This paper reviews some of our recent work which demonstrated that substitutional doping of a‐Si can a‐Ge is possible. Bothn‐ andp‐type specimens of a‐Si can be prepared with room temperature conductivities varying over some ten orders of magnitude, corresponding to a shift in the Fermi level position of about 1.2 eV. This control of the properties has enabled the density of states to be determined over an increased energy range. The results of thermoelectric power measurements onn‐type samples are described and discussed in terms of the normal electronic contribution for extended state conduction and a phonon drag effect. The doping of a‐Si also suggests device applications and the recent production of an amorphousp‐njunction is briefly discussed.