Patent Publication Number: US-6992930-B2

Title: Semiconductor memory device, method for driving the same and portable electronic apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is related to Japanese patent application No. 2003-130656 filed on May 8, 2003, whose priority is claimed under 35 USC § 119, the disclosure of which is incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconductor memory device using a nonvolatile memory, a method for driving the semiconductor memory device and a portable electronic apparatus. 
   2. Description of the Related Art 
   Conventionally, a flash memory is typically used as a nonvolatile memory. 
   In a flash memory, as shown in  FIG. 24 , a floating gate  902 , an insulating film  907  and a word line (control gate)  903  are formed in this order on a semiconductor substrate  901  via a gate insulating film. On both sides of the floating gate  902 , a source line  904  and a bit line  905  are formed by a diffusion region, thereby constructing a memory cell. A device isolation region  906  is formed around the memory cell (see, for example, Japanese Unexamined Patent Publication No. Hei 5-304277 (1993)). 
   The memory cell retains data according to a charge amount in the floating gate  902 . In a memory cell array constructed by arranging memory cells, an operation of rewriting/reading a desired memory cell can be performed by selecting a specific word line and a specific bit line and applying a predetermined voltage. 
   In such a flash memory, when a charge amount in the floating gate changes, a drain current (Id)-gate voltage (Vg) characteristic as shown in  FIG. 25  is displayed. When the amount of negative charges in the floating gate increases, the threshold increases, and an Id-Vg curve shifts substantially in parallel in the direction of increasing Vg. 
   In such a flash memory, however, it is necessary to dispose the insulating film  907  for separating the floating gate  902  and the word line  903  from the functional viewpoint. In addition, in order to prevent leakage of charges from the floating gate  902 , it is difficult to reduce the thickness of the gate insulating film. It is therefore difficult to effectively reduce the thickness of the insulating film  907  and the gate insulating film, and it disturbs reduction in size of the memory cell. 
   SUMMARY OF THE INVENTION 
   The present invention has been achieved in consideration of the above mentioned problems and its object is to provide a semiconductor memory device including a nonvolatile memory cell which facilitates its microfabrication, a method for driving the semiconductor memory device, and a portable electronic apparatus. 
   The present invention provides a method for driving a semiconductor memory device comprising a memory array having a plurality of memory cells arranged in rows and columns, each memory cell includes a gate electrode formed on a semiconductor layer via a gate insulating film, a channel region disposed below the gate electrode, a source and a drain as diffusion regions disposed on both sides of the channel region and having a conductive type opposite to that of the channel region, and memory functional units formed on both sides of the gate electrode and having a function of retaining charges, the method comprising the steps of: selecting a row line connected to the gate electrode of the memory cell to be selected; grounding a first column line connected to the source of the memory cell to be selected; and applying a first potential to a second column line and a second potential to a third column line at the same time, wherein the second column line is connected to the drain of the memory cell to be selected and the third column line is connected to the drain of a memory cell adjacent to the memory cell to be selected while other row lines in the memory array are separated from the second and third column lines, and the first potential is applied by a first circuit, the second potential is applied by a second circuit, the first potential conditionally allows a reading current to flow via the memory cell to be selected, and a value of the current indicates the contents of the memory cell to be selected. 
   In the driving method according to the present invention, the first and second potentials may be substantially equal in value to each other. 
   The second circuit may have an output impedance lower than that of the first circuit. 
   The memory functional unit of the memory cell may include a film having a surface substantially parallel with a surface of the gate insulating film and having the function of retaining charges. 
   The memory cell may include a film having a surface substantially parallel with a surface of the gate insulating film and having the function of retaining charges; and an insulating film for separating the film from the channel region or the semiconductor layer, the insulating film having a thickness thinner than the gate insulating film and not less than 0.8 nm. 
   The semiconductor layer in the memory cell may have a region in the vicinity of the diffusion region, the region having a concentration higher than a portion in the vicinity of a surface of the semiconductor layer below the gate electrode. 
   At least a part of the memory functional unit of the memory cell may overlap a part of the diffusion region. 
   The present invention also provides a semiconductor memory device comprising: a memory array having a plurality of memory cells arranged in rows and columns, each of the memory cells including a gate electrode formed on a semiconductor layer via a gate insulating film, a channel region disposed below the gate electrode, first and second diffusion regions disposed on both sides of the channel region and having a conductive type opposite to that of the channel region, and memory functional units formed on both sides of the gate electrode and having a function of retaining charges; a plurality of row lines, in which the gate electrodes in all of the memory cells in the same row are connected to a common row line; first and second column lines, to which the first and second diffusion regions in all of the cells in the same column are connected, respectively; row decoding means for selecting a row line connected to a memory cell to be selected; and column decoding means for grounding a second column line connected to the memory cell to be selected and for connecting a first column line connected to the memory cell to be selected to a first potential applied from a first circuit, so as to conditionally allow a current to flow via the memory cell to be selected, thereby accessing the content of the memory cell to be selected, wherein the column decoding means connects another second column line connected to a memory cell adjacent to the memory cell to be selected to a second potential to be applied from a second circuit while all of other first and second column lines in the memory array are separated from the first and second potentials, so as to prevent any interference from the adjacent memory cell during an access period of the memory cell to be selected. 
   The first and second potentials may be substantially equal in value to each other, for protecting the selected cell from a parasitic current during a reading processing period. 
   The second circuit may have an output impedance lower than that of the first circuit. 
   The semiconductor memory device may further comprise a plurality of path gate transistors, which is connected between the adjacent first and second column lines to selectively de-bias the memory array during a writing period to avoid a trouble of adjacent cells. 
   The memory functional unit of the memory cell may include a film having a surface substantially parallel with a surface of the gate insulating film and having the function of retaining charges. 
   The memory cell may include a film having a surface substantially parallel with a surface of the gate insulating film and having the function of retaining charges; and an insulating film for separating the film from the channel region or the semiconductor layer, the insulating film having a thickness thinner than the gate insulating film and not less than 0.8 nm. 
   The semiconductor layer in the memory cell may have a region in the vicinity of the diffusion region, the region having a concentration higher than a portion in the vicinity of the surface of the semiconductor layer below the gate electrode. 
   At least a part of the memory functional unit of the memory cell may overlap a part of the diffusion region. 
   The present invention also provides a method for driving a semiconductor memory device comprising a plurality of memory cells arranged in rows and columns, each of the memory cells including a gate electrode formed on a semiconductor layer via a gate insulating film, a channel region disposed below the gate electrode, first and second diffusion regions disposed on both sides of the channel region and having a conductive type opposite to that of the channel region, and memory functional units formed on both sides of the gate electrode and having a function of retaining charges, and a plurality of row lines including first and second lines, in which the gate electrodes in all of the memory cells in the same row are connected to the same row line and first and second diffusion regions in all of the cells in the same column line are connected to the corresponding first and second column lines, respectively, the method comprising the steps of: applying a first potential to a row line connected to a memory cell to be selected, so as to connect all of the first and second column lines on one side of the memory cell to be selected to a first node; connecting all of the first and second column lines on the other side of the memory cell to be selected to a second node; grounding the first node; and increasing a potential of the second node up to a second potential at a predetermined speed, so as to tunnel electrons to the gate electrode of the memory cell to be selected, wherein the predetermined speed is controlled in such a manner as to prevent data stored in adjacent cells from being disturbed by a potential between the first and second column lines connected to the selected memory cell. 
   The predetermined speed may be 10 volt/μsec or less. 
   The first potential may be about 12 V. 
   The second potential may be about 7 V. 
   The memory functional unit of the memory cell may include a film having a surface substantially parallel with a surface of the gate insulating film and having the function of retaining charges. 
   The memory cell may include a film having a surface substantially parallel with a surface of the gate insulating film and having the function of retaining charges; and an insulating film for separating the film from the channel region or the semiconductor layer, the insulating film having a thickness thinner than the gate insulating film and not less than 0.8 nm. 
   The semiconductor layer in the memory cell may have a region in the vicinity of the diffusion region, the region having a concentration higher than a portion in the vicinity of a surface of the semiconductor layer below the gate electrode. 
   At least a part of the memory functional unit of the memory cell may overlap a part of the diffusion region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic sectional view showing a main part of a memory cell (first embodiment) in a semiconductor memory device according to the present invention; 
       FIGS. 2A and 2B  are schematic sectional views each showing a main part of a modification of the memory cell (first embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 3  is a diagram for describing a writing operation of the memory cell (first embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 4  is a diagram for describing a writing operation of the memory cell (first embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 5  is a diagram for describing an erasing operation of the memory cell (first embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 6  is a diagram for describing an erasing operation of the memory cell (first embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 7  is a diagram for describing a reading operation of the memory cell (first embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 8  is a schematic sectional view showing a main part of a memory cell (second embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 9  is an enlarged schematic sectional view of the main part shown in  FIG. 8 ; 
       FIG. 10  is an enlarged schematic sectional view of a modification of the main part shown in  FIG. 8 ; 
       FIG. 11  is a graph showing electric characteristics of the memory cell (second embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 12  is a schematic sectional view showing a main part of a modification of the memory cell (second embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 13  is a schematic sectional view showing a main part of a memory cell (third embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 14  is a schematic sectional view showing a main part of a memory cell (fourth embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 15  is a schematic sectional view showing a main part of a memory cell (fifth embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 16  is a schematic sectional view showing a main part of a memory cell (sixth embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 17  is a schematic sectional view showing a main part of a memory cell (seventh embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 18  is a schematic sectional view showing a main part of a memory cell (eighth embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 19  is a graph showing electric characteristics of a memory cell (ninth embodiment) in the semiconductor memory device according to the present invention; 
       FIG. 20  is a circuit diagram showing a main part of a semiconductor memory device (tenth embodiment) according to the present invention; 
       FIG. 21  is a circuit diagram showing a main part of the semiconductor memory device (tenth embodiment) according to the present invention; 
       FIG. 22  is a schematic configuration diagram showing a liquid crystal display device (eleventh embodiment) in which the semiconductor memory device according to the present invention is assembled; 
       FIG. 23  is a schematic configuration diagram showing a portable electronic apparatus (twelfth embodiment) in which the semiconductor memory device according to the present invention is assembled; 
       FIG. 24  is a schematic sectional view showing a main part of a conventional flash memory; and 
       FIG. 25  is a graph showing electric characteristics of a conventional flash memory. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   According to the present invention, the memory cell forming a memory array is mainly constructed by a semiconductor layer, a gate insulating film, a gate electrode, a channel region, a diffusion region and a memory functional unit. Herein, the channel region is normally a region of the same conductive type as that of the semiconductor layer and denotes a region immediately below the gate electrode. The diffusion region denotes a region of the conductive type opposite to that of the channel region. 
   Specifically, the memory cell in the present invention may be constructed by a region of a first conductive type as a diffusion region, a region of a second conductive type as a channel region, a memory functional unit disposed across a border of the regions of the first and second conductive types, and an electrode provided via a gate insulating film. It is suitable that the memory cell of the present invention is constructed by a gate electrode formed on a gate insulating film, two memory functional units formed on both sides of the gate electrode, two diffusion regions disposed on the opposite sides of the gate electrode of the memory functional units, and a channel region disposed below the gate electrode. 
   In the semiconductor device of the present invention, the semiconductor layer is formed on the semiconductor substrate, preferably, on a well region of the first conductive type formed in the semiconductor substrate. 
   The semiconductor substrate is not particularly limited as long as it can be used for a semiconductor device, and examples thereof include a bulk substrate made of an element semiconductor such as silicon, germanium or the like or a compound semiconductor such as silicon germanium, GaAs, InGaAs, ZnSe or GaN. As a substrate having a semiconductor layer on its surface, various substrates such as an SOI (Silicon on Insulator) substrate, an SOS substrate and a multilayer SOI substrate, or a glass or plastic substrate having thereon a semiconductor layer may be used. In particular, a silicon substrate and an SOI substrate having a semiconductor layer on its surface are preferable. The semiconductor substrate or semiconductor layer may be single crystal (formed by, for example, epitaxial growth), polycrystal, or amorphous although an amount of current flowing therein varies a little. 
   Preferably, a device isolation region is formed on the semiconductor layer. Further, a single layer or multilayer structure may be formed by a combination of devices such as a transistor, a capacitor and a resistor, a circuit formed by the devices, a semiconductor device, and an interlayer insulating film. The device isolation region can be formed by any of various device isolation films such as an LOCOS film, a trench oxide film and an STI film. The semiconductor layer may be of the P or N conductive type. At least one well region of the first conductive type (P or N type) is preferably formed in the semiconductor layer. As impurity concentration in the semiconductor layer and the well region, impurity concentration which is within a known range in this field can be used. In the case of using the SOI substrate as the semiconductor layer, the well region may be formed in the surface semiconductor layer and a body region may be provided below a channel region. 
   The gate insulating film is not particularly limited as long as it is usually used for a semiconductor device, and an example thereof include a single-layer film or a laminated film of an insulating film such as a silicon oxide film or a silicon nitride film, or a high dielectric constant film such as an aluminum oxide film, a titanium oxide film, a tantalum oxide film or a hafnium oxide film. Particularly, a silicon oxide film is preferable. The gate insulating film has a thickness of, for example, about 1 to 20 nm, preferably, about 1 to 6 nm. The gate insulating film may be formed only immediately below the gate electrode or formed so as to be larger (wider) than the gate electrode. 
   The gate electrode is formed in a shape which is usually used for a semiconductor device or a shape having a recess in a lower end portion on the gate insulating film. The gate electrode is preferably formed in an integral form without being separated by a single-layered or multilayer conductive film. The gate electrode may be disposed in a state where it is separated by a single-layered or multilayer conductive film. The gate electrode may have a side-wall insulating film on its sidewalls. Usually, the gate electrode is not particularly limited as long as it is used for a semiconductor device, and an example of thereof includes a conductive film such as a single-layered or multilayer film made of polysilicon, a metal such as copper or aluminum, a high-refractory metal such as tungsten, titanium or tantalum, and a silicide or the like with the high refractory metal. Suitable thickness of the gate electrode is, for example, about 50 to 400 nm. Below the gate electrode, a channel region is formed. 
   Preferably, the gate electrode is formed only on the sidewalls of the memory functional unit or does not cover the top part of the memory functional unit. By such arrangement, a contact plug can be disposed closer to the gate electrode, so that reduction in the size of the memory cell is facilitated. It is easy to manufacture the memory cell having such simple arrangement, so that the yield can be improved. 
   The memory functional unit has at least the function of retaining charges (hereinafter, described as “charge retaining function”). In other words, the memory functional unit has the function of accumulating and retaining charges, the function of trapping charges or the function of holding a charge polarization state. The function is exhibited, for example, when the memory functional unit includes a film or region having the charge retaining function. Examples of elements having the above function include: silicon nitride; silicon; a silicate glass including impurity such as phosphorus or boron; silicon carbide; alumina; a high dielectric material such as hafnium oxide, zirconium oxide or tantalum oxide; zinc oxide; ferroelectric; metals, and the like. Therefore, the memory functional unit can be formed by, for example, a single-layered or laminated structure of: an insulating film including a silicon nitride film; an insulating film having therein a conductive film or a semiconductor layer; an insulating film including at least one conductor or semiconductor dot; or an insulating film including a ferroelectric film of which inner charge is polarized by an electric field and in which the polarized state is held. Particularly, the silicon nitride film is preferable for the reason that the silicon nitride film can obtain a large hysteretic characteristic since a number of levels of trapping charges exist. In addition, the charge retention time is long and a problem of charge leakage due to occurrence of a leak path does not occur, so that the retention characteristics are good. Further, silicon nitride is a material which is used as standard in an LSI process. 
   By using the insulating film including a film having the charge retaining function such as a silicon nitride film as the memory functional unit, reliability of storage and retention can be increased. Since the silicon nitride film is an insulator, even in the case where a charge leak occurs in part of the silicon nitride film, the charges in the whole silicon nitride film are not lost immediately. In the case of arranging a plurality of memory cells, even when the distance between the memory cells is shortened and neighboring memory cells come into contact with each other, unlike the case where the memory functional units are made of conductors, information stored in the memory functional units is not lost. Further, a contact plug can be disposed closer to the memory functional unit. In some cases, the contact plug can be disposed so as to be overlapped with the memory functional unit. Thus, reduction in size of the memory cell is facilitated. 
   In order to increase the reliability of storage and retention, the film having the charge retaining function does not always have to have a film shape. Preferably, films having the charge retaining function exist discretely in an insulating film. Concretely, it is preferable that the films having the charge retaining function in the shape of dots be spread in a material which is hard to retain charges, for example, in a silicon oxide. 
   In the case of using a conductive film or semiconductor layer as the charge retaining film, preferably, the conductive film or semiconductor layer is disposed via an insulating film so that the charge retaining film is not in direct contact with the semiconductor layer (semiconductor substrate, well region, body region, source/drain regions or diffusion region) or a gate electrode. For example, a laminated structure of the conductive film and the insulating film, a structure in which conductive films in the form of dots are spread in the insulating film, a structure in which the conductive film is disposed in a part of a sidewall insulating film formed on sidewalls of the gate, and the like can be mentioned. 
   It is preferable to use the insulating film having therein the conductive film or semiconductor layer as a memory functional unit for the reason that an amount of injecting charges into the conductor or semiconductor can be freely controlled and multilevel values can be easily obtained. 
   Further, it is preferable to use the insulating film including at least one conductor or semiconductor dot as the memory functional unit for the reason that it becomes easier to perform writing and erasing by direct tunneling of charges, and reduction in power consumption can be achieved. 
   Alternately, as a memory functional unit, a ferroelectric film such as PZT or PLZT in which the polarization direction changes according to the electric field may be used. In this case, charges are substantially generated in the surface of the ferroelectric film by the polarization and are held in that state. It is therefore preferable since the ferroelectric film can obtain a hysteresis characteristic similar to that of a film to which charges are supplied from the outside of the film having the memory function and which traps charges. In addition, it is unnecessary to inject charges from the outside of the film in order to retain charges in the ferroelectric film, and the hysteresis characteristic can be obtained only by the polarization of the charge in the film, so that writing/erasing can be performed at high speed. 
   As the insulating film constructing the memory functional unit, a film having a region or function of suppressing escape of charges is suitable. An example of a film having the function of suppressing escape of charges includes a silicon oxide film. 
   The charge retaining film included in the memory functional unit is disposed on both sides of the gate electrode directly or via an insulating film, and is disposed on the semiconductor layer (semiconductor substrate, well region, body region or source/drain region, or diffusion region) directly or via a gate insulating film. Preferably, the charge retaining film on both sides of the gate electrode is formed so as to cover all or part of the sidewalls of the gate electrode directly or via the insulating film. In an application example, in the case where the gate electrode has a recess in its lower end, the charge retaining film may be formed so as to completely or partially bury the recess directly or via an insulating film. 
   The diffusion regions can function as source and drain regions and have the conductive type opposite to that of the semiconductor layer or well region. In the junction between the diffusion region and the semiconductor layer or well region, preferably, impurity concentration is high for the reason that hot electrons or hot holes are generated efficiently with low voltage, and high-speed operation can be performed with lower voltage. The junction depth of the diffusion region is not particularly limited but can be appropriately adjusted in accordance with the performance or the like of a semiconductor memory device to be obtained. In the case of using an SOI substrate as a semiconductor substrate, the diffusion region may have a junction depth smaller than the thickness of the surface semiconductor layer. It is preferable that the diffusion region has junction depth substantially the same as that of the surface semiconductor layer. 
   The diffusion region may be disposed so as to overlap with an end of the gate electrode, so as to match an end of the gate electrode, or so as to be offset from an end of the gate electrode. The case of offset is particularly preferable because easiness of inversion of the offset region below the charge retaining film largely changes in accordance with an amount of charges accumulated in the memory functional unit when voltage is applied to the gate electrode, the memory effect increases, and a short channel effect is reduced. However, when the diffusion region is offset too much, drive current between the diffusion regions (source and drain) decreases conspicuously. Therefore, it is preferable that the offset amount, that is, the distance to the diffusion area closer to one of the gate electrode ends in the gate length direction is shorter than the thickness of the charge retaining film extending in the direction parallel with the gate length direction. It is particularly preferable that at least a part of the film or region having the charge retaining function in the memory functional unit is overlapped with part of the diffusion region. This is because the essence of the memory cell as a component of the semiconductor memory device is to rewrite stored information by an electric field which is applied across the memory functional unit in accordance with the voltage difference between the gate electrode which exists only in the sidewall part of the memory functional unit and the diffusion region. 
   A part of the diffusion region may extend at a level higher than the surface of the channel region or the under face of the gate insulating film. In this case, it is suitable that, on the diffusion region formed in the semiconductor substrate, the conductive film integrated with the diffusion region is laminated. The conductive film is made of semiconductor such as polysilicon or amorphous silicon, silicide, the above-described metals, high-refractory metals, or the like. In particular, polysilicon is preferred. Since impurity diffusion speed of polysilicon is much faster than that of the semiconductor layer, it is easy to make the junction depth of the diffusion region in the semiconductor layer shallow and to suppress the short channel effect. In this case, preferably, a part of the diffusion region is disposed so as to sandwich at least a part of the memory functional unit in cooperation with the gate electrode. 
   The memory cell of the present invention can be formed by a normal semiconductor process, for example, a method similar to the method of forming the sidewall spacer having the single-layer or laminated structure on the sidewalls of the gate electrode. Concrete examples of the method include; a method of forming the gate electrode, after that, forming a single-layer film or laminated film including the charge retaining film such as a film having the function of retaining charges (hereinafter, described as “charge retaining film”), charge retaining film/insulating film, insulating film/charge retaining film, or insulating film/charge retaining film/insulating film, and etching back the formed film under suitable conditions so as to leave the films in a sidewall spacer shape; a method of forming an insulating film or charge retaining film, etching back the film under suitable conditions so as to leave the film in the sidewall spacer shape, further forming the charge retaining film or insulating film, and similarly etching back the film so as to leave the film in the sidewall spacer shape; a method of applying or depositing an insulating film material in which particles made of a charge retaining material are spread on the semiconductor layer including the gate electrode and etching back the material under suitable conditions so as to leave the insulating film material in a sidewall spacer shape; and a method of forming a gate electrode, after that, forming the single-layer film or laminated film, and patterning the film with a mask. According to another method, before the gate electrode is formed, charge retaining film, charge retaining film/insulating film, insulating film/charge retaining film, insulating film/charge retaining film/insulating film, or the like is formed. An opening is formed in a region which becomes the channel region of the films, a gate electrode material film is formed on the entire surface of the opening, and the gate electrode material film is patterned in a shape including the opening and larger than the opening. 
   One example of a method for forming the memory cell according to the present invention will now be described. 
   First, a gate insulating film and a gate electrode are formed on a semiconductor substrate in accordance with known procedures. Subsequently, a silicon oxide film having a thickness of 0.8 to 20 nm, more preferably 3 to 10 nm is formed by thermal oxidation or deposited by CVD (Chemical Vapor Deposition) over the entire semiconductor substrate. Next, a silicon nitride film having a thickness of 2 to 15 nm, more preferably 3 to 10 nm is deposited by the CVD over the entire silicon oxide film. Moreover, another silicon oxide film having a thickness of 20 to 70 nm is deposited by the CVD over the entire silicon nitride film. 
   Subsequently, the silicon oxide film/silicon nitride film/silicon oxide film are etched back by anisotropic etching, thereby forming a memory functional unit optimum for storing data on the sidewall of the gate electrode in the form of a sidewall spacer. 
   Thereafter, ions are injected while using the gate electrode and the memory functional unit in the form of the sidewall spacer as masks, thereby forming a diffusion layer region (source/drain region). After that, a silicide process or an upper wiring process may be performed in accordance with known procedures. 
   In the case of constructing the memory cell array by arranging memory cells of the present invention, the best mode of the memory cell satisfies all of the requirements: for example, (1) the gate electrodes of a plurality of memory cells are integrated and have the function of a word line; (2) the memory functional units are formed on both sides of the word line; (3) an insulator, particularly, a silicon nitride film retains charges in the memory functional unit; (4) the memory functional unit is constructed by an ONO (Oxide Nitride Oxide) film and the silicon nitride film has a surface substantially parallel with the surface of the gate insulating film; (5) a silicon nitride film in the memory functional unit is isolated from a word line and a channel region via a silicon oxide film; (6) the silicon nitride film and a diffusion region in the memory functional unit are overlapped; (7) the thickness of the insulating film separating the silicon nitride film having the surface which is substantially parallel with the surface of the gate insulating film from the channel region or semiconductor layer and the thickness of the gate insulating film are different from each other; (8) an operation of writing/erasing one memory cell is performed by a single word line; (9) there is no electrode (word line) having the function of assisting the writing/erasing operation on the memory functional unit; and (10) in a portion in contact with the diffusion region immediately below the memory functional unit, a region of high concentration of impurity whose conductive type is opposite to that of the diffusion region is provided. It is sufficient for the memory cell to satisfy even one of the requirements. 
   A particularly preferable combination of the requirements is, for example, (3) an insulator, particularly, a silicon nitride film retains charges in the memory functional unit, (6) the insulating film (silicon nitride film) and the diffusion region in the memory functional unit are overlapped, and (9) there is no electrode (word line) having the function of assisting the writing/erasing operation on the memory functional unit. 
   In the case where the memory cell satisfies the requirements (3) and (9), it is very useful for the following reasons. First, the bit line contact can be disposed closer to the memory functional unit on the word line sidewall or even when the distance between memory cells is shortened, a plurality of memory functional units do not interfere with each other, and stored information can be held. Therefore, reduction in size of the memory cell is facilitated. In the case where the charge retaining region in the memory functional unit is made of a conductor, as the distance between memory cells decreases, interference occurs between the charge retaining regions due to capacitive coupling, so that stored information cannot be held. 
   In the case where the charge retaining region in the memory functional unit is made of an insulator (for example, a silicon nitride film), it becomes unnecessary to make the memory functional unit independent for each memory cell. For example, the memory functional units formed on both sides of a single word line shared by a plurality of memory cells do not have to be isolated for each memory cell. The memory functional units formed on both sides of one word line can be shared by a plurality of memory cells sharing the word line. Consequently, a photo etching process for isolating the memory functional unit becomes unnecessary, and the manufacturing process is simplified. 
   Further, a margin for positioning in the photolithography process and a margin for film reduction by etching become unnecessary, so that the margin between neighboring memory cells can be reduced. Therefore, as compared with the case where the charge retaining region in the memory functional unit is made of a conductor (for example, polysilicon film), even when the memory functional unit is formed at the same microfabrication level, a memory cell occupied area can be reduced. In the case where the charge retaining region in the memory functional unit is made of a conductor, the photo etching process for isolating the memory functional unit for each memory cell is preferable, and a margin for positioning in the photolithography process and a margin for film reduction by etching are preferable. 
   Moreover, since the electrode having the function of assisting the writing and erasing operations does not exist on the memory functional unit and the device structure is simple, the number of processes decreases, so that the yield can be increased. Therefore, it facilitates formation with a transistor as a component of a logic circuit or an analog circuit, and a cheap semiconductor memory device can be obtained. 
   The present invention is more useful in the case where not only the requirements (3) and (9) but also the requirement (6) are satisfied. 
   Specifically, by overlapping the charge retaining region in the memory functional unit and the diffusion region, writing and erasing can be performed with a very low voltage. Concretely, with a low voltage of 5 V or less, the writing and erasing operations can be performed. The action is a very large effect also from the viewpoint of circuit designing. Since it is unnecessary to generate a high voltage in a chip unlike a flash memory, a charge pumping circuit requiring a large occupation area can be omitted or its scale can be reduced. Particularly, when a memory of small-scale capacity is provided for adjustment in a logic LSI, as for an occupied area in a memory part, an occupation area of peripheral circuits for driving a memory cell is dominant more than that of a memory cell. Consequently, omission or down sizing of the charge pumping circuit for a memory cell is most effective to reduce the chip size. 
   On the other hand, in the case where the requirement (3) is not satisfied, that is, in the case where a conductor retains charges in the memory functional unit, even when the requirement (6) is not satisfied, specifically, even when the conductor in the memory functional unit and the diffusion region do not overlap with each other, writing operation can be performed. This is because that the conductor in the memory functional unit assists writing operation by capacitive coupling with the gate electrode. 
   In the case where the requirement (9) is not satisfied, specifically, in the case where the electrode having the function of assisting the writing and erasing operations exists on the memory functional unit, even when the requirement (6) is not satisfied, specifically, even when the insulator in the memory functional unit and the diffusion region do not overlap with each other, writing operation can be performed. 
   In the semiconductor memory device of the present invention, a transistor may be connected in series with one of or both sides of a memory cell, or the memory cell may be mounted on the same chip with a logic transistor. In such a case, the semiconductor device of the present invention, particularly, the memory cell can be formed by a process having high compatibility with a process of forming a normal standard transistor such as a transistor or a logic transistor, they can be formed simultaneously. Therefore, a process of forming both the memory cell and a transistor or a logic transistor is very simple and, as a result, a cheap embedded device can be obtained. 
   In the semiconductor memory device of the present invention, the memory cell can store information of two or more values in one memory functional unit. Thus, the memory cell can function as a memory cell for storing information of four or more values. The memory cell may store binary data only. The memory cell is also allowed to function as a memory cell having the functions of both a selection transistor and a memory transistor by a variable resistance effect of the memory functional unit. 
   The semiconductor memory device of the present invention can be widely applied by being combined with a logic device, a logic circuit or the like to: a data processing system such as a personal computer, a note-sized computer, a laptop computer, a personal assistant/transmitter, a mini computer, a workstation, a main frame, a multiprocessor/computer, a computer system of any other type, or the like; an electronic part as a component of the data processing system, such as a CPU, a memory or a data memory device; a communication apparatus such as a telephone, a PHS, a modem or a router; an image display apparatus such as a display panel or a projector; an office apparatus such as a printer, a scanner or a copier; an image pickup apparatus such as a video camera or a digital camera; an entertainment apparatus such as a game machine or a music player; an information apparatus such as a portable information terminal, a watch or an electronic dictionary; a vehicle-mounted apparatus such as a car navigation system or a car audio system; an AV apparatus for recording/reproducing information such as a motion picture, a still picture or music; an appliance such as a washing machine, a microwave, a refrigerator, a rice cooker, a dish washer, a vacuum cleaner or an air conditioner; a health managing apparatus such as a massage device, a bathroom scale or a manometer; and a portable memory device such as an IC card or a memory card. Particularly, it is effective to apply the semiconductor memory device to portable electronic apparatuses such as portable telephone, portable information terminal, IC card, memory card, portable computer, portable game machine, digital camera, portable motion picture player, portable music player, electronic dictionary and watch. The semiconductor memory device of the present invention may be provided as at least a part of a control circuit or a data storing circuit of an electronic device or, as necessary, detachably assembled. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of the semiconductor memory device, the driving method therefor, and the portable electronic apparatus of the present invention will be described in detail with reference to the drawings. 
   First Embodiment 
   A semiconductor memory device of a first embodiment has a memory cell  1  as shown in FIG.  1 . 
   The memory cell  1  has a gate electrode  104  formed on a P-type well region  102  formed on the surface of a semiconductor substrate  101  via a gate insulating film  103 . On the top face and side faces of the gate electrode  104 , a silicon nitride film  109  having a trap level of retaining charges and serving as a charge retaining film is disposed. In the silicon nitride film  109 , parts of both sidewalls of the gate electrode  104  serve as memory functional units  105   a  and  105   b  for actually retaining charges. The memory functional unit refers to a part in which charges are actually accumulated by rewriting operation in the memory functional unit or the charge retaining film. In the P-type well region  102  on both sides of the gate electrode  104 , N-type diffusion regions  107   a  and  107   b  functioning as a source region and a drain region, respectively, are formed. Each of the diffusion regions  107   a  and  107   b  has an offset structure. Specifically, the diffusion regions  107   a  and  107   b  do not reach a region  121  below the gate electrode  104 , and offset regions  120  below the charge retaining film construct part of the channel region. 
   The memory functional units  105   a  and  105   b  for substantially retaining charges are the parts on both sidewalls of the gate electrode  104 . It is therefore sufficient that the silicon nitride film  109  is formed only in regions corresponding to the parts (see FIG.  2 A). Each of the memory functional units  105   a  and  105   b  may have a structure in which fine particles  111  each made of a conductor or semiconductor and having a nanometer size are distributed like discrete points in an insulating film  112  (see FIG.  2 B). When the fine particle  111  has a size less than 1 nm, a quantum effect is too large, so that it becomes hard for charges to go through the dots. When the size exceeds 10 nm, a conspicuous quantum effect does not appear at room temperature. Therefore, the diameter of the fine particle  111  is preferably in a range from 1 nm to 10 nm. The silicon nitride film  109  serving as a charge retaining film may be formed in a sidewall spacer shape on a side face of the gate electrode (see FIG.  3 ). 
   The principle of the writing operation of the memory cell will be described with reference to  FIGS. 3 and 4 . The case where whole memory functional units  131   a  and  131   b  have the function of retaining charges will be described. “Writing” denotes herein injection of electrons into the memory functional units  131   a  and  131   b  when the memory cell is of the N channel type. Hereinafter, on assumption that the memory cell is of the N channel type, description will be given. 
   In order to inject electrons (write) the second memory functional unit  131   b , as shown in  FIG. 3 , the first diffusion region  107   a  of the N type is set as a source electrode, and the second diffusion region  107   b  of the N type is set as a drain electrode. For example, 0 V is applied to the first diffusion region  107   a  and the P-type well region  102 , +5 V is applied to the second diffusion region  107   b , and +5 V is applied to the gate electrode  104 . Under such voltage parameters, an inversion layer  226  extends from the first diffusion region  107   a  (source electrode) but does not reach the second diffusion region  107   b  (drain electrode), and a pinch off point occurs. Electrons are accelerated from the pinch-off point to the second diffusion region  107   b  (drain electrode) by a high electric field, and become so-called hot electrons (high-energy conduction electrons). By injection of the hot electrons into the second memory functional unit  131   b , writing is performed. Since hot electrons are not generated in the vicinity of the first memory functional unit  131   a , writing is not performed. 
   On the other hand, in order to inject electrons (write) into the first memory functional unit  131   a , as shown in  FIG. 4 , the second diffusion region  107   a  is set as the source electrode, and the first diffusion region  107   a  is set as the drain electrode. For example, 0 V is applied to the second diffusion region  107   b  and the P-type well region  102 , +5 V is applied to the first diffusion region  107   a , and +5 V is applied to the gate electrode  104 . By interchanging the source and drain regions so as to be different from the case of injecting electrons into the second memory functional unit  131   b , electrons are injected into the first memory functional unit  131   a  and writing can be performed. 
   The principle of erasing operation of the memory cell will now be described with reference to  FIGS. 5 and 6 . 
   In a first method of erasing information stored in the first memory functional unit  131   a , by applying positive voltage (for example, +5 V) to the first diffusion region  107   a  and applying 0 V to the P-type well region  102  as shown in  FIG. 5 , the PN junction between the first diffusion region  107   a  and the P-type well region  102  is reverse-biased and, further, negative voltage (for example, −5 V) is applied to the gate electrode  104 . At this time, in the vicinity of the gate electrode  104  in the PN junction, due to the influence of the gate electrode to which the negative voltage is applied, particularly, gradient of potential becomes sharp. Consequently, hot holes (positive holes of high energy) are generated on the side of the P-type well region  102  of the PN junction by interband tunneling. The hot holes are attracted toward the gate electrode  104  having a negative potential and, as a result, the holes are injected to the first memory functional unit  131   a . In such a manner, information in the first memory functional unit  131   a  is erased. At this time, to the second diffusion region  107   b , it is sufficient to apply 0 V. 
   In the case of erasing information stored in the second memory functional unit  131   b , the above-described operation is performed while interchanging the potential of the first diffusion region and that of the second diffusion region. 
   In a second method of erasing information stored in the first memory functional unit  131   a , as shown in  FIG. 6 , positive voltage (for example, +4 V) is applied to the first diffusion region  107   a , 0 V is applied to the second diffusion region  107   b , negative voltage (for example, −4 V) is applied to the gate electrode  104 , and positive voltage (for example, +0.8 V) is applied to the P-type well region  102 . At this time, forward voltage is applied between the P-type well region  102  and the second diffusion region  107   b , and electrons are injected to the P-type well region  102 . The injected electrons are diffused to the PN junction between the P-type well region  102  and the first diffusion region  107   a , where the electrons are accelerated by a strong electric field, thereby becoming hot electrons. By the hot electrons, an electron-hole pair is generated in the PN junction. Specifically, by applying forward voltage between the P-type well region  102  and the second diffusion region  107   b , electrons injected in the P-type well region  102  become a trigger, and hot holes are generated in the PN junction positioned on the opposite side. The hot holes generated in the PN junction are attracted toward the gate electrode  104  having the negative potential and, as a result, positive holes are injected into the first memory functional unit  131   a.    
   According to the method, also in the case where only voltage insufficient to generate hot holes by interband tunneling is applied to the PN junction between the P-type well region and the first diffusion region  107   a , electrons injected from the second diffusion region  107   b  become a trigger to generate an electron-positive hole pair in the PN junction, thereby enabling hot holes to be generated. Therefore, voltage in the erasing operation can be decreased. Particularly, in the case where the offset region  120  (see  FIG. 1 ) exists, an effect that the gradient of potential in the PN junction becomes sharp by the gate electrode to which the negative potential is applied is low. Consequently, although it is difficult to generate hot holes by interband tunneling, by the second method, the disadvantage is overcome and the erasing operation can realized with low voltage. 
   In the case of erasing information stored in the first memory functional unit  131   a , +5 V has to be applied to the first diffusion region  107   a  in the first erasing method whereas +4 V is sufficient in the second erasing method. As described above, according to the second method, the voltage at the time of erasing can be decreased, so that power consumption can be reduced and deterioration of the memory cell due to hot carriers can be suppressed. 
   In any of the erasing methods, over-erasure does not occur easily in the memory cell. The over-erasure herein denotes a phenomenon that as the amount of positive holes accumulated in the memory functional unit increases, the threshold decreases without saturation. The over-erasure is a big issue in an EEPROM typified by a flash memory. Particularly, in the case where the threshold becomes negative, critical malfunctioning that selection of a memory cell becomes impossible occurs. On the other hand, in the memory cell in the semiconductor memory device of the present invention, also in the case where a large amount of positive holes are accumulated in the memory functional unit, only electrons are induced below the memory functional unit but an influence is hardly exerted to the potential in the channel region below the gate insulating film. Since the threshold at the time of erasing is determined by the potential below the gate insulating film, occurrence of over-erasure is suppressed. 
   The memory cell includes a gate electrode formed on a semiconductor layer via a gate insulating film, a channel region disposed under the gate electrode, diffusion regions disposed on both sides of the channel region, and memory functional units formed on both sides of the gate electrode and having the function of retaining charges. The memory functional units are separated by the gate electrode from each other. It is therefore easy to reduce the thickness of the gate insulating film as compared with the conventional flash memory. As a result, it is possible to easily apply an electric field, which is stronger than conventional one, from the gate electrode to the PN junction, and to realize a high-speed erasing operation with low voltage. 
   In addition, it is desired that a charge retaining region in the memory functional unit overlaps with the diffusion region. By overlapping the diffusion region with the charge retaining region in the memory functional unit, hot holes are generated in a first erasing method and distance between the PN junction, in which an electron-hole couple is generated in a second erasing method, and the charge retaining region becomes shorter, so that it is possible to realize a high-speed erasing operation with low voltage. 
   Further, the principle of reading operation of the memory cell will be described with reference to FIG.  7 . 
   In the case of reading information stored in the first memory functional unit  131   a , the first diffusion region  107   a  is set as a source electrode, the second diffusion region  107   b  is set as a drain electrode, and the transistor is allowed to operate in a saturated region. For example, 0 V is applied to the first diffusion region  107   a  and the P-type well region  102 , +1.8 V is applied to the second diffusion region  107   b , and +2 V is applied to the gate electrode  104 . In the case where electrons are not accumulated in the first memory functional unit  131   a  at this time, drain current is apt to flow. On the other hand, in the case where electrons are accumulated in the first memory functional unit  131   a , an inversion layer is not easily formed in the vicinity of the first memory functional unit  131   a , so that the drain current is not apt to flow. Therefore, by detecting the drain current, information stored in the first memory functional unit  131   a  can be read. In the case of applying a voltage so as to perform the pinch-off operation, thereby reading information, it is possible to determine with higher accuracy the state of charge accumulation in the first memory functional unit  131   a  without influence of the presence/absence of charge accumulation in the second memory functional unit  131   b.    
   In the case of reading information stored in the second memory functional unit  131   b , the second diffusion region  107   b  is set as a source electrode, the first diffusion region  107   a  is set as a drain electrode, and the transistor is operated. It is sufficient to apply, for example, 0 V to the second diffusion region  107   b  and the P-type well region  102 , +1.8 V to the first diffusion region  107   a , and +2 V to the gate electrode  104 . By interchanging the source and drain regions of the case of reading information stored in the first memory functional unit  131   a , information stored in the second memory functional unit  131   b  can be read. 
   In the case where a channel region (offset region  120 ) which is not covered with the gate electrode  104  remains, in the channel region which is not covered with the gate electrode  104 , an inversion layer is dissipated or formed according to the presence/absence of excessive charges in the memory functional units  131   a  and  131   b  and, as a result, large hysteresis (change in the threshold) is obtained. However, when the offset region  120  is too wide, the drain current largely decreases and reading speed becomes much slower. Therefore, it is preferable to determine the width of the offset region  120  so as to obtain sufficient hysteresis and reading speed. 
   Also in the case where the diffusion regions  107   a  and  107   b  reach ends of the gate electrode  104 , that is, the diffusion regions  107   a  and  107   b  overlap with the gate electrode  104 , the threshold of the transistor hardly changes by the writing operation. However, parasitic resistance at the source/drain ends largely changes, and the drain current largely decreases (by equal to or more than one digit). Therefore, reading can be performed by detecting the drain current, and the function as a memory can be obtained. In the case where a larger memory hysteresis effect is necessary, it is preferable that the diffusion regions  107   a  and  107   b  and the gate electrode  104  are not overlapped (offset region  120  exists). 
   By the above operating method, two bits can be written/erased selectively per one transistor. By connecting a word line WL to the gate electrode  104  of the memory cell, connecting a first bit line BL 1  to the first diffusion region  107   a , connecting a second bit line BL 2  to the second diffusion region  107   b , and arranging memory cells, a memory cell array can be constructed. 
   In the above-described operating method, by interchanging the source electrode and the drain electrode, writing and erasing of two bits per one transistor are performed. Alternately, by fixing the source electrode and the drain electrode, the transistor may operate as a 1-bit memory. In this case, common fixed voltage can be applied to one of the source and drain regions, so that the number of bit lines connected to the source/drain regions can be reduced to the half. 
   As obvious from the above description, in the memory cell in the semiconductor memory device of the present invention, the memory functional unit is formed independently of the gate insulating film, and is formed on both sides of the gate electrode, so that 2-bit operation is possible. Since each memory functional unit is isolated by the gate electrode, interference at the time of rewriting is effectively suppressed. Further, since the gate insulating film is isolated from the memory functional unit, it can be formed thinly and a short channel effect can be suppressed. Therefore, reduction in size of the memory cell and, accordingly, the semiconductor memory device can be achieved easily. 
   Second Embodiment 
   A memory cell in a semiconductor memory device according to a second embodiment has a configuration substantially similar to that of the memory cell  1  of  FIG. 1  except that, as shown in  FIG. 8 , each of memory functional units  261  and  262  is constructed by a charge retaining region (which is a charge accumulating region and may be a film having the function of retaining charges) and a region for suppressing escape of charges (or a film having the function of suppressing escape of charges). 
   From the viewpoint of improving a memory retention characteristic, preferably, the memory functional unit includes a charge retaining film having the function of retaining charges and an insulating film. In the second embodiment, a silicon nitride film  242  having a level of trapping charges is used as the charge retaining film, and silicon oxide films  241  and  243  having the function of preventing dissipation of charges accumulated in the charge retaining are used as insulating films. The memory functional unit includes the charge retaining film and the insulating films, thereby preventing dissipation of charges, and the retention characteristic can be improved. As compared with the case where the memory functional unit is constructed only by the charge retaining film, the volume of the charge retaining film can be appropriately reduced, movement of charges in the charge retaining film is regulated, and occurrence of a characteristic change due to charge movement during retention of information can be suppressed. Further, by employing the structure in which the silicon nitride film  242  is sandwiched by the silicon oxide films  241  and  243 , charge injecting efficiency at the time of rewriting operation becomes high, so that higher-speed operation can be performed. In the memory cell, the silicon nitride film  242  may be replaced with a ferroelectric. 
   The regions for retaining charges (silicon nitride films  242 ) in the memory functional units  261  and  262  overlap with diffusion regions  212  and  213 . The overlap denotes herein that at least a part of the region for retaining charges (silicon nitride film  242 ) exists over at least a part of the diffusion regions  212  and  213 . A reference numeral  211  denotes a semiconductor substrate, a reference numeral  214  denotes a gate insulating film, a reference numeral  217  denotes a gate electrode, and a reference numeral  271  indicates an offset region between the gate electrode  217  and the diffusion regions  212  and  213 . Although not shown, the surface of the semiconductor substrate  211  under the gate insulating film  214  serves as a channel region. 
   An effect obtained when the silicon nitride films  242  as regions for retaining charges in the memory functional units  261  and  262  overlap with the diffusion regions  212  and  213  will be described. 
   As shown in  FIG. 9 , in an area around the memory functional unit  262 , when an offset amount between the gate electrode  217  and the diffusion region  213  is W 1  and the width of the memory functional unit  262  in a cross section in the channel length direction of the gate electrode is W 2 , the overlap amount between the memory functional unit  262  and the diffusion region  213  is expressed as W 2 −W 1 . It is important herein that the memory functional unit  262  constructed by the silicon oxide film  242  in the memory functional unit  262  overlaps with the diffusion region  213 , that is, the relation of W 2 &gt;W 1  is satisfied. 
   In  FIG. 9 , an end on the side apart from the gate electrode  217  of the silicon nitride film  242  in the memory functional unit  262  matches with the end of the memory functional unit  262  on the side apart from the gate electrode  217 , so that the width of the memory functional unit  262  is defined as W 2 . 
   As shown in  FIG. 10 , when the end on the side apart from the gate electrode of a silicon nitride film  242   a  in a memory functional unit  262   a  does not match with the end of the memory functional unit  262   a  on the side apart from the gate electrode, W 2  may be defined as a distance from the gate electrode end to an end on the side apart from the gate electrode of the silicon nitride film  242   a.    
     FIG. 11  shows drain current Id when the width W 2  of the memory functional unit  262  is fixed to 100 nm and the offset amount W 1  is changed in the structure of the memory cell of FIG.  9 . Herein, the drain current was obtained by device simulation on assumption that the memory functional unit  262  is in erasing state (holes are accumulated), and the diffusion regions  212  and  213  serve as the source electrode and the drain electrode, respectively. 
   As obvious from  FIG. 11 , in the range where W 1  is 100 nm or more (that is, the silicon nitride film  242  and the diffusion region  213  do not overlap with each other), the drain current sharply decreases. Since the drain current value is substantially proportional to the reading operation speed, the performance of the memory sharply deteriorates with W 1  of 100 nm or more. On the other hand, in the range where the silicon nitride film  242  and the diffusion region  213  overlap with each other, decrease in the drain current is gentle. Therefore, in the case of considering also variations in mass production, if at least a part of the silicon nitride film  242  as the film having the function of retaining charges does not overlap with the source and drain regions, it is difficult to obtain the memory function in reality. 
   On the basis of the result of the device simulation, by fixing W 2  to 100 nm and setting W 1  to 60 nm and 100 nm as design values, memory cell arrays were produced. In the case where W 1  is 60 nm, the silicon nitride film  242  and the diffusion regions  212  and  213  overlap with each other by 40 nm as a design value. In the case where W 1  is 100 nm, there is no overlap as a design value. Reading time of the memory cell arrays was measured and worst cases considering variations were compared with each other. In the where W 1  is set to 60 nm as a design value, read access time is 100 times as fast as that of the other case. In practice, the read access time is preferably 100 n/sec or less per one bit. When W 1 =W 2 , the condition cannot be achieved. In the case of considering manufacture variations as well, it is more preferable that (W 2 −W 1 )&gt;10 nm. 
   To read information stored in the memory functional unit  261  (region  281 ), in a manner similar to the first embodiment, it is preferable to set the diffusion region  212  as a source electrode, set the diffusion region  213  as a drain region, and form a pinch-off point on the side closer to the drain region in the channel region. Specifically, at the time of reading information stored in one of the two memory functional units, it is preferable to form the pinch-off point in a region closer to the other memory functional unit, in the channel region. With the arrangement, irrespective of a storage state of the memory functional unit  262 , information stored in the memory functional unit  261  can be detected with high sensitivity, and it is a large factor to achieve 2-bit operation. 
   On the other hand, in the case of storing information only one of two memory functional units or in the case of using the two memory functional units in the same storage state, it is not always necessary to form the pinch-off point at the time of reading. 
   Although not shown in  FIG. 8 , it is preferable to form a well region (P-type well in the case of the N channel device) in the surface of the semiconductor substrate  211 . By forming the well region, it becomes easy to control the other electric characteristics (withstand voltage, junction capacitance and short-channel effect) while setting the impurity concentration in the channel region optimum to the memory operations (rewriting operation and reading operation). 
   The memory functional unit preferably includes the charge retaining film disposed substantially in parallel with the gate insulating film surface. In other words, it is preferable that the level of the top face of the charge retaining film in the memory functional unit is positioned parallel to the level of the top face of the gate insulating film  214 . Concretely, as shown in  FIG. 12 , the silicon nitride film  242   a  as a charge retaining film of the memory functional unit  262  has a surface substantially parallel with the surface of the gate insulating film  214 . In other words, it is preferable that the silicon nitride film  242   a  is formed at a level parallel to the level corresponding to the surface of the gate insulating film  214 . 
   By the existence of the silicon nitride film  242   a  substantially parallel to the surface of the gate insulating film  214  in the memory functional unit  262 , formation easiness of the inversion layer in the offset region  271  can be effectively controlled in accordance with an amount of charges accumulated in the silicon nitride film  242   a . Thus, the memory effect can be increased. By forming the silicon nitride film  242   a  substantially in parallel with the surface of the gate insulating film  214 , even in the case where the offset amount (W 1 ) varies, a change in the memory effect can be maintained relatively small, and variations of the memory effect can be suppressed. Moreover, movement of the charges upward in the silicon nitride film  242   a  is suppressed, and occurrence of a characteristic change due to the charge movement during retention of information can be suppressed. 
   Preferably, the memory functional unit  262  includes an insulating film (for example, portion on the offset region  271  in the silicon oxide film  244 ) for separating the silicon nitride film  242   a  which is substantially parallel to the surface of the gate insulating film  214  and the channel region (or well region). By the insulating film, dissipation of the charges accumulated in the charge retaining film is suppressed and a memory cell having a better retention characteristic can be obtained. 
   By controlling the thickness of the silicon nitride film  242   a  and controlling the thickness of the insulating film below the silicon nitride film  242   a  (portion on the offset region  271  in the silicon oxide film  244 ) to be constant, the distance from the surface of the semiconductor substrate to charges accumulated in the charge retaining film can be maintained substantially constant. To be specific, the distance from the surface of the semiconductor substrate to the charges accumulated in the charge retaining film can be controlled in a range from the minimum thickness value of the insulating film under the silicon nitride film  242   a  to the sum of the maximum thickness value of the insulating film under the silicon nitride film  242   a  and the maximum thickness value of the silicon nitride film  242   a . Consequently, density of electric lines of force generated by the charges accumulated in the silicon nitride film  242   a  can be substantially controlled, and variations in the memory effect of the memory cell can be reduced very much. 
   Third Embodiment 
   The memory functional unit  262  in a semiconductor memory device of a third embodiment has a shape in which the silicon nitride film  242  as a charge retaining film has substantially uniform thickness and is disposed substantially in parallel with the surface of the gate insulating film  214  as shown in  FIG. 13  (region  281 ) and, further, substantially in parallel with a side face of the gate electrode  217  (region  282 ). 
   In the case where positive voltage is applied to the gate electrode  217 , an electric line  283  of force in the memory functional unit  262  passes the silicon nitride film  242  twice (regions  282  and  281 ) as shown by an arrow. When negative voltage is applied to the gate electrode  217 , the direction of the electric line of force becomes opposite. Herein, the dielectric constant of the silicon nitride film  242  is about 6, and that of silicon oxide films  241  and  243  is about 4. Therefore, effective dielectric constant of the memory functional unit  262  in the direction of the electric line  283  of force is higher and the potential difference at both ends of the electric line of force can be reduced more as compared with the case where only the region  281  of the charge retaining film exists. In other words, a large part of the voltage applied to the gate electrode  217  is used to enhance the electric field in the offset region  271 . 
   The reason why charges are injected to the silicon nitride film  242  in the rewriting operation is because generated charges are attracted by the electric field in the offset region  271 . Therefore, by including the charge retaining film shown by the arrow  282 , charges injected into the memory functional unit  262  increase in the rewriting operation, and the rewriting speed increases. 
   In the case where the portion of the silicon oxide film  243  is also the silicon nitride film, that is, in the case where the level of the charge retaining film is not parallel with the level corresponding to the surface of the gate insulating film  214 , upward movement of charges in the silicon nitride film becomes conspicuous, and the retention characteristic deteriorates. 
   More preferably, in place of the silicon nitride film, the charge retaining film is made of a high dielectric such as hafnium oxide having a very high dielectric constant. 
   It is preferable that the memory functional unit further includes an insulating film (portion on the offset region  271  in the silicon oxide film  241 ) for separating the charge retaining film substantially parallel to the surface of the gate insulating film and the channel region (or well region). By the insulating film, dissipation of charges accumulated in the charge retaining film is suppressed, and the retention characteristic can be further improved. 
   Preferably, the memory functional unit further includes an insulating film (portion in contact with the gate electrode  217  in the silicon oxide film  241 ) for separating the gate electrode and the charge retaining film extended substantially parallel with the side face of the gate electrode. The insulating film prevents injection of charges from the gate electrode into the charge retaining film and accordingly prevents a change in the electric characteristics. Thus, the reliability of the memory cell can be improved. 
   Further, in a manner similar to the second embodiment, it is preferable to control the thickness of the insulating film under the silicon nitride film  242  (portion on the offset region  271  in the silicon oxide film  241 ) to be constant and to control the thickness of the insulating film on the side face of the gate electrode (portion in contact with the gate electrode  217  in the silicon oxide film  241 ) to be constant. Consequently, the density of the electric lines of force generated by the charges accumulated in the silicon nitride film  242  can be substantially controlled, and charge leak can be prevented. 
   Fourth Embodiment 
   In a fourth embodiment, optimization of the gate electrode, the memory functional unit, and the distance between the source and drain regions of a memory cell in a semiconductor memory device will be described. 
   As shown in  FIG. 14 , a reference character A denotes length of the gate electrode in a cut surface in the channel length direction, a reference character B denotes the distance between the source and drain regions (channel length), and a reference character C denotes the distance from the end of one of memory functional units to the end of the other memory functional unit, that is, the distance between the end (on the side far from the gate electrode) of a film having the function of retaining charges in one of memory functional units to the end (on the side apart from the gate electrode) of a film having the function of retaining charges in the other memory functional unit in a cut surface in the channel length direction. 
   In such a memory cell, B&lt;C is preferable. By satisfying such a relation, the offset regions  271  exist between the portion under the gate electrode  217  in the channel region and the diffusion regions  212  and  213 . Consequently, easiness of inversion effectively fluctuates in the whole offset regions  271  by charges accumulated in the memory functional units  261  and  262  (silicon nitride films  242 ). Therefore, the memory effect increases and, particularly, higher-speed reading operation is realized. 
   In the case where the gate electrode  217  and the diffusion regions  212  and  213  are offset from each other, that is, in the case where the relation of A&lt;B is satisfied, easiness of inversion in the offset region when voltage is applied to the gate electrode largely varies according to an amount of charges accumulated in the memory functional unit, so that the memory effect increases, and the short channel effect can be reduced. 
   However, as long as the memory effect appears, the offset region  271  does not always have to exist. Also in the case where the offset region  271  does not exist, if the impurity concentration in the diffusion regions  212  and  213  is sufficiently low, the memory effect can be exhibited in the memory functional units  261  and  262  (silicon nitride films  242 ). 
   Therefore, A&lt;B&lt;C is the most preferable. 
   Fifth Embodiment 
   A memory cell of a semiconductor memory device in a fifth embodiment has a substantially similar configuration to that of the second embodiment except that an SOI substrate is used as the semiconductor substrate in the second embodiment as shown in FIG.  15 . 
   In the memory cell, a buried oxide film  288  is formed on a semiconductor substrate  286 , and an SOI layer is formed on the buried oxide film  288 . In the SOI layer, the diffusion regions  212  and  213  are formed and the other region is a body region  287 . 
   By the memory cell as well, action and effect similar to those of the memory cell of the second embodiment are obtained. Further, junction capacitance between the diffusion regions  212  and  213  and the body region  287  can be remarkably reduced, so that higher-speed operation and lower power consumption of the device can be achieved. 
   Sixth Embodiment 
   A memory cell in a semiconductor memory device in a sixth embodiment has, as shown in  FIG. 16 , a configuration substantially similar to that of the memory cell of the second embodiment except that a P-type high-concentration region  291  is added adjacent to the channel sides of the N-type diffusion regions  212  and  213 . 
   Specifically, the concentration of a P-type impurity (for example, boron) in the P-type high-concentration region  291  is higher than that of a P-type impurity in a region  292 . Suitable P-type impurity concentration int he P-type high-concetration region  291  is, for example, about 5×10 17  to 1×10 19  cm −3 . The P-type impurity concentration of the region  292  can be set to, for example, 5×10 16  to 1×10 18  cm −3 . 
   By providing the P-type high-concentration region  291 , the junction between the diffusion regions  212  and  213  and the semiconductor substrate  211  becomes sharp below the memory functional units  261  and  262 . Consequently, hot carriers are easily generated in the writing and erasing operations, the voltage of the writing and erasing operations can be decreased or the writing operation and the erasing operation can be performed at high speed. Moreover, since the impurity concentration in the region  292  is relatively low, the threshold when the memory is in the erasing state is low, and the drain current is large. Consequently, the reading speed is improved. Therefore, the memory cell with low rewriting voltage or high rewriting speed and high reading speed can be obtained. 
   In  FIG. 16 , by providing the P-type high-concentration region  291  in the vicinity of the source/drain regions and below the memory functional unit (that is, not immediately below the gate electrode), the threshold of the whole transistor remarkably increases. The degree of increase is much higher than that in the case where the P-type high-concentration region  291  is positioned immediately below the gate electrode. In the case where write charges (electrons when the transistor is of the N-channel type) are accumulated in the memory functional unit, the difference becomes larger. On the other hand, in the case where sufficient erasing charges (positive holes when the transistor is of the N-channel type) are accumulated in the memory functional unit, the threshold of the whole transistor decreases to a threshold determined by the impurity concentration in the channel region (region  292 ) below the gate electrode. That is, the threshold in the erasing operation does not depend on the impurity concentration of the P-type high-concentration region  291  whereas the threshold in the writing operation is largely influenced. Therefore, by disposing the P-type high-concentration region  291  under the memory functional unit and in the vicinity of the source/drain regions, only the threshold in the writing operation largely fluctuates, and the memory effect (the difference between the threshold in the writing operation and that in the erasing operation) can be remarkably increased. 
   Seventh Embodiment 
   A memory cell in a semiconductor memory device of a seventh embodiment has a configuration substantially similar to that of the second embodiment except that, as shown in  FIG. 17 , the thickness (T 1 ) of an insulating film separating the charge retaining film (silicon nitride film  242 ) and the channel region or well region is smaller than the thickness (T 2 ) of the gate insulating film. 
   The thickness T 2  of the gate insulating film  214  has the lower limit value from the demand of withstand voltage at the time of rewriting operation of the memory. However, the thickness T 1  of the insulating film can be made smaller than T 2  irrespective of the demand of withstand voltage. 
   The flexibility of designing with respect to T 1  is high in the memory cell for the following reason. 
   In the memory cell, the insulating film for separating the charge retaining film and the channel region or well region is not sandwiched by the gate electrode and the channel region or well region. Consequently, to the insulating film for separating the charge retaining film and the channel region or well region, a high electric field acting between the gate electrode and the channel region or well region does not directly act, but a relatively low electric field spreading from the gate electrode in the lateral direction acts. Consequently, irrespective of the demand of withstand voltage to the gate insulating film, T 1  can be made smaller than T 2 . 
   By making T 1  thinner, injection of charges into the memory functional unit becomes easier, the voltage of the writing operation and the erasing operation is decreased or the writing operation and erasing operation can be performed at high speed. Since the amount of charges induced in the channel region or well region when charges are accumulated in the silicon nitride film  242  increases, the memory effect can be increased. 
   The electric lines of force in the memory functional unit include a short one which does not pass through the silicon nitride film  242  as shown by an arrow  284  in FIG.  13 . On the relatively short electric line of force, electric field intensity is relatively high, so that the electric field along the electric line of power plays a big role in the rewriting operation. By reducing T 1 , the silicon nitride film  242  is positioned downward in the figure, and the electric line of force indicated by the arrow  283  passes through the silicon nitride film. Consequently, the effective dielectric constant in the memory functional unit along the electric line  284  of force increases, and the potential difference at both ends of the electric line of force can be further decreased. Therefore, a large part of the voltage applied to the gate electrode  217  is used to increase the electric field in the offset region, and the writing operation and the erasing operation become faster. 
   In contrast, for example, in an EEPROM typified by a flash memory, the insulating film separating the floating gate and the channel region or well region is sandwiched by the gate electrode (control gate) and the channel region or well region, so that a high electric field from the gate electrode directly acts. Therefore, in an EEPROM, the thickness of the insulating film separating the floating gate and the channel region or well region is regulated, and optimization of the function of the memory cell is inhibited. 
   As obvious from the above, by setting T 1 &lt;T 2 , without deteriorating the withstand voltage performance of the memory, the voltage of the writing and erasing operations is decreased, or the writing operation and erasing operation are performed at high speed and, further, the memory effect can be increased. More preferably, the thickness T 1  of the insulating film is 0.8 nm or more at which uniformity or quality by a manufacturing process can be maintained at a predetermined level and which is the limitation that the retention characteristic does not deteriorate extremely. 
   Concretely, in the case of a liquid crystal driver LSI requiring high withstand voltage in a design rule, to drive the liquid crystal panel TFT, voltage of 15 to 18 V at the maximum is required, so that the gate oxide film cannot be thinned normally. In the case of mounting a nonvolatile memory for image adjustment on the liquid crystal driver LSI, in the memory cell of the present invention, the thickness of the insulating film separating the charge retaining film (silicon nitride film  242 ) and the channel region or well region can be designed optimally independently of the thickness of the gate insulating film. For example, the thickness can be individually set as T 1 =20 nm and T 2 =10 nm for a memory cell having a gate electrode length (word line width) of 250 nm, so that a memory cell having high writing efficiency can be realized (the reason why the short channel effect is not produced when T 1  is larger than the thickness of a normal logic transistor is because the source and drain regions are offset from the gate electrode). 
   Eighth Embodiment 
   A memory cell in a semiconductor memory device of an eighth embodiment has a configuration substantially similar to that of the second embodiment except that, as shown in  FIG. 18 , the thickness (T 1 ) of the insulating film separating the charge retaining film (silicon nitride film  242 ) and the channel region or well region is larger than the thickness (T 2 ) of the gate insulating film. 
   The thickness T 2  of the gate insulating film  214  has an upper limit value due to demand of preventing a short channel effect of the device. However, the thickness T 1  of the insulating film can be made larger than T 2  irrespective of the demand of preventing the short channel effect. Specifically, when reduction in scaling progresses (when reduction in thickness of the gate insulating film progresses), the thickness of the insulating film separating the charge retaining film (silicon nitride film  242 ) and the channel region or well region can be designed optimally independent of the gate insulating film thickness. Thus, an effect that the memory functional unit does not disturb scaling is obtained. 
   The reason why flexibility of designing T 1  is high in the memory cell is that, as described already, the insulating film separating the charge retaining film and the channel region or well region is not sandwiched by the gate electrode and the channel region or well region. Consequently, irrespective of the demand of preventing the short channel effect for the gate insulating film, T 1  can be made thicker than T 2 . 
   By making T 1  thicker, dissipation of charges accumulated in the memory functional unit can be prevented and the retention characteristic of the memory can be improved. 
   Therefore, by setting T 1 &gt;T 2 , the retention characteristic can be improved without deteriorating the short channel effect of the memory. 
   The thickness T 1  of the insulating film is, preferably, 20 nm or less in consideration of decrease in rewriting speed. 
   Concretely, in a conventional nonvolatile memory typified by a flash memory, a selection gate electrode serves as a write erase gate electrode, and a gate insulating film (including a floating gate) corresponding to the write erase gate electrode also serves as a charge accumulating film. Since a demand for size reduction (thinning of a film is indispensable to suppress short channel effect) and a demand for assuring reliability (to suppress leak of retained charges, the thickness of the insulating film separating the floating gate and the channel region or well region cannot be reduced to about 7 nm or less) are contradictory, it is difficult to reduce the size. Actually, according to the ITRS (International Technology Roadmap for Semiconductors), there is no prospect of reduction in a physical gate length of about 0.2 micron or less. In the memory cell, since T 1  and T 2  can be individually designed as described above, size reduction is made possible. 
   For example, for a memory cell having a gate electrode length (word line width) of 45 nm, T 2 =4 nm and T 1 =7 nm are individually set, and a memory cell in which the short channel effect is not produced can be realized. The reason why the short channel effect is not produced even when T 2  is set to be thicker than the thickness of a normal logic transistor is because the source/drain regions are offset from the gate electrode. 
   Since the source/drain regions are offset from the gate electrode in the memory cell, as compared with a normal logic transistor, reduction in size is further facilitated. 
   Since the electrode for assisting writing and erasing does not exist in the upper part of the memory functional unit, a high electric field acting between the electrode for assisting writing and erasing and the channel region or well region does not directly act on the insulating film separating the charge retaining film and the channel region or well region, but only a relatively low electric field which spreads in the horizontal direction from the gate electrode acts. Consequently, the memory cell having a gate length which is reduced to be equal to or less than the gate length of a logic transistor of the same process generation can be realized. 
   Ninth Embodiment 
   A ninth embodiment relates to a change in the electric characteristic at the time of rewriting a memory cell of a semiconductor memory device. 
   In an N-channel type memory cell, when an amount of charges in a memory functional unit changes, a drain current (Id)-gate voltage (Vg) characteristic (actual measurement value) as shown in  FIG. 19  is exhibited. 
   As obvious from  FIG. 19 , in the case of performing a writing operation in an erasing state (solid line), not only the threshold simply increases, but also the gradient of a graph remarkably decreases in a sub-threshold region. Consequently, also in a region where a gate voltage (Vg) is relatively high, the drain current ratio between the erasing state and the writing state is high. For example, also at Vg=2.5V, the current ratio of two digits or more is maintained. The characteristic is largely different from that in the case of a flash memory (FIG.  25 ). 
   Appearance of such a characteristic is a peculiar phenomenon which occurs since the gate electrode and the diffusion region are offset from each other, and the gate electric field does not easily reach the offset region. When the memory cell is in a writing state, even when a positive voltage is applied to the gate electrode, an inversion layer is extremely hard to be formed in the offset region under the memory functional unit. This is the cause that the gradient of the Id-Vg curve is gentle in the sub-threshold region in the writing state. 
   On the other hand, when the memory cell is in an erasing state, electrons of high density are induced in the offset region. Further, when 0 V is applied to the gate electrode (that is, when the gate electrode is in an off state), electrons are not induced in the channel below the gate electrode (consequently, an off-state current is small). This is the cause that the gradient of the Id-Vg curve is sharp in the sub-threshold region in the erasing state, and current increasing rate (conductance) is high in the region of the threshold or more. 
   As obviously understood from the above, in the memory cell in the semiconductor memory device of the present invention, the drain current ratio between the writing operation and the erasing operation can be particularly made high. 
   Tenth Embodiment 
   A tenth embodiment relates to a semiconductor memory device and a method for driving the semiconductor memory device, in which a memory array is configured by arranging the plurality of memory cells described in the first to ninth embodiments and a drive circuit for the memory array is additionally provided. 
   Specifically, a memory cell in the memory array in this embodiment includes a gate electrode formed on a semiconductor layer via a gate insulating film, a channel region disposed below the gate electrode, diffusion regions disposed on both sides of the channel region and having a conductive type opposite to that of the channel region, and memory functional units formed on both sides of the gate electrode and having a function of retaining charges. 
   A memory array shown in  FIG. 20  includes memory cells C consisting of a row array and a column array. A gate of the memory cell C in one row is connected to a row line W. A source of the memory cell C in one column is connected to a bit line B 2 , and its drain is connected to a bit line B 3 . 
   In order to read the content of a cell Cb, the cell is first selected by setting a word line Wa at a high positive potential. At the same time, a source line B 2   b  is grounded via a transistor Tr 2 . Another bit line right to the cell Cb (for example, a bit line B 3   b ) is turned into a floating state. A row line B 3   a  is connected to a read/drain bias potential, which is applied to a node B, via a transistor Tr 1 . An adjacent column line B 2   a  is connected not to the same potential as that of the node B but to another read/drain bias potential, which is applied to a node A, in the method shown in FIG.  20 . The node A is connected to the adjacent bit line B 2   a  via a transistor Tr 3 . All of other bit lines left to the column B 2   a  remain floating. 
   The read/drain bias potential applied to the node A is equal in value to the potential to be applied to the node B, for example, 1.2 V. A reading current all can be secured to flow via the cell Cb without bypassing the cell Ca by applying the potentials of the same value. Consequently, a value of a current Ir precisely corresponds to the content of the cell Cb. In other word, a circuit for applying the potential to the node A is different from a circuit for applying the potential to the node B. 
   A Y decoder circuit in this embodiment shown in  FIG. 21  has source bit lines B 2  and drain bit lines B 3 , across which a transistor  426  is connected. A gate of each of the transistors  426  is connected to a signal line YD along a line  427 . The lines B 2  and B 3 , of course, include the column line in the memory array shown in FIG.  20 . The transistor  426  includes an inside column path gate transistor, for separating various column lines during a variety of writing processing periods. The function of the transistor  426  will be described later. 
   Each of the drain bit lines B 3  is ended at a source of a transistor  441 . A drain of the transistor  441  is connected to a read/drain bias voltage B along a line  430 . A gate of each of the transistors  441  is connected to a bus  429  (represented by reference character YB). The decoder shown in  FIG. 21  contains an array of 8 lines therein. The bus  429  has 8 bits, each of which is connected to the gate of each of the transistors  441 . During a reading processing period, each of the lines of the bus  429  is actuated; thus, the selected drain line B 3  is connected to the B potential. 
   In the same manner, each of the source lines B 2  is connected to one terminal of a transistor  442 , the other terminal of which is connected to a source/read bias line (represented by reference character D). A gate of each of the transistors  442  is connected to a bus  433  (represented by reference character YA). Normally, the line D is connected to a ground potential during the reading period, and then, a proper line YA is increased in potential, so that the selected source line B 2  is grounded via the transistor  442 . A second read/drain bias voltage A is provided along a line  434  shown in  FIG. 21 , and is connected to a source of a transistor  443 . A drain of each of the transistors  443  is connected to each of the source bit lines B 2 . A gate of the transistor  443  is connected along a selection line bus  435  (represented by reference character YC). When the contents of the memory cell are read, the selection lines YA and YB allow the source and drain bit lines to be connected to the ground potential and the B potential, respectively. One line of the YC bus is used for selecting the source column line, which is located immediately next to the drain line connected to the B potential at this time. The selected source column line is connected to the potential A on the line  434 . As described above, the potentials A and B are supplied via the different nodes by the different circuits, respectively, although they are equal in value to each other. 
   The source column line B 2  supplies a potential V to a line  440  via each of P-channel field effect transistors  446 . An n-well of each of the P-channel field effect transistors  446  also is connected to the potential V along the line  440 . A gate of each of the transistors  446  is connected along a bus  438  (represented by reference character YE). The transistor  446  is provided for erasing process; therefore, the contents of all of the memory arrays can be erased at the same time. For example, in order to erase the contents of the memory array shown in  FIG. 21 , the potential V is connected to each of the column lines B 2  by grounding the gate of the transistor  446 . Simultaneously, all of the word lines W (see  FIG. 20 ) in the array are grounded. As a result, the contents of all of the arrays are erased. Of course, there are many other methods for erasing the memory cells in the array. 
   During a writing period in the memory cell, the source is grounded, and further, a writing voltage is applied to the drain within a range of 4 V to 10 V and to the gate within a range of 3 V to 9 V. Herein, it is supposed to apply 5 V to the drain and 5 V to the gate. A writing voltage is applied, so that thermoelectrons are generated in a high electric field. The thermoelectrons adhere to the memory functional unit (i.e., the sidewall) at the cell. The term of “a writing trouble” signifies that writing is performed with respect to the adjacent cell along the same line to a similar extent, although not intended. For example, if the potential of the word line on a given row is increased up to a writing potential of 5 V, the potential of a gate of a cell immediately adjacent to a selected cell (i.e., a cell to be read) also becomes 5 V. The adjacent cell also shares the drain line whose potential is increased up to about 5 V, so that there is a possibility of generation of a high electric field over the channel of the adjacent cell. As a consequence, the writing is parasitically performed with respect to the adjacent memory cell. If the source and drain lines of the adjacent cell can be held at substantially the same potential, it is possible to definitely avoid the parasitic writing. In order to avoid a writing trouble with respect to the adjacent cell on the same row line as the selected cell, a de-bias mechanism incorporating the inside column path gate having a slowly inclined selection column voltage is used in this embodiment. 
   As shown in  FIG. 21 , each of the plurality of n-channel field effect transistors  426  has a gate connected to the 16-bit bus  427  (represented by reference character YD). Each of the transistors  426  is connected between the source bit line B 2  and the drain bit line B 3 . During the writing period of the selected cell (for example, the cell Cb) in the column array, the source line B 2  is first grounded via the transistor Tr 2  (see FIG.  20 ). All of the column lines of the other sources and drains on the other side of that array (that is, the line B 3   b  and the like in  FIG. 20 ) also are grounded by increasing the potential of the gate of the transistor  426  right to the selected cell. Consequently, the ground potential connected to the source bit line B 2  is transmitted to the other column line in the array. 
   A similar approach is adopted to de-bias all of the column lines left to the selected memory cell. That is to say, the writing potential connected to the drain column line B 3   a  is transmitted to all of the column lines left to the selected memory cell via the transistors  426  by increasing the potential of the gate of the transistor  426  left to the selected cell to a high value. 
   In other words, when the writing is carried out with respect to the selected memory cell, all of the column lines on the side of the source of the selected cell are grounded while all of the column lines on the side of the drain of the selected cell are connected to a writing voltage of about 5 V. The de-biasing in this manner can effectively prevent any generation of the high electric field over the adjacent cell. As described above, not-intended writing is readily carried out at the cell by the high electric field between the source/drain terminals of the adjacent cell. 
   It is necessary to recognize that each of the transistors  426  has a given resistance incidental to the channel region and each of the column lines has an incidental capacitance. As a result, if the writing potential applied to the drain column line B 3  is increased very fast (that is, at a high rising speed), there is produced a marked difference in voltage. It is supposed what happens when the line B 3   a  shown in  FIG. 20  abruptly rises up to a writing voltage of 5 V. The potential of the adjacent column line B 2   a  is delayed in comparison with that of the line B 3   a  due to the respective resistances and capacitances incidental to the transistor  426  and the column lines B 2  and B 3 . Consequently, a signal path has characteristics of a standard transmission bus. 
   The propagation delay of the writing potential causes an inclination of a voltage between the adjacent column lines B 3   a  and B 2   a . If the difference in potential is sufficiently large, the writing is carried out with respect to the adjacent cell Ca. Ideally, the difference in potential between the two column lines connected to a not-selected cell must be zero, and at least, it must become smaller than a predetermined limit. The limit signifies a largest electric field strength which can be allowed before the writing is carried out. In order to maintain the difference in potential smaller than the allowable limit, a writing potential to be applied to the drain bias line B 3  is increased at a controlled speed. An inclination speed of about 10 volt/μsec or less is suitable for avoiding the writing trouble. If the speed is lower than that value, the voltages of the connected column lines rise up substantially at the same time. That is to say, if the speed is lower than 10 volt/μsec, a difference in potential generated between any two adjacent column lines is insufficient for parasitically performing the writing in the memory cell. It is understood that the transistor  426  should be optionally eliminated when the parasitic writing is ignored or the speed is not critical. 
   Eleventh Embodiment 
   As an application example of the semiconductor memory device, for example, as shown in  FIG. 22 , a rewritable nonvolatile memory for image adjustment of a liquid crystal panel can be mentioned. 
   A liquid crystal panel  1001  is driven by a liquid crystal driver  1002 . In the liquid crystal driver  1002 , a nonvolatile memory  1003 , an SRAM  1004  and a liquid crystal driver circuit  1005  are provided. The nonvolatile memory  1003  is constructed by the memory cell of the present invention, more preferably, any of the semiconductor memory devices of the first to ninth embodiments. The nonvolatile memory  1003  can be rewritten from the outside. 
   Information stored in the nonvolatile memory  1003  is transferred to the SRAM  1004  at the time of turn-on of the power source of an apparatus. The liquid crystal driver circuit  1005  can read stored information from the SRAM  1004  as necessary. By providing the SRAM, high reading speed of stored information can be achieved. 
   The liquid crystal driver  1002  may be externally attached to the liquid crystal panel  1001  as shown in  FIG. 22  or formed on the liquid crystal panel  1001 . 
   In a liquid crystal panel, tones displayed by applying voltages in multiple grades to pixels are changed. The relation between the given voltage and the displayed tone varies according to products. Consequently, information for correcting variations in each product after completion of the product is stored and correction is made on the basis of the information, thereby enabling the picture qualities of products to be made uniform. It is therefore preferable to mound a rewritable nonvolatile memory for storing correction information. As the nonvolatile memory, it is preferable to use the memory cell of the present invention. Particularly, it is preferable to use any of the semiconductor memory devices of the first to ninth embodiments in which memory cells of the present invention are integrated. 
   Twelfth Embodiment 
     FIG. 23  shows a portable telephone as a portable electronic apparatus in which the semiconductor memory device is assembled. 
   The portable telephone is constructed mainly by a control circuit  811 , a battery  812 , an RF (radio frequency) circuit  813 , a display  814 , an antenna  815 , a signal line  816 , a power source line  817  and the like. In the control circuit  811 , the semiconductor memory device of the present invention is assembled. The control circuit  811  is preferably an integrated circuit using cells having the same structure as a memory circuit cell and a logic circuit cell as described in the tenth embodiment. It facilitates fabrication of the integrated circuit, and the manufacturing cost of the portable electronic apparatus can be particularly reduced. 
   By using the semiconductor memory device capable of performing high-speed reading operation and whose process of mounting a memory part and a logic circuit part simultaneously is easy for a portable electronic apparatus, the operation speed of the portable electronic apparatus is increased, and the manufacturing cost can be reduced. Thus, a cheap, high-reliability, and high-performance portable electronic apparatus can be obtained. 
   It is possible to prevent any parasitic current at the adjacent cell in the memory array by the semiconductor memory device and the method for driving the semiconductor memory device according to the present invention. 
   Furthermore, according to the present invention, the memory cell includes a gate electrode formed on a semiconductor layer via a gate insulating film, a channel region disposed below the gate electrode, diffusion regions disposed on both sides of the channel region and having a conductive type opposite to that of the channel region, and memory functional units formed on both sides of the gate electrode and having a function of retaining charges. Consequently, a memory cell forming process has a very high affinity with a normal transistor forming process. Therefore, it is possible to remarkably reduce the number of masks and the number of processes in comparison with the case of the combination with a peripheral circuit including normal transistors by using a conventional flash memory as a nonvolatile memory cell. As a consequence, it is possible to enhance the yield in production of a chip and reduce a cost. 
   The memory functional unit of the memory cell includes a film having a surface substantially parallel with a surface of the gate insulating film and having the function of retaining charges. Consequently, it is possible to suppress variations of the memory effect, so as to increase the probability of a success in a rewriting operation. Thus, it is possible to set a maximum pulse value to a value smaller than that in the prior art, thereby completing the rewriting operation in a short period of time. In addition, it is possible to decrease the possibility of generation of a deficient rewriting operation. 
   The semiconductor layer in the memory cell has a region in the vicinity of the diffusion region is higher in concentration than a region in the vicinity of a surface of the semiconductor layer below the gate electrode. Consequently, the memory effect is enhanced, and further, the rewriting speed is increased. The memory cell includes a film having a surface substantially parallel with a surface of the gate insulating film and having the function of retaining charges and an insulating film for separating the film from the channel region or the semiconductor layer, the insulating film having a thickness thinner than the gate insulating film and not less than 0.8 nm. Therefore, the increase in charges induced in the channel region or the well region enhances the memory effect, and further, facilitates the implantation of the charges into the memory functional unit, thereby increasing the rewriting speed. 
   Consequently, since the memory effect is enhanced in the memory cell, it is possible to take a large design margin for determining the program (writing) state or the erasing state, so as to increase the probability of the success in the rewriting operation. Therefore, it is possible to set the maximum pulse value smaller than that in the prior art, so as to complete the rewriting operation in a short period of time. Moreover, it is possible to decrease the possibility of the generation of the deficient rewriting operation. 
   The memory functional unit of the memory cell includes a charge retention film extending substantially parallel with the side surface of the gate electrode, thereby increasing the rewriting speed. 
   In this manner, since the rewriting speed in the memory cell according to the present invention is higher than that in the prior art, it is possible to complete the rewriting operation in a shorter period of time than in the conventional flash memory. 
   Since at least a part of the memory functional unit in the memory cell overlaps a part of the diffusion region, the erasing operation can be performed at a negative voltage of an absolute value lower than that in the prior art. 
   Since the electronic apparatus, in particular, the portable electronic apparatus according to the present invention is provided with the above-described semiconductor memory device, it is possible to facilitate the process of combining the memory and the logic circuit, to increase the operating speed of the electronic apparatus, to reduce the manufacturing cost, and to provide the display device of a high reliability at a reduced cost.