Abstract: Thin film transistors in which the active layer is a film of an organic semiconductor with a structure having two or three six-membered, fused aromatic rings with two five-membered, heterocyclic aromatic rings fused thereto. The five-membered rings are either substituted or unsubstituted. If substituted, the substituents are either alkyl or alkoxyalkyl with about two to about 18 carbon atoms. The organic semiconductor compound has a field-effect mobility greater than 10.sup.-3 cm.sup.2 /Vs and a conductivity less than about 10.sup.-6 S/cm at 20.degree. C. Thin film devices made of these materials have an on/off ratio of at least about 100.