I. Introduction
The numerous possible applications in the terahertz (THz) range, such as medical/security imaging, spectroscopy, and communication, have attracted intensive research interest toward THz solid-state integrated electronics [1], [2]. In these applications, THz signal sources are needed to provide the output signal in a transmitter or provide the local oscillator signal in a heterodyne transceiver [3]. Conventionally, THz circuits are mainly based on III–V technologies, which are expensive and cannot be integrated with high-density digital circuits. SiGe technologies are also good candidates in designing THz signal sources. Research in recent years has reported the good performance of THz signal sources in SiGe technologies [3]–[10]. CMOS technologies have the advantage of low cost in high-volume production and full integration with digital circuits. However, the low and low supply voltage of CMOS technologies make it more challenging to design THz signal sources in CMOS technologies compared to SiGe and III–V technologies.