This paper presents a Doherty power amplifier (PA) in 22nm FD-SOI for achieving high output power and high backoff efficiency for 28GHz 5G communications. The output stage of the main PA and the auxiliary PA both use a stacked-FET topology for high output power without posing a reliability issue. A dedicated transformer-based output matching network is proposed to achieve a true Doherty load modulation while maintaining a compact layout. The PA is fabricated in a 22nm FD-SOI technology with a core area of 0.2mm2. At 28GHz, the measured saturated output power (Psat), 1dB output compression point (P1dB), and peak power-added efficiency (PAE) are 22.5dBm, 21.1dBm, and 28.5%, respectively. State-of-the-art ITRS figure-of-merit (FOM) and power density are achieved. The measured 6dB power back-off (PBO) PAE is 22.1%, which results in a PBO enhancement ratio of 1.56/3.12 with respect to a class-B/class-A PA. The proposed PA can support 2.4Gb/s 64-QAM and 0.8Gb/s 256-QAM signal with a competitive average PAE.