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We present an ultralow-power millimeter-sized ultrasonic wake-up receiver (WuRx) that uses precharged ultrasonic transducers to wirelessly receive wake-up signatures. Our WuRx takes advantage of the low carrier frequency afforded by the low speed of sound to achieve high impedance, and thus high sensitivity, operation in a millimeter-sized form factor. We discuss the system-level considerations that arise from the use of ultrasound as a mode of signature transmission, including the choice of operating frequency and the effect of transducer miniaturization on WuRx sensitivity. We describe the design of the receiver electronics and support our case for ultrasonic wake-up with thorough wireless measurements of error rate and electrical characterization of multiple dice from the same wafer lot. The receiver electronics are fabricated in 65-nm CMOS technology, while the receiver transducer is fabricated by means of a custom in-house process. The receiver achieves a sensitivity of −59.7 dBm at a bit error rate (BER) of 10−3 with a power consumption of 8 nW and a footprint of 14.5 mm2. Our benchmarking shows that the proposed WuRx achieves the lowest system area/aperture of all RF and acoustic WuRxs in the literature (1.45 $\times $ smaller than the state of the art) while maintaining competitive sensitivity and power consumption. [ABSTRACT FROM AUTHOR]