I. Introduction
Terahertz RF front ends have achieved much attention due to their potential applications in communications and radars [1], [2], especially when the IEEE 802.15.3d standard was proposed [3]. However, when the operating frequency rises above 300 GHz, the highly integrated, low-cost, and easily large-scale integration silicon-based process is limited by their low cutoff frequency (), and the output power of the RF front-end is limited. One effective solution is to integrate amplifiers based on III–V process, i.e., indium phosphide (InP). In addition, the antenna above 300 GHz is a critical component to radiate the power into the free space.