A dense network of single wall carbon nanotubes (SWNTs) grown on a substrate is treated as a thin film to fabricate into a field effect transistor (FET) that can be made, for the first time in the literature, exhibiting either unipolar p-type, n-type or ambipolar characteristics by proper annealing for different gases. Different annealing conditions are made to find the clues or mechanisms leading to different types of transistors. The anneal with either O(2) or N(2) gas for the case when the SWNT network is covered with a HfO(2) layer on top can make either donor or acceptor trap states to occur at the SWNT film/HfO(2) interface. These trap states can accumulate either positive or negative charges during charge transfer process and cause suppression of either electron or hole conduction in the SWNT film and lead to either p-type or n-type FET. The SWNT FET fabricated is shown to have a much superior electronic properties than any of the FET made by single SWNT(1) or amorphous silicon thin film transistors(2) due to interconnection of the metal-semiconducting SWNT network. Fabrication of different types of SWNT Film FET allows fabrication of complementary SWNT FETs that can be used to assemble simple integrated circuits or logic devices in additional to the thin film transistors. Finally, the fabrication process used is simple (IC compatible) and cost effective which has the potential for later use in mass production. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.096112jes] All rights reserved.