We report on threefold-stacked vertically aligned InP/GaInP quantum dot injection lasers emitting in the visible part of the spectrum (690–705 nm) with a low threshold current density ofjth=172 A/cm2at 90 K showing a thermally activated increase towards higher temperatures. We derived an activation energy for this behavior, which is found to be just one half of the energetic distance between the dot transition energy and the wetting layer band gap. Thus, we identify thermal evaporation of carriers out of the dots and into the wetting layer states as the process responsible for the increase in the threshold current. The nonradiative carrier lifetime in the wetting layer(&tgr;nrWL)is estimated to be approximately 250–400 ps. ©1998 American Institute of Physics.