Semiconductor device and its mfg. method

A semiconductor device includes a first-conductivity-type semiconductor substrate (101), a second-conductivity-type source layer (102) formed in a surface in the semiconductor substrate (101), a second-conductivity-type drain layer (103) formed in the surface in the semiconductor substrate (101) to oppose the source layer (102) with a predetermined interval, a gate insulating film (104) formed on the semiconductor substrate (101), a gate layer (105) formed on the gate insulating film (104) between the source layer (102) and drain layer (103), source electrodes (108) formed on the semiconductor substrate (101) and electrically connected to the source layer (102), a drain electrode (109) formed on the semiconductor substrate (101) and electrically connected to the drain layer (103), a gate electrode (107) formed on the semiconductor substrate (101) and electrically connected to the gate layer (105), and a capacitance adjusting electrode (114) formed on the gate insulating film (104).