Critical behaviors indicating an insulator-metal (IM) transition are observed in poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) [PBTTT] in ionic-liquid-gated transistors. At room temperature, a maximum channel conductivity of 300 S cm(-1) is achieved at the doping concentration of 10(21) cm(-3). The conductivity shows a very weak temperature dependence; the conductivity at 5K is only 1.6 times lower than that at 250K. The signature of the IM transition at low temperatures is evidenced by the results of Zabrodskii plot analysis. The IM transition is benefitted by the semicrystalline lamellar structure of PBTTT enhanced by the substrate treatment with a self-assembled monolayer. (C) 2015 The Japan Society of Applied Physics