The Radiofrequency (RF) energy harvesting is a widespread technique to extend the lifetime of devices in low power applications such as Wireless Sensor Networks (WSNs). Among the key building blocks featuring the RF harvester, the rectifier converts the incident AC signal into a DC output voltage. This paper presents the comparison between a multi-stage traditional CMOS voltage multiplier and a multi-stage voltage multiplier using bulk biasing to overcome the threshold voltage drop. The two rectifiers operate at 900MHz and 2.4GHz ISM bands and are prototyped using a standard CMOS 130nm process. The bulk-biased rectifier achieves a significant increase in power efficiency and low voltage-drop. Considering a case of study for supplying a load of 3.6 µW, the overall power efficiency reaches 48%, which is an improvement of 17% compared with the traditional circuit.