We report our study on the current-voltage characteristics of single layer devices employing pure and heavily tetrathianaphthacene (TTN)-doped tris(8-hydroxyquinoline) aluminum (Alq(3)) films as active layers. Sandwich structures with magnesium (Mg) top and bottom electrodes were employed in the devices. In the undoped device, the current increases exponentially with the applied voltage, and is concluded to be limited by the carrier injection at the Mg/Alq(3) interface with a small injection barrier. With the same device structure, both the bulk-doped and interface-doped devices show a bulk-limited current flow, which is much larger than that in the undoped device and behaves as trap-free space-charge-limited current (SCLC) with a field-independent mobility. The improvement of the current is ascribed to the large enhancement of carrier injection by the doping. An increase of the magnitude of the interfacial dipole has been proposed as the possible cause for the enhancement of carrier injection.