  Logarithmic voltage profile characteristic to two-dimensional current flows
has been reported for several semiconductor surfaces. We analyse this
phenomenon within a simple model of accumulation and inversion layers. Prompted
by numerical analysis of several conductivity profiles we point out how the
logarithmic voltage drop may appear in the case of three-dimensional samples.
It may or may not be observed both for the accumulation and the inversion
layers. Further, the numerical data suggest that the surface conductivity at
large distances is governed by very few parameters.
