Research and development on the next-generation information and communications infrastructure, “Beyond 5G/6G”, which is expected to be the foundation of all industries and societies in the 2030s, is becoming active. It will be achieved by the integration of several scientific fields, and vacuum and surface sciences would also make an important contribution for the element technologies. This article briefly explains the intention and purpose of the special issue that focus on research and development of elemental technologies toward Beyond 5G.

This report reviews fabrication processes and epitaxial growth technology of InP-based high electron mobility transistors (InP-HEMTs) and Tera-Hertz monolithic ICs (THz-ICs) in order to develop 300-GHz-band frequency, which is one of the candidates for “Beyond 5G/6G” network. We also introduce a successful result of a wireless transmission with 300-GHz 16-QAM format with InP-HEMT-based THz-ICs.

Toward realization of ultra-high-speed and low-power-consumption photonic transceiver, heterogeneous integration technology and photonic devices using the technology is reviewed. First, the reason why the heterogeneous integration technology should be needed is explained. Next, bonding technology, fabrication process and characteristics of hybrid photonic devices are explained.

In the beyond 5G/6G or further future wireless communication system, the terahertz (THz) frequency band will be used for realizing ultra-high data rate of over 100 Gbps with very wide bandwidth. Resonant-tunneling-diode (RTD) THz oscillators are a strong candidate of THz signal sources and can be utilized in the THz communication system. In this paper, we report on a recent progress on our RTD terahertz oscillators and introduce the demonstrations of THz communications with the RTD devices. Until now, owing to continuous development of RTD oscillators, a fundamental oscillation up to 1.98 THz and a high-power operation of 0.7 mW at 1 THz have been achieved. Using the high frequency modulation characteristics, THz communications with several tens Gbps have been demonstrated using various type of RTD oscillators.

The doubly clamped MEMS beam resonators have typical resonance frequencies in the range of several hundred kHz to several MHz, and high Q-factors of several thousand to ten thousand even at room temperature. We study terahertz sensing applications of MEMS resonators by utilizing the facts that 1) the mechanical resonance frequency changes very sensitively due to the thermal expansion of the beam and 2) the heat capacitance of the MEMS beam structure is very small, which is suitable for high-speed operation. In this article, we introduce a terahertz detector based on a new operation principle that reads a slight temperature rise due to terahertz absorption as a resonance frequency shift of the MEMS beam resonator fabricated with semiconductor heterostructures. We show that the present MEMS detectors are faster by 10 to 100 times than conventional room temperature thermal sensors.

The communication capacity of the 5th generation (5G) is not enough communication traffic which increases to practical use service using 5G. Therefore terahertz band (THz-band) wireless communications that can communicate with large capacity is in demand in “Beyond 5G”. The traveling wave tube (TWT) with the folded wave guide slow-wave structure is the most promising THz-band device. We are developing a 300 GHz TWT and this paper presents the development status of a 300 GHz-band TWT.