"In the last section, we looked at the p-n junction. More efficient recombination of electron-hole pairs can be acheived by incorporation of a thin layer of semiconductor material, either p or n type semiconductor with a smaller energy gap than the cladding layers, to form a double heterostructure. (More on this in the future). As the active layer thickness in a double heterostructure becomes close to the De-Broglie wavelength (about 10nm for semiconductor laser devices) quantum effects become apparent."