Different aspects of the metastable character of Si‐related localized states in modulation‐doped heterostructures of GaAs/GaAlAs have been studied. Hydrostatic pressure was used to change the two‐dimensional electron‐gas concentration at the active interface of the system. The performed studies give evidence that below a critical temperature ofTc=135 K for a given sample, at the same temperature and pressure conditions, an arbitrary carrier concentration in the quantum well can be obtained depending on the pressure at which the sample was cooled.