An InP Distributed Active Isolator With >30-dB Isolation Over 215-GHz Bandwidth for Sub-THz Wideband Transceivers

This article presents a 5–220-GHz active isolator with >50 to >30 dB reverse isolation to protect local oscillators in sub-THz wideband transceivers. The isolator design adopts a Darlington distributed amplifier topology comprised of five high-isolation stages to achieve a flat forward transmission and wideband input/output matchings. To achieve a high isolation across the wide bandwidth, the isolator codesigns stacked heterojunction bipolar transistor (HBT) topology and high-isolation layout structures, building high-isolation stages for the complete isolator. A new layout arrangement using low-crosstalk transmission lines, high-isolation cell shielding, and coplanar waveguides with continuous ground planes is presented and implemented to mitigate couplings that severely degrade isolation at sub-THz frequencies. The indium phosphide active isolator is fabricated and characterized. The prototype exhibits a forward gain of 10 dB and a reverse isolation of >50, >37, and >30 dB up to 40, 190, and 220 GHz, respectively. The maximum saturated power and the output 1-dB compression point are 15.8 and 12.2 dBm, respectively. From 120 to 170 GHz, the output third-order intercept point is from 21.4 to 16.1 dBm. The isolator consumes 45 mA of current with an active area size of 0.176 mm².