Abstract:
A metal oxide semiconductor transistor includes a semiconductor substrate; a source area formed in a device area of the semiconductor substrate; a drain area formed in the device area; a gate layer formed on and across the device area between the source area and the drain area; a control gate layer; and a diffusion area formed in the device area between the gate area and the control gate area. The control gate layer has a first part including a first end of the control gate layer and a second part including a second end of the control gate layer. The first part is formed on the device area between the source area and the gate layer. The first end is disposed so that there is a gap between the first end and an edge of the device area.

Description:
BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates to a metal oxide semiconductor (MOS) transistor in which a gain coefficient of a field-effect transistor can be modulated. The invention particularly relates to the MOS transistor to be a basic device which realizes new-type LSI devices such as self-optimizing LSI and a self-adapting LSI in which high performance of LSI is realized by optimizing individual transistors after manufacturing LSI in a large-scale and highly integrated semiconductor integrated circuit (LSI) device in the future. 
     2) Description of the Related Art 
     Recent LSI devices are large scaled and highly integrated according to development of miniaturized devices, and a system-on-chip is realized. As a result, it is being required to integrate a large variety of functional circuits in chips. In the design of such large-scale LSI devices, it is particularly important to optimize operation timing or the like between the functional circuits in order to properly operate the many integrated functional circuits. 
     On the other hand, the performance of LSI device has been improved mainly by miniaturization of devices for 30 or more years since its invention. In these days where various physical limits become obvious in the miniaturization of devices, however, it becomes extremely difficult to manufacture integrated circuit devices stably and uniformly. 
     As a result, the design of the LSI devices requires measures that secures an operating margin in order to cover a process fluctuation which cannot be avoided in the manufacturing process of LSIs. The measures that secures the operating margin interferes with heightening of performance of the large-scale LSI devices according to diversification and enlargement of the devices. 
     In such future LSI devices, non-uniformity of device characteristics for each LSI chip,.such as dispersion (distribution) of intra-chip device characteristics and a median fluctuation (shift) of device characteristics due to process fluctuation, and difficulty of LSI physical design (performance optimization design) due to the non-uniformity become obvious. For this reason, a performance heightening method for LSI devices which depends only on the miniaturization of devices is reaching a limit. 
     In order to improve the high performance of LSI devices, it is indispensable to establish new LSI design and manufacturing method in which the dispersion of the device characteristics larger than a certain level is premised. As one method relating to the LSI design and manufacturing method in which the dispersion of the device characteristics larger than a certain level is premised, a method of providing a self-adjusting function into LSI chips is considered. 
     Specifically, in such a method, electrical characteristics adjustment based on setting of a size (a gate length and a gate width) in individual field-effect transistors (e.g. MOS transistors), which is conventionally performed at the final LSI design step (physical design), is designed to be performed automatically by each chip itself after manufacturing of LSIs. As a result, the electrical characteristics of individual MOS transistors in the LSI chips are optimized so that the chip performance is heightened. 
     In order that the LSI chips realize the self-adjustment performance, it is necessary to provide a design that a program or electrical dynamics can automatically adjust the electrical characteristics into LSI chips. In order to realize this design, therefore, at least some kind of means that electrically modulates the electrical characteristics is essential, and technical development of such means gives the key to realize the self-adjustment function. 
     The method of electrically modulating the electrical characteristics which can be realized by using a conventional art is explained below. In the conventional art, when the electrical characteristics are electrically modulated, a method using a circuit configuration and a method of modulating characteristics of devices are mainly adopted. 
     The method using the circuit configuration includes a method of establishing a circuit configuration such that a plurality of MOS transistors are used and a number of their parallel connection is switched by an electric switch as explained in CIRCUIT  1  to CIRCUIT  4 , for example. According to this method, effective electrical characteristics (gain coefficient) when the entire circuit is regarded as one MOS transistor can be modulated. The method that realizes the circuit is, however, very inefficient from viewpoints of adjustment accuracy and a circuit scale as explained below. 
     A configuration (CIRCUIT  1 ) such that two MOS transistors are connected in parallel is considered. In CIRCUIT  1 , a normal signal voltage is applied to a gate electrode of one MOS transistor, and a signal voltage and an OFF voltage for OFF operation are switched so as to be applied to a gate electrode of the other MOS transistor. 
     According to this CIRCUIT  1 , when a switch connects the signal voltage with the gate electrode of the other MOS transistor, the two MOS transistors connected in parallel-serve as one MOS transistor in this circuit. Further, when the switch connects the OFF voltage with the gate electrode of the other MOS transistor, only one MOS transistor functions in this circuit. As a result, the substantial gain coefficient of the MOS transistor can be modulated. 
     A configuration (CIRCUIT  2 ) such that five MOS transistors are connected in parallel is considered. In CIRCUIT  2 , a normal signal voltage is applied to a gate electrode of one MOS transistor, and a signal voltage and an OFF voltage for OFF operation are switched so as to be applied to gate electrodes of the other four MOS transistors. 
     According to this CIRCUIT  2 , sixteen variations can be realized by states of four switches. That is to say, the gain coefficients of the four MOS transistors are set to be a multiple of a power of 2, so that coefficients of 16 degrees can be set with equal intervals. 
     A configuration (CIRCUIT  3 ) such that two MOS transistors are connected in series is considered. In CIRCUIT  3 , a normal signal voltage is applied to a gate electrode of one MOS transistor, and a signal voltage and an ON voltage for ON operation are switched so as to be applied to a gate electrode of the other MOS transistor. 
     According to this CIRCUIT  3 , when a switch connects the signal voltage with the gate electrode of the other MOS transistor, the two MOS transistors are connected in series and perform the same operation. For this reason, the two MOS transistors serve as one normal MOS transistor in this circuit. Further, when the switch connects the ON voltage with the gate electrode of the other MOS transistor, this circuit serves as a circuit in which the one MOS transistor is connected with ON resistance of the other MOS transistor in series. 
     A configuration (CIRCUIT  4 ) such that two MOS transistors are connected in series is considered. In CIRCUIT  4 , a normal signal voltage is applied to a gate electrode of one MOS transistor, and a control voltage which changes the ON resistance is applied to the gate electrode of the other MOS transistor. This circuit serves as a circuit that adjusts the resistance connected with the one MOS transistor in series. 
     The switch normally includes a complementary MOS (CMOS) switch which is connected with a P-channel MOS (hereinafter, “PMOS”) transistor and an N-channel MOS (hereinafter, “NMOS”) transistor in parallel, an inverter which generates a gate signal of the CMOS switch, and a latch circuit which holds a state of the switch. Totally about 24 MOS transistors are necessary. 
     In the circuit configuration examples adopting the series connection in CIRCUIT  1  and CIRCUIT  2 , therefore, a trade-off relationship is established between accuracy of the characteristics adjustment and circuit scale, and this causes a problem that the circuit scale becomes large when the adjustment accuracy is heightened. 
     Further, in the circuit configuration examples adopting the parallel connection in CIRCUIT  3  and CIRCUIT  4 , the circuit scale becomes large, and a resistance component which causes nonlinear characteristics of an input signal intervenes in series. For this reason, an effective characteristics adjustment range is limited. 
     The circuit configurations that modulate the electrical characteristics of the transistor has essential restriction such that devices whose number is several times or several-dozen times a number of devices to be adjusted should be required. These configurations hardly fit in packaging of the self-adjustment function which promotes high integration and heightens the performance of the LSI devices. 
     In prior MOS transistors, it is not easy to change the electrical characteristics after manufacturing LSI, but electrical characteristics of devices can be modulated by controlling a back gate voltage. The electrical characteristics of MOS transistors are roughly explained. 
     The electrical characteristics of MOS transistors can be expressed by the following equations in which Ids is a source-drain current, Vds is a source-drain voltage, Vgs is a gate voltage, Vt is a threshold voltage, and β is a gain coefficient. The equations (1) and (2) below represent that no short channel effect or the like for simplicity is not produced.
 
 Vds&gt;Vgs−Vt, Ids ≈β( Vgs−Vt ) 2 /2   (1)
 
 Vds≦Vgs−Vt, Ids ≈β(( Vgs−Vt ) Vds−Vds   2 /2)   (2)
 
     The gain coefficient β can be expressed by the following equation (3) in which W is a gate width, L is a gate length, Tox is a thickness of a gate insulating film, μ is a carrier mobility, and ε is permittivity of the gate insulating film.
 
β≈με W /( L·Tox )  (3)
 
     As understood from the equations (1) and (2), the electrical characteristics of MOS transistors depend on the threshold voltage Vt. After LSI is manufactured, the threshold voltage Vt can be changed by controlling a back gate voltage. In a method of changing the electrical characteristics of MOS transistor after the manufacturing of LSI using the conventional art, therefore, the back gate voltage is changed so that the threshold voltage Vt is modulated. 
     A reverse bias relationship should be, however, maintained between the back gate voltage and the source-drain voltage, and additionally the back gate voltages should be electrically separated from each other for each device to be modulated. For this reason, this method is inadequate to high integration. 
     Further, a change in the threshold voltage Vt cannot influence the source-drain current Ids only according to a difference between the threshold voltage Vt and the gate voltage Vgs. For this reason, it is difficult that the electrical characteristics of MOS transistors are modulated dynamically only by changing the threshold voltage Vt. 
     That is to say, the system that modulates the transistor electrical characteristics by changing the threshold value Vt using the conventional art hardly fits in the packaging of the self-adjustment function which promotes high integration and heightens the performance of the LSI devices. This is because of inhibition of an integration degree and fragility of a modulation degree due to the separation of back gate. 
     In the conventional art, it is not easy to provide the self-adjustment function in a highly integrated manner, or to change the electrical characteristics after the manufacturing of LSI. It is, therefore, desired to develop new devices that can modulate the electrical characteristics dynamically without inhibiting high integration. 
     In the equation (3), since the carrier mobility μ, the permittivity ε, and the thickness of the gate insulating film Tox are, generally constant, the gain coefficient β can be set by a ratio of the gate width W to the gate length L. The electrical characteristics of MbS transistors which can be set in the physical design of LSI devices are, therefore, the gain coefficient β. 
     When the gain coefficient β can be modulated, as is clear from the equations, the source-drain current Ids can be strongly influenced in proportion to the product of the gate voltage Vgs and the gain coefficient β. For this reason, the electrical characteristics of MOS transistors can be modulated dynamically. That is to say, the gain coefficient β can be electrically modulated to be about several times or several dozen times, so that correction of dispersion of the device characteristics, automatic compensation of a load change, and the like which match the modulation can be made after the manufacturing of LSI devices. 
     At this time, it is important for basic devices for active LSI to be capable of analog-modulating the gain coefficient β with a compact device size which does not inhibit high integration. 
     From such a viewpoint, the inventor has devised a semiconductor device which is capable of modulating a gain coefficient of a field-effect transistor (see Japanese Patent Application Laid-Open No. 2002-222944). The semiconductor device in Japanese Patent Application Laid-Open No. 2002-222944 is called as a gain coefficient variable MOS transistor, and its summary is explained. 
     Configurational characteristics of the gain coefficient variable MOS transistor are such that a control gate is additionally arranged in a gate area (main gate) in a prior MOS transistor in a slanted manner. That is to say, the gain coefficient variable MOS transistor is characterized in that a triangular area is formed on a source area side and a drain area side which are not overlapped with the main gate in a channel area under a control gate, and these areas form a parallelogram in a state that they sandwiching the main gate. 
     The modulation characteristics of the gain coefficient β can be set by device shape parameters (a gate width W and a gate length L of the main gate, and an angle θ formed between the main gate and the control gate). 
     According to this configuration, a direction of an electric field with respect to the gate channel can be controlled by a voltage of the control gate. That is to say, the voltage of the control gate is adjusted, and a conductance of the control gate channel is changed with respect to a conductance of the main gate, so that the effective gate length L and gate width W can be analog-modulated. As a result, the gain coefficient β can be analog-modulated. 
     The gain coefficient variable MOS transistor is incorporated into LSI, therefore, so that characteristics of a device can be adjusted dynamically by on-chip itself. As a result, a mechanism, that automatically corrects an operation timing between built-in functional circuits due to enlargement of LSI and dispersion of the device characteristics which increases due to miniaturization of devices, can be realized in a highly integrated manner. 
     In the semiconductor device in Japanese Patent-Application Laid-Open No. 2002-222944, however, since the control gate is arranged so as to be overlapped with the main gate, it is necessary to additionally provide the second gate layer which can be electrically separated from the main gate. As a result, a number of the manufacturing steps of LSI mounted with the semiconductor device increases in comparison with the normal CMOS process, and thus a manufacturing cost increases. 
     Further, in the semiconductor device in Japanese Patent Application Laid-Open No. 2002-222944, the control gate which forms a certain angle with respect to the main gate is additionally provided in order to form the rectangular area on the main gate on the source area side and the drain area side. For this reason, the size of the device becomes large. 
     Further, the modulation characteristics of the gain coefficient in the semiconductor device are determined by a conductance ratio of the main gate to the control gate. For this reason, as the conductance of the main gate becomes smaller, the modulation degree of the gain coefficient becomes smaller. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least solve the problems in the conventional technology. 
     A metal oxide semiconductor transistor according to one aspect of the present invention includes a semiconductor substrate; a source area formed in a device area of the semiconductor substrate; a drain area formed in the device area; a gate layer formed on and across the device area between the source area and the drain area; a control gate layer; and a diffusion area formed in the device area between the gate area and the control gate area. The control gate layer has a first part including a first end of the control gate layer and a second part including a second end of the control gate layer. The first part is formed on the device area between the gate layer and at least one of the source area and the drain area, and the first end is disposed so that there is a gap between the first end and an edge of the device area. 
     A metal oxide semiconductor transistor according to another aspect of the present invention includes a semiconductor substrate; a source area formed in a device area of the semiconductor substrate; a drain area formed in the device area; a gate layer formed on and across the device area between the source area and the drain area; and a control channel area formed in the device area between the gate layer and at least one of the source area and the drain area. The control channel area has a threshold value that gradually changes in a longitudinal direction of the gate layer. 
     The other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are top pattern diagrams illustrating a configuration of a MOS transistor according to a first embodiment of the present invention; 
         FIG. 2  is a diagram explaining shape parameters defining characteristics of the MOS transistor shown in  FIG. 1B ; 
         FIG. 3  is an equivalent circuit diagram in which the MOS transistor shown in  FIG. 1B  is expressed by an electronic circuit; 
         FIGS. 4A  to  4 C are equivalent circuit diagrams when a channel conductance of a control gate in the MOS transistor shown in  FIG. 1B  changes with respect to a channel conductance of a main gate; 
         FIGS. 5A  to  5 C are diagrams explaining a gate channel width modulating operation realized by the MOS transistor shown in  FIG. 1B ; 
         FIGS. 6A and 6B  are top pattern diagrams illustrating a configuration of a MOS transistor according to a second embodiment of the present invention; 
         FIG. 7  is a top pattern diagrams illustrating a configuration of a MOS transistor according to a third embodiment of the present invention; 
         FIG. 8  is a top pattern diagrams illustrating a configuration of a MOS transistor according to a fourth embodiment of the present invention; 
         FIG. 9  is a top pattern diagrams illustrating a configuration of a MOS transistor according to a fifth embodiment of the present invention; 
         FIG. 10  is a cross-sectional view taken along line X—X of  FIG. 9 ; 
         FIG. 11  is a top view illustrating a configuration of a MOS transistor according to a sixth embodiment of the present invention; 
         FIG. 12  is a cross-sectional view taken along line XII—XII of the MOS transistor shown in  FIG. 11 ; 
         FIG. 13  is a cross-sectional view taken along line XIII—XIII of the MOS transistor shown in  FIG. 11 ; 
         FIG. 14  is a diagram illustrating a drain current characteristic example per unit gate width controlled by the control gate of the MOS transistor shown in  FIG. 1 ; 
         FIG. 15  is a diagram illustrating shape parameters defining the characteristics of the MOS transistor shown in  FIG. 1 ; 
         FIGS. 16A  to  16 C are diagrams explaining gate channel width modulation realized by the MOS transistor shown in  FIG. 11 ; 
         FIG. 17  is a main sectional view (cross-sectional view taken along line B-B′ of  FIG. 11 ) illustrating a configuration of a MOS transistor according to a seventh embodiment of the present invention; 
         FIG. 18  is a top view illustrating a configuration of a MOS transistor according to an eighth embodiment of the present invention; and 
         FIG. 19  is a top view illustrating a configuration of a MOS transistor according to a ninth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of a MOS transistor relating to the present invention will be explained in detail below with reference to the accompanying drawings. 
       FIGS. 1A and 1B  are top pattern views illustrating a configuration of a MOS transistor according to a first embodiment of the present invention. In  FIGS. 1A and 1B , as is well known, the MOS transistor is configured so that a channel area  12  in gate areas  1   a  and  1   b  is arranged between a source area  2  and a drain area  3  (a central position) so as to cross a channel formed between the source area  2  and the drain area  3  (hereinafter, “gate channel”). A contact forming area is extended on one end of the channel area  12  (upper portion in the drawing), so that a contact  4   a  is composing an electrode is provided. Contacts  4   b  and  4   c  composing electrodes are provided in the source area  2  and the drain area  3 . 
     In such the MOS transistor according to the first embodiment, as shown in  FIG. 1A , the gate area  1   a  is called as a main gate  1   a  hereinafter, and control gates  5   a  and  5   b  are formed by a gate layer similar to the main gate  1   a  on both sides of the channel area  12  of the main gate  1   a  in a gate lengthwise direction, namely, on a side of the source area  2  and a side of the drain area  3 . In other words, the main gate  1   a  and the control gates  5   a  and  5   b  are formed on a common plane. The gate layer is generally formed by a polysilicon or a metal layer. 
     The control gates  5   a  and  5   b  include control gate areas  6   a  and  6   b  arranged parallel with the channel area  12  of the main gate  1   a,  and contact forming areas  7   a  and  7   b  extended on one ends of control gate areas  6   a  and  6   b  (lower portion in the drawing). 
     Contacts  4   d  and  4   e  composing electrodes are provided on the contact forming areas  7   a  and  7   b.    FIGS. 1A and 1B  illustrate examples that the contact forming areas  7   a  and  7   b  and the contact forming area of the main gate  1   a  are formed on sides opposite to each other with respect to a line segment which connects the source area  2  with the drain area  3 . 
     The control gate areas  6   a  and  6   b  are formed so that diffusion areas of gaps  8   a  and  8   b  are provided between side edges of the control gate areas  6   a  and  6   b  in its lengthwise direction and a side edge of the channel area  12  of the main gate  1   a  in a lengthwise direction. 
     The control gate areas  6   a  and  6   b  are not formed on the entire gate width in the channel area  12  of the main gate  1   a,  namely, across the entire width of the gate channel. Hiatus portions  9   a  and  9   b  where the control gates are not present, however, formed on one end sides of the gate channel width, i.e., the other ends (upper portion in the drawing) of the control gate areas  6   a  and  6   b  in the illustrated example, namely, on the sides of the contact  4   a  of the main gate  1   a,  respectively. 
     In  FIG. 1B , an entire portion, where the contact forming areas  7   a  and  7   b  of the control gates  5   a  and  5   b  in  FIG. 1A  are unified, is used as the control gate  10 , and it has one contact  4   f.  That is to say, the control gate  10  includes control gate areas  10   a  and  10   b  which are arranged parallel on both sides of the channel area  12  of the main gate  1   b  in the gate lengthwise direction with the gaps  8   a  and  8   b  are being formed, and a contact forming area  10   c  which connects one ends of the control gate areas  10   a  and  10   b.  The hiatus portions  9   a  and  9   b  are provided on the other ends (upper portion in the drawing) of the control gate areas  10   a  and  10   b.  Hereinafter, the configuration of  FIG. 1B  is adopted for convenience of the explanation. 
     An operating principle which modulates the gain coefficient of the MOS transistor having the above configuration is explained below with reference to  FIG. 1B  to FIG.  5 .  FIG. 2  is a diagram explaining shape parameters for defining characteristics of the MOS transistor shown in FIG.  1 B.  FIG. 3  is an equivalent circuit diagram in which the MOS transistor shown in  FIG. 1B  is expressed by an electronic circuit.  FIGS. 4A  to  4 C are equivalent circuit diagrams when a channel conductance of the control gate in the MOS transistor shown in  FIG. 1B  changes with respect to a channel conductance of the main gate.  FIGS. 5A  to  5 C are diagrams explaining a gate channel width modulating operation realized by the MOS transistor shown in FIG.  1 B. 
     According to this configuration, a conductance (1/resistance) of the control channel, which is formed under the control gate areas  10   a  and  10   b,  can be controlled by a voltage to be applied to the control gate  10 . As a result, an effective channel width of the gate channel formed under the channel area  12  of the main gate  1   b  is modulated so that the gain coefficient β can be modulated by a voltage. 
     Channel modulation characteristics to be realized can be set by the shape parameters shown in FIG.  2 . In  FIG. 2 , the shape parameters include a gate length L and a gate width W in the channel area  12  of the main gate  1   b,  a gate length Lc of the control gate areas  10   a  and  10   b  in the control gate  10 , a gap Sv between the control gate areas  10   a  and  10   b  and the channel area  12  in the main gate  1   b  (diffusion area), and a gap Sc between the hiatus portion where the control gate areas  10   a  and  10   b  are not present. These parameters are adjusted so that the modulation characteristics of the gain coefficient β can be designed. 
     In order to ease understanding of the operating principle, the equivalent circuit of the MOS transistor shown in  FIG. 1B  is considered. In the MOS transistor shown in  FIG. 1B , the control gate areas  10   a  and  10   b  can be considered as variable resistors. Further, a resistance component (fixed resistor) which depends on the diffusion area is present between the control gate areas  10   a  and  10   b  and the channel area  12  of the main gate  1   b.  Since the other ends of the control gate areas  10   a  and  10   b  are dropped out, the source area  2  and the drain area  3  are connected directly with the channel area  12  on the hiatus portions. The MOS transistor shown in  FIG. 1B  can be, therefore, represented as shown in  FIG. 3 , for example. Since accuracy is not important in the understanding of the operating principle, the resistors are present discretely so that the circuit configuration is simplified in FIG.  3 . 
     As shown in  FIG. 3 , some portions of the source area  2  and the drain area  3  are connected directly with the channel area  12 . A plurality of variable resistors Rc representing the control gate area  10   a  are connected in parallel between the channel area  2  and the source area  2 . Similarly, a plurality of variable resistors Rc representing the control gate area  10   b  are connected in parallel between the channel area  12  and the drain area  3 . Further, a plurality of fixed resistors Rs are connected in series between the channel area  12  and the control gate area  10   a.  Similarly, a plurality of fixed resistors Rs are connected in series between the channel area  12  and the control gate area  10   b.    
     A principle that the channel width of the main gate is modulated by the channel conductance of the control gate is explained with reference to  FIGS. 4A and 5A .  FIGS. 4A and 5A  illustrate the case where the channel conductances Gc (control gate) of the control gate areas  10   a  and  10   b  are extremely larger than the channel conductance Gc (gate) of the channel area  12  of the main gate  1   b.    FIGS. 4B and 5B  illustrate the case where both the conductances are equal.  FIGS. 4C and 5C  illustrate the case where the channel conductances Gc (control gate) of the control gate areas  10   a  and  10   b  are extremely smaller than the channel conductance Gc (gate) of the channel area  12  of the main gate  1   b.    
     When the channel conductances Gc (control gate) of the control gate areas  10   a  and  10   b  are extremely larger than the channel conductance Gc (gate) of the channel area  12  of the main gate  1   b  (Gc (gate)&lt;&lt;Gc (control gate), as shown in  FIG. 4A , it is regarded that the variable resistances Rc are regarded as not being present. As shown in  FIG. 5A , therefore, a width of an effective channel  13   a  in the channel area  12  of the main gate  1  spreads over approximately entire width of the channel area  12 . For this reason, this width is comparatively large. 
     In the equivalent circuit shown in  FIG. 4A , a general resistance of the fixed resistors Rs becomes larger from the top to the bottom in the drawing in a gate channel other than a portion which is connected directly with the source area  2  and the drain area  3 . In the effective channel  13   a  shown in  FIG. 5A , therefore, a color is darkened so that large electric current flows in the other ends of the control gate areas  10   a  and  10   b  (upper portion in the drawing). A black color is thinner towards the one ends of the control gate areas  10   a  and  10   b  (lower portion in the drawing), and this represents that the flowing electric current is gradually small. 
     When the channel conductances Gc (control gate) of the control gate areas  10   a  and  10   b  are extremely smaller than the channel conductance Gc (gate) of the channel area  12  of the main gate  1   b  (Gc (gate)&gt;&gt;Gc (control gate), the variable resistors Rc have extremely large resistance. For this reason, in  FIG. 4C , the connection can be regarded as being cut off between the channel area  12  and the source area  2  and between the channel area  12  and the drain area  3 . 
     In this case, electric current flows only in the area which is connected directly with the source area  2  and the drain area  3 . As shown in  FIG. 5C , therefore, the width of an effective channel  13   c  in the channel area  12  of the main gate  1   b  becomes an area corresponding to the hiatus portions being present on the other ends (upper portion in the drawing) of the control gate areas  10   a  and  10   b  in the channel area  12 , respectively. As a result, the width is considerably narrow. 
     When the channel conductance Gc (control gate) of the control gate areas  10   a  and  10   b  are equal with the channel conductance Gc (gate) of the channel area  12  of the main gate  1   b  (Gc (gate)=Gc (control gate), as shown in  FIG. 4B , the variable resistors Rc can be regarded as being fixed resistors. In this case, as shown in  FIG. 5B , the width of an effective channel  13   b  in the channel area  12  of the main gate  1   b  becomes an intermediate channel width. 
     The modulation degree of the channel width depends on resistor components (Rs) of the diffusion area as the source-drain area sandwiched between the channel area  12  of the main gate  1   b  and the control gate areas  10   a  and  10   b  of the control gate  10 , and channel resistor components (Rc) of the control gate areas  10   a  and  10   b.  In general, as the resistance components (Rs) of the source-drain area are higher, or as a change in the channel resistance components (Rc) is larger, the modulation degree of the channel width of the main gate becomes larger. 
     In the MOS transistor according to the first embodiment, the effective channel width of the gate channel can be adjusted by a voltage to be applied to the control gate. That is to say, analog modulation can be made on the drain current characteristics electrically. For this reason, the gain coefficient β of the MOS transistor can be modulated. 
     An electric power consumed by the modulation of the gain coefficient β is only a leak current from the control gate. This electric current is very small, and this does not substantially become a problem. 
       FIGS. 6A and 6B  are top pattern diagrams illustrating the configuration of a MOS transistor according to a second embodiment of the present invention. The second embodiment explains the MOS transistor in which the control gate is provided on one side of the main gate in the gate lengthwise direction, namely, any one of the sides of the source area and the drain area in the MOS transistor shown in FIG.  1 A. 
     In  FIG. 6A , the control gate  15  is formed by a gate layer which is the similar to the main gate  1   a  between the main gate  1   a  and the source area  2 . The control gate  15  includes the control gate area  15   a,  and the contact forming area  15   b  which is extended to one end of the control gate area  15   a.    
     The control gate area  15   a  is arranged parallel in a state that the gap  16  is provided between the control gate area  15   a  and the channel area  12  of the main gate  1   a,  and the hiatus portion  17  of a predetermined gap is provided on the other end.  FIG. 6A  illustrates an example in which the contact forming area  15   b  provided with the contact  4   g  forming the electrode is arranged on the same side as the contact forming area of the main gate  1   a.    
     In  FIG. 6B , the control gate  20  is formed by the gate layer similar to the main gate  1   a  between the main gate  1   a  and the drain area  3 . The control gate  20  includes the control gate area  20   a,  and the contact forming area  20   b  which is extended to one end of the control gate area  20   a.    
     The control gate area  20   a  is arranged parallel in a state that the gap  21  is provided between the control gate area  20   a  and the channel area  12  of the main gate  1   a , and the hiatus portion  22  of a predetermined gap is provided on the other end. The  FIG. 6B  illustrates an example in which the contact forming area  20   b  provided with the contact  4   h  forming the electrode is arranged on the opposite side to the contact forming area of the main gate  1   a  with respect to a line segment which connects the source area  2  and the drain area  3 . 
     Also in the MOS transistor in which the control gate is provided on one end of the main gate  1   a  in the gate lengthwise direction, as explained with reference to  FIGS. 4A  to  5 C, the effective channel width in the channel area  12  of the main gate  1   a  can be modulated by adjusting the voltage to be applied to the control gate  15  (or control gate  20 ). As a result, the gain coefficient β of the MOS transistor can be modulated. 
       FIG. 7  is a top pattern diagram illustrating the configuration of a MOS transistor according to a third embodiment of the present invention. The third embodiment explains another configuration example of the MOS transistor in which the control gates are provided on both sides of the main gate in the gate lengthwise direction, namely, on the sides of the source area and the drain area, respectively, in the MOS transistor shown in FIG.  1 B. 
     In  FIG. 7 , the control gate  25  includes the control gate areas  25   a  and  25   b,  and the contact forming area  25   c  in the gate lengthwise direction in the channel area  12  of the main gate  1   b.  The control gate area  25   a  is arranged so as to surround the source area  2 . The control gate area  25   b  is arranged so as to surround the drain area  3 . The contact forming area  25   c  connects the control gate areas  25   a  and  25   b.    
     The control gate areas  25   a  and  25   b  have side edges which are parallel with the gaps  26   a  and  26   b  being provided between the side edges and the channel area  12 . The side edges are provided with the hiatus portions  27   a  and  27   b  of predetermined gaps in a substantially central position of the gate width of the channel area  12 , namely, a position corresponding to the approximately central position of the gate channel width. 
     The contact forming area  25   c  is provided with the contact  4   k  composing the electrode.  FIG. 7  illustrates an example in which the contact forming area  25   c  of the control gate  25  and the contact forming area of the main gate  1   b  are arranged on opposite sides with respect to a line segment which connects the source area  2  and the drain area  3 . 
     That is to say, in the first and the second embodiments, the control gate areas of the control gate are provided so as to have the hiatus portions on their one ends of the gate channel width, but in the third embodiment, the control gate areas are formed so as to have the hiatus portions in the substantially central position of the gate channel with. In response to the second embodiment, only one of the control gate areas  25   a  and  25   b  shown in  FIG. 7  may be provided. 
     In such the MOS transistor having such a configuration, as explained with reference to  FIGS. 4A  to  5 C, the effective channel width in the channel area  12  of the main gate  1   b  can be modulated by adjusting the voltage to be applied to the control gate  25 . As a result, the gain coefficient β of the MOS transistor can be modulated. 
     According to the third embodiment, an area where an electric current is concentrated is always a central portion of the gate channel, and it is separated from a device separating wall. For this reason, this central portion is hardly influenced by defect or electric charges present in an interface of the separating wall, and thus the electrical characteristics with less dispersion can be realized. 
       FIG. 8  is a top pattern diagram illustrating the configuration of a MOS transistor according to a fourth embodiment of the present invention. The fourth embodiment explains still another configuration example of the MOS transistor in which the control gates are provided on both the sides of the main gate in the gate lengthwise direction, namely, on the sides of the source area and the drain area, respectively, in the MOS transistor shown in FIG.  1 A. 
     The MOS transistor shown in  FIG. 8  includes the control gate  15  shown in FIG.  6 A and the control gate  20  shown in FIG.  6 B. That is to say, in the fourth embodiment, the control gates are formed so as to have the hiatus portions on both the ends of the gate channel width. 
     In  FIG. 8 , the control gates  31  and  32  are formed by the gate layer similar to the main gate  1   a  on both the sides of the gate lengthwise direction in the channel area  12  of the main gate  1   a.    
     The control gate  31  arranged between the main gate  1   a  and the source area  2  includes the control gate area  31   a,  and the contact forming area  31   b  which is extended to one end (upper portion in the drawing) in the control gate area  31   a.    
     The control gate area  31   a  is arranged parallel so that the gap  33   a  is provided between the control gate area  31   a  and the channel area  12  of the main gate  1   a.  The hiatus portion  34   a  of a predetermined gap is provided on the other end (lower portion in the drawing).  FIG. 8  illustrates an example in which the contact forming area  31   b  provided with the contact  4   m  forming the electrode is arranged on the same side as the contact forming area of the main gate  1   a.    
     The control gate  32  arranged between the main gate  1   a  and the drain area  3  includes the control gate area  32   a,  and the contact forming area  32   b  which is extended to one end (lower portion in the drawing) of the control gate area  32   a.    
     The control gate area  32   a  is arranged parallel so that the gap  33   b  is provided between the control gate area  32   a  and the channel area  12  of the main gate  1   a.  The hiatus portion  34   b  of a predetermined gap is provided on the other end (upper portion in the drawing).  FIG. 8  illustrates an example in which the contact forming area  32   b  provided with the contact  4   n  forming the electrode is arranged on the opposite side to the contact forming area of the main gate  1   a  with respect to the line segment which connects the source area  2  and the drain area  3 . 
     In the MOS transistor, in which the control gates are formed so as to have the hiatus portions on both the ends of the gate channel width, as explained with reference to  FIGS. 4A  to  5 C, the effective channel width in the channel area  12  of the main gate  1   b  can be modulated by adjusting the voltage to be applied to the control gate  25 . As a result, the gain coefficient β of the MOS transistor can be modulated. 
     Further, according to the fourth embodiment, since the channel length is modulated besides the modulation of the channel width of the main gate, the gain coefficient β can be modulated dynamically. 
       FIG. 9  is a top pattern diagram illustrating the configuration of a MOS transistor according to a fifth embodiment of the present invention.  FIG. 10  is a cross-sectional view taken along line X—X shown in FIG.  9 . The fifth embodiment explains still another configuration example of the MOS transistor in which the control gates are located on opposite sides of the main gate, in the lengthwise direction of the gate, namely, on the source area and the drain area sides in the MOS transistor shown in FIG.  1 B. 
     In  FIG. 9 , the diffusion areas  36  between the channel area  12  of the main gate  1   b  and the control gate areas  10   a  and  10   b  of the control gate  10  is impurity diffusion areas which are similar to the source area  2  and the drain area  3 . In the fifth embodiment, however, impurity concentration of the diffusion areas  36  is lower than impurity concentration of the source area  2  and the drain area  3 . 
     As shown in  FIG. 10 , the channel area  12  of the main gate  1   b  and the control gate areas  10   a  and  10   b  of the control gate  10  are formed on an upper surface of a board (well area) so as to be separated from each other by an insulating layer  38 . The diffusion areas  36  between the channel area  12  and the control gate areas  10   a  and  10   b  of the control gate  10  are formed on the upper surface of the base (well area)  37  when the source area  2  and the drain area  3  are formed. 
     At this time, the impurity concentration of the diffusion areas  36  can be set to be lower than the impurity concentration of the source area  2  and the drain area  3  by a method of making an injection amount of the impurity different by introducing a photo mask for separating the respective areas. 
     In another method, a new mask is not added, but the gaps between the channel area  12  and the control gate areas  10   a  and  10   b  of the control gate  10  (Sv in  FIG. 2 ) are optimized. As a result, an LDD (Lightly Doped Drain) configuration can be utilized, so that the impurity concentration of the diffusion areas  36  can be set to be lower than that of the source area  2  and the drain area  3 . 
     When the impurity concentration of the diffusion areas  36  is set to be lower than that of the source area  2  and the drain area  3 , the value of the fixed resistors Rs shown in  FIG. 3  can be heightened. For this reason, the modulation degree of the channel width can be further increased. 
     The fifth embodiment illustrates the example of application to the first embodiment, but, needless to say, the fifth embodiment can be applied similarly to the second to the fourth embodiments. 
     In the MOS transistor of the present invention, the control gate can be formed by the same gate layer as that of the main gate differently from the MOS transistor filed before by the inventors. For this reason, the MOS transistor can be manufactured without changing the prior LSI manufacturing process at all, so that an increase in the manufacturing cost can be suppressed. 
     Further, in the MOS transistor of the present invention, since the NMOS transistor and the PMOS transistor can be realized by the same configuration, this MOS transistor can be easily adopted into a CMOS circuit. 
       FIGS. 11  to  13  are pattern diagrams illustrating the configuration of a MOS transistor according to a sixth embodiment of the present invention.  FIG. 11  is a top view.  FIG. 12  is a cross-sectional view taken along line XII—XII of FIG.  11 .  FIG. 13  is a cross-sectional view taken along line XIII—XIII of FIG.  11 . 
     In  FIG. 11 , as is well known, the configuration of the MOS transistor is such that the gate area  1  is provided between the source area  2  and the drain area  3  (central position) so as to cross a channel formed between the source area  2  and the drain area  3  (hereinafter, “gate channel”). The contacts  4   a,    4   b,  and  4   c  composing the electrode are provided on the gate area  1 , the source area  2  and the drain area  3 , respectively. 
     In the MOS transistor according to the sixth embodiment, the gate area  1  is called as the main gate  1  in the MOS transistor, and the control gate  5  is provided so as to cover the main gate  1 . As a result, the control gate channel areas  50  are formed so as to protrude on both ends of the main gate  1  in the gate lengthwise direction, namely, on the sides of the source area  2  and the drain area  3 . 
     The control gate  5  is provided with the contact  4   d  composing the electrode.  FIG. 11  illustrates an example in which an arrangement area of the contact  4   d  in the control gate  5  and an arrangement area of the contact  4   a  in the main gate  1 are formed on opposite sides with respect to the line segment which connects the source area  2  and the drain area  3 . 
     As shown in  FIG. 12  which is a cross-sectional view parallel with the gate channel, the main gate  1  is separated from the control gate  5  by the insulating film layer  38  so as to be electrically disconnected. The channel impurity diffusion areas  51 , which compose the control gate channel areas  50  formed so as to protrude on both the sides of the main gate  1  in the gate lengthwise direction, are formed on the surface of the board (well area)  37 . 
     The impurity concentration of the channel impurity diffusion areas  51  is not uniform in the gate widthwise direction of the main gate  1 . As shown in  FIG. 13  which is a cross-sectional view of the control gate channel area  50  vertical to the gate channel, the impurity concentration has non-uniformity such that it changes successively from one end to the other end.  FIG. 13  illustrates that as the black color is darker, the impurity concentration becomes lower. 
     As a result, a threshold value of the control gate channel areas  50  (hereinafter, “threshold value of the control gate 5”) is not uniform in the gate widthwise direction of the main gate  1 , namely, in a direction which crosses the gate channel. The threshold value has unidirectional change characteristics such that it changes successively from one end to the other end. 
       FIG. 11  illustrates that as the black color becomes darker, the threshold value of the control gate  5  becomes lower. That is to say, the darkest black color on the side of the contact  4   a  of the main gate  1  represents that the threshold value is minimum. The lightest black color on the side of the contact  4   d  of the control gate  5  represents that the threshold value is maximum. 
     The non-uniformity of the threshold value in the control gate  5  is realized by distributing the impurity concentration of the channel impurity diffusion areas  51  spatially in the gate widthwise direction of the main gate  1 . The shape of the control gate  5  is, therefore, arbitrary. In general, the shape of the control gate  5  depends on the shape of the main gate  1 , and thus is mostly a rectangular shape. 
     The operating principle which modulates the gain coefficient of the MOS transistor having the above configuration is explained below with reference to  FIGS. 11  to  16 C.  FIG. 14  is a diagram illustrating an example of drain current characteristics per unit gate width controlled by the control gate shown in FIG.  11 .  FIG. 15  is a diagram explaining shape parameters defining the characteristics of the MOS transistor shown in FIG.  11 .  FIG. 16  is a diagram explaining the gate channel width modulating operation realized by the MOS transistor shown in FIG.  11 . 
     The threshold value of the control gate  5  has the unidirectional change characteristics such that the threshold value changes successively from one end to the other end in the gate widthwise direction of the main gate  1 . For this reason, when the control voltage to be applied to the control gate  5  is changed, the width of the channels formed on the control gate channel areas  50  changes according to the distribution of the threshold value. The channels formed on the control gate channel areas  50  at this time become the effective channels in the gate area  1 . 
     In  FIG. 14 , a control gate voltage Vcg of the control gate  5  is read along an axis of abscissas. A drain current Id per unit gate width for each gate threshold value Vt is read along an axis of ordinate. The left one of the three lines shown in  FIG. 14  represents a characteristic curve for the lowest gate threshold value, and the right one for the highest gate threshold value. In other words, the density of each line indicates a gradation (corresponding to the gate threshold value) of the control gate channel area  50  shown in FIG.  11 . 
     As shown in  FIG. 14 , as the threshold value Vt of the control gate  5  becomes lower, the drain current Id per unit gate width flows more from the low value of the control gate voltage Vcg. As the threshold value Vt becomes higher, the control gate voltage Vcg becomes higher. If not, the drain current Id on such a portion does not flow. The drain current Id flows only on a portion where the control gate voltage Vcg exceeds the threshold value Vt. 
     That is to say, the effective channel width of the main gate  1  can be modulated by the control voltage Vcg to be given to the control gate  5 , so that the gain coefficient β of the MOS transistor can be modulated. The channel modulation characteristics to be realized can be set by the distribution of the threshold value of the control gate  5  and the shape parameters shown in FIG.  15 . 
     In  FIG. 15 , as the shape parameters, the gate length L and the gate width W of the main gate  1 , and the gate length Lc of the control gate are used. When they are modulated, so that the modulation characteristics of the gain coefficient β can be designed. In general, as the gate length Lc of the control gate  5  which is the control channel length is smaller, or a change amount in the distribution, of the threshold value of the control gate  5  is larger, the modulation degree of the gain coefficient β becomes larger. 
     The effective channel width in the main gate  1  is, as shown in  FIGS. 16A  to  16 C, for example, modulated by the control gate voltage Vcg. In  FIG. 16 , the maximum threshold value of the control gate  5  is represented by Vtmax, the minimum value is represented by Vtmin, and an intermediate value therebetween is represented by Vtmiddle. 
     As explained with reference to  FIG. 11 , the threshold value of the control gate  5  becomes gradually lower toward the contact  4   a  of the main gate  1 . The conductance of the main gate  1  is larger in a portion where the threshold value is lower. In  FIGS. 16A  to  16 C, therefore, the effective channel of the main gate  1  is formed on the side of the contact  4   a  of the main gate  1 . 
       FIG. 16A  illustrates that the control gate voltage Vcg is comparatively high (Vcg&gt;Vtmax). In this case, the channel is formed in the most part of the control gate channel areas  50  in the control gate  5 . As a result, the width of the effective channel  61  formed on the side of the contact  4   a  of the main gate  1  is comparatively larger towards the portion where the threshold value is higher, and the gain coefficient β becomes large. Further, the drain current in the main gate  1  flows a lot in the portion where the threshold value is low. Since the width of the effective channel  61 , however, becomes comparatively larger towards the portion where the threshold value is higher, the distribution of the current is proportional to the enlargement of the width. That is to say, a change range of the drain current becomes wide. 
       FIG. 16C  illustrates that the control gate electrode Vcg is comparatively low (Vtmiddle&gt;Vcg&gt;Vtmin). In this case, the channel is formed only in the portion in the control gate channel area  5  of the control gate  5  where the threshold value is lower than the intermediate value Vtmiddle. That is to say, the channel width on the control gate channel forming area  50  becomes considerably narrow. As a result, the width of the effective channel  62  formed on the side of the contact  4   a  of the main gate  1  falls within a range that the threshold value is low, namely, becomes comparatively small, so that the gain coefficient β becomes small. The change range of the drain current becomes considerably narrow. 
       FIG. 16B  illustrates that the control gate voltage Vcg is middle (Vtmax&gt;Vcg&gt;Vtmiddle). In this case, the channel is formed up to a portion in the control gate channel forming area  50  where the threshold value is nearly the intermediate value. That is to say, the width of the channel formed on the control gate channel forming area  50  becomes large to an extent of the middle between the case of FIG.  16 C and the case of FIG.  16 A. As a result, the width of the effective channel  63  formed on the side of the contact  4   a  in the main gate  1  becomes slightly larger towards the portion where the threshold value is high than the case of FIG.  16 C. For this reason, the gain coefficient β is the middle between  FIGS. 16A and 16B . 
     In the MOS transistor of the present invention, the width of the channel formed on the control gate channel area  50  can be changed by the voltage to be applied to the control gate  5 . As a result, since the effective channel width of the main gate  1  is modulated, the gain coefficient β of the MOS transistor to be a basis, namely, the drain current characteristics can be modulated successively. 
     The MOS transistor of the present invention adopts the configuration system which does not require the formation of a triangular area by the control gate channel required in the configuration system of the MOS transistor filed before by the inventors. For this reason, the device size can be reduced. 
     In the MOS transistor filed before by the inventors, the direction of the electric field to be applied to the gate channel is changed by the control gate voltage, so that the effective channel width of the main gate is modulated. In the MOS transistor of the present invention, however, the effective channel width of the main gate  1  is modulated by adjusting the width of the channel formed on the control gate channel area  50 . For this reason, the modulation degree of the gain coefficient β is secured regardless of the conductance of the main gate  1 , and thus the modulation characteristics of the gain coefficient β does not strongly depend on the gate voltage. 
     Further, since the MOS transistor of the present invention can be realized in both NMOS transistors and PMOS transistors, this can be adopted into CMOS circuits. 
     The electric power consumed by the modulation of the gain coefficient β is obtained only by the leak current from the control gate  5 . This electric current is extremely weak, and thus this does not substantially become a problem. 
       FIG. 17  is a main section cross-sectional view (cross-sectional view taken along line B-B′ of  FIG. 11 ) illustrating the configuration of a MOS transistor according to a seventh embodiment of the present invention. In  FIG. 17 , like or equivalent components as or to those shown in  FIG. 11  are designated by like reference numerals. The portions relating to the seventh embodiment are mainly explained here. 
     The seventh embodiment illustrates another configuration example which realizes spatial non-uniformity of the control gate threshold value. In the MOS transistor of the seventh embodiment, as shown in  FIG. 17 , the surface of the board  37  is covered with not the insulating film layer  38  but a gate insulating film  71  in the control gate channel areas  50  shown in  FIG. 11. A  thickness of the gate insulating film  71  is spatially non-uniform, namely, the gate insulating film  71  is formed so as to become gradually thicker from one end to the other end. The impurity concentration of the channel impurity diffusion area  72  formed on the surface of the board  37  is spatially uniform. 
     According to such a configuration, the threshold value of the control gate  5  can be spatially non-uniform, and thus the modulation effect similar to that in the sixth embodiment can be expected. In addition, in the seventh embodiment, the spatial non-uniformity of the distribution of the threshold value can be realized while the impurity concentration of the control gate channel area is uniform. For this reason, a photo mask for forming the distribution of the concentration is not required, and thus the manufacturing cost and the manufacturing steps can be reduced. 
       FIG. 18  is a top view illustrating the configuration of a MOS transistor according to an eighth embodiment of the present invention. In  FIG. 18 , like or equivalent components as or to those in the configuration of  FIG. 11  are designated by like reference numerals. The portions relating to the eighth embodiment are mainly explained here. 
     The eighth embodiment explains still another configuration example which realizes the spatial non-uniformity of the control gate threshold value. As shown in  FIG. 18 , in the eight embodiment, the control gate  81  is provided instead of the control gate  5  shown in FIG.  11 . 
     The spatial distribution of the unidirectional change characteristics of the threshold value is not successively changed in the control gate channel areas  82  of the control gate  81 , but it is set so as to change stepwise. The stepwise change is equal in both sides of the main gate  1  in the gate lengthwise direction. 
     Specifically, as shown in  FIG. 18 , for example, the threshold value Vt of the control gate  81  changes in both the sides of the main gate  1  in the gate lengthwise direction as follows. The threshold value Vt is low in the gate width direction, namely, on the side of the contact  4   a  of the main gate  1 . Vt is high on the side of the contact  4   d  of the control gate  81 , and Vt is middle between the contacts  4   a  and  4   d.    
     The control gate  81  having different threshold values can be realized by a method of changing the channel impurity concentration in each area as explained in the sixth embodiment or a method of changing the thickness of the gate insulating film as explained in the seventh embodiment. 
     According to the eighth embodiment, the modulation effect similar to that in the sixth embodiment can be obtained, and further it is not necessary to develop a special new means which forms the distribution of the threshold value of the control gate stepwise. As a result, a method of changing the distribution differently using a prior mask can be used. 
       FIG. 19  is a top view illustrating the configuration of a MOS transistor according to a ninth embodiment of the present invention. In  FIG. 19 , like or equivalent components as or to those in the configuration of  FIG. 11  are designated by like reference numerals. The portions relating to the ninth embodiment are mainly explained here. 
     The ninth embodiment explains still another configuration example which realizes the spatial non-uniformity of the control gate threshold value. As shown in  FIG. 19 , in the ninth embodiment, the control gate  91  is provided instead of the control gate  5  shown in FIG.  11 . 
     The spatial distribution of the unidirectional change characteristics of the threshold value is not successively changed in the control gate channel areas  92  and  93  in the control gate  91 , but it is set so as to change stepwise. The stepwise change is made in the gate widthwise direction on both the sides of the main gate  1  in the gate lengthwise direction in the opposite order to that in the eighth embodiment. 
     Specifically, the threshold value Vt of the control gate  91  is low on the side of the contact  4   a  of the main gate  1 , high on the side of the contact  4   d  of the control gate  91 , and middle between the contacts  4   a  and  4   d  in the control gate channel area  93  as shown in  FIG. 19 , for example. 
     On the contrary, the threshold value Vt is high on the side of the contact  4   a  of the main gate  1 , Vt is low on the side of the contact  4   d  of the control gate  41 , and Vt is middle between the contacts  4   a  and  4   d  in the control gate channel area  92 . 
     According to the ninth embodiment, the modulation effect similar to the sixth embodiment is obtained, and further not only the channel width of the main gate but also the channel length are modulated. For this reason, the gain coefficient β can be modulated more dynamically. 
     The ninth embodiment explained the example applied to the eighth embodiment, but this can be applied also to the sixth and the seventh embodiments. The sixth to the ninth embodiments explain that the control gate channel area is formed on both the sides of the main gate in the gate lengthwise direction. The present invention is, however, not limited to this, and the control gate channel area may be formed on one side of the main gate in the gate lengthwise direction, namely, any one of the source area side or the drain area side. This also produces the similar effect. 
     The MOS transistor of the present invention has such characteristics that its size is more compact and the power consumption is less than the MOS transistor filed before by the inventors. For this reason, a mechanism circuit, in which the electrical characteristics are automatically adjusted by the on-chip and the characteristics dispersion is corrected, can be packaged into every LSI device with high density. 
     That is to say, the characteristics dispersion due to the miniaturization of the device prohibiting future improvement in high performance of the large-scale LSI devices, deterioration in the performance due to non-uniformity of the characteristics such as a fluctuation in the device characteristics due to a fluctuation in the process, difficulty with LSI physical design, and the like can be greatly alleviated. 
     The device configuration technique of the present invention is, therefore, expected to contribute to realization of new type LSI devices based on completely new design concept such as a self-optimizing LSI device and a self-adapting LSI device which allow reasonably large dispersion of the device characteristics. 
     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.