The bulk trap density of tris-(8-hydroxyquinoline) aluminum was evaluated by the differential method, and was found to be approximately 10(16) cm(-3) at the Fermi level of 0.45-0.60 eV below the lowest unoccupied molecular orbital (LUMO). This value increased with an increase in the Fermi energy. The activation energy of the current density changed with the applied voltage, i.e., traps of various energy levels contribute to conduction. The existence of a shallow trap at the Fermi level of 0.55 eV below the LUMO was confirmed. (C) 2002 American Institute of Physics.