Electronic devices may have thin films which are conductors (metals), insulators (dielectrics) or semiconductors. Since the electrical conductivity is low, gases are dielectrics or insulators. They must function as conductors, semiconductors, dielectrics, or insulators. Other polymers are used mostly as insulators and dielectrics. It also forms within a polyethylene dielectric (as well as many other solid dielectrics). Organic electronics not only includes organic semiconductors, but also organic dielectrics, conductors and light emitters. Other potential materials include microwave dielectrics and carbon nanotube (CNT) composites. SWNTs are compatible with field-effect transistor (FET) architectures and high-k dielectrics. The different dielectrics of the many capacitor types show great differences in temperature dependence. Air, nitrogen and sulfur hexafluoride are the three most commonly used gaseous dielectrics. The related piezoelectric effect occurs only in a particular class of dielectrics. The surface states model can be used to explain electron transfer between two dielectrics. Several solid dielectrics are available, including paper, plastic, glass, mica and ceramic. He discovered this type of polarization in other materials including a class of dielectrics. For non-magnetic dielectrics, this equation is equivalent to the Fresnel equations.