  We study resonant tunneling in B-$\delta$-doped diodes grown by Si-molecular
beam epitaxy. A Thomas-Fermi approach is used to obtain the conduction-band
modulation. Using a scalar Hamiltonian within the effective-mass approximation
we demonstrate that the occurrence of negative differential resistance (NDR)
only involves conduction-band states, whereas interband tunneling effects seem
to be negligible. Our theoretical results are in very good agreement with
recent experimental observations of NDR in this type of diodes.
