This paper presents an ultra-low-power wake-up receiver (WuRx) for use in the worldwide recognized 2.4 GHz ISM band. The WuRx features a fully-differential complementary current-reuse RF detector with an embedded impedance matching network. The proposed WuRx does not require power-hungry RF amplification and frequency down-conversion circuits, so the power consumption can be significantly reduced. The measurement results show that the proposed receiver achieves a sensitivity of -50 dBm and has a power consumption of just 4.5 μW in a 0.18-μm CMOS technology. Consequently, it provides a promising solution for a wide range of low-power wireless sensor network and wireless body area network applications.