Rectifiers in CMOS for RFID Applications

Abstract: Radio frequency identification (RFID) technology is used today in a number of different areas, such as logistics, supply chain management, access control and environmental monitoring. The ability to store information electronically in small tags that can be read wirelessly has great potential. Recently, research on RFID technology has focused on sensor-tags, localization techniques, antennas and propagation, data security, communication protocols and circuit design for the tags and the readers. In a typical RFID system, a passive tag is powered up remotely by a radio frequency signal sent from a reader unit. The RF signal received by the tag antenna is converted to a DC-supply voltage in the rectifier in the analog front-end of the tag. To avoid loss in the rectifying operation, low-voltage Schottky diodes are often used in a multi-stage rectifier. However, using Schottky diodes is not cost-efficient because these diodes must be designed in advanced semiconductor processes. Because one of the demands on future RFID technology is to reduce the cost, efficient rectifiers that can be integrated in a low cost semiconductor process is highly desirable. For this reason, different rectifiers in standard CMOS has been proposed. This thesis discuss recent work as well as present new ideas on rectifiers in CMOS that have the potential to replace Schottky diodes in low-power, multistage rectifiers for magnetically coupled RFID systems. As a brief summary of this thesis, Part I includes a theoretical analysis of the RF to DC generation block. The analysis illustrates how different properties, such as voltage and power conversion efficiency of the rectifier, the Q factor of the resonance circuit and coupling coefficient between coil antennas, affect the tag DC generation. In Part II, paper A discusses the limitations with the CMOS crossconnected bridge rectifier and proposes a modified bridge with active diodes to improve rectifier performance. The proposed bridge was manufactured, and an evaluation of the chip show good agrement between simulated and measured performance. Paper B presents a theoretical model for diode connected MOS transistors with internal threshold cancelation (ITC), as well as a design procedure that describes how to optimize a rectifier based on MOS ITC diodes. In Paper C a highly efficient active MOS diode is presented that can be used in multi-stage low-power rectifiers. In addition, this study shows that active diodes in CMOS can be designed to have a diode voltage drop below 100 mV that consumes a small amount of μW. These results are promising in the improvement and cost reduction of inductively coupled RFID systems. The work in this thesis has shown that highly efficient RF to DC conversion can be achieved in CMOS rectifiers for low power applications. New techniques in CMOS have been demonstrated with the potential to replace Schottky diodes in RFID rectifiers.