5.2.3 Doping

As already stated, the background concentration of impurities in the non intentionally doped layers determines the Hall mobility. Changes of the activation of carriers in the $\delta$-doping due to a strong Si-F bound is suggested in both wet etched InAlAs/InGaAs HEMTs [71] and for dry etched AlGaAs/InGaAs HEMTs [127]. A number of reports state that processes that extensively use flourine, improve performance and reliability problems when reducing the use of flourine in the process, e.g. [137]. Thus, aggressive etching techniques statistically influence the number of carriers available. Furthermore in [153], the variations of the sheet charge concentration ${\it n}_{\mathrm{sheet}}$ are attributed to the molecular beam epitaxy (MBE) growth non-uniformities.