High-mobility organic single-crystal transistors are constructed with silicon dioxide, a polymeric insulator (polyvinylphenol), and an organic insulating single-crystal (diphenylanthracene). All the single-crystal devices show high-mobility p exceeding 10 cm(2)/V s with relatively low-density carriers of similar to 10(11) cm(-2), though mu is significantly reduced at one-order higher carrier density. Among the three, the diphenylanthracene device holds relatively high-mobility even at carrier density of similar to 10(12) cm(-2), so that it realizes the maximum conductivity ever achieved for organic transistors. Employing simultaneous measurements of Hall-effect and conductivity of the field-effect carriers, the result is attributed to different extent of randomness in the surface potential on the gate insulators. (C) 2007 Elsevier Ltd. All rights reserved.