Abstract: When the SiGe HBT is in the normally amplified condition, there is more movable charge in emitter and collector space charge regions. Thus differential capacitance should be adopted when considering their junction capacitance. Additionally the width of the junction is largely related to teh current density, so three cases exist according to the current density. Based on the consideration of carrier transit of teh SiGe HBT, the B-E junction capacitance model related to carrier distribution and the B-C junction capacitance model under different current densities are established and analysed.

Key words: SiGe HBT, junction capacitance, differential capacitance, mobile charge