Fibers having an electrically conductive outer surface and having an average diameter of less than about 5 millimeters; and a dielectric polymeric layer comprising a polymer having a main polymer chain on the outer surface, the dielectric polymeric layer having a thickness of less than about 50 microns, the main polymer chain comprising carbon. Fiber transistors having an on/off ratio of at least about 10. Techniques for making fibers and fiber transistors.