This article describes a fully integrated CMOS radio frequency energy-harvesting (RFEH) front end. It features an on-chip stacked step-up transformer integrated with a cross-coupled differential drive (CCDD) rectifier to enhance the input sensitivity. The transformer also serves as an on-chip balun for the CCDD rectifier. The CCDD rectifier innovates a gate-biasing technique and realizes coupling capacitors at the end of each stage to increase the subsequent stage biasing. Here, our RFEH front end operating at 900 MHz achieves an improved sensitivity of -20 and -19.2 dBm at the 1-V output for no-load and a 1-MΩ load, respectively.