The spectral dependence of extrinsic photoresponse is reported for measurements at 8 K on ‘‘fat‐gate’’ metal‐semiconductor field‐effect transistors (MESFETs) made by silicon implantation into undoped semi‐insulating GaAs, and for similarly implanted conductive layer samples. The interest, for the field‐effect transistors (FETs), lies in any evidence concerning deep‐level flaw contributions to a space charge at the channel–substrate interface. A large photosensitivity seen in the 1.0–1.5‐eV range was identified as associated with photoionization of EL2 midgap donors, from its spectrum and photobleaching characteristics. Peaks of photoresponse were noted also at 0.54, 0.66, 0.79, and 0.89 eV. The 0.89‐eV peak can be photobleached, and is attributed to EL2+photoneutralization at sites in the interface region. The same region is postulated for the sites causing the 0.54‐eV response, postanneal complexes dervied from the ‘‘Uband’’ of implant damage states. The origins are as yet undetermined for the 0.79‐eV and the (very weak) 0.66‐eV response regions.