Impact of Bias Conditions on Total Ionizing Dose Effects of $^{60}{\hbox{Co}}\gamma $ in SiGe HBT

The effect of bias condition on total ionizing dose (TID) of Silicon–Germanium heterojunction bipolar transistors (SiGe HBTs) is investigated. The SiGe HBTs are set at forward, saturated, cutoff, and all-grounded biases during
$^{60}{\hbox{Co}}\gamma $
irradiation. After each irradiation stops, the forward Gummel characteristic and inverse Gummel characteristic are measured, and a 3-D simulation of TID for SiGe HBT is performed. The mechanism of TID in different bias conditions is obtained by analyzing normalized excess base current. The results show that the TID damages are different at various irradiation biases of the SiGe HBT, and the worst bias between forward and inverse Gummel characteristics exhibits inconsistently. The reason could be attributed to different defects produced and accumulated in oxide layers by irradiation at various bias conditions. To be specific, the oxide trap charges (
${{\hbox{N}}_{\rm ot}}$
) in emitter/base (E/B) Spacer is important to the forward Gummel characteristic, yet the
${{\hbox{N}}_{\rm ot}}$
in LOCOS determines the inverse Gummel characteristic. However, after long time irradiation, the interface states (
${{\hbox{N}}_{\rm it}}$
) both in E/B Spacer and LOCOS dominate the damage to the SiGe HBT despite in forward Gummel mode or inverse Gummel mode.