This article introduces an ultra-low-power (ULP) active-RF tag for long-range event-driven Internet-of-Things (IoT) applications. The tag employs the following: 1) a complementary Class-B/-C voltage-controlled oscillator–power amplifier (VCO–PA) with an optimum-sized loop antenna and a co-designed transmit/receive (T/R) switch to maximize the efficiency of the transmitter (TX); 2) an antenna–transceiver (TRX) co-designed RF interface replacing the function of the off-chip matching network (MN) while enabling the frequency tunability and a 2-D wake-up scheme for the receiver (RX); and 3) a frequency-locked loop (FLL) plus injection locking calibration scheme for the TRX to guarantee both the frequency accuracy and tuning range. Prototyped in 65-nm CMOS, the TX achieves a 20.8% efficiency at −20.3-dBm equivalent isotropic radiated power (EIRP). The RX achieves a sensitivity of −60.4 dBm at a data rate of 1 kb/s while can operate from 790 to 925 MHz with 17.8-nW power consumption.