preferred embodiments of the present invention will now be described with reference to the accompanying drawings . first of all , fig1 and 2 collectively show a power ( high breakdown voltage ) vertical mosfet of a trench gate structure according to a first embodiment of the present invention , wherein fig1 is a cross sectional view of a power ( high breakdown voltage ) vertical mosfet of a trench gate structure according to the first embodiment of the present invention , and fig2 is a plan view showing a gate pattern on the upper surface of the mosfet shown in fig1 and a polysilicon pattern within the trench , and showing particularly the relationship between these gate pattern and the polysilicon pattern . in other words , fig1 and 2 show a unit cell portion of a semiconductor device . as shown in fig1 a trench gate structure that permits improving the gate density is employed in the power vertical mosfet in order to improve the on - voltage characteristics . an n - type epitaxial semiconductor layer is formed on a first main surface ( upper surface ) of an n - type silicon semiconductor substrate 11 . formed in the n - type epitaxial semiconductor layer are an n - type drain region ( n − - type drain region ) 6 , a p - type base region 5 , and an n - type source region ( n + - type source region ) 4 . the p - type base region 5 is formed in a surface region of the n - type drain region 6 . the n - type source region 4 is formed in a surface region of the p - type base region 5 . each of the p - type base region 5 and the n - type source region 4 is formed by an impurity diffusion . a gate trench is formed in a manner to extend downward from the surface of the n - type source region 4 through the n - type source region 4 and the p - type base region 5 to reach an inner region of the n - type drain region 6 , and a gate insulating film 7 such as a silicon oxide film is formed on the side wall and the bottom of the trench gate . a gate electrode 8 made of , for example , polysilicon is buried in the trench . the gate insulating film 7 and the gate electrode 8 are covered with an interlayer insulating film 3 such as a silicon oxide film . a barrier metal layer 2 is formed on the entire substrate surface including the interlayer insulating film 3 . further , a source electrode 1 consisting of a metal electrode such as an aluminum electrode is formed on the source region 4 and the base region 5 with the barrier metal layer 2 interposed therebetween so as to be connected to the source region 4 and the base region 5 . still further , a drain electrode 12 , which is a metal electrode , is formed on a second main surface ( back surface ) of the semiconductor substrate 11 . a p - type contact region ( p + - type contact region ) 14 consisting of a diffusion region of a high impurity concentration is formed in a surface region of the p - type base region 5 . also formed is a contact trench extending downward from the surface of the p - type base region 5 through the p - type contact region 14 and the p - type base region 5 to reach an inner region of the n - type drain region 6 . a p - type impurity diffusion region ( p − - type diffusion region ) 9 having an impurity concentration lower than that in the p - type base region 5 is formed on the side wall and the bottom of that portion of the contact trench which is positioned within the n - type drain region 6 . a conductive film 10 such as a polysilicon film is buried in the contact trench . in this embodiment , a conductive film of polysilicon is buried in each of the gate trench and the contact trench . however , the conductive film buried in the trench is not limited to the polysilicon film . it is also possible to bury in the trench another conductive material such as amorphous silicon or sipos ( amorphous sio ), or an insulating film such as a cvd silicon oxide film . further , it is possible to form an oxide film such as a silicon oxide film on the side wall and the bottom of the contact trench and to bury a conductive film such as a polysilicon film or an amorphous silicon film within the trench having the oxide film formed therein . fig1 shows a depletion layer 13 generated when a reverse bias is applied between the source electrode and the drain electrode of the mosfet of the construction shown in fig1 . in the depletion layer , an electric field is dispersed in the corner portion of the trench gate as denoted by arrows . to be more specific , since the p - type impurity diffusion layer 9 having a low impurity concentration , which is formed in the periphery of the side wall of the contact trench , causes the depletion layer to grow in a direction perpendicular to the gate trench during application of the reverse bias , the electric field concentration on the corner portion of the gate trench is relaxed so as to improve the breakdown voltage . by this technology , the breakdown voltage fully comparable with that in the prior art can be obtained even if the impurity concentration is increased in the epitaxial growth layer of silicon , making it possible to lower the resistance of the epitaxial layer , compared with the prior art . it should also be noted that , since a deep extended base region is partially formed in the base region because of the formation of the contact trench , a low resistivity layer of hole is formed . since the formation of the low resistivity layer facilitates the flow of the hole current in the step of the switching off , the switching time can be shortened . how to form the impurity diffusion layer 9 of a low impurity concentration will now be described . in the first step , the base region 5 and the source region 4 are formed by an impurity diffusion , followed by forming a contact trench deeper than the gate trench in the base region 5 . in other words , formed is a contact trench extending from the surface of the base region 5 through the base region 5 to reach an inner region of the n - type drain region . then , the material deposited on the side wall of the contact trench is removed , followed by implanting an impurity such as boron by a rotary implantation ( implanting angle of 7 °) under an accelerating energy of 60 kev and at a dose of 2e12 / cm 2 . then , the implanted boron ions are diffused at 1100 ° c . for 30 minutes under an n 2 atmosphere , followed by changing the atmosphere to n 2 / o 2 of 10 / 1 and carrying out the diffusion at 1100 ° c . for 2 hours , so as to form the impurity ( boron ) diffusion region 9 of a low impurity concentration . then , the oxide film formed inside the contact trench is removed , followed by burying polysilicon within the contact trench . further , the buried polysilicon is etched back by a cde ( chemical dry etching ) process . the subsequent steps are equal to those of the conventional method . the semiconductor device thus manufactured was found to exhibit an on - resistance of 2 . 6ω ( where vg = 15v and vd = 2v ), which is about 1 / 4 . 6 of 12ω for the prior art . fig2 shows that the gates are arranged in parallel . alternatively , it is possible for the gates to be arranged in the shape of a ladder as shown in fig3 . fig3 is a plan view directed to a modification of the first embodiment and showing a gate pattern on the upper surface of a vertical mosfet and a polysilicon pattern within the trench , and showing particularly the relationship between these gate pattern and the polysilicon pattern . fig4 shows a power mosfet of a planar structure according to a second embodiment of the present invention . fig4 is a cross sectional view showing a power mosfet of a planar structure according to the second embodiment of the present invention . in other words , fig4 shows a unit cell portion of a semiconductor device . as shown in the drawing , an n - type epitaxial semiconductor layer ( n - type drain region ) 26 is formed on a first main surface ( upper surface ) of an n - type silicon semiconductor substrate 211 . a p - type base region 25 is formed in a surface region of the n - type drain region 26 . also , an n - type source region 24 is formed in a surface region of the p - type base region 25 . a gate insulating film 27 such as a silicon oxide film is formed on the n - type source region 24 , the p - type base region 25 and the n - type drain region 26 except the region where a source electrode is to be formed . also , a gate electrode 28 consisting of , for example , polysilicon is formed on the gate insulating film 27 . these gate insulating film 27 and gate electrode 28 are covered with an interlayer insulating film 23 such as a silicon oxide film . a source electrode 21 consisting of a metal electrode such as an aluminum electrode is formed on the n - type source region 24 and the p - type base region 25 so as to be electrically connected to the n - type source region 24 and the p - type base region 25 . further , a drain electrode 212 is formed on a second main surface ( back surface ) of the semiconductor substrate 211 . further , formed is a contact trench extending from the surface of the base region 25 through the base region 25 to reach an inner region of the n - type drain region . still further , a p - type impurity diffusion region ( p − - type diffusion region ) 29 having an impurity concentration lower than that in the p - type base region 25 is formed in the side wall and the bottom of that portion of the contact trench which is positioned within the n - type drain region 26 . a conductive film 20 such as a polysilicon film is buried in the contact trench . in this embodiment , a conductive film of polysilicon is buried in the contact trench . however , the conductive film buried in the trench is not limited to the polysilicon film . it is also possible to bury in the trench another conductive material such as amorphous silicon or sipos ( amorphous sio ), or an insulating film such as a cvd silicon oxide film . further , it is possible to form an oxide film such as a silicon oxide film on the side wall and the bottom of the contact trench and to bury a conductive film such as a polysilicon film or an amorphous silicon film within the trench having the oxide film formed therein . how to form the impurity diffusion region 29 of a low impurity concentration will now be described . in the first step , the base region 25 and the source region 24 are formed by an impurity diffusion , followed by forming a contact trench deeper than the gate trench in the base region 25 . in other words , formed is a contact trench extending from the surface of the base region 25 through the base region 25 to reach an inner region of the n - type drain region . then , the material deposited on the side wall of the contact trench is removed , followed by implanting an impurity such as boron by a rotary implantation ( implanting angle of 7 °) under an accelerating energy of 60 kev and at a dose of 2e12 / cm 2 . then , the implanted boron ions are diffused at 1100 ° c . for 30 minutes under an n 2 atmosphere , followed by changing the atmosphere to n 2 / o 2 of 10 / 1 and carrying out the diffusion at 1100 ° c . for 2 hours , so as to form the impurity ( boron ) diffusion region 29 of a low impurity concentration . then , the oxide film formed inside the contact trench is removed , followed by burying polysilicon within the contact trench . further , the buried polysilicon is etched back by a cde ( chemical dry etching ) process . the subsequent steps are equal to those of the conventional method . in this embodiment , an electric field is dispersed in the extended portion of the p - type base region 25 . to be more specific , since the p - type impurity diffusion layer 29 having a low impurity concentration , which is formed in the periphery of the side wall of the contact trench , causes the depletion layer to grow in a direction perpendicular to the gate trench during application of the reverse bias , the electric field concentration on the extended portion of the p - type base region 25 is relaxed so as to improve the breakdown voltage . by this technology , the breakdown voltage fully comparable with that in the prior art can be obtained even if the impurity concentration is increased in the epitaxial growth layer of silicon , making it possible to lower the resistance of the epitaxial layer , compared with the prior art . it should also be noted that , since a deep extended base region is partially formed in the p - type base region 25 because of the formation of the contact trench , a low resistivity layer of hole is formed . since the formation of the low resistivity layer facilitates the flow of the hole current in the step of the switching off , the switching time can be shortened . a power ( high breakdown voltage ) igbt of a trench gate structure according to a third embodiment of the present invention will now be described with reference to fig5 . fig5 is a cross sectional view showing a power igbt of a trench gate structure according to a third embodiment of the present invention . in other words , fig5 shows a unit cell portion of a semiconductor device . the power igbt employs a trench gate structure that permits improving the gate density for improving the on - voltage characteristics . a buffer layer 315 consisting of an n + - type epitaxial silicon semiconductor layer is formed on a first main surface ( upper surface ) of a p - type silicon semiconductor substrate 311 , and an n - type epitaxial semiconductor layer is formed on the buffer layer 315 . the impurity concentration in the buffer layer 315 is higher than that in the n - type epitaxial semiconductor layer formed on the buffer layer 315 . an n - type drain region ( n − - type drain region ) 36 , a p - type base region 35 , and an n - type source region ( n − - type source region ) 34 are formed in the n - type epitaxial semiconductor layer positioned on the buffer layer 315 . the p - type base region 35 is formed in a surface region of the n − - type drain region 36 . the n - type source region 34 is formed in a surface region of the p - type base region 35 . each of these p - type base region 35 and n - type source region 34 is formed by impurity diffusion . also formed is a trench gate extending from the surface of the n - type source region 34 to reach the n - type drain region 36 . a gate insulating film 37 consisting of , for example , a silicon oxide film is formed on the side wall and the bottom of the trench gate . also , a gate electrode 38 made of , for example , polysilicon is buried in the gate trench . the gate insulating film 37 and the gate electrode 38 are covered with an interlayer insulating film 33 such as a silicon oxide film . further , a source electrode 31 consisting of a metal electrode such as an aluminum electrode is formed on the source region 34 and the base region 35 with a barrier metal layer 32 interposed therebetween so as to be electrically connected to the source region 34 and the base region 35 . still further , a drain electrode 312 , which is a metal electrode , is formed on a second main surface ( back surface ) of the semiconductor substrate 311 . a p - type contact region ( p + - type contact region ) 314 consisting of an impurity diffusion region of a high impurity concentration is formed in a surface region of the p - type base region 35 . also formed is a contact trench extending downward from the surface of the p - type base region 35 through the p - type contact region 14 and the p - type base region 5 to reach an inner region of the n - type drain region 36 . further , a p - type impurity diffusion region ( p − - type impurity diffusion region ) 39 having an impurity concentration lower than that in the p - type base region 35 is formed on the side wall and the bottom of that portion of the contact trench which is positioned within the n - type drain region 36 . a conductive film 30 made of , for example , polysilicon is buried in the contact trench . it is possible to form an oxide film such as a silicon oxide film on the side wall and bottom within the contact trench and to bury a conductive film such as a polysilicon film or an amorphous silicon film within the trench surrounded by the oxide film . in this embodiment , a conductive film of polysilicon is buried in each of the gate trench and the contact trench . however , the conductive film buried in the trench is not limited to the polysilicon film . it is also possible to bury in the trench another conductive material such as amorphous silicon or sipos ( amorphous sio ), or an insulating film such as a cvd silicon oxide film . further , it is possible to form an oxide film such as a silicon oxide film on the side wall and the bottom of the contact trench and to bury a conductive film such as a polysilicon film or an amorphous silicon film within the trench surrounded by the oxide film . also , in the igbt according to this embodiment , the impurity diffusion layer 39 of a low impurity concentration , which is formed in the periphery of the side wall of the contact trench , causes the depletion layer to grow in a direction perpendicular to the gate trench during application of the reverse bias , with the result that the electric field concentration on the corner portion of the gate trench is relaxed so as to improve the breakdown voltage . by this technology , the breakdown voltage fully comparable with that in the prior art can be obtained even if the impurity concentration is increased in the epitaxial growth layer of silicon , making it possible to lower the resistance of the epitaxial layer , compared with the prior art . it should also be noted that , since a deep extended base region is partially formed in the p - type base region because of the formation of the contact trench , a low resistivity layer of hole is formed . since the formation of the low resistivity layer facilitates the flow of the hole current in the step of the switching off , the switching time can be shortened . fig6 shows a power mosfet of a planar structure according to a fourth embodiment of the present invention . in other words , fig6 shows a unit cell portion of a semiconductor device . a buffer layer 415 consisting of an n + - type epitaxial silicon semiconductor layer is formed on a first main surface ( upper surface ) of a p - type silicon semiconductor substrate 411 , and an n - type epitaxial semiconductor layer ( n − - type drain region ) 46 is formed on the buffer layer 415 . the impurity concentration in the buffer layer 415 is higher than that in the n - type epitaxial semiconductor layer 46 formed on the buffer layer 415 . a p - type base region 45 is formed in a surface region of the n - type drain region 46 . also , an n - type source region 44 is formed in a surface region of the p - type base region 45 . a gate insulating film 47 such as a silicon oxide film is formed on the n - type source region 44 , the p - type base region 45 and the n - type drain region 46 except the region where a source electrode is to be formed . also , a gate electrode 48 consisting of , for example , polysilicon is formed on the gate insulating film 47 . these gate insulating film 47 and gate electrode 48 are covered with an interlayer insulating film 43 such as a silicon oxide film . a source electrode 41 consisting of a metal electrode such as an aluminum electrode is formed on the n - type source region 44 and the p - type base region 45 so as to be electrically connected to the n - type source region 44 and the p - type base region 45 . further , a drain electrode 412 is formed on a second main surface ( back surface ) of the semiconductor substrate 411 . further , formed is a contact trench extending from the surface of the base region 45 through the base region 45 to reach an inner region of the n - type drain region 46 . still further , a p - type impurity diffusion region ( p − - type diffusion region ) 49 having an impurity concentration lower than that in the p - type base region 45 is formed in the side wall and the bottom of that portion of the contact trench which is positioned within the n - type drain region 46 . a conductive film 40 such as a polysilicon film is buried in the contact trench . in this embodiment , a conductive film of polysilicon is buried in the contact trench . however , the conductive film buried in the trench is not limited to the polysilicon film . it is also possible to bury in the trench another conductive material such as amorphous silicon or sipos ( amorphous sio ), or an insulating film such as a cvd silicon oxide film . further , it is possible to form an oxide film such as a silicon oxide film on the side wall and the bottom of the contact trench and to bury a conductive film such as a polysilicon film or an amorphous silicon film within the trench having the oxide film formed therein . also in this embodiment , an electric field is dispersed in the extended portion of the p - type base region 45 . to be more specific , since the p - type impurity diffusion layer 49 having a low impurity concentration , which is formed in the periphery of the side wall of the contact trench , causes the depletion layer to grow in a direction perpendicular to the gate trench during application of the reverse bias , the electric field concentration on the extended portion of the p - type base region 45 is relaxed so as to improve the breakdown voltage . by this technology , the breakdown voltage fully comparable with that in the prior art can be obtained even if the impurity concentration is increased in the epitaxial growth layer of silicon , making it possible to lower the resistance of the epitaxial layer , compared with the prior art . it should also be noted that , since a deep extended base region is partially formed in the p - type base region 45 because of the formation of the contact trench , a low resistivity layer of hole is formed . since the formation of the low resistivity layer facilitates the flow of the hole current in the step of the switching off , the switching time can be shortened . to reiterate , in the present invention , the impurity diffusion layer of a low impurity concentration , which is formed in the periphery of the side wall of the contact trench , causes the depletion layer to grow in a direction perpendicular to the gate trench during application of the reverse bias , with the result that the electric field concentration on the corner portion of the gate trench or on the extended portion of the base region is relaxed so as to improve the breakdown voltage . by this technology , the breakdown voltage fully comparable with that in the prior art can be obtained even if the impurity concentration is increased in the epitaxial growth layer of silicon , making it possible to lower the resistance of the epitaxial layer , compared with the prior art . it should also be noted that , since a deep extended base region is partially formed in the p - type base region , a low resistivity layer of hole is formed . since the formation of the low resistivity layer facilitates the flow of the hole current in the step of the switching off , the switching time can be shortened . additional advantages and modifications will readily occur to those skilled in the art . therefore , the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein . accordingly , various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents .