Once principal components are obtained, these main components are naturally subject to interpretation. Initially, it is not determined at all, that the principal components have a direct physical meaning, they primarily simplify the data set. Only secondary exclusions and comparisons allow for their interpretation. Factor 1 can be considered to be strongly influenced by the sheet charge density , if not the sheet carrier density itself. The following reasoning explains this idea:

- is strongly negatively correlated to and , which is true for Factor 1.
- is negatively correlated to , that is increasing leads lower breakdown voltage, which is one reason to reduce the -doping in power HEMTs.
- Both and are positively correlated to .
- An increased sheet charge density leads to an increase of , which is true for Factor 1.

For Factor 2, no direct physical correlation could be found.

More generally, the variations in III-V HEMTs processing are different from those typical from silicon CMOS due to the wafer size. For 2-4 inch wafers the statistical sample is smaller and singular events and systematic changes mix. A list of typical variations is given in the following, some origins of these variations are given in the next section.

- the gate-to-channel separation [50].
- the channel sheet carrier concentration [153].
- the gate length .
- the alignment of the gate contact [153].
- the alignment of additional masks for double recess HEMTs.
- the etch depth for the cap etching of the second recess.

Stepping from 4 inch to 6 and 8 inch wafers the importance shifts from singular events to statistical variations due to the increase of the cell numbers in the sample.

2001-12-21