A simple and accurate closed-form expressed for the threshold-current density Jth in symmetric DH structures is presented. The novel expresion allows analytical solutions for do, the active-layer thickness corresponding to minimum threshold-current density. Optimization of the cavity thickness for minimum Jth is presented for wide variations in cavity length (100–500 μm) and facet reflectivity values. The analytical formulas are applied to the AlGaAs/GaAs system and extended to the InGaAsP/InP system. By using previously published experimental results, a linear gain-current relationship is estimated for InGaAsP (λ=1.2–1.3 μm), and thus it is found that do should vary between 0.12 and 0.20 μm as the cavity parameters (length and facet reflectivity) change; and that minimum Jth values should be comparable to the Jth values for AlGaAs lasers with the same cavity parameters.