In heterostructures, mobile electrons tend concentrate in one layer or the other layer. These heterostructure-driven concentrations improve the efficiency of semiconductor LED’s, and are absolutely essential for semiconductor lasers.

But for different crystal layers to bond perfectly to one another, their atomic spacing’s / bond lengths must be essentially identical. The best example of this is crystals of GaAs and AlAs. In this model, the dark blue atoms of the top layer could be Ga, the lighter blue atoms of the bottom layer, Al. But in this particular case, the orange atoms and yellow atoms of both layers would be As.