The space‐charge layer at ap‐njunction is a region of low concentration of electrons and holes, and therefore exhibits less absorption than the bulk of the semiconductor for radiation of sufficiently long wavelengths. Thus the space‐charge region of ap‐njunction provides an optical window of a width which can be modulated by an applied field. A quantitative estimate of this effect is given. Because of an obvious relation to unipolar transistors, the name ``gate modulation'' is proposed, as opposed to the ``injection modulation'' discussed previously.