Distributed IoT devices require energy harvesting solutions for autonomous operation. Among the popular ambient energy sources (solar, wind, vibration), RF energy is special because of the transmission by humans while the other sources mainly emerge from Nature. In other words, when compared with other sources RF energy is easier to control. In an RF energy harvesting system the rectifier is the critical component. This paper addresses first the comparison between cross-connected and diode-connected CMOS rectifiers most commonly used in the energy harvesting system. Then, we discuss VTH compensation in the rectifier with adaptive bias tuning methods to improve the PCE performance of CMOS rectifiers.