AbstractThe theory of impurity‐mediated free‐carrier absorption in n‐GaAs bulk material is extended to quasi‐two‐dimensional structures. The surfaces are treated within the classical infinite‐barrier model and the electron—electron interaction within the random‐phase approximation. Screening due to phonons is also considered. Using temperature‐dependent Green's functions and starting from the Kubo formula a general expression is derived for the free‐carrier absorption coefficient including both bulk and surface contributions. In thick n‐GaAs slabs coupled phonon—plasmon bulk excitations give rise to distinct peaks in the absorption spectrum. These structures are increasingly washed out with decreasing thickness. For comparison impurity‐mediated free‐carrier absorption spectra of non‐interacting electrons have also been calculated. At and below the bulk plasmon frequency the absorption is reduced due to screening. The absorption increases on the other hand wit