Scientists at Infineon Technologies, Erlangen, Germany, have fabricated organic transistors and circuits on commercially available cotton-fiber paper--a neat trick, considering paper is hardly an ideal circuit substrate. Its fibrous surface is rough, whereas a typical semiconductor substrate such as silicon is smooth; it soaks up processing chemicals like a sponge; and it would burst into flames at normal semiconductor fabrication temperatures.
Infineon worked around the combustion problem by relying on the semiconducting organic material pentacene, which is workable at relatively low temperatures. And the researchers solved the sponginess problem by coating the paper with a polymer film less than a micrometer thick. The paper's inherent roughness forced Infineon to make the transistors about 100 times as large as those you'd find on a silicon circuit, resulting in much slower switching times.
Organic Electronics on Paper , by Florian Ender et al., Applied Physics Letters, 5 April 2004, pp. 2673-75.