We have studied how midwave infrared (MWIR) photo- and electroluminescence of type-II "W" InAs/InGaSb/InAs/AlGaAsSb quantum well structures depend on molecular beam epitaxy growth conditions and substrate material. All samples were grown with Sb-4, in contrast to most recent reports that use Sb-2. Resulting devices represent the highest reported external differential efficiency for molecular beam epitaxy grown light emitting diodes emitting in the 4.3-4.6 mu m wavelength range in the continuous wave mode at the room temperature. Another important aspect of the work is the finding that MWIR emitters on InAs substrates are superior to those on conventionally used GaSb substrates. (c) 2006 American Vacuum Society.