By changing the packing motif of the conjugated cores and the thin-film microstructures, unipolar organic semiconductors may be converted into ambipolar materials. A combined experimental and theoretical investigation is conducted on the thin-film organic field-effect transistors (OFETs) of three organic semiconductors that have the same conjugated core structure of s-indaceno[1,2-b: 5,6-b'] dithiophene-4,9-dione but with different n-alkyl groups. The optical and electrochemical measurements suggest that the three organic semiconductors have very similar energy levels; however, their OFETs exhibit dramatically different transport characteristics. Transistors based on compound 1a or 1c show ambipolar transport properties, while those based on compound 1b show p-type unipolar behavior. Specifically, compound 1c is characterized as a good ambipolar semiconductor with the highest electron mobility of 0.22 cm 2 V-1 s(-1) and the highest hole mobility of 0.03 cm 2 V-1 s(-1). Complementary metal oxide semiconductor (CMOS) inverters incorporated with compound 1c show sharp inversions with high gains above 50. Theoretical investigations reveal that the drastic difference in the transport properties of the three materials is due to the difference in their molecular packing and film microstructures.