Abstract:
An object of the present invention is to provide a semiconductor device which enables to reduce the device area, while securing the breakdown voltage between the drain and the source of each MOS transistor for the semiconductor device including plural MOS transistors, which are arrayed adjacently each other, with different types of channel conductivity. The semiconductor device includes a semiconductor substrate, a buried oxide film and a semiconductor layer, and furthermore the semiconductor layer has an island-like semiconductor layer, in which a MOS transistor is formed, the MOS transistor has a source region, and a drain region that is positioned in the periphery of the source region, an island-like semiconductor layer, in which a MOS transistor is formed, the MOS transistor has a drain region, and a source region that is positioned in the periphery of the drain region, an isolation trench which isolates the former island-like semiconductor layer from other portions of the semiconductor layer, an isolation trench which isolates the latter island-like semiconductor layer from other portions of the semiconductor layer, and a buffer region, in which the electric potential is fixed to the lowest electric potential in a circuit, which prevents an electrical interference occurred between transistors.

Description:
BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates to a semiconductor device, and in particular to a Metal Oxide Semiconductor (MOS) transistor formed in a Silicon-on-Insulator (SOI) substrate. 
   (2) Description of the Related Art 
   In recent years, a semiconductor device, in which an N-channel MOS transistor and a P-channel MOS transistor are formed in an SOI substrate, is utilized for various applications. Particularly, a semiconductor device, which adopts a MOS transistor having an offset structure, is used for a drive circuit with a high breakdown voltage. 
   In fact, in a semiconductor device including plural MOS transistors on a semiconductor substrate, a buffer region is formed between transistors, so that each transistor is not affected by an electrical interference from an adjacent transistor. In addition, an electric potential applied to the buffer region is actually specified to the same electric potential applied to a source of the adjacent MOS transistor, so as to improve the breakdown voltage between a drain and the source in the MOS transistor. The MOS transistor using such method is disclosed for example in Japanese Laid-Open Patent Application No. H11-330383. 
   Hereafter the exemplified conventional MOS transistor disclosed in Japanese Laid-Open Patent Application No. H11-330383 is described referring to  FIG. 1 .  FIG. 1  is a cross-sectional diagram of a P-channel MOS transistor formed in an SOI substrate. 
   As shown in  FIG. 1 , the aforesaid conventional P-channel MOS transistor includes an SOI substrate which has a semiconductor substrate  101 , a buried oxide film  102  formed on the semiconductor substrate  101 , and a semiconductor layer  103  formed on the buried oxide film  102 . The semiconductor layer  103  has an island-like semiconductor layer  103   a,  of which a P-channel MOS transistor structure is formed, isolated from other elements forming regions by an isolation trench  104 . In addition, an I layer with an extremely low concentration impurity, which functions as an intrinsic semiconductor layer substantially, is formed in the region adjacent to the buried oxide film  102  in the semiconductor layer  103 . The I layer functions as an electric field alleviation layer. 
   A drain region  105 , which is a P-type impurity layer with a low concentration impurity, is formed in the island-like semiconductor layer  103   a.  A drain contact region  106 , which is a P-type impurity layer with a high concentration impurity, is formed on the surface of the drain region  105 , and a drain electrode  106   a  is placed on the drain contact region  106 . In addition, a ring-shaped gate electrode  107  which is made of polycrystalline silicon, and a ring-shaped body region  108 , which is an N-type impurity layer with a low concentration impurity, are formed centering on the drain region  105  in the periphery of the drain region  105 . A ring-shaped source region  109 , which is a P-type impurity layer with a high concentration impurity, and a ring-shaped body contact region  110 , which is an N-type impurity layer with a high concentration impurity, are formed in the body region  108 . 
   A source electrode  109   a  is placed on the source region  109  and the body contact region  110 , and the source region  109  and the body region  108  are electrically connected by the source electrode  109   a.  In addition, a LOCOS oxide film  111  for alleviating electric field concentration is formed in a predetermined part in the island-like semiconductor layer  103   a.    
   A buffer region  112 , which is an N-type impurity layer with a low concentration impurity, is formed in the outer periphery region of the island-like semiconductor layer  103   a  in other words in the outer periphery region adjacent to the island-like semiconductor layer  103   a  across the isolation trench  104 , so as not to be affected by an electrical interference from the other adjacent elements. A buffer contact layer  113 , which is an N-type impurity layer with a high concentration impurity, is formed on the surface of the buffer region  112 , and a buffer electrode  113   a  is placed on the buffer contact layer  113 . 
   Accordingly, the conventional P-channel MOS transistor having the aforesaid structure is characterized in that the drain region  105  is formed in the center of the island-like semiconductor layer  103   a,  the source region  109  and the body region  108  are formed in the outer periphery of the drain region  105 , and a connection unit  114  is placed so as to make the electric potential in the buffer electrode  113   a  the same as the electric potential in the source electrode  109   a.    
     FIG. 2  is a drawing showing a part of a potential distribution (dotted and dashed lines) in the case where a high electric potential of positive polarity is applied to the source electrode  109   a,  while the electric potential of the drain electrode  106   a  is specified to a ground electric potential in the P-channel MOS transistor having the aforesaid structure. As shown in  FIG. 2 , a high electric potential of positive polarity with the same electric potential as the source electrode  109   a  is applied to the buffer electrode  113   a  through the connection unit  114 , so that an electric potential difference between the buffer region  112  and the source region  109  is not generated. Thus an occurrence of an avalanche breakdown between the isolation trench  104  and the source region  109  can be prevented. As a result the breakdown voltage is determined based on the potential distribution in the drain region  105  in the conventional MOS transistor. 
     FIG. 3  is a drawing showing a part of the potential distribution (dotted and dashed lines) in the case where a high electric potential of positive polarity is applied to the source electrode  109   a,  while the electric potentials of the buffer electrode  113   a  and the drain electrode  106   a  are specified to a ground electric potential in a P-channel MOS transistor without the connection unit  114 . In Japanese Laid-Open Patent Application No. H11-330383, it is disclosed that this technology is generally used. However, the body region  108  as the N-type impurity layer is applied with an electric potential which is a higher electric potential than the electric potential of the buffer region  112 , so that a depletion layer grows in the body region  108  as the N-type impurity layer. Thus, the breakdown voltage, which is supposed to be determined based on the potential distribution in the drain region  105 , is actually determined based on the electric potential concentration generated by a voltage between the drain and the source in the surface region (region A in  FIG. 3 ) between the source region  109  and the isolation trench  104 . The electric potential in the surface region (region A) is concentrated and the electric field becomes exceptionally large, so that the electric potential of this case might cause a lowering of the breakdown voltage between the drain and the source in the MOS transistor. 
   In such a case, a conceivable method is to make the distance between the isolation trench  104  and the source region  109  longer so as to prevent an occurrence of the avalanche breakdown caused by the potential concentration in the surface region (region A in  FIG. 3 ) between the isolation trench  104  and the source region  109 . However, there exists a problem that the device area is increased by this method. Thus, it can be expected in the conventional MOS transistor shown in  FIG. 1  that the occurrence of the avalanche breakdown in the surface region between the isolation trench  104  and the source region  109  can be prevented without making the distance between the isolation trench  104  and the source region  109  longer, by placing the connection unit  114  for making the potentials of the buffer region  112  and the source region  109  the same, so that the breakdown voltage between the drain and the source can be improved. 
   SUMMARY OF THE INVENTION 
   However, in the case where an N-channel MOS transistor is placed adjacent to a P-channel MOS transistor as the conventional technology, there exists a problem that the device area is increased. Hereafter, the reason of the problem is described. 
     FIG. 4  is a cross-sectional diagram of a semiconductor device including a P-channel MOS transistor having the structure as shown in  FIG. 1  and an N-channel MOS transistor placed adjacent to the P-channel MOS transistor. 
   The P-channel MOS transistor structure is formed in an island-like semiconductor layer  103   a  in a semiconductor layer  103 , and the N-channel MOS transistor structure is formed in an island-like semiconductor layer  103   b  in the semiconductor layer  103  which is isolated from the island-like semiconductor layer  103   a.    
   A drain region  125  is formed in the center of the island-like semiconductor layer  103   b,  and a body region  128 , on which a source region  129  and a body contact region  130  are formed, is placed in the outer periphery region of the drain region  125 . 
   A drain contact region  126  is formed on the surface of the drain region  125 , and a drain electrode  106   b  is placed on the drain contact region  126 . A source electrode  109   b  is placed on the source region  129  and the body contact region  130 . A LOCOS oxide film  131  for alleviating an electric field is formed in a predetermined part in the island-like semiconductor layer  103   b.  A gate electrode  127  is formed on the island-like semiconductor layer  103   b.    
   A buffer region  132 , for not to be affected by an electric interference from other adjacent elements such as the P-channel MOS transistor, is formed in the outer periphery region of the island-like semiconductor layer  103   b,  in other words in the outer periphery region adjacent to the island-like semiconductor layer  103   b  across an isolation trench  124 . On the surface of the buffer region  132 , a buffer contact layer  133  is formed, and a buffer electrode  113   b  is placed on a buffer contact layer  133 . Here, the buffer electrode  113   b  and the source electrode  109   b  are connected through a connection unit  134 . 
   According to the semiconductor device having the aforesaid structure, the same electric potential with the source region  109  of the P-channel MOS transistor is applied to the buffer region  112  adjacent to the P-channel MOS transistor, while the same electric potential with the source region  129  of the N-channel MOS transistor is applied to the buffer region  132  adjacent to the N-channel MOS transistor. In the generally used circuit like a CMOS circuit, a voltage of positive polarity for example power supply voltage is applied to the source region  109  of the P-channel MOS transistor, while a ground electric potential is applied to the source region of the N-channel MOS transistor for example. Thus, the electric potential in the buffer region  112  adjacent to the P-channel MOS transistor and the electric potential in the buffer region  132  adjacent to the N-channel MOS transistor are different, so that it is not possible to share the buffer region for the both transistors. The fact results in that an isolation trench  115  needs to be newly placed between the both buffer regions; therefore the device area is increased unexpectedly. 
   In order to avoid the aforesaid problem, it is necessary to remove the connection unit  114  and the connection unit  134 , and then to fix the electric potential in the buffer region  112  of the P-channel MOS transistor and the electric potential in the buffer region  132  of the N-channel MOS transistor to the same electric potential. For example, under a condition that the electric potential in the both buffer regions are fixed to the ground electric potential, as shown in  FIG. 3 , in the case where a high electric potential of positive polarity is applied to the source region  109  of the P-channel MOS transistor, the breakdown voltage is determined based on the concentration of the electric potential in the surface region between the source region and the isolation trench (region A in  FIG. 3 ). The fact results in a lowering of the breakdown voltage of the P-channel MOS transistor. On the contrary, under a condition that the voltage is fixed to a high electric potential of positive polarity in the both buffer regions, a lowering of the breakdown voltage of the N-channel MOS transistor is caused by the same reason with the aforesaid case. 
   In view of the problems, the object of the present invention is to provide a semiconductor device which enables to reduce the device area, while securing the breakdown voltage between the drain and the source of each MOS transistor for the semiconductor device including plural MOS transistors, which are arrayed adjacently each other, with different types of channel conductivity. 
   In order to achieve the aforesaid object, the semiconductor device according to the present invention is characterized in that it includes: a semiconductor substrate; a buried oxide film formed on the semiconductor substrate; and a semiconductor layer formed on the buried oxide film. The semiconductor layer has: a first island-like semiconductor layer, in which a first MOS transistor of a first conductivity type is formed, the first MOS transistor having a) a first body region, b) a first source region that is positioned in the first body region, c) and a first drain region that is positioned in the periphery of the first body region; a second island-like semiconductor layer, in which a second MOS transistor of a second conductivity type is formed, the second MOS transistor having a) a second drain region, b) a second body region that is positioned in the periphery of the second drain region, and c) a second source region that is positioned in the second body region; a first isolation trench, positioned in the periphery of the first island-like semiconductor layer, which isolates the first island-like semiconductor layer from other portions of the semiconductor layer; a second isolation trench, positioned in the periphery of the second island-like semiconductor layer, which isolates the second island-like semiconductor layer from other portions of the semiconductor layer; and a buffer region, formed between the first isolation trench and the second isolation trench, which prevents an electrical interference occurred between the first MOS transistor and the second MOS transistor. The electric potential of the buffer region is fixed to one of the lowest electric potential or the highest electric potential in a circuit. 
   In the aforesaid configuration, it is preferable that the first MOS transistor is a P-channel MOS transistor and the second MOS transistor is an N-channel MOS transistor, and the electric potential of the buffer region is fixed to the lowest electric potential in the circuit. 
   In the aforesaid configuration, it is preferable that the first MOS transistor is the N-channel MOS transistor and the second MOS transistor is the P-channel MOS transistor, and the electric potential of the buffer region is fixed to the highest electric potential in the circuit. 
   In the aforesaid configuration, it is preferable that the semiconductor layer further includes a drain buffer region between the first drain region and the first isolation trench, with a lower concentration impurity than the first drain region. 
   In the aforesaid configuration, it is preferable that the first drain region is adjacent to the first isolation trench in the semiconductor layer. 
   Thus, it is possible to share the buffer region for isolating the P-channel MOS transistor from the other elements and for isolating the N-channel MOS transistor from the other elements, so that the device area can be reduced. In addition, the electric potential applied to the buffer region and the drain can be reduced, so that the distance between the isolation trench and the source region can be shorter. Further in the P-channel MOS transistor and the N-channel MOS transistor, the conductivity types of the regions adjacent to the isolation trench are the same, and either the lowest electric potential or the highest electric potential in the circuit is applied to the shared buffer region, so that the lowering of the breakdown voltage between the drain and the source in the MOS transistor can be prevented. 
   As described above, according to the present invention it is possible to share the buffer region for isolating the P-channel MOS transistor from other elements and for isolating the N-channel MOS transistor from the other elements, so that an additional isolation trench between the two buffer regions is not needed and the device area can be reduced. In addition, the electric potential applied to the buffer region and the drain can be reduced, so that the distance between the isolation trench and the source region can be shorter. Further in the P-channel MOS transistor and the N-channel MOS transistor, the conductivity types of the regions adjacent to the isolation trench are the same, and also the electric potential applied to the shared buffer region is fixed to the lowest potential in the circuit, so that the lowering of the breakdown voltage between the drain and the source in the MOS transistor can be prevented. In fact, there is an effect that the device area can be reduced, while the breakdown voltage between the drain and the source can be maintained. 
   Further Information About Technical Background to This Application 
   The disclosure of Japanese Patent Application No. 2005-142232 filed on May 16, 2005 including specification, drawings and claims is incorporated herein by reference in its entirety. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the invention. 
     In the Drawings: 
       FIG. 1  is a cross-sectional diagram of a conventional P-channel MOS transistor disclosed in Japanese Laid-Open Patent Application No. H11-330383; 
       FIG. 2  is a cross-sectional diagram of a MOS transistor showing a potential distribution in the case where a ground electric potential is applied to a drain electrode and a high electric potential of positive polarity is applied to the source electrode in the MOS transistor in  FIG. 1 ; 
       FIG. 3  is a cross-sectional diagram of a MOS transistor showing a potential distribution in the case where a ground electric potential is applied to the buffer electrode and the drain electrode, and a high electric potential of positive polarity is applied to the source electrode under a condition that a connection unit is not placed in the MOS transistor shown in  FIG. 1 ; 
       FIG. 4  is a cross sectional diagram of a semiconductor device, in which the P-channel MOS transistor shown in  FIG. 2  and an N-channel MOS transistor adjacent to the P-channel MOS transistor, are formed; 
       FIG. 5  is a cross sectional diagram of the P-channel MOS transistor structuring a semiconductor device according to the embodiment for the present invention; 
       FIG. 6  is a cross sectional diagram of the MOS transistor showing the potential distribution in the case where the lowest electric potential in a circuit is applied to the buffer electrode, and a high electric potential of positive polarity is applied to the drain electrode in the P-channel MOS transistor according to the embodiment; 
       FIG. 7  is a cross sectional diagram of the semiconductor device according to the embodiment; and 
       FIG. 8  is a schematic top view of the semiconductor device of the embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Hereafter, a semiconductor device of an embodiment for the present invention will be described referring to the drawings. However the present invention is not limited merely to the embodiment mentioned below. 
   The semiconductor device of the embodiment includes a P-channel MOS transistor and an N-channel MOS transistor adjacent to the P-channel MOS transistor. 
     FIG. 5  is a cross sectional diagram of the P-channel MOS transistor structuring a semiconductor device according to the embodiment for the present invention. 
   As shown in  FIG. 5 , the P-channel MOS transistor includes an SOI substrate which has a semiconductor substrate  1 , a buried oxide film  2  of 1 to 3 μm in thickness placed on the semiconductor substrate  1 , and a P-type semiconductor layer  3  of 3 to 5 μm in thickness placed on the buried oxide film  2 . An island-like semiconductor layer  3   a,  in which the P-channel MOS transistor structure is formed, isolated from other portions of the semiconductor layer  3  by an isolation trench  4  in the outer periphery region of the island-like semiconductor layer  3   a,  is formed in the semiconductor layer  3 . 
   A body region  8 , which is an N-type impurity layer with a low concentration impurity, is formed in the center of the island-like semiconductor layer  3   a.  A source region  9 , which is a P-type impurity layer with a high concentration impurity, and a body contact region  10 , which is the N-type impurity layer with a higher concentration impurity than the body region  8  are formed in the body region  8 . A source electrode  9   a  is placed on the source region  9  and the body contact region  10 . The source region  9  and the body region  8  are electrically connected by the source electrode  9   a.  In addition, a gate electrode  7 , which is made of polycrystalline silicon, a drain region  5  as the P-type impurity layer with a low concentration impurity and a drain buffer region  5   a  as the P-type impurity layer with a lower concentration impurity than the drain region  5  are formed centering on the body region  8  in the outer periphery of the body region  8 . In fact, the drain region  5  and the drain buffer  5   a  are positioned on the opposite side of the source region  9  across the body region  8 . A drain contact region  6 , which is the P-type impurity layer with a higher concentration impurity than the drain region  5 , is formed on the surface of the drain region  5 , and furthermore a drain electrode  6   a  is placed on the surface of the drain contact region  6 . In addition, a LOCOS oxide film  11  of 300 to 500 μm in thickness for alleviating electric field is formed on a predetermined part in the island-like semiconductor layer  3   a,  and the LOCOS oxide film  11  of the present embodiment operates as a gate oxide film. 
   A buffer region  12 , which is the P-type impurity layer with a low concentration impurity, is formed in the outer periphery region of the island-like semiconductor layer  3   a,  that is in the outer periphery region adjacent to the island-like semiconductor layer  3   a  across the isolation trench  4  so as not to be affected by an electrical interference from other adjacent elements. On the surface of the buffer region  12 , a buffer contact layer  13 , which is the P-type impurity layer with a higher concentration impurity than the buffer region  12 , is formed, and a buffer electrode  13   a  is placed on the buffer contact layer  13 . 
   Accordingly, the semiconductor device of the present embodiment is characterized in that in the P-channel MOS transistor the source region  9  is formed in the center of the island-like semiconductor layer  3   a,  the drain region  5  and the drain buffer region  5   a  are formed centering on the source region  9  in the outer periphery of the source region  9 , and the electric potential of the buffer electrode  13   a,  which is adjacent to the drain region  5  and the drain buffer region  5   a  across the isolation trench  4 , is fixed to the lowest electric potential in the circuit. 
   This structure enables to prevent an occurrence of an avalanche breakdown in the surface region between the isolation trench  4  and the drain contact region  6  without increasing the distance between the isolation trench  4  and the drain contact region  6 , and also enables to prevent a decrease of a breakdown voltage between the drain and the source. The reason why such preventions can be realized will be described hereafter. 
   According to the generally used circuit like a CMOS circuit, a high electric potential of positive polarity is applied to the source electrode  9   a  of the P-channel MOS transistor, and the state of the drain electrode  6   a  ranges from a state where a high voltage of positive polarity is applied to a state where the lowest voltage in the circuit is applied. Considering that the electric potential of the buffer electrode  13   a  is fixed to the lowest electric potential in the circuit, in the case where a high electric potential of positive polarity is applied to the drain electrode  6   a,  the electric potential difference between a) the buffer region  12  and b) the drain region  5  and the drain buffer region  5   a  is increased to the maximum. Therefore, the occurrence of the avalanche breakdown needs to be considered. 
     FIG. 6  shows a part of the potential distribution (dotted and dashed lines) under a state that the electric potential between the buffer region  12  and the drain region  5  is increased, for example under a state that the gate of the P-channel MOS transistor is on, a high electric potential of positive polarity is applied to the source electrode  9   a,  and an electric potential of the drain electrode  6   a  is fixed to an electric potential between a higher electric potential than the buffer electrode  13   a  and a lower electric potential than the source electrode  9   a.  As shown in  FIG. 6 , the distribution of the potential ranges between the drain region  5  and the buffer region  12  (region B in  FIG. 6 ). The voltage added between the buffer region  12  and the drain region  5  includes a part of the voltage added between the drain region  5  and the body region  8 , so that the entire voltage applied to the drain electrode  6   a  is not added between the drain region  5  and the buffer region  12 . Thus, the voltage added between the buffer region  12  and the drain region  5  is the maximum in the case where the electric potential of the drain electrode  6   a  is an intermediate between the potential of the source electrode  9   a  and the potential of the buffer electrode  13   a.  The reason of the fact is that in the case where the electric potentials of the drain electrode  6   a  and the source electrode  9   a  are almost the same, there is no voltage added between the drain region  5  and the body region  8 . In addition, in the case where the electric potentials of the drain electrode  6   a  and the buffer electrode  13   a  are almost the same, there is no voltage added between the drain region  5  and the buffer region  12 . Therefore, the distance between the drain region  5  and the isolation trench  4  may be fixed to a value so as not to cause the avalanche breakdown in the aforesaid intermediate electric potential, so that the aforesaid distance can be shortened, and further the increase of the chip size can be restrained since a buffer region connected to the drain electrode  6   a  with the same electric potential is not necessary to be newly formed. 
   It should be noted that the drain region  5  is formed in the drain buffer region  5   a,  and the drain buffer region  5   a  is formed between the drain region  5  and the isolation trench  4  in  FIG. 6 . Alternatively, the drain region  5  may be formed adjacent to the isolation trench  4 . In this case, the aforesaid distance can be shorter, so that it is possible to further restrain the increase of the chip size. 
   In the case where the lowest electric potential in the circuit is applied to the drain electrode  6   a,  there does not exist a problem, because the electric potential difference between the drain region  5  and the drain buffer region  5   a,  and the buffer region  12  is not generated. 
   In fact, the potential distribution is affected by neither the high electric potential of positive polarity nor the lowest electric potential in the circuit applied to the drain electrode  6   a  in the P-channel MOS transistor having the aforesaid structure. Thus, the occurrence of the avalanche breakdown in the surface region between the isolation trench  4  and the drain contact region  6  can be prevented without increasing the distance between the isolation trench  4  and the drain contact region  6 , so that the decrease of the breakdown voltage between the drain and the source can be prevented. 
   Next, a semiconductor device structure, in which the aforesaid P-channel MOS transistor and the N-channel MOS transistor adjacent to the P-channel MOS transistor are formed, will be described hereafter.  FIG. 7  is a cross sectional diagram of the semiconductor device and  FIG. 8  is a schematic top view of the semiconductor device to describe the layout plan. Note that the shaded areas in the drawing indicate the isolation trenches  4 . 
   The P-channel MOS transistor structure is formed in the island-like semiconductor layer  3   a  in the semiconductor layer  3 , while the N-channel MOS transistor structure is formed in the island-like semiconductor layer  3   b  which is isolated from other portions of the semiconductor layer  3  by the isolation trench  24 . 
   A drain region  25  which is the N-type impurity layer with a low concentration impurity is formed, and also a drain contact region  26  which is the N-type impurity layer with a higher concentration impurity than the drain region  25  is formed in the center of the island-like semiconductor  3   b.  A drain electrode  6   b  is placed on the drain contact region  26 . In addition, a body region  28  which is the P-type impurity layer with a low concentration impurity is formed centering on the drain region  25  in the periphery of the drain region  25 . A source region  29  which is the N-type impurity layer with a high concentration impurity, and a body contact region  30  which is the P-type impurity layer with a higher concentration impurity than the body region  28  is formed in the body region  28 . In fact, the source region  29  is positioned facing the drain region  25  across the body region  28  in the periphery of the drain region  25 . A source electrode  9   b  is placed on the source region  29  and the body contact region  30 . The source region  29  and the body region  28  are electrically connected by the source electrode  9   b.  A LOCOS oxide film  31  for alleviating electric field is formed in a predetermined part in the island-like semiconductor layer  3   b,  and a gate electrode  27  is placed on the island-like semiconductor layer  3   b.    
   A buffer region  12 , which is the P-type impurity layer with a low concentration impurity, is formed in the outer periphery region of the island-like semiconductor layer  3   b,  in other words in the outer periphery region adjacent to the island-like semiconductor layer  3   b  across the isolation trench  24  in the periphery of the island-like semiconductor layer  3   b,  so as not to be affected by an electrical interference from other adjacent elements such as the P-channel MOS transistor. In fact, the buffer region  12  is formed between the isolation trench  4  of the P-channel MOS transistor and the isolation trench  24  of the N-channel MOS transistor (region C in  FIG. 7 ). On the surface of the buffer region  12 , the buffer contact layer  13 , which is the P-type impurity layer with a higher concentration impurity than the buffer region  12 , is formed, and the buffer electrode  13   a  is placed on the buffer contact layer  13 . 
   The electric potential of the buffer electrode  13   a  of the N-channel MOS transistor is fixed to the lowest electric potential in the circuit like the buffer electrode  13   a  of the P-channel MOS transistor. 
   According to the generally used circuit like the CMOS circuit, a high electric potential of positive polarity is applied to the drain electrode  6   b  of the N-channel MOS transistor, and the electric potential of the source electrode  9   b  is fixed to an intermediate electric potential of positive polarity in the circuit or the lowest electric potential in the circuit. Considering that the electric potential of the buffer electrode  13   a,  which is adjacent to the island-like semiconductor layer  3   b  across the isolation trench  24 , is fixed to the lowest electric potential in the circuit, in the case where an intermediate electric potential of positive polarity is applied to the source electrode  9   b,  the electric potential difference between the buffer region  12  and the source region  29  is increased to the maximum, so that the occurrence of the avalanche breakdown between the isolation trench  24  and the source region  29  needs to be considered. 
   However, according to the semiconductor device having the aforesaid structure, the body region  28  as the P-type impurity layer is applied with an electric potential which is a higher electric potential than the electric potential of the buffer region  12 , so that a depletion layer on the surface of the isolation trench  24  grows and extends over to the buffer region  12  side, not to the body region  28  side. Thus, the voltage added between the source region  29  and the buffer region  12  is added to the depletion layer extended to the buffer region  12  side and the isolation trench  24 , so that the electric potential is concentrated to the surface region of the buffer region  12 . As a result, the potential distribution at the body region  28  side of the N-channel MOS transistor is not affected by the voltage. 
   On the other hand, in the case where the lowest electric potential in the circuit is applied to the source electrode  9   b,  the electric potential difference between the drain region  25  and the buffer region  12  is not generated. 
   Accordingly, the potential distribution of the N-channel MOS transistor is affected by neither an intermediate electric potential of positive polarity nor the lowest electric potential in the circuit applied to the source electrode  9   b  in the N-channel MOS transistor having the aforesaid structure. Thus, the occurrence of the avalanche breakdown in the surface region between the isolation trench  24  and the source region  29  can be prevented without increasing the distance between the isolation trench  24  and the source region  29 , so that the decrease of the breakdown voltage between the drain and the source can be prevented. 
   It should be noted that the electric potential of the semiconductor substrate  1  is assumed to be fixed to the lowest electric potential in the circuit in the present embodiment. In fact, it is not limited to the exemplified electric potential of the present embodiment, and the electric potential is not necessary to be the lowest electric potential in the circuit. The electric potential which does not decrease the breakdown voltage between the drain and the source can be used actually. 
   As described hereinbefore, according to the semiconductor device of the present embodiment, in the semiconductor device in which the N-channel MOS transistor adjacent to the P-channel MOS transistor are formed, the conductivity types are fixed to the P type for the regions adjacent to the isolation trench of the P-channel MOS transistor and to the isolation trench of the N-channel MOS transistor, and also the electric potentials for the buffer electrodes of the N-channel MOS transistor and the P-channel MOS transistor are both fixed to the lowest electric potential in the circuit, so that the decrease of the both breakdown voltages between the drain and the source can be prevented. Additionally, the buffer regions of the N-channel MOS transistor and the P-channel MOS transistor can be shared, so that an additional isolation trench between the two buffer regions is not needed, and also the device area can be reduced. Thus, the semiconductor device, which enables to reduce the device area while securing the breakdown voltage between the drain and the source, can be implemented. 
   It should be noted that according to the present embodiment, the drain region is formed in the outer periphery of the source region in the P-channel MOS transistor, and the source region is formed in the outer periphery of the drain region in the N-channel MOS transistor, and also the body region is formed between the source region and the isolation trench, and the electric potentials of the buffer regions of the both MOS transistors are fixed to the lowest electric potential in the circuit. However it is evident that in the case where the source region is formed in the outer periphery of the drain region of the P-channel MOS transistor, the drain region is formed in the outer periphery of the source region of the N-channel MOS transistor region, the body region is formed between the drain region and the isolation trench, and the electric potentials of the buffer regions of the both MOS transistors are fixed to the highest electric potential, the same effect can be expected. 
   Although only an exemplary embodiment of this invention has been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention. 
   INDUSTRIAL APPLICABILITY 
   The present invention is applicable to a semiconductor device, and particularly to a semiconductor device and the like which are used for a driving circuit with a high breakdown voltage.