Despite polypyrrole having higher conductivities compared to polythiophenes, pyrrole based materials have garnered less attention in the organic electronics community due to their instability in air. Construction of stable pyrrolic units could be achieved by fusing relatively unstable pyrrole with stable aromatic rings. In this report, we discuss and compare the organic field-effect transistor performances of the smallest S,N-heteroacene and O,N-heteroacene, thieno[3,2-b]pyrrole and furo[3,2-b]pyrrole, respectively, in donor-acceptor-donor type organic semiconducting small molecules. Since both building blocks are highly electron-rich, thiophene flanked 5,6-difluorobenzo[c][1,2,5]thiadiazole is employed as a central electron-withdrawing unit. The small molecule containing thieno[3,2-b]pyrrole exhibit moderate hole mobilities (10-3 cm2 V-1 s-1) irrespective of the annealing temperature. In contrast, the small molecule bearing furo[3,2-b]pyrrole is completely inactive in field-effect transistors. To the best of our knowledge this is the first report to compare the smallest units of S,N and O,N-heteroacenes for organic field-effect transistors.