Why is there no internal collector resistance in HICUM?

The internal (or epi-) collector region in a bipolar (hetero-)junction transistor strongly determines the bias dependent terminal behavior. Most compact transistor model attempt to capture the current-voltage behavior of the epi-collector by a special equivalent circuit element (and associated analytical formulations) in addition to the usual transfer current source representing just the base region. This paper provides (i) theoretical proof that a separate representation of the epi-collector region in an equivalent circuit is neither required nor necessarily advantageous in terms of accuracy and (ii) a physics-based guide on how to model the epi-collector consistently if a separate description is still desired. The theoretical results are validated by numerical device simulation.