In contradiction to a number of publications the
surface charge density or corresponding surface
potential [50] is not considered an arbitrary fitting
parameter in this work. A strong pinning of the surface prevails
for the SiN/AlGaAs interface which significantly interferes with
the transport near the surface [116]. Typical depletion
depths even for doping concentrations of
cm amount to 10 nm, as can be
calculated with a one-dimensional *Schrödinger Poisson*
solver [95]. Given the scaled gate geometries, this
depletion interferes with channel transport. The everlasting
attempts to process stable GaAs based Metal Insulator (MIS)
transistors, e.g. [229], shows that it is nearly
impossible to depin the Fermi level at SiN/GaAs based III-V
semiconductor interfaces, which is a prerequisite to obtain a
channel as in a Si MOSFET. For III-V semiconductors
Table 3.37 gives a collection of pinning levels taken
from literature, always measured from the valence band edge.

For the most relevant barrier materials AlGaAs and cap material GaAs the Fermi level is typically pinned near midgap. For InAlAs the level are typical higher than midgap.

2001-12-21