The undesired behavior of the substrate significantly affects the output impedance of the device; hence degrades circuit performance mainly in the high frequency regime. Therefore, for high-speed and RF circuits, collector-substrate modeling has to be sufficiently accurate. In this paper, an improved collector-substrate equivalent circuit model is proposed. The circuit model elements are physics based and are calculated from technological data. The validity of the equivalent circuit has been verified by on-wafer measurements of an SiGe HBT fabricated in B55 technology up to 330 GHz, the highest frequency reported so far for collector-substrate modeling. The proposed substrate network can be considered as an extension of the latest large-signal HICUM model (L2v2.4). © 2019 IEEE.