Ionizing-radiation induced degradation of SiGe HBTs

Ionizing radiation induced degradation of 30% SiGe HBTs was studied by exposing them to Co⁶⁰ γ-radiation under bias upto a total dose of 3.2 Mrads. The devices were characterized immediately after each dose using: 1) Gummel measurements 2) Reverse bias E-B and C-B leakage currents and 3) E-B and C-B reverse capacitance measurements. To find out the effect of emitter geometry, devices with different perimeter to area ratios were investigated. The results indicate that the devices are quite rad-hard and the damage is qualitatively similar to that of Si BJTs. The base current I_B is observed to increase with dose without any increase in collector current I_C, thus causing degradation in gain I_C/ I_B. Also devices with higher P/A ratio show more degradation compared to the degradation of devices with lower P/A ratio. The reverse C-B and E-B capacitance and E-B and C-B leakage currents do not change due to irradiation. The experimentally observed degradation of SiGe HBTs can be explained considering positive oxide charge generation and interface states generation in the oxide over the E-B junction. This would cause excess recombination in the base region close to the surface and near the E-B junction causing increase of I_B. However, no change in the E-B reverse bias leakage current indicates that the generation of interface states is negligible and the degradation is primarily because of the positive charge generation in the oxide.