InP HBTs with fmax crossing 600 GHz can enable high-efficiency and broadband power amplifiers at mm-Wave frequencies for applications in communication, sensing and imaging. InP PA design has traditionally relied on a classical microwave approach with transmission-line based impedance matching and power combining. On the other hand, silicon-based PAs, at RF and mm-Wave frequencies, have evolved tremendously over the last two decades, employing passive transformer and inductor-based impedance matching and power combining, allowing such topologies to be compact and efficient. In this paper, we present for the first time InP topologies incorporating the following 1) efficient and compact transformer-based power combining, 2) linearity enhancement technique to reduce AM-PM distortion and 3) second harmonic load pull impedance exploiting the common mode impedance synthesis at the transformer. This work presents a transformer-based push-pull InP power amplifier in 0.25 μm technology across 42-62 GHz demonstrating a peak PAE of 39.5% and peak Psat of 20.6 dBm. The PA supports QPSK signals with 4 GHz bandwidth (data rate of 8 Gb/s) at 55 GHz with superior linearity and adjacent channel leakage ratio (ACLR) of -31.84 dBc. To the best of the authors' knowledge, this work presents one of the highest efficiency and highest linearity mmWave PAs in InP technology.