Undoped and Fe-doped liquid encapsulated Czochralski (LEC) InP has been studied by Hall effect, current–voltage (I–V), and infrared absorption (IR) spectroscopy. The results indicate that a native hydrogen vacancy complex donor defect exists in as-grown LEC InP. By studying the IR results, it is found that the concentration of this donor defect in Fe-doped InP is much higher than that in undoped InP. This result is consistent with the observation that a much higher concentration of Fe2+ than the apparent net donor concentration is needed to achieve the semi-insulating (SI) property in InP. By studying the I–V and IR results of Fe-doped InP wafers sliced from different positions on an ingot, the high concentration of Fe2+ is found to correlate with the existence of this hydrogen complex. The concentration of this donor defect is high in wafers from the top of an ingot. Correspondingly, a higher concentration of Fe2+ can be detected in these wafers. These results reveal the influence of the complex defect on the compensation and uniformity of Fe-doped SI InP materials. © 2001 American Institute of Physics.