The behavior of Ti silicidation on Si/SiGe/Si base and its effect on base resistance and fmax in SiGe hetero-junction bipolar transistors

The relation between Ti silicidation and base resistance in SiGe hetero-junction bipolar transistors (HBT) was investigated. The Ti layer deposited on the Si/SiGe/Si base converted to Ti silicide during two-step annealing. The thickness of the Ti silicide, which was identified as the Ti(Si1-xGex) phase of uniform composition, abruptly increased over the annealing temperature of 650/850 °C, and as a result it accomplished a very low extrinsic base resistance. The Ti silicidation affected the base resistance of real devices (RB), which was extracted from simulating the electrical data of SiGe HBTs such as I –V curves, forward Gummel plots, forward current gain curves, and s-parameter plots. It was shown that the RB was compatible with the theoretical relation which included the small-signal unity-gain frequency (fT), the maximum oscillation frequency (fmax) and RB. fmax varied more sensitively with RB than fT, which was due to the inherent property of fmax being inversely proportional to √RB. The fmax of the SiGe HBT reached 47.4 GHz when Ti silicidation was performed at the annealing temperature of 650/850 °C. This silicidation condition is thought to be an appropriate temperature for Ti silicidation applicable to SiGe HBT fabrication. © 2001 Kluwer Academic Publishers