84 A Brief Review of Technology4.7.2 Shot NoiseShot noise occurs at both the base and the collector, and is due to the discrete na-ture of charge carriers as they pass a potential barrier, such as a pn junction. That is to say that even though we think about current as a continuous flow, it is actually made up of many electrons (charge carriers) that move through the conductor. If the electrons encounter a barrier they must cross, then at any given instant a differ-ent number of electrons will cross that barrier even though on average they cross at the rate of the current flow. This random process is called shot noise and is usually expressed in amperes per root hertz. The base shot noise is described by: bn2 BiqI= (4.22)and the collector shot noise is described by: cn2 CiqI= (4.23)where IB and IC are the base and collector bias currents, respectively. The frequency spectrum of shot noise is white.4.7.3 1/f NoiseThis type of noise is also called flicker noise, or excess noise. 1/f noise is due to variation in the conduction mechanism, for example fluctuations of surface effects (such as the filling and emptying of traps) and of recombination and generation mechanisms. Typically, the power spectral density of 1/f noise is inversely propor-tional to the frequency and is given by the following equation:Figure 4.9 fT as a function of currents for different transistor size relative to a unit transistor size of 1x.