A new physical approach for the design of mid-IR lasers operating at 3–5μm based on type II heterojunctions with effective electron–hole confinement owing to a large asymmetric band-offset at the interface (ΔEC>0.6eV and ΔEV>0.35eV) has been proposed. The creation of high barriers for carriers leads to their strong accumulation in the active region and increases the quantum emission efficiency of the spatially separated electrons and holes across the heteroboundary due to a tunnel-injection radiative recombination mechanism within the device. An extremely weak reduction of the electroluminescence (EL) intensity for the interface tunnelling-assisted emission band with increasing temperature from 77 to 300K was observed. This coherent emission (λ=3.146μm at 77K) was totally polarised in the plane perpendicular to the p–n heterojunction plane, which means the laser emission was TM-polarised due to tunnelling-assisted light-hole–electron recombination across the interface.