We have presented experimental results and theoretical estimations of the electronic structure of the GaAs quantum dots (QD) in a GaSb matrix which have large enough lattice-mismatch (similar to7%) to introduce self-organization of QD nanostructures optically active in mid-IR range. This nanostructure is the only type-II QD system with the electron confinement, which meets the 3D-confinement conditions due to anomalously small lateral sizes and height of the QDs. The reason of the occurrence of such small QDs is still questionable, although the tensile stress in a thin GaAs layer, driving the self-organization process in this case, may be responsible for that. (C) 2004 Elsevier B.V. All rights reserved.