Abstract: A new silicon based semiconductor device comprises a layer of amorphous silicon in which the density of energy states in the energy gap has been reduced by hydrogenation; this layer is deposited on a layer of a hydrogen-containing substrate material that can supply hydrogen in atomic form to the amorphous silicon. In processes of the invention the silicon layer and a substrate layer are hydrogenated separately to permit optimum hydrogenation; the silicon layer may be deposited without hydrogenation and hydrogenated subsequently with hydrogen from the substrate material. A specific example consists of a layer of hydrogenated amorphous silicon of about 1 micrometer thickness deposited on a hydrogen-containing chromium layer which is itself deposited on a carrier, the silicon then forming the active element of a photovoltaic cell particularly functional as a solar cell.