Very thin mixed group-V layers deposited by molecular beam epitaxy (MBE) enable the precise bandgap engineering required to meet the demands of modern devices. In optoelectronic devices it is important to control material gain (wavefunction overlap) in addition to the bandgap. For example, the MBE growth of small bandgap GaAsSb on GaAs has application in 1300 nm datacom lasers. When comparing GaAs1-xSbx and InxGa1-xAs, the bulk material bandgaps are very similar for x<0.5, however, for the same strain levels on the GaAs, much longer emission wavelengths can be reached using GaAsSb. This is attributed to a much smaller conduction band offset for the antimonides and a large bandgap bowing parameter for pseudomorphic GaAsSb on GaAs. Furthermore, the conduction band offset is reported to be type-II for large Sb compositions, allowing access to still longer wavelengths. The cost of achieving longer wavelengths through type-II band alignments is a reduction in material gain due to poor electron confinement. Therefore the band alignment must be carefully considered when engineering active materials for optoelectronic devices.