The invention relates to a photocathode for the infra-red range having a plurality of layers of semi-conductive and conductive material. The photocathode is transparent and sensitive in a spectral range of between approx. 1 and 20 .mu.m. This is achieved by the following layer structure:p.sub.1 : a highly doped p-layern.sub.2 : a highly doped n-layeri.sub.3 : an intrinsic layerp.sub.4 : a highly doped p-layerm.sub.5 : a thin metal layer, preferably of an atomic layer of Cs.The spectral sensitivity can be adjusted by applying a negative bias voltage to the layer p.sub.1 with respect to the layer P.sub.4. When this happens, the Fermi level of the layer p.sub.2 is shifted and the work function of the electrons is reduced.