Progress in development of the resonant tunneling diodes as promising compact sources at the THz gap bottom

In recent years remarkable progress has been made in filling the “terahertz gap” of compact sources from the “optical” side using quantum cascade lasers and difference frequency generation systems. On the “electronic” side the range up to 0.5 THz is covered by several devices like Gunn diodes, IMPATT diodes and HEMT transistors. However for the most difficult central range 0.5-2 THz, which includes frequencies for detection of explosives and several airborne threats, along with the popular semi-compact Schottky diodes based frequency multiplier systems, the resonant tunneling diodes (RTD) as micrometer-size range ultrafast electronic devices able to operate at room temperature may offer a real solution for the design of compact portable equipment. It has been predicted that frequency limit for operation of the RTDs may extend up to 3 THz if the problem of the low, microwatt-range output power could be solved. Over the last 10 years remarkable progress has been achieved in increasing the output power of RTDs by almost 2 orders of magnitude, and in extending the operation frequencies from earlier 0.7 THz range to the values near 2 THz, thus making RTDs competitive with the Schottky diode based multipliers. The chapter compares the RTDs with other compact THz sources and discusses the design approaches that have yielded remarkable increase of power and frequency of RTDs.