We characterize the resistivity of InP buffer layers deposited by metal-organic vapor-phase epitaxy for the fabrication of Al-free GaInP/GaInAs high electron mobility transistors (HEMTs). Achieving highly-resistive InP buffer layers on semi-insulating (SI) InP:Fe substrates has long been recognized to be challenging. This is particularly true in HEMT applications because such devices are especially sensitive to the deleterious effects of buffer leakage currents. Our experiments show that impurities arising from the SI InP substrate as well as from reactor parts produce an overall n-type doping concentration of n=1–10×1016 cm-3 near the buffer/substrate interface, which decays exponentially to a level of 4×1014 cm-3 after approximately 1 μm of InP growth. This nonintentional impurity incorporation leads to a parasitic channel in the buffer and a current leakage path for the HEMT structures, regardless of the growth conditions used. Highly-insulating buffer layers could however be obtained with InP doped by Fe at a concentration of 6×1016 cm-3 in a thin region near the InP:Fe substrate. The sheet resistance consequently increased from RS=3 000 Ω/◻ for the not intentionally doped InP layers to RS=9.4×107 Ω/◻ when Fe-doping is used in the buffer layers, a value suitable for the realization of high-speed HEMTs. As a demonstration vehicle, Al-free pseudomorphic T-gate GaInP/GaInAs HEMTs with a 100 nm footprint were fabricated and achieved a cutoff frequency of fT=fMAX"- - 3c;250 GHz based on a still nonoptimized channel structure featuring a mobility and sheet carrier concentration of 10 000 cm2/V s and 1012 cm-2, respectively. The present work differentiates itself from previous Fe doping studies of InP by clarifying and quantifying the physical processes leading to parasitic conduction in not intentionally doped InP buffer layers grown on InP:Fe substrates.