Rectifiers in CMOS for RFID Applications

Abstract: Radio frequency identification (RFID) technology is used today in a number of differentareas, such as logistics, supply chain management, access control and environmentalmonitoring. The ability to store information electronically in small tags that can be readwirelessly has great potential. Recently, research on RFID technology has focused onsensor-tags, localization techniques, antennas and propagation, data security, communicationprotocols and circuit design for the tags and the readers.In a typical RFID system, a passive tag is powered up remotely by a radio frequencysignal sent from a reader unit. The RF signal received by the tag antenna is convertedto a DC-supply voltage in the rectifier in the analog front-end of the tag. To avoid lossin the rectifying operation, low-voltage Schottky diodes are often used in a multi-stagerectifier. However, using Schottky diodes is not cost-efficient because these diodes mustbe designed in advanced semiconductor processes. Because one of the demands on futureRFID technology is to reduce the cost, efficient rectifiers that can be integrated in alow cost semiconductor process is highly desirable. For this reason, different rectifiers instandard CMOS has been proposed.This thesis discuss recent work as well as present new ideas on rectifiers in CMOSthat have the potential to replace Schottky diodes in low-power, multistage rectifiers formagnetically coupled RFID systems. As a brief summary of this thesis, Part I includesa theoretical analysis of the RF to DC generation block. The analysis illustrates howdifferent properties, such as voltage and power conversion efficiency of the rectifier, the Qfactor of the resonance circuit and coupling coefficient between coil antennas, affect thetag DC generation. In Part II, paper A discusses the limitations with the CMOS crossconnectedbridge rectifier and proposes a modified bridge with active diodes to improverectifier performance. The proposed bridge was manufactured, and an evaluation ofthe chip show good agrement between simulated and measured performance. Paper Bpresents a theoretical model for diode connected MOS transistors with internal thresholdcancelation (ITC), as well as a design procedure that describes how to optimize a rectifierbased on MOS ITC diodes. In Paper C a highly efficient active MOS diode is presentedthat can be used in multi-stage low-power rectifiers. In addition, this study shows thatactive diodes in CMOS can be designed to have a diode voltage drop below 100 mV thatconsumes a small amount of μW. These results are promising in the improvement andcost reduction of inductively coupled RFID systems.The work in this thesis has shown that highly efficient RF to DC conversion can beachieved in CMOS rectifiers for low power applications. New techniques in CMOS havebeen demonstrated with the potential to replace Schottky diodes in RFID rectifiers.