SEMICONDUCTOR DEVICE

PROBLEM TO BE SOLVED: To increase a current amplification factor of a bipolar transistor in a semiconductor device using a substrate having an insulation layer on a bottom part of a semiconductor layer in a formation region of the bipolar transistor.SOLUTION: A bipolar transistor comprises: a collector 5, a base 7, an emitter 9, a base high concentration ohmic diffusion layer 11, and a collector high concentration ohmic diffusion layer 13, which are formed in a semiconductor layer 1c; and a gate insulation film 15 and a gate electrode 17, which are formed on the semiconductor layer 1c. The base 7 is formed at a depth reaching an insulation layer 1b, formed so as to partially overlap the gate electrode 17 when viewed from above and formed under the gate electrode 17 adjacently to the collector 5. The base 7 has a concentration gradient in which a P-type impurity concentration decreases with distance from an end of the gate electrode 17 on the base 7 side toward the collector 5 side.