A reference-less ultra-low-power wake-up receiver (WuRx) is proposed for the highly integrated Internet of things applications. The WuRx utilises a direct radio-frequency envelope detection technique with input synchronised switching to suppress 1/f noise and enhance the sensitivity. Moreover, injection-locked clock and data recovery circuitry is used to perform on-chip clock generation. A prototype 2.4 GHz WuRx is designed and implemented with 90 nm complementary metal-oxidesemiconductor technology. The chip area is 1.0 × 0.95 mm 2. The measurement results show that the WuRx consumes 25 μW and achieves a sensitivity of −51.5 dBm for 200 kbps data rate.