Junction capacitance in the SiGe HBT based on movable charge

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 the current density, so three cases exist according to the current density. Based on the consideration of carrier transit of the 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.