A semiconductor device has a structure reducing resistances to a high frequency current. The semiconductor device includes a semi-insulating substrate, a first n-type layer made of a compound semiconductor, and a first p-type layer made of a compound semiconductor in which a signal current flows in a lateral direction, parallel to the semi-insulating substrate. The first p-type layer is sandwiched between the semi-insulating substrate and the first n-type layer. A second n type layer made of a compound semiconductor is between the semi-insulating substrate and the first p type layer. An alternating current component of the signal current flows through the second n type layer.
