The theory of semiconductor lasers, both double heterostructures (DHs) and quantum structures, which was developed in earlier chapters, considered direct-bandgap semiconductors. It is well known that although Si is the most used material for electronics, it is not suitable for photonic device applications, particularly in the form of emitters, due to the indirect nature of its bandgap. The same is true for Ge, the other Group IV semiconductor that is used in electronic devices.