Diode and Transistor Noise
In active devices, aside from the thermal noise due to resistive elements, we have two additional sources of electronic noise: (1) shot noise in PN junctions, and (2) flicker noise in MOSFETs.
Diode Noise
Shot noise is the random movement of quantized charges flowing through a forward-biased PN junction. The shot noise power in a diode, shown in Fig. 1, is given by:
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(1)
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Where , is the DC current flowing through the diode, and is the observation bandwidth. Shot noise has a white power spectral density similar to thermal noise, however, it is independent of temperature and instead, is proportional to the DC diode current. Since noise, in general, can be considered a "small signal", we normally include the noise generators in the diode small signal model.
BJT Noise
In a bipolar junction transistor (BJT), both PN junctions produce shot noise, which we model as:
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(2)
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(3)
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Fig. 2 shows the small signal model of the BJT, showing the shot noise generators, and , and the thermal noise generators for the physical bulk and terminal resistances, , , and . Note that the small signal resistances and are not physical resistors. These small signal resistors are used to model mechanisms such as recombination and base-width modulation, and thus, do not generate noise.