As 5G becomes more popular, the development of 6G, the next generation of communication technology, is also in full swing. Whether it is China, the US, the EU or Japan and South Korea, the leading countries in the field of communications are all increasing their investment in 6G research and development. According to current R&D targets, 6G is expected to achieve more than ten times the communication speed of 5G and the standard is expected to be launched around 2026. In terms of related applications, 6G is expected to continue on the path of 5G and will cover personal communications while continuing to cover IoT and smart industrial applications, including ultra-high-speed in-plant wireless access.
In order to support greater communication rates, channel capacity must increase accordingly according to Shannon’s Law, which generally means an increase in communication bandwidth is required. The most direct way to increase communication bandwidth is to increase the carrier frequency, and this is why terahertz has received particular attention in the 6G sector. Generally speaking, terahertz (THz) refers to the frequency band with a carrier frequency in the range of 300 GHz – 3 THz, while sub-THz refers to the frequency band around 100 GHz – 300 GHz. In the context of 6G, on the other hand, terahertz generally refers to both the THz band and the sub-THz band.
Currently, various countries are actively developing 6G-related terahertz technology and are opening up relevant frequency bands. China started a working group to promote the development of 6G technology as early as the end of 2019, while Huawei also announced earlier this year a prototype 6G system with a communication range of 500 metres using terahertz technology; the United States also decided to open up the 95 GHz – 3 THz 6G experimental spectrum in 2019; the South Korean government is investing heavily in 6G research and development, and both Samsung and LG are also actively developing related technologies. LG announced at the beginning of September this year that it had collaborated with the Fraunhofer Institute in Germany to realise a prototype terahertz communication system with an output power of up to 20 dBm over a distance of 200 metres.
In summary, we believe that with the rise of 6G technology, in order to meet the demand for high communication rates, the carrier frequency continues to rise to the terahertz band will become a key technology for 6G, while the related semiconductor chips and systems will be the core of supporting terahertz and 6G communications.
Current status and foresight of semiconductor terahertz communication chips
As mentioned above, terahertz communication chips will be the technical core of 6G. Terahertz communication-related chips can be divided into two main categories, one is RF chips, while other is baseband chips.
As far as RF chips are concerned, terahertz first needs circuits that can operate in the high-frequency band (terahertz band) and have a large bandwidth. In order to meet this requirement, the current terahertz RF chips for long-range communication mainly use the III-V semiconductors HEMT and HBT transistors to achieve RF-related work. III-V semiconductors have a high operating frequency, a large operating bandwidth and a high output power to meet the main needs of terahertz band communication. Since the first step in terahertz communication is still inter-base station communication, we believe that terahertz-implemented RF chips will be the semiconductor technology of choice for terahertz long-range communication chips in the coming years.
Outside of III-V semiconductors, terahertz communication technologies using CMOS and SiGe, which are silicon-based materials, are also flourishing. Compared to III-V semiconductors, CMOS and SiGe chips have the advantage of high integration and low cost and have therefore gained the attention of academia and industry alike. For terahertz, the main challenges for CMOS and SiGe are the low transistor cut-off frequency and the low operating bandwidth. The low cut-off frequency means that CMOS and SiGe chips can operate in the terahertz band, but their output power will be low, which means that long-range communication is difficult to achieve. to achieve communication. At present, the use of CMOS and SiGe chips for terahertz communication is still mainly for short-range communication (e.g. within a range of about 1 m). Looking ahead, the development of CMOS and SiGe for terahertz communications will be mainly at the circuit level and system level, where improvements in semiconductor processes do not currently improve the performance of CMOS/SiGe circuits in the terahertz band (e.g. CMOS is one of the best process nodes for the terahertz band at 65nm).
In addition to RF, another very important chip in the field of terahertz communications will be the baseband chip. While the standards for 6G are not yet defined, the current discussion on baseband is mainly about how to generate modulation of high-speed signals (e.g. if 6G needs to achieve over 100Gbps transmission in the terahertz band, how to achieve such high-speed modulation signals) and the related control of the RF circuitry, e.g. linearisation techniques. For high-speed communications, how to increase the speed of digital signal processing and how to improve the performance of digital-to-analogue conversions such as ultra-high-speed ADCs/DACs will be major topics. In addition, terahertz communication is still trying to increase the communication distance, or rather how to increase the effective output power of RF circuits is also currently a very important topic, so relevant digital auxiliary technologies, such as amplifier linearisation techniques, will also play a very important role in the field of terahertz.
Another RF-related semiconductor technology that deserves attention is packaging technology. In the field of circuitry, the integration of III-V and CMOS using advanced packaging technology is also a technology that allows both III-V and CMOS to take advantage of each other’s strengths, but how to ensure that the losses in the relevant systems are controlled in the terahertz band will be a topic of great interest. In addition, and more importantly, the smaller wavelengths in the terahertz band allow for smaller antenna sizes and therefore the possibility of using advanced packaging techniques to package multiple (>10) RF chips together in arrays to achieve high-performance beamforming to further enhance system performance. We believe that advanced packaging will play an enabling role in such miniaturised RF arrays to support the development of 6G terahertz technology.
Imaging is another potential area for terahertz chips
In addition to communications, another major application for terahertz chips is imaging. The main feature of terahertz is that it can penetrate some barriers that conventional light cannot while being able to sensitively detect metallic objects, thus having great promise for applications in areas such as security. At the same time, compared to previous millimetre wave-based security imaging technologies, terahertz has a shorter wavelength and can achieve a larger bandwidth, so imaging accuracy is better than millimetre wave imaging.
Unlike communications, imaging does not require long transmission distances, so terahertz imaging can be implemented using silicon-based chips. In addition, the use of CMOS/SiGe for terahertz imaging is also promising in terms of cost, as there is a need for miniaturisation and large-scale deployment for placement and imaging.
Currently, terahertz imaging chips implemented using CMOS/SiGe typically operate at 100 – 400 GHz with bandwidths up to 100 GHz, thus enabling very high precision imaging. We believe that there is considerable future upside for terahertz circuits in this area, including the integration of more sophisticated imaging algorithms (e.g. compressed sensing, etc.), the integration of more complex array systems, etc. Imaging technology will be the most critical application of terahertz in the future along with 6G communications, thus driving further development of terahertz chips and systems. Terahertz is set to become another promising frequency band after the millimetre wave, with related chip technologies and market applications to look forward to.
