This chapter addresses the electronic properties of metallic carbon nanotubes and discusses the most important device made from a semiconducting nanotube, the carbon nanotube field effect transistor and its applications. It outlines the differences between Schottky barrier devices and bulk-switching devices and show how doping can be used to build high-performance transistors and logic from carbon nanotubes. The carbon nanotube field effect transistor, which utilizes a semiconducting carbon nanotube to switch, is a device that may have the potential to become technologically important for the semiconductor industry. Nanotubes that are dispersed in a solution may be deposited at any stage in the field effect transistor (FET) fabrication process as long as the solvent can be tolerated. S. Heinze et al.have modeled the carbon nanotube FET in various geometries, such as single or double gate, thin leads, thick leads, and needle-like leads, and found that the exact contact geometry does make a difference.