We report the characteristics of molecular-beam epitaxy grown InAs on highly lattice mismatched (001) GaP substrates. Strain relaxation in this system occurs at low thickness by the generation of a periodic two-dimensional square grid network of 90 degrees misfit dislocations at the heterointerface. The very high interface dislocation density (similar to 10(13) intersections/cm(2)) exerts a unique influence on the electronic properties of the system. An extended defect structure at the intersection of 90 degrees misfit dislocations is proposed to act as an ordered structural donor source. Hall effect measurements indicate that this source is fully ionized with a constant sheet carrier concentration of 10(13) cm(-2), irrespective of the InAs layer thickness. and exhibits no freeze out at low temperatures. We have also demonstrated that the electron mobility increases significantly with InAs layer thickness, reaching values in excess of 10 000 cm(2)/V s in nominally undoped layers. The high threading dislocation density (similar to 10(10) cm(-2)) in the InAs epilayers does not appear to have a deleterious effect on the transport properties of majority carrier electrons.