Exact Solutions of Two-Band Models of Graded-Gap Superlattices
  We have theoretically investigated two-band models of graded-gap superlattices within the envelope-function approximation. Assuming that the gap varies linearly with spatial coordinate, we are able to find exact solutions of the corresponding Dirac-like equation describing the conduction- and valence-band envelope-functions. The dispersion relation inside allowed miniband of the superlattice may be expressed in terms of confluent hypergeometric functions in a closed form. 
B. Mendez and F. Dominguez-Adame (Departamento de Fisica de Materiales   Universidad Complutense, E-28040 Madrid, Spain)