SIMS depth profiles of Si and Al as well as Hall mobility data are reported for inverted high electron mobility transistor (I-HEMT) structures grown by MBE at different substrate temperatures, T s , between 350 and 750°C. To obtain both high electron mobility μ(cm 2 (Vs) −1) and high electron density of the two-dimensional electron gas, the doped AlGaAs barrier was separated from the undoped GaAs layer by a 5 nm undoped AlGaAs spacer. By high resolution SIMS depth profiling this spacer was indeed found to be free of Si (detection limit 1×10 17 atoms cm −3) for structures grown at T s ⪕ 650°C . For T s = 700° C, however, the space contains more than 10 17 atoms cm −3. For T s = 750° C the heterojunction appears broadened and the spacer is doped with Si up to 1×10 18 atoms cm −3. The low temperature Hall data are in accordance with these observations. The highest mobility ( μ = 61,000 at T = 18 K) was recorded for T s = 500° C, and was found to decrease slowly to μ = 14,000 at T s = 700° C, then dropping abruptly to μ = 2300 for T s = 750° C. It is concluded that charge carrier scattering by ionized Si, which has migrated into the nominally undoped spacer, degrades the electrical performance of I-HEMTs grown at T s ⪖ 500° C. A control experiment demonstrated that the mechanism of Si migration is not diffusion, but segregation. At T s = 750° C a deterioration of the heterojunction and/ or a migration induced doping of the active channel is indicated.