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High-quality crystals of the organic molecular semiconductors tetracene and pentacene were used to prepare metal-insulator-semiconductor (MIS) structures exhibiting hole and electron mobilities exceeding 10 ^sup 4^ square centimeters per volt per second. The carrier concentration in the channel region of these ambipolar field-effect devices was controlled by the applied gate voltage. Well-defined Shubnikov-de Haas oscillations and quantized Hall plateaus were observed for two-dimensional carrier densities in the range of 10 ^sup 11^ per square centimeter. Fractional quantum Hall states were observed in tetracene crystals at temperatures as high as ~2 kelvin.
The quantum Hall effect (QHE) (1), in which the Hall resistance R ^sub xy^, of a quasi-two-dimensional (21)) electron or hole gas becomes quantized with values R ^sub xy^ = h/e ^sup 2^ j (where h is Planck's constant, e is the electron charge, and j is an integer), has been observed in a variety of inorganic semiconductors, such as Si, GaAs, InAs, and InP. At higher magnetic fields, fractional quantum Hall states where j is not an integer have also been observed (2). A QHE-like state was also seen in organic materials such as Bechgaard salts (TMTSF) ^sub 2^X (where TMTSF is tetramethyl tetraselenafulvalene and X = CIO ^sub 4^, ReO ^sub 4^, or PF ^sub 6^) (3-5). However, in...