Impurity bands and sequential resonant tunneling in the presence of   terahertz fields
  A theoretical and experimental study of transport in a low doped multiple quantum well structure shows that impurity bands are essential in understanding the electronic transport both with and without terahertz irradiation. A full self-contained model, which involves only the nominal parameters of the sample, agrees quantitatively with a wide range of experiments. 
Andreas Wacker, Antti-Pekka Jauho, Stefan Zeuner, and S. James Allen