We report quantum efficiencies exceeding unity for monochromatic red illumination in hydrogenated amorphous silicon (a-Si : H) pin diodes and a-Si : H/monocrystalline Si heterojunction solar cells. The values depend on the spatial size of the illumination spot which we systematically varied by alignment of different masks. The apparently large quantum efficiencies larger than unity are easily achieved for low photon flux if the illumination spot is allowed to be larger than the contact which defines the nominal conduction cross section. We present an equivalent circuit model which tracks the contributions of lateral photocurrents resulting from photogeneration outside the contact area. This additional current is effectively blocked by the voltage drop in the lateral direction across the collecting doped layer at high photon flux. True values of quantum efficiencies are always smaller than unity if restriction of the illumination spot to the contact area eliminates any lateral current contributions.