It is demonstrated that the mobility and number fluctuations in an n-channel metal oxide semiconductor field effect transistor (n-MOSFET), which has a shallow-doped channel, can be separately measured. The mobility fluctuation becomes the dominant source of noise in the case where a negative gate voltage is applied. This is because carriers are not near the Si/SiO interface. If a positive gate voltage is applied to the MOSFET, carriers exist near the Si/SiO interface, so that noise due to the number fluctuation becomes dominant.

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