The results of bromine doping of transparent and conducting single wall carbon nanotube (SWNT) thin films are described. Br profoundly effects the density of states (DOS) of SWNTs which leads to dramatic improvement in the electrical properties. The authors show that the role of the Br is not only in shifting the Fermi level but also in forming acceptor sites in metallic SWNTs. These modifications of the DOS through bromination lead to simultaneous increase in both the on/off ratio and mobility of thin film transistors. Furthermore, the transistor characteristics of Br-functionalized SWNTs are similar in air, inert atmosphere, and vacuum. (c) 2007 American Institute of Physics.