  We have evaluated the density of interface trap states (Dit) at the surface
of a GaN/AlGaN/GaN heterojunction by the previously described gated van der
Pauw experiments, as well as by a UV assisted gated van der Pauw method,
described in this article. The obtained Dit values are about two orders of
magnitude lower than assumed by the surface-donor theory and three orders of
magnitude lower than required to compensate the polarization surface charge in
GaN. Previous experimental studies using a variety of other techniques reported
similarly low Dit values. We hence conclude that variable midgap surface-charge
is not responsible for the formation of the two-dimensional electron gas, and
cannot compensate for the large surface polarization charge in GaN. A yet
unexplained polarization self-compensating (PSC) surface charge must be invoked
to account for experiments. A few comments about the physical nature of the
proposed PSC charge are provided.
