  Physical self-adjoint extensions and their spectra of the simplest
one-dimensional Hamiltonian operator in which the mass is constant except for a
finite jump at one point of the real axis are correctly found. Some
self-adjoint extensions are used to model different kinds of semiconductor
heterojunctions within the effective-mass approximation. Their properties and
relation to different boundary conditions on envelope wave functions are
studied. The limiting case of equal masses (with no mass jump) are reviewed.
