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Recent research has explored the spatiotemporal modulation of permittivity to break Lorentz reciprocity in a manner compatible with integrated-circuit fabrication. However, permittivity modulation is inherently weak and accompanied by loss due to carrier injection, particularly at higher frequencies, resulting in large insertion loss, size, and/or narrow operation bandwidths. Here, we show that the presence of absorption in an integrated electronic circuit may be counter-intuitively used to our advantage to realize a new generation of magnet-free non-reciprocal components. We exploit the fact that conductivity in semiconductors provides a modulation index several orders of magnitude larger than permittivity. While directly associated with loss in static systems, we show that properly synchronized conductivity modulation enables loss-free, compact and extremely broadband non-reciprocity. We apply these concepts to obtain a wide range of responses, from isolation to gyration and circulation, and verify our findings by realizing a millimeter-wave (25 GHz) circulator fully integrated in complementary metal-oxide-semiconductor technology.

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Multiband FDD operation requires numerous off-chip duplexers, which limit form factor. Widely-tunable low-noise RF active self-interference cancellation (SIC) (e.g. [1]) is a step towards enabling duplexers with reduced TX/RX isolation as well as adjacent-channel full duplex. However, SIC bandwidth (BW) is limited to a few MHz due to the selectivity of the high-Q duplexer isolation. Recent works suggest that same-channel full duplex (SC-FD) can greatly improve network performance [2][3][4]. SIC must be pursued in RF, analog/mixedsignal and digital for SC-FD to achieve the >100dB of SIC required, as filtering the SI is not an option. However, SC-FD self-interference channels can be frequency-selective due to ambient reflections [3], requiring silicon-averse bulky delay lines to replicate the delays in the RF SIC path [2].

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