Radio Frequency (RF) energy harvesting (EH) has drawn significant attention due to the development of Internet of Things (IoT) applications. This paper first reviews the design challenges on high efficiency and wide input power range integrated RF EH rectifiers. Then the design considerations on these challenges have been discussed, including the multi-path rectification, the input impedance mismatch reduction, and efficiency enhancement on single rectification path. Finally, we suggest a multi-path topology with class-DE structure for each single path, combining the above techniques that are compatible to each other.