ABSTRACT
251Traditionally, terahertz (THz) applications have been developed using photonic-based systems with THz time-domain spectroscopy, parametric light sources, and single traveling carrier photodiodes. However, compact THz sources using semiconductor electronic/optical devices have recently been developed [1], and a highly compact THz application system is expected to be realized in the near future. Among such electronic THz sources, resonant tunneling diodes (RTDs) have long been known for their high-frequency operation. Initially, a high-frequency oscillator was developed using waveguide-type resonators, and in 1991, 712 GHz fundamental oscillation was achieved [2]. Research on planar integration of the oscillator circuit was then conducted, and a 650 GHz fundamental oscillation was obtained [3]. Subsequently, further efforts to develop RTD oscillators have been conducted. Since the frequency was updated in 2009 [4], it has been continuously improved [5–12], and a fundamental oscillation frequency of approximately 2 THz has 252been achieved, which is the highest oscillation frequency among room-temperature electronic single oscillators [12]. A milliwatt-class output power was also obtained at approximately 1 THz with a large-scale oscillator array [13]. Alongside a high-frequency and high output power, various characteristics, including wide frequency-tuning and high-frequency modulation, have been developed. Utilizing these characteristics, a wide range of applications, such as wireless communication, imaging, radar, sensing, and analysis, have been conducted using RTD devices. This chapter summarizes the characteristics and functions of RTD oscillators and their applications.
