Hard x-ray photoelectron spectroscopy (HAXPES) has been used to study metal-oxide-semiconductor (MOS) structures fabricated with both high (Ni) and low (Al) work-function metals on 8-nm thick Al${}_{2}$O${}_{3}$ dielectric layers, deposited on sulfur passivated $n$- and $p$-doped GaAs substrates. A binding energy difference of 0.6 eV was measured between the GaAs core levels of the $n$- and $p$-doped substrates in the absence of gate metals, indicating different Fermi level positions in the band gap. Subsequent photoemission measurements made on the MOS structures with the different work-function metals displayed very limited change in the GaAs core level binding energies, indicating that the movement of the Fermi level at the Al${}_{2}$O${}_{3}$/GaAs interface is restricted. Using a combination of HAXPES measurements and theoretical calculations, the Fermi level positions in the band gap have been determined to be in the range of 0.4\char21{}0.75 eV and 0.8\char21{}1.11 eV above the valence band maximum for $p$- and $n$-type GaAs, respectively. Analysis of capacitance voltage (C-V) measurements on identically prepared samples yield very similar Fermi level positions at zero applied gate bias. The C-V analysis also indicates a higher interface defect density ($D$${}_{it}$) in the upper half of the GaAs bandgap.