Millimeter-wave spectrum covers the frequency range of 30 – 300 GHz, which corresponds to the wavelength of 1 cm – 1 mm. Traditionally, this frequency range was considered exceptionally high and thus the application was limited to special-purpose areas such as military or space-related applications. However, with the recent rapid progress in the development of low cost Si-based technologies, the performance of Si RFCMOS and SiGe technologies became comparable to those of III-V technologies and now their device speed has reached several hundreds of GHz. Such technological advances, combined with the ever-increasing demand for raised system operation speed and wider bandwidth, have enabled the various commercial applications of millimeter-wave spectrum. There are growing applications on WPAN (Wireless Personal Area Network) systems at 60 GHz, automotive radar systems at 77 GHz, image sensing systems at 94 GHz, etc. Furthermore, the recently emerging 5G mobile communication systems is expected to be based on mm-wave or near-mm-wave frequency ranges. In HSISL, various research activities are going on related to the key millimeter-wave circuits such as low noise amplifiers (LNAs), voltage-controlled oscillators (VCOs), mixers, and their integration for mm-wave systems with focus on communication and imaging applications, all based on Si-based technologies.

The terahertz spectrum is roughly defined as the frequency range of 0.3 – 30 THz and falls on roughly between the microwave and optical bands. Hence, it generally indicates the band with even higher frequencies than the millimeter-wave band, widely overlapping with sub-mm-wave band. Compared to the neighboring microwave and optical bands, which have been extensively exploited for a myriad of applications, the terahertz band has long remained as a territory only scarcely explored. For this reason, the spectrum has popularly been called the ‘terahertz gap’. There have been two approaches for the terahertz band development: ‘downward’ approach from the optics and ‘upward’ approach from the electronics. Although the downward approach has been more widely used, there are recent efforts for the upward approach based on the electronics owing to the progress in the high speed semiconductor technologies. The systems based on this electronics-based approach is expected to more compact and compatible with existing electronic systems, making it more attractive for practical and commercial applications. HSISL has taken this upward approach to the terahertz applications and is working on implementation of basic circuit blocks such as LNAs, VCOs, mixers, switches, detectors, frequency multipliers, etc. Furthermore, they are integrated, together with antennas, to realize on-chip THz systems such as transceivers and imagers, and various experiments based on the completed THz systems are also going on.