The physical mechanisms of the breakdown of carrier confinement are considered using both the thermionic‐emission and diffusion models. It is shown that, for most practical AlGaAs/GaAs DH lasers, the diffusion current is responsible for carrier leakage. The thermionic emission of minority carriers is important when the confinement barrier or the mobility is very large. The theory presented here is also applicable to structures other than the AlGaAs/GaAs laser.