ABSTRACT
The major noise sources in a bipolar transistor are the base resistance thermal noise, or Johnson noise,
the base current shot noise, and the collector current shot noise. The base resistance thermal noise
is typically described by a noise voltage with a spectral density of 4kTR, and the shot noise is described
by a spectral density of 2qI, with I as the DC base current or collector current. These descriptions
are based on macroscopic views. The standard derivation of the magic 2qI shot noise assumes a
Poisson stream of an elementary charge q. These charges need to overcome a potential barrier, and
thus flow in a completely uncorrelated manner. In a bipolar transistor, the base current shot noise 2qIB results from the flow of base majority holes across the EB junction potential barrier. The reason that
IB appears in the base shot noise is that the amount of hole current overcoming the EB barrier is
determined by the minority hole current in the emitter, IB. Similarly, the collector current shot noise
results from the flow of emitter majority electrons over the EB junction potential barrier, and has a
spectral density of 2qIC.
