We report on the investigation of the room‐temperature optoelectronic behavior of a metal–semiconductor–metal two‐dimensional electron gas photodiode based on the two‐dimensional electron gas of a high electron mobility transistor structure. The photodetector is fabricated in the InP/InGaAs material system, without use of Al‐containing layers. Optoelectronic measurements on a device with a finger spacing of 3 &mgr;m show a full width at half‐maximum (FWHM) of the pulse response of ≤60 ps, which is the resolution limit of our measurement equipment. Low‐temperature measurements at 40 K with electro‐optical sampling at a wavelength of 890 nm show a FWHM of 1 ps. ©1995 American Institute of Physics.