Wake-up receivers (WuRXs) are low-power radios that continuously monitor the RF environment to wake up a higher-power radio upon detection of a predetermined RF signature. Prior-art WuRXs have 100s of kHz of bandwidth with low signature-to-wake-up-signal latency to help synchronize communication amongst nominally asynchronous wireless devices. However, applications such as unattended ground sensors and smart home appliances wake-up infrequently in an event-driven manner, and thus WuRX bandwidth and latency are less critical; instead, the most important metrics are power consumption and sensitivity. Unfortunately, current state-of-the-art WuRXs utilizing direct envelope-detecting and IF/uncertain-IF architectures achieve only modest sensitivity at low-power (e.g., -39dBm at 104nW ), or achieve excellent sensitivity at higher-power (e.g., -97 dBm at 99 μW [3]) via active IF gain elements. Neither approach meets the needs of next-generation event-driven sensing networks.This paper presents a 0.4 V 113.5 MHz OOK-modulated WuRX that achieves -69 dBm sensitivity with only 4.5 nW of power by: reducing the baseband signal bandwidth to 300 Hz, suitable for many event-driven applications, to aggressively filter noise; employing a high-Q transformer and filter that passively amplifies the voltage of the incoming RF waveform by 25dB and filters adjacent channel noise and interferers; simultaneously demodulating and amplifying the wake-up signal via a high-impedance dynamic threshold MOS (DTMOS) envelope detector (ED) with subthreshold active-inductor biasing; digitizing the ED output via a regenerative comparator with kickback elimination; generating the baseband clock via a 0.9pJ/cycle 1.1nW relaxation oscillator; decoding the received OOK signal modulated with a custom 16b codeword using a high-Vt subthreshold digital baseband correlator; and operating all circuits at 0.4V to minimize static and dynamic power.