An insulating layer in a field effect transistor is formed of superfine ceramic particles dispersed in a polymeric matrix. The characteristics of the insulating layer can be changed by varying the mix of ceramic particles and matrix components. Appropriate selection of components can provide a high dielectric constant material which is not subject to pinholes, has a high voltage breakdown and is chemically resistant. The material can be applied at relatively low processing temperatures, using a wide range of coating techniques, and is highly suited for use with polymeric substrates.