“…Their (0.1-10) × 10 12 Hz frequency range is suitable for a wide variety of applications, such as wireless communications, spectroscopy, biomedical and molecular imaging and high-speed signal processing. Consequently, Gunn diodes [37,38], resonant tunnelling diodes [39,40], quantum cascade lasers [41,42] and emitters based on transistors [43,44] have been investigated for use in THz oscillators, and a high electron mobility transistor [45][46][47], an HBT [48,49] operated at several hundred GHz and a metal-insulator-metal THz detector [50,51] have been developed.…”