A nitride based heterojunction semiconductor device includes a gallium nitride (GaN) layer disposed on a substrate, an aluminum (Al)-doped GaN layer disposed on the GaN layer, a Schottky electrode disposed in a first area on the Al-doped GaN layer, an AlGaN layer disposed in a second area on the Al-doped GaN layer, and an ohmic electrode disposed on the AlGaN layer. The first area is different from the second area.