= Solution
A <double heterostructure laser> places a lower-<band gap> active layer between wider-<band gap> layers with suitable <type-I band alignment>. The <band offsets> confine injected <electrons> and <electron holes> in the same small volume, increasing their overlap and reducing escape. Usually the active layer also has larger <refractive index>, so the same sandwich forms an optical <waveguide>. Optical confinement increases the gain-overlap factor $\Gamma$, while carrier confinement lowers the injection needed to reach the required <quasi-Fermi-level splitting>. These two distinct mechanisms reduce threshold current and heating. A <band gap> difference alone does not establish the sign of both confinement barriers; suitable <band offsets> are essential.
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