# Broadband Millimeter-Wave Front-End Module Design Considerations in FD-SOI CMOS vs. GaN HEMTs
## 5 Conclusions  
For mm-Wave phased array systems operating in the SHF and EHF bands, HPBW will be an important factor in determining their robustness for reliable communication, especially when LOS blockers are present. Reducing the size of the antenna arrays will increase their HPBW, alleviating some of the LOS-blocking problems. Therefore, high-power GaN FEMs providing greater \(P_{OUT,SAT}\) can be used in place of Si FEMs to reach the same Tx EIRP and with smaller array sizes and improved LOS blocking. The CW measurement data collected in our lab on our CMOS FD-SOI PA and our medium-power GaN PA support this concept, with our medium-power GaN PAs expected to fit into an array with \(\sim\)2\(\times\) wider HPBW and x4 reduction in channel numbers than an array using our CMOS PAs. We expect HPBW to be further widened by \(\sim\)3\(\times\) for GaN compared to Si using our Watt-level GaN PA, as the \(P_{OUT,SAT}\) of these Watt-level GaN FEMs appears  
Figure 10: HPBW plotted with \(P_{OUT,AVE}\)/_element_ of an \(N\)-element 2-\(D\) uniform square \(\sqrt{N}\times\sqrt{N}\) array assuming spacing between the elements is \(d=\lambda/2\) with the beam pointing towards broadside plotted, targeting a Tx EIRP of 65 dBm (antenna gain per element = 5 dBi; one FEM per element). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Units** & **All Silicon** & **GaN + Silicon** \\ \hline Average Output Power per FEM & dBm & \(\sim\)**2** & \(\sim\)**12 (medium power)**//\(\sim\)**18 (Watt-level)** \\ \hline Single Antenna Element Gain & dBi & 5 & 5 \\ \hline Array Size & - & **1024** (\(>\)768) & **256**//**128** \\ \hline Average Output Power, All FEMs & dBm & \(\sim\)30 & \(\sim\)36//\(\sim\)39 \\ \hline Antenna Array Gain & dBi & \(\sim\)35 & \(\sim\)29//\(\sim\)26 \\ \hline Tx EIRP & dBm & 65 & 65 \\ \hline
**Tx Array HPBW** & deg. & **3.2\({}^{\circ}\)** & **6.3\({}^{\circ}\)**//**9\({}^{\circ}\)** \\ \hline \hline \end{tabular} Bold text highlights key differences between 5G arrays designed using our Si vs. GaN PAs.  
\end{table} 
Table 3: Design comparison of potential Si vs. GaN mm-Wave 5G arrays using our FEMs. to be \(\sim\)15 dB greater vs. Si FEMs (e.g., Table 2 and Figure 7(b)). The two-stack Watt-level GaN PA has been designed in the same \"T3\" process from HRL as our medium-power PA, and its output power should be able to reach \(\sim\)1 W based on the post-layout PEX EM simulations that were seen to match up quite closely with the measurements taken on our medium-power GaN PA at the same frequencies. The increase in HPBW for mm-Wave antenna arrays with fewer GaN FEMs is expected to alleviate the thorny LOS-blocking problems with \(\times\)8 reduced channel numbers and improve the system's robustness vs. using an all-silicon array. Additionally, there might even be cost-saving benefits with reduced channel elements, especially if the GaN on silicon technology can be matured to reduce the GaN die cost [4,49]. Conceptualization, C.S., J.C.M. and D.Y.C.L. ; methodology, C.S., J.C.M. and D.Y.C.L. ; software, C.S. ; validation, C.S. and D.Y.C.L. ; formal analysis, C.S. ; investigation, C.S. and D.Y.C.L. ; resources, D.Y.C.L. and J.L. ; data curation, C.S. ; writing--original draft preparation, C.S. ; writing--review and editing, C.S. and D.Y.C.L. ; visualization, C.S. and D.Y.C.L. ; supervision, D.Y.C.L. ; project administration, D.Y.C.L. ; funding acquisition, D.Y.C.L. All authors have read and agreed to the published version of the manuscript. This material is based on research sponsored by the Air Force Research Laboratory (AFRL) and the Defense Advanced Research Projects Agency (DARPA) under Grant Number FA8650-19-1-7902. The US Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright notation therein. The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author. The authors would like to thank HRL Laboratories for their support and technical assistance.