The most important uncertainty of the transport parameters, i.e. mobility in quantum well layers, is due to the long term statistical variations of MBE growth. Although growth parameters and background concentrations in principal are controlled to high precision yielding high low field mobilities for a long time [312], industrial mass production of MBE layers with high throughput will lead to statistical variations, as growth parameters are not primarily optimized for highest mobility. The changes are mostly due to the variations of the background concentration and the interface roughness [312]. Damage and trap introduction by dry etching [88] is a secondary concern. For heterostructures the effect of etching on the mobility has been demonstrated in [32].