In article 4hFLc.141721$JR4.5322@attbi_s54,
"Rick Karlquist N6RK" writes:
You didn't even mention base spreading resistance, a major
figure of merit for RF transistors. Newer designers have
considerably lower r-sub-bb-prime than legacy transistors.
Look up the hybrid-pi transistor model. SiGe technology's
claim to fame is low base spreading resistance.

What you say about rbb is true, but SiGe has other 'cool'
characteristics, some derived from low rbb, and some due
to other effects of the 'strained' silicon?... Ignoring
the rbb itself, SiGe tends also to have very low 1/f noise
and good LF noise in general. Also, it tends to have high
Beta (at LF.) So, where an RF transistor might tend to have
a Beta of 20-50, an SiGE part might be 100-300 or higher.

With the combo of the low rbb (and low 1/f noise), along with
the high Beta, the total amount of input current noise and
input voltage noise is damned low.

SiGe would make good oscillators (for less PM noise) and
of course, good preamps. This is one case where GaAs FETS
that are very fast, might be undesirable because of their
worse 1/F noise characteristics.

One big disadvantage of the typical SiGe transistors is
that their breakdown voltage is low. However, the tradeoff
of breakdown voltage is BETTER for a given frequency response
and Beta than a normal Si transistor.

The SiGe transistors are also not very expensive. A part that
works well with reasonably low distortion and reasonably low
noise figure at 600MHz would be significantly less than $1.00.
Unless the transistor is too fast for a given layout, SiGe
can be used at low frequencies (e.g. VHF) while still avoiding
the low frequency noise problems that are common from GaAs FETS
and even other fast BJTs.

John