A thin film transistor comprises a layer of organic semiconductor material comprising a tetracarboxylic diimide naphthalene-based compound having, attached to each of the imide nitrogen atoms, an aromatic moiety, at least one of which moieties is substituted with at least one electron donating group. Such transistors can further comprise spaced apart first and second contact means or electrodes in contact with said material. Further disclosed is a process for fabricating an organic thin-film transistor device, preferably by sublimation deposition onto a substrate, wherein the substrate temperature is no more than 100° C.