The features of the ion-solid interaction most often used in microfabrication are implantation, sputtering (ion milling), and induced chemical reaction. The novelty of focused ion beam fabrication is that most of these processes can now be carried out with better than 0.1 μm resolution without the use of mask or resist. For implantation the species available include the dopants of Si and GaAs (such as B, As, Be, and Si). Energies up to 300 keV are achieved if doubly ionized species are used. The focused ion beam systems can deliver a desired dose with 0.1 μm accuracy aligned to existing features on the wafer. So far, a number of applications of this unique maskless resistless patterning capability have been reported, including: threshold adjust of transistors, transistors fabricated with graded doping profiles and tunable Gunn diodes. In addition, focused ion beams promise to be competitive with e-beams for pattern writing in resist.