FREQUENCY DOMAIN ANALYSIS OF PERIODICALLY SWITCHED LINEAR CIRCUITS - Computer Methods for Analysis of Mixed-Mode Switching Circuits - page 275

For MOSFET transistors, it was found that the mobility fluctuation
model only holds when the devices are operated in the triode region
where the inverse layer can be approximated by a homogeneous re-
sistor. The density fluctuation theory, on the other hand, predicts
noise accurately in all regions of MOSFET transistors.
Flicker noise is always associated with a direct current and is often
modeled as a stationary process with power spectral density given by
[2]
where I is the average current, is a process and temperature
dependent constant, and are constants whose values are in the
ranges of and and is frequency. For most
electronic devices, flicker noise surpasses thermal and shot noise at
low frequencies. Extensive experiments show that there is no change
in the shape of the power spectral density of flicker noise even at
extremely low frequencies [96]. The upper frequency limit of flicker
noise is difficult to detect as it is usually masked by the floor of ther-
mal noise. The corner frequency, defined as the frequency at which
the power spectral density of thermal/shot noise and that of flicker
noise intercept, is often used as the upper bound of the frequency of
flicker noise for MOSFETs and is in the range of several MHz.
4.3 Noise Equivalent Circuits
The equivalent circuits of integrated devices with noise sources in-
cluded are used in noise analysis of electronic circuits.
Resistors : Physical resistors such as diffusion resistors and polysilicon
resistors, generate thermal noise. At low frequencies, a resistor can be
modeled as a hypothetical noise-free resistor in series with a random
voltage generator or a noise-free conductor in parallel with a random
current generator, as shown in Fig.11.24. At high frequencies, the
parasitic capacitances associated with the resistors must also be taken
into account.