A semiconductor device that generates or detects terahertz waves includes a semiconductor layer that has a gain of the generated or detected terahertz waves; a first electrode connected to the semiconductor layer; a second electrode that is arranged at a side opposite to the side at which the first electrode is arranged with respect to the semiconductor layer and that is electrically connected to the semiconductor layer; a third electrode electrically connected to the second electrode; and a dielectric layer that is arranged around the semiconductor layer and the second electrode and between the first electrode and the third electrode and that is thicker than the semiconductor layer. The dielectric layer includes an area including a conductor electrically connecting the second electrode to the third electrode. The area is filled with the conductor.

An oscillation element that oscillates an electromagnetic wave includes a negative resistance element and a resonator including a first conductor and a second conductor, in which the negative resistance element and the resonator are arranged on a substrate, the negative resistance element is electrically connected to the first conductor and the second conductor, the first conductor and the second conductor are capacitively coupled to each other, and when a capacitance between the first conductor and the second conductor is set as C, an inductance of the first conductor and the second conductor is set as L.sub.1, a speed of the oscillated electromagnetic wave in vacuum is set as C.sub.0, a relative dielectric constant of the substrate is set as .sub.r, and a diagonal line length of the substrate is set as d, a series resonant frequency f.sub.1 of the resonator satisfies f.sub.1=1/{2(L.sub.1C)}, and f.sub.1<C.sub.0/[d{(1+.sub.r)/2}].