Material selection for double heterojunction lasers--A higher bandgap does not necessarily mean lower refractive index

Double heterojunction (DH) lasers are composed of a lower energy bandgap active layer sandwiched between higher bandgap cladding layers, and the refractive index of the active layer has to be higher than that of the cladding layer. A number of published papers are based on the belief that higher bandgap materials necessarily have lower refractive indexes, so that the correct-bandgap DH structure automatically satisfies the requirement for the refractive index. This paper points out that this belief appears to be incorrect, and presents a series of examples in support of this statement.