I. Introduction
Terahertz monolithic integrated circuits (TMICs) have been a hot research issue for their promising applications in high-data-rate communication and high resolution imaging system. Power amplifier is an essential constituent part in many successful systems. Combiners used in these power amplifiers and small-signal amplifiers are the Wilkinson power combiner [1], [2], Dolph–Chebyshev combiner [3]–[5], and Tee-junction combiner [4]–[7]. These -band amplifiers are with high small-signal gain and high output power with advanced 0.25- InP double heterojunction bipolar transistor (DHBT) and 35-nm InP HEMT technologies. The lack of beyond 200-GHz workable power amplifier limits the use of the low-cost 0.5- InP HBT/DHBT technology which does not require complex fabrication steps such as e-beam lithography and refractory contact metal deposition.