A 900-MHz radio-frequency energy harvester is designed and implemented for remotely powered devices. An efficient power path structure with adaptive control is proposed to maintain close-to-optimal power conversion efficiency. The control circuit consumes low power by adopting a novel open-loop asynchronous implementation and duty-cycled operation. A startup circuit is also presented. Measurement results show minimum input available power for startup of -17 dBm. With input available power of -12 dBm and load resistance of 144 kΩ, the measured output power is 27.8 μW, giving an output voltage of 2 V. This corresponds to an end-to-end power conversion efficiency of 44.1%. The harvester consumes quiescent current of 1.56 μA at output voltage of 2 V.