  Silicon-germanium heterojunction bipolar transistors (HBTs) are of interest
as low-noise microwave amplifiers due to their competitive noise performance
and low cost relative to III-V devices. The fundamental noise performance
limits of HBTs are thus of interest, and several studies report that
quasiballistic electron transport across the base is a mechanism leading to
cryogenic non-ideal IV characteristics that affects these limits. However, this
conclusion has not been rigorously tested against theoretical predictions
because prior studies modeled electron transport with empirical approaches or
approximate solutions of the Boltzmann equation. Here, we study non-diffusive
transport in narrow-base SiGe HBTs using an exact, semi-analytic solution of
the Boltzmann equation based on an asymptotic expansion approach. We find that
the computed transport characteristics are inconsistent with experiment,
implying that quasiballistic electron transport is unlikely to be the origin of
cryogenic non-ideal IV characteristics. Our work helps to identify the
mechanisms governing the lower limits of the microwave noise figure of
cryogenic HBT amplifiers.
