The current gain was obtained at a

The current gain was obtained at a fixed base-emitter voltage (VBE = 0.65 V) before and after irradiation. The variation in reciprocal of the current gain (Δ(1/β)) is defined as the reciprocal of current gain after irradiation subtracting the value before irradiation, Δ(1/β) = 1/βirradiated − 1/βpre. Fig. 3 demonstrates that Δ(1/β) verse irradiation fluence for the irradiated transistors. As shown in Fig. 3, Δ(1/β) increases super-linearly at lower fluence, while becomes a linear curve at higher fluence (>4 × 1011 cm−2). As discussed in previous result [9], when the ionizing effects are dominated at low fluence, and leading to a super-linear behavior. At higher fluence, the degradation curve gives a linear response that follows the Messenger-Spratt equation [1]. Based on the discussion in Ref. [9], it can be deduced that during the ionization/displacement combined irradiation in NPN BJTs, the displacement dose needs to be higher than a certain value (designated as critical fluence), in order to produce sufficient displacement damage defects to reduce the minority carrier lifetime. The critical fluence might depend on the structure of the BJTs and the characteristics of the incident charged particles. Therefore, the principal damage is ionization damage in the PNP BJTs at lower radiation fluences, and when the proton fluence is higher than the critical fluence, the displacement damage gives the major contribution to current gain degradation in BJTs.