The electron densities in the channel of Si delta -doped InGaAs-InAlAs high electron mobility transistors grown on InP by molecular-beam epitaxy have been investigated by 1.4 K Shubnikov-de Haas (S-dH) measurements. Growth procedures have been developed that result in (i) minimized spreading of the Si delta doping, (ii) significantly improved transfer of electrons from the Si donors to the InGaAs channel, (iii) reduced parallel conduction associated with electrons remaining in the vicinity of the Si donors, and (iv) high sheet density-mobility products (n(H) X mu (H)), important for a low access resistance into the channel. The Fermi energy of the free InAlAs surface has also been estimated by comparing self-consistent calculations of the channel electron density with the S-dH data from samples with progressively thinner InAlAs Schottky layers. The Fermi energy is found to be pinned at similar to0.6 eV below the conduction band edge, in agreement with the value deduced from photoreflectance spectrometry