fig1 shows the components of a composite / semiconductor / glass material with the glass substrate 21 and the semiconductor layer 25 . the semiconductor layer 25 and the glass substrate 21 are placed one on the other with their surfaces 23 and 22 in contact as described , for example , in the earlier federal republic of germany patent application no . p 28 42 492 . 2 corresponding to u . s . pat . application ser . no . 80 , 948 , filed oct . 1 , 1979 , now u . s . pat . no . 4 , 311 , 743 , and are connected together by application of pressure and heat . this type of composite / semiconductor / glass material is used as a photocathode , for example , in an image converter or image intensifier tube . in order to achieve an improvement in the optoelectronic properties of these cathodes , an attempt must be made to reduce the recombination of minority charge carriers at the interface layer between the semiconductor layer and the glass substrate . this can be achieved , in accordance with the invention , by the semiconductor layer 25 having layer regions 24 , 26 which have different proportions of doping agent . in the layer region 26 of the semiconductor layer 25 which is remote from the interface between the semiconductor layer and the glass substrate , the concentration of the doping agent is constant and has the value n = n o . starting from this value the proportion of the doping agent increases continuously in the direction of the surface normals of the semiconductor layer 25 as the spacing from the semiconductor / glass interface 22 , 23 decreases , in a part 24 of the semiconductor layer 25 , up to a value of n = n s at the surface 23 of the semiconductor layer 25 adjacent the glass substrate 21 . the curve for the proportion of doping agent , n , is shown as a function of the spacing z from the interface area between the semiconductor layer 25 and the glass substrate 21 in fig3 . particularly good properties are achieved in the composite / semiconductor / glass material if the concentration n s of doping agent at the outer surface 23 of the semiconductor layer 24 , adjacent the glass substrate , is more than approximately thirty ( 30 ) times the doping agent concentration n o in the region 26 of the semiconductor layer 25 . the minority charge carriers ( electrons ) are reflected by the layer 24 of the semiconductor layer 25 because of the distribution of the doping agent shown in fig3 . this is made clear by the diagram of the energy band of the semiconductor layer shown in fig2 . in this figure , the fermi level is designated 30 . the reference numbers 31 and 32 represent the edge of the valency band or the edge of the conduction band respectively . this type of arrangement of the energy level is a p - doped semiconductor layer causes the minority charge carriers ( electrons ) to be reflected at the layer 24 with the result that a reduction in the recombination of minority charge carriers is achieved in the border region between the semiconductor layer 25 and the glass substrate 21 . the thickness of the layer 24 is preferrably chosen between 10 and 100 nanometers . the proportions of doping agent may be introduced into the layer 24 by means of ion implantation in a particularly favourable manner . however the curve for the proportion of doping agent in semiconductor layer 25 may be affected by a diffusion process . in a preferred manner the diffusion is implemented so that the outer surface of the semiconductor layer 25 which limits the layer 24 is covered by a silicon dioxide layer enriched with a doping substance at least during the diffusion process . this silicon dioxide layer is removed in a manner known per se once the diffusion process has been completed . * it will be understood that the above description of the present invention is susceptible to various modification changes and adaptions .