hereinafter , a preferred embodiment of the present invention is described with reference to the drawings . fig1 a and 1b are schematic sectional views showing a resistor of a semiconductor device according to a first embodiment of the present invention . fig1 b is a view showing the semiconductor device of fig1 a , which is viewed from above . fig1 a is a sectional view of fig1 b , which is taken along a line a - a ′ of fig1 b . a p - type epitaxial layer 202 is formed on an n - type silicon semiconductor substrate 201 , and two types of trenches 205 and 206 having different depths are formed in a portion of the p - type epitaxial layer 202 , which is not a resistor . the depth of the trench 205 is less than a full depth of the p - type epitaxial layer 202 , and there remains the p - type epitaxial layer 202 on a bottom of the trench 205 . the depth of the trench 206 is deeper than the depth of the trench 205 , and the trench 206 reaches the n - type silicon semiconductor substrate 201 located under the p - type epitaxial layer 202 and divides the p - type epitaxial layers 202 adjacent to each other . further , the trenches 205 and 206 are filled with an insulating film 203 for electrically insulating resistors , and an epitaxial layer 207 which becomes a resistor is provided with p - type heavily - doped regions 204 whose impurity concentration is increased so as to have sufficient contact with an aluminum wire . the p - type epitaxial layer 202 which becomes a resistor has a three - dimensional u - shape . note that the n - type silicon semiconductor substrate does not always need to be used , and an n - type epitaxial layer may be formed using a p - type silicon semiconductor substrate to form the resistor . the p - type heavily - doped region 204 is formed of bf 2 , and an impurity concentration during the formation is 1e15 atm / cm 3 . note that bf 2 does not always need to be used , and b may be used . the impurity concentration during the formation does not always need to be 1e15 atm / cm 3 , and if equal to or more than 1e15 atm / cm 3 , sufficient contact can be obtained . the trench includes the first trench 205 and the second trench 206 . the second trench 206 is formed to electrically insulate the adjacent resistors from each other , while the first trench 205 is formed to three - dimensionally arrange the resistor . depending on the depth of the first trench 205 , a thickness and a length of the resistor can be varied with ease , whereby the resistor can be easily set to have an appropriate resistance value . when the p - type epitaxial layer 202 has a thickness ranging from 0 . 1 μm to 10 μm , and more preferably from 0 . 5 μm to 5 . 0 μm , the resistor with high specific accuracy can be formed , and thus the aforementioned first object can be achieved . if the thickness of the p - type epitaxial layer 202 is thin , the resistor with a large resistance value cannot be formed in a small area , and thus the thickness of the epitaxial layer may be equal to or more than 0 . 1 μm , and preferably equal to or more than 0 . 5 μm . meanwhile , when the thickness thereof is large , processing accuracy of the trench decreases , and the stable resistance value cannot be obtained . in particular , sufficient characteristics cannot be obtained in a circuit such as a breeder resistance circuit , in which high specific accuracy is required , and thus the thickness of the epitaxial layer 202 may be equal to or less than 10 μm , and preferably equal to or less than 5 . 0 μm . further , a specific resistance of the p - type epitaxial layer 202 is set to 0 . 01 to 150 ωcm , and preferably 1 to 50 ωcm . the first object is attained with the configuration described above . if the specific resistance of the epitaxial layer is extremely small , the resistor with a large resistance value cannot be formed in a small area , and thus the specific resistance thereof may be set to equal to or more than 0 . 01 ωcm , and preferably equal to or more than 1 ωcm . meanwhile , extremely large specific resistance causes large variations in the resistance value , whereby a stable resistance value cannot be obtained . in particular , the sufficient characteristics cannot be obtained in the circuit such as the breeder resistance circuit , in which high specific accuracy is required , and hence the specific resistance of the epitaxial layer may be set to equal to or less than 150 ωcm , and more preferably equal to or less than 50 ωcm .