Field-effect transistors (FETs) have been fabricated with thin films of a series of 2,9-dialkylated phenanthro [1,2-b:8,7-b'] dithiophene derivatives (C-n-PDTs). The FET characteristics of C-n-PDT thin-film FETs with an SiO2 gate dielectric as well as high-k gate dielectrics were recorded, and the dependence of the field-effect mobility, mu, on the number (n) of carbon atoms in the alkyl chains was investigated, showing that the 2,9-didodecylphenanthro[1,2-b:8,7-b'] dithiophene (C-12-PDT) thin-film FET displays superior properties, with mu s as high as 1.8 cm(2) V-1 s(-1) for the SiO2 gate dielectric and 2.2 cm(2) V-1 s(-1) for the HfO2 gate dielectric. The average mu values, <mu >, reach 1.1(5) and 1.8(6) cm(2) V-1 s(-1), respectively, for the SiO2 and ZrO2 gate dielectrics. Low-voltage operation, showing an absolute average threshold voltage <|V-th|> of similar to 11 V, was implemented, together with the above high <mu > of similar to 2 cm(2) V-1 s(-1). Also, a flexible FET was fabricated with a parylene gate dielectric. The results of this study show the potential of the C-12-PDT molecule for application in a high-performance transistor.