Applications of terahertz (THz) and sub-THz technology require portable, inexpensive, efficient, and reliable THz electronics. This stimulated the search for novel semiconductor materials for THz applications. This paper evaluates the potential of ultra-wide bandgap semiconductors (UWBGS) for THz and sub-THz applications. The wide bandgap is the consequence of strong bonds between the nearest neighbors in UWBGS, such as diamond, boron nitride, and aluminum nitride. This is the consequence of small atomic radii of carbon and nitrogen forming intimate contacts with their nearest neighbors. The resulting high energies of optical phonons in these materials lead to superior transport properties, including high breakdown voltages and long mean free paths of electrons in the conduction band and holes in the valence band. These factors make ultra-wideband semiconductors prime candidates for high-power and high-frequency applications. The analysis of the cutoff frequencies and quality factor of plasma oscillations show that diamond has a high potential as an active material for Terahertz Field Effect Transistors (TeraFETs) generating THz and sub-THz radiation, including radiation in the 300 GHz range, which is of special interest for 6G communications. Boron nitride should enable high cutoff frequencies of operation in collision-dominated regimes.