The concept of non-​ionizing energy loss (NIEL) has been found useful for characterizing displacement damage defects in materials and devices. When NPN power transistors (2N6688 manufd. by BEL, India) are exposed for 110 MeV Si8+ ion irradn. in the fluence range 5 × 109 to 1 × 1013 ions cm-​2 at room temp. (300 K) and at liq. nitrogen temp. (77 K) cause functional failure due to surface and bulk defects. The output collector characteristics are studied as a function of total ionizing dose (TID) and total displacement damage dose (Dd) obtained using TRIM Monte Carlo code. It is obsd. that the shift in the output satn. voltage is considerably less for heavy ion irradn. compared to lighter ions like lithium ion irradn. The gain of the transistor degrades with ion irradn. Base reverse biased leakage current (BRBLC) increases with increase in ion fluence. The obsd. results are almost independent of the irradn. temp. These studies help to improve the device fabrication technol. to make Radiation Hard Devices for advanced applications.