Environmental Engineering Reference
In-Depth Information
Bipolar Junction Transistors
: Bipolar junction transistors (BJTs)
are widely used for high-frequency and high-speed applications due to
their large intrinsic There are four main noise sources associated
with a BJT : (1) thermal noise generated by the base resistance, (2)
shot noise in the base current, (3) flicker noise in the base current, and
(4) shot noise in the collector current. Flicker noise of BJTs arises
mainly due to the generation/recombination process in the emitter-
base depletion region and the trap/de-trap of carriers by the oxide
located above the emitter-base junction. The former usually predom-
inates over the later [97]. As compared with MOSFET transistors,
noise of BJTs is much smaller and manifests itself in frequency
regions several decades lower [98]. The base resistance consists of the
intrinsic base resistance and extrinsic base resistance. The intrinsic
base resistance is usually larger than the extrinsic [97]. However, due
to the effect of current crowding [2], the intrinsic resistance and the
associated thermal noise can be reduced by a large collector current.
The extrinsic base resistance is made up of the bulk and contact re-
sistances. It can be reduced by increasing the number of contacts in
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