Abstract
The in‐plane photoconductivity of n‐modulation‐doped quantum well structures of GaAs/AlGaAs is experimentally studied as a function of the incident photon energy. Negative photoconductivity with surprising dependences on background illumination, temperature, and electric field is observed for photon energies slightly below the band gap of GaAs. The effect may be due to optical filling of ionized deep donors in AlGaAs as well as to an optical backgating effect. High positive photoconductivity is present above the band gaps—in contrast to theoretical expectations from previous optical transport experiments.