We use a theoretical model of semiconductor lasers to calculate threshold current density (Jth), optical gain (g), and spontaneous emission rate (rsp) as a function of the two heterojunction band offsets between Zn1−uCduSe(well)/ZnSzSe1−z(guiding) layers and ZnSzSe1−z(guiding)/ Zn1−xMgxSySe1−y(cladding) layers. We find that for the conduction‐band offset between the well and the guiding layers ranging from ∼0.55&Dgr;Egto ∼0.75&Dgr;Eg(&Dgr;Egbeing the energy band‐gap difference), there is very little change inJth,g,rsp. Furthermore, these three quantities are very weakly dependent on the conduction‐band offset between the guiding and the cladding layers ranging from 0.2&Dgr;Egto 0.5&Dgr;Eg. However, if the offset between the well and the guiding layers is increased from ∼0.75&Dgr;Egto ∼0.8&Dgr;Eg, the calculations indicate a sharp drop inJth,g, andrsp. We explain this behavior as due to the unbinding of the ground‐state light‐hole subband as the valence band offset is reduced. ©1996 American Institute of Physics.