Provided are a semiconductor device and a bidirectional field effect transistor which can easily overcome the tradeoff relation between the high voltage resistance and high speed in the semiconductor device using a polarization super junction, realize both the high voltage resistance and elimination of the occurrence of current collapse, operate at a high speed, and further the loss is low. The semiconductor device comprises a polarization super junction region and a p-electrode contact region. The polarization super junction region comprises an undoped GaN layer 11, an undoped AlxGa1-xN layer 12 with a thickness not smaller than 25 nm and not larger than 47 nm and 0.17≤x≤0.35, an undoped GaN layer 13 and a p-type GaN layer 14. When the reduced thickness tR is defined as tR=u+v(1+w×10−18) for the thickness u [nm] of the undoped GaN layer 13, the thickness v [nm] and the Mg concentration w [cm−3] of the p-type GaN layer 14, tR≥0.864/(x−0.134)+46.0 [nm] is satisfied. The p-electrode contact region comprises a p-type GaN contact layer formed to be in contact with the p-type GaN layer 14 and a p-electrode that is in contact with the p-type GaN contact layer.