If you click on the reference to "shot noise" in my previous post you will get numerous hits on an explanation. Basically it's due to the discrete charge of each electron as it crosses a barrier, such as a semiconductor junction. The statistical variation in the number of electrons crossing the barrier at any instant in time creates a variation in the voltage created by the sum of all the electrons, which we call noise.
Shot noise has nothing to do with whether a device is unipolar or bipolar. A vacuum tube (valve) has shot noise even though it is a unipolar device that uses only electrons.
A BJT will have Thermal noise for any resistance it has between terminals, but that is normally much less than the shot noise.