The electrons and holes recombine in the intrinsic semiconductor emitting photons. If the width of the intrinsic region is reduced to the order of the de Broglie wavelength, the energies in the intrinsic region no longer become continuous but become discrete. (Actually, they are not continuous but the energy levels are very close together so we think of them as being continuous.)
In the context of semiconductors, electrons and holes recombine, producing photons. However, when the width of the intrinsic region is minimized to match the de Broglie wavelength, the energy levels within this region transition from being nearly continuous to being clearly distinct.