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Existing and emerging applications of terahertz technology in imaging, medicine, biology, space exploration, covert communications, compact radar ranges, industrial controls, terahertz microscopy, terahertz tomography, and homeland security have stimulated intensive research effort in photonics and electronics technologies bracketing the famous terahertz gap from the high and low frequency sides, respectively. Cutoff frequencies and maximum frequencies of operation of InGaAs-based heterostructure bipolar transistors and high electron mobility transistors are now approaching or even exceeding 600 GHz. New ideas of using plasma resonances of 2D electrons for tunable detection and emission of terahertz radiation are being explored and proven experimentally. Plasma effects in polarization-induced electrons and holes in granular pyroelectric/semiconductor heterostructures hold promise of an active THz medium tunable by external electric field or light.