GaAs single heterojunction laser diodes were prepared by epitaxially growing all the layers of the structure. The doping in then‐type confining layer and thep‐type active layer were varied as was the thickness of the active layer. The effect of these device parameters on the temperature variation of the threshold current and efficiency was determined. A correlation was observed between an abrupt increase in the threshold current on one hand and the onset of optical pulse delay effects on the other hand. Coincident changes in the spectrum and far‐field pattern were also observed. A dielectric slab model is used to explain the experimental results. It is also postulated that there was a strong saturable absorption mechanism associated with then‐type region.