A theoretical prediction that the lasing gain must increase by breaking the electrically neutral condition in the active layer is given. A possible structure to give a very low threshold current less than 1 mA was discussed.

The effects of impurity atoms and energy gap profiles in the optical guiding layers of GaAs-AlGaAs Separate Confinement Hetero (SCH) structure on the threshold current density are theoretically analyzed. Impurity doping in the optical guiding layers is not effective in reducing the threshold current, because impurity atoms induce non-effective current in those layers. The threshold current densities in three types of band gap profiles of the optical guiding layers are examined to find the most profitable structre of the SCH laser. Examined shapes of the profile are step, parabolic and linear types. The lowest threshold current densities calculated in each profile are almost identical if the design parameters such as the thickness of each layer and the alloy ratio are suitably chosen.