This paper presents an ultra-low power wake-up receiver (WuRx) based on direct active-RF detection architecture. A noise-cancelling technique is implemented on the RF envelope detector to minimize the noise due to the circuit components and to obtain a higher signal-to-noise ratio while maintaining its power within tens of microwatts. Compared to other wake-up receivers, this work focused on designing the receiver with reduced system complexity through a single-ended topology and minimized number of circuit components; thus, ultra-low power operation is attained. At 2.4 GHz RF operating frequency, a - 50 dBm sensitivity is achieved with 100 kbps data rate. Power consumption is measured as 13.7 μW from a 1 V supply voltage, obtaining 138 pJ/bit energy efficiency. The prototype receiver was designed using a 65 nm CMOS technology process and has a layout area of 0.194 mm².