Excess current was obtained in GaAs/InAs quantum dot structures at low temperatures and low current levels. This excess current exhibited instabilities with changing the bias, and over the time. It has been concluded that the excess current is a C minority injection current connected with recombination through defects originated from the formation of QDs. The instabilities are connected with unstable occupation of energy levels induced by the above defects, which depend on temperature and on the current level. (C) 2002 Elsevier Science B.V. All rights reserved.