Single active‐layer visible‐spectrum In1−xGaxP1−zAszheterostructure lasers are described that exhibit quantum size effects (QSE) in photoluminescence and by means of diode injection (&lgr;<6300 A˚). Liquid‐phase epitaxial (LPE) quaternary layers as thin as ∼300 A˚ have been realized. It is demonstrated that single thin‐layer recombination wells (Lz<1000 A˚) are very effective in the capture and recombination of carriers, even if injection occurs at some distance (a<1 &mgr;m) from the thin layer. On the other hand, optical confinement and absorption are weak in diodes with single active layers thin enough to exhibit QSE. This behavior indicates that the type of thin‐active‐layer diodes described here are not optimum as lasers, but may lead to improved light emitters.