PURPOSE: To improve high frequency characteristics and breakdown strength by preparing an N-type island region and at the same time making the thickness of an insulating film below a gate electrode thinner than those of insulating films at other parts.

CONSTITUTION: Even though an arrangement of a N-type island region 7 makes P-type regions 5 and 5' deeper, it also makes a length between N-type source regions 6 and 6' of P-type base regions 5 and 5' that result in a real channel region below a gate electrode 9 and the N-type island region 7 shorter and further, makes a part below the N-type source regions 6 and 6' of the P-type regions 5 and 5' thicker. Since the real channel region is short and lower of the N-type source regions 6 and 6' of the p-type regions 5 and 5' can be thicker, the inner resistance can be reduced to obtain favorable high frequency characteristics. Subsequently, it is difficult for a depletion layer developed from the P-type regions 5 and 5' in the N-type island region 7 but the gate electrode 9 is so located on the island region 7 that it is easy for the depletion layer to extend in the island region. Then, in addition to permitting a drain current to be taken out by passing through the P-type regions 5 and 5', the above arrangement makes it possible to perform high breakdown strengthening.