A photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode that consists of an input window in the form of a disk made from sapphire, layers of heteroepitaxial structure of gallium nitride compounds as a semi-transparent photocathode grown on the inner surface of the input window, and an element for connecting the input window with a vacuum photoelectronic device housing, which is vacuum-tight fixed on the outer surface of the input window at its periphery. The element for connecting of the input window with the vacuum photoelectronic device housing is made of a bimetal, in which a layer that is not in contact with the outer surface of the input window consists of a material with a temperature coefficient of linear expansion that differs from the temperature coefficient of linear expansion of sapphire by no more than 10% in the temperature range from 20° C. to 200° C.