Microcapacitors for Energy Storage: General Characteristics and Overview of Recent Progress

Autonomous sensor systems and networks are of increasing demand toward the realization of the Internet of Things (IoT). In this respect, integrated devices for energy storage and management, such as microcapacitors and microbatteries, are intensively studied. Their integration with Si electronics is highly desirable, as it offers important advantages related to fabrication simplicity and cost reduction. Moreover, microcapacitors as compared with microbatteries offer additional advantages, including a theoretically infinite number of charge/discharge cycles, which makes them more appropriate for truly autonomous systems. They thus constitute the focus herein. Their basic characteristics for use in energy storage are discussed, and their interdependence is demonstrated, along with the demonstration that those characteristics cannot be independently optimized for energy storage. The two types of existing microcapacitors, namely the solid-state microcapacitors and the microsupercapacitors, are presented in terms of their fabrication and integration schemes and their energy-storage characteristics. Their advantages and disadvantages for the specific applications are analyzed. The usefulness of solid-state microcapacitors in the above application, despite their significantly lower capacitance density compared with microsupercapacitors, is demonstrated, attributed to their integration maturity with standard complimentary metal-oxide-semiconductor (CMOS) processing, in addition to their larger charging voltages and maximum frequencies of operation.