In recent years, the terahertz plasma effects in high-mobility electronic systems have attracted much attention theoretically and experimentally. Plasma excitation in the terahertz regime can be used for generation, detection, and frequency multiplication of terahertz radiation. A channel of a field-effect transistor with sufficiently high electron mobility can serve as a resonant cavity for the plasma oscillation. When the signal period is in the vicinity of the electron transit time, self-excitation of plasma oscillation can occur. If the typical plasma frequency is in the terahertz regime, the phenomenon can be used as a terahertz source.