  I present a theory of electron dynamics in semiconductors with slowly varying
composition. I show that the frequency-dependent conductivity, required for the
description of transport and optical properties, can be obtained from a
knowledge of the band structures and momentum matrix elements of homogeneous
semiconductor alloys. New sum rules for the electronic oscillator strengths,
which apply within a given energy band or between any two bands, are derived,
and a general expression for the width of the intraband absorption peak is
given. Finally, the low-frequency dynamics is discussed, and a correspondence
with the semiclassical motion is established.
