The photoresponse of In0.5Ga0.5As metal-semiconductor-metal photodetectors is related to the presence of a hole trap. Detectors made from material grown with an In0.5Al0.5As buffer layer had no measurable trap density when examined using deep-level transient spectroscopy, and the full width half maximum (FWHM) of the photoresponse was 80 ps at 5 V bias for 3 mu m interdigitated fingers and spacings. Detectors made from material grown without an In0.5Al0.5As buffer layer had a hole trap and a FWHM photoresponse of 220 ps. This deep hole trap is likely related to impurities that diffused upward from an interface of an InP substrate and an InGaAs epilayer.