Interest in power harvesting for Radio Frequency Identification Tag (RFID) application has been rapidly growing. Currently the major challenge is to reduce the cost, decrease the size, and improve the efficiency and the sensitivity of the low power harvesting devices. This monograph presents a high efficiency RF to DC converter. The technique exploits an internal Vth cancellation along with a leakage current reducer. The work demonstrates methods to reduce the threshold voltage of a transistor and reducing its leakage current. It further discusses techniques to reduce sub-threshold leakage current and enhance the power conversion efficiency.