This paper discusses the design of highly efficient and monolithic medium-power RF Class E SiGe power amplifiers (PAs) in IBM 7HP SiGe BiCMOS technology at both 900MHz and 2.4GHz for wireless sensor applications. Without needing off-chip on-board matching, we achieved high power-added-efficiency (PAE) for the single-stage Class E SiGe PAs at ∼70% (900MHz) and ∼60% (2.4GHz), respectively. Using large number of downbonds at the emitter node of the PA, optimal device sizing and layout, and careful circuit design with bondwire tank inductors, maximum PAE of 62% at 2.4GHz is obtained, which performance rivals that of commercially-available IH-V PA modules. Taking advantages of the higher output power with breakdown robustness and the excellent PAE for SiGe PAs vs. CMOS PAs, one can not only shrink the battery size and therefore sensor volume, but also reduce the number of nodes required in a wireless sensor network to bring down system cost and simplify data fusion. With improved understanding of on-chip PA loss mechanisms, it is likely that we can push these high-efficient SiGe PAs into higher frequencies of operation (say 10GHz) to utilize smaller antenna size, enabling new and exciting wireless sensor network applications.