Abstract: The results from self-consistent k· p computation of optical gain characteristics of AlSb/InGaAsP/GaAsSb type-II ultra-thin quantum-well heterostructures show a marked improvement in optical gain as compared to the InGaAsP/GaAsSb type-II ultra-thin quantum well heterostructures. The AlSb/InGaAsP/GaAsSb type-II ultra-thin quantum well heterostructures were designed to obtain enhanced optical gain as compared to InGaAsP/GaAsSb type-II quantum well heterostructures. An improvement in optical gain of 948 cm- 1 and a shift in peak energy of 0.03 eV is attributed to interband resonant tunnelling effect and the band alignment due to the presence of GaAsSb layer. Also, a narrower optical gain spectrum is observed in AlSb / In 0.5Ga 0.5As 0.8P 0.2/ Ga As 0.5Sb 0.5 QW heterostructure as compared to the In 0.5Ga 0.5As 0.8P 0.2/ Ga As 0.5Sb 0.5 QW heterostructures. Furthermore, the effect of variation in well width has been studied at AlSb layer thickness 1 nm for optical gain enhancement and wavefunction confinement where the resonant tunnelling effect is observed. Graphic Abstract: Fig. Optical gain in AlSb/ In0.5Ga0.5As0.8P0.2/ GaAs0.5Sb0.5 heterostructures [Figure not available: see fulltext.] © 2021, The Author(s), under exclusive licence to EDP Sciences, SIF and Springer-Verlag GmbH Germany, part of Springer Nature.