We report electronic Raman scattering measurements of the plasmon mode in a single GaAs/AlGaAs heterojunction, with a &dgr; layer of acceptors in the GaAs buffer a well‐defined distance from the interface. Under illumination above the band gap of the AlGaAs barrier, a dynamic charge‐transfer effect occurs in which the quasi‐two‐dimensional electron concentration of the hetrojunction decreases. From Raman measurements of the plasmon mode we directly determine the change in carrier concentration with excess illumination. We obtain a time of &tgr;=120 ps for the transfer of electrons from the AlGaAs barrier into the two‐dimensional channel.