A semiconductor element for generating or detecting a terahertz wave is provided. The element includes a substrate, a first electrode, a semiconductor layer disposed between the substrate and the first electrode and including a gain medium for the terahertz wave, a dielectric layer disposed to cover the substrate, and a second electrode disposed on the dielectric layer and connected to the first electrode via an opening provided in the dielectric layer. A portion of the second electrode disposed in the opening includes a first inclined portion, a second inclined portion disposed between the first inclined portion and the first electrode and is less inclined than the first inclined portion, and an intermediate portion connecting the first and second inclined portions. The intermediate portion includes a planar terrace and the terrace is less inclined than the first and second inclined portions.