A study for the optimization of HgCdTe heterostructure lasers for applications as midinfrared wavelength sources has been carried out. Structures are examined to emit photons at 2.5 and 4.5 &mgr;m at 77 K. For the 2.5 &mgr;m case, it is found that a quantum‐well laser with well width of 200 A˚ in a separate confinement structure is optimum. For the 4.5 &mgr;m case the optimum structure is one with a 1000 A˚ active region. For the 4.5 &mgr;m case the high carrier density at threshold in quantum wells and the consequent high Auger rates do not allow the decrease of threshold current with smaller well sizes. This result is rather general for narrow‐gap zinc‐blende semiconductors and represents a cautionary warning against the commonly held belief that narrow quantum wells will always improve threshold currents.