The present disclosure describes additives that attenuate a specific transport channel in ambipolar semiconductors to achieve unipolar characteristics. Carrier selective traps are included in the ambipolar semiconductors and are chosen on the basis of energetic preferences for holes or electrons and the relative positions of the molecular orbital energies of host polymer and the dopants. In one embodiment, a composition of matter useful as a current transport region in an organic semiconductor device comprises a semiconducting polymer; and means for accepting holes (e.g., a hole trapping compound) injected into the current transport region so as to impede conduction of the holes in the semiconducting polymer. This simple solution-processable method can improve the on and off current ratios (ION/IOFF) of OFETs by up to three orders of magnitude. Moreover, the treatment yields tailored blends that can be used to fabricate complementary inverters with excellent gain and low-power characteristics.