Theory of noise in metal oxide semiconductor devices

Noise in MOS diodes arises from different sources: fluctuations in occupation of surface states, shot noise, and leakage noise. Fluctuations in the occupation of surface states produce changes in the surface space-charge distribution which in turn produce currents. Shot noise is produced by fluctuations of the individual drift and diffusion flows toward the surface. Leakage noise is associated with the small flow of current through the oxide. In MOS triodes these three mechanisms give rise to gate noise and thus input noise in the amplifier, but the first one produces an important indirect effect. Fluctuations in the occupation of interface states result in modulation of the channel conductance. At low frequencies this modulation is the dominant effect, giving rise to a noise power spectrum which resembles noise. At high frequencies, where only thermal noise in the channel and input noise are of importance, MOS triodes are similar to junction field effect devices from the noise point of view.