## 1 Introduction  
Worldwide digitalization has led to an explosion of mobile data traffic within the recent fourth generation (4G) and long term evolution (LTE) telecommunication generations. To ensure the enormous user experience demands for future digital systems and services are met, 5G/6G is expected to provide 1000\(\times\) more capacity [1]. To achieve this enormous increase in data rates, several wideband millimeter-wave (mmWave) frequency bands between 24-53 GHz for 5G commercial wireless telecommunication have been allocated by the third generation partnership project (3 GPP) new radio (NR) standard [2], and frequencies above 100 GHz for 6G are being discussed [3, 4]. However, at mmWave frequencies, we must overcome not only the increasing path losses but also the decreased antenna size. In fact, it has been proposed that large phased arrays with RF beamformers are needed to provide enough antenna gain and spatial coverage [5, 6, 7, 8]. This results in a dramatic change in the transceiver implementation and raises severe design challenges. First, the size of an antenna array at mmWave frequencies is comparable to the wavelength (antenna elements are spaced by \(\lambda/2\)), and at 30 GHz, \(\lambda\) is \(\approx\) 1 cm. This results in the requirement that the transmitter should be small and highly integrated. Second, in large phased arrays, the required output power levels for each antenna decrease as the number of antennas increases. This results in the fact that, in phased arrays, each antenna is preceded by a small or medium power amplifier (PA), which is preferably integrated in the transceiver RFIC [9, 10, 11, 12, 13, 14, 15, 16]. Third, due to the high modulation schemes and high PAPR (peak to average power), wideband up to 400 MHz 256-QAM based OFDM signals are proposed, and the specified linearity requirements are very strict, as can be seen in Table 1. As a result, the phase linearity of the transmitter needs to be small. On the other hand, the wider FR2 bands' adjacent channel power ratio (ACPR) requirements shown in Table 2 are more relaxed and allow more spectral distortion [17]. To achieve linearity, the PA is usually backed off at around 10 dB from its peak power. As efficiency is proportional to the output amplitude, the PA operates inefficiently most of the time. In traditional macro base stations (BS), this is solved by massive digital predistortion (DPD), which linearizes a single and large efficiency-enhanced PA (conventional BS power consumption is highly dominated by the PA). With the introduction of mmWave phased arrays, the number of PAs is so large that linearizing each of them separately is simply not cost-effective; instead, either analog or averaged effects are needed [18].  
This paper describes a highly linear and compact four-stack PA compatible for 3 GPP/NR FR2 bands n258 and n257 that is implemented using GLOBAL FOUNDRIES 45 nm CMOS SOI technology. The achievable output power is limited by the nominal VDD, which is 1 V in 45 nm CMOS SOI. Fortunately, transistor stacking is possible in the SOI technology, which allows higher operating voltages to be used and thus higher output power. In addition, with compact input and output matching and a distributed transistor core, the total size and thus parasitics of the PA are minimized, providing higher gain.  
This paper is an extended version of [19]. The structure and the design flow of the proposed stacked PA is presented in detail with an illustration of the actual layout in Section 2. Section 3 shows the measurement setup in detail. Measured results with simulations are shown and compared against the state-of-the-art in Section 4. Conclusions are presented in Section 5.