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Summary form only given. In spite of the recent progress in quantum-cascade lasers, vacuum microelectronics, and transistors based on Si, InP and GaN, the search for compact THz sources continues. One alternative approach involves hetero-structures of two-dimensional (2D) layers which exploit the unique properties of graphene, silicene, germanene, BN, MoS2, NbSe2, etc. For example, basically, graphene is a conductor, 2D BN is an insulator, 2D MoS2 is a semiconductor, and 2D NbSe2 is a superconductor. The quality of graphene grown on hexagonal BN substrates is already approaching that of exfoliated graphene, while 2D BN of variable quality has been grown on graphene. Still, challenges remain in growing 2D layers on top of each other without metal catalysts by using van der Waals epitaxy and other techniques for uniform growth over large areas. Another alternative approach involves complex oxides and chalcogenides of transition metals, which can be deposited on almost any substrate with fault tolerance and radiation hardness. For example, ZnO transistors have already been demonstrated at microwave frequencies, while the interface of LaAlO3 and SrTiO3 has been found to contain a 2D electron density of 1014/cm2, which is two orders of magnitude higher than that in Si and InP transistors and one order of magnitude higher than that in GaN transistors. However, the electron mobility generally decreases with increasing ionicity of these ionic compounds. Yet, with correlated states and transport for electrons at such high concentrations, they may collectively exhibit high mobility. Other novel phenomena such as metal-insulator transition and topologically preserved states can also be exploited.