Filament formation through self-focusing of light in semiconductor lasers leads to beam quality degradation, as well as lifetime and facet degradation. In this paper, we rederive an expression for the growth rate of sinusoidal perturbations, or filaments superimposed on the steady-state field in a semiconductor laser. We note that while the mode gain decreases with the confinement factor in a linear manner, the gain for filaments decreases far more rapidly. This agrees with recent observations of improved beam quality in broad-area semiconductor lasers with lowered confinement factors. Finally, we describe design rules for low-confinement-factor semiconductor lasers offering low-filamentation and greatly improved beam quality.