  Achieving non-reciprocity and building nonreciprocal components through
spatio-temporal modulation of material properties has attracted a lot of
attention in the recent past as an alternative to the more traditional approach
of exploiting Faraday rotation in magnetic materials. In this letter, we review
recent research on spatio-temporal conductivity-modulation, which enables
low-loss, small-footprint, wide-bandwidth and high-power-handling
non-reciprocal components operating from radio frequencies (RF) to
millimeter-waves (mm-waves) and integrated in a CMOS platform. Four generations
of non-reciprocal circulators and circulator-based systems will be reviewed. We
will also discuss metrics of performance that are important for wireless
applications and standards, and introduce a new antenna (ANT) interface
efficiency figure of merit ($\eta_{ANT}$) to enable a fair comparison between
various types of antenna interfaces.
