Millimeter-wave and THz frequencies (f > 100 GHz) remain one of the most underdeveloped frequency ranges, even though potential applications in remote sensing, spectroscopy, plasma diagnostics, and communications are obviously great. This is because the millimeter wave and far-infrared frequency range falls between two other frequency ranges in which conventional semiconductor devices are usually operated. One is the microwave frequency range, and the other is the near-infrared and optical frequency range. Semiconductor devices which utilize the classical diffusive transport of electrons, such as diodes and transistors, have a high frequency limit. This limit is set by the transient time and parasitic RC time constants. Currently, electron mobility and the smallest feature size which can be fabricated by lithography limit the frequency range to below several hundred GHz. Semiconductor devices based on quantum mechanical interband transitions, however, are limited to frequencies higher than those corresponding to the semiconductor energy gap, which is higher than 10 THz for most bulk semiconductors. Therefore, a large gap exists from 100 GHz to 10 THz in which very few devices are available.