We report on the optimization of InP-based InxGa1-xAs/InyAl1-yAs pseudomorphic high electron mobility transistor (PHEMT) structures to achieve the highest possible two-dimensional-electron gas (2DEG) density and mobility. The layer structures are grown by solid-source molecular beam epitaxy with a valved phosphorus cracker cell. The single-side-doped PHEMT structure with a delta -doping concentration of 6 X 10(12) cm(-2) exhibits a 2DEG sheet density of 3.93 X 10(12) cm(-2) with a mobility of 11100 cm(2)/Vs at 300 K. The double-side-doped PHEMT structure with a bottom delta -doping concentration of 1 x 10(12) cm(-2) and a top delta -doping concentration of 5 X 10(12) cm(-2) gives a 2DEG sheet density of 4.57 x 10(12) cm(-2) with a mobility of 10 900 cm(2)/V s at 300 K. The electrical, optical and structural properties of the PHEMT structures were characterized by Hall, photoluminescence, and x-ray diffraction measurements.