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A new RF switch architecture with ultrahigh isolation and possible gain is proposed, analyzed, and demonstrated using 0.18-μm BiCMOS technology. The new RF switch architecture achieves an ultrahigh isolation through implementation of a new RF leaking cancellation technique in which the RF leaking signal is suppressed by combining with its replica using a balun. Its isolation is substantially higher than that produced by a conventional switch topology. An analysis of the new active balun employed in the proposed RF switch is also conducted, showing its distinguished characteristic of good balance across ultra-wide frequency ranges, making possible not only the achievement of extremely high isolation, but also ultra-wideband isolation for the RF switch. Additionally, the active balun also provides some gain to compensate for the inherent loss of the RF switch. The newly designed 0.18-μm BiCMOS RF switch exhibits an ultra-broadband performance from 10 to 38 GHz with - 2.6-dB loss to 0.4-dB gain, isolation from 40 to about 70 dB, and input return loss from 8 to 20 dB under small-signal conditions. Within 35.5-38.5 GHz, its isolation reaches extremely high values, with the highest isolation around 70 dB at 36 GHz, approaching the measurable limit of the vector network analyzer. Measured insertion loss and isolation under large-signal conditions at 35 GHz show around 1-2 and 51.5 dB, respectively. The RF switch consumes a dc current of only 8 mA from a 1.8-V source. The extremely high isolation achievable by the new RF switch demonstrates the possibility of pushing RF system performance limited by switch isolation to a next level.