The dependence of electron diffusion length on the excitation and impurity compensation in thepregion of a GaAs junction laser has been calculated from the band theory of impure semiconductors. Using a screened potential for each impurity atom, the average room‐temperature diffusion length in a typical diffused diode is found to be 1.4 &mgr; at low injection limit. The diffusion length increases with increase in excitation because of the enhancement in carrier screening and carrier diffusion energy. This effect can be used to explain the much lower threshold current in an AlxGa1−xAs&sngbnd;GaAs heterostructure laser and an abnormally high threshold current in a shallow diffused homostructure laser. Furthermore, the electron diffusion length is found to be larger for the region near the junction due to longer electron lifetime in this region. This also explains the observation that in a normal diffused junction laser, the active region occurs somewhere in thepregion but not right next to the space‐charge region.