Commercial bipolar junction transistor (2N 2219A. npn) irradiated with 150 MeV Cu11+-ions with fluence of the order 10(12) ions cm(-2), is Studied for radiation induced gain degradation and deep level defects. 1-V measurements are made to study the gain degradation as a function of ion fluence. The properties such as activation energy, trap concentration and capture cross-section of deep levels are studied by deep level transient spectroscopy (DLTS). Minority carrier trap levels with energies ranging from E-C - 0.164 eV to E-C - 0.695 eV are observed in the base-collector junction of the transistor. Majority carrier trap levels are also observed with energies ranging from E-V + 0.203 eV to E-V + 0.526 eV. The irradiated transistor is subjected to isothermal and isochronal annealing. The defects are seen to anneal above 350 degrees C. The defects generated in the base region of the transistor by displacement damage appear to be responsible for transistor gain degradation. (C) 2008 Elsevier Ltd. All rights reserved.