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 
-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
are attributed to the molecular beam epitaxy
(MBE) growth non-uniformities.