# A Perspective on Terahertz Next-Generation Wireless Communications
## 3 Terahertz Devices
Fortunately, the baseband circuits directly benefit from continuous efforts to scale CMOS devices for high speed and low-power operation. For instance, the performance and energy efficiency of analog to digital converters (ADC) has been continuously improved [66]. While baseband circuits certainly need wide bandwidth to retain the benefit of terahertz carriers, they do not need to operate themselves at terahertz frequencies. This relaxes some of the difficult electronics constraints that terahertz waves impose. Unfortunately, front-end circuits do not always gain performance from CMOS scaling. Rather, the lower supply voltage and the pronounced interconnect parasitics of scaled CMOS processes deteriorate the front-end performance. The frequency where the maximum achievable power gain becomes unity, \(f_{\text{MAX}}\) of CMOS devices currently peaks at 350 GHz [67; 68]. This means that it is almost impossible to develop basic building blocks for terahertz front-ends, such as amplifiers and oscillators. However, other integrated circuit (IC) technologies can be considered as alternatives, and it is not uncommon to use multiple technologies to build communication devices. As a matter of fact, mobile handsets, which have very limited circuit-board area and power budget, employ compound semiconductor and micro-electro-mechanical components in their frontends to meet performance targets [69; 70; 71]. Indium-phosphide (InP) technologies offer the highest \(f_{\text{MAX}}\) reaching beyond 1 THz [64; 72]. Indeed, a 50-Gbps QPSK transceiver has been demonstrated using InP technology [51]. However, its yield and manufacturing capacity have not been proven in commercial large-volume applications yet. Gallium-nitride (GaN) technology is also promising for infrastructure thanks to its inherent high-power handling and generation capability. A 2 W power amplifier IC and a 37 W module employing the IC was demonstrated up to 100 GHz using a 0.15 \(\upmu\)m GaN process [52; 73]. Its manufacturability is expected to be improved as GaN technologies are being adopted into 5G infrastructure [74]. Finally, silicon germanium (SiGe) technologies can be considered for mobile handsets. The \(f_{\text{MAX}}\) of SiGe has reached 700 GHz and, importantly, it can support integration with CMOS and large-volume \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & \multirow{2}{*}{**Technologies**} & \multirow{2}{*}{\(f_{\text{MAX}}\)} & \multicolumn{2}{c}{**Frontend IC Performance**} & \multirow{2}{*}{**Comm. System Demonstration**} & \multirow{2}{*}{**Note**} \\ \cline{3-4} \cline{6-7}  & & & \(P_{\text{sat}}\) & & & **Noise Figure** & \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Compound Semiconductor \\ \end{tabular} } & HDF & \(>\) 15 THz & \(-\) 2 dBm at 850 GHz & 127 dB at 850 GHz [50] & 50 Gbps (QPSK at 300 GHz [51] & Highest \(f_{\text{MAX}}\) \\  & HEMT/HBT & \(>\) 15 THz & [56] & 127 dB & 50 Gbps (QPSK at 300 GHz [51] & Highest \(f_{\text{MAX}}\) \\  & \multirow{2}{*}{GaN HEMT} & \multirow{2}{*}{\(>\) 20 GHz} & 33 dBm at 100 GHz & & & Highest \(f_{\text{MAX}}\) \\  & & & [52] & & & \\ \cline{2-7}  & GaAs SBD & \(>\)3 THz (\(f_{\text{1}}\)) & \(-\)14 dB at 2 THz (hmm) & 14 dB at 2 THz (mirror 18d) & Highest frequency & \\  & & & (hmm). & & & operation \\ \hline \multirow{2}{*}{Silcom} & SGe HBT & \(>\) 700 GHz & 9.6 dBm at 215 GHz & 549 & 90 Gbps QAM at 230 GHz [50] & Medium volume \\  & & & [53] & 11 dB at 253 GHz [55] & & & \\ \cline{2-7}  & & 4.6 GHz at 210 GHz & 9 dB at 200 GHz (NA) [18] 14 dB at & & 105 Gbps QAM at 300 GHz [56] 30 Gbps QPSK at 300 & Large volume \\ \cline{2-7}  & CMOS FET & \(>\) 450 GHz & \(\left(\text{PA}\right)\) [56] & 250 GHz (mirror 18d) [59] & & & \\ \cline{2-7}  & & 300 GHz (Hmm) & \(\left[\text{57}\right]\) & 250 GHz (mirror 18d) [59] & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of leading electronic terahertz front-ends and whole communication systems. manufacturing [75; 76].