# Key Technologies for THz Wireless Link by Silicon CMOS Integrated Circuits
## 2 300 GHz Band Transceiver Using Silicon Integrated Circuits
### Integrated Circuit Realizing Terahertz Transceiver and Its Characteristics  
The frequency used for 300 GHz band communication is 100 times higher than that of widely used microwave communication. Therefore, it is necessary for the circuit to operate at ultra-high frequency. The realization of terahertz wireless communication has two approaches: all electronic and photonic-electronic hybrid. The latter covers the frequency band exceeding 500 GHz that is unachievable with the all-electronic approach, and which is desirable for downsizing and power  
Figure 1: Evolution of data rate of wired communication and wireless communication. The evolution of the data rate of wireless communication is faster than that of wired communication, and is expected to reach 100 gigabits per second in 2020. Furthermore, if we continue evolving at this speed, the data rate of wireless communication will be equivalent to wired communication in 2030.  
reduction of the total communication system [6]. For this reason, the photonic-electronic hybrid approach has been studied previously to generate terahertz signals. However, research using integrated circuits, which is based on the all-electronic approach, has been undertaken recently. Integrated circuits are broadly classified into compound semiconductor integrated circuits using indium phosphide and gallium arsenide, and silicon integrated circuits fabricated on silicon substrates. In addition, silicon integrated circuits include silicon germanium (SiGe) BiCMOS (integrated circuits integrate bipolar-junction transistors and CMOS transistors in a single device) integrated circuits and CMOS integrated circuits. The difference between these integrated circuits is the maximum oscillation frequency \(f_{\text{max}}\) of the transistor and the degree of integration. In accordance with [PERSON]'s law, the performance of devices using compound semiconductors as well as silicon has been improved with the benefit of miniaturization. If capacitance and resistance are reduced by reducing the gate length (or base thickness) of the transistor, the high frequency performance is improved. The \(f_{\text{max}}\) of the latest compound semiconductor transistor exceeds 1 THz [7,8], and \(f_{\text{max}}\) of the SiGe bipolar transistor exceeds 500 GHz [9]. However, since the influence of the tunnel current and the variation due to the reduction of the power supply voltage becomes large, it is impossible to reduce the simple three-dimensional size, and the influence of the parasitic capacitance and the parasitic resistance prevents the high frequency performance of the transistor from being further improved. In the digital circuit, even if the performance of the transistor is not improved, the circuit performance is improved because the wiring length is shortened due to miniaturization of metal layers. On the other hand, \(f_{\text{max}}\) of a transistor is important in a circuit handling high frequencies, especially terahertz frequencies. In the CMOS integrated circuit, \(f_{\text{max}}\) remains at around 300 GHz after the 65 nm process, and the performance of the transistor is not significantly improved. On the other hand, digital baseband logic can be integrated in BiCMOS integrated circuits and CMOS integrated circuits, whereas digital circuits cannot be integrated in compound semiconductor integrated circuits. In particular, large-scale digital circuits can be integrated in CMOS integrated circuits, which are becoming more miniaturized than BiCMOS circuits, so there is a possibility to realize a single chip terahertz transceiver device in the future. Although performance is inferior to that of compound semiconductors and SiGe integrated circuits, if the terahertz transceiver can be realized with CMOS integrated circuits, the application range will extend to portable devices. However, CMOS transceivers made of transistors with \(f_{\text{max}}\) around 300 GHz cannot integrate power amplifiers in transmitters and low-noise amplifiers in receivers, which are generally essential for transceivers.