Intersubband transitions in GaAs/AlxGa1−xAs quantum wells have been studied as a function of the doping level by Raman scattering and infrared absorption. Then‐type dopant concentration placed in the well was varied between 1×1018and 8×1018cm−3. With increasing doping level Raman scattering reveals a frequency down‐shift of the single‐particle intersubband transition and a shift to higher frequencies of the collective intersubband plasmon‐phonon mode. The resonance in infrared absorption follows closely the collective mode observed in Raman scattering, demonstrating clearly that the doping‐dependent depolarization shift of the absorption peak is an important parameter that must be taken into account in device design. Possible models for the frequency down‐shift of the single‐particle transition are also discussed.