In this study NPN rf power transistors were irradiated by 175 MeV Ni13+ ions in the dose range of 100 krad to 100 Mrad. Their characteristics such as excess base current (Delta I-B = I-Bpost - I-Bpre), dc current gain (h(FE)), transconductance (g(m)) and collector-saturation current (I-CSat) were studied before and after irradiation. The damage factor (K) for h(FE) was calculated using Messenger-Spratt relation. The base current (I-B) was found to increase significantly after irradiation and in turn decreases the h(FE). The g(m) and collector current (I-C) in the saturation region (I-CSat) were found to decrease with increase in radiation dose. The results obtained here were also compared with that obtained by 140 MeV Si10+ ions, 100 MeV F8+ ions, 95 MeV O7+ ions and 50 MeV Li3+ ions irradiation studies in the same dose ranges to understand the LET effects. The recovery in the I-V characteristics of irradiated NPN transistors were studied by isothermal and isochronal annealing methods. (C) 2012 Elsevier B.V. All rights reserved.