3.6.3 Surface Potentials

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 ${\it N}_{\mathrm{D}}= 3\times10^{18}$cm$^{-3}$ amount to $\approx$ 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.

Table 3.37: Energies of the pinned Fermi level.

Material	$E_F$	References
	[eV]
n-GaAs	0.61,0.72	[256,323]
Al$_{x}$Ga$_{1-x}$As	0.8	[256]
In$_{0.52}$Al$_{0.48}$As/SiN	0.8-1	[18]
In$_y$Al$_{1-y}$As/SiN 0.42 $\leq$y $\leq$ 0.57	0.9-1.3	[66]
In$_{0.53}$Ga$_{0.47}$As/SiN	0.37-0.77	[18]

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.