Using current noise measurement techniques, the authors have studied the effects of electron beam exposure on field effect transistors based on carbon nanotube channels. In the case of p-type semiconducting nanotubes, the authors find that high doses induce a potential barrier along the channel, and transport is dominated by the tunneling events across this barrier. The authors suggest that the barrier is induced by charges trapped in the underlying SiO(2) barrier. Complementary studies on metallic nanotubes do not exhibit this behavior. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3517517]