  We describe a mechanism, which links the long-range potential fluctuations
induced by charged defects to the low frequency resistance noise widely known
as 1/f noise. This mechanism is amenable to the first principles microscopic
calculation of the noise spectrum, which includes the absolute noise intensity.
We have performed such a calculation for the thin films of hydrogenated
amorphous silicon (a-Si:H) under the condition that current flows perpendicular
to the plane of the films, and found a very good agreement between the
theoretical noise intensity and the measured one. The mechanism described is
quite general. It should be present in a broad class of systems containing
poorly screened charged defects.
