A semiconductor device includes a semiconductor substrate of n-type silicon including, in an upper portion thereof, a first polarity inversion region and a second polarity inversion regions spaced from each other and doped with a p-type impurity. A first HFET including a first active layer and a second HFET including a second active layer both made of a group III-V nitride semiconductor are independently formed on the respective polarity inversion regions in the semiconductor substrate, and the HFETs are electrically connected to each other through interconnects.