Patent Publication Number: US-6982906-B2

Title: Electrically programmable and electrically erasable semiconductor memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is related to Japanese patent application No.2003-131984 filed on May 9, 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 and, more particularly, to an electrically programmable and electrically erasable semiconductor memory device. More specifically, the present invention relates to: a semiconductor memory device obtained by arranging semiconductor memory cells each including 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 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 the function of retaining charges; and to a display device and a portable electronic apparatus each having the semiconductor memory device. The present invention concerns a verifying operation at the time of performing a data writing/erasing process on such a semiconductor memory device. 
   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. 22 , 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. 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, by selecting a specific word line and a specific bit line and applying a predetermined voltage, an operation of rewriting/reading a desired memory cell can be performed. 
   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. 23  is displayed. In the figure, a solid line shows the characteristic in a writing state, and a dashed line shows the characteristic in an erasing state. When the amount of negative charges in the floating gate increases, the threshold increases, and an Id-Vg curve shifts almost 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 such circumstances and its object is to provide a finer nonvolatile semiconductor memory device. 
   Another object of the present invention is to improve speed in a writing/erasing process on a semiconductor memory device and the reliability, and to realize lower power consumption. 
   The present invention provides a semiconductor memory device comprising: an electrically programmable and erasable nonvolatile memory cell which includes a plurality of memory cells requiring a first potential for reading data and a second potential for programming data, the second potential being higher than the first potential, and which is assembled on a substrate; latch means for receiving data and temporarily storing the data; pulse generation means for generating a pulse used for programming data into a memory cell, the pulse generation means being coupled in order to receive the second potential; comparator means for comparing data in the latch means with data in a memory cell; and control means for controlling the pulse generation means to repeatedly generate a pulse until the data in the latch means matches the data in the memory cell, the control means being coupled to the comparator means and the pulse generation means, wherein the control means controls the pulse generation means so that the pulse is repeatedly generated until data is programmed in a memory cell, and the nonvolatile 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 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 the function of retaining charges. 
   With the configuration, even when a pulse shorter than a conventionally used pulse is used, a failure in writing and erasing processes on the memory cell can be prevented, and the speed of the writing and erasing processes can be improved. 
   The control means may make the pulse generation means erase data in a memory cell prior to writing data to the memory cell. 
   The control means may generate a signal indicating that data is written into a memory cell, thereby enabling the latch means to receive new data. 
   Further, the pulse generation means may generate a pulse of which leading edge is inclined. 
   Pulse duration time may be substantially shorter than 10 ms. 
   Further, when data is read from a memory cell, the reference potential may be applied to the memory cell and, during comparison by the comparator means, a potential other than the reference potential may be applied to ensure that data in a memory cell can be read in adverse environments. 
   The present invention also provides an electrically programmable and erasable semiconductor memory device comprising: an electrically programmable and erasable nonvolatile memory cell which uses a plurality of memory cells requiring a first potential for reading data and a second potential for programming/erasing data, the second potential being higher than the first potential, and using a reference potential when data is read from a memory cell, and is assembled on a substrate; first circuit means for erasing data in a memory cell, applying the first potential different from the reference potential to the memory cell, and reading the data from the memory cell while applying the first potential to ensure that data in the memory cell is erased; and second circuit means for writing data into a memory cell, applying the second potential which is different from the reference potential and the first potential to the memory cell, and reading the data from the memory cell while applying the second potential to check whether the data is written in the memory cell or not, wherein the nonvolatile memory device 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 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 the function of retaining charges, and the data in the memory cell is read by a potential other than the reference potential in order to ensure that the memory cell is correctly programmed. 
   The reference potential may be a positive potential, the first potential may be higher than the reference potential, and the second potential may be lower than the reference potential. 
   The semiconductor memory device may further comprises: latch means for receiving data and temporarily storing the data; pulse generation means for generating a pulse used for programming data into a memory cell, the pulse generation means being coupled to receive the second potential; comparator means for comparing data in the latch means with data in a memory cell; and control means for controlling the pulse generation means to repeatedly generate a pulse until the data in the latch means matches the data in the memory cell, the control means being coupled to the comparator means and the pulse generation means. 

   
     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 schematic configuration diagram showing a liquid crystal display (eleventh embodiment) in which the semiconductor memory device according to the present invention is assembled; 
       FIG. 21  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. 22  is a schematic sectional view showing a main part of a conventional flash memory; 
       FIG. 23  is a graph showing electric characteristics of a conventional flash memory; 
       FIG. 24  is a circuit diagram of a pulse generator used in a semiconductor memory device according to the present invention; 
       FIG. 25  is a block diagram showing a circuit for verifying data in the semiconductor memory device according to the present invention; 
       FIG. 26  is a circuit diagram showing a circuit for generating two reference potentials of a high reference potential and a low reference potential used at the time of verifying data; 
       FIG. 27  shows a control circuit of the circuit in  FIG. 26 ; and 
       FIG. 28  is a waveform chart showing a verifying operation signal in the semiconductor memory device according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A semiconductor memory device according to the present invention is mainly constructed by a nonvolatile memory cell, first and second load cells, a reference cell, and a program circuit for programming the reference cell into a reference state. Programming denotes herein to set a state where a desired amount of charges is accumulated in the nonvolatile memory cell and the reference cell. The program circuit is a circuit provided for accumulating a desired amount of charges in the nonvolatile memory cell and the reference cell. The semiconductor memory device according to the present invention basically employs an MOS circuit and, preferably, all of circuits including the MOS circuit are mounted on a single semiconductor substrate. 
   The nonvolatile memory cell in the semiconductor memory device according to the present invention 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. 
   Concretely, the nonvolatile memory cell of 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. Hereinafter, the nonvolatile memory cell of the present invention will be referred to as a sidewall memory cell. 
   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 an example thereof includes 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. 
   On the semiconductor layer, preferably, a device isolation region is formed. 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. In the semiconductor layer, preferably, at least one well region of the first conductive type (P or N type) is formed. 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 formed preferably 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, for example, 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 sidewall 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 sidewall memory cells, even when the distance between the sidewall 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. 
   Alternatively, 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 almost 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 important 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 sidewall 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 sidewall memory cell according to the present invention will now be described. First, the gate insulating film and the gate electrode are formed on the 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 the 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 sidewall memory cells, the best mode of the sidewall memory cell satisfies all of the requirements: for example, (1) the gate electrodes of a plurality of sidewall 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 almost 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 almost 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 sidewall 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 sidewall 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 sidewall 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 sidewall memory cell. For example, the memory functional units formed on both sides of a single word line shared by a plurality of sidewall memory cells do not have to be isolated for each sidewall memory cell. The memory functional units formed on both sides of one word line can be shared by a plurality of sidewall 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 sidewall 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 sidewall 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 sidewall memory cell is necessary, and a margin for positioning in the photolithography process and a margin for film reduction by etching are necessary. 
   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 sidewall memory cell is dominant more than that of a sidewall memory cell. Consequently, omission or down sizing of the charge pumping circuit for a sidewall 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 sidewall memory cell, or the sidewall 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 sidewall 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 sidewall 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 sidewall memory cell can store information of two or more values in one memory functional unit. Thus, the sidewall memory cell can function as a memory cell for storing information of four or more values. The sidewall memory cell may store binary data only. The sidewall 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 display device 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 sidewall memory-cell  1  as shown in  FIG. 1 . 
   The sidewall 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. 2A ). 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. 2B ). 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 sidewall 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 sidewall memory cell is of the N channel type. Hereinafter, on assumption that sidewall 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 sidewall 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. 
   Further, the principle of reading operation of the sidewall 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. The presence/absence of charge accumulation in the second memory functional unit  131   b  does not exert an influence on the drain current since the pinch-off point occurs in the area in the vicinity of the drain. 
   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, 0V 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 sidewall 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 sidewall memory cells, a sidewall 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 sidewall 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 sidewall memory cell in a semiconductor memory device according to a second embodiment has a configuration substantially similar to that of the sidewall 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 apart 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 sidewall 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 almost 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, sidewall 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 sidewall 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 almost 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 almost 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  almost 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  almost 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 almost 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 sidewall 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 almost 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 almost controlled, and variations in the memory effect of the sidewall 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 almost uniform thickness and is disposed almost in parallel with the surface of the gate insulating film  214  as shown in  FIG. 13  (region  281 ) and, further, almost 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 almost 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 almost 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 sidewall 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 almost 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 sidewall 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 sidewall 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 sidewall 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 sidewall 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 sidewall memory cell as well, action and effect similar to those of the sidewall 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 sidewall 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 sidewall 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 in the P-type high-concentration 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 sidewall 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 sidewall 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 sidewall memory cell for the following reason. 
   In the sidewall 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 sidewall 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 sidewall memory cell, 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 sidewall memory cell having a gate electrode length (word line width) of 250 nm, so that a sidewall 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 sidewall 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 sidewall 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 sidewall 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 sidewall 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 sidewall 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 sidewall 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 sidewall memory cell of a semiconductor memory device. 
   In an N-channel type sidewall 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. 22 ). 
   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 sidewall 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 sidewall 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. 
   As described above in the first to ninth embodiments, the sidewall memory cell has an insulating film for insulating a film having a surface almost parallel with a surface of a gate insulating film and having the function of retaining charges from a channel region or a semiconductor layer. The insulating film is thinner than the gate insulating film and has a thickness of 0.8 nm or more, thereby facilitating injection of charges to a memory functional unit. Consequently, the writing operation can be performed at higher speed, and time for writing a reference cell can be shortened. 
   The memory functional unit of the sidewall memory cell includes the film having the surface almost parallel with the surface of the gate insulating film and having the function of retaining charges, thereby enabling variations in memory effects to be suppressed. In the embodiment using such a sidewall memory cell, a design margin for variations can be largely set and designing is facilitated. 
   In the sidewall memory cell, the charge retaining film in the memory functional unit is the insulating film. Thus, the sidewall memory cell is resistive to a charge leak, and has an excellent charge retention characteristic. Since the sidewall memory cell having the excellent charge retention characteristic is used and the current of a reference cell using the same sidewall memory cell is accurately set, reading can be performed for a longer period. 
   The sidewall memory cell includes the insulating film for separating a film having a surface almost parallel with the surface of the gate insulating film and having the function of retaining charges from a channel region or a semiconductor layer, and the insulating film is thicker than the gate insulating film and has a thickness of 20 nm or less, so that the charge retention characteristic is excellent. Since the sidewall memory cell having the excellent charge retention characteristic is used and current of a reference cell using the same sidewall memory cell is accurately set, reading can be performed for a longer period. 
   The memory functional unit of the sidewall memory cell includes the film having the surface almost parallel with the surface of the gate insulating film and having the function of retaining charges, thereby suppressing a characteristic change during retention. As described above, since the sidewall memory cell having the excellent charge retention characteristic is used and the current of the reference cell using the same sidewall memory cell is accurately set, reading operation can be performed for a longer period. 
   Tenth Embodiment 
   In a tenth embodiment, a verifying operation performed at the time of, for example, writing data in a semiconductor memory device in which a plurality of sidewall memory cells described in the first to ninth embodiments are arranged will be described. An example of the verifying operation of actually verifying whether writing is performed normally or not by temporarily latching data to be written, applying a write pulse to a selected memory cell and, after that, comparing the latched data with data in the selected memory cell will be described here. 
   In the case where a result of the verification is “success”, the writing operation is finished at that time point. In the case of “fail”, a similar verifying process of applying a write pulse again to a selected memory cell is repeated until the result indicates “success”. 
   The verifying process can be used not only for the data writing process but also a data erasing process. 
   By performing such a verifying process, a failure in writing and erasing due to insufficient pulse application time can be prevented. Consequently, the time of pulse application to a memory cell does not have to have a margin, so that write and erase time can be shortened. 
   The verifying process will be described below. 
   In this embodiment, in the sidewall memory, 5 V (Vcc) is used during a normal read cycle. In the case where a higher potential is necessary to program a memory cell, 7 V-potential (Vpp) is applied to the memory from the outside. The Vpp potential is always applied to a memory even in the read cycle. All of operations of switching the potential are performed in the memory. 
   The term “programming” denotes here erasing and/or writing of data. In order to erase data in a memory cell, a negative voltage is applied to a control gate, and a drain terminal of the-memory cell is grounded. When a high voltage pulse is applied to a drain terminal, binary data “0” is written into the memory cell. 
     FIG. 24  shows a pulse generator for generating a pulse to be applied to a memory cell. 
   The pulse generator supplies an output pulse onto a line  10  used for both erasing and writing. The pulse is generated when a control signal on a line  14  is at the high level. When the control signal is at a lower level, a depletion mode transistor  13  does not operate, so that a node  37  in the pulse generator is decoupled from the high voltage potential (Vpp). Rather, a transistor  36  decreases the gate of a depletion mode transistor  12  to the Vcc potential, and the node  37  is coupled to Vcc via the transistor  12 . In the case where the control signal is at the high level, a transistor  35  operates to decouple the node  37  from the Vcc potential, thereby increasing the node  37  to the Vpp potential. 
   The pulse generator in  FIG. 24  uses three comparators  20 ,  21  and  24 . The comparator  20  compares the potential on a node S with 2.5 V. When the potential on the node S reaches 2.5 V, the level of an O 1  signal becomes high (increases to Vpp) to make a transistor  17  operate. The comparator  21  compares the potential on the node S with 2 V. When the potential on the node S reaches 2.0 V, the level of an O 2  signal becomes high to make a transistor  25  operate. The comparator  24  compares the potential on a node T with 6 V. When the potential on the node T reaches 6 V, the level of an O 3  signal becomes high. Two signals are output from the comparator  24 . An output increased to 5 V is coupled to the gate of a transistor  34  and the other output which has increased to the Vpp potential is coupled to the gate of a transistor  19  via a line  26 . 
   The Vpp potential on the node  37  is, first, coupled to the line  10  via a depletion mode transistor  11 . The gate of the transistor  11  is controlled by the transistors  17  and  19  and a transistor  18  as will be described below. The line  10  is increased to the Vcc potential via a transistor  16 . 
     FIG. 28  shows an example of signal waveforms at the nodes in  FIG. 24 . It is assumed that the potential of the control signal on the line  14  rises. The potential on the node  37  in  FIG. 24  is increased to Vpp as shown in  FIG. 28 . This happens during an erasing or writing operation. In the other case, that is, during a reading operation, the potential on the node is at Vcc. 
   Since the node S is charged by a constant current source  30 , the potential on the node S rises toward the Vpp potential. A relatively large external capacitor  33  is charged. Relatively slow charging of the capacitor  33  makes the leading edge of an output waveform “inclined”. Since the potential on the node S rises, the transistor  18  operates and the potential on the node T is increased. Subsequently, the transistor  11  is operated and the potential on the line  10  increases (see  FIG. 28  for the waveforms of signals on the nodes S and T and the line  10 ). The potential on the node T continuously rises until it reaches 6 V. When it reaches 6 V, the level of the O 3  signal becomes high to make the transistors  19  and  34  conductive. When a constant current source  32  starts operating, since  12  is about twice as large as I 1 , the potential on the node S decreases as shown in  FIG. 28 . On the other hand, the potential on the node T is maintained at the high level via the transistor  19  and does not decrease. When the potential on the node S reaches 2.5 V, as shown by the O 1  signal, an output is generated from the comparator  20 . The output makes the transistors  16  and  17  operate. At this time, the output signal sharply decreases in period  22 . As shown by the waveform of the potential on the node T, the potential on the gate of the transistor  11  also sharply decreases. When the potential on the node S reaches 2 V, an output of the comparator  21  operates the transistor  25  and makes a node  27  discharged. At this time, the transistor  34  is interrupted, thereby preventing the potential on the node S from decreasing. After completion of the period  22  in  FIG. 28 , when the control signal is at the high level, the potential on the node S rises again and a pulse is generated. 
   As shown in  FIG. 28 , an output pulse V P/E  has an inclined leading edge and is at the high level for about 2 ms. A waveform having an inclined leading edge is desirable to program (both erase and write) a memory cell in a sidewall memory of the present invention. In most of the cases, only a 2 ms pulse is necessary to program a memory cell. During the period  22 , as will be described below, a check is made to see whether data has been erased or written from/to a memory cell. Data stored in a memory cell is read from the memory cell and is subjected to comparison during the period  22 . If the data is not compared with a desired programming, an additional pulse is generated and data in the memory cell is checked again. This operation will be described more specifically by using  FIG. 25 . As long as the control signal remains at the high level, a pulse is generated. When the potential of the control signal decreases, the potential on the output line  10  in the pulse generator remains at Vcc. 
   In the case of programming the sidewall memory, first, new data is latched. Specifically, eight bits are latched into the buffer at once. First, the data is ignored and an erasing cycle starts. A check is made to see whether data in the memory cell has been erased or not, that is, binary data “1” is programmed in the memory cell or not. After that, the data is examined. When all of data is the binary data “1”, further programming is not performed. The memory is used as a read only memory by a signal READY or another memory cell may be programmed. In the case where erasing is not successful, that is, when not all of input data is the binary data “1”, writing is performed to program the memory cell with the binary data “0”. The data in the memory cell is compared again with the latched data. Until the programming is completed, short program pulses are generated continuously. 
     FIG. 25  shows a circuit related to the programming. Reference numeral  40  in  FIG. 25  is a single sense amplifier connected to a selected memory cell. Further, a single output buffer  41  for receiving both an output enable signal and an input enable signal is provided. A data-in buffer and latch circuit  44  is connected to a line  47  like the output buffer  41 . Input and output data is multiplexed. The data-in buffer and latch circuit  44  gives data&#39;s complement to a NAND gate  43 . An output of the gate  43  is input to one of input terminals of an exclusive OR gate  42 . To the other input terminal of the gate  42 , data detected by the sense amplifier  40  is input. In a byte-wide memory, for eight sense amplifiers  40 , eight buffers  41 , eight data-in buffer and latch circuits  44 , and eight gates  42  and  43 , one of input terminals of each of eight gates  43  connected to a node  50  is used as shown by lines  48 . 
   Each of outputs indicated as Ai (A 0  to A 7 ) of the gate  42  is connected to a NOR gate  60 . A O 1/  signal generated in the pulse generator of  FIG. 24  is delayed a little by a delay  61  and the resultant signal is input to the gate  60 . An output of the gate  60  is connected to the gate of a transistor  59 . The transistor  59  is connected to the node  50  via inverters  56  and  57 . The node  50  is selectively connected to the ground via a transistor  58 . A signal on the node  50  is connected as an additional input to a NAND gate  49 . The signal on the node  50  and the signal&#39;s complement are used for the circuit of  FIG. 27  as will be described later. A signal from the NAND gate  49  is supplied via the inverter  61  and a latch circuit  52  and is output as a signal READY on a line  54 . 
   The control logic  53  as a part of the timing circuit for the sidewall memory supplies some control signals including an erase signal (ERASE) and a control signal (CONTROL). 
   The circuit of  FIG. 25  stops except for the case where the sense amplifier  40  detects data and outputs the data to the output line  47  via the data output buffer  41 . 
   It is now assumed that a memory cell is to be programmed. At this time, in the beginning, the node  50  has the ground potential and the inverters  56  and  57  operate as latches. When the potential of a signal WE/ decreases, the address of a memory cell to be selected is latched. When the level of the signal becomes high, input data is latched by the data-in buffer and latch circuit  44  and the inverters  56  and  57  serving as latches and stored. Since programming is performed simultaneously, the potential of the signal WE/ becomes high and data is latched. After that, the erase signal is supplied from the control logic  53 . By the erase signal, the pulse generator shown in  FIG. 24  is connected to the control gate of the addressed memory cell. The drain terminal of the memory cell is connected to the ground, and data in the memory cell is erased by a pulse from the pulse generator of  FIG. 24 . The level of the control signal on the line  14  becomes high, and the pulse is generated as shown in  FIG. 28 . 
   Since the node  50  is at the low level during the erasing operation, the conditions of the NAND gate  43  are not satisfied and data in the latch  44  is therefore ignored. The high-level signal is supplied to an exclusive OR gate  42  from the NAND gate  43 . It is assumed that at least one of eight memory cells addressed includes binary data “0”. When a first pulse is generated from the pulse generator, the data in the memory cell is to be erased. In the period  22  in  FIG. 28 , after generation of each of pulses from the pulse generator, the sense amplifier  40  detects data in the memory cells and the detected data is transmitted to the gate  42  and similar gates. At this time, data in the memory cells is verified. In the case where all of signals input to the gate  42  have the high level and it shows that erasing has been performed, an output potential of the gate  42  decreases. In the above state, the input signals A 0  to A 7  to be connected to the gate  60  have the low level. In the period  22 , the signal O 1/  is also at the low level. Consequently, an output signal from the gate  60  is at the high level. When the transistor  59  operates, the potential on the node  50  is increased the high level by the inverters  56  and  57  and latched at the high level. 
   If the first pulse of the pulse generator of  FIG. 24  has not erase data of all of the eight memory cells, at least one of the input signals A 0  to A 7  in the gate  60  remains at the high level, thereby preventing the transistor  59  from erroneously operating. In this state, the erase signal and the control signal from the control logic remain at the high level. Until the erasing is completed, the pulse generation is repeated by the pulse generator of  FIG. 24 . 
   Once the erasing is completed, the potential on the node  50  rises, so that the data from the data-in buffer and latch circuit  44  is immediately supplied to the exclusive OR gate  42  via the gate  43 . In the case where the data-in buffer and latch circuit  44  includes binary data “1”, an output signal of the gate  42  remains at the low level. When all of the input signals A 0  to A 7  to the gate  60  remain at the low level, the output signal of the gate  42  remains at the high level. Consequently, all of the conditions in the gate  49  are satisfied. An output signal of the gate  49  increases, the output potential is latched by the latch  52 , and the signal READY is supplied onto the line  54 . The signal READY connected to the control logic  53  decreases the potential of the control signal on the line  14  and enables whether the sidewall memory to receive additional data for programming or to operate in a read only mode. 
   If the data&#39;s complement from the latch circuit  44  and the data detected by the sense amplifier  40  are compared and do not match each other, an output of at least one of the gates  42  remains at the high level. After the erasing and verifying operations, one of inputs to the gate  60  is at the high level, so that the output from the gate  60  has the low level. At this time, although the potential on the node  50  is at the high level, the output of the gate  60  is at the low level, so that the signal READY is not generated. Although the control signal from the control logic  53  remains at the high level, pulses are generated for writing rather than erasing. The pulses are supplied to the drain terminal of a memory cell to be programmed with binary data “0”. The pulses are repeatedly generated until the memory cells are programmed with the binary data “0”. At this time, all of input signals to the gate  60  have the low level and all of input signals to the gate  49  have the high level, so that the signal READY is supplied onto the line  54 . Further, once the signal READY is output, the memory cells are programmed and the data is verified in the memory cells. 
   During the period in which data is read from both of the sidewall memory of the present invention and 2816, the control gate receives the reference potential. The potential is generated on the substrate from a dummy memory cell of a known method. In the present invention, during the binary data “1” is verified after erasure, the reference potential increases by about ½ volt. By the increases, a memory cell of which data is erased a little is started. After that, an additional pulse is generated and, as a result, complete erasing operation is executed. Similarly, after writing, the reference potential decreases by about ½ volt during the verifying operation. A memory cell programmed a little does not operate any more. By generating an additional pulse, the writing is ensured. 
     FIGS. 26 and 27  show circuits for supplying a higher reference potential and a lower reference potential during verification. In  FIG. 26 , the reference potential on a line  91  is supplied to a memory cell such as a memory cell  70 . During a normal reading cycle, signals C+ and C− are at the low level. The signal C+ on a line  89  prevents a transistor  73  from operating. Since a signal C +/  on the line  87  is at the high level, a transistor  74  operates and a transistor  72  is prevented from operating. The signal C− on a line  90  prevents a transistor  76  from operating, even if a transistor  75  slightly shows conductivity, the line  91  is not influenced. Therefore, the potential on the line  91  remains at V REF . In  FIG. 27 , in the reading operation, the signal O 1/  is at the high level and both transistors  80  and  83  operate. The line  90  is consequently connected to the ground via the transistor  80 , thereby reliably setting the signal C− to the low level. Similarly, the transistor  83  operates, so that the line  89  has the ground potential, and the signal C+ is reliably set to the low level. Since a transistor  85  does not operate, the potential on the line  87  is increased to V cc  via a transistor  86  and the potential of the signal C +/  is increased. 
   After erasing and during erasing verification, the potential of the signal O 1/  decreases, and the potential A on the node  50  in  FIG. 25  remains at the low level. A transistor  81  operates and the line  90  is maintained at the ground potential. However, the transistor  83  and a transistor  84  do not operate during the period  22  in  FIG. 28 , the line  89  is connected to Vcc via a transistor  82 , and the potential on the line  87  is decreased. In  FIG. 26 , the transistor  76  remains in the off state but the transistor  73  operates and the transistor  74  is turned off. Consequently, the gate of the transistor  72  has about 3 V. The potential on the line  91  is increased by the depletion mode transistor  72  by about ½ volt. 
   During verification after writing, the potential A is at the high level and a potential A/ is at the low level. Since both of the transistors  80  and  81  do not operate, the signal C− is at the high level. Since the transistor  84  operates, the signal C+ is at the low level and the signal C +/  is at the high level. In  FIG. 26 , the transistor  76  operates and the transistor  72  is prevented from operating. Therefore, a current path exists from the line  91  to the ground at which the potential on the line  91  is decreased by about ½ volt. Thus, the potential on the control gate of a memory cell is low during the write verification. 
   The erase verifying and write verifying cycles as described above enable a programming pulse which is shorter than a conventional one to be used (for example, 2 ms shorter than 10 ms). In most of the cases, only a single short pulse is necessary. In the case where a memory cell is not properly programmed with a single pulse, the pulse is repeatedly output until the memory cell is programmed. Since the pulse is repeatedly output, programming can be performed in shorter time. Further, by performing automatic on-chip verification, the memory of the present invention becomes more usable. With a higher reference potential and a lower reference potential used during the verification, a slightly programmed memory cell can be specified, so that automatic re-programming can be performed. 
   Eleventh Embodiment 
   As an application example of the semiconductor memory device, for example, as shown in  FIG. 20 , 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 sidewall memory cell, 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. 20  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 sidewall memory cell and, particularly, a semiconductor memory device described in the first to ninth embodiments in which the sidewall memory cells are arranged. 
   Twelfth Embodiment 
     FIG. 21  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. 
   According to the present invention, by using a pulse shorter than a conventionally used pulse in order to program a memory cell, a failure in writing and erasing due to insufficient pulse application time can be prevented, and the speed of the writing process and the erasing process on a nonvolatile memory cell can be increased. Therefore, the operation of programming a memory cell can be performed at higher speed. 
   Since a cell having a memory functional unit is used as the memory cell, the yield and reliability can be improved, and the power consumption can be reduced. 
   In the sidewall memory cell as a component of the semiconductor memory device, the memory function of the memory functional unit and the transistor operation function of the gate insulating film are separated from each other. Consequently, while maintaining the sufficient memory function, it is easy to reduce the thickness of the gate insulating film and suppress a short channel effect. Further, as compared with an EEPROM, the value of current flowing between the diffusion regions changes due to rewriting more largely. Therefore, the writing state and erasing state of the semiconductor memory device can be easily discriminated from each other.