This work presents a design approach of a fully integrated Doherty power amplifier (PA) based on a 250nm Gallium-Nitride (GaN) MMIC process. This ultra-compact circuit is designed for massive MIMO base stations in the foreseen European 5G band from 3.4 to 3.8 GHz. Due to the significantly higher instantaneous bandwidth of up to 100MHz and a peak-to-average ratio (PAPR) of 8.5 dB in this telecommunication service, methods for increasing bandwidth while maintaining energy efficiency in the output back-off (OBO) were investigated. A power added efficiency (PAE) of 55% at 38.5dBm and 46% at 30dBm output power was measured. Furthermore, the phase sensitivity of the main and peak path is analysed, which is essential for ensuring the Doherty characteristics.