Abstract:
A semiconductor device has: a main circuit including a plurality of MOS transistors operating at a first voltage; a memory requiring an operation at a second voltage higher than the first voltage; and a drive circuit for driving the memory, the drive circuit comprising one well, two or more MOS transistors in a cascade connection formed in the well, and well contact or contacts formed between MOS transistors in the well and on both outer sides of the cascade connection, or formed only between MOS transistors, or formed on both outer sides of the cascade connection, or formed only outside a drain of MOS transistors in the cascade connection. The semiconductor device is provided which integrates a memory requiring a high voltage, can simplify manufacture processes for a memory drive circuit and suppress an increase in an occupation area in chip of the memory drive circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on and claims priority of Japanese Patent Application No. 2004-316790 filed on Oct. 29, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     A) Field of the Invention  
         [0003]     The present invention relates to a semiconductor device, and more particularly to a semiconductor device having a main circuit operating at a first voltage and a memory requiring an operation at a second voltage higher than the first voltage.  
         [0004]     B) Description of the Related Art  
         [0005]     An operation voltage of a semiconductor integrated circuit is lowering from 3 V to 2.5 V and to 1.25 V. A flash memory requires a high voltage of about 10 V. Because of its non-volatility performances, a flash memory is used in various logical integrated circuits. For example, the application range is broadening to logical integrated circuits mixedly mounting flash memories, such as a field programmable gate array (FPGA) and a complex programmable logic device (CPLD). MOS transistors for controlling a memory requiring a high voltage such as a flash memory cell require a very high voltage. In order to realize a very high breakdown voltage of a MOS transistor, some design is necessary for determining an impurity concentration distribution of drain regions and wells, resulting in a considerable increase in the number of processes. In order not to increase the number of processes, it is desired to use general MOS transistors and operate the MOS transistors at a voltage higher than the drain breakdown voltage.  
         [0006]     Techniques have been proposed which operate a MOS transistor at a voltage higher than the drain breakdown voltage. For example, two or more MOS transistors each formed in an independent well are connected in a cascade. As each MOS transistor is formed separately in an independent well, the device area becomes very large. If a plurality of MOS transistors are formed in one well and connected in a cascade, the breakdown voltage tends to be lowered. For these techniques, for example, refer to Japanese Patent Laid-open Publications No. 2000-323584 and HEI-11-133926.  
       SUMMARY OF THE INVENTION  
       [0007]     An object of the present invention is to provide a multi-voltage semiconductor device having a high breakdown voltage portion and being capable of suppressing an increase in an occupation area in chip.  
         [0008]     Another object of the present invention is to provide a semiconductor device mixedly mounting memories which require a high voltage, being capable of simplifying manufacture steps for a memory drive circuit and suppressing an increase in an occupation area of the memory drive circuit.  
         [0009]     According to one aspect of the present invention, there is provided a semiconductor device comprising: a main circuit including a plurality of MOS transistors operating at a first voltage; a memory requiring an operation at a second voltage higher than the first voltage; and a drive circuit for driving the memory, the drive circuit comprising one well, two or more MOS transistors in a cascade connection formed in the well, and a well contact formed between MOS transistors in the well and on both outer sides of the cascade connection, or formed only between MOS transistors, or formed on both outer sides of the cascade connection, or formed only outside a drain of MOS transistors in the cascade connection.  
         [0010]     As a plurality of transistors are formed in one well and connected in cascade, the breakdown voltage of transistors change with the layout of well contacts. By properly selecting the layout of well contacts, a transistor breakdown voltage can be improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIGS. 1A and 1B  are a plan view showing an example of the structure of a single transistor and a graph showing the characteristics of the transistor.  
         [0012]      FIGS. 2A, 2B  and  2 C are a plan view, a cross sectional view and an equivalent circuit showing a semiconductor integrated circuit integrating flash memories.  
         [0013]      FIGS. 3A  to  3 E are cross sectional views showing the structures of samples.  
         [0014]      FIGS. 4A  to  4 F are cross sectional views illustrating the processes of forming a sample.  
         [0015]      FIG. 5  is a graph showing the characteristics of samples obtained through simulation.  
         [0016]      FIGS. 6A  to  6 C are a cross sectional view and plan views showing a first embodiment.  
         [0017]      FIG. 7  is a cross sectional view showing the structure of a flash memory circuit according to the first embodiment.  
         [0018]      FIGS. 8A and 8B  are a cross sectional view and a plan view showing a second embodiment.  
         [0019]      FIGS. 9A, 9B  and  9 C are a cross sectional view and plan views showing a third embodiment.  
         [0020]      FIGS. 10A and 10B  are a cross sectional view and a plan view showing a fourth embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]      FIGS. 1A and 1B  are a plan view showing the structure of a general single transistor and a graph showing its characteristics.  
         [0022]     As shown in  FIG. 1A , a p-type well Wp is formed in a silicon substrate and an element isolation region made of shallow trench isolation (STI) defines an active region ARn in which an n-channel MOS transistor is formed and a p-type region WCp in which well contacts are formed. A gate electrode G is formed traversing the active region, and a gate contact GC provides a contact with the gate electrode G. In the regions on both sides of the gate electrode G, n-type impurity ions are doped to form an n-type source region Sn and an n-type drain region Dn. Source contacts SC and drain contacts DC are formed in the source and drain regions, respectively. Well contacts WC are formed in the well contact region WCp.  FIG. 1B  shows the characteristics of this transistor, with the gate length being set to 1 μm and a distance between the active region ARn and well contact region WCp being set to 2 μm.  
         [0023]     In  FIG. 1B , the abscissa represents a source-drain voltage Vds in the unit of V, and the ordinate represents a drain current Ids in the unit of A. A curve VG 0  indicates a drain current at a gate voltage of 0 V, and curves VG 1  to VG 10  indicate drain currents at the gate voltages of 1 V to 10 V, respectively.  
         [0024]     Breakdown voltages of a MOS transistor have two breakdown voltages: a breakdown voltage (non-operation breakdown voltage) in a transistor off-state and a breakdown voltage (operation breakdown voltage) in a transistor on-state. The drain breakdown voltage at the gate voltage of 0 V is the non-operation breakdown voltage, and is over 10 V in the graph shown in  FIG. 1B . The characteristics at the gate voltage of 1V or higher, particularly the characteristics at VG 2  or higher with abrupt rise of the drain current, are the characteristics in operation, and the voltage at which the drain current abruptly rises is the operation breakdown voltage. In the characteristics shown in  FIG. 1B , at the gate voltage of 3 V or higher, the drain current increases greatly at the drain voltage of about 8 V and the operation breakdown voltage is lower than 9 V. The operation breakdown voltage in the on-state becomes in some cases lower than the non-operation breakdown voltage in the off-state.  
         [0025]     For the operation breakdown voltage, a snap-back phenomenon showing a negative resistance is observed in some cases. The snap-back phenomenon appears when a parasitic bipolar transistor of source-well-drain turns on because avalanche breakdown occurs due to a high electric field near the drain and the well potential rises.  
         [0026]      FIG. 2A  shows an example of the structure of a semiconductor logical integrated circuit mixedly mounting flash memories. A logic CMOS circuit  111  operating at a low voltage (LV: e.g., 1.25 V) is disposed in the central area of a semiconductor chip  110 , and disposed on both sides of the logic CMOS circuit are an input/output n-channel MOS transistor (NMOS) circuit  112  and an input/output p-channel MOS transistor (PMOS) circuit  113  operating at a low voltage (LV: e.g., 1.25 V) and a middle voltage (MV: e.g., 3 V), respectively. A flash memory circuit  116  is disposed in upper and lower areas of the semiconductor chip  110 . An NMOS circuit  117  and a PMOS circuit  118  for controlling flash memory cells are disposed on the inner side of the flash memory circuit. The NMOS circuit  117  and PMOS circuit  118  are constituted of high voltage (HV: e.g., 5 V) transistors.  
         [0027]     A flash memory cell FMC is required to operate at a high voltage of about 10 V. The flash memory cell control circuits  117  and  118  are also required to operate at similar high voltage.  
         [0028]      FIG. 2B  is a schematic cross sectional view showing the structures of high voltage transistors to be used for controlling the flash memory cells of the semiconductor integrated circuit shown in  FIG. 2A  and low voltage transistors of the logical circuit.  FIG. 2B  shows from the left side a flash memory cell FMC, a high voltage n-channel MOS transistor HV-NMOS, a high voltage p-channel MOS transistor HV-PMOS, a low voltage n-channel MOS transistor LV-NMOS and a low voltage p-channel MOS transistor LV-PMOS.  
         [0029]     LV-PMOS is formed in a first n-type well Wn 1 , and LV-NMOS is formed in a first p-type well Wp 1 . HV-PMOS is formed in a second n-type well Wn 2 , and HV-NMOS is formed in a second p-type well Wp 2 . The first and second n-type wells Wn 1  and Wn 2  have the same well impurity concentration, and the first and second p-type wells Wp 1  and Wp 2  have the same well impurity concentration. The flash memory cell FMC is formed in a third p-type well Wp 3 . The second p-type well Wp 2  and third p-type well Wp 3  are formed in a third n-type well Wn 3 .  
         [0030]     The well of a high voltage transistor assuming a 5 V operation can be formed at the same well impurity concentration as that of low voltage transistors LV-NMOS and LV-PMOS. The well of a transistor assuming a 10 V operation is required to have a lower impurity concentration. The well of a middle voltage transistor assuming a 3 V operation can be formed at the same well impurity concentration as that of a 5 V operation transistor. If transistors constituting the flash memory control circuits  117  and  118  for controlling flash memory cells  116  can be formed by the high voltage operation transistors shown in  FIG. 2B , the number of processes can be reduced. In this case, the structure having an operation breakdown voltage of 10 V or higher is necessary.  
         [0031]      FIG. 2C  is an equivalent circuit of the control circuit for controlling a flash memory cell.  
         [0032]     In erasing data in a flash memory cell, a voltage of 10 V is applied to the well of the flash memory cell. A voltage of 10 V is applied from a pump circuit to a VPUMP terminal. A flash memory cell control circuit is shown by broken lines which is constituted of one p-channel MOS transistor PMOS and one n-channel MOS transistor NMOS. First, the p-channel MOS transistor PMOS is turned on and a voltage of 10 V is applied to the well of the flash memory cell. After the erase operation, the p-channel MOS transistor PMOS is turned off and the n-channel MOS transistor NMOS is turned on to drain charges in the well of the flash memory to the ground potential. The potential at the well becomes 0 V.  
         [0033]     As the p-channel MOS transistor PMOS is turned on to start the erase operation, a voltage of −10 V is applied relative to the drain of PMOS. It is therefore necessary to set the operation breakdown voltage of PMOS to 10 V or higher. PMOS having the characteristics shown in  FIG. 1B  has an insufficient operation breakdown voltage.  
         [0034]     As NMOS is turned on after the erase operation to drain charges in the well of the flash memory, a voltage of 10 V is applied to the drain of NMOS. It is therefore necessary to set the operation breakdown voltage of NMOS to 10 V or higher. The characteristics shown in  FIG. 1B  show an insufficient operation breakdown voltage.  
         [0035]     In order to overcome this problem, instead of using a single p-channel MOS transistor PMOS, two cascade-connected p-channel MOS transistors PMOS 1  and PMOS 2  are used, and instead of using a single n-channel MOS transistor NMOS, two cascade-connected n-channel MOS transistors NMOS 1  and NMOS 2  are used. As two cascade-connected transistors are used, an occupation area in chip increases more than using one transistor. In order to suppress an increase in the occupation area in chip, two transistors are disposed in the same well.  
         [0036]     The layout of STI and well contacts along a cross section of a central channel in the well was changed in various ways and breakdown voltages were obtained through two-dimensional simulation.  
         [0037]      FIGS. 3A  to  3 E show the cross sectional structures of five types of samples S 1  to S 5 .  
         [0038]      FIG. 3A  shows the cross sectional structure of a first sample S 1 . A p-type well Wp is formed in a deep n-type well DWn. On both sides of the well, well contact regions WCp 1  and WCp 2  having a width of 0.2 μm are formed outside shallow trench isolations STI 1  and STI 2  having a width of 0.3 μm. A well contact region WCp 3  having a width of 0.8 μm is formed in the well central region between STI 3  and STI 4  having a width of 1.6 μm. An NMOS  1  is formed between STI 1  and STI 3 , having n-type source/drain regions and a gate length of 1.5 μm, and an NMOS  2  is formed between STI 4  and STI 2 , having n-type source/drain regions and a gate length of 1.5 μm.  
         [0039]     Adjacent current terminals of both NMOS&#39;s are connected to form a cascade connection structure. The remaining current terminal of NMOS 1  is used as a source S and the remaining current terminal of NMOS 2  is used as a drain D. Electrodes are connected to the source S, drain D and each well contact region. In the following description, the elements represented by the same symbol have the same size, excepting the wells.  
         [0040]      FIG. 3B  shows the cross sectional structure of a second sample S 2 . This structure corresponds to the structure that both STI 1  and STI 2  and the well contact regions WCp 1  and WCp 2  of the sample S 1  are removed. The length of the well can be reduced by 1 μm.  
         [0041]      FIG. 3C  shows the cross sectional structure of a third sample S 3 . This structure corresponds to the structure that the central well contact region WCp 3  and STI 3  and STI 4  on opposite sides of the first sample are removed to combine the adjacent source/drain regions of NMOS 1  and NMOS 2 . The length of the well can be reduced by 4 μm.  
         [0042]      FIG. 3D  shows the cross sectional structure of a fourth sample S 4 . This structure corresponds to the structure that the well contact region WCp 1  of the third sample S 3  on the left side (on the source side of the cascade connection) is omitted. The well contact region exists only on the drain D side of the cascade connection.  
         [0043]      FIG. 3E  shows the cross sectional structure of a fifth sample S 5 . This structure corresponds to the structure that the well contact region WCp 2  of the third sample S 3  on the right side (on the drain side of the cascade connection) is omitted. The well contact region exists only on the source S side of the cascade connection.  
         [0044]     For the comparison, a sample S 0  was formed which has one NMOS formed in one well. In this structure, only the left side portion from the center of the central well contact region WCp 3  of the first sample S 1  is disposed in the well.  
         [0045]      FIGS. 4A  to  4 F are cross sectional views illustrating a process flow of the first sample S 1 . For the samples S 2  to S 5 , the processes omitting the corresponding parts are executed.  
         [0046]     As shown in  FIG. 4A , a shallow trench isolation STI is formed in an Si substrate. A deep n-type well DWn is formed by implanting n-type impurity ions P +  at an acceleration energy of 2 MeV and a dose of 1 to 3 E13 cm −2  (E is an exponent notation, E13 is equal to ×10 13 , this notation is applied also in the following). A p-type well Wp is formed by implanting p-type impurity ions B +  at an acceleration energy of 400 to 500 keV and a dose of 1 to 2 E13 cm −2 , and again at an acceleration energy of 100 keV and a dose of 2 to 4 E12 cm −2 .  
         [0047]     As shown in  FIG. 4B , the silicon surface is thermally oxidized to form a gate oxide film having a thickness of 17 nm. A polysilicon layer is deposited by CVD to a thickness of 180 nm on the oxide film, and patterned at a gate length of 1.5 μm by photolithography and etching to form gate electrodes G.  
         [0048]     As shown in  FIG. 4C , by using a photomask, n-type impurity ions As +  are implanted into a transistor region at an acceleration energy of 100 to 150 keV and a dose of 1 to 3 E13 cm −2  to form low impurity concentration source/drain regions LDD. In a well contact region, p-type impurity ions BF 2   +  are implanted at an acceleration energy of 50 to 100 keV and a dose of 1 to 2 E13 cm −2  to form p-type well contact regions WCp. If a CMOS is to be formed, ion implantation for p-type well contact regions can be performed by the same ion implantation as PMOS LDD ion implantation.  
         [0049]     As shown in  FIG. 4D , a silicon oxide layer having a thickness of about 130 nm is deposited by CVD on the Si substrate surface, and etched back by anisotropic etching to form side wall spacers SW having a thickness of about 130 nm on the gate electrode side walls.  
         [0050]     As shown in  FIG. 4E , by using a photomask, high impurity concentration source/drain regions S/D are formed by implanting n-type impurity ions P +  at an acceleration energy of 15 to 20 keV and a dose of 1 to 2 E15 cm −2 , and again at an acceleration energy of 10 to 15 keV and a dose of 5 to 8 E15 cm 2 . In the well contact regions, p-type impurity ions B +  are implanted at an acceleration energy of 5 to 8 keV and a dose of 2 to 5 E13 cm −2  to form p-type contact regions WCp. If a CMOS is to be formed, ion implantation for the well contact regions can be performed by the same ion implantation as PMOS S/D ion implantation.  
         [0051]     As shown in  FIG. 4F , electrodes are formed on the well contact regions WCp and the source/drain regions on opposite sides, and an interlayer insulating film IL is formed on the substrate. A wiring for connecting inner side source/drain regions is formed. The Si substrate at the level lower than the impurity concentration peak of the deep n-type well DWn in terms of simulation is removed, and an electrode is formed on its surface.  
         [0052]     As application voltages in terms of simulation, 1.2 V was applied to the deep n-type well DWn, 0V was applied to the p-type well from the well contact regions, 0 V was applied to the source S, and 5 V was applied to the gate G and a voltage applied to the drain D was swept.  
         [0053]      FIG. 5  is a graph showing the simulation results. The abscissa represents a sweep voltage Vd applied to the drain D in the unit of V, and the ordinate represents a drain current Id in the unit of A. Curves s 0  to s 5  indicate the characteristics of the samples S 0  to S 5 , respectively. The characteristics s 0  of the single transistor have the lowest breakdown voltage which does not exceed 10 V. The characteristics of the sample S 5  having well contacts only outside the source S have the next lowest breakdown voltage which is about higher or lower than 10 V.  
         [0054]     The characteristics s 1  of the first sample S 1  have a highest breakdown voltage which readily exceeds 13 V. Although the lower the breakdown voltage becomes, the more the structure is simplified as in the case of the sample S 2 , S 3  and S 4  and the more the occupation area in chip is reduced, the breakdown voltage is still higher than that of the sample S 5 . It is therefore preferable to use the structures of the samples S 1  to S 4  if two or more transistors MOS transistors are formed in one well. The samples S 1  to S 3  among others flow a stable drain current at 11 V or lower.  
         [0055]      FIGS. 6A and 6B  show a semiconductor device using the structure of the first sample S 1  according to the first embodiment.  FIG. 6A  is a cross sectional view and  FIG. 6B  is a plan view. Formed in an NMOS region of a p-type silicon substrate by executing the processes shown in  FIGS. 4A  to  4 F are: an element isolation region  11 ; a deep n-type well  12 ; a p-type well  13 , gate insulating films  15 ; n-type gate electrodes  16   n ; side wall spacers  17 ; n-type LDD regions  18   n ; n-type source/drain regions  19   n ; p-type well contact regions  20   p ; and wirings  21 . Formed in a PMOS region by using the same processes are: an element isolation region  11 : gate insulating films  15 , side wall spacers  17 ; and wirings  21 . In the PMOS region, by using the opposite conductivity type in the ion implantation processes, there are formed: an n-type well  14 ; p-type gate electrodes  16   p ; p-type LDD regions  18   p ; p-type source/drain regions  19   p  and  n -type well contact regions  20   n . As described previously, ion implantation can be used in common to both the source-drain regions and well contact regions.  
         [0056]     As shown in  FIG. 6B , in the p-type well  13 , two n-channel MOS transistors NMOS 1  and NMOS 2  having the same structure as that of the sample S 1  are formed and cascade-connected, and in the n-type well  14 , two p-channel MOS transistors PMOS 1  and PMOS 2  having the same structure as that of the sample S 1  are formed and cascade-connected. The source/drain regions on opposite sides of each cascade connection are called a source S and a drain D. The well contact regions are formed between two transistors (WCp 3  and WCn 3 ) and outside the source and drain (WCp 1 , WCp 2 , WCn 1  and WCn 2 ).  
         [0057]     Breakdown occurs when the potential of the well rises because of an impact ionization multiplication mechanism and a lateral type parasitic bipolar transistor of source-well-drain becomes conductive. By disposing well contacts outside each MOS transistor and between MOS transistors, it is possible to prevent the potential of the well near the source from being raised and to raise the breakdown voltage because the lateral type bipolar transistor will be become conductive.  
         [0058]      FIG. 6C  shows a modification. The cross sectional structure along a current direction is similar to that shown in  FIG. 6A . In this structure, three well contact regions WCp 1 , WCp 2  and WCp 3  and three well contact regions WCn 1 , WCn 2  and WCn 3  are coupled by well contact regions disposed in the upper and lower areas, to form continuous well contact regions. By disposing the well contact regions also at the sides of, or side by side with, the transistors, the potential of the well can be stabilized and the breakdown voltage can further be improved. Although it is not necessarily required to surround the transistor with the well contact regions, the reliability is expected to be improved by surrounding the transistor with the well contact regions.  
         [0059]      FIG. 7  shows the structure of the flash memory circuit shown in  FIG. 2C  by using the structure shown in  FIG. 6A . The cascade circuit shown in  FIG. 6A  is formed in the left area of  FIG. 7 , and flash memory cells FMC are formed in the right area. In the flash memory area, a deep n-type well  25  is formed, and in this n-type well, a p-type well  26  is formed. In the p-type well  26 , a number of n-channel FMC&#39;s and p-type well contact regions  27  are formed. Outputs of cascade connections are applied via wirings  23 . An output of each flash memory cell FMC is supplied via a bit line BL.  
         [0060]      FIGS. 8A and 8B  show a semiconductor device using the structure of the second sample S 2  according to the second embodiment.  
         [0061]     As shown in  FIG. 8A , well contact regions  20   p  and  20   n  are formed only between two cascade-connected transistors. By omitting STI and well contact regions outside the source S and drain D, the lateral size as viewed in  FIG. 8A  can be made small.  
         [0062]      FIG. 8B  shows a plan layout. In a p-type well  13 , two n-channel MOS transistors NMOS 1  and NMOS 2  are formed, and p-type well contact regions WCp are formed between the MOS transistors. The inner source/drain regions are connected by a wiring, and the outer source/drain regions form a source S and a drain D. In an n-type well  14 , two p-channel MOS transistors PMOS 1  and PMOS 2  are formed, and n-type well contact regions WCn are formed between the MOS transistors. The inner source/drain regions are connected by a wiring, and the outer source/drain regions form a source S and a drain D.  
         [0063]     In the circuit shown in  FIG. 7 , as the left side cascade connection circuit is replaced with the cascade connection circuit of this embodiment, a flash memory circuit can be formed.  
         [0064]      FIGS. 9A and 9B  show the structure of a semiconductor device using the structure of the third sample S 3  according to the third embodiment.  
         [0065]     As shown in  FIG. 9A , the source/drain regions of two adjacent transistors are integrated to form a common region  18   f , and well contact regions  20   p  and  20   n  are formed only outside a source S and a drain D. By omitting STI and well contact regions between the two transistors, the lateral size as viewed in  FIG. 9A  can be made small.  
         [0066]      FIG. 9B  shows a plan layout. In a p-type well  13 , two n-channel MOS transistors NMOS 1  and NMOS 2  are formed merging the adjacent source/drain regions, and p-type well contact regions WCp 1  and WCp 2  are formed on the both outer sides of the MOS transistors. The outer source/drain regions form a source S and a drain D. In an n-type well  14 , two p-channel MOS transistors PMOS 1  and PMOS 2  are formed merging the adjacent source/drain regions, and n-type well contact regions WCn 1  and WCn 2  are formed on the both outer sides of the MOS transistors. The outer source/drain regions form a source S and a drain D.  
         [0067]     In the circuit shown in  FIG. 7 , as the left side cascade connection circuit is replaced with the cascade connection circuit of this embodiment, a flash memory circuit can be formed.  
         [0068]      FIG. 9C  shows a modification. The cross sectional structure along a current direction is similar to that shown in  FIG. 9A . In this structure, two well contact regions WCp 1  and WCp 2  of two transistors and two well contact regions WCn 1  and WCn 2  of two transistors are coupled by well contact regions disposed in the upper and lower areas, to form continuous well contact regions WCp and WCn. By disposing the well contact regions also at the sides of the transistors, the potential of the well can be stabilized and the breakdown voltage can further be improved. Although it is not necessarily required to surround the transistor with the well contact regions, the reliability is expected to be improved by surrounding the transistor with the well contact regions.  
         [0069]      FIGS. 10A and 10B  show the structure of a semiconductor device using the structure of the fourth sample S 4  according to the fourth embodiment.  
         [0070]     As shown in  FIG. 10A , transistors have the same structure as that shown in  FIG. 9A . Well contact regions  20   p  and  20   n  are formed only outside the drain of two cascade-connected transistors. As compared to  FIG. 9A , since STI and well contact regions outside the source S are omitted, the lateral size as viewed in  FIG. 10A  can be made small.  
         [0071]      FIG. 10B  shows a plan layout. In a p-type well  13 , two n-channel MOS transistors NMOS 1  and NMOS 2  are formed merging the inner source/drain regions, and the outer source/drain regions form a source S and a drain D. Outside the drain D, p-type well contact regions WCp are formed. In an n-type well  14 , two p-channel MOS transistors PMOS 1  and PMOS 2  are formed merging the inner source/drain regions, and the outer source/drain regions form a source S and a drain D. Outside the drain D, n-type well contact regions WCn are formed.  
         [0072]     In the circuit shown in  FIG. 7 , as the left side cascade connection circuit is replaced with the cascade connection circuit of this embodiment, a flash memory circuit can be formed.  
         [0073]     The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. For example, the cascade connection in an n-type well may have a different structure from that of the cascade connection in a p-type well, for example, the structure of the first sample in the n-type well and the structure of the second sample in the p-type well. A cascade connection made of discrete transistors may be used for one of the n- and p-type transistors. The well contact regions at the sides of a cascade connection similar to that of the modifications of the first and third embodiments may be adopted in the second and fourth embodiments. It will be apparent to those skilled in the art that other various modifications, improvements, combinations, and the like can be made.