Comprehensive Overview and Mitigation Strategies of Nonlinear Transistor Behavior in CMOS Ambient RF Energy Harvesting Systems

This article presents a comprehensive overview of nonlinear CMOS transistor circuits, their uses, effects, and techniques in ambient RF energy harvesting (RFEH) systems. The proliferation of wireless-enabled Internet of Things (IoT) devices and wireless sensor nodes (WSNs), along with their expanding wireless infrastructure, has created a favorable environment where ambient RF energy can serve as a primary power source. The trend towards full integration of RFEH systems for miniaturization and cost reduction relies heavily on CMOS technology, which enables the development of efficient, compact, and cost-effective solutions. The inherent nonlinearity of the CMOS transistor is central to the functionality of various circuits in RFEH systems; however, these nonlinear characteristics can also introduce undesirable effects when under constantly varying circuit conditions. While existing surveys typically classify RFEH designs by topology or other specific parameters, this review presents a novel framework centered on the analysis and mitigation of these undesirable effects. It starts with a brief overview of the components within an RFEH system, including antenna, rectifier, voltage regulators and energy storage devices, before diving into the circuit exploits of nonlinear CMOS transistors for enhancing RFEH system performance. The fundamental operation of these nonlinear transistors in different applications and the causes of undesirable effects are analyzed in detail, preparing readers for the subsequent categorized review of state-of-the-art techniques. By consolidating foundational concepts with a cause-and-effect analysis of mitigation techniques, this review provides a holistic understanding of CMOSbased RFEH systems and identifies critical research gaps for future innovation.