The effects of mixed-mode stress damage on the DC performance of SiGe HBT devices were studied, and the changes in 1/f noise characteristics of the devices before and after the mixed-mode damage were compared. It shows that the mixed mode damage produces an interfacial defect Pb at the Si/SiO2 interface, which leads to the increase of the base current in low injection level. On the other hand, the base current in the medium injection region decreases due to the passivation of H atoms of the dangling bonds at polysilicon grain boundries, which leads to the enhancement of current gain. However, the mixed-mode damage defects are mainly located near the intrinsic Fermi level within the silicon band gap, which leads the increase of the SRH recombination component of base current. As a result, the low frequency noise characteristics of the device are not changed.