# Broadband Millimeter-Wave Front-End Module Design Considerations in FD-SOI CMOS vs. GaN HEMTs
## 1 Introduction  
Advanced Gallium Nitride (GaN) high-electron-mobility transistors (HEMTs) will be of great importance in the realization of radio frequency (RF) front-end modules (FEMs) for beyond 5G (B5G) and 6 th generation (6G) technologies, as they can attain higher levels of RF output power (\(P_{OUT}\)) and power-added efficiency (PAE) compared to silicon-based technologies at millimeter wave (mm-Wave) and Tera-Hertz (THz) frequencies [1, 2]. GaN HEMTs use a two-dimensional electron gas (2 DEG) to provide high mobility (\(\mu\)), and their high peak saturation velocity (\(v_{sat}\)) makes them well suited for higher frequency operations. Additionally, GaN HEMTs show improved current-handling capabilities and superior ruggedness compared to Si counterparts [3]. Power amplifiers (PAs) often usethe most power in an RF FEM and can consume roughly half the power in a mm-Wave phased array system [4], so it is imperative that they are designed to be as efficient and robust as possible to avoid reliability and thermal issues [5]. GaN devices are traditionally used for higher voltage/power operations, but they also show great promise in various lower voltage/power applications, such as for mm-Wave 5G, and especially if GaN on Si technologies can be matured to reduce the die cost [1; 6; 7]. Multiple-input-multiple-output (MIMO) antennas are implemented in many mm-Wave systems and integrated with digital control systems and FEMs to reach a certain level of transmitter (Tx) equivalent isotropic radiated power (EIRP) [4]. To combat the problem of increasing path loss at mm-Wave frequencies vs. at the traditional sub-6 GHz banded, silicon-based phased array systems can use larger antenna arrays to increase their gain, but this will reduce the half power beam width (HPBW) to form a \"pencil-beam\", exacerbating the effect of line-of-sight (LOS) blocking. Alternatively, GaN FEMs can be used to increase the output power of each antenna element, allowing a smaller antenna array with a larger HPBW to alleviate the thorny LOS blockers while maintaining the required level of Tx EIRP [6; 8; 9]. This work presents a simulated system-level antenna array design, along with two monolithic broadband mm-Wave PA design using post-layout parasitic-extracted (PEX) electromagnetic (EM) simulation vs. measurement data, targeted to cover key portions of the 5G FR2-band (i.e., 24.25-52.6 GHz). The FR2-band is part of both the super-high-frequency (SHF, i.e., 3-30 GHz) band and the extremely-high-frequency (EHF, i.e., 30-300 GHz) band, where the RF signals have wavelengths that can range from centimeters to millimeters in length. The SHF band is used mainly for wireless communications, while, traditionally, the EHF band is used more for astronomy and remote sensing, since there is greater path loss due to the absorption in the atmosphere and it also can suffer considerably from non-line-of-sight (NLOS)-blocking effects [8; 10]. However, 5G and 6G technologies are attempting to leverage using the EHF band for communications as well, since the bandwidth and data rate of RF signals increases with frequency [1; 2]. Our previous mm-Wave PA designs have shown quite good agreement among PEX EM simulations and measurements [11; 12; 13; 14; 15; 16]. One PA shown in this paper is designed in the 22 nm fully depleted silicon on insulator (FD-SOI) CMOS \"22 FDX\" technology from Global Foundries (GF), and the other is designed in an advanced 40 nm GaN HEMT \"T3\" technology from HRL Laboratories. Additionally, we show PEX EM simulations on a third higher power \"Watt-level\" GaN PA we designed. Our broadband mm-Wave PAs cover portions of both the SHF and EHF frequency bands and are compared with other broadband PAs designed in Si [17; 18; 19; 20] and GaN [21; 22; 23] technologies. References [17; 21; 22] are part of the SHF band, and [18; 19; 20; 23] reach into the EHF band.