This paper presents an ultra-wideband (UWB) medium power amplifier (MPA) and a broadband high-power power amplifier (HPA) operating at the 5G/6G frequency bands. By using $0.15~\mu \text{m}$ GaAs pseudomorphic high electron mobility transistor (pHEMT) technology process, the proposed UWB MPA delivers an average small-signal gain of 16.5 dB, a saturation output power ( $\text{P}_{\mathrm {sat}}$ ) of 24 dBm, and a peak power-added efficiency (PAE) over 24% from 24 to 38 GHz with a chip area of $2\times1$ mm2. The broadband HPA demonstrates a 17-dB average small-signal gain, 29-dBm $\text{P}_{\mathrm {sat}}$ , and a PAE over 28% from 24 to 32 GHz with a $2.4\times1.1$ mm2 chip size. The measurement results have demonstrated the great potential of the proposed PA for 5G/6G millimeter-wave applications.