  MnAs/GaAs superlattices, made by $\delta$-doping GaAs with Mn, are known as
digital ferromagnetic heterostructures. Here we present a theoretical density
functional study of the electronic, magnetic and transport properties of such
heterostructures. In the absence of intrinsic donors these systems show an half
metallic density of states, with an exchange interaction much stronger than
that of a random alloy with the same Mn concentration. {\it Ab initio}
ballistic transport calculations show that the carriers with energies close to
the Fermi energy are strongly confined within a few monolayers around the MnAs
plane. This strong confinement is responsible for the large exchange coupling.
Therefore the system can be described as a two dimensional half metal with
large conductance in the MnAs plane and small conductance in the perpendicular
direction.
