Elektret

An electret, sometimes referred to as an elektret in German, is a dielectric material that can retain a quasi-permanent electric charge or polarization after being charged or poled. In electrets, charges are trapped in deep energy wells or dipoles are oriented in such a way that a surface potential remains for extended periods, providing a stable internal electric field without continuous power input.

Common electret materials include polymers such as polypropylene, PTFE, PMMA, polycarbonate, and fluoropolymers like PVDF. Ceramic and inorganic electrets, including silicon nitride and thin oxide layers, are used for high-temperature or specialized applications. Material choice depends on stability under operating temperature, humidity, and mechanical stress, as well as the desired electrical properties.

Electret charge can be created by charging the dielectric under a high electric field through methods such as corona discharge, electron or ion beam charging, and poling of polymers while heated. Some electrets also rely on triboelectric effects or charge injection during deposition. The resulting quasi-permanent surface potential or embedded charge enables various sensing and electrostatic functions.

The best-known application is the electret microphone, where a permanently charged dielectric forms a capacitor with a movable diaphragm to convert sound into an electrical signal without an external bias. Electrets are also used in capacitive sensors, microelectromechanical systems (MEMS), and some energy-harvesting or charge-storage devices. Their performance depends on the stability of the trapped charge, which can be affected by temperature, humidity, and mechanical handling, posing limitations for long-term reliability compared with other transduction materials.