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Noise sources in MOSFET transistors


The noise sources in a MOS transistor are:
thermal noise in the channel,
1/f noise,
Noise in the resistive poly gate,
noise due to the distributed substrate resistance,
shotnoise associated with the leakage current of the drain source reverse
diodes.
For normal use, only the first two items are important. The other noise sources
must be taken into account for very low noise applications.
1.1. Channel thermal noise.
A MOSFET has in normal working order an inverse resistive channel
between the drain and the source. The gate voltage forms with minority
carriers the channel. In the extreme case when the drain source voltage

1/f Noise in MOS Transistors.
1/f noise has been observed in all kind of devices, from homogeneous metal
film resistors and different kind of resistors to semiconductor devices and
even in chemical concentrated cells. Because 1/f noise is well spread over the
components, people think that there a fundamental physical mechanism is
behind it. Till now such a mechanism is not yet found.
There is a lot of experimental evidence that several mechanisms are involved
in generating 1/f noise.
The MOS transistor has the highest 1/f noise of all active semiconductors, due
to their surface conduction mechanism.
The result is: several theory’s and physical models competing together to
explain the 1/f noise in a MOSFET. These theory’s and models differ in detail
but are all based on the mobility fluctuation model expressed by the Hooge
empirical relation, and the carrier density or number fluctuation model first
introduced by McWhorter

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