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:

${\overline {i_{n}^{2}}}=2qI_{D}B$

(1)

Where $q=1.602\times 10^{-19}\,\mathrm {C}$, $I_{D}$ is the DC current flowing through the diode, and $B$ 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:

${\overline {i_{b}^{2}}}=2qI_{B}B$

(2)

${\overline {i_{c}^{2}}}=2qI_{C}B$

(3)

Fig. 2 shows the small signal model of the BJT, showing the shot noise generators, ${\overline {i_{b}^{2}}}$ and ${\overline {i_{c}^{2}}}$, and the thermal noise generators for the physical bulk and terminal resistances, $r_{b}$, $r_{e}$, and $r_{c}$. Note that the small signal resistances $r_{\pi }$ and $r_{o}$ 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.