Silicon NPN overlay RF power high speed commercial bipolar junction transistors (BJTs) find applications in military, space and communication equipments. Here we report the effect of 50MeV Li3+ ion irradiation in the fluence range 1×1011–1.8×1012ionscm−2 on NPN power transistor. The range (R), electronic energy loss (Se), nuclear energy loss (Sn), total ionizing dose (TID) and total displacement damage (Dd) in the silicon target are calculated from TRIM Monte Carlo Code. Output resistance is 3.568×104Ω for unirradiated device and it increases to 6×107Ω as the fluence is increased from 1×1011 to 1.8×1012ionscm−2. The capacitance of the emitter-base junction of the transistor decreases and dielectric loss of the emitter-base junction increases with increase in ion fluence. The built in voltage of the unirradiated sample is 0.5V and it shifts to 0.4V after irradiation at fluence of 1.8×1012ionscm−2 and the corresponding doping density reduced to 5.758×1016cm−3. The charge carrier removal rate varies linearly with the increase in ion fluence.