribution can be obtained by the writing method of the present invention. 
         [0155]    After the writing operation is carried out for the time tw, the writing operation is finished in the manner described below. First, in the control circuit, the off signal is inputted as the write/read signal Sw/r, and the on signal is inputted as the discharge signal Sdc, so that the potentials of all the word lines WL are connected to GND. At the same time, the potentials Vsel of all the selection liens SL are made 3 V, and after the control gate electrodes of all the memory transistor are connected to GND, the potentials Vsel of all the selection lines SL are connected to 0 V. At the same time, the off signal is inputted as the discharge signal Sdc. 
         [0156]    Incidentally, in the writing operation, in the memory cells (2, 1) to (m, 1) in which writing is not carried out, the potential Vsel of the selection line is made 0 V, and the selection transistor is turned of In this case, although the control gate electrodes of the memory transistors constituting the memory cells (2, 1) to (m, 1) are brought into the floating states, as described above, the potential of the control gate is 0 V, and the threshold voltage Vth of the memory transistor is made uniform to be positive, so that the memory transistor is turned off and there is no fear of erroneous operation. With respect to the memory cells other than the first column, since the control circuits  1804 ( 2 ) to  1804 ( n ) are in the non-selection state, and both the bit line and the word line are in the floating state, there is no problem of erroneous operation. 
         [0157]    Of course, in the column in which the control circuit  1804 ( i ) (i is an integer of 1 to n) is in the non-selection state, a circuit may be constructed such that the bit line and the word line have 0 V. 
         [0158]    Next, a readout operation will be described. In the readout operation, the off signal is inputted as the erase signal Serase, and the on signal is inputted as the write/read signal Sw/r and the readout signal Sread. As a result, a circuit structure concerned with the readout operation to the memory cell (1, 1) can be expressed as in  FIG. 21 . In  FIG. 21 , reference character VWL designates a potential of the word line; VBL, a potential of the bit line; Vsc, a potential of the common source line; Vsel, a potential of the selection line; Vout, a readout potential to be outputted; and Sdc, a discharge signal. Besides, a potential Vread is inputted to an operational amplifier  2101 , and a constant current I is supplied from a constant current source  2102 . The circuit diagram shown in  FIG. 21  is coincident with the circuit diagram shown in  FIG. 1  except for a selection transistor STr, a discharge circuit, and an output portion of the readout potential Vout, and the operation principle is the same as that already explained. 
         [0159]    The operation voltage at the time of readout may be made, for example, Sdc=0 V, Vsel=5 V, Vsc=GND, Vread=1 V, and I=1 mA. We may consider that the operation point of the memory transistor in this case is substantially the same as the example (see  FIG. 11 ) shown in this embodiment mode. It is necessary that the operation voltage at the time of readout satisfies the conditions under which hot electrons are not generated by impact ionization. For that purpose, it is appropriate that Vread is set low. 
         [0160]    Data stored in the memory cell can be read out by reading the readout potential Vout under this operation voltage. This readout operation uses a phenomenon that when the threshold voltage of the memory transistor is increased by ΔVth, the control gate voltage of the memory transistor, that is, the readout potential Vout is also increased by ΔVth through negative feedback characteristics. 
         [0161]    Also after the readout operation is carried out, the control gate electrodes of all the memory transistors are connected to GND by a discharge operation. First, in the control circuit, the off signal is inputted as the write/read signal Sw/r, and the on signal is inputted as the discharge signal Sdc, so that the potentials of all the word lines WL are connected to GND. At the same time, the potentials Vsel of all the selection lines SL are made 3V, and after the control electrodes of all the memory transistors are connected to GND, the potentials Vsel of all the selection lines SL are connected to 0 V. At the same time, the off signal is inputted as the discharge signal Sdc. 
         [0162]    Incidentally, in the memory cells (2, 1) to (m, 1) in which readout is not carried out, the potentials Vsel of the selection lines are made 0 V, and the selection transistors are turned off. As a result, the memory transistors constituting the memory cells (2, 1) to (m, 1) are turned off, and the readout potential Vout is determined only by the memory cell (1, 1), so that there is no fear of erroneous readout. Besides, since the operation voltage is low, there is no problem of erroneous writing as well. With respect to the memory cells other than the first column, the control circuits  1804 ( 2 ) to  1804 ( n ) are in the non-selection state, and both the bit line and the word line are in the floating state, so that there is no problem of erroneous operation. 
         [0163]    Incidentally, in this embodiment, although the method of reading out the word line potential is adopted as the readout method, a method of reading out a bit line potential can also be used. 
         [0164]    Finally, an erase operation will be described. At the time of the erase operation, the on signal is inputted as the erase signal Serase, and the off signal is inputted as the write/read signal Sw/r and the readout signal Sread. Besides, the off signal is inputted as the discharge signal Sdc. As a result, a circuit structure concerned with the erase operation to the memory cells (1, 1) to (m, 1) is expressed as shown in  FIG. 22 . In  FIG. 22 , reference character VWL designates a potential of the word line; Vsc, a potential of the common source line; and Vsel(1) to Vsel(m), potentials of the selection lines. As shown in  FIG. 22 , in the erase operation, an operation amplifier and a constant current source do not participate. 
         [0165]    The operation voltage at the time of erasing may be made, for example, VWL=−8 V, the potentials Vsel of all the selection lines=0 V, and a substrate potential (or well potential)=8 V. As a result, the selection transistors constituting the first column memory cells are turned on, and a high potential difference of about 16 V is generated between the control gate and the substrate of the memory transistor. As a result, charges stored in the floating gate are drawn into the substrate through the FN tunnel current. Incidentally, the bit line is in the floating state. With respect to the memory cells other than the first column, the control circuits  1804 ( 2 ) to  1804 ( n ) are in the non-selection state, and the word lines are in the floating state, so that there is no problem of erroneous erasure. Incidentally, instead of making the substrate potential 8 V, it is also possible to provide a partially overlapping region (overlap region) between the source region of the memory transistor and the floating gate to make the common source potential Vsc 8V. 
         [0166]    In order to decrease the distribution width after the writing operation in which the verify operation is not carried out, it is naturally necessary that the distribution width of the threshold voltage before the writing is small. However, in the foregoing erase operation, it is generally difficult to realize the narrow distribution width. Then, in this embodiment, after the erase operation through the FN tunnel, verify writing into the erase state is newly carried out. It is preferable that the threshold voltage of the erase state is 0 V or higher, and is made a value slightly higher than that of the distribution after erasure. Incidentally, in the verify writing into the erase state, a well-known circuit structure and a well-known operation method may be used. After the verify writing into the erase state, the potentials of the control gates of all the memory cells are made 0 V by the discharge operation. 
         [0167]    In the manner described above, the writing operation, the readout operation, and the erase operation of the non-volatile memory of the present invention are carried out. 
         [0168]    In this embodiment, although the two-value non-volatile memory has been described, since the feature of the non-volatile memory of the present invention is in the high accuracy threshold controllability, the effect is rather exhibited in the multi-value non-volatile memory in which the margin of the threshold distribution is small. By combining this embodiment with the embodiment 1, it becomes possible to realize the non-volatile memory in which the writing time is short and the multi-value degree is high. 
         [0169]    Incidentally, in this embodiment, although the writing method in which the verify operation is not used has been described, of course, verify writing may be carried out. Besides, in this embodiment, the method of writing and readout to the memory cell (1, 1) and simultaneous erasing of the first column memory cells (1, 1) to (m, 1) have been described, however these operations can be simultaneously carried out over a plurality of columns. The application of these is as described in this embodiment mode. 
         [0170]    Incidentally, in this embodiment, although the threshold voltage of the memory transistor in the erase state is made positive, the present invention is not limited to this case. In the case where the threshold voltage of the memory transistor in the erase state is negative, the potential of the control gate is made lower than the threshold voltage of the erase state by the discharge operation, so that an operation can be carried out in exactly the same manner as this embodiment. In addition, it is also effective to make the memory transistor have a split gate structure. 
       Embodiment 3 
       [0171]    The present invention can be applied to various memory elements including a charge storage region between an active region and a control gate electrode. Especially, the present invention can also be applied to a memory element in which a semiconductor cluster layer, a metal cluster layer, or a nitride film is provided as the charge storage region. These memory elements are characterized in that a region for storing an electric charge is spatially discretely provided. 
         [0172]    A memory element using a semiconductor or conductor cluster layer as a region for storing an electric charge is disclosed in, for example, Japanese Patent Publication No. Sho. 49-22356. Its typical sectional structure is shown in  FIG. 26A . A memory element shown in  FIG. 26A  is a memory transistor in which a first insulating film  2602 , a cluster layer  2603 , a second insulating film  2605 , and a control gate electrode  2606  are successively stacked on an active region  2601 . The cluster layer  2603  is a layer constituted by discrete block-like semiconductor or conductor (called cluster), and this discrete cluster  2604  functions as a charge-trapped center. 
         [0173]    As a memory element using a nitride film as a region for storing an electric charge, MNOS (Metal-Nitride-Oxide-Semiconductor), MONOS (Metal-Oxide-Nitride-Oxide-Semiconductor), and the like are known. Typical sectional structures of the MNOS and the MONOS are shown in  FIGS. 26B and 26C , respectively. In the memory element shown in  FIG. 26B , an oxide film  2608 , a nitride film  2609 , and a control gate electrode  2610  are stacked on an active region  2607 . In the memory element shown in  FIG. 26C , an oxide film  2612 , a nitride film  2613 , an oxide film  2614 , and a control gate electrode  2615  are stacked on an active region  2611 . In either case, spatially discrete impurity levels in the nitride film function as charge-trapped centers. 
         [0174]    Also in the case where such a memory element is applied to the non-volatile memory of the present invention, the circuit structure and the operation method explained in this embodiment mode can be used as they are. Of course, it is preferable that optimum values are used for the operation voltage and the operation current in each element. 
         [0175]    In the case where an element in which a region for storing an electric charge is discretely provided is used as a memory element, there is an effect that a charge holding characteristic is hardly influenced by a defect of a tunnel oxide film or a pin hole. For example, like a memory transistor including a floating gate as shown in  FIG. 26D , in the case where a region for storing an electric charge is continuously provided, if one pin hole exists in the tunnel oxide film, since all charges stored in the floating gate leak through the pin hole, it has a large influence on the charge holding characteristic of the memory transistor. However, in the case where an element in which regions for storing electric charges are discretely provided is used, a region receiving the influence of the pin hole is limited, and many charge storage regions do not receive the influence of the pin hole, so that the influence on the charge holding characteristic of the memory element is little. 
         [0176]    Incidentally, the memory transistor including the floating gate is a memory transistor in which as shown in  FIG. 26D , a first insulating film  2617 , a floating gate electrode  2618  made of a semiconductor film or a conductive film, a second insulating film  2619 , and a control gate electrode  2620  are successively stacked over an active region  2616 . 
         [0177]    In the case where the memory element of this embodiment is applied to the non-volatile memory of the present invention, it is preferable to apply it especially to a multi-value memory. Since the multi-value non-volatile memory does not have a large margin of threshold voltages expressing different states, it is well known that high accuracy threshold control at the time of writing or erasing, and an excellent charge holding characteristic become necessary. Since the non-volatile memory of the present invention is excellent in the controllability of threshold voltage, the present invention is especially effective for the multi-value non-volatile memory, as already described in this embodiment mode and the embodiment 1. 
         [0178]    On the other hand, with respect to the charge holding characteristic which is another problem of the multi-value non-volatile memory, it becomes possible to make a substantial improvement by using the memory element of this embodiment. Thus, it can be said that the memory element described in this embodiment is especially preferable when the present invention is applied to the multi-value non-volatile memory. 
         [0179]    Incidentally, the structure of this embodiment can be freely combined with any structure of the embodiments 1 and 2. 
       Embodiment 4 
       [0180]    In this embodiment, a microprocessor such as an RISC processor and an ASIC processor will be described in which nonvolatile memories of the present invention are integrated on one chip. 
         [0181]      FIG. 23  shows an exemplary microprocessor. The microprocessor is typically composed of a CPU core  2301 , a flash memory  2304  (which may be a RAM), a clock controller  2303 , a cache memory  2302 , a cache controller  2305 , a serial interface  2306 , an I/O port  2307 , and the like. Needless to say, the microprocessor shown in  FIG. 23  is a simplified example, and an actual microprocessor is various designed depending upon its use. 
         [0182]    In the microprocessor shown in  FIG. 23 , the CPU core  2301 , the clock controller  2303 , the cache controller  2305 , the serial interface  2306 , and the I/O port  2307  are composed of a CMOS circuit. As the flash memory  2304 , the nonvolatile memory of the present invention is used, and a multi-valued flash memory utilizing a pinning memory transistor is used. As the circuit configuration of the flash memory  2304 , that shown in Embodiment modes and Embodiments 1 to 3 may be used, or the other circuit configurations may be used. It is also possible to utilize the nonvolatile memory of the present invention for the cache memory  2302 . 
         [0183]    Further, the microprocessor of this embodiment can be used any structure of combination of Embodiments 1 to 3. 
       Embodiment 5 
       [0184]    The nonvolatile memory of the present invention can be incorporated into electronic equipment in various fields, as a recording medium for storing/reading data. In this embodiment, such electronic equipment will be described. 
         [0185]    Examples of the electronic equipment that can utilize the nonvolatile memory of the present invention include a display, a video camera, a digital camera, a head mount display, a DVD player, a game machine, a goggle-type display, a car navigation, an acoustic reproducing apparatus (a car mounted audio, etc.), a personal computer, a portable information terminal (a mobile computer, a mobile phone, an electronic dictionary, etc.), and the like.  FIGS. 24A-24F  and  25 A- 25 B illustrate examples thereof. 
         [0186]      FIG. 24A  shows a display that includes a housing  2401 , a supporter  2402 , a display portion  2403 , and the like. The present invention is connected to the display portion  2403  and other signal control circuits, and used for correcting an image signal and storing processing data. 
         [0187]      FIG. 24B  shows a video camera that is composed of a body  2411 , a display portion  2412 , a voice input portion  2413 , an operation switch  2414 , a battery  2415 , and an image-receiving portion  2416 . The present invention is incorporated into a built-in LSI substrate and used for storing image data and the like. 
         [0188]      FIG. 24C  shows a part (right chip side) of a head mount display including a body  2421 , a signal cable  2422 , a head portion fixing band  2423 , a display portion  2424 , an optical system  2425 , a display apparatus  2426 , and the like. The present invention is connected to the display apparatus  2426  and other signal control circuits, and used for correcting an image signal and storing processing data. 
         [0189]      FIG. 24D  shows an image reproducing apparatus (more specifically, a DVD reproducing apparatus) provided with a recording medium, which is composed of a body  2431 , a recording medium  2432 , an operation switch  2433 , a display portion (a)  2434 , a display portion (b)  2435 , and the like. This apparatus uses as a recording medium a digital versatile disc (DVD), a compact disc (CD), and the like so as to allow a user to listen to music, see movies, play games, and browse the Internet. The present invention is incorporated into a built-in LSI substrate and used for storing image data and processing data. 
         [0190]      FIG. 24E  shows a goggle type display that includes a body  2441 , a display portion  2442 , and an arm portion  2443 . The present invention is connected to the display portion  2442  and other signal control circuits, and used for correcting an image signal and storing processing data. 
         [0191]      FIG. 24F  shows a personal computer that is composed of a body  2451 , a housing  2452 , a display portion  2453 , a keyboard  2454 , and the like. The present invention is incorporated into a built-in LSI substrate, and used for storing processing data and image data. 
         [0192]      FIG. 25A  shows a mobile phone that includes a body  2501 , a voice output portion  2502 , a voice input portion  2503 , a display portion  2504 , an operation switch  2505 , and an antenna  2506 . The present invention is incorporated into a built-in LSI substrate, and used for adding an address function of recording telephone numbers. 
         [0193]      FIG. 25B  shows an acoustic reproducing apparatus (more specifically, a car audio system) which includes a body  2511 , a display portion  2512 , and operation switches  2513  and  2514 . The present invention is incorporated into a built-in LSI substrate, and used for storing image data and processing data. Furthermore, in this embodiment, a car mounted audio system is illustrated. However, a portable or domestic acoustic reproducing apparatus may be used. 
         [0194]    As described above, the application range of the present invention is very wide, so that the present invention is applicable to electronic equipment in various fields. Furthermore, the electronic equipment of this embodiment can also be realized even by using the configuration formed of any combination of Embodiments 1 to 3. 
         [0195]    In the non-volatile memory of the present invention, by controlling a drain voltage and a drain current of a memory element, it becomes possible to carry out high accuracy threshold control in a writing operation. 
         [0196]    As a result, it becomes possible to decrease the number of verify times at verify writing, and further, a writing method without using a verify system becomes possible, so that as compared with the prior art, it becomes possible to realize substantial shortening of a writing time. 
         [0197]    In the case where the present invention is applied especially to a multi-value non-volatile memory, an effect of shortening of a writing time is large. Further, in the case where a transistor including discrete charge storage regions is used as a memory element, it is possible to provide a multi-value non-volatile memory which is excellent in charge holding characteristic and also in threshold controllability at the time of writing. 
         [0198]    By mounting the non-volatile memory of the present invention excellent in the threshold controllability, it is possible to provide a semiconductor device including a non-volatile memory capable of carrying out a high speed writing and erasing operation. As a result, an electronic equipment which incorporates the non-volatile memory of the present invention can shorten a waiting time for data processing. Further, the non-volatile memory of the present invention can be adjusted to high speed CPU.